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1156873 | https://en.wikipedia.org/wiki/Volatile%20memory | Volatile memory | Volatile memory, in contrast to non-volatile memory, is computer memory that requires power to maintain the stored information; it retains its contents while powered on but when the power is interrupted, the stored data is quickly lost.
Volatile memory has several uses including as primary storage. In addition to usually being faster than forms of mass storage such as a hard disk drive, volatility can protect sensitive information, as it becomes unavailable on power-down. Most general-purpose random-access memory (RAM) is volatile.
Types
There are two kinds of volatile RAM: dynamic and static. Even though both types need continuous electrical current to retain data, there are some important differences between them.
Dynamic RAM (DRAM) is very popular due to its cost-effectiveness. DRAM stores each bit of information in a different capacitor within the integrated circuit. DRAM chips need just one single capacitor and one transistor to store each bit of information. This makes it space-efficient and inexpensive.
The main advantage of static RAM (SRAM) is that it is much faster than dynamic RAM. Its disadvantage is its high price. SRAM does not need continuous electrical refreshes, but it still requires constant current to sustain the difference in voltage. Every single bit in a static RAM chip needs a cell of six transistors, whereas dynamic RAM requires only one capacitor and one transistor. As a result, SRAM is unable to accomplish the storage capabilities of the DRAM family. SRAM is commonly used as CPU cache and for processor registers and in networking devices.
| Technology | Volatile memory | null |
1156934 | https://en.wikipedia.org/wiki/Ammonium%20carbonate | Ammonium carbonate | Ammonium carbonate is a chemical compound with the chemical formula . It is an ammonium salt of carbonic acid. It is composed of ammonium cations and carbonate anions . Since ammonium carbonate readily degrades to gaseous ammonia and carbon dioxide upon heating, it is used as a leavening agent and also as smelling salt. It is also known as baker's ammonia and is a predecessor to the more modern leavening agents baking soda and baking powder. It is a component of what was formerly known as sal volatile and salt of hartshorn, and produces a pungent smell when baked. It comes in the form of a white powder or block, with a molar mass of 96.09 g/mol and a density of 1.50 g/cm3. It is a strong electrolyte.
Production
Ammonium carbonate is produced by combining carbon dioxide and aqueous ammonia. About 80,000 tons/year were produced as of 1997.
An orthorhombic ammonium carbonate monohydrate is known (). It crystallizes in an ammonia solution exposed in a carbon dioxide-rich atmosphere.
Decomposition
Ammonium carbonate slowly decomposes at standard temperature and pressure through two pathways. Thus any initially pure sample of ammonium carbonate will soon become a mixture including various byproducts.
Ammonium carbonate can spontaneously decompose into ammonium bicarbonate and ammonia:
Which further decomposes to carbon dioxide, water and another molecule of ammonia:
Uses
Leavening agent
Ammonium carbonate may be used as a leavening agent in traditional recipes, particularly those from northern Europe and Scandinavia (e.g. Amerikaner, Speculoos, Tunnbröd or Lebkuchen). It was the precursor to today's more commonly used baking powder.
Originally made from ground deer horn and called hartshorn, today it is called baker's ammonia. It is prepared by the sublimation of a mixture of ammonium sulfate and calcium carbonate and occurs as a white powder or a hard, white or translucent mass. It acts as a heat activated leavening agent and breaks down into carbon dioxide (leavening), ammonia (which needs to dissipate) and water. It is sometimes combined with sodium bicarbonate double acting baking powder and to help mask any ammonia smell not baked out.
It also serves as an acidity regulator and has the E number E503. It can be replaced with baking powder, but this may affect both the taste and texture of the finished product. Baker's ammonia should be used to create thin dry baked goods like crackers and cookies. This allows the strong ammonia smell to bake out. It should not be used to make moist baked items like cake since ammonia is hydrophilic and will leave a strong bitter taste.
Its use as a leavening agent, with associated controversy, goes back centuries:
Other uses
Ammonium carbonate is the main component of smelling salts, although the commercial scale of their production is small. Buckley's cough syrup from Canada today uses ammonium carbonate as an active ingredient intended to help relieve symptoms of bronchitis. It is also used as an emetic. It is also found in smokeless tobacco products, such as Skoal, and it is used in aqueous solution as a photographic lens cleaning agent, such as Eastman Kodak's "Kodak Lens Cleaner."
It is also used for luring of apple maggots in Washington State, to monitor the spread of the infestation and adjust the borders of the Apple Maggot Quarantine Area.
| Physical sciences | Carbonic oxyanions | Chemistry |
1156993 | https://en.wikipedia.org/wiki/Rolling | Rolling | Rolling is a type of motion that combines rotation (commonly, of an axially symmetric object) and translation of that object with respect to a surface (either one or the other moves), such that, if ideal conditions exist, the two are in contact with each other without sliding.
Rolling where there is no sliding is referred to as pure rolling. By definition, there is no sliding when there is a frame of reference in which all points of contact on the rolling object have the same velocity as their counterparts on the surface on which the object rolls; in particular, for a frame of reference in which the rolling plane is at rest (see animation), the instantaneous velocity of all the points of contact (for instance, a generating line segment of a cylinder) of the rolling object is zero.
In practice, due to small deformations near the contact area, some sliding and energy dissipation occurs. Nevertheless, the resulting rolling resistance is much lower than sliding friction, and thus, rolling objects typically require much less energy to be moved than sliding ones. As a result, such objects will more easily move, if they experience a force with a component along the surface, for instance gravity on a tilted surface, wind, pushing, pulling, or torque from an engine. Unlike cylindrical axially symmetric objects, the rolling motion of a cone is such that while rolling on a flat surface, its center of gravity performs a circular motion, rather than a linear motion. Rolling objects are not necessarily axially-symmetrical. Two well known non-axially-symmetrical rollers are the Reuleaux triangle and the Meissner bodies. The oloid and the sphericon are members of a special family of developable rollers that develop their entire surface when rolling down a flat plane. Objects with corners, such as dice, roll by successive rotations about the edge or corner which is in contact with the surface. The construction of a specific surface allows even a perfect square wheel to roll with its centroid at constant height above a reference plane.
Applications
Most land vehicles use wheels and therefore rolling for displacement. Slip should be kept to a minimum (approximating pure rolling), otherwise loss of control and an accident may result. This may happen when the road is covered in snow, sand, or oil, when taking a turn at high speed or attempting to brake or accelerate suddenly.
One of the most practical applications of rolling objects is the use of rolling-element bearings, such as ball bearings, in rotating devices. Made of metal, the rolling elements are usually encased between two rings that can rotate independently of each other. In most mechanisms, the inner ring is attached to a stationary shaft (or axle). Thus, while the inner ring is stationary, the outer ring is free to move with very little friction. This is the basis for which almost all motors (such as those found in ceiling fans, cars, drills, etc.) rely on to operate. Alternatively, the outer ring may be attached to a fixed support bracket, allowing the inner ring to support an axle, allowing for rotational freedom of an axle. The amount of friction on the mechanism's parts depends on the quality of the ball bearings and how much lubrication is in the mechanism.
Rolling objects are also frequently used as tools for transportation. One of the most basic ways is by placing a (usually flat) object on a series of lined-up rollers, or wheels. The object on the wheels can be moved along them in a straight line, as long as the wheels are continuously replaced in the front (see history of bearings). This method of primitive transportation is efficient when no other machinery is available. Today, the most practical application of objects on wheels are cars, trains, and other human transportation vehicles.
Rolling is used to apply normal forces to a moving line of contact in various processes, for example in metalworking, printing, rubber manufacturing, painting.
Rigid bodies
The simplest case of rolling is that of a rigid body rolling without slipping along a flat surface with its axis parallel to the surface (or equivalently: perpendicular to the surface normal).
The trajectory of any point is a trochoid; in particular, the trajectory of any point in the object axis is a line, while the trajectory of any point in the object rim is a cycloid.
The velocity of any point in the rolling object is given by , where is the displacement between the particle and the rolling object's contact point (or line) with the surface, and ω is the angular velocity vector. Thus, despite that rolling is different from rotation around a fixed axis, the instantaneous velocity of all particles of the rolling object is the same as if it was rotating around an axis that passes through the point of contact with the same angular velocity.
Any point in the rolling object farther from the axis than the point of contact will temporarily move opposite to the direction of the overall motion when it is below the level of the rolling surface (for example, any point in the part of the flange of a train wheel that is below the rail).
Energy
Since kinetic energy is entirely a function of an object mass and velocity, the above result may be used with the parallel axis theorem to obtain the kinetic energy associated with simple rolling
Forces and acceleration
Differentiating the relation between linear and angular velocity, , with respect to time gives a formula relating linear and angular acceleration . Applying Newton's second law:
It follows that to accelerate the object, both a net force and a torque are required. When external force with no torque acts on the rolling object‐surface system, there will be a tangential force at the point of contact between the surface and rolling object that provides the required torque as long as the motion is pure rolling; this force is usually static friction, for example, between the road and a wheel or between a bowling lane and a bowling ball. When static friction isn't enough, the friction becomes dynamic friction and slipping happens. The tangential force is opposite in direction to the external force, and therefore partially cancels it. The resulting net force and acceleration are:
has dimension of mass, and it is the mass that would have a rotational inertia at distance from an axis of rotation. Therefore, the term may be thought of as the mass with linear inertia equivalent to the rolling object rotational inertia (around its center of mass). The action of the external force upon an object in simple rotation may be conceptualized as accelerating the sum of the real mass and the virtual mass that represents the rotational inertia, which is . Since the work done by the external force is split between overcoming the translational and rotational inertia, the external force results in a smaller net force by the dimensionless multiplicative factor where represents the ratio of the aforesaid virtual mass to the object actual mass and it is equal to where is the radius of gyration corresponding to the object rotational inertia in pure rotation (not the rotational inertia in pure rolling). The square power is due to the fact rotational inertia of a point mass varies proportionally to the square of its distance to the axis.
In the specific case of an object rolling in an inclined plane which experiences only static friction, normal force and its own weight, (air drag is absent) the acceleration in the direction of rolling down the slope is:
is specific to the object shape and mass distribution, it does not depend on scale or density. However, it will vary if the object is made to roll with different radiuses; for instance, it varies between a train wheel set rolling normally (by its tire), and by its axle. It follows that given a reference rolling object, another object bigger or with different density will roll with the same acceleration. This behavior is the same as that of an object in free fall or an object sliding without friction (instead of rolling) down an inclined plane.
Deformable bodies
When an axisymmetric deformable body contacts a surface, an interface is formed through which normal and shear forces may be transmitted. For example, a tire contacting the road carries the weight (normal load) of the car as well as any shear forces arising due to acceleration, braking or steering. The deformations and motions in a steady rolling body can be efficiently characterized using an Eulerian description of rigid body rotation and a Lagrangian description of deformation. This approach greatly simplifies analysis by eliminating time-dependence, resulting in displacement, velocity, stress and strain fields that vary only spatially. Analysis procedures for finite element analysis of steady state rolling were first developed by Padovan, and are now featured in several commercial codes.
| Physical sciences | Classical mechanics | Physics |
11551222 | https://en.wikipedia.org/wiki/Heat%20pump%20and%20refrigeration%20cycle | Heat pump and refrigeration cycle | Thermodynamic heat pump cycles or refrigeration cycles are the conceptual and mathematical models for heat pump, air conditioning and refrigeration systems. A heat pump is a mechanical system that transmits heat from one location (the "source") at a certain temperature to another location (the "sink" or "heat sink") at a higher temperature. Thus a heat pump may be thought of as a "heater" if the objective is to warm the heat sink (as when warming the inside of a home on a cold day), or a "refrigerator" or "cooler" if the objective is to cool the heat source (as in the normal operation of a freezer). The operating principles in both cases are the same; energy is used to move heat from a colder place to a warmer place.
Thermodynamic cycles
According to the second law of thermodynamics, heat cannot spontaneously flow from a colder location to a hotter area; work is required to achieve this. An air conditioner requires work to cool a living space, moving heat from the interior being cooled (the heat source) to the outdoors (the heat sink). Similarly, a refrigerator moves heat from inside the cold icebox (the heat source) to the warmer room-temperature air of the kitchen (the heat sink). The operating principle of an ideal heat engine was described mathematically using the Carnot cycle by Sadi Carnot in 1824. An ideal refrigerator or heat pump can be thought of as an ideal heat engine that is operating in a reverse Carnot cycle.
Heat pump cycles and refrigeration cycles can be classified as vapor compression, vapor absorption, gas cycle, or Stirling cycle types.
Vapor-compression cycle
The vapor-compression cycle is used by many refrigeration, air conditioning, and other cooling applications and also within heat pump for heating applications. There are two heat exchangers, one being the condenser, which is hotter and releases heat, and the other being the evaporator, which is colder and accepts heat. For applications which need to operate in both heating and cooling modes, a reversing valve is used to switch the roles of these two heat exchangers.
At the start of the thermodynamic cycle the refrigerant enters the compressor as a low pressure and low temperature vapor. In heat pumps, this refrigerant is typically R32 refrigerant or R290 refrigerant. Then the pressure is increased and the refrigerant leaves as a higher temperature and higher pressure superheated gas. This hot pressurised gas then passes through the condenser where it releases heat to the surroundings as it cools and condenses completely. The cooler high-pressure liquid next passes through the expansion valve (throttle valve) which reduces the pressure abruptly causing the temperature to drop dramatically. The cold low pressure mixture of liquid and vapor next travels through the evaporator where it vaporizes completely as it accepts heat from the surroundings before returning to the compressor as a low pressure low temperature gas to start the cycle again.
Some simpler applications with fixed operating temperatures, such as a domestic refrigerator, may use a fixed speed compressor and fixed aperture expansion valve. Applications that need to operate at a high coefficient of performance in very varied conditions, as is the case with heat pumps where external temperatures and internal heat demand vary considerably through the seasons, typically use a variable speed inverter compressor and an adjustable expansion valve to control the pressures of the cycle more accurately.
The above discussion is based on the ideal vapor-compression refrigeration cycle and does not take into account real-world effects like frictional pressure drop in the system, slight thermodynamic irreversibility during the compression of the refrigerant vapor, or non-ideal gas behavior (if any).
Vapor absorption cycle
In the early years of the twentieth century, the vapor absorption cycle using water-ammonia systems was popular and widely used but, after the development of the vapor compression cycle, it lost much of its importance because of its low coefficient of performance (about one fifth of that of the vapor compression cycle). Nowadays, the vapor absorption cycle is used only where heat is more readily available than electricity, such as industrial waste heat, solar thermal energy by solar collectors, or off-the-grid refrigeration in recreational vehicles.
The absorption cycle is similar to the compression cycle, but depends on the partial pressure of the refrigerant vapor. In the absorption system, the compressor is replaced by an absorber and a generator. The absorber dissolves the refrigerant in a suitable liquid (dilute solution) and therefore the dilute solution becomes a strong solution. In the generator, on heat addition, the temperature increases, and with it, the partial pressure of the refrigerant vapor is released from the strong solution. However, the generator requires a heat source, which would consume energy unless waste heat is used. In an absorption refrigerator, a suitable combination of refrigerant and absorbent is used. The most common combinations are ammonia (refrigerant) and water (absorbent), and water (refrigerant) and lithium bromide (absorbent).
Absorption refrigeration systems can be powered by combustion of fossil fuels (e.g., coal, oil, natural gas, etc.) or renewable energy (e.g., waste-heat recovery, biomass combustion, or solar energy).
Gas cycle
When the working fluid is a gas that is compressed and expanded but does not change phase, the refrigeration cycle is called a gas cycle. Air is most often this working fluid. As there is no condensation and evaporation intended in a gas cycle, components corresponding to the condenser and evaporator in a vapor compression cycle are the hot and cold gas-to-gas heat exchangers.
For given extreme temperatures, a gas cycle may be less efficient than a vapor compression cycle because the gas cycle works on the reverse Brayton cycle instead of the reverse Rankine cycle. As such, the working fluid never receives or rejects heat at constant temperature. In the gas cycle, the refrigeration effect is equal to the product of the specific heat of the gas and the rise in temperature of the gas in the low temperature side. Therefore, for the same cooling load, gas refrigeration cycle machines require a larger mass flow rate, which in turn increases their size.
Because of their lower efficiency and larger bulk, air cycle coolers are not often applied in terrestrial refrigeration. The air cycle machine is very common, however, on gas turbine-powered jet airliners since compressed air is readily available from the engines' compressor sections. These jet aircraft's cooling and ventilation units also serve the purpose of heating and pressurizing the aircraft cabin.
Stirling engine
The Stirling cycle heat engine can be driven in reverse, using a mechanical energy input to drive heat transfer in a reversed direction (i.e. a heat pump, or refrigerator). There are several design configurations for such devices that can be built. Several such setups require rotary or sliding seals, which can introduce difficult tradeoffs between frictional losses and refrigerant leakage.
Reversed Carnot cycle
The Carnot cycle, which has a quantum equivalent, is reversible so the four processes that comprise it, two isothermal and two isentropic, can also be reversed. When a Carnot cycle runs in reverse, it is called a reverse Carnot cycle. A refrigerator or heat pump that acts according to the reversed Carnot cycle is called a Carnot refrigerator or Carnot heat pump, respectively. In the first stage of this cycle, the refrigerant absorbs heat isothermally from a low-temperature source, , in the amount . Next, the refrigerant is compressed isentropically (adiabatically, without heat transfer) and its temperature rises to that of the high-temperature source, . Then at this high temperature, the refrigerant isothermally rejects heat in the amount (negative according to the sign convention for heat lost by the system). Also during this stage, the refrigerant changes from a saturated vapor to a saturated liquid in the condenser. Lastly, the refrigerant expands isentropically until its temperature falls to that of the low-temperature source, .
Absorption-compression heat pump
An absorption-compression heat pump (ACHP) is a device that integrate an electric compressor in an absorption heat pump. In some cases this is obtained by combining a vapor-compression heat pump and an absorption heat pump. It is also referred to as a hybrid heat pump which is however a broader field. Thanks to this integration, the device can obtain cooling and heating effects using both thermal and electrical energy sources. This type of systems is well coupled with cogeneration systems where both heat and electricity are produced. Depending on the configuration, the system can maximise heating and cooling production from a given amount of fuel, or can improve the temperature (hence the quality) of waste heat from other processes. This second use is the most studied one and has been applied to several industrial applications.
Coefficient of performance
The merit of a refrigerator or heat pump is given by a parameter called the coefficient of performance (COP). The equation is:
where
is the useful heat given off or taken up by the considered system.
is the net work done on the considered system in one cycle.
The detailed COP of a refrigerator is given by the following equation:
The COP of a heat pump (sometimes referred to as coefficient of amplification COA) is given by the following equations, where the first law of thermodynamics: and was used in one of the last steps:
Both the COP of a refrigerator and a heat pump can be greater than one. Combining these two equations results in:
for fixed values of and .
This implies that will be greater than one because will be a positive quantity. In a worst-case scenario, the heat pump will supply as much energy as it consumes, making it act as a resistance heater. However, in reality, as in home heating, some of is lost to the outside air through piping, insulation, etc., thus making the drop below unity when the outside air temperature is too low.
For Carnot refrigerators and heat pumps, the COP can be expressed in terms of temperatures:
These are the upper limits for the COP of any system operating between and .
| Physical sciences | Thermodynamics | Physics |
10115658 | https://en.wikipedia.org/wiki/Molecular%20self-assembly | Molecular self-assembly | In chemistry and materials science, molecular self-assembly is the process by which molecules adopt a defined arrangement without guidance or management from an outside source. There are two types of self-assembly: intermolecular and intramolecular. Commonly, the term molecular self-assembly refers to the former, while the latter is more commonly called folding.
Supramolecular systems
Molecular self-assembly is a key concept in supramolecular chemistry. This is because assembly of molecules in such systems is directed through non-covalent interactions (e.g., hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, pi-stacking interactions, and/or electrostatic) as well as electromagnetic interactions. Common examples include the formation of colloids, biomolecular condensates, micelles, vesicles, liquid crystal phases, and Langmuir monolayers by surfactant molecules. Further examples of supramolecular assemblies demonstrate that a variety of different shapes and sizes can be obtained using molecular self-assembly.
Molecular self-assembly allows the construction of challenging molecular topologies. One example is Borromean rings, interlocking rings wherein removal of one ring unlocks each of the other rings. DNA has been used to prepare a molecular analog of Borromean rings. More recently, a similar structure has been prepared using non-biological building blocks.
Biological systems
Molecular self-assembly underlies the construction of biologic macromolecular assemblies and biomolecular condensates in living organisms, and so is crucial to the function of cells. It is exhibited in the self-assembly of lipids to form the membrane, the formation of double helical DNA through hydrogen bonding of the individual strands, and the assembly of proteins to form quaternary structures. Molecular self-assembly of incorrectly folded proteins into insoluble amyloid fibers is responsible for infectious prion-related neurodegenerative diseases. Molecular self-assembly of nanoscale structures plays a role in the growth of the remarkable β-keratin lamellae/setae/spatulae structures used to give geckos the ability to climb walls and adhere to ceilings and rock overhangs.
Protein multimers
When multiple copies of a polypeptide encoded by a gene self-assemble to form a complex, this protein structure is referred to as a "multimer". Genes that encode multimer-forming polypeptides appear to be common. When a multimer is formed from polypeptides produced by two different mutant alleles of a particular gene, the mixed multimer may exhibit greater functional activity than the unmixed multimers formed by each of the mutants alone. In such a case, the phenomenon is referred to as intragenic complementation. Jehle pointed out that, when immersed in a liquid and intermingled with other molecules, charge fluctuation forces favor the association of identical molecules as nearest neighbors.
Nanotechnology
Molecular self-assembly is an important aspect of bottom-up approaches to nanotechnology. Using molecular self-assembly, the final (desired) structure is programmed in the shape and functional groups of the molecules. Self-assembly is referred to as a 'bottom-up' manufacturing technique in contrast to a 'top-down' technique such as lithography where the desired final structure is carved from a larger block of matter. In the speculative vision of molecular nanotechnology, microchips of the future might be made by molecular self-assembly. An advantage to constructing nanostructure using molecular self-assembly for biological materials is that they will degrade back into individual molecules that can be broken down by the body.
DNA nanotechnology
DNA nanotechnology is an area of current research that uses the bottom-up, self-assembly approach for nanotechnological goals. DNA nanotechnology uses the unique molecular recognition properties of DNA and other nucleic acids to create self-assembling branched DNA complexes with useful properties. DNA is thus used as a structural material rather than as a carrier of biological information, to make structures such as complex 2D and 3D lattices (both tile-based as well as using the "DNA origami" method) and three-dimensional structures in the shapes of polyhedra. These DNA structures have also been used as templates in the assembly of other molecules such as gold nanoparticles and streptavidin proteins.
Two-dimensional monolayers
The spontaneous assembly of a single layer of molecules at interfaces is usually referred to as two-dimensional self-assembly. One of the common examples of such assemblies are Langmuir-Blodgett monolayers and multilayers of surfactants. Non-surface active molecules can assemble into ordered structures as well. Early direct proofs showing that non-surface active molecules can assemble into higher-order architectures at solid interfaces came with the development of scanning tunneling microscopy and shortly thereafter. Eventually two strategies became popular for the self-assembly of 2D architectures, namely self-assembly following ultra-high-vacuum deposition and annealing and self-assembly at the solid-liquid interface. The design of molecules and conditions leading to the formation of highly-crystalline architectures is considered today a form of 2D crystal engineering at the nanoscopic scale.
| Physical sciences | Supramolecular chemistry | Chemistry |
19178886 | https://en.wikipedia.org/wiki/Protist | Protist | A protist ( ) or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus. Protists do not form a natural group, or clade, but are a polyphyletic grouping of several independent clades that evolved from the last eukaryotic common ancestor.
Protists were historically regarded as a separate taxonomic kingdom known as Protista or Protoctista. With the advent of phylogenetic analysis and electron microscopy studies, the use of Protista as a formal taxon was gradually abandoned. In modern classifications, protists are spread across several eukaryotic clades called supergroups, such as Archaeplastida (photoautotrophs that includes land plants), SAR, Obazoa (which includes fungi and animals), Amoebozoa and Excavata.
Protists represent an extremely large genetic and ecological diversity in all environments, including extreme habitats. Their diversity, larger than for all other eukaryotes, has only been discovered in recent decades through the study of environmental DNA and is still in the process of being fully described. They are present in all ecosystems as important components of the biogeochemical cycles and trophic webs. They exist abundantly and ubiquitously in a variety of forms that evolved multiple times independently, such as free-living algae, amoebae and slime moulds, or as important parasites. Together, they compose an amount of biomass that doubles that of animals. They exhibit varied types of nutrition (such as phototrophy, phagotrophy or osmotrophy), sometimes combining them (in mixotrophy). They present unique adaptations not present in multicellular animals, fungi or land plants. The study of protists is termed protistology.
Definition
Protists are a diverse group of eukaryotes (organisms whose cells possess a nucleus) that are primarily single-celled and microscopic but exhibit a wide variety of shapes and life strategies. They have different life cycles, trophic levels, modes of locomotion, and cellular structures. Although most protists are unicellular, there is a considerable range of multicellularity amongst them; some form colonies or multicellular structures visible to the naked eye. The term 'protist' is defined as a paraphyletic group of all eukaryotes that are not animals, plants or fungi, the three traditional eukaryotic kingdoms. Because of this definition by exclusion, the term includes the ancestors of those three kingdoms.
The names of some protists (called ambiregnal protists), because of their mixture of traits similar to both animals and plants or fungi (e.g., slime molds and flagellated algae like euglenids), have been published under either or both of the botanical (ICN) and the zoological (ICZN) codes of nomenclature.
Common types
Protists display a wide range of distinct morphological types that have been used to classify them for practical purposes, although most of these categories do not represent evolutionary cohesive lineages or clades and have instead evolved independently several times. The most recognizable types are:
Amoebae. Characterized by their irregular, flexible shapes, these protists move by extending portions of their cytoplasm, known as pseudopodia, to crawl along surfaces. Many groups of amoebae are naked, but testate amoebae and foraminifera grow a shell around their cell made from digested material or surrounding debris. Some, known as radiolarians and heliozoans, have special spherical shapes with microtubule-supported pseudopodia radiating from the cell. Some amoebae are capable of producing stalked multicellular stages that bear spores, often by aggregating together; these are known as slime molds. The main clades containing amoebae are Amoebozoa (including various slime molds and testate amoebae) and Rhizaria (including famous groups such as foraminifera and radiolarians, as well as a few testate amoebae). Even some individual amoebae can grow to giant sizes visible to the naked eye.
Flagellates. These protists are equipped with one or more whip-like appendages called cilia, undulipodia or eukaryotic flagella, which enable them to swim or glide freely through the environment. Flagellates are found in all lineages, reflecting that the common ancestor of all living eukaryotes was a flagellate. They usually exhibit two cilia (e.g., in Provora, Telonemia, Stramenopiles, Alveolata, Obazoa and most excavates), but there are a number of flagellate groups with a high number of cilia (such as Hemimastigophora and other excavates). Some groups, such as the well-known ciliates and the parasitic opalinids, have a cell surface covered in rows of cilia that beat rhythmically. A few groups of amoebae have retained their flagella, making them amoeboflagellates.
Algae. They are the photosynthetic protists, and can be found in most of the main clades, completely intermingled with heterotrophic protists which are traditionally called protozoa. Algae exhibit the most diverse range of morphologies, from single flagellated or coccoid cells (e.g., cryptophytes, haptophytes, dinoflagellates, chromerids, many green algae, ochrophytes, euglenophytes) to amoeboid cells (chlorarachniophytes) to colonial and multicellular macroscopic forms (e.g., red algae, some green algae, and some ochrophytes such as kelp).
Fungus-like protists. Several clades of protists have evolved an appearance similar to fungi through hyphae-like structures and a saprophytic nutrition. They have evolved multiple times, often very distantly from true fungi (e.g., the oomycetes, labyrinthulomycetes and hyphochytrids, in Stramenopiles; the myxomycetes, in Amoebozoa; the phytomyxeans, in Rhizaria; the perkinsozoans, in Alveolata).
Sporozoa. This category traditionally included parasitic protists that reproduced via spores (the apicomplexans, microsporidians, myxozoans and ascetosporeans). Its current use is restricted to the apicomplexans, such as Plasmodium falciparum, the cause of malaria.
Diversity
The species diversity of protists is severely underestimated by traditional methods that differentiate species based on morphological characteristics. The number of described protist species is very low (ranging from 26,000 to over 76,000) in comparison to the diversity of plants, animals and fungi, which are historically and biologically well-known and studied. The predicted number of species also varies greatly, ranging from 1.4×10 to 1.6×10, and in several groups the number of predicted species is arbitrarily doubled. Most of these predictions are highly subjective. Molecular techniques such as environmental DNA barcoding have revealed a vast diversity of undescribed protists that accounts for the majority of eukaryotic sequences or operational taxonomic units (OTUs), dwarfing those from plants, animals and fungi. As such, it is considered that protists dominate eukaryotic diversity.
The evolutionary relationships of protists have been explained through molecular phylogenetics, the sequencing of entire genomes and transcriptomes, and electron microscopy studies of the flagellar apparatus and cytoskeleton. New major lineages of protists and novel biodiversity continue to be discovered, resulting in dramatic changes to the eukaryotic tree of life. The newest classification systems of eukaryotes do not recognize the formal taxonomic ranks (kingdom, phylum, class, order...) and instead only recognize clades of related organisms, making the classification more stable in the long term and easier to update. In this new cladistic scheme, the protists are divided into various branches informally named supergroups. Most photosynthetic eukaryotes fall under the Diaphoretickes clade, which contains the supergroups Archaeplastida (which includes plants) and TSAR (including Telonemia, Stramenopiles, Alveolata and Rhizaria), as well as the phyla Cryptista and Haptista. The animals and fungi fall into the Amorphea supergroup, which contains the phylum Amoebozoa and several other protist lineages. Various groups of eukaryotes with primitive cell architecture are collectively known as the Excavata.
Excavata
Excavata is a group that encompasses diverse protists, mostly flagellates, ranging from aerobic and anaerobic predators to phototrophs and heterotrophs. The common name 'excavate' refers to the common characteristic of a ventral groove in the cell used for suspension feeding, which is considered to be an ancestral trait present in the last eukaryotic common ancestor. The Excavata is composed of three clades: Discoba, Metamonada and Malawimonadida, each including 'typical excavates' that are free-living phagotrophic flagellates with the characteristic ventral groove. According to most phylogenetic analyses, this group is paraphyletic, with some analyses placing the root of the eukaryote tree within Metamonada.
Discoba includes three major groups: Jakobida, Euglenozoa and Percolozoa. Jakobida are a small group (~20 species) of free-living heterotrophic flagellates, with two cilia, that primarily eat bacteria through suspension feeding; most are aquatic aerobes, with some anaerobic species, found in marine, brackish or fresh water. They are best known for their bacterial-like mitochondrial genomes. Euglenozoa is a rich (>2,000 species) group of flagellates with very different lifestyles, including: the free-living heterotrophic (both osmo- and phagotrophic) and photosynthetic euglenids (e.g., the euglenophytes, with chloroplasts originated from green algae); the free-living and parasitic kinetoplastids (such as Trypanosoma cruzi, the agent of Chagas disease); the deep-sea anaerobic symbiontids; and the elusive diplonemids. Percolozoa (~150 species) are a collection of amoebae, flagellates and amoeboflagellates with complex life cycles, among which are some slime molds (acrasids). The two clades Euglenozoa and Percolozoa are sister taxa, united under the name Discicristata, in reference to their mitochondrial cristae shaped like discs. The species Tsukubamonas globosa is a free-living flagellate whose precise position within Discoba is not yet settled, but is probably more closely related to Discicristata than to Jakobida.
The metamonads (Metamonada) are a phylum of completely anaerobic or microaerophilic protozoa, primarily flagellates. Some are gut symbionts of animals such as termites, others are free-living, and others are parasitic. They include three main clades: Fornicata, Parabasalia and Preaxostyla. Fornicata (>140 species) encompasses the diplomonads, with two nuclei (e.g., Giardia, genus of well-known parasites of humans), and several smaller groups of free-living, commensal and parasitic protists (e.g., Carpediemonas, retortamonads). Parabasalia (>460 species) is a varied group of anaerobic, mostly endobiotic organisms, ranging from small parasites (like Trichomonas vaginalis, another human pathogen) to giant intestinal symbionts with numerous flagella and nuclei found in wood-eating termites and cockroaches. Preaxostyla (~140 species) includes the anaerobic and endobiotic oxymonads, with modified (or completely lost) mitochondria, and two genera of free-living microaerophilic bacterivorous flagellates Trimastix and Paratrimastix, with typical excavate morphology. Two genera of anaerobic flagellates of recent description and unique cell architecture, Barthelona and Skoliomonas, are closely related to the Fornicata.
The malawimonads (Malawimonadida) are a small group (three species) of freshwater or marine suspension-feeding bacterivorous flagellates with typical excavate appearance, closely resembling Jakobida and some metamonads but not phylogenetically close to either in most analyses.
Diaphoretickes
Diaphoretickes includes nearly all photosynthetic eukaryotes. Within this clade, the TSAR supergroup gathers a colossal diversity of protists. The most basal branching member of the TSAR clade is Telonemia, a small (seven species) phylum of obscure phagotrophic predatory flagellates, found in marine and freshwater environments. They share some cellular similarities with the remaining three clades: Rhizaria, Alveolata and stramenopiles, collectively known as the SAR supergroup. Another highly diverse clade within Diaphoretickes is Archaeplastida, which houses land plants and a variety of algae. In addition, two smaller groups, Haptista and Cryptista, also belong to Diaphoretickes.
Stramenopiles
The stramenopiles, also known as Heterokonta, are characterized by the presence of two cilia, one of which bears many short, straw-like hairs (mastigonemes). They include one clade of phototrophs and numerous clades of heterotrophs, present in virtually all habitats. Stramenopiles include two usually well-supported clades, Bigyra and Gyrista, although the monophyly of Bigyra is being questioned. Branching outside both Bigyra and Gyrista is a single species of enigmatic heterotrophic flagellates, Platysulcus tardus. Much of the diversity of heterotrophic stramenopiles is still uncharacterized, known almost entirely from lineages of genetic sequences known as MASTs (MArine STramenopiles), of which only a few species have been described.
The phylum Gyrista includes the photosynthetic Ochrophyta or Heterokontophyta (>23,000 species), which contain chloroplasts originated from a red alga. Among these are many lineages of algae that encompass a wide range of structures and morphologies. The three most diverse ochrophyte classes are: the diatoms, unicellular or colonial organisms encased in silica cell walls (frustules) that exhibit widely different shapes and ornamentations, responsible for a big portion of the oxygen produced worldwide, and comprising much of the marine phytoplankton; the brown algae, filamentous or 'truly' multicellular (with differentiated tissues) macroalgae that constitute the basis of many temperate and cold marine ecosystems, such as kelp forests; and the golden algae, unicellular or colonial flagellates that are mostly present in freshwater habitats. Inside Gyrista, the sister clade to Ochrophyta are the predominantly osmotrophic and filamentous pseudofungi (>1,200 species), which include three distinct lineages: the parasitic oomycetes or water moulds (e.g., Phytophthora infestans, the agent behind the Irish Potato Famine), which encompass most of the pseudofungi species; the less diverse non-parasitic hyphochytrids that maintain a fungus-like lifestyle; and the bigyromonads, a group of bacterivorous or eukaryovorous phagotrophs. A small group of heliozoan-like heterotrophic amoebae, Actinophryida, has an uncertain position, either within or as the sister taxon of Ochrophyta.
The little studied phylum Bigyra is an assemblage of exclusively heterotrophic organisms, most of which are free-living. It includes the labyrinthulomycetes, among which are single-celled amoeboid phagotrophs, mixotrophs, and fungus-like filamentous heterotrophs that create slime networks to move and absorb nutrients, as well as some parasites and a few testate amoebae (Amphitremida). Also included in Bigyra are the bicosoecids, phagotrophic flagellates that consume bacteria, and the closely related Placidozoa, which consists of several groups of heterotrophic flagellates (e.g., the deep-sea halophilic Placididea) as well as the intestinal commensals known as Opalinata (e.g., the human parasite Blastocystis, and the highly unusual opalinids, composed of giant cells with numerous nuclei and cilia, originally misclassified as ciliates).
Alveolata
The alveolates (Alveolata) are characterized by the presence of cortical alveoli, cytoplasmic sacs underlying the cell membrane of unknown physiological function. Among them are three of the most well-known groups of protists: apicomplexans, dinoflagellates and ciliates. The ciliates (Ciliophora) are a highly diverse (>8,000 species) and probably the most thoroughly studied group of protists. They are mostly free-living microbes characterized by large cells covered in rows of cilia and containing two kinds of nuclei, micronucleus and macronucleus (e.g., Paramecium, a model organism). Free-living ciliates are usually the top heterotrophs and predators in microbial food webs, feeding on bacteria and smaller eukaryotes, present in a variety of ecosystems, although a few species are kleptoplastic. Others are parasitic of numerous animals. Ciliates have a basal position in the evolution of alveolates, together with a few species of heterotrophic flagellates with two cilia collectively known as colponemids.
The remaining alveolates are grouped under the clade Myzozoa, whose common ancestor acquired chloroplasts through a secondary endosymbiosis from a red alga. One branch of Myzozoa contains the apicomplexans and their closest relatives, a small clade of flagellates known as Chrompodellida where phototrophic and heterotrophic flagellates, called chromerids and colpodellids respectively, are evolutionarily intermingled. In contrast, the apicomplexans (Apicomplexa) are a large (>6,000 species) and highly specialized group of obligate parasites who have all secondarily lost their photosynthetic ability (e.g., Plasmodium falciparum, cause of malaria). Their adult stages absorb nutrients from the host through the cell membrane, and they reproduce between hosts via sporozoites, which exhibit an organelle complex (the apicoplast) evolved from non-photosynthetic chloroplasts.
The other branch of Myzozoa contains the dinoflagellates and their closest relatives, the perkinsids (Perkinsozoa), a small group (26 species) of aquatic intracellular parasites which have lost their photosynthetic ability similarly to apicomplexans. They reproduce through flagellated spores that infect dinoflagellates, molluscs and fish. In contrast, the dinoflagellates (Dinoflagellata) are a highly diversified (~4,500 species) group of aquatic algae that have mostly retained their chloroplasts, although many lineages have lost their own and instead either live as heterotrophs or reacquire new chloroplasts from other sources, including tertiary endosymbiosis and kleptoplasty. Most dinoflagellates are free-living and compose an important portion of phytoplankton, as well as a major cause of harmful algal blooms due to their toxicity; some live as symbionts of corals, allowing the creation of coral reefs. Dinoflagellates exhibit a diversity of cellular structures, such as complex eyelike ocelli, specialized vacuoles, bioluminescent organelles, and a wall surrounding the cell known as the theca.
Rhizaria
Rhizaria is a lineage of morphologically diverse organisms, composed almost entirely of unicellular heterotrophic amoebae, flagellates and amoeboflagellates, commonly with reticulose (net-like) or filose (thread-like) pseudopodia for feeding and locomotion. It was the last supergroup to be described, because it lacks any defining characteristic and was discovered exclusively through molecular phylogenetics. Three major clades are included, namely the phyla Cercozoa, Endomyxa and Retaria.
Retaria contains the most familiar rhizarians: forams and radiolarians, two groups of large free-living marine amoebae with pseudopodia supported by microtubules, many of which are macroscopic. The radiolarians (Radiolaria) are a diverse group (>1,000 living species) of amoebae, often bearing delicate and intricate siliceous skeletons. The forams (Foraminifera) are also diverse (>6,700 living species), and most of them are encased in multichambered tests constructed from calcium carbonate or agglutinated mineral particles. Both groups have a rich fossil record, with tens of thousands of described fossil species.
Cercozoa (also known as Filosa) is an assemblage of free-living protists with very different morphologies. Cercozoan amoeboflagellates are important predators of other microbes in terrestrial habitats and the plant microbiota (e.g., cercomonads and paracercomonads and glissomonads, collectively known as class Sarcomonadea), and a few can generate slime molds (e.g., Helkesea). Many cercozoans are testate or scale-bearing amoebae, namely the elusive Kraken and the two classes Imbricatea (e.g., the euglyphids) and Thecofilosea. Thecofilosea also contains the Phaeodaria (~400–500 species), a group of skeleton-bearing marine amoebae previously classified as radiolarians, and both classes include some non-scaly naked flagellates (e.g., spongomonads in Imbricatea and thaumatomonads in Thecofilosea). Among the basal-branching cercozoans are the pseudopodia-lacking thecate flagellates of Metromonadea, the heliozoan-like Granofilosea and the photosynthetic chlorarachniophytes, whose chloroplasts originated from a secondary endosymbiosis with a green alga.
Endomyxa contains two major clades of parasitic protists: Ascetosporea are sporozoan-type parasites of marine invertebrates, while Phytomyxea are obligate pathogens of plants and algae, divided into the terrestrial plasmodiophorids and the marine phagomyxids. Also included in Endomyxa are the order of predatory amoebae Vampyrellida (48 species) and two genera of marine amoebae, the thecate Gromia and the naked Filoreta.
Besides these three phyla, Rhizaria includes numerous enigmatic and understudied lineages of uncertain evolutionary position. One such clade is the Gymnosphaerida, which includes heliozoan-type protists. Several clades labeled as Novel Clades (NC) are entirely composed of environmental DNA from uncultured protists, although a few have slowly been resolved over the decades with the description of new taxa (e.g., Tremulida and Aquavolonida, formerly NC11 and NC10 respectively, with a deep-branching position in Rhizaria).
Haptista and Cryptista
Haptista and Cryptista are two similar phyla of single-celled protists previously thought to be closely related, and collectively known as Hacrobia. However, the monophyly of Hacrobia was disproven, as the two groups originated independently. Molecular analyses place Cryptista next to Archaeplastida, forming the hypothesized "CAM" clade, and Haptista next to the TSAR clade.
The phylum Haptista includes two distinct clades with mineralized scales: haptophytes and centrohelids. The haptophytes (Haptophyta) are a group of over 500 living species of flagellated or coccoid algae that have acquired chloroplasts from a secondary endosymbiosis. They are mostly marine, comprise an important portion of oceanic plankton, and include the coccolithophores, whose calcified scales ('coccoliths') contribute to the formation of sedimentary rocks and the biogeochemical cycles of carbon and calcium. Some species are capable of forming toxic blooms. The centrohelids (Centroplasthelida) are a small (~95 species) but widespread group of heterotrophic heliozoan-type amoebae, usually covered in scale-bearng mucous, that form an important component of benthic food webs of aquatic habitats, both marine and freshwater.
The phylum Cryptista is a clade of three distinct groups of unicellular protists: cryptomonads, katablepharids, and the species Palpitomonas bilix. The cryptomonads (>100 species), also known as cryptophytes, are flagellated algae found in aquatic habitats of diverse salinity, characterized by extrusive organelles or extrusomes called ejectisomes. Their chloroplasts, of red algal origin, contain a nucleomorph, a remnant of the eukaryotic nucleus belonging to the endosymbiotic red alga. The katablepharids, the closest relatives of cryptomonads, are heterotrophic flagellates with two cilia, also characterized by ejectisomes. The species Palpitomonas bilix is the most basal-branching member of Cryptista, a marine heterotrophic flagellate with two cilia, but unlike the remaining members it lacks ejectisomes.
Archaeplastida
Archaeplastida is the clade containing those photosynthetic groups whose plastids were likely obtained through a single event of primary endosymbiosis with a cyanobacterium. It contains land plants (Embryophyta) and a big portion of the diversity of algae, most of which are the green algae, from which plants evolved, and the red algae. A third lineage of algae, the glaucophytes (25 species), contains rare and obscure species found in surfaces of freshwater and terrestrial habitats.
The red algae or Rhodophyta (>7,100 species) are a group of diverse morphologies, ranging from single cells to multicellular filaments to giant pseudoparenchymatous thalli, all without flagella. They lack chlorophyll and only harvest light energy through phycobiliproteins. Their life cycles are varied and may include two or three generations. They are present in terrestrial, freshwater and primarily marine habitats, from the intertidal zone to deep waters; some are calcified and are vital components of marine ecosystems such as coral reefs. Closely related to the red algae are two small lineages of non-photosynthetic predatory flagellates: the freshwater and marine Rhodelphidia (3 species), which still retain genetic evidence of relic plastids; and the marine Picozoa (1 species), which lack any remains of plastids. The evolutionary position of Picozoa may indicate that there have been two separate events of primary endosymbiosis, as opposed to one.
The green algae, unlike the monophyletic glaucophytes and rhodophytes, are a paraphyletic group from which land plants evolved. Together they compose the Chloroplastida or Viridiplantae clade. The earliest branching member is the phylum Prasinodermophyta (ten species), whose members are exclusively marine coccoid cells or small macroscopic thalli. The remaining green algae are distributed in two major clades. One clade is the phylum Chlorophyta (>7,900 species), which includes numerous lineages of scaly unicellular flagellate algae known collectively as prasinophytes along with the Prasinodermophyta, but also includes a variety of morphologies such as coccoids, palmelloids, colonies, and macroscopic filamentous, foliose or tubular thalli, present in aquatic and terrestrial habitats. The opposed clade is Streptophyta, which contains the land plants and a paraphyletic group of green algae collectively known as phylum Charophyta, composed of several classes: Zygnematophyceae (>4,300 species), containing unicellular, colonial and filamentous flagella-lacking organisms found almost exclusively in freshwater habitats; Charophyceae (450 living species), also known as stoneworts, consisting of complex multicellular thalli only found in freshwater habitats; Klebsormidiophyceae (52 species), with unbranched filamentous thalli; Coleochaetophyceae (36 species), containing branched filamentous thalli; Mesostigmatophyceae, composed of a single species of scaly flagellates; and Chlorokybophyceae (five species), with sarcinoid forms.
Amorphea
Amorphea is a group of exclusively heterotrophic organisms. It contains the fungi and animals, as well as most slime moulds, many amoebae and some flagellates. Many of its protist members exhibit complex life cycles with different levels of multicellularity. Amorphea is roughly equivalent to the concept of 'unikonts', meaning 'single cilium', although it currently contains several organisms with more cilia. It is defined as the smallest clade containing the groups Amoebozoa (containing mostly slime moulds and amoebae) and Opisthokonta (containing fungi, animals, and their closest relatives). The closest relatives of Opisthokonta are two small lineages of single-celled protists with two cilia: the flagellate Apusomonadida (28 species) and the amoeboflagellate anaerobic Breviatea (four species). Together with opisthokonts, these two groups form the clade Obazoa, the sister clade to Amoebozoa.
Amoebozoa
The phylum Amoebozoa (2,400 species) is a large group of morphologically diverse phagotrophic protists, mostly amoebae. A considerable portion of amoebozoans are lobose amoebae, meaning they produce round, blunt-ended pseudopods. It includes the 'archetypal' amoebae, known as the naked lobose amoebae or 'gymnamoebae' (such as Amoeba itself), among which is a genus of sorocarp-forming slime moulds, Copromyxa. Some gymnamoebae are important pathogens to animals (e.g., Acanthamoeba). Other relevant lobose amoebae are the Arcellinida, a diverse order of testate amoebae and one of the most conspicuous protist groups overall. The remaining, non-lobose amoebozoans include the Eumycetozoa or 'true slime moulds', comprising the sorocarp-producing bacterivorous dictyostelids and the sporocarp-producing omnivorous myxogastrids and protosporangiids. Due to the fungus-like appearance of their fruiting bodies, eumycetozoans are often studied by mycologists. Closely related to the eumycetozoans are two lineages: the Variosea, a heterogeneous assortment of amoeboid, reticulate or flagellated organisms (including some sorocarp-producing organisms); and the anaerobic Archamoebae, some of which live as intestinal symbionts of some animals (e.g., Entamoeba).
Opisthokonta
Opisthokonta includes the animal and fungal kingdoms, as well as their closest protist relatives. The branch leading to the fungi is known as Nucletmycea or Holomycota, while the branch leading to the animals is called Holozoa. The Holomycota includes the closest relatives of fungi, the nucleariids, a small group (~50 species) of free-living naked or scale-bearing phagotrophic amoebae with filose pseudopodia, some of which can aggregate into slime moulds. Within the wider definition of fungi, three groups are studied as protists by some authors: Aphelida (15 species), Rozellida (27 species) and Microsporidia (~1,300 species), collectively known as Opisthosporidia, as opposed to the 'true' or osmotrophic fungi. Both aphelids and rozellids are single-celled phagotrophic flagellates that feed in an endobiotic manner, penetrating the cells of their respective hosts. Microsporidians are obligate intracellular parasites that feed through osmotrophy, much like true fungi. Aphelids and true fungi are closest relatives, and generally feed on cellulose-walled organisms (many algae and plants). Conversely, rozellids and microsporidians form a separate clade, and generally feed on chitin-walled organisms (fungi and animals).
The Holozoa includes various lineages with complex life cycles involving different cell types and associated with the origin of animal multicellularity. The closest relatives to animals are the choanoflagellates (~360 species), free-living flagellates that feed through a collar of microvilli surrounding a larger cilium and often form colonies. The Ichthyosporea (>40 species), otherwise known as mesomycetozoans, are a group of fungus-like pathogenic holozoans specialized in infecting fish and other animals. The Filasterea (six species) are a heterogeneous group of free-living, endosymbiotic, or parasitic amoebae or flagellates. Lastly, the Pluriformea are two species of free-living holozoans with life cycles that include multicellular aggregates. An elusive flagellate species Tunicaraptor unikontum has an uncertain evolutionary position among these holozoan groups.
Orphan groups
Several smaller lineages do not belong to any of the three main supergroups, and instead have a deep-branching "kingdom-level" position in eukaryote evolution. They are usually poorly known groups with limited data and few species, often referred to as "orphan groups". The CRuMs clade, containing the free-swimming Collodictyonidae (seven species) with two to four cilia, the amoeboid Rigifilidae (two species) with filose pseudopodia, and the gliding Mantamonadidae (three species) with two cilia, are the sister clade of Amorphea. The Ancyromonadida (35 species) are aquatic gliding flagellates with two cilia, positioned near Amorphea and CRuMs. The Hemimastigophora (ten species), or hemimastigotes, are predatory flagellates with a distinctive cell morphology and two rows of around a dozen flagella. The Provora (eight species) are predatory flagellates with an unremarkable morphology similar to that of excavates and other flagellates with two cilia. Both Hemimastigophora and Provora were thought to be related to or within Diaphoretickes, although further analyses have placed them in a separate clade along with a mysterious species of predatory protists, Meteora sporadica. This species has a remarkable morphology: they lack flagella, are bilaterally symmetrical, project a pair of lateral "arms" that swing back and forth, and contain a system of motility unlike any other.
There are also many genera of uncertain affiliation among eukaryotes because their DNA has not been sequenced, and consequently their phylogenetic affinities are unknown. One enigmatic heliozoan species is so large that it does not match the description of any known genus, and was consequently transferred to a separate genus Berkeleyaesol with an unclear position, although it probably belongs to Diaphoretickes along with all other heliozoa. The organism Parakaryon is harder to place, as it shares traits from both prokaryotes and eukaryotes.
Biology
In general, protists have typical eukaryotic cells that follow the same principles of biology described for those cells within the "higher" eukaryotes (animals, fungi and plants). However, many have evolved a variety of unique physiological adaptations that do not appear in the remaining eukaryotes, and in fact protists encompass almost all of the broad spectrum of biological characteristics expected in eukaryotes.
Nutrition
Protists display a wide variety of food preferences and feeding mechanisms. According to the source of their nutrients, they can be divided into autotrophs (producers) and heterotrophs (consumers). Autotrophic protists synthesize their own organic compounds from inorganic substrates through the process of photosynthesis, using light as the source of energy; accordingly, they are also known as phototrophs.
Heterotrophic protists obtain organic molecules synthesized by other organisms, and can be further divided according to the size of their nutrients. Those that feed on soluble molecules or macromolecules under 0.5 μm in size are called osmotrophs, and they absorb them by diffusion, ciliary pits, transport proteins of the cell membrane, and a type of endocytosis (i.e., invagination of the cell membrane into vacuoles, called endosomes) known as pinocytosis or fluid-phase endocytosis. Those that feed on organic particles over 0.5 μm in size or entire cells are called phagotrophs, and they ingest them through a type of endocytosis known as phagocytosis. Endocytosis is considered one of the most important adaptations in the origin of eukaryotes because it increased the potential food supply, and phagocytosis allowed the endosymbiosis and development of mitochondria and chloroplasts. In both osmotrophs and phagotrophs, endocytosis is often restricted to a specific region of the cell membrane, known as the cytostome, which may be followed by a cytopharynx, a specialized tract supported by microtubules.
Osmotrophy
Osmotrophic protists acquire soluble nutrients through membrane channels and carriers, but also through different types of pinocytosis. Macropinocytosis involves the folding of membrane into ruffles, which creates large (0.2 to 1.0 μm) vacuoles. Micropinocytosis involves smaller vesicles that are usually formed by clathrin. In both scenarios, the vesicles merge into a digestive vacuole or endosome where digestion takes place. Some osmotrophs, called saprotrophs or lysotrophs, perform external digestion by releasing enzymes into the environment and decomposing organic matter into simpler molecules that can be absorbed. This external digestion has a distinct advantage: it allows greater control over the substances that are allowed to enter the cell, thus minimizing the intake of harmful substances or infection.
Probably all eukaryotes are capable of osmotrophy, but some have no alternative of acquiring nutrients. Obligate osmotrophs and saprotrophs include some euglenids, some green algae, the human parasite Blastocystis, some metamonads, the parasitic trypanosomatids, and the fungus-like oomycetes and hyphochytrids.
Phagotrophy
Phagotrophic feeding consists of two phases: the concentration of food particles in the environment, and the phagocytosis, which encloses the food particle in a vacuole (the phagosome) where digestion takes place. In ciliates and most phagotrophic flagellates, digestion occurs at the oral region or cytostome, which is covered by a single membrane from which vacuoles are formed; the phagosomes then may be shuttled to the interior of the cell along the cytopharynx. In amoebae, phagocytosis takes place anywhere on the cell surface. The average food particle size is around one tenth the size of the protist cell.
Phagotrophic protists can be further classified according to how they approach the nutrients. The filter feeders acquire small, suspended food particles or prokaryotic cells and accumulate them by filtration into the cytostome (e.g., choanoflagellates, some chrysomonads, most ciliates); filter-feeding flagellates accumulate particles by propelling them with a flagellum through a collar of rigid tentacles or pseudopodia that act as a filter, while filter-feeding ciliates generate water currents through cilia and membranelle zones surrounding the cytostome. The raptorial feeders (e.g., bicosoecids, chrysomonads, kinetoplastids, some euglenids, many dinoflagellates and ciliates), instead of retaining all particles in bulk, capture each particle individually. Among raptorial protists, the grazers search and ingest prey from surfaces covered with potential food items such as bacterial lawns, while the predators actively pursue scarce prey. Predators that feed on filamentous algae or fungal hyphae either swallow the filaments entirely or penetrate the cell wall and ingest the cytoplasm (e.g., Viridiraptoridae). Predators may have adaptations to hunt prey, such as 'toxicysts' that immobilize prey cells. Certain ciliates have developed a specialized kind of raptorial feeding called histophagy, where they attack damaged but live animals (e.g., annelids and small crustaceans), enter the wounds, and ingest animal tissue. Large raptorial amoebae enclose their prey in a "food cup" of pseudopodia, prior to the formation of the food vacuole. Lastly, diffusion feeders (e.g., heliozoa, foraminifera and many other amoebae, suctorian ciliates) engulf prey that happen to collide with their pseudopods or, in the case of ciliates, tentacles that carry toxicysts or extrusomes to immobilize the prey.
Consumers of prokaryotes are popularly called bacterivores (e.g., most amoebae), while consumers (including osmotrophic parasites) of eukaryotes are known as eukaryovores. In particular, eukaryovores that feed on unicellular protists are cytotrophs (e.g., colponemids, colpodellids, many amoebae, some ciliates); those that feed on fungi are mycophages or mycotrophs (e.g., the ciliate family Grossglockneriidae of obligate mycophages); those that prey on nematodes are nematophages; and those that feed on algae are phycotrophs (e.g., vampyrellids).
Mixotrophy
Most autotrophic protists are mixotrophs and combine photosynthesis with phagocytosis. They are classified into various functional groups or 'mixotypes'. Constitutive mixotrophs have the innate ability to photosynthesize through already present chloroplasts, and have diverse feeding behaviors, as some require phototrophy, others phagotrophy, and others are obligate mixotrophs (e.g., nanoflagellates such as some haptophytes and dinoflagellates). Non-constitutive mixotrophs acquire the ability to photosynthesize by stealing chloroplasts from their prey, a process known as kleptoplasty. Non-constitutives can be divided into two: generalists, which can steal chloroplasts from a variety of prey (e.g., oligotrich ciliates), or specialists, which can only acquire chloroplasts from a few specific prey (e.g., Rapaza viridis can only steal from Tetraselmis cells). The specialists are further divided into two types: plastidic, which contain differentiated plastids (e.g., Mesodinium, Dinophysis), and endosymbiotic, which contain whole endosymbionts (e.g., mixotrophic Rhizaria such as Foraminifera and Radiolaria, dinoflagellates like Noctiluca).
Among exclusively heterotrophic protists, variation of nutritional modes is also observed. The diplonemids, which inhabit deep waters where photosynthesis is absent, can flexibly switch between osmotrophy and bacterivory depending on the environmental conditions.
Osmoregulation
Many freshwater protists need to osmoregulate (i.e., remove excess water volume to adjust the ion concentrations) because non-saline water enters in excess by osmosis from the environment and by endocytosis when feeding. Osmoregulation is done through active ion transporters of the cell membrane and through contractile vacuoles, specialized organelles that periodically excrete fluid high in potassium and sodium through a cycle of diastole and systole. The cycle stops when the cells are placed in a medium with different salinity, until the cell adapts.
The contractile vacuoles are surrounded by the spongiome, an array of cytoplasmic vesicles or tubes that slowly collect fluid from the cytoplasm into the vacuole. The vacuoles then contract and discharge the fluid outside of the cell through a pore. The contractile mechanism varies depending on the protist: in ciliates, the spongiome is composed of irregular tubules and actin filaments wind around the pore and over the vacuole surface, together with microtubules; in most flagellates and amoebae, the spongiome is composed of both vesicles and tubules; in dinoflagellates, a flagellar rootlet branches to form a contractile sheath around the vacuole (known as pusule). The location and amount also varies: unicellular flagellated algae (cryptomonads, euglenids, prasinophytes, golden algae, haptophytes, etc.) typically have a single contractile vacuole in a fixed position; naked amoebae have numerous small vesicles that fuse into one vacuole and then split again after excretion. Marine or parasitic protists (e.g., metamonads), as well as those with rigid cell walls, lack these vacuoles.
Respiration
The last eukaryotic common ancestor was aerobic, bearing mitochondria for oxidative metabolism. Many lineages of free-living and parasitic protists have independently evolved and adapted to inhabit anaerobic or microaerophilic habitats, by modifying the early mitochondria into hydrogenosomes, organelles that generate ATP anaerobically through fermentation of pyruvate. In a parallel manner, in the microaerophilic trypanosomatid protists, the fermentative glycosome evolved from the peroxisome.
Sensory perception
Many flagellates and probably all motile algae exhibit a positive phototaxis (i.e. they swim or glide toward a source of light). For this purpose, they exhibit three kinds of photoreceptors or "eyespots": (1) receptors with light antennae, found in many green algae, dinoflagellates and cryptophytes; (2) receptors with opaque screens; and (3) complex ocelloids with intracellular lenses, found in one group of predatory dinoflagellates, the Warnowiaceae. Additionally, some ciliates orient themselves in relation to the Earth's gravitational field while moving (geotaxis), and others swim in relation to the concentration of dissolved oxygen in the water.
Endosymbionts
Protists have an accentuated tendency to include endosymbionts in their cells, and these have produced new physiological opportunities. Some associations are more permanent, such as Paramecium bursaria and its endosymbiont Chlorella; others more transient. Many protists contain captured chloroplasts, chloroplast-mitochondrial complexes, and even eyespots from algae. The xenosomes are bacterial endosymbionts found in ciliates, sometimes with a methanogenic role inside anaerobic ciliates.
Life cycle and reproduction
Protists exhibit a large range of life cycles and strategies involving multiple stages of different morphologies which have allowed them to thrive in most environments. Nevertheless, most of the knowledge concerning protist life cycles concerns model organisms and important parasites. Free-living uncultivated protists represent the majority, but knowledge on their life cycles remains fragmentary.
Asexual reproduction
Protists typically reproduce asexually under favorable environmental conditions, allowing for rapid exponential population growth with minimal genetic diversification. This asexual reproduction, occurs through mitosis and has historically been regarded as the primary reproductive mode in protists. This process is also known as vegetative reproduction, as it is only performed by the 'vegetative stage' or individual.
Unicellular protists often multiply via binary fission, similarly to bacteria. They can also divide through budding, similarly to yeasts, or through multiple fissions, a process known as schizogony. In multicellular protists, vegetative reproduction can take the form of fragmentation of body parts, or specialized propagules composed of numerous cells (e.g., in red algae).
Sexual reproduction
While asexual reproduction remains the most common strategy among protists, sexual reproduction is also a fundamental characteristic of eukaryotes. Sexual reproduction involves meiosis (a specialized nuclear division enabling genetic recombination) and syngamy (the fusion of nuclei from two parents). These processes are thought to have been present in the last eukaryotic common ancestor, which likely had the ability to reproduce sexually on a facultative (non-obligate) basis. Even protists that no longer reproduce sexually still retain a core set of meiosis-related genes, reflecting their descent from sexual ancestors. For example, although amoebae are traditionally considered asexual organisms, most asexual amoebae likely arose recently and independently from sexually reproducing amoeboid ancestors. Even in the early 20th century, some researchers interpreted phenomena related to chromidia (chromatin granules free in the cytoplasm) in amoebae as sexual reproduction.
Basic sexual cycles
Every sexual cycle involves the events of syngamy and meiosis, which increase or decrease the ploidy (i.e., number of chromosome sets, represented by the letter n), respectively. Syngamy implies the fusion of two haploid (1n) reproductive cells, known as gametes, which generates a diploid (2n) cell called zygote. The diploid cell then undergoes meiosis to generate haploid cells. Depending on which cells compose the individual or vegetative stage (i.e., the stage that grows by mitosis), there are three distinguishable sexual cycles observed in free-living protists:
In the haploid cycle, the individual is haploid and differentiates into haploid gametes through mitosis. The gametes fuse into a zygote which immediately undergoes meiosis to generate new haploid individuals. This is the case for some green algae (namely Volvocales), many dinoflagellates, some metamonads, and apicomplexans.
In the diploid cycle, the individual is diploid and undergoes meiosis to generate haploid gametes, which in turn fuse with others to form a zygote that develops into a new individual. This is the case for some metamonads, heliozoans, many green algae, diatoms, and ciliates, as well as animals. Instead of generating gametes, ciliates divide their diploid micronucleus into two haploid nuclei, exchange one of them by conjugation with another ciliate, and fuse the two nuclei into a new diploid nucleus.
In the haplo-diploid cycle, there are two alternating generations of individuals. One generation is the diploid 'agamont', which undergoes meiosis to generate haploid cells (spores) that develop into the other generation, the haploid 'gamont'. The gamont then generates gametes by mitosis, which in turn fuse to form the zygote that develops into the agamont. This is the case for many foraminifera and many algae, as well as land plants. There are three modes of this cycle depending on the relative growth and lifespan of one generation compared to the other: haploid-dominant, diploid-dominant, or equally dominant generations. Brown algae exhibit the full range of these modes.
Free-living protists tend to reproduce sexually under stressful conditions, such as starvation or heat shock. Oxidative stress, which leads to DNA damage, also appears to be an important factor in the induction of sex in protists.
Sexual cycles in pathogenic protists
Pathogenic protists tend to have extremely complex life cycles that involve multiple forms of the organism, some of which reproduce sexually and others asexually. The stages that feed and multiply inside the host are generally known as trophozoites (), but the names of each stage vary depending on the protist group. For example:
In apicomplexans, a haploid sporozoite is released into the host, penetrates a host cell, begins the infection and transforms into a meront that grows and asexually divides into numerous merozoites (a schizogony called merogony); each merozoite continues the infection by multiplying. Eventually, the merozoites differentiate (gamogony) into female (macrogametocytes) and male (microgametocytes) that generate gametes, which in turn fuse (sporogony) into a diploid zygote that grows into a sporocyst. The sporocyst then undergoes meiosis to form sporozoites that transmit the infection.
In phytomyxeans, the diploid primary zoospores enter the host, encyst, and penetrate cells as a uninucleate protoplast or plasmodium. Inside the cells, the protoplast grows into a multinucleate zoosporangium, which then divides into secondary zoospores that infect more cells. These multiply into thick-walled resting spores that begin meiosis and divide into binucleate resting spores; one nucleus is lost, and the spores hatch as primary zoospores.
Some protist pathogens undergo asexual reproduction in a wide variety of organisms – which act as secondary or intermediate hosts – but can undergo sexual reproduction only in the primary or definitive host (e.g., Toxoplasma gondii in felids such as domestic cats). Others, such as Leishmania, are capable of performing syngamy in the secondary vector. In apicomplexans, sexual reproduction is obligatory for parasite transmission.
Despite undergoing sexual reproduction, it is unclear how frequently there is genetic exchange between different strains of pathogenic protists, as most populations may be clonal lines that rarely exchange genes with other members of their species.
Ecology
Protists are indispensable to modern ecosystems worldwide. They also have been the only eukaryotic component of all ecosystems for much of Earth's history, which allowed them to evolve a vast functional diversity that explains their critical ecological significance. They are essential as primary producers, as intermediates in multiple trophic levels, as key regulating parasites or parasitoids, and as partners in diverse symbioses.
Habitat diversity
Protists are abundant and diverse in nearly all habitats. They contribute 4 gigatons (Gt) to Earth's biomass—double that of animals (2 Gt), but less than 1% of the total. Combined, protists, animals, archaea (7 Gt), and fungi (12 Gt) make up less than 10% of global biomass, with plants (450 Gt) and bacteria (70 Gt) dominating. Protist diversity, as detected through environmental DNA surveys, is vast in every sampled environment, but it is mostly undescribed. The richest protist communities appear in soils, followed by oceanic and lastly freshwater habitats, mostly as part of the plankton. Freshwater protist communities are characterized by a higher "beta diversity" (i.e. highly heterogeneous between samples) than soil and marine plankton. The high diversity can be a result of the hydrological dynamic of recruiting organisms from different habitats through extreme floods. Soil-dwelling protist communities are ecologically the richest, possibly be due to the complex and highly dynamic distribution of water in the sediment, which creates extremely heterogenous environmental conditions. The constantly changing environment promotes the activity of only one part of the community at a time, while the rest remains inactive; this phenomenon promotes high microbial diversity in prokaryotes as well as protists.
Primary producers
Microscopic phototrophic protists (or microalgae) are the main contributors to the biomass and primary production in nearly all aquatic environments, where they are collectively known as phytoplankton (together with cyanobacteria). In marine phytoplankton, the smallest fractions, the picoplankton (<2 μm) and nanoplankton (2–20 μm), are dominated by several different algae (prymnesiophytes, pelagophytes, prasinophytes); fractions larger than 5 μm are instead dominated by diatoms and dinoflagellates. In freshwater phytoplankton, golden algae, cryptophytes and dinoflagellates are the most abundant groups. Altogether, they are responsible for almost half of the global primary production. They are the main providers of much of the energy and organic matter used by bacteria, archaea, and higher trophic levels (zooplankton and fish), including essential nutrients such as fatty acids. Their abundance in the oceans depends mostly on the availability of inorganic nutrients, rather than temperature or sunlight; they are most abundant in coastal waters that receive nutrient-rich run-off from land, and areas where nutrient-rich deep ocean water reaches the surface, namely the upwelling zones in the arctic oceans and along continental margins. In freshwater habitats, most phototrophic protists are mixotrophic, meaning they also behave as consumers, while strict consumers (heterotrophs) are less abundant.
Macroalgae (namely red algae, green algae and brown algae), unlike phytoplankton, generally require a fixation point, which limits their marine distribution to coastal waters, and particularly to rocky substrates. They support numerous herbivorous animals, especially benthic ones, as both food and refuge from predators. Some communities of seaweeds exist adrift on the ocean surface, serving as a refuge and means of dispersal for associated organisms.
Phototrophic protists are as abundant in soils as their aquatic counterparts. Given the importance of aquatic algae, soil algae may provide a larger contribution to the global carbon cycle than previously thought, but the magnitude of their carbon fixation has yet to be quantified. Most soil algae are stramenopiles (diatoms, xanthophytes and eustigmatophytes) and archaeplastids (green algae). There is also presence of environmental DNA from dinoflagellates and haptophytes in soil, but no living forms have been seen.
Consumers
Phagotrophic protists are the most diverse functional group in all ecosystems, primarily represented by cercozoans (dominant in freshwater and soils), radiolarians (dominant in oceans), non-photosynthetic stramenopiles (with higher abundance in soils than in oceans), and ciliates.
Contrary to the common division between phytoplankton and zooplankton, much of the marine plankton is composed of mixotrophic protists, which pose a largely underestimated importance and abundance (around 12% of all marine environmental DNA sequences). Mixotrophs have varied presence due to seasonal abundance and depending on their specific type of mixotrophy. Constitutive mixotrophs are present in almost the entire range of oceanic conditions, from eutrophic shallow habitats to oligotrophic subtropical waters but mostly dominating the photic zone, and they account for most of the predation of bacteria. They are also responsible for harmful algal blooms. Plastidic and generalist non-constitutive mixotrophs have similar biogeographies and low abundance, mostly found in eutrophic coastal waters, with generalist ciliates dominating up to half of ciliate communities in the photic zone. Lastly, endosymbiotic mixotrophs are by far the most widespread and abundant non-constitutive type, representing over 90% of all mixotroph sequences (mostly radiolarians).
In the trophic webs of soils, protists are the main consumers of both bacteria and fungi, the two main pathways of nutrient flow towards higher trophic levels. Amoeboflagellates like the glissomonads and cercomonads are among the most abundant soil protists: they possess both flagella and pseudopodia, a morphological variability well suited for foraging between soil particles. Testate amoebae are also acclimated to the soil environment, as their shells protect against desiccation. As bacterial grazers, they have a significant role in the foodweb: they excrete nitrogen in the form of NH, making it available to plants and other microbes. Traditionally, protists were considered primarily bacterivorous due to biases in cultivation techniques, but many (e.g., vampyrellids, cercomonads, gymnamoebae, testate amoebae, small flagellates) are omnivores that feed on a wide range of soil eukaryotes, including fungi and even some animals such as nematodes. Bacterivorous and mycophagous protists amount to similar biomasses.
Decomposers
Necrophagy (the degradation of dead biomass) among microbes is mainly attributed to bacteria and fungi, but protists have a still poorly recognized role as decomposers with specialized lytic enzymes. In soils, fungus-like protists and slime molds (e.g., oomycetes, myxomycetes, acrasids) are present abundantly as osmotrophs and saprotrophs. In marine and estuarine environments, the well-studied thraustochytrids (part of labyrinthulomycetes) are relevant saprotrophs that decompose various substrates, including dead plant and animal tissue. Various ciliates and testate amoebae scavenge on dead animals. Some nucleariid amoebae specifically consume the contents of dead or damaged cells, but not healthy cells. However, all these examples are only facultative necrophages that also feed on live prey. In contrast, the algivorous cercozoan family Viridiraptoridae, present in shallow bog waters, are broad-range but sophisticated necrophages that feed on a variety of exclusively dead algae, potentially fulfilling an important role in cleaning up the environment and releasing nutrients for live microbes.
Parasites and pathogens
Parasitic protists occupy around 15–20% of all environmental DNA in marine and soil systems, but only around 5% in freshwater systems, where chytrid fungi likely fill that ecological niche. In oceanic systems, parasitoids (i.e. those which kill their hosts, e.g. Syndiniales) are more abundant. In freshwater ecosystems, parasitoids are mainly Perkinsea and Syndiniales (Alveolata), while true parasites (i.e. those which do not kill their hosts) in freshwater are mostly oomycetes, Apicomplexa and Ichthyosporea. In soil ecosystems, true parasites are primarily animal-hosted apicomplexans and plant-hosted oomycetes and plasmodiophorids. In Neotropical forest soils, apicomplexans dominate eukaryotic diversity and have an important role as parasites of small invertebrates, while oomycetes are very scarce in contrast.
Some protists are significant parasites of animals (e.g.; five species of the parasitic genus Plasmodium cause malaria in humans and many others cause similar diseases in other vertebrates), plants (the oomycete Phytophthora infestans causes late blight in potatoes) or even of other protists. Around 100 protist species can infect humans.
Biogeochemical cycles
Marine protists have a fundamental impact on biogeochemical cycles, particularly the carbon cycle. As phytoplankton, they fix as much carbon as all terrestrial plants combined. Soil protists, particularly testate amoebae, contribute to the silica cycle as much as forest trees through the biomineralization of their shells.
History of protist classification
Early classifications
From the start of the 18th century, the popular term "infusion animals" (later infusoria) referred to protists, bacteria and small invertebrate animals. In the mid-18th century, while Swedish scientist Carl von Linnaeus largely ignored the protists, his Danish contemporary Otto Friedrich Müller was the first to introduce protists to the binomial nomenclature system.
In the early 19th century, German naturalist Georg August Goldfuss introduced Protozoa (meaning 'early animals') as a class within Kingdom Animalia, to refer to four very different groups: Infusoria (ciliates), corals, phytozoa (such as Cryptomonas) and jellyfish. Later, in 1845, Carl Theodor von Siebold was the first to establish Protozoa as a phylum of exclusively unicellular animals consisting of two classes: Infusoria (ciliates) and Rhizopoda (amoebae, foraminifera). Other scientists did not consider all of them part of the animal kingdom, and by the middle of the century they were regarded within the groupings of Protozoa (early animals), Protophyta (early plants), Phytozoa (animal-like plants) and Bacteria (mostly considered plants). Microscopic organisms were increasingly constrained in the plant/animal dichotomy. In 1858, the palaeontolgist Richard Owen was the first to define Protozoa as a separate kingdom of eukaryotic organisms, with "nucleated cells" and the "common organic characters" of plants and animals, although he also included sponges within protozoa.
In 1860, British naturalist John Hogg proposed Protoctista (meaning 'first-created beings') as the name for a fourth kingdom of nature (the other kingdoms being Linnaeus' plant, animal and mineral) which comprised all the lower, primitive organisms, including protophyta, protozoa and sponges, at the merging bases of the plant and animal kingdoms.
In 1866, the 'father of protistology', German scientist Ernst Haeckel, addressed the problem of classifying all these organisms as a mixture of animal and vegetable characters, and proposed Protistenreich (Kingdom Protista) as the third kingdom of life, comprising primitive forms that were "neither animals nor plants". He grouped both bacteria and eukaryotes, both unicellular and multicellular organisms, as Protista. He retained the Infusoria in the animal kingdom, until German zoologist Otto Bütschli demonstrated that they were unicellular. At first, he included sponges and fungi, but in later publications he explicitly restricted Protista to predominantly unicellular organisms or colonies incapable of forming tissues. He clearly separated Protista from true animals on the basis that the defining character of protists was the absence of sexual reproduction, while the defining character of animals was the blastula stage of animal development. He also returned the terms Protozoa and Protophyta as subkingdoms of Protista.
End of the animal-plant dichotomy
Bütschli considered the kingdom to be too polyphyletic and rejected the inclusion of bacteria. He fragmented the kingdom into protozoa (only nucleated, unicellular animal-like organisms), while bacteria and the protophyta were a separate grouping. This strengthened the old dichotomy of protozoa/protophyta from German scientist Carl Theodor von Siebold, and the German naturalists asserted this view over the worldwide scientific community by the turn of the century. However, British biologist C. Clifford Dobell in 1911 brought attention to the fact that protists functioned very differently compared to the animal and vegetable cellular organization, and gave importance to Protista as a group with a different organization that he called "acellularity", shifting away from the dogma of German cell theory. He coined the term protistology and solidified it as a branch of study independent from zoology and botany.
In 1938, American biologist Herbert Copeland resurrected Hogg's label, arguing that Haeckel's term Protista included anucleated microbes such as bacteria, which the term Protoctista (meaning "first established beings") did not. Under his four-kingdom classification (Monera, Protoctista, Plantae, Animalia), the protists and bacteria were finally split apart, recognizing the difference between anucleate (prokaryotic) and nucleate (eukaryotic) organisms. To firmly separate protists from plants, he followed Haeckel's blastular definition of true animals, and proposed defining true plants as those with chlorophyll a and b, carotene, xanthophyll and production of starch. He also was the first to recognize that the unicellular/multicellular dichotomy was invalid. Still, he kept fungi within Protoctista, together with red algae, brown algae and protozoans. This classification was the basis for Whittaker's later definition of Fungi, Animalia, Plantae and Protista as the four kingdoms of life.
In the popular five-kingdom scheme published by American plant ecologist Robert Whittaker in 1969, Protista was defined as eukaryotic "organisms which are unicellular or unicellular-colonial and which form no tissues". Just as the prokaryotic/eukaryotic division was becoming mainstream, Whittaker, after a decade from Copeland's system, recognized the fundamental division of life between the prokaryotic Monera and the eukaryotic kingdoms: Animalia (ingestion), Plantae (photosynthesis), Fungi (absorption) and the remaining Protista.
In the five-kingdom system of American evolutionary biologist Lynn Margulis, the term "protist" was reserved for microscopic organisms, while the more inclusive kingdom Protoctista (or protoctists) included certain large multicellular eukaryotes, such as kelp, red algae, and slime molds. Some use the term protist interchangeably with Margulis' protoctist, to encompass both single-celled and multicellular eukaryotes, including those that form specialized tissues but do not fit into any of the other traditional kingdoms.
Advances in electron microscopy and molecular phylogenetics
The five-kingdom model remained the accepted classification until the development of molecular phylogenetics in the late 20th century, when it became apparent that protists are a paraphyletic group from which animals, fungi and plants evolved, and the three-domain system (Bacteria, Archaea, Eukarya) became prevalent. Today, protists are not treated as a formal taxon, but the term is commonly used for convenience in two ways:
Phylogenetic definition: protists are a paraphyletic group. A protist is any eukaryote that is not an animal, land plant or fungus, thus excluding many unicellular groups like the fungal Microsporidia, Chytridiomycetes and yeasts, and the non-unicellular Myxozoan animals included in Protista in the past.
Functional definition: protists are essentially those eukaryotes that are never multicellular, that either exist as independent cells, or if they occur in colonies, do not show differentiation into tissues. While in popular usage, this definition excludes the variety of non-colonial multicellularity types that protists exhibit, such as aggregative (e.g., choanoflagellates) or complex multicellularity (e.g., brown algae).
There is, however, one classification of protists based on traditional ranks that lasted until the 21st century. The British protozoologist Thomas Cavalier-Smith, since 1998, developed a six-kingdom model: Bacteria, Animalia, Plantae, Fungi, Protozoa and Chromista. In his context, paraphyletic groups take preference over clades: both protist kingdoms Protozoa and Chromista contain paraphyletic phyla such as Apusozoa, Eolouka or Opisthosporidia. Additionally, red and green algae are considered true plants, while the fungal groups Microsporidia, Rozellida and Aphelida are considered protozoans under the phylum Opisthosporidia. This scheme endured until 2021, the year of his last publication.
Fossil record
Paleo- and Mesoproterozoic
Before the existence of plants, animals and fungi, all eukaryotes were protists. Modern or crown-group eukaryotes originated from the last eukaryotic common ancestor (LECA) and emerged between 1600 and 2400 million years ago (Ma), during the Paleoproterozoic and Mesoproterozoic eras. However, the fossil record through this time is scarce and dominated by stem-group eukaryotes, extinct lineages preceding LECA. These lineages displayed early eukaryotic traits like flexible cell membranes and complex cell wall ornamentations, which require a flexible endomembrane system, but they lacked crown-group eukaryotes' advanced sterols (e.g., cholesterol), and instead produced simpler protosterols that require less oxygen during biosynthesis. Examples of these are: Trachyhystrichosphaera and Leiosphaeridia dated at 1100 Ma, Satka dated at 1300 Ma, Tappania and Shuiyousphaeridium dated at 1600 Ma, Grypania dated at 1800–1900 Ma, and Valeria which ranges from 1650 to 700 Ma.
Crown-group eukaryotes achieved significant morphological and ecological diversity before 1000 Ma, with multicellular algae capable of sexual reproduction and unicellular protists exhibiting modern phagocytosis and locomotion. Their advanced but metabolically expensive sterols likely provided numerous evolutionary advantages due to the increased membrane flexibility, including resilience to osmotic shock during dessication and rehydration cycles, extreme temperatures, UV light exposure, and protection against changing oxygen levels. These adaptations allowed crown-group eukaryotes to colonize diverse and harsh environments (e.g., mudflats, rivers, agitated shorelines and land). In contrast, stem-group eukaryotes occupied the low-oxygen marine waters as anaerobes. The oldest definitive crown-group eukaryotic fossils include Rafatazmia and Ramathallus, both putative red algae, dated at 1600 Ma.
Neoproterozoic
As oxygen levels rose during the Tonian period, crown-group eukaryotes outcompeted stem-group eukaryotes, expanding into oxygen-rich marine environments that supported an aerobic metabolism enabled by their mitochondria. Stem-group eukaryotes may have gone extinct due to competition and the extreme climatic changes of the Cryogenian glaciations and subsequent global warming, cementing the dominance of crown-group eukaryotes. Crown-group eukaryotes began to appear abundantly in this era, fueled by the proliferation of red algae. The oldest fossils assigned to modern eukaryotic groups include two photosynthetic protists: the multicellular red alga Bangiomorpha (1047 Ma), and the chlorophyte green alga Proterocladus (1000 Ma). Also included are the oldest fossils of Opisthokonta: Ourasphaira giraldae (1010–890 Ma), interpreted as the earliest fungus, and Bicellum brasieri (1000 Ma), the earliest holozoan, showing traits associated with complex multicellularity.
Abundant fossils of heterotrophic protists appear significantly later, parallel to the emergence of fungi. Vase-shaped microfossils (VSMs), widespread rocks dated at 780–720 Ma (Tonian to Cryogenian), have been described as a variety of organisms across the decades (e.g., algae, chitinozoans, tintinnids), but current scientific consensus relates most VSMs to testate amoebae. As such, VSMs comprise the oldest known fossils of both filose (Cercozoa) and lobose (Amoebozoa) testate amoebae.
After the Gaskiers glaciation of the Late Ediacaran (~579 Ma), fossils of heterotrophic protists undergo diversification. Some fossils similar to VSMs are interpreted as the oldest fossils of Foraminifera dated at 548 Ma (e.g., Protolagena), but their foraminiferal affinity is doubtful. Other microfossils that are possibly foraminifera include some poorly preserved tubular shells from 716–635 Ma rocks.
Phanerozoic
Radiolarian shells appear abundantly in the fossil record since the Middle Cambrian. Definitive radiolarian fossils have been found in rocks as old as the Early Cambrian period (~540 Ma), with records from older Precambrian rocks disregarded due to the lack of reliable fossils. Shortly afterwards, the oldest convincible foraminifera shells appear at around 525 Ma.
| Biology and health sciences | Other organisms | null |
19178965 | https://en.wikipedia.org/wiki/Fungus | Fungus | A fungus (: fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as one of the traditional eukaryotic kingdoms, along with Animalia, Plantae, and either Protista or Protozoa and Chromista.
A characteristic that places fungi in a different kingdom from plants, bacteria, and some protists is chitin in their cell walls. Fungi, like animals, are heterotrophs; they acquire their food by absorbing dissolved molecules, typically by secreting digestive enzymes into their environment. Fungi do not photosynthesize. Growth is their means of mobility, except for spores (a few of which are flagellated), which may travel through the air or water. Fungi are the principal decomposers in ecological systems. These and other differences place fungi in a single group of related organisms, named the Eumycota (true fungi or Eumycetes), that share a common ancestor (i.e. they form a monophyletic group), an interpretation that is also strongly supported by molecular phylogenetics. This fungal group is distinct from the structurally similar myxomycetes (slime molds) and oomycetes (water molds). The discipline of biology devoted to the study of fungi is known as mycology (from the Greek , mushroom). In the past, mycology was regarded as a branch of botany, although it is now known that fungi are genetically more closely related to animals than to plants.
Abundant worldwide, most fungi are inconspicuous because of the small size of their structures, and their cryptic lifestyles in soil or on dead matter. Fungi include symbionts of plants, animals, or other fungi and also parasites. They may become noticeable when fruiting, either as mushrooms or as molds. Fungi perform an essential role in the decomposition of organic matter and have fundamental roles in nutrient cycling and exchange in the environment. They have long been used as a direct source of human food, in the form of mushrooms and truffles; as a leavening agent for bread; and in the fermentation of various food products, such as wine, beer, and soy sauce. Since the 1940s, fungi have been used for the production of antibiotics, and, more recently, various enzymes produced by fungi are used industrially and in detergents. Fungi are also used as biological pesticides to control weeds, plant diseases, and insect pests. Many species produce bioactive compounds called mycotoxins, such as alkaloids and polyketides, that are toxic to animals, including humans. The fruiting structures of a few species contain psychotropic compounds and are consumed recreationally or in traditional spiritual ceremonies. Fungi can break down manufactured materials and buildings, and become significant pathogens of humans and other animals. Losses of crops due to fungal diseases (e.g., rice blast disease) or food spoilage can have a large impact on human food supplies and local economies.
The fungus kingdom encompasses an enormous diversity of taxa with varied ecologies, life cycle strategies, and morphologies ranging from unicellular aquatic chytrids to large mushrooms. However, little is known of the true biodiversity of the fungus kingdom, which has been estimated at 2.2 million to 3.8 million species. Of these, only about 148,000 have been described, with over 8,000 species known to be detrimental to plants and at least 300 that can be pathogenic to humans. Ever since the pioneering 18th and 19th century taxonomical works of Carl Linnaeus, Christiaan Hendrik Persoon, and Elias Magnus Fries, fungi have been classified according to their morphology (e.g., characteristics such as spore color or microscopic features) or physiology. Advances in molecular genetics have opened the way for DNA analysis to be incorporated into taxonomy, which has sometimes challenged the historical groupings based on morphology and other traits. Phylogenetic studies published in the first decade of the 21st century have helped reshape the classification within the fungi kingdom, which is divided into one subkingdom, seven phyla, and ten subphyla.
Etymology
The English word fungus is directly adopted from the Latin fungus (mushroom), used in the writings of Horace and Pliny. This in turn is derived from the Greek word sphongos (σφόγγος 'sponge'), which refers to the macroscopic structures and morphology of mushrooms and molds; the root is also used in other languages, such as the German Schwamm ('sponge') and Schimmel ('mold').
The word mycology is derived from the Greek (μύκης 'mushroom') and logos (λόγος 'discourse'). It denotes the scientific study of fungi. The Latin adjectival form of "mycology" (mycologicæ) appeared as early as 1796 in a book on the subject by Christiaan Hendrik Persoon. The word appeared in English as early as 1824 in a book by Robert Kaye Greville. In 1836 the English naturalist Miles Joseph Berkeley's publication The English Flora of Sir James Edward Smith, Vol. 5. also refers to mycology as the study of fungi.
A group of all the fungi present in a particular region is known as mycobiota (plural noun, no singular). The term mycota is often used for this purpose, but many authors use it as a synonym of Fungi. The word funga has been proposed as a less ambiguous term morphologically similar to fauna and flora. The Species Survival Commission (SSC) of the International Union for Conservation of Nature (IUCN) in August 2021 asked that the phrase fauna and flora be replaced by fauna, flora, and funga.
Characteristics
Before the introduction of molecular methods for phylogenetic analysis, taxonomists considered fungi to be members of the plant kingdom because of similarities in lifestyle: both fungi and plants are mainly immobile, and have similarities in general morphology and growth habitat. Although inaccurate, the common misconception that fungi are plants persists among the general public due to their historical classification, as well as several similarities. Like plants, fungi often grow in soil and, in the case of mushrooms, form conspicuous fruit bodies, which sometimes resemble plants such as mosses. The fungi are now considered a separate kingdom, distinct from both plants and animals, from which they appear to have diverged around one billion years ago (around the start of the Neoproterozoic Era). Some morphological, biochemical, and genetic features are shared with other organisms, while others are unique to the fungi, clearly separating them from the other kingdoms:
Shared features:
With other eukaryotes: Fungal cells contain membrane-bound nuclei with chromosomes that contain DNA with noncoding regions called introns and coding regions called exons. Fungi have membrane-bound cytoplasmic organelles such as mitochondria, sterol-containing membranes, and ribosomes of the 80S type. They have a characteristic range of soluble carbohydrates and storage compounds, including sugar alcohols (e.g., mannitol), disaccharides, (e.g., trehalose), and polysaccharides (e.g., glycogen, which is also found in animals).
With animals: Fungi lack chloroplasts and are heterotrophic organisms and so require preformed organic compounds as energy sources.
With plants: Fungi have a cell wall and vacuoles. They reproduce by both sexual and asexual means, and like basal plant groups (such as ferns and mosses) produce spores. Similar to mosses and algae, fungi typically have haploid nuclei.
With euglenoids and bacteria: Higher fungi, euglenoids, and some bacteria produce the amino acid L-lysine in specific biosynthesis steps, called the α-aminoadipate pathway.
The cells of most fungi grow as tubular, elongated, and thread-like (filamentous) structures called hyphae, which may contain multiple nuclei and extend by growing at their tips. Each tip contains a set of aggregated vesicles—cellular structures consisting of proteins, lipids, and other organic molecules—called the Spitzenkörper. Both fungi and oomycetes grow as filamentous hyphal cells. In contrast, similar-looking organisms, such as filamentous green algae, grow by repeated cell division within a chain of cells. There are also single-celled fungi (yeasts) that do not form hyphae, and some fungi have both hyphal and yeast forms.
In common with some plant and animal species, more than one hundred fungal species display bioluminescence.
Unique features:
Some species grow as unicellular yeasts that reproduce by budding or fission. Dimorphic fungi can switch between a yeast phase and a hyphal phase in response to environmental conditions.
The fungal cell wall is made of a chitin-glucan complex; while glucans are also found in plants and chitin in the exoskeleton of arthropods, fungi are the only organisms that combine these two structural molecules in their cell wall. Unlike those of plants and oomycetes, fungal cell walls do not contain cellulose.
Most fungi lack an efficient system for the long-distance transport of water and nutrients, such as the xylem and phloem in many plants. To overcome this limitation, some fungi, such as Armillaria, form rhizomorphs, which resemble and perform functions similar to the roots of plants. As eukaryotes, fungi possess a biosynthetic pathway for producing terpenes that uses mevalonic acid and pyrophosphate as chemical building blocks. Plants and some other organisms have an additional terpene biosynthesis pathway in their chloroplasts, a structure that fungi and animals do not have. Fungi produce several secondary metabolites that are similar or identical in structure to those made by plants. Many of the plant and fungal enzymes that make these compounds differ from each other in sequence and other characteristics, which indicates separate origins and convergent evolution of these enzymes in the fungi and plants.
Diversity
Fungi have a worldwide distribution, and grow in a wide range of habitats, including extreme environments such as deserts or areas with high salt concentrations or ionizing radiation, as well as in deep sea sediments. Some can survive the intense UV and cosmic radiation encountered during space travel. Most grow in terrestrial environments, though several species live partly or solely in aquatic habitats, such as the chytrid fungi Batrachochytrium dendrobatidis and B. salamandrivorans, parasites that have been responsible for a worldwide decline in amphibian populations. These organisms spend part of their life cycle as a motile zoospore, enabling them to propel themselves through water and enter their amphibian host. Other examples of aquatic fungi include those living in hydrothermal areas of the ocean.
around 148,000 species of fungi have been described by taxonomists, but the global biodiversity of the fungus kingdom is not fully understood. A 2017 estimate suggests there may be between 2.2 and 3.8 million species. The number of new fungi species discovered yearly has increased from 1,000 to 1,500 per year about 10 years ago, to about 2,000 with a peak of more than 2,500 species in 2016. In the year 2019, 1,882 new species of fungi were described, and it was estimated that more than 90% of fungi remain unknown. The following year, 2,905 new species were described—the highest annual record of new fungus names. In mycology, species have historically been distinguished by a variety of methods and concepts. Classification based on morphological characteristics, such as the size and shape of spores or fruiting structures, has traditionally dominated fungal taxonomy. Species may also be distinguished by their biochemical and physiological characteristics, such as their ability to metabolize certain biochemicals, or their reaction to chemical tests. The biological species concept discriminates species based on their ability to mate. The application of molecular tools, such as DNA sequencing and phylogenetic analysis, to study diversity has greatly enhanced the resolution and added robustness to estimates of genetic diversity within various taxonomic groups.
Mycology
Mycology is the branch of biology concerned with the systematic study of fungi, including their genetic and biochemical properties, their taxonomy, and their use to humans as a source of medicine, food, and psychotropic substances consumed for religious purposes, as well as their dangers, such as poisoning or infection. The field of phytopathology, the study of plant diseases, is closely related because many plant pathogens are fungi.
The use of fungi by humans dates back to prehistory; Ötzi the Iceman, a well-preserved mummy of a 5,300-year-old Neolithic man found frozen in the Austrian Alps, carried two species of polypore mushrooms that may have been used as tinder (Fomes fomentarius), or for medicinal purposes (Piptoporus betulinus). Ancient peoples have used fungi as food sources—often unknowingly—for millennia, in the preparation of leavened bread and fermented juices. Some of the oldest written records contain references to the destruction of crops that were probably caused by pathogenic fungi.
History
Mycology became a systematic science after the development of the microscope in the 17th century. Although fungal spores were first observed by Giambattista della Porta in 1588, the seminal work in the development of mycology is considered to be the publication of Pier Antonio Micheli's 1729 work Nova plantarum genera. Micheli not only observed spores but also showed that, under the proper conditions, they could be induced into growing into the same species of fungi from which they originated. Extending the use of the binomial system of nomenclature introduced by Carl Linnaeus in his Species plantarum (1753), the Dutch Christiaan Hendrik Persoon (1761–1836) established the first classification of mushrooms with such skill as to be considered a founder of modern mycology. Later, Elias Magnus Fries (1794–1878) further elaborated the classification of fungi, using spore color and microscopic characteristics, methods still used by taxonomists today. Other notable early contributors to mycology in the 17th–19th and early 20th centuries include Miles Joseph Berkeley, August Carl Joseph Corda, Anton de Bary, the brothers Louis René and Charles Tulasne, Arthur H. R. Buller, Curtis G. Lloyd, and Pier Andrea Saccardo. In the 20th and 21st centuries, advances in biochemistry, genetics, molecular biology, biotechnology, DNA sequencing, and phylogenetic analysis have provided new insights into fungal relationships and biodiversity, and have challenged traditional morphology-based groupings in fungal taxonomy.
Morphology
Microscopic structures
Most fungi grow as hyphae, which are cylindrical, thread-like structures 2–10μm in diameter and up to several centimeters in length. Hyphae grow at their tips (apices); new hyphae are typically formed by emergence of new tips along existing hyphae by a process called branching, or occasionally growing hyphal tips fork, giving rise to two parallel-growing hyphae. Hyphae also sometimes fuse when they come into contact, a process called hyphal fusion (or anastomosis). These growth processes lead to the development of a mycelium, an interconnected network of hyphae. Hyphae can be either septate or coenocytic. Septate hyphae are divided into compartments separated by cross walls (internal cell walls, called septa, that are formed at right angles to the cell wall giving the hypha its shape), with each compartment containing one or more nuclei; coenocytic hyphae are not compartmentalized. Septa have pores that allow cytoplasm, organelles, and sometimes nuclei to pass through; an example is the dolipore septum in fungi of the phylum Basidiomycota. Coenocytic hyphae are in essence multinucleate supercells.
Many species have developed specialized hyphal structures for nutrient uptake from living hosts; examples include haustoria in plant-parasitic species of most fungal phyla, and arbuscules of several mycorrhizal fungi, which penetrate into the host cells to consume nutrients.
Although fungi are opisthokonts—a grouping of evolutionarily related organisms broadly characterized by a single posterior flagellum—all phyla except for the chytrids have lost their posterior flagella. Fungi are unusual among the eukaryotes in having a cell wall that, in addition to glucans (e.g., β-1,3-glucan) and other typical components, also contains the biopolymer chitin.
Macroscopic structures
Fungal mycelia can become visible to the naked eye, for example, on various surfaces and substrates, such as damp walls and spoiled food, where they are commonly called molds. Mycelia grown on solid agar media in laboratory petri dishes are usually referred to as colonies. These colonies can exhibit growth shapes and colors (due to spores or pigmentation) that can be used as diagnostic features in the identification of species or groups. Some individual fungal colonies can reach extraordinary dimensions and ages as in the case of a clonal colony of Armillaria solidipes, which extends over an area of more than 900ha (3.5 square miles), with an estimated age of nearly 9,000years.
The apothecium—a specialized structure important in sexual reproduction in the ascomycetes—is a cup-shaped fruit body that is often macroscopic and holds the hymenium, a layer of tissue containing the spore-bearing cells. The fruit bodies of the basidiomycetes (basidiocarps) and some ascomycetes can sometimes grow very large, and many are well known as mushrooms.
Growth and physiology
The growth of fungi as hyphae on or in solid substrates or as single cells in aquatic environments is adapted for the efficient extraction of nutrients, because these growth forms have high surface area to volume ratios. Hyphae are specifically adapted for growth on solid surfaces, and to invade substrates and tissues. They can exert large penetrative mechanical forces; for example, many plant pathogens, including Magnaporthe grisea, form a structure called an appressorium that evolved to puncture plant tissues. The pressure generated by the appressorium, directed against the plant epidermis, can exceed . The filamentous fungus Paecilomyces lilacinus uses a similar structure to penetrate the eggs of nematodes.
The mechanical pressure exerted by the appressorium is generated from physiological processes that increase intracellular turgor by producing osmolytes such as glycerol. Adaptations such as these are complemented by hydrolytic enzymes secreted into the environment to digest large organic molecules—such as polysaccharides, proteins, and lipids—into smaller molecules that may then be absorbed as nutrients. The vast majority of filamentous fungi grow in a polar fashion (extending in one direction) by elongation at the tip (apex) of the hypha. Other forms of fungal growth include intercalary extension (longitudinal expansion of hyphal compartments that are below the apex) as in the case of some endophytic fungi, or growth by volume expansion during the development of mushroom stipes and other large organs. Growth of fungi as multicellular structures consisting of somatic and reproductive cells—a feature independently evolved in animals and plants—has several functions, including the development of fruit bodies for dissemination of sexual spores (see above) and biofilms for substrate colonization and intercellular communication.
Fungi are traditionally considered heterotrophs, organisms that rely solely on carbon fixed by other organisms for metabolism. Fungi have evolved a high degree of metabolic versatility that allows them to use a diverse range of organic substrates for growth, including simple compounds such as nitrate, ammonia, acetate, or ethanol. In some species the pigment melanin may play a role in extracting energy from ionizing radiation, such as gamma radiation. This form of "radiotrophic" growth has been described for only a few species, the effects on growth rates are small, and the underlying biophysical and biochemical processes are not well known. This process might bear similarity to CO2 fixation via visible light, but instead uses ionizing radiation as a source of energy.
Reproduction
Fungal reproduction is complex, reflecting the differences in lifestyles and genetic makeup within this diverse kingdom of organisms. It is estimated that a third of all fungi reproduce using more than one method of propagation; for example, reproduction may occur in two well-differentiated stages within the life cycle of a species, the teleomorph (sexual reproduction) and the anamorph (asexual reproduction). Environmental conditions trigger genetically determined developmental states that lead to the creation of specialized structures for sexual or asexual reproduction. These structures aid reproduction by efficiently dispersing spores or spore-containing propagules.
Asexual reproduction
Asexual reproduction occurs via vegetative spores (conidia) or through mycelial fragmentation. Mycelial fragmentation occurs when a fungal mycelium separates into pieces, and each component grows into a separate mycelium. Mycelial fragmentation and vegetative spores maintain clonal populations adapted to a specific niche, and allow more rapid dispersal than sexual reproduction. The "Fungi imperfecti" (fungi lacking the perfect or sexual stage) or Deuteromycota comprise all the species that lack an observable sexual cycle. Deuteromycota (alternatively known as Deuteromycetes, conidial fungi, or mitosporic fungi) is not an accepted taxonomic clade and is now taken to mean simply fungi that lack a known sexual stage.
Sexual reproduction
Sexual reproduction with meiosis has been directly observed in all fungal phyla except Glomeromycota (genetic analysis suggests meiosis in Glomeromycota as well). It differs in many aspects from sexual reproduction in animals or plants. Differences also exist between fungal groups and can be used to discriminate species by morphological differences in sexual structures and reproductive strategies. Mating experiments between fungal isolates may identify species on the basis of biological species concepts. The major fungal groupings have initially been delineated based on the morphology of their sexual structures and spores; for example, the spore-containing structures, asci and basidia, can be used in the identification of ascomycetes and basidiomycetes, respectively. Fungi employ two mating systems: heterothallic species allow mating only between individuals of the opposite mating type, whereas homothallic species can mate, and sexually reproduce, with any other individual or itself.
Most fungi have both a haploid and a diploid stage in their life cycles. In sexually reproducing fungi, compatible individuals may combine by fusing their hyphae together into an interconnected network; this process, anastomosis, is required for the initiation of the sexual cycle. Many ascomycetes and basidiomycetes go through a dikaryotic stage, in which the nuclei inherited from the two parents do not combine immediately after cell fusion, but remain separate in the hyphal cells (see heterokaryosis).
In ascomycetes, dikaryotic hyphae of the hymenium (the spore-bearing tissue layer) form a characteristic hook (crozier) at the hyphal septum. During cell division, the formation of the hook ensures proper distribution of the newly divided nuclei into the apical and basal hyphal compartments. An ascus (plural asci) is then formed, in which karyogamy (nuclear fusion) occurs. Asci are embedded in an ascocarp, or fruiting body. Karyogamy in the asci is followed immediately by meiosis and the production of ascospores. After dispersal, the ascospores may germinate and form a new haploid mycelium.
Sexual reproduction in basidiomycetes is similar to that of the ascomycetes. Compatible haploid hyphae fuse to produce a dikaryotic mycelium. However, the dikaryotic phase is more extensive in the basidiomycetes, often also present in the vegetatively growing mycelium. A specialized anatomical structure, called a clamp connection, is formed at each hyphal septum. As with the structurally similar hook in the ascomycetes, the clamp connection in the basidiomycetes is required for controlled transfer of nuclei during cell division, to maintain the dikaryotic stage with two genetically different nuclei in each hyphal compartment. A basidiocarp is formed in which club-like structures known as basidia generate haploid basidiospores after karyogamy and meiosis. The most commonly known basidiocarps are mushrooms, but they may also take other forms (see Morphology section).
In fungi formerly classified as Zygomycota, haploid hyphae of two individuals fuse, forming a gametangium, a specialized cell structure that becomes a fertile gamete-producing cell. The gametangium develops into a zygospore, a thick-walled spore formed by the union of gametes. When the zygospore germinates, it undergoes meiosis, generating new haploid hyphae, which may then form asexual sporangiospores. These sporangiospores allow the fungus to rapidly disperse and germinate into new genetically identical haploid fungal mycelia.
Spore dispersal
The spores of most of the researched species of fungi are transported by wind. Such species often produce dry or hydrophobic spores that do not absorb water and are readily scattered by raindrops, for example. In other species, both asexual and sexual spores or sporangiospores are often actively dispersed by forcible ejection from their reproductive structures. This ejection ensures exit of the spores from the reproductive structures as well as traveling through the air over long distances.
Specialized mechanical and physiological mechanisms, as well as spore surface structures (such as hydrophobins), enable efficient spore ejection. For example, the structure of the spore-bearing cells in some ascomycete species is such that the buildup of substances affecting cell volume and fluid balance enables the explosive discharge of spores into the air. The forcible discharge of single spores termed ballistospores involves formation of a small drop of water (Buller's drop), which upon contact with the spore leads to its projectile release with an initial acceleration of more than 10,000g; the net result is that the spore is ejected 0.01–0.02cm, sufficient distance for it to fall through the gills or pores into the air below. Other fungi, like the puffballs, rely on alternative mechanisms for spore release, such as external mechanical forces. The hydnoid fungi (tooth fungi) produce spores on pendant, tooth-like or spine-like projections. The bird's nest fungi use the force of falling water drops to liberate the spores from cup-shaped fruiting bodies. Another strategy is seen in the stinkhorns, a group of fungi with lively colors and putrid odor that attract insects to disperse their spores.
Homothallism
In homothallic sexual reproduction, two haploid nuclei derived from the same individual fuse to form a zygote that can then undergo meiosis. Homothallic fungi include species with an Aspergillus-like asexual stage (anamorphs) occurring in numerous different genera, several species of the ascomycete genus Cochliobolus, and the ascomycete Pneumocystis jirovecii. The earliest mode of sexual reproduction among eukaryotes was likely homothallism, that is, self-fertile unisexual reproduction.
Other sexual processes
Besides regular sexual reproduction with meiosis, certain fungi, such as those in the genera Penicillium and Aspergillus, may exchange genetic material via parasexual processes, initiated by anastomosis between hyphae and plasmogamy of fungal cells. The frequency and relative importance of parasexual events is unclear and may be lower than other sexual processes. It is known to play a role in intraspecific hybridization and is likely required for hybridization between species, which has been associated with major events in fungal evolution.
Evolution
In contrast to plants and animals, the early fossil record of the fungi is meager. Factors that likely contribute to the under-representation of fungal species among fossils include the nature of fungal fruiting bodies, which are soft, fleshy, and easily degradable tissues, and the microscopic dimensions of most fungal structures, which therefore are not readily evident. Fungal fossils are difficult to distinguish from those of other microbes, and are most easily identified when they resemble extant fungi. Often recovered from a permineralized plant or animal host, these samples are typically studied by making thin-section preparations that can be examined with light microscopy or transmission electron microscopy. Researchers study compression fossils by dissolving the surrounding matrix with acid and then using light or scanning electron microscopy to examine surface details.
The earliest fossils possessing features typical of fungi date to the Paleoproterozoic era, some (Ma); these multicellular benthic organisms had filamentous structures capable of anastomosis. Other studies (2009) estimate the arrival of fungal organisms at about 760–1060Ma on the basis of comparisons of the rate of evolution in closely related groups. The oldest fossilizied mycelium to be identified from its molecular composition is between 715 and 810 million years old. For much of the Paleozoic Era (542–251Ma), the fungi appear to have been aquatic and consisted of organisms similar to the extant chytrids in having flagellum-bearing spores. The evolutionary adaptation from an aquatic to a terrestrial lifestyle necessitated a diversification of ecological strategies for obtaining nutrients, including parasitism, saprobism, and the development of mutualistic relationships such as mycorrhiza and lichenization. Studies suggest that the ancestral ecological state of the Ascomycota was saprobism, and that independent lichenization events have occurred multiple times.
In May 2019, scientists reported the discovery of a fossilized fungus, named Ourasphaira giraldae, in the Canadian Arctic, that may have grown on land a billion years ago, well before plants were living on land. Pyritized fungus-like microfossils preserved in the basal Ediacaran Doushantuo Formation (~635 Ma) have been reported in South China. Earlier, it had been presumed that the fungi colonized the land during the Cambrian (542–488.3Ma), also long before land plants. Fossilized hyphae and spores recovered from the Ordovician of Wisconsin (460Ma) resemble modern-day Glomerales, and existed at a time when the land flora likely consisted of only non-vascular bryophyte-like plants. Prototaxites, which was probably a fungus or lichen, would have been the tallest organism of the late Silurian and early Devonian. Fungal fossils do not become common and uncontroversial until the early Devonian (416–359.2Ma), when they occur abundantly in the Rhynie chert, mostly as Zygomycota and Chytridiomycota. At about this same time, approximately 400Ma, the Ascomycota and Basidiomycota diverged, and all modern classes of fungi were present by the Late Carboniferous (Pennsylvanian, 318.1–299Ma).
Lichens formed a component of the early terrestrial ecosystems, and the estimated age of the oldest terrestrial lichen fossil is 415Ma; this date roughly corresponds to the age of the oldest known sporocarp fossil, a Paleopyrenomycites species found in the Rhynie Chert. The oldest fossil with microscopic features resembling modern-day basidiomycetes is Palaeoancistrus, found permineralized with a fern from the Pennsylvanian. Rare in the fossil record are the Homobasidiomycetes (a taxon roughly equivalent to the mushroom-producing species of the Agaricomycetes). Two amber-preserved specimens provide evidence that the earliest known mushroom-forming fungi (the extinct species Archaeomarasmius leggetti) appeared during the late Cretaceous, 90Ma.
Some time after the Permian–Triassic extinction event (251.4Ma), a fungal spike (originally thought to be an extraordinary abundance of fungal spores in sediments) formed, suggesting that fungi were the dominant life form at this time, representing nearly 100% of the available fossil record for this period. However, the relative proportion of fungal spores relative to spores formed by algal species is difficult to assess, the spike did not appear worldwide, and in many places it did not fall on the Permian–Triassic boundary.
Sixty-five million years ago, immediately after the Cretaceous–Paleogene extinction event that famously killed off most dinosaurs, there was a dramatic increase in evidence of fungi; apparently the death of most plant and animal species led to a huge fungal bloom like "a massive compost heap".
Taxonomy
Although commonly included in botany curricula and textbooks, fungi are more closely related to animals than to plants and are placed with the animals in the monophyletic group of opisthokonts. Analyses using molecular phylogenetics support a monophyletic origin of fungi. The taxonomy of fungi is in a state of constant flux, especially due to research based on DNA comparisons. These current phylogenetic analyses often overturn classifications based on older and sometimes less discriminative methods based on morphological features and biological species concepts obtained from experimental matings.
There is no unique generally accepted system at the higher taxonomic levels and there are frequent name changes at every level, from species upwards. Efforts among researchers are now underway to establish and encourage usage of a unified and more consistent nomenclature. Until relatively recent (2012) changes to the International Code of Nomenclature for algae, fungi and plants, fungal species could also have multiple scientific names depending on their life cycle and mode (sexual or asexual) of reproduction. Web sites such as Index Fungorum and MycoBank are officially recognized nomenclatural repositories and list current names of fungal species (with cross-references to older synonyms).
The 2007 classification of Kingdom Fungi is the result of a large-scale collaborative research effort involving dozens of mycologists and other scientists working on fungal taxonomy. It recognizes seven phyla, two of which—the Ascomycota and the Basidiomycota—are contained within a branch representing subkingdom Dikarya, the most species rich and familiar group, including all the mushrooms, most food-spoilage molds, most plant pathogenic fungi, and the beer, wine, and bread yeasts. The accompanying cladogram depicts the major fungal taxa and their relationship to opisthokont and unikont organisms, based on the work of Philippe Silar, "The Mycota: A Comprehensive Treatise on Fungi as Experimental Systems for Basic and Applied Research" and Tedersoo et al. 2018. The lengths of the branches are not proportional to evolutionary distances.
Taxonomic groups
The major phyla (sometimes called divisions) of fungi have been classified mainly on the basis of characteristics of their sexual reproductive structures. , nine major lineages have been identified: Opisthosporidia, Chytridiomycota, Neocallimastigomycota, Blastocladiomycota, Zoopagomycotina, Mucoromycota, Glomeromycota, Ascomycota, and Basidiomycota.
Phylogenetic analysis has demonstrated that the Microsporidia, unicellular parasites of animals and protists, are fairly recent and highly derived endobiotic fungi (living within the tissue of another species). Previously considered to be "primitive" protozoa, they are now thought to be either a basal branch of the Fungi, or a sister group–each other's closest evolutionary relative.
The Chytridiomycota are commonly known as chytrids. These fungi are distributed worldwide. Chytrids and their close relatives Neocallimastigomycota and Blastocladiomycota (below) are the only fungi with active motility, producing zoospores that are capable of active movement through aqueous phases with a single flagellum, leading early taxonomists to classify them as protists. Molecular phylogenies, inferred from rRNA sequences in ribosomes, suggest that the Chytrids are a basal group divergent from the other fungal phyla, consisting of four major clades with suggestive evidence for paraphyly or possibly polyphyly.
The Blastocladiomycota were previously considered a taxonomic clade within the Chytridiomycota. Molecular data and ultrastructural characteristics, however, place the Blastocladiomycota as a sister clade to the Zygomycota, Glomeromycota, and Dikarya (Ascomycota and Basidiomycota). The blastocladiomycetes are saprotrophs, feeding on decomposing organic matter, and they are parasites of all eukaryotic groups. Unlike their close relatives, the chytrids, most of which exhibit zygotic meiosis, the blastocladiomycetes undergo sporic meiosis.
The Neocallimastigomycota were earlier placed in the phylum Chytridiomycota. Members of this small phylum are anaerobic organisms, living in the digestive system of larger herbivorous mammals and in other terrestrial and aquatic environments enriched in cellulose (e.g., domestic waste landfill sites). They lack mitochondria but contain hydrogenosomes of mitochondrial origin. As in the related chrytrids, neocallimastigomycetes form zoospores that are posteriorly uniflagellate or polyflagellate.
Members of the Glomeromycota form arbuscular mycorrhizae, a form of mutualist symbiosis wherein fungal hyphae invade plant root cells and both species benefit from the resulting increased supply of nutrients. All known Glomeromycota species reproduce asexually. The symbiotic association between the Glomeromycota and plants is ancient, with evidence dating to 400 million years ago. Formerly part of the Zygomycota (commonly known as 'sugar' and 'pin' molds), the Glomeromycota were elevated to phylum status in 2001 and now replace the older phylum Zygomycota. Fungi that were placed in the Zygomycota are now being reassigned to the Glomeromycota, or the subphyla incertae sedis Mucoromycotina, Kickxellomycotina, the Zoopagomycotina and the Entomophthoromycotina. Some well-known examples of fungi formerly in the Zygomycota include black bread mold (Rhizopus stolonifer), and Pilobolus species, capable of ejecting spores several meters through the air. Medically relevant genera include Mucor, Rhizomucor, and Rhizopus.
The Ascomycota, commonly known as sac fungi or ascomycetes, constitute the largest taxonomic group within the Eumycota. These fungi form meiotic spores called ascospores, which are enclosed in a special sac-like structure called an ascus. This phylum includes morels, a few mushrooms and truffles, unicellular yeasts (e.g., of the genera Saccharomyces, Kluyveromyces, Pichia, and Candida), and many filamentous fungi living as saprotrophs, parasites, and mutualistic symbionts (e.g. lichens). Prominent and important genera of filamentous ascomycetes include Aspergillus, Penicillium, Fusarium, and Claviceps. Many ascomycete species have only been observed undergoing asexual reproduction (called anamorphic species), but analysis of molecular data has often been able to identify their closest teleomorphs in the Ascomycota. Because the products of meiosis are retained within the sac-like ascus, ascomycetes have been used for elucidating principles of genetics and heredity (e.g., Neurospora crassa).
Members of the Basidiomycota, commonly known as the club fungi or basidiomycetes, produce meiospores called basidiospores on club-like stalks called basidia. Most common mushrooms belong to this group, as well as rust and smut fungi, which are major pathogens of grains. Other important basidiomycetes include the maize pathogen Ustilago maydis, human commensal species of the genus Malassezia, and the opportunistic human pathogen, Cryptococcus neoformans.
Fungus-like organisms
Because of similarities in morphology and lifestyle, the slime molds (mycetozoans, plasmodiophorids, acrasids, Fonticula, and labyrinthulids, now in Amoebozoa, Rhizaria, Excavata, Cristidiscoidea, and Stramenopiles, respectively), water molds (oomycetes) and hyphochytrids (both Stramenopiles) were formerly classified in the kingdom Fungi, in groups like Mastigomycotina, Gymnomycota and Phycomycetes. The slime molds were studied also as protozoans, leading to an ambiregnal, duplicated taxonomy.
Unlike true fungi, the cell walls of oomycetes contain cellulose and lack chitin. Hyphochytrids have both chitin and cellulose. Slime molds lack a cell wall during the assimilative phase (except labyrinthulids, which have a wall of scales), and take in nutrients by ingestion (phagocytosis, except labyrinthulids) rather than absorption (osmotrophy, as fungi, labyrinthulids, oomycetes and hyphochytrids). Neither water molds nor slime molds are closely related to the true fungi, and, therefore, taxonomists no longer group them in the kingdom Fungi. Nonetheless, studies of the oomycetes and myxomycetes are still often included in mycology textbooks and primary research literature.
The Eccrinales and Amoebidiales are opisthokont protists, previously thought to be zygomycete fungi. Other groups now in Opisthokonta (e.g., Corallochytrium, Ichthyosporea) were also at given time classified as fungi. The genus Blastocystis, now in Stramenopiles, was originally classified as a yeast. Ellobiopsis, now in Alveolata, was considered a chytrid. The bacteria were also included in fungi in some classifications, as the group Schizomycetes.
The Rozellida clade, including the "ex-chytrid" Rozella, is a genetically disparate group known mostly from environmental DNA sequences that is a sister group to fungi. Members of the group that have been isolated lack the chitinous cell wall that is characteristic of fungi. Alternatively, Rozella can be classified as a basal fungal group.
The nucleariids may be the next sister group to the eumycete clade, and as such could be included in an expanded fungal kingdom.
Many Actinomycetales (Actinomycetota), a group with many filamentous bacteria, were also long believed to be fungi.
Ecology
Although often inconspicuous, fungi occur in every environment on Earth and play very important roles in most ecosystems. Along with bacteria, fungi are the major decomposers in most terrestrial (and some aquatic) ecosystems, and therefore play a critical role in biogeochemical cycles and in many food webs. As decomposers, they play an essential role in nutrient cycling, especially as saprotrophs and symbionts, degrading organic matter to inorganic molecules, which can then re-enter anabolic metabolic pathways in plants or other organisms.
Symbiosis
Many fungi have important symbiotic relationships with organisms from most if not all kingdoms. These interactions can be mutualistic or antagonistic in nature, or in the case of commensal fungi are of no apparent benefit or detriment to the host.
With plants
Mycorrhizal symbiosis between plants and fungi is one of the most well-known plant–fungus associations and is of significant importance for plant growth and persistence in many ecosystems; over 90% of all plant species engage in mycorrhizal relationships with fungi and are dependent upon this relationship for survival.
The mycorrhizal symbiosis is ancient, dating back to at least 400 million years. It often increases the plant's uptake of inorganic compounds, such as nitrate and phosphate from soils having low concentrations of these key plant nutrients. The fungal partners may also mediate plant-to-plant transfer of carbohydrates and other nutrients. Such mycorrhizal communities are called "common mycorrhizal networks". A special case of mycorrhiza is myco-heterotrophy, whereby the plant parasitizes the fungus, obtaining all of its nutrients from its fungal symbiont. Some fungal species inhabit the tissues inside roots, stems, and leaves, in which case they are called endophytes. Similar to mycorrhiza, endophytic colonization by fungi may benefit both symbionts; for example, endophytes of grasses impart to their host increased resistance to herbivores and other environmental stresses and receive food and shelter from the plant in return.
With algae and cyanobacteria
Lichens are a symbiotic relationship between fungi and photosynthetic algae or cyanobacteria. The photosynthetic partner in the relationship is referred to in lichen terminology as a "photobiont". The fungal part of the relationship is composed mostly of various species of ascomycetes and a few basidiomycetes. Lichens occur in every ecosystem on all continents, play a key role in soil formation and the initiation of biological succession, and are prominent in some extreme environments, including polar, alpine, and semiarid desert regions. They are able to grow on inhospitable surfaces, including bare soil, rocks, tree bark, wood, shells, barnacles and leaves. As in mycorrhizas, the photobiont provides sugars and other carbohydrates via photosynthesis to the fungus, while the fungus provides minerals and water to the photobiont. The functions of both symbiotic organisms are so closely intertwined that they function almost as a single organism; in most cases the resulting organism differs greatly from the individual components. Lichenization is a common mode of nutrition for fungi; around 27% of known fungi—more than 19,400 species—are lichenized. Characteristics common to most lichens include obtaining organic carbon by photosynthesis, slow growth, small size, long life, long-lasting (seasonal) vegetative reproductive structures, mineral nutrition obtained largely from airborne sources, and greater tolerance of desiccation than most other photosynthetic organisms in the same habitat.
With insects
Many insects also engage in mutualistic relationships with fungi. Several groups of ants cultivate fungi in the order Chaetothyriales for several purposes: as a food source, as a structural component of their nests, and as a part of an ant/plant symbiosis in the domatia (tiny chambers in plants that house arthropods). Ambrosia beetles cultivate various species of fungi in the bark of trees that they infest. Likewise, females of several wood wasp species (genus Sirex) inject their eggs together with spores of the wood-rotting fungus Amylostereum areolatum into the sapwood of pine trees; the growth of the fungus provides ideal nutritional conditions for the development of the wasp larvae. At least one species of stingless bee has a relationship with a fungus in the genus Monascus, where the larvae consume and depend on fungus transferred from old to new nests. Termites on the African savannah are also known to cultivate fungi, and yeasts of the genera Candida and Lachancea inhabit the gut of a wide range of insects, including neuropterans, beetles, and cockroaches; it is not known whether these fungi benefit their hosts. Fungi growing in dead wood are essential for xylophagous insects (e.g. woodboring beetles). They deliver nutrients needed by xylophages to nutritionally scarce dead wood. Thanks to this nutritional enrichment the larvae of the woodboring insect is able to grow and develop to adulthood. The larvae of many families of fungicolous flies, particularly those within the superfamily Sciaroidea such as the Mycetophilidae and some Keroplatidae feed on fungal fruiting bodies and sterile mycorrhizae.
As parasites
Many fungi are parasites on plants, animals (including humans), and other fungi. Serious pathogens of many cultivated plants causing extensive damage and losses to agriculture and forestry include the rice blast fungus Magnaporthe oryzae, tree pathogens such as Ophiostoma ulmi and Ophiostoma novo-ulmi causing Dutch elm disease, Cryphonectria parasitica responsible for chestnut blight, and Phymatotrichopsis omnivora causing Texas Root Rot, and plant pathogens in the genera Fusarium, Ustilago, Alternaria, and Cochliobolus. Some carnivorous fungi, like Paecilomyces lilacinus, are predators of nematodes, which they capture using an array of specialized structures such as constricting rings or adhesive nets. Many fungi that are plant pathogens, such as Magnaporthe oryzae, can switch from being biotrophic (parasitic on living plants) to being necrotrophic (feeding on the dead tissues of plants they have killed). This same principle is applied to fungi-feeding parasites, including Asterotremella albida, which feeds on the fruit bodies of other fungi both while they are living and after they are dead.
Some fungi alter the behavior of their animal hosts in ways that spread their spores more effectively (also called "active host transmission"). Examples include Ophiocordyceps unilateralis and possibly the extinct Allocordyceps.
As pathogens
Some fungi can cause serious diseases in humans, several of which may be fatal if untreated. These include aspergillosis, candidiasis, coccidioidomycosis, cryptococcosis, histoplasmosis, mycetomas, and paracoccidioidomycosis. Furthermore, a person with immunodeficiency is more susceptible to disease by genera such as Aspergillus, Candida, Cryptoccocus, Histoplasma, and Pneumocystis. Other fungi can attack eyes, nails, hair, and especially skin, the so-called dermatophytic and keratinophilic fungi, and cause local infections such as ringworm and athlete's foot. Fungal spores are also a cause of allergies, and fungi from different taxonomic groups can evoke allergic reactions.
As targets of mycoparasites
Organisms that parasitize fungi are known as mycoparasitic organisms. About 300 species of fungi and fungus-like organisms, belonging to 13 classes and 113 genera, are used as biocontrol agents against plant fungal diseases. Fungi can also act as mycoparasites or antagonists of other fungi, such as Hypomyces chrysospermus, which grows on bolete mushrooms.
Fungi can also become the target of infection by mycoviruses.
Communication
There appears to be electrical communication between fungi in word-like components according to spiking characteristics.
Possible impact on climate
According to a study published in the academic journal Current Biology, fungi can soak from the atmosphere around 36% of global fossil fuel greenhouse gas emissions.
Mycotoxins
Many fungi produce biologically active compounds, several of which are toxic to animals or plants and are therefore called mycotoxins. Of particular relevance to humans are mycotoxins produced by molds causing food spoilage, and poisonous mushrooms (see above). Particularly infamous are the lethal amatoxins in some Amanita mushrooms, and ergot alkaloids, which have a long history of causing serious epidemics of ergotism (St Anthony's Fire) in people consuming rye or related cereals contaminated with sclerotia of the ergot fungus, Claviceps purpurea. Other notable mycotoxins include the aflatoxins, which are insidious liver toxins and highly carcinogenic metabolites produced by certain Aspergillus species often growing in or on grains and nuts consumed by humans, ochratoxins, patulin, and trichothecenes (e.g., T-2 mycotoxin) and fumonisins, which have significant impact on human food supplies or animal livestock.
Mycotoxins are secondary metabolites (or natural products), and research has established the existence of biochemical pathways solely for the purpose of producing mycotoxins and other natural products in fungi. Mycotoxins may provide fitness benefits in terms of physiological adaptation, competition with other microbes and fungi, and protection from consumption (fungivory). Many fungal secondary metabolites (or derivatives) are used medically, as described under Human use below.
Pathogenic mechanisms
Ustilago maydis is a pathogenic plant fungus that causes smut disease in maize and teosinte. Plants have evolved efficient defense systems against pathogenic microbes such as U. maydis. A rapid defense reaction after pathogen attack is the oxidative burst where the plant produces reactive oxygen species at the site of the attempted invasion. U. maydis can respond to the oxidative burst with an oxidative stress response, regulated by the gene YAP1. The response protects U. maydis from the host defense, and is necessary for the pathogen's virulence. Furthermore, U. maydis has a well-established recombinational DNA repair system which acts during mitosis and meiosis. The system may assist the pathogen in surviving DNA damage arising from the host plant's oxidative defensive response to infection.
Cryptococcus neoformans is an encapsulated yeast that can live in both plants and animals. C.neoformans usually infects the lungs, where it is phagocytosed by alveolar macrophages. Some C.neoformans can survive inside macrophages, which appears to be the basis for latency, disseminated disease, and resistance to antifungal agents. One mechanism by which C.neoformans survives the hostile macrophage environment is by up-regulating the expression of genes involved in the oxidative stress response. Another mechanism involves meiosis. The majority of C.neoformans are mating "type a". Filaments of mating "type a" ordinarily have haploid nuclei, but they can become diploid (perhaps by endoduplication or by stimulated nuclear fusion) to form blastospores. The diploid nuclei of blastospores can undergo meiosis, including recombination, to form haploid basidiospores that can be dispersed. This process is referred to as monokaryotic fruiting. This process requires a gene called DMC1, which is a conserved homologue of genes recA in bacteria and RAD51 in eukaryotes, that mediates homologous chromosome pairing during meiosis and repair of DNA double-strand breaks. Thus, C.neoformans can undergo a meiosis, monokaryotic fruiting, that promotes recombinational repair in the oxidative, DNA damaging environment of the host macrophage, and the repair capability may contribute to its virulence.
Human use
The human use of fungi for food preparation or preservation and other purposes is extensive and has a long history. Mushroom farming and mushroom gathering are large industries in many countries. The study of the historical uses and sociological impact of fungi is known as ethnomycology. Because of the capacity of this group to produce an enormous range of natural products with antimicrobial or other biological activities, many species have long been used or are being developed for industrial production of antibiotics, vitamins, and anti-cancer and cholesterol-lowering drugs. Methods have been developed for genetic engineering of fungi, enabling metabolic engineering of fungal species. For example, genetic modification of yeast species—which are easy to grow at fast rates in large fermentation vessels—has opened up ways of pharmaceutical production that are potentially more efficient than production by the original source organisms. Fungi-based industries are sometimes considered to be a major part of a growing bioeconomy, with applications under research and development including use for textiles, meat substitution and general fungal biotechnology.
Therapeutic uses
Modern chemotherapeutics
Many species produce metabolites that are major sources of pharmacologically active drugs.
Antibiotics
Particularly important are the antibiotics, including the penicillins, a structurally related group of β-lactam antibiotics that are synthesized from small peptides. Although naturally occurring penicillins such as penicillin G (produced by Penicillium chrysogenum) have a relatively narrow spectrum of biological activity, a wide range of other penicillins can be produced by chemical modification of the natural penicillins. Modern penicillins are semisynthetic compounds, obtained initially from fermentation cultures, but then structurally altered for specific desirable properties. Other antibiotics produced by fungi include: ciclosporin, commonly used as an immunosuppressant during transplant surgery; and fusidic acid, used to help control infection from methicillin-resistant Staphylococcus aureus bacteria. Widespread use of antibiotics for the treatment of bacterial diseases, such as tuberculosis, syphilis, leprosy, and others began in the early 20th century and continues to date. In nature, antibiotics of fungal or bacterial origin appear to play a dual role: at high concentrations they act as chemical defense against competition with other microorganisms in species-rich environments, such as the rhizosphere, and at low concentrations as quorum-sensing molecules for intra- or interspecies signaling.
Other
Other drugs produced by fungi include griseofulvin isolated from Penicillium griseofulvum, used to treat fungal infections, and statins (HMG-CoA reductase inhibitors), used to inhibit cholesterol synthesis. Examples of statins found in fungi include mevastatin from Penicillium citrinum and lovastatin from Aspergillus terreus and the oyster mushroom. Psilocybin from fungi is investigated for therapeutic use and appears to cause global increases in brain network integration. Fungi produce compounds that inhibit viruses and cancer cells. Specific metabolites, such as polysaccharide-K, ergotamine, and β-lactam antibiotics, are routinely used in clinical medicine. The shiitake mushroom is a source of lentinan, a clinical drug approved for use in cancer treatments in several countries, including Japan. In Europe and Japan, polysaccharide-K (brand name Krestin), a chemical derived from Trametes versicolor, is an approved adjuvant for cancer therapy.
Traditional medicine
Certain mushrooms are used as supposed therapeutics in folk medicine practices, such as traditional Chinese medicine. Mushrooms with a history of such use include Agaricus subrufescens, Ganoderma lucidum, and Ophiocordyceps sinensis.
Cultured foods
Baker's yeast or Saccharomyces cerevisiae, a unicellular fungus, is used to make bread and other wheat-based products, such as pizza dough and dumplings. Yeast species of the genus Saccharomyces are also used to produce alcoholic beverages through fermentation. Shoyu koji mold (Aspergillus oryzae) is an essential ingredient in brewing shoyu (soy sauce) and sake, and the preparation of miso, while Rhizopus species are used for making tempeh. Several of these fungi are domesticated species that were bred or selected according to their capacity to ferment food without producing harmful mycotoxins (see below), which are produced by very closely related Aspergilli. Quorn, a meat alternative, is made from Fusarium venenatum.
In food
Edible mushrooms include commercially raised and wild-harvested fungi. Agaricus bisporus, sold as button mushrooms when small or Portobello mushrooms when larger, is the most widely cultivated species in the West, used in salads, soups, and many other dishes. Many Asian fungi are commercially grown and have increased in popularity in the West. They are often available fresh in grocery stores and markets, including straw mushrooms (Volvariella volvacea), oyster mushrooms (Pleurotus ostreatus), shiitakes (Lentinula edodes), and enokitake (Flammulina spp.).
Many other mushroom species are harvested from the wild for personal consumption or commercial sale. Milk mushrooms, morels, chanterelles, truffles, black trumpets, and porcini mushrooms (Boletus edulis) (also known as king boletes) demand a high price on the market. They are often used in gourmet dishes.
Certain types of cheeses require inoculation of milk curds with fungal species that impart a unique flavor and texture to the cheese. Examples include the blue color in cheeses such as Stilton or Roquefort, which are made by inoculation with Penicillium roqueforti. Molds used in cheese production are non-toxic and are thus safe for human consumption; however, mycotoxins (e.g., aflatoxins, roquefortine C, patulin, or others) may accumulate because of growth of other fungi during cheese ripening or storage.
Poisonous fungi
Many mushroom species are poisonous to humans and cause a range of reactions including slight digestive problems, allergic reactions, hallucinations, severe organ failure, and death. Genera with mushrooms containing deadly toxins include Conocybe, Galerina, Lepiota and the most infamous, Amanita. The latter genus includes the destroying angel (A.virosa) and the death cap (A.phalloides), the most common cause of deadly mushroom poisoning. The false morel (Gyromitra esculenta) is occasionally considered a delicacy when cooked, yet can be highly toxic when eaten raw. Tricholoma equestre was considered edible until it was implicated in serious poisonings causing rhabdomyolysis. Fly agaric mushrooms (Amanita muscaria) also cause occasional non-fatal poisonings, mostly as a result of ingestion for its hallucinogenic properties. Historically, fly agaric was used by different peoples in Europe and Asia and its present usage for religious or shamanic purposes is reported from some ethnic groups such as the Koryak people of northeastern Siberia.
As it is difficult to accurately identify a safe mushroom without proper training and knowledge, it is often advised to assume that a wild mushroom is poisonous and not to consume it.
Pest control
In agriculture, fungi may be useful if they actively compete for nutrients and space with pathogenic microorganisms such as bacteria or other fungi via the competitive exclusion principle, or if they are parasites of these pathogens. For example, certain species eliminate or suppress the growth of harmful plant pathogens, such as insects, mites, weeds, nematodes, and other fungi that cause diseases of important crop plants. This has generated strong interest in practical applications that use these fungi in the biological control of these agricultural pests. Entomopathogenic fungi can be used as biopesticides, as they actively kill insects. Examples that have been used as biological insecticides are Beauveria bassiana, Metarhizium spp., Hirsutella spp., Paecilomyces (Isaria) spp., and Lecanicillium lecanii. Endophytic fungi of grasses of the genus Epichloë, such as E. coenophiala, produce alkaloids that are toxic to a range of invertebrate and vertebrate herbivores. These alkaloids protect grass plants from herbivory, but several endophyte alkaloids can poison grazing animals, such as cattle and sheep. Infecting cultivars of pasture or forage grasses with Epichloë endophytes is one approach being used in grass breeding programs; the fungal strains are selected for producing only alkaloids that increase resistance to herbivores such as insects, while being non-toxic to livestock.
Bioremediation
Certain fungi, in particular white-rot fungi, can degrade insecticides, herbicides, pentachlorophenol, creosote, coal tars, and heavy fuels and turn them into carbon dioxide, water, and basic elements. Fungi have been shown to biomineralize uranium oxides, suggesting they may have application in the bioremediation of radioactively polluted sites.
Model organisms
Several pivotal discoveries in biology were made by researchers using fungi as model organisms, that is, fungi that grow and sexually reproduce rapidly in the laboratory. For example, the one gene-one enzyme hypothesis was formulated by scientists using the bread mold Neurospora crassa to test their biochemical theories. Other important model fungi are Aspergillus nidulans and the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe, each of which with a long history of use to investigate issues in eukaryotic cell biology and genetics, such as cell cycle regulation, chromatin structure, and gene regulation. Other fungal models have emerged that address specific biological questions relevant to medicine, plant pathology, and industrial uses; examples include Candida albicans, a dimorphic, opportunistic human pathogen, Magnaporthe grisea, a plant pathogen, and Pichia pastoris, a yeast widely used for eukaryotic protein production.
Others
Fungi are used extensively to produce industrial chemicals like citric, gluconic, lactic, and malic acids, and industrial enzymes, such as lipases used in biological detergents, cellulases used in making cellulosic ethanol and stonewashed jeans, and amylases, invertases, proteases, and xylanases.
| Biology and health sciences | Biology | null |
19179023 | https://en.wikipedia.org/wiki/Protozoa | Protozoa | Protozoa (: protozoan or protozoon; alternative plural: protozoans) are a polyphyletic group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic debris. Historically, protozoans were regarded as "one-celled animals".
When first introduced by Georg Goldfuss, in 1818, the taxon Protozoa was erected as a class within the Animalia, with the word 'protozoa' meaning "first animals", because they often possess animal-like behaviours, such as motility and predation, and lack a cell wall, as found in plants and many algae.
This classification remained widespread in the 19th and early 20th century, and even became elevated to a variety of higher ranks, including phylum, subkingdom, kingdom, and then sometimes included within the similarly paraphyletic Protoctista or Protista.
By the 1970s, it became usual to require that all taxa be monophyletic (derived from a common ancestor that would also be regarded as protozoan), and holophyletic (containing all of the known descendants of that common ancestor). The taxon 'Protozoa' fails to meet these standards, so grouping protozoa with animals, and treating them as closely related, became no longer justifiable.
The term continues to be used in a loose way to describe single-celled protists (that is, eukaryotes that are not animals, plants, or fungi) that feed by heterotrophy. Traditional textbook examples of protozoa are Amoeba, Paramecium, Euglena and Trypanosoma.
History of classification
The word "protozoa" (singular protozoon) was coined in 1818 by zoologist Georg August Goldfuss (=Goldfuß), as the Greek equivalent of the German , meaning "primitive, or original animals" ( 'proto-' + 'animal'). Goldfuss created Protozoa as a class containing what he believed to be the simplest animals. Originally, the group included not only single-celled microorganisms but also some "lower" multicellular animals, such as rotifers, corals, sponges, jellyfish, bryozoans and polychaete worms. The term Protozoa is formed from the Greek words (), meaning "first", and (), plural of (), meaning "animal".
In 1848, with better microscopes and Theodor Schwann and Matthias Schleiden's cell theory, the zoologist C. T. von Siebold proposed that the bodies of protozoa such as ciliates and amoebae consisted of single cells, similar to those from which the multicellular tissues of plants and animals were constructed. Von Siebold redefined Protozoa to include only such unicellular forms, to the exclusion of all Metazoa (animals). At the same time, he raised the group to the level of a phylum containing two broad classes of microorganisms: Infusoria (mostly ciliates) and flagellates (flagellated protists and amoebae). The definition of Protozoa as a phylum or sub-kingdom composed of "unicellular animals" was adopted by the zoologist Otto Bütschli—celebrated at his centenary as the "architect of protozoology".
As a phylum under Animalia, the Protozoa were firmly rooted in a simplistic "two-kingdom" concept of life, according to which all living beings were classified as either animals or plants. As long as this scheme remained dominant, the protozoa were understood to be animals and studied in departments of Zoology, while photosynthetic microorganisms and microscopic fungi—the so-called Protophyta—were assigned to the Plants, and studied in departments of Botany.
Criticism of this system began in the latter half of the 19th century, with the realization that many organisms met the criteria for inclusion among both plants and animals. For example, the algae Euglena and Dinobryon have chloroplasts for photosynthesis, like plants, but can also feed on organic matter and are motile, like animals. In 1860, John Hogg argued against the use of "protozoa", on the grounds that "naturalists are divided in opinion—and probably some will ever continue so—whether many of these organisms or living beings, are animals or plants." As an alternative, he proposed a new kingdom called Primigenum, consisting of both the protozoa and unicellular algae, which he combined under the name "Protoctista". In Hoggs's conception, the animal and plant kingdoms were likened to two great "pyramids" blending at their bases in the Kingdom Primigenum.
In 1866, Ernst Haeckel proposed a third kingdom of life, which he named Protista. At first, Haeckel included a few multicellular organisms in this kingdom, but in later work, he restricted the Protista to single-celled organisms, or simple colonies whose individual cells are not differentiated into different kinds of tissues.
Despite these proposals, Protozoa emerged as the preferred taxonomic placement for heterotrophic microorganisms such as amoebae and ciliates, and remained so for more than a century. In the course of the 20th century, the old "two kingdom" system began to weaken, with the growing awareness that fungi did not belong among the plants, and that most of the unicellular protozoa were no more closely related to the animals than they were to the plants. By mid-century, some biologists, such as Herbert Copeland, Robert H. Whittaker and Lynn Margulis, advocated the revival of Haeckel's Protista or Hogg's Protoctista as a kingdom-level eukaryotic group, alongside Plants, Animals and Fungi. A variety of multi-kingdom systems were proposed, and the Kingdoms Protista and Protoctista became established in biology texts and curricula.
By 1954, Protozoa were classified as "unicellular animals", as distinct from the "Protophyta", single-celled photosynthetic algae, which were considered primitive plants. In the system of classification published in 1964 by B.M. Honigsberg and colleagues, the phylum Protozoa was divided according to the means of locomotion, such as by cilia or flagella.
Despite awareness that the traditional Protozoa was not a clade, a natural group with a common ancestor, some authors have continued to use the name, while applying it to differing scopes of organisms. In a series of classifications by Thomas Cavalier-Smith and collaborators since 1981, the taxon Protozoa was applied to certain groups of eukaryotes, and ranked as a kingdom. A scheme presented by Ruggiero et al. in 2015, placed eight not closely related phyla within Kingdom Protozoa: Euglenozoa, Amoebozoa, Metamonada, Choanozoa sensu Cavalier-Smith, Loukozoa, Percolozoa, Microsporidia and Sulcozoa. This approach excludes several major groups traditionally placed among the protozoa, such as the ciliates, dinoflagellates, foraminifera, and the parasitic apicomplexans, which were moved to other groups such as Alveolata and Stramenopiles, under the polyphyletic Chromista. The Protozoa in this scheme were paraphyletic, because it excluded some descendants of Protozoa.
The continued use by some of the 'Protozoa' in its old sense highlights the uncertainty as to what is meant by the word 'Protozoa', the need for disambiguating statements such as "in the sense intended by Goldfuß", and the problems that arise when new meanings are given to familiar taxonomic terms. Some authors classify Protozoa as a subgroup of mostly motile Protists. Others class any unicellular eukaryotic microorganism as Protists, and make no reference to 'Protozoa'. In 2005, members of the Society of Protozoologists voted to change its name to the International Society of Protistologists.
In the system of eukaryote classification published by the International Society of Protistologists in 2012, members of the old phylum Protozoa have been distributed among a variety of supergroups.
Phylogenetic distribution
Protists are distributed across all major groups of eukaryotes, including those that contain multicellular algae, green plants, animals, and fungi. If photosynthetic and fungal protists are distinguished from protozoa, they appear as shown in the phylogenetic tree of eukaryotic groups. The Metamonada are hard to place, being sister possibly to Discoba, possibly to Malawimonada.
Characteristics
Reproduction
Reproduction in Protozoa can be sexual or asexual. Most Protozoa reproduce asexually through binary fission.
Many parasitic Protozoa reproduce both asexually and sexually. However, sexual reproduction is rare among free-living protozoa and it usually occurs when food is scarce or the environment changes drastically. Both isogamy and anisogamy occur in Protozoa, anisogamy being the more common form of sexual reproduction.
Size
Protozoans, as traditionally defined, range in size from as little as 1 micrometre to several millimetres, or more. Among the largest are the deep-sea–dwelling xenophyophores, single-celled foraminifera whose shells can reach 20 cm in diameter.
Habitat
Free-living protozoa are common and often abundant in fresh, brackish and salt water, as well as other moist environments, such as soils and mosses. Some species thrive in extreme environments such as hot springs and hypersaline lakes and lagoons. All protozoa require a moist habitat; however, some can survive for long periods of time in dry environments, by forming resting cysts that enable them to remain dormant until conditions improve.
Feeding
All protozoa are heterotrophic, deriving nutrients from other organisms, either by ingesting them whole by phagocytosis or taking up dissolved organic matter or micro-particles (osmotrophy). Phagocytosis may involve engulfing organic particles with pseudopodia (as amoebae do), taking in food through a specialized mouth-like aperture called a cytostome, or using stiffened ingestion organelles
Parasitic protozoa use a wide variety of feeding strategies, and some may change methods of feeding in different phases of their life cycle. For instance, the malaria parasite Plasmodium feeds by pinocytosis during its immature trophozoite stage of life (ring phase), but develops a dedicated feeding organelle (cytostome) as it matures within a host's red blood cell.
Protozoa may also live as mixotrophs, combining a heterotrophic diet with some form of autotrophy. Some protozoa form close associations with symbiotic photosynthetic algae (zoochlorellae), which live and grow within the membranes of the larger cell and provide nutrients to the host. The algae are not digested, but reproduce and are distributed between division products. The organism may benefit at times by deriving some of its nutrients from the algal endosymbionts or by surviving anoxic conditions because of the oxygen produced by algal photosynthesis. Some protozoans practice kleptoplasty, stealing chloroplasts from prey organisms and maintaining them within their own cell bodies as they continue to produce nutrients through photosynthesis. The ciliate Mesodinium rubrum retains functioning plastids from the cryptophyte algae on which it feeds, using them to nourish themselves by autotrophy. The symbionts may be passed along to dinoflagellates of the genus Dinophysis, which prey on Mesodinium rubrum but keep the enslaved plastids for themselves. Within Dinophysis, these plastids can continue to function for months.
Motility
Organisms traditionally classified as protozoa are abundant in aqueous environments and soil, occupying a range of trophic levels. The group includes flagellates (which move with the help of undulating and beating flagella). Ciliates (which move by using hair-like structures called cilia) and amoebae (which move by the use of temporary extensions of cytoplasm called pseudopodia). Many protozoa, such as the agents of amoebic meningitis, use both pseudopodia and flagella. Some protozoa attach to the substrate or form cysts, so they do not move around (sessile). Most sessile protozoa are able to move around at some stage in the life cycle, such as after cell division. The term 'theront' has been used for actively motile phases, as opposed to 'trophont' or 'trophozoite' that refers to feeding stages.
Walls, pellicles, scales, and skeletons
Unlike plants, fungi and most types of algae, most protozoa do not have a rigid external cell wall but are usually enveloped by elastic structures of membranes that permit movement of the cell. In some protozoa, such as the ciliates and euglenozoans, the outer membrane of the cell is supported by a cytoskeletal infrastructure, which may be referred to as a "pellicle". The pellicle gives shape to the cell, especially during locomotion. Pellicles of protozoan organisms vary from flexible and elastic to fairly rigid. In ciliates and Apicomplexa, the pellicle includes a layer of closely packed vesicles called alveoli. In euglenids, the pellicle is formed from protein strips arranged spirally along the length of the body. Familiar examples of protists with a pellicle are the euglenoids and the ciliate Paramecium. In some protozoa, the pellicle hosts epibiotic bacteria that adhere to the surface by their fimbriae (attachment pili).
Some protozoa live within loricasloose fitting but not fully intact enclosures. For example, many collar flagellates (Choanoflagellates) have an organic lorica or a lorica made from silicous sectretions. Loricas are also common among some green euglenids, various ciliates (such as the folliculinids, various testate amoebae and foraminifera. The surfaces of a variety of protozoa are covered with a layer of scales and or spicules. Examples include the amoeba Cochliopodium, many centrohelid heliozoa, synurophytes. The layer is often assumed to have a protective role. In some, such as the actinophryid heliozoa, the scales only form when the organism encysts. The bodies of some protozoa are supported internally by rigid, often inorganic, elements (as in Acantharea, Pylocystinea, Phaeodareacollectively the 'Radiolaria', and Ebriida).
Life cycle
Protozoa mostly reproduce asexually by binary fission or multiple fission. Many protozoa also exchange genetic material by sexual means (typically, through conjugation), but this is generally decoupled from reproduction. Meiotic sex is widespread among eukaryotes, and must have originated early in their evolution, as it has been found in many protozoan lineages that diverged early in eukaryotic evolution.
Aging
In the well-studied protozoan species Paramecium tetraurelia, the asexual line undergoes clonal aging, loses vitality and expires after about 200 fissions if the cells fail to undergo autogamy or conjugation. The functional basis for clonal aging was clarified by transplantation experiments of Aufderheide in 1986. These experiments demonstrated that the macronucleus, and not the cytoplasm, is responsible for clonal aging.
Additional experiments by Smith-Sonneborn, Holmes and Holmes, and Gilley and Blackburn showed that, during clonal aging, DNA damage increases dramatically. Thus, DNA damage in the macronucleus appears to be the principal cause of clonal aging in P. tetraurelia. In this single-celled protozoan, aging appears to proceed in a manner similar to that of multicellular eukaryotes (see DNA damage theory of aging).
Ecology
Free-living
Free-living protozoa are found in almost all ecosystems that contain free water, permanently or temporarily. They have a critical role in the mobilization of nutrients in ecosystems. Within the microbial food web they include the most important bacterivores. In part, they facilitate the transfer of bacterial and algal production to successive trophic levels, but also they solubilize the nutrients within microbial biomass, allowing stimulation of microbial growth. As consumers, protozoa prey upon unicellular or filamentous algae, bacteria, microfungi, and micro-carrion. In the context of older ecological models of the micro- and meiofauna, protozoa may be a food source for microinvertebrates.
Most species of free-living protozoa live in similar habitats in all parts of the world.
Parasitism
Many protozoan pathogens are human parasites, causing serious diseases such as malaria, giardiasis, toxoplasmosis, and sleeping sickness. Some of these protozoa have two-phase life cycles, alternating between proliferative stages (e.g., trophozoites) and resting cysts, enabling them to survive harsh conditions.
Commensalism
A wide range of protozoa live commensally in the rumens of ruminant animals, such as cattle and sheep. These include flagellates, such as Trichomonas, and ciliated protozoa, such as Isotricha and Entodinium. The ciliate subclass Astomatia is composed entirely of mouthless symbionts adapted for life in the guts of annelid worms.
Mutualism
Association between protozoan symbionts and their host organisms can be mutually beneficial. Flagellated protozoa such as Trichonympha and Pyrsonympha inhabit the guts of termites, where they enable their insect host to digest wood by helping to break down complex sugars into smaller, more easily digested molecules.
| Biology and health sciences | Eukaryotes | Plants |
19179592 | https://en.wikipedia.org/wiki/Archaea | Archaea | Archaea ( ) is a domain of organisms. Traditionally, Archaea only included its prokaryotic members, but this since has been found to be paraphyletic, as eukaryotes are now known to have evolved from archaea. Even though the domain Archaea includes eukaryotes, the term "archaea" (: archaeon , from the Greek "ἀρχαῖον", which means ancient) in English still generally refers specifically to prokaryotic members of Archaea. Archaea were initially classified as bacteria, receiving the name archaebacteria (, in the Archaebacteria kingdom), but this term has fallen out of use.
Archaeal cells have unique properties separating them from Bacteria and Eukaryota. Archaea are further divided into multiple recognized phyla. Classification is difficult because most have not been isolated in a laboratory and have been detected only by their gene sequences in environmental samples. It is unknown if they are able to produce endospores.
Archaea and bacteria are generally similar in size and shape, although a few archaea have very different shapes, such as the flat, square cells of Haloquadratum walsbyi. Despite this morphological similarity to bacteria, archaea possess genes and several metabolic pathways that are more closely related to those of eukaryotes, notably for the enzymes involved in transcription and translation. Other aspects of archaeal biochemistry are unique, such as their reliance on ether lipids in their cell membranes, including archaeols. Archaea use more diverse energy sources than eukaryotes, ranging from organic compounds such as sugars, to ammonia, metal ions or even hydrogen gas. The salt-tolerant Haloarchaea use sunlight as an energy source, and other species of archaea fix carbon (autotrophy), but unlike cyanobacteria, no known species of archaea does both. Archaea reproduce asexually by binary fission, fragmentation, or budding; unlike bacteria, no known species of Archaea form endospores.
The first observed archaea were extremophiles, living in extreme environments such as hot springs and salt lakes with no other organisms. Improved molecular detection tools led to the discovery of archaea in almost every habitat, including soil, oceans, and marshlands. Archaea are particularly numerous in the oceans, and the archaea in plankton may be one of the most abundant groups of organisms on the planet.
Archaea are a major part of Earth's life. They are part of the microbiota of all organisms. In the human microbiome, they are important in the gut, mouth, and on the skin. Their morphological, metabolic, and geographical diversity permits them to play multiple ecological roles: carbon fixation; nitrogen cycling; organic compound turnover; and maintaining microbial symbiotic and syntrophic communities, for example.
No clear examples of archaeal pathogens or parasites are known. Instead they are often mutualists or commensals, such as the methanogens (methane-producing strains) that inhabit the gastrointestinal tract in humans and ruminants, where their vast numbers facilitate digestion. Methanogens are also used in biogas production and sewage treatment, and biotechnology exploits enzymes from extremophile archaea that can endure high temperatures and organic solvents.
Discovery and classification
Early concept
For much of the 20th century, prokaryotes were regarded as a single group of organisms and classified based on their biochemistry, morphology and metabolism. Microbiologists tried to classify microorganisms based on the structures of their cell walls, their shapes, and the substances they consume. In 1965, Emile Zuckerkandl and Linus Pauling instead proposed using the sequences of the genes in different prokaryotes to work out how they are related to each other. This phylogenetic approach is the main method used today.
Archaea were first classified separately from bacteria in 1977 by Carl Woese and George E. Fox, based on their ribosomal RNA (rRNA) genes. (At that time only the methanogens were known). They called these groups the Urkingdoms of Archaebacteria and Eubacteria, though other researchers treated them as kingdoms or subkingdoms. Woese and Fox gave the first evidence for Archaebacteria as a separate "line of descent": 1. lack of peptidoglycan in their cell walls, 2. two unusual coenzymes, 3. results of 16S ribosomal RNA gene sequencing. To emphasize this difference, Woese, Otto Kandler and Mark Wheelis later proposed reclassifying organisms into three natural domains known as the three-domain system: the Eukarya, the Bacteria and the Archaea, in what is now known as the Woesian Revolution.
The word archaea comes from the Ancient Greek , meaning "ancient things", as the first representatives of the domain Archaea were methanogens and it was assumed that their metabolism reflected Earth's primitive atmosphere and the organisms' antiquity, but as new habitats were studied, more organisms were discovered. Extreme halophilic and hyperthermophilic microbes were also included in Archaea. For a long time, archaea were seen as extremophiles that exist only in extreme habitats such as hot springs and salt lakes, but by the end of the 20th century, archaea had been identified in non-extreme environments as well. Today, they are known to be a large and diverse group of organisms abundantly distributed throughout nature. This new appreciation of the importance and ubiquity of archaea came from using polymerase chain reaction (PCR) to detect prokaryotes from environmental samples (such as water or soil) by multiplying their ribosomal genes. This allows the detection and identification of organisms that have not been cultured in the laboratory.
Classification
The classification of archaea, and of prokaryotes in general, is a rapidly moving and contentious field. Current classification systems aim to organize archaea into groups of organisms that share structural features and common ancestors. These classifications rely heavily on the use of the sequence of ribosomal RNA genes to reveal relationships among organisms (molecular phylogenetics). Most of the culturable and well-investigated species of archaea are members of two main phyla, the "Euryarchaeota" and the Thermoproteota (formerly Crenarchaeota). Other groups have been tentatively created, such as the peculiar species Nanoarchaeum equitans — discovered in 2003 and assigned its own phylum, the "Nanoarchaeota". A new phylum "Korarchaeota" has also been proposed, containing a small group of unusual thermophilic species sharing features of both the main phyla, but most closely related to the Thermoproteota. Other detected species of archaea are only distantly related to any of these groups, such as the Archaeal Richmond Mine acidophilic nanoorganisms (ARMAN, comprising Micrarchaeota and Parvarchaeota), which were discovered in 2006 and are some of the smallest organisms known.
A superphylum – TACK – which includes the Thaumarchaeota (now Nitrososphaerota), "Aigarchaeota", Crenarchaeota (now Thermoproteota), and "Korarchaeota" was proposed in 2011 to be related to the origin of eukaryotes. In 2017, the newly discovered and newly named Asgard superphylum was proposed to be more closely related to the original eukaryote and a sister group to TACK.
In 2013, the superphylum DPANN was proposed to group "Nanoarchaeota", "Nanohaloarchaeota", Archaeal Richmond Mine acidophilic nanoorganisms (ARMAN, comprising "Micrarchaeota" and "Parvarchaeota"), and other similar archaea. This archaeal superphylum encompasses at least 10 different lineages and includes organisms with extremely small cell and genome sizes and limited metabolic capabilities. Therefore, DPANN may include members obligately dependent on symbiotic interactions, and may even include novel parasites. However, other phylogenetic analyses found that DPANN does not form a monophyletic group, and that the apparent grouping is caused by long branch attraction (LBA), suggesting that all these lineages belong to "Euryarchaeota".
Phylogeny
According to Tom A. Williams et al. 2017, Castelle & Banfield (2018) and GTDB release 09-RS220 (24 April 2024):
Concept of species
The classification of archaea into species is also controversial. Ernst Mayr's species definition — a reproductively isolated group of interbreeding organisms — does not apply, as archaea reproduce only asexually.
Archaea show high levels of horizontal gene transfer between lineages. Some researchers suggest that individuals can be grouped into species-like populations given highly similar genomes and infrequent gene transfer to/from cells with less-related genomes, as in the genus Ferroplasma. On the other hand, studies in Halorubrum found significant genetic transfer to/from less-related populations, limiting the criterion's applicability. Some researchers question whether such species designations have practical meaning.
Current knowledge on genetic diversity in archaeans is fragmentary, so the total number of species cannot be estimated with any accuracy. Estimates of the number of phyla range from 18 to 23, of which only 8 have representatives that have been cultured and studied directly. Many of these hypothesized groups are known from a single rRNA sequence, so the level of diversity remains obscure. This situation is also seen in the Bacteria; many uncultured microbes present similar issues with characterization.
Prokaryotic phyla
Valid phyla
The following phyla have been validly published according to the Bacteriological Code:
Halobacteriota
Methanobacteriota
Microcaldota
Nanobdellota
Promethearchaeota
Thermoproteota (syn. Nitrososphaerota)
Thermoplasmatota
Provisional phyla
The following phyla have been proposed, but have not been validly published according to the Bacteriological Code (including those that have candidatus status):
"Candidatus Aenigmarchaeota"
"Candidatus Aigarchaeota"
"Candidatus Altiarchaeota"
"Candidatus Asgardaeota"
"Candidatus Bathyarchaeota"
"Candidatus Brockarchaeota"
"Candidatus Diapherotrites"
"Candidatus Geoarchaeota"
"Candidatus Hadarchaeota"
"Candidatus Hadesarchaeota"
"Candidatus Halobacterota"
"Candidatus Heimdallarchaeota"
"Candidatus Helarchaeota"
"Candidatus Huberarchaeota"
"Candidatus Hydrothermarchaeota"
"Candidatus Korarchaeota"
"Candidatus Lokiarchaeota"
"Candidatus Mamarchaeota"
"Candidatus Marsarchaeota"
"Candidatus Micrarchaeota"
"Candidatus Nanoarchaeota"
"Candidatus Nanohaloarchaeota"
"Candidatus Nezhaarchaeota"
"Candidatus Njordarchaeota"
"Candidatus Odinarchaeota"
"Candidatus Pacearchaeota"
"Candidatus Parvarchaeota"
"Candidatus Thermoplasmatota"
"Candidatus Thorarchaeota"
"Candidatus Undinarchaeota"
"Candidatus Verstraetearchaeota"
"Candidatus Woesearchaeota"
Origin and evolution
The age of the Earth is about 4.54 billion years. Scientific evidence suggests that life began on Earth at least 3.5 billion years ago. The earliest evidence for life on Earth is graphite found to be biogenic in 3.7-billion-year-old metasedimentary rocks discovered in Western Greenland and microbial mat fossils found in 3.48-billion-year-old sandstone discovered in Western Australia. In 2015, possible remains of biotic matter were found in 4.1-billion-year-old rocks in Western Australia.
Although probable prokaryotic cell fossils date to almost 3.5 billion years ago, most prokaryotes do not have distinctive morphologies, and fossil shapes cannot be used to identify them as archaea. Instead, chemical fossils of unique lipids are more informative because such compounds do not occur in other organisms. Some publications suggest that archaeal or eukaryotic lipid remains are present in shales dating from 2.7 billion years ago, though such data have since been questioned. These lipids have also been detected in even older rocks from west Greenland. The oldest such traces come from the Isua district, which includes Earth's oldest known sediments, formed 3.8 billion years ago. The archaeal lineage may be the most ancient that exists on Earth.
Woese argued that the bacteria, archaea, and eukaryotes represent separate lines of descent that diverged early on from an ancestral colony of organisms. One possibility is that this occurred before the evolution of cells, when the lack of a typical cell membrane allowed unrestricted lateral gene transfer, and that the common ancestors of the three domains arose by fixation of specific subsets of genes. It is possible that the last common ancestor of bacteria and archaea was a thermophile, which raises the possibility that lower temperatures are "extreme environments" for archaea, and organisms that live in cooler environments appeared only later. Since archaea and bacteria are no more related to each other than they are to eukaryotes, the term prokaryote may suggest a false similarity between them. However, structural and functional similarities between lineages often occur because of shared ancestral traits or evolutionary convergence. These similarities are known as a grade, and prokaryotes are best thought of as a grade of life, characterized by such features as an absence of membrane-bound organelles.
Comparison with other domains
The following table compares some major characteristics of the three domains, to illustrate their similarities and differences.
Archaea were split off as a third domain because of the large differences in their ribosomal RNA structure. The particular molecule 16S rRNA is key to the production of proteins in all organisms. Because this function is so central to life, organisms with mutations in their 16S rRNA are unlikely to survive, leading to great (but not absolute) stability in the structure of this polynucleotide over generations. 16S rRNA is large enough to show organism-specific variations, but still small enough to be compared quickly. In 1977, Carl Woese, a microbiologist studying the genetic sequences of organisms, developed a new comparison method that involved splitting the RNA into fragments that could be sorted and compared with other fragments from other organisms. The more similar the patterns between species, the more closely they are related.
Woese used his new rRNA comparison method to categorize and contrast different organisms. He compared a variety of species and happened upon a group of methanogens with rRNA vastly different from any known prokaryotes or eukaryotes. These methanogens were much more similar to each other than to other organisms, leading Woese to propose the new domain of Archaea. His experiments showed that the archaea were genetically more similar to eukaryotes than prokaryotes, even though they were more similar to prokaryotes in structure. This led to the conclusion that Archaea and Eukarya shared a common ancestor more recent than Eukarya and Bacteria. The development of the nucleus occurred after the split between Bacteria and this common ancestor.
One property unique to archaea is the abundant use of ether-linked lipids in their cell membranes. Ether linkages are more chemically stable than the ester linkages found in bacteria and eukarya, which may be a contributing factor to the ability of many archaea to survive in extreme environments that place heavy stress on cell membranes, such as extreme heat and salinity. Comparative analysis of archaeal genomes has also identified several molecular conserved signature indels and signature proteins uniquely present in either all archaea or different main groups within archaea. Another unique feature of archaea, found in no other organisms, is methanogenesis (the metabolic production of methane). Methanogenic archaea play a pivotal role in ecosystems with organisms that derive energy from oxidation of methane, many of which are bacteria, as they are often a major source of methane in such environments and can play a role as primary producers. Methanogens also play a critical role in the carbon cycle, breaking down organic carbon into methane, which is also a major greenhouse gas.
This difference in the biochemical structure of Bacteria and Archaea has been explained by researchers through evolutionary processes. It is theorized that both domains originated at deep sea alkaline hydrothermal vents. At least twice, microbes evolved lipid biosynthesis and cell wall biochemistry. It has been suggested that the last universal common ancestor was a non-free-living organism. It may have had a permeable membrane composed of bacterial simple chain amphiphiles (fatty acids), including archaeal simple chain amphiphiles (isoprenoids). These stabilize fatty acid membranes in seawater; this property may have driven the divergence of bacterial and archaeal membranes, "with the later biosynthesis of phospholipids giving rise to the unique G1P and G3P headgroups of archaea and bacteria respectively. If so, the properties conferred by membrane isoprenoids place the lipid divide as early as the origin of life".
Relationship to bacteria
The relationships among the three domains are of central importance for understanding the origin of life. Most of the metabolic pathways, which are the object of the majority of an organism's genes, are common between Archaea and Bacteria, while most genes involved in genome expression are common between Archaea and Eukarya. Within prokaryotes, archaeal cell structure is most similar to that of gram-positive bacteria, largely because both have a single lipid bilayer and usually contain a thick sacculus (exoskeleton) of varying chemical composition. In some phylogenetic trees based upon different gene/protein sequences of prokaryotic homologs, the archaeal homologs are more closely related to those of gram-positive bacteria. Archaea and gram-positive bacteria also share conserved indels in a number of important proteins, such as Hsp70 and glutamine synthetase I; but the phylogeny of these genes was interpreted to reveal interdomain gene transfer, and might not reflect the organismal relationship(s).
It has been proposed that the archaea evolved from Gram-positive bacteria in response to antibiotic selection pressure. This is suggested by the observation that archaea are resistant to a wide variety of antibiotics that are produced primarily by Gram-positive bacteria, and that these antibiotics act primarily on the genes that distinguish archaea from bacteria. The proposal is that the selective pressure towards resistance generated by the gram-positive antibiotics was eventually sufficient to cause extensive changes in many of the antibiotics' target genes, and that these strains represented the common ancestors of present-day Archaea. The evolution of Archaea in response to antibiotic selection, or any other competitive selective pressure, could also explain their adaptation to extreme environments (such as high temperature or acidity) as the result of a search for unoccupied niches to escape from antibiotic-producing organisms; Cavalier-Smith has made a similar suggestion, the Neomura hypothesis. This proposal is also supported by other work investigating protein structural relationships and studies that suggest that gram-positive bacteria may constitute the earliest branching lineages within the prokaryotes.
Relation to eukaryotes
The evolutionary relationship between archaea and eukaryotes remains unclear. Aside from the similarities in cell structure and function that are discussed below, many genetic trees group the two.
Complicating factors include claims that the relationship between eukaryotes and the archaeal phylum Thermoproteota is closer than the relationship between the "Euryarchaeota" and the phylum Thermoproteota and the presence of archaea-like genes in certain bacteria, such as Thermotoga maritima, from horizontal gene transfer. The standard hypothesis states that the ancestor of the eukaryotes diverged early from the Archaea, and that eukaryotes arose through symbiogenesis, the fusion of an archaean and a eubacterium, which formed the mitochondria; this hypothesis explains the genetic similarities between the groups. The eocyte hypothesis instead posits that Eukaryota emerged relatively late from the Archaea.
A lineage of archaea discovered in 2015, Lokiarchaeum (of the proposed new phylum "Lokiarchaeota"), named for a hydrothermal vent called Loki's Castle in the Arctic Ocean, was found to be the most closely related to eukaryotes known at that time. It has been called a transitional organism between prokaryotes and eukaryotes.
Several sister phyla of "Lokiarchaeota" have since been found ("Thorarchaeota", "Odinarchaeota", "Heimdallarchaeota"), all together comprising a newly proposed supergroup Asgard.
Details of the relation of Asgard members and eukaryotes are still under consideration, although, in January 2020, scientists reported that Candidatus Prometheoarchaeum syntrophicum, a type of Asgard archaea, may be a possible link between simple prokaryotic and complex eukaryotic microorganisms about two billion years ago.
Morphology
Individual archaea range from 0.1 micrometers (μm) to over 15 μm in diameter, and occur in various shapes, commonly as spheres, rods, spirals or plates. Other morphologies in the Thermoproteota include irregularly shaped lobed cells in Sulfolobus, needle-like filaments that are less than half a micrometer in diameter in Thermofilum, and almost perfectly rectangular rods in Thermoproteus and Pyrobaculum. Archaea in the genus Haloquadratum such as Haloquadratum walsbyi are flat, square specimens that live in hypersaline pools. These unusual shapes are probably maintained by both their cell walls and a prokaryotic cytoskeleton. Proteins related to the cytoskeleton components of other organisms exist in archaea, and filaments form within their cells, but in contrast with other organisms, these cellular structures are poorly understood. In Thermoplasma and Ferroplasma the lack of a cell wall means that the cells have irregular shapes, and can resemble amoebae.
Some species form aggregates or filaments of cells up to 200 μm long. These organisms can be prominent in biofilms. Notably, aggregates of Thermococcus coalescens cells fuse together in culture, forming single giant cells. Archaea in the genus Pyrodictium produce an elaborate multicell colony involving arrays of long, thin hollow tubes called cannulae that stick out from the cells' surfaces and connect them into a dense bush-like agglomeration. The function of these cannulae is not settled, but they may allow communication or nutrient exchange with neighbors. Multi-species colonies exist, such as the "string-of-pearls" community that was discovered in 2001 in a German swamp. Round whitish colonies of a novel Euryarchaeota species are spaced along thin filaments that can range up to long; these filaments are made of a particular bacteria species.
Structure, composition development, and operation
Archaea and bacteria have generally similar cell structure, but cell composition and organization set the archaea apart. Like bacteria, archaea lack interior membranes and organelles. Like bacteria, the cell membranes of archaea are usually bounded by a cell wall and they swim using one or more flagella. Structurally, archaea are most similar to gram-positive bacteria. Most have a single plasma membrane and cell wall, and lack a periplasmic space; the exception to this general rule is Ignicoccus, which possess a particularly large periplasm that contains membrane-bound vesicles and is enclosed by an outer membrane.
Cell wall and archaella
Most archaea (but not Thermoplasma and Ferroplasma) possess a cell wall. In most archaea, the wall is assembled from surface-layer proteins, which form an S-layer. An S-layer is a rigid array of protein molecules that cover the outside of the cell (like chain mail). This layer provides both chemical and physical protection, and can prevent macromolecules from contacting the cell membrane. Unlike bacteria, archaea lack peptidoglycan in their cell walls. Methanobacteriales do have cell walls containing pseudopeptidoglycan, which resembles eubacterial peptidoglycan in morphology, function, and physical structure, but pseudopeptidoglycan is distinct in chemical structure; it lacks D-amino acids and N-acetylmuramic acid, substituting the latter with N-Acetyltalosaminuronic acid.
Archaeal flagella are known as archaella, that operate like bacterial flagella – their long stalks are driven by rotatory motors at the base. These motors are powered by a proton gradient across the membrane, but archaella are notably different in composition and development. The two types of flagella evolved from different ancestors. The bacterial flagellum shares a common ancestor with the type III secretion system, while archaeal flagella appear to have evolved from bacterial type IV pili. In contrast with the bacterial flagellum, which is hollow and assembled by subunits moving up the central pore to the tip of the flagella, archaeal flagella are synthesized by adding subunits at the base.
Membranes
Archaeal membranes are made of molecules that are distinctly different from those in all other life forms, showing that archaea are related only distantly to bacteria and eukaryotes. In all organisms, cell membranes are made of molecules known as phospholipids. These molecules possess both a polar part that dissolves in water (the phosphate "head"), and a "greasy" non-polar part that does not (the lipid tail). These dissimilar parts are connected by a glycerol moiety. In water, phospholipids cluster, with the heads facing the water and the tails facing away from it. The major structure in cell membranes is a double layer of these phospholipids, which is called a lipid bilayer.
The phospholipids of archaea are unusual in four ways:
They have membranes composed of glycerol-ether lipids, whereas bacteria and eukaryotes have membranes composed mainly of glycerol-ester lipids. The difference is the type of bond that joins the lipids to the glycerol moiety; the two types are shown in yellow in the figure at the right. In ester lipids, this is an ester bond, whereas in ether lipids this is an ether bond.
The stereochemistry of the archaeal glycerol moiety is the mirror image of that found in other organisms. The glycerol moiety can occur in two forms that are mirror images of one another, called enantiomers. Just as a right hand does not fit easily into a left-handed glove, enantiomers of one type generally cannot be used or made by enzymes adapted for the other. The archaeal phospholipids are built on a backbone of sn-glycerol-1-phosphate, which is an enantiomer of sn-glycerol-3-phosphate, the phospholipid backbone found in bacteria and eukaryotes. This suggests that archaea use entirely different enzymes for synthesizing phospholipids as compared to bacteria and eukaryotes. Such enzymes developed very early in life's history, indicating an early split from the other two domains.
Archaeal lipid tails differ from those of other organisms in that they are based upon long isoprenoid chains with multiple side-branches, sometimes with cyclopropane or cyclohexane rings. By contrast, the fatty acids in the membranes of other organisms have straight chains without side branches or rings. Although isoprenoids play an important role in the biochemistry of many organisms, only the archaea use them to make phospholipids. These branched chains may help prevent archaeal membranes from leaking at high temperatures.
In some archaea, the lipid bilayer is replaced by a monolayer. In effect, the archaea fuse the tails of two phospholipid molecules into a single molecule with two polar heads (a bolaamphiphile); this fusion may make their membranes more rigid and better able to resist harsh environments. For example, the lipids in Ferroplasma are of this type, which is thought to aid this organism's survival in its highly acidic habitat.
Metabolism
Archaea exhibit a great variety of chemical reactions in their metabolism and use many sources of energy. These reactions are classified into nutritional groups, depending on energy and carbon sources. Some archaea obtain energy from inorganic compounds such as sulfur or ammonia (they are chemotrophs). These include nitrifiers, methanogens and anaerobic methane oxidisers. In these reactions, one compound passes electrons to another (in a redox reaction), releasing energy to fuel the cell's activities. One compound acts as an electron donor and one as an electron acceptor. The energy released is used to generate adenosine triphosphate (ATP) through chemiosmosis, the same basic process that happens in the mitochondrion of eukaryotic cells.
Other groups of archaea use sunlight as a source of energy (they are phototrophs), but oxygen–generating photosynthesis does not occur in any of these organisms. Many basic metabolic pathways are shared among all forms of life; for example, archaea use a modified form of glycolysis (the Entner–Doudoroff pathway) and either a complete or partial citric acid cycle. These similarities to other organisms probably reflect both early origins in the history of life and their high level of efficiency.
Some Euryarchaeota are methanogens (archaea that produce methane as a result of metabolism) living in anaerobic environments, such as swamps. This form of metabolism evolved early, and it is even possible that the first free-living organism was a methanogen. A common reaction involves the use of carbon dioxide as an electron acceptor to oxidize hydrogen. Methanogenesis involves a range of coenzymes that are unique to these archaea, such as coenzyme M and methanofuran. Other organic compounds such as alcohols, acetic acid or formic acid are used as alternative electron acceptors by methanogens. These reactions are common in gut-dwelling archaea. Acetic acid is also broken down into methane and carbon dioxide directly, by acetotrophic archaea. These acetotrophs are archaea in the order Methanosarcinales, and are a major part of the communities of microorganisms that produce biogas.
Other archaea use in the atmosphere as a source of carbon, in a process called carbon fixation (they are autotrophs). This process involves either a highly modified form of the Calvin cycle or another metabolic pathway called the 3-hydroxypropionate/ 4-hydroxybutyrate cycle. The Thermoproteota also use the reverse Krebs cycle while the "Euryarchaeota" also use the reductive acetyl-CoA pathway. Carbon fixation is powered by inorganic energy sources. No known archaea carry out photosynthesis (Halobacterium is the only known phototroph archeon but it uses an alternative process to photosynthesis). Archaeal energy sources are extremely diverse, and range from the oxidation of ammonia by the Nitrosopumilales to the oxidation of hydrogen sulfide or elemental sulfur by species of Sulfolobus, using either oxygen or metal ions as electron acceptors.
Phototrophic archaea use light to produce chemical energy in the form of ATP. In the Halobacteria, light-activated ion pumps like bacteriorhodopsin and halorhodopsin generate ion gradients by pumping ions out of and into the cell across the plasma membrane. The energy stored in these electrochemical gradients is then converted into ATP by ATP synthase. This process is a form of photophosphorylation. The ability of these light-driven pumps to move ions across membranes depends on light-driven changes in the structure of a retinol cofactor buried in the center of the protein.
Genetics
Archaea usually have a single circular chromosome, but many euryarchaea have been shown to bear multiple copies of this chromosome. The largest known archaeal genome as of 2002 was 5,751,492 base pairs in Methanosarcina acetivorans. The tiny 490,885 base-pair genome of Nanoarchaeum equitans is one-tenth of this size and the smallest archaeal genome known; it is estimated to contain only 537 protein-encoding genes. Smaller independent pieces of DNA, called plasmids, are also found in archaea. Plasmids may be transferred between cells by physical contact, in a process that may be similar to bacterial conjugation.
Archaea are genetically distinct from bacteria and eukaryotes, with up to 15% of the proteins encoded by any one archaeal genome being unique to the domain, although most of these unique genes have no known function. Of the remainder of the unique proteins that have an identified function, most belong to the Euryarchaeota and are involved in methanogenesis. The proteins that archaea, bacteria and eukaryotes share form a common core of cell function, relating mostly to transcription, translation, and nucleotide metabolism. Other characteristic archaeal features are the organization of genes of related function – such as enzymes that catalyze steps in the same metabolic pathway into novel operons, and large differences in tRNA genes and their aminoacyl tRNA synthetases.
Transcription in archaea more closely resembles eukaryotic than bacterial transcription, with the archaeal RNA polymerase being very close to its equivalent in eukaryotes, while archaeal translation shows signs of both bacterial and eukaryotic equivalents. Although archaea have only one type of RNA polymerase, its structure and function in transcription seems to be close to that of the eukaryotic RNA polymerase II, with similar protein assemblies (the general transcription factors) directing the binding of the RNA polymerase to a gene's promoter, but other archaeal transcription factors are closer to those found in bacteria. Post-transcriptional modification is simpler than in eukaryotes, since most archaeal genes lack introns, although there are many introns in their transfer RNA and ribosomal RNA genes, and introns may occur in a few protein-encoding genes.
Gene transfer and genetic exchange
Haloferax volcanii, an extreme halophilic archaeon, forms cytoplasmic bridges between cells that appear to be used for transfer of DNA from one cell to another in either direction.
When the hyperthermophilic archaea Sulfolobus solfataricus and Sulfolobus acidocaldarius are exposed to DNA-damaging UV irradiation or to the agents bleomycin or mitomycin C, species-specific cellular aggregation is induced. Aggregation in S. solfataricus could not be induced by other physical stressors, such as pH or temperature shift, suggesting that aggregation is induced specifically by DNA damage. Ajon et al. showed that UV-induced cellular aggregation mediates chromosomal marker exchange with high frequency in S. acidocaldarius. Recombination rates exceeded those of uninduced cultures by up to three orders of magnitude. Frols et al. and Ajon et al. hypothesized that cellular aggregation enhances species-specific DNA transfer between Sulfolobus cells in order to provide increased repair of damaged DNA by means of homologous recombination. This response may be a primitive form of sexual interaction similar to the more well-studied bacterial transformation systems that are also associated with species-specific DNA transfer between cells leading to homologous recombinational repair of DNA damage.
Archaeal viruses
Archaea are the target of a number of viruses in a diverse virosphere distinct from bacterial and eukaryotic viruses. They have been organized into 15–18 DNA-based families so far, but multiple species remain un-isolated and await classification. These families can be informally divided into two groups: archaea-specific and cosmopolitan. Archaeal-specific viruses target only archaean species and currently include 12 families. Numerous unique, previously unidentified viral structures have been observed in this group, including: bottle-shaped, spindle-shaped, coil-shaped, and droplet-shaped viruses. While the reproductive cycles and genomic mechanisms of archaea-specific species may be similar to other viruses, they bear unique characteristics that were specifically developed due to the morphology of host cells they infect. Their virus release mechanisms differ from that of other phages. Bacteriophages generally undergo either lytic pathways, lysogenic pathways, or (rarely) a mix of the two. Most archaea-specific viral strains maintain a stable, somewhat lysogenic, relationship with their hosts – appearing as a chronic infection. This involves the gradual, and continuous, production and release of virions without killing the host cell. Prangishyili (2013) noted that it has been hypothesized that tailed archaeal phages originated from bacteriophages capable of infecting haloarchaeal species. If the hypothesis is correct, it can be concluded that other double-stranded DNA viruses that make up the rest of the archaea-specific group are their own unique group in the global viral community. Krupovic et al. (2018) states that the high levels of horizontal gene transfer, rapid mutation rates in viral genomes, and lack of universal gene sequences have led researchers to perceive the evolutionary pathway of archaeal viruses as a network. The lack of similarities among phylogenetic markers in this network and the global virosphere, as well as external linkages to non-viral elements, may suggest that some species of archaea specific viruses evolved from non-viral mobile genetic elements (MGE).
These viruses have been studied in most detail in thermophilics, particularly the orders Sulfolobales and Thermoproteales. Two groups of single-stranded DNA viruses that infect archaea have been recently isolated. One group is exemplified by the Halorubrum pleomorphic virus 1 (Pleolipoviridae) infecting halophilic archaea, and the other one by the Aeropyrum coil-shaped virus (Spiraviridae) infecting a hyperthermophilic (optimal growth at 90–95 °C) host. Notably, the latter virus has the largest currently reported ssDNA genome. Defenses against these viruses may involve RNA interference from repetitive DNA sequences that are related to the genes of the viruses.
Reproduction
Archaea reproduce asexually by binary or multiple fission, fragmentation, or budding; mitosis and meiosis do not occur, so if a species of archaea exists in more than one form, all have the same genetic material. Cell division is controlled in a cell cycle; after the cell's chromosome is replicated and the two daughter chromosomes separate, the cell divides. In the genus Sulfolobus, the cycle has characteristics that are similar to both bacterial and eukaryotic systems. The chromosomes replicate from multiple starting points (origins of replication) using DNA polymerases that resemble the equivalent eukaryotic enzymes.
In Euryarchaeota the cell division protein FtsZ, which forms a contracting ring around the cell, and the components of the septum that is constructed across the center of the cell, are similar to their bacterial equivalents. In cren- and thaumarchaea, the cell division machinery Cdv fulfills a similar role. This machinery is related to the eukaryotic ESCRT-III machinery which, while best known for its role in cell sorting, also has been seen to fulfill a role in separation between divided cell, suggesting an ancestral role in cell division.
Both bacteria and eukaryotes, but not archaea, make spores. Some species of Haloarchaea undergo phenotypic switching and grow as several different cell types, including thick-walled structures that are resistant to osmotic shock and allow the archaea to survive in water at low salt concentrations, but these are not reproductive structures and may instead help them reach new habitats.
Behavior
Communication
Quorum sensing was originally thought to not exist in Archaea, but recent studies have shown evidence of some species being able to perform cross-talk through quorum sensing. Other studies have shown syntrophic interactions between archaea and bacteria during biofilm growth. Although research is limited in archaeal quorum sensing, some studies have uncovered LuxR proteins in archaeal species, displaying similarities with bacteria LuxR, and ultimately allowing for the detection of small molecules that are used in high density communication. Similarly to bacteria, Archaea LuxR solos have shown to bind to AHLs (lactones) and non-AHLs ligans, which is a large part in performing intraspecies, interspecies, and interkingdom communication through quorum sensing.
Ecology
Habitats
Archaea exist in a broad range of habitats, and are now recognized as a major part of global ecosystems, and may represent about 20% of microbial cells in the oceans. However, the first-discovered archaeans were extremophiles. Indeed, some archaea survive high temperatures, often above , as found in geysers, black smokers, and oil wells. Other common habitats include very cold habitats and highly saline, acidic, or alkaline water, but archaea include mesophiles that grow in mild conditions, in swamps and marshland, sewage, the oceans, the intestinal tract of animals, and soils. Similar to PGPR, Archaea are now considered as a source of plant growth promotion as well.
Extremophile archaea are members of four main physiological groups. These are the halophiles, thermophiles, alkaliphiles, and acidophiles. These groups are not comprehensive or phylum-specific, nor are they mutually exclusive, since some archaea belong to several groups. Nonetheless, they are a useful starting point for classification.
Halophiles, including the genus Halobacterium, live in extremely saline environments such as salt lakes and outnumber their bacterial counterparts at salinities greater than 20–25%. Thermophiles grow best at temperatures above , in places such as hot springs; hyperthermophilic archaea grow optimally at temperatures greater than . The archaeal Methanopyrus kandleri Strain 116 can even reproduce at , the highest recorded temperature of any organism.
Other archaea exist in very acidic or alkaline conditions. For example, one of the most extreme archaean acidophiles is Picrophilus torridus, which grows at pH 0, which is equivalent to thriving in 1.2 molar sulfuric acid.
This resistance to extreme environments has made archaea the focus of speculation about the possible properties of extraterrestrial life. Some extremophile habitats are not dissimilar to those on Mars, leading to the suggestion that viable microbes could be transferred between planets in meteorites.
Recently, several studies have shown that archaea exist not only in mesophilic and thermophilic environments but are also present, sometimes in high numbers, at low temperatures as well. For example, archaea are common in cold oceanic environments such as polar seas. Even more significant are the large numbers of archaea found throughout the world's oceans in non-extreme habitats among the plankton community (as part of the picoplankton). Although these archaea can be present in extremely high numbers (up to 40% of the microbial biomass), almost none of these species have been isolated and studied in pure culture. Consequently, our understanding of the role of archaea in ocean ecology is rudimentary, so their full influence on global biogeochemical cycles remains largely unexplored. Some marine Thermoproteota are capable of nitrification, suggesting these organisms may affect the oceanic nitrogen cycle, although these oceanic Thermoproteota may also use other sources of energy.
Vast numbers of archaea are also found in the sediments that cover the sea floor, with these organisms making up the majority of living cells at depths over 1 meter below the ocean bottom. It has been demonstrated that in all oceanic surface sediments (from 1,000- to 10,000-m water depth), the impact of viral infection is higher on archaea than on bacteria and virus-induced lysis of archaea accounts for up to one-third of the total microbial biomass killed, resulting in the release of ~0.3 to 0.5 gigatons of carbon per year globally.
Role in chemical cycling
Archaea recycle elements such as carbon, nitrogen, and sulfur through their various habitats. Archaea carry out many steps in the nitrogen cycle. This includes both reactions that remove nitrogen from ecosystems (such as nitrate-based respiration and denitrification) as well as processes that introduce nitrogen (such as nitrate assimilation and nitrogen fixation).
Researchers recently discovered archaeal involvement in ammonia oxidation reactions. These reactions are particularly important in the oceans. The archaea also appear crucial for ammonia oxidation in soils. They produce nitrite, which other microbes then oxidize to nitrate. Plants and other organisms consume the latter.
In the sulfur cycle, archaea that grow by oxidizing sulfur compounds release this element from rocks, making it available to other organisms, but the archaea that do this, such as Sulfolobus, produce sulfuric acid as a waste product, and the growth of these organisms in abandoned mines can contribute to acid mine drainage and other environmental damage.
In the carbon cycle, methanogen archaea remove hydrogen and play an important role in the decay of organic matter by the populations of microorganisms that act as decomposers in anaerobic ecosystems, such as sediments, marshes, and sewage-treatment works.
Interactions with other organisms
The well-characterized interactions between archaea and other organisms are either mutual or commensal. There are no clear examples of known archaeal pathogens or parasites, but some species of methanogens have been suggested to be involved in infections in the mouth, and Nanoarchaeum equitans may be a parasite of another species of archaea, since it only survives and reproduces within the cells of the Crenarchaeon Ignicoccus hospitalis, and appears to offer no benefit to its host.
Mutualism
Mutualism is an interaction between individuals of different species that results in positive (beneficial) effects on per capita reproduction and/or survival of the interacting populations. One well-understood example of mutualism is the interaction between protozoa and methanogenic archaea in the digestive tracts of animals that digest cellulose, such as ruminants and termites. In these anaerobic environments, protozoa break down plant cellulose to obtain energy. This process releases hydrogen as a waste product, but high levels of hydrogen reduce energy production. When methanogens convert hydrogen to methane, protozoa benefit from more energy.
In anaerobic protozoa, such as Plagiopyla frontata, Trimyema, Heterometopus and Metopus contortus, archaea reside inside the protozoa and consume hydrogen produced in their hydrogenosomes. Archaea associate with larger organisms, too. For example, the marine archaean Cenarchaeum symbiosum is an endosymbiont of the sponge Axinella mexicana.
Commensalism
Some archaea are commensals, benefiting from an association without helping or harming the other organism. For example, the methanogen Methanobrevibacter smithii is by far the most common archaean in the human flora, making up about one in ten of the prokaryotes in the human gut. In termites and in humans, these methanogens may in fact be mutualists, interacting with other microbes in the gut to aid digestion. Archaean communities associate with a range of other organisms, such as on the surface of corals, and in the region of soil that surrounds plant roots (the rhizosphere).
Parasitism
Although Archaea do not have a historical reputation of being pathogens, Archaea are often found with similar genomes to more common pathogens like E. coli, showing metabolic links and evolutionary history with today's pathogens. Archaea have been inconsistently detected in clinical studies because of the lack of categorization of Archaea into more specific species.
Significance in technology and industry
Extremophile archaea, particularly those resistant either to heat or to extremes of acidity and alkalinity, are a source of enzymes that function under these harsh conditions. These enzymes have found many uses. For example, thermostable DNA polymerases, such as the Pfu DNA polymerase from Pyrococcus furiosus, revolutionized molecular biology by allowing the polymerase chain reaction to be used in research as a simple and rapid technique for cloning DNA. In industry, amylases, galactosidases and pullulanases in other species of Pyrococcus that function at over allow food processing at high temperatures, such as the production of low lactose milk and whey. Enzymes from these thermophilic archaea also tend to be very stable in organic solvents, allowing their use in environmentally friendly processes in green chemistry that synthesize organic compounds. This stability makes them easier to use in structural biology. Consequently, the counterparts of bacterial or eukaryotic enzymes from extremophile archaea are often used in structural studies.
In contrast with the range of applications of archaean enzymes, the use of the organisms themselves in biotechnology is less developed. Methanogenic archaea are a vital part of sewage treatment, since they are part of the community of microorganisms that carry out anaerobic digestion and produce biogas. In mineral processing, acidophilic archaea display promise for the extraction of metals from ores, including gold, cobalt and copper.
Archaea host a new class of potentially useful antibiotics. A few of these archaeocins have been characterized, but hundreds more are believed to exist, especially within Haloarchaea and Sulfolobus. These compounds differ in structure from bacterial antibiotics, so they may have novel modes of action. In addition, they may allow the creation of new selectable markers for use in archaeal molecular biology.
| Biology and health sciences | Biology | null |
19179706 | https://en.wikipedia.org/wiki/Abiogenesis | Abiogenesis | Abiogenesis is the natural process by which life arises from non-living matter, such as simple organic compounds. The prevailing scientific hypothesis is that the transition from non-living to living entities on Earth was not a single event, but a process of increasing complexity involving the formation of a habitable planet, the prebiotic synthesis of organic molecules, molecular self-replication, self-assembly, autocatalysis, and the emergence of cell membranes. The transition from non-life to life has never been observed experimentally, but many proposals have been made for different stages of the process.
The study of abiogenesis aims to determine how pre-life chemical reactions gave rise to life under conditions strikingly different from those on Earth today. It primarily uses tools from biology and chemistry, with more recent approaches attempting a synthesis of many sciences. Life functions through the specialized chemistry of carbon and water, and builds largely upon four key families of chemicals: lipids for cell membranes, carbohydrates such as sugars, amino acids for protein metabolism, and nucleic acid DNA and RNA for the mechanisms of heredity. Any successful theory of abiogenesis must explain the origins and interactions of these classes of molecules.
Many approaches to abiogenesis investigate how self-replicating molecules, or their components, came into existence. Researchers generally think that current life descends from an RNA world, although other self-replicating and self-catalyzing molecules may have preceded RNA. Other approaches ("metabolism-first" hypotheses) focus on understanding how catalysis in chemical systems on the early Earth might have provided the precursor molecules necessary for self-replication. The classic 1952 Miller–Urey experiment demonstrated that most amino acids, the chemical constituents of proteins, can be synthesized from inorganic compounds under conditions intended to replicate those of the early Earth. External sources of energy may have triggered these reactions, including lightning, radiation, atmospheric entries of micro-meteorites, and implosion of bubbles in sea and ocean waves. More recent research has found amino acids in meteorites, comets, asteroids, and star-forming regions of space.
While the last universal common ancestor of all modern organisms (LUCA) is thought to have been quite different from the origin of life, investigations into LUCA can guide research into early universal characteristics. A genomics approach has sought to characterize LUCA by identifying the genes shared by Archaea and Bacteria, members of the two major branches of life (with Eukaryotes included in the archaean branch in the two-domain system). It appears there are 60 proteins common to all life and 355 prokaryotic genes that trace to LUCA; their functions imply that the LUCA was anaerobic with the Wood–Ljungdahl pathway, deriving energy by chemiosmosis, and maintaining its hereditary material with DNA, the genetic code, and ribosomes. Although the LUCA lived over 4 billion years ago (4 Gya), researchers believe it was far from the first form of life. Earlier cells might have had a leaky membrane and been powered by a naturally occurring proton gradient near a deep-sea white smoker hydrothermal vent.
Earth remains the only place in the universe known to harbor life. Geochemical and fossil evidence from the Earth informs most studies of abiogenesis. The Earth was formed at 4.54 Gya, and the earliest evidence of life on Earth dates from at least 3.8 Gya from Western Australia. Some studies have suggested that fossil micro-organisms may have lived within hydrothermal vent precipitates dated 3.77 to 4.28 Gya from Quebec, soon after ocean formation 4.4 Gya during the Hadean.
Overview
Life consists of reproduction with (heritable) variations. NASA defines life as "a self-sustaining chemical system capable of Darwinian [i.e., biological] evolution." Such a system is complex; the last universal common ancestor (LUCA), presumably a single-celled organism which lived some 4 billion years ago, already had hundreds of genes encoded in the DNA genetic code that is universal today. That in turn implies a suite of cellular machinery including messenger RNA, transfer RNA, and ribosomes to translate the code into proteins. Those proteins included enzymes to operate its anaerobic respiration via the Wood–Ljungdahl metabolic pathway, and a DNA polymerase to replicate its genetic material.
The challenge for abiogenesis (origin of life) researchers is to explain how such a complex and tightly interlinked system could develop by evolutionary steps, as at first sight all its parts are necessary to enable it to function. For example, a cell, whether the LUCA or in a modern organism, copies its DNA with the DNA polymerase enzyme, which is itself produced by translating the DNA polymerase gene in the DNA. Neither the enzyme nor the DNA can be produced without the other. The likely answer to this challenge is that the evolutionary process could have involved molecular self-replication, self-assembly such as of cell membranes, and autocatalysis via RNA ribozymes. Nonetheless, the transition of non-life to life has never been observed experimentally, nor has there been a satisfactory chemical explanation.
The preconditions to the development of a living cell like the LUCA are clear enough, though disputed in their details: a habitable world is formed with a supply of minerals and liquid water. Prebiotic synthesis creates a range of simple organic compounds, which are assembled into polymers such as proteins and RNA. On the other side, the process after the LUCA is readily understood: biological evolution caused the development of a wide range of species with varied forms and biochemical capabilities. However, the derivation of living things such as LUCA from simple components is far from understood.
Although Earth remains the only place where life is known, the science of astrobiology seeks evidence of life on other planets. The 2015 NASA strategy on the origin of life aimed to solve the puzzle by identifying interactions, intermediary structures and functions, energy sources, and environmental factors that contributed to the diversity, selection, and replication of evolvable macromolecular systems, and mapping the chemical landscape of potential primordial informational polymers. The advent of polymers that could replicate, store genetic information, and exhibit properties subject to selection was, it suggested, most likely a critical step in the emergence of prebiotic chemical evolution. Those polymers derived, in turn, from simple organic compounds such as nucleobases, amino acids, and sugars that could have been formed by reactions in the environment. A successful theory of the origin of life must explain how all these chemicals came into being.
Pre-1960s conceptual history
Spontaneous generation
One ancient view of the origin of life, from Aristotle until the 19th century, is of spontaneous generation. This theory held that "lower" animals such as insects were generated by decaying organic substances, and that life arose by chance. This was questioned from the 17th century, in works like Thomas Browne's Pseudodoxia Epidemica. In 1665, Robert Hooke published the first drawings of a microorganism. In 1676, Antonie van Leeuwenhoek drew and described microorganisms, probably protozoa and bacteria. Van Leeuwenhoek disagreed with spontaneous generation, and by the 1680s convinced himself, using experiments ranging from sealed and open meat incubation and the close study of insect reproduction, that the theory was incorrect. In 1668 Francesco Redi showed that no maggots appeared in meat when flies were prevented from laying eggs. By the middle of the 19th century, spontaneous generation was considered disproven.
Panspermia
Dating back to Anaxagoras in the 5th century BC, panspermia is the idea that life originated elsewhere in the universe and came to Earth. The modern version of panspermia holds that life may have been distributed to Earth by meteoroids, asteroids, comets or planetoids. It does not attempt to explain how life originated, but shifts the origin of life to another heavenly body. The advantage is that life is not required to have formed on each planet it occurs on, but rather in a more limited set of locations, or even a single location, and then spread about the galaxy to other star systems via cometary or meteorite impact. Panspermia did not get much scientific support and deflects the need of an answer instead of explaining observable phenomena. Although interest in panspermia grew when traces of organic materials were found in meteorites, it is currently accepted that life started locally on Earth.
"A warm little pond": primordial soup
The idea that life originated from non-living matter in slow stages appeared in Herbert Spencer's 1864–1867 book Principles of Biology, and in William Turner Thiselton-Dyer's 1879 paper "On spontaneous generation and evolution". On 1 February 1871 Charles Darwin wrote about these publications to Joseph Hooker, and set out his own speculation, suggesting that the original spark of life may have begun in a "warm little pond, with all sorts of ammonia and phosphoric salts, light, heat, electricity, , present, that a compound was chemically formed ready to undergo still more complex changes." Darwin went on to explain that "at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed."
Alexander Oparin in 1924 and J. B. S. Haldane in 1929 proposed that the first molecules constituting the earliest cells slowly self-organized from a primordial soup, and this theory is called the Oparin–Haldane hypothesis. Haldane suggested that the Earth's prebiotic oceans consisted of a "hot dilute soup" in which organic compounds could have formed. J. D. Bernal showed that such mechanisms could form most of the necessary molecules for life from inorganic precursors. In 1967, he suggested three "stages": the origin of biological monomers; the origin of biological polymers; and the evolution from molecules to cells.
Miller–Urey experiment
In 1952, Stanley Miller and Harold Urey carried out a chemical experiment to demonstrate how organic molecules could have formed spontaneously from inorganic precursors under prebiotic conditions like those posited by the Oparin–Haldane hypothesis. It used a highly reducing (lacking oxygen) mixture of gases—methane, ammonia, and hydrogen, as well as water vapor—to form simple organic monomers such as amino acids. Bernal said of the Miller–Urey experiment that "it is not enough to explain the formation of such molecules, what is necessary, is a physical-chemical explanation of the origins of these molecules that suggests the presence of suitable sources and sinks for free energy." However, current scientific consensus describes the primitive atmosphere as weakly reducing or neutral, diminishing the amount and variety of amino acids that could be produced. The addition of iron and carbonate minerals, present in early oceans, however, produces a diverse array of amino acids. Later work has focused on two other potential reducing environments: outer space and deep-sea hydrothermal vents.
Producing a habitable Earth
Evolutionary history
Early universe with first stars
Soon after the Big Bang, which occurred roughly 14 Gya, the only chemical elements present in the universe were hydrogen, helium, and lithium, the three lightest atoms in the periodic table. These elements gradually accreted and began orbiting in disks of gas and dust. Gravitational accretion of material at the hot and dense centers of these protoplanetary disks formed stars by the fusion of hydrogen. Early stars were massive and short-lived, producing all the heavier elements through stellar nucleosynthesis. Element formation through stellar nucleosynthesis proceeds to its most stable element Iron-56. Heavier elements were formed during supernovae at the end of a stars lifecycle. Carbon, currently the fourth most abundant chemical element in the universe (after hydrogen, helium, and oxygen), was formed mainly in white dwarf stars, particularly those bigger than twice the mass of the sun. As these stars reached the end of their lifecycles, they ejected these heavier elements, among them carbon and oxygen, throughout the universe. These heavier elements allowed for the formation of new objects, including rocky planets and other bodies. According to the nebular hypothesis, the formation and evolution of the Solar System began 4.6 Gya with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened into a protoplanetary disk out of which the planets, moons, asteroids, and other small Solar System bodies formed.
Emergence of Earth
The age of the Earth is 4.54 Gya as found by radiometric dating of calcium-aluminium-rich inclusions in carbonaceous chrondrite meteorites, the oldest material in the Solar System. Earth, during the Hadean eon (from its formation until 4.031 Gya,) was at first inhospitable to any living organisms. During its formation, the Earth lost a significant part of its initial mass, and consequentially lacked the gravity to hold molecular hydrogen and the bulk of the original inert gases. Soon after initial accretion of Earth at 4.48 Ga, its collision with Theia, a hypothesised impactor, is thought to have created the ejected debris that would eventually form the Moon. This impact would have removed the Earth's primary atmosphere, leaving behind clouds of viscous silicates and carbon dioxide. This unstable atmosphere was short-lived and condensed shortly after to form the bulk silicate Earth, leaving behind an atmosphere largely consisting of water vapor, nitrogen, and carbon dioxide, with smaller amounts of carbon monoxide, hydrogen, and sulfur compounds. The solution of carbon dioxide in water is thought to have made the seas slightly acidic, with a pH of about 5.5.
Condensation to form liquid oceans is theorised to have occurred as early as the Moon-forming impact. This scenario has found support from the dating of 4.404 Gya zircon crystals with high δ18O values from metamorphosed quartzite of Mount Narryer in Western Australia. The Hadean atmosphere has been characterized as a "gigantic, productive outdoor chemical laboratory," similar to volcanic gases today which still support some abiotic chemistry. Despite the likely increased volcanism from early plate tectonics, the Earth may have been a predominantly water world between 4.4 and 4.3 Gya. It is debated whether or not crust was exposed above this ocean due to uncertainties of what early plate tectonics looked like. For early life to have developed, it is generally thought that a land setting is required, so this question is essential to determining when in Earth's history life evolved. Immediately after the Moon-forming impact, Earth likely had little if any continental crust, a turbulent atmosphere, and a hydrosphere subject to intense ultraviolet light from a T Tauri stage Sun. It was also affected by cosmic radiation, and continued asteroid and comet impacts. Despite all this, niche environments likely existed conducive to life on Earth in the Late-Hadean to Early-Archaean.
The Late Heavy Bombardment hypothesis posits that a period of intense impact occurred at 4.1 to 3.8 Gya during the Hadean and early Archean eons. Originally it was thought that the Late Heavy Bombardment was a single cataclysmic impact event occurring at 3.9 Gya; this would have had the potential to sterilise all life on Earth by volatilising liquid oceans and blocking the Sun needed for photosynthesising primary producers, pushing back the earliest possible emergence of life to after the Late Heavy Bombardment. However, more recent research questioned both the intensity of the Late Heavy Bombardment as well as its potential for sterilisation. Uncertainties as to whether Late Heavy Bombardment was one giant impact or a period of greater impact rates greatly changed the implication of its destructive power. The 3.9 Ga date arose from dating of Apollo mission sample returns collected mostly near the Imbrium Basin, biasing the age of recorded impacts. Impact modelling of the lunar surface reveals that rather than a cataclysmic event at 3.9 Ga, multiple small-scale, short-lived periods of bombardment likely occurred. Terrestrial data backs this idea by showing multiple periods of ejecta in the rock record both before and after the 3.9 Ga marker, suggesting that the early Earth was subject to continuous impacts that would not have had as great an impact on extinction as previously thought. If the Late Heavy Bombardment was not a single cataclysmic event, the emergence of life could have taken place far before 3.9 Ga.
If life evolved in the ocean at depths of more than ten meters, it would have been shielded both from late impacts and the then high levels of ultraviolet radiation from the sun. Geothermically heated oceanic crust could have yielded far more organic compounds through deep hydrothermal vents than the Miller–Urey experiments indicated. The available energy is maximized at 100–150 °C, the temperatures at which hyperthermophilic bacteria and thermoacidophilic archaea live.
Earliest evidence of life
The timing at which life emerged on Earth is most likely between 3.48 and 4.32 Gya. Minimum age estimates are based on evidence from the geologic rock record. In 2017, the earliest physical evidence of life so far found was reported to consist of microbialites in the Nuvvuagittuq Greenstone Belt of Northern Quebec, in banded iron formation rocks at least 3.77 and possibly as old as 4.32 Gya. The micro-organisms could have lived within hydrothermal vent precipitates, soon after the 4.4 Gya formation of oceans during the Hadean. The microbes resembled modern hydrothermal vent bacteria, supporting the view that abiogenesis began in such an environment. However, later research disputed this interpretation of the data, stating that the observations may be better explained by abiotic processes in silica-rich waters, "chemical gardens," circulating hydrothermal fluids, or volcanic ejecta.
Biogenic graphite has been found in 3.7 Gya metasedimentary rocks from southwestern Greenland and in microbial mat fossils from 3.49 Gya cherts in the Pilbara region of Western Australia. Evidence of early life in rocks from Akilia Island, near the Isua supracrustal belt in southwestern Greenland, dating to 3.7 Gya, have shown biogenic carbon isotopes. In other parts of the Isua supracrustal belt, graphite inclusions trapped within garnet crystals are connected to the other elements of life: oxygen, nitrogen, and possibly phosphorus in the form of phosphate, providing further evidence for life 3.7 Gya. In the Pilbara region of Western Australia, compelling evidence of early life was found in pyrite-bearing sandstone in a fossilized beach, with rounded tubular cells that oxidized sulfur by photosynthesis in the absence of oxygen. Carbon isotope ratios on graphite inclusions from the Jack Hills zircons suggest that life could have existed on Earth from 4.1 Gya.
The Pilbara region of Western Australia contains the Dresser Formation with rocks 3.48 Gya, including layered structures called stromatolites. Their modern counterparts are created by photosynthetic micro-organisms including cyanobacteria. These lie within undeformed hydrothermal-sedimentary strata; their texture indicates a biogenic origin. Parts of the Dresser formation preserve hot springs on land, but other regions seem to have been shallow seas. A molecular clock analysis suggests the LUCA emerged prior to 3.9 Gya.
Producing molecules: prebiotic synthesis
All chemical elements derive from stellar nucleosynthesis except for hydrogen and some helium and lithium. Basic chemical ingredients of life – the carbon-hydrogen molecule (CH), the carbon-hydrogen positive ion (CH+) and the carbon ion (C+) – can be produced by ultraviolet light from stars. Complex molecules, including organic molecules, form naturally both in space and on planets. Organic molecules on the early Earth could have had either terrestrial origins, with organic molecule synthesis driven by impact shocks or by other energy sources, such as ultraviolet light, redox coupling, or electrical discharges; or extraterrestrial origins (pseudo-panspermia), with organic molecules formed in interstellar dust clouds raining down on to the planet.
Observed extraterrestrial organic molecules
An organic compound is a chemical whose molecules contain carbon. Carbon is abundant in the Sun, stars, comets, and in the atmospheres of most planets of the Solar System. Organic compounds are relatively common in space, formed by "factories of complex molecular synthesis" which occur in molecular clouds and circumstellar envelopes, and chemically evolve after reactions are initiated mostly by ionizing radiation. Purine and pyrimidine nucleobases including guanine, adenine, cytosine, uracil, and thymine have been found in meteorites. These could have provided the materials for DNA and RNA to form on the early Earth. The amino acid glycine was found in material ejected from comet Wild 2; it had earlier been detected in meteorites. Comets are encrusted with dark material, thought to be a tar-like organic substance formed from simple carbon compounds under ionizing radiation. A rain of material from comets could have brought such complex organic molecules to Earth. It is estimated that during the Late Heavy Bombardment, meteorites may have delivered up to five million tons of organic prebiotic elements to Earth per year. Currently 40,000 tons of cosmic dust falls to Earth each year.
Polycyclic aromatic hydrocarbons
Polycyclic aromatic hydrocarbons (PAH) are the most common and abundant polyatomic molecules in the observable universe, and are a major store of carbon. They seem to have formed shortly after the Big Bang, and are associated with new stars and exoplanets. They are a likely constituent of Earth's primordial sea. PAHs have been detected in nebulae, and in the interstellar medium, in comets, and in meteorites.
A star, HH 46-IR, resembling the sun early in its life, is surrounded by a disk of material which contains molecules including cyanide compounds, hydrocarbons, and carbon monoxide. PAHs in the interstellar medium can be transformed through hydrogenation, oxygenation, and hydroxylation to more complex organic compounds used in living cells.
Nucleobases and nucleotides
Organic compounds introduced on Earth by interstellar dust particles can help to form complex molecules, thanks to their peculiar surface-catalytic activities. The RNA component uracil and related molecules, including xanthine, in the Murchison meteorite were likely formed extraterrestrially, as suggested by studies of 12C/13C isotopic ratios. NASA studies of meteorites suggest that all four DNA nucleobases (adenine, guanine and related organic molecules) have been formed in outer space. The cosmic dust permeating the universe contains complex organics ("amorphous organic solids with a mixed aromatic–aliphatic structure") that could be created rapidly by stars. Glycolaldehyde, a sugar molecule and RNA precursor, has been detected in regions of space including around protostars and on meteorites.
Laboratory synthesis
As early as the 1860s, experiments demonstrated that biologically relevant molecules can be produced from interaction of simple carbon sources with abundant inorganic catalysts. The spontaneous formation of complex polymers from abiotically generated monomers under the conditions posited by the "soup" theory is not straightforward. Besides the necessary basic organic monomers, compounds that would have prohibited the formation of polymers were also formed in high concentration during the Miller–Urey and Joan Oró experiments. Biology uses essentially 20 amino acids for its coded protein enzymes, representing a very small subset of the structurally possible products. Since life tends to use whatever is available, an explanation is needed for why the set used is so small. Formamide is attractive as a medium that potentially provided a source of amino acid derivatives from simple aldehyde and nitrile feedstocks.
Sugars
Alexander Butlerov showed in 1861 that the formose reaction created sugars including tetroses, pentoses, and hexoses when formaldehyde is heated under basic conditions with divalent metal ions like calcium. R. Breslow proposed that the reaction was autocatalytic in 1959.
Nucleobases
Nucleobases, such as guanine and adenine, can be synthesized from simple carbon and nitrogen sources, such as hydrogen cyanide (HCN) and ammonia. Formamide produces all four ribonucleotides when warmed with terrestrial minerals. Formamide is ubiquitous in the Universe, produced by the reaction of water and HCN. It can be concentrated by the evaporation of water. HCN is poisonous only to aerobic organisms (eukaryotes and aerobic bacteria), which did not yet exist. It can play roles in other chemical processes such as the synthesis of the amino acid glycine.
DNA and RNA components including uracil, cytosine and thymine can be synthesized under outer space conditions, using starting chemicals such as pyrimidine found in meteorites. Pyrimidine may have been formed in red giant stars or in interstellar dust and gas clouds. All four RNA-bases may be synthesized from formamide in high-energy density events like extraterrestrial impacts.
Other pathways for synthesizing bases from inorganic materials have been reported. Freezing temperatures are advantageous for the synthesis of purines, due to the concentrating effect for key precursors such as hydrogen cyanide. However, while adenine and guanine require freezing conditions for synthesis, cytosine and uracil may require boiling temperatures. Seven amino acids and eleven types of nucleobases formed in ice when ammonia and cyanide were left in a freezer for 25 years. S-triazines (alternative nucleobases), pyrimidines including cytosine and uracil, and adenine can be synthesized by subjecting a urea solution to freeze-thaw cycles under a reductive atmosphere, with spark discharges as an energy source. The explanation given for the unusual speed of these reactions at such a low temperature is eutectic freezing, which crowds impurities in microscopic pockets of liquid within the ice, causing the molecules to collide more often.
Peptides
Prebiotic peptide synthesis is proposed to have occurred through a number of possible routes. Some center on high temperature/concentration conditions in which condensation becomes energetically favorable, while others focus on the availability of plausible prebiotic condensing agents.
Experimental evidence for the formation of peptides in uniquely concentrated environments is bolstered by work suggesting that wet-dry cycles and the presence of specific salts can greatly increase spontaneous condensation of glycine into poly-glycine chains. Other work suggests that while mineral surfaces, such as those of pyrite, calcite, and rutile catalyze peptide condensation, they also catalyze their hydrolysis. The authors suggest that additional chemical activation or coupling would be necessary to produce peptides at sufficient concentrations. Thus, mineral surface catalysis, while important, is not sufficient alone for peptide synthesis.
Many prebiotically plausible condensing/activating agents have been identified, including the following: cyanamide, dicyanamide, dicyandiamide, diaminomaleonitrile, urea, trimetaphosphate, NaCl, CuCl2, (Ni,Fe)S, CO, carbonyl sulfide (COS), carbon disulfide (CS2), SO2, and diammonium phosphate (DAP).
An experiment reported in 2024 used a sapphire substrate with a web of thin cracks under a heat flow, similar to the environment of deep-ocean vents, as a mechanism to separate and concentrate prebiotically relevant building blocks from a dilute mixture, purifying their concentration by up to three orders of magnitude. The authors propose this as a plausible model for the origin of complex biopolymers. This presents another physical process that allows for concentrated peptide precursors to combine in the right conditions. A similar role of increasing amino acid concentration has been suggested for clays as well.
While all of these scenarios involve the condensation of amino acids, the prebiotic synthesis of peptides from simpler molecules such as CO, NH3 and C, skipping the step of amino acid formation, is very efficient.
Producing suitable vesicles
The largest unanswered question in evolution is how simple protocells first arose and differed in reproductive contribution to the following generation, thus initiating the evolution of life. The lipid world theory postulates that the first self-replicating object was lipid-like. Phospholipids form lipid bilayers in water while under agitation—the same structure as in cell membranes. These molecules were not present on early Earth, but other amphiphilic long-chain molecules also form membranes. These bodies may expand by insertion of additional lipids, and may spontaneously split into two offspring of similar size and composition. Lipid bodies may have provided sheltering envelopes for information storage, allowing the evolution and preservation of polymers like RNA that store information. Only one or two types of amphiphiles have been studied which might have led to the development of vesicles. There is an enormous number of possible arrangements of lipid bilayer membranes, and those with the best reproductive characteristics would have converged toward a hypercycle reaction, a positive feedback composed of two mutual catalysts represented by a membrane site and a specific compound trapped in the vesicle. Such site/compound pairs are transmissible to the daughter vesicles leading to the emergence of distinct lineages of vesicles, which would have allowed natural selection.
A protocell is a self-organized, self-ordered, spherical collection of lipids proposed as a stepping-stone to the origin of life. A functional protocell has (as of 2014) not yet been achieved in a laboratory setting. Self-assembled vesicles are essential components of primitive cells. The theory of classical irreversible thermodynamics treats self-assembly under a generalized chemical potential within the framework of dissipative systems. The second law of thermodynamics requires that overall entropy increases, yet life is distinguished by its great degree of organization. Therefore, a boundary is needed to separate ordered life processes from chaotic non-living matter.
Irene Chen and Jack W. Szostak suggest that elementary protocells can give rise to cellular behaviors including primitive forms of differential reproduction, competition, and energy storage. Competition for membrane molecules would favor stabilized membranes, suggesting a selective advantage for the evolution of cross-linked fatty acids and even the phospholipids of today. Such micro-encapsulation would allow for metabolism within the membrane and the exchange of small molecules, while retaining large biomolecules inside. Such a membrane is needed for a cell to create its own electrochemical gradient to store energy by pumping ions across the membrane. Fatty acid vesicles in conditions relevant to alkaline hydrothermal vents can be stabilized by isoprenoids which are synthesized by the formose reaction; the advantages and disadvantages of isoprenoids incorporated within the lipid bilayer in different microenvironments might have led to the divergence of the membranes of archaea and bacteria.
Laboratory experiments have shown that vesicles can undergo an evolutionary process under pressure cycling conditions. Simulating the systemic environment in tectonic fault zones within the Earth's crust, pressure cycling leads to the periodic formation of vesicles. Under the same conditions, random peptide chains are being formed, which are being continuously selected for their ability to integrate into the vesicle membrane. A further selection of the vesicles for their stability potentially leads to the development of functional peptide structures, associated with an increase in the survival rate of the vesicles.
Producing biology
Energy and entropy
Life requires a loss of entropy, or disorder, as molecules organize themselves into living matter. At the same time, the emergence of life is associated with the formation of structures beyond a certain threshold of complexity. The emergence of life with increasing order and complexity does not contradict the second law of thermodynamics, which states that overall entropy never decreases, since a living organism creates order in some places (e.g. its living body) at the expense of an increase of entropy elsewhere (e.g. heat and waste production).
Multiple sources of energy were available for chemical reactions on the early Earth. Heat from geothermal processes is a standard energy source for chemistry. Other examples include sunlight, lightning, atmospheric entries of micro-meteorites, and implosion of bubbles in sea and ocean waves. This has been confirmed by experiments and simulations.
Unfavorable reactions can be driven by highly favorable ones, as in the case of iron-sulfur chemistry. For example, this was probably important for carbon fixation. Carbon fixation by reaction of CO2 with H2S via iron-sulfur chemistry is favorable, and occurs at neutral pH and 100 °C. Iron-sulfur surfaces, which are abundant near hydrothermal vents, can drive the production of small amounts of amino acids and other biomolecules.
Chemiosmosis
In 1961, Peter Mitchell proposed chemiosmosis as a cell's primary system of energy conversion. The mechanism, now ubiquitous in living cells, powers energy conversion in micro-organisms and in the mitochondria of eukaryotes, making it a likely candidate for early life. Mitochondria produce adenosine triphosphate (ATP), the energy currency of the cell used to drive cellular processes such as chemical syntheses. The mechanism of ATP synthesis involves a closed membrane in which the ATP synthase enzyme is embedded. The energy required to release strongly bound ATP has its origin in protons that move across the membrane. In modern cells, those proton movements are caused by the pumping of ions across the membrane, maintaining an electrochemical gradient. In the first organisms, the gradient could have been provided by the difference in chemical composition between the flow from a hydrothermal vent and the surrounding seawater, or perhaps meteoric quinones that were conducive to the development of chemiosmotic energy across lipid membranes if at a terrestrial origin.
PAH world hypothesis
The PAH world hypothesis posits polycyclic aromatic hydrocarbons as precursors to the RNA world.
The RNA world
The RNA world hypothesis describes an early Earth with self-replicating and catalytic RNA but no DNA or proteins. Many researchers concur that an RNA world must have preceded the DNA-based life that now dominates. However, RNA-based life may not have been the first to exist. Another model echoes Darwin's "warm little pond" with cycles of wetting and drying.
RNA is central to the translation process. Small RNAs can catalyze all the chemical groups and information transfers required for life. RNA both expresses and maintains genetic information in modern organisms; and the chemical components of RNA are easily synthesized under the conditions that approximated the early Earth, which were very different from those that prevail today. The structure of the ribosome has been called the "smoking gun", with a central core of RNA and no amino acid side chains within 18 Å of the active site that catalyzes peptide bond formation.
The concept of the RNA world was proposed in 1962 by Alexander Rich, and the term was coined by Walter Gilbert in 1986. There were initial difficulties in the explanation of the abiotic synthesis of the nucleotides cytosine and uracil. Subsequent research has shown possible routes of synthesis; for example, formamide produces all four ribonucleotides and other biological molecules when warmed in the presence of various terrestrial minerals.
RNA replicase can function as both code and catalyst for further RNA replication, i.e. it can be autocatalytic. Jack Szostak has shown that certain catalytic RNAs can join smaller RNA sequences together, creating the potential for self-replication. The RNA replication systems, which include two ribozymes that catalyze each other's synthesis, showed a doubling time of the product of about one hour, and were subject to natural selection under the experimental conditions. If such conditions were present on early Earth, then natural selection would favor the proliferation of such autocatalytic sets, to which further functionalities could be added. Self-assembly of RNA may occur spontaneously in hydrothermal vents. A preliminary form of tRNA could have assembled into such a replicator molecule.
Possible precursors to protein synthesis include the synthesis of short peptide cofactors or the self-catalysing duplication of RNA. It is likely that the ancestral ribosome was composed entirely of RNA, although some roles have since been taken over by proteins. Major remaining questions on this topic include identifying the selective force for the evolution of the ribosome and determining how the genetic code arose.
Eugene Koonin has argued that "no compelling scenarios currently exist for the origin of replication and translation, the key processes that together comprise the core of biological systems and the apparent pre-requisite of biological evolution. The RNA World concept might offer the best chance for the resolution of this conundrum but so far cannot adequately account for the emergence of an efficient RNA replicase or the translation system."
From RNA to directed protein synthesis
In line with the RNA world hypothesis, much of modern biology's templated protein biosynthesis is done by RNA molecules—namely tRNAs and the ribosome (consisting of both protein and rRNA components). The most central reaction of peptide bond synthesis is understood to be carried out by base catalysis by the 23S rRNA domain V. Experimental evidence has demonstrated successful di- and tripeptide synthesis with a system consisting of only aminoacyl phosphate adaptors and RNA guides, which could be a possible stepping stone between an RNA world and modern protein synthesis. Aminoacylation ribozymes that can charge tRNAs with their cognate amino acids have also been selected in in vitro experimentation. The authors also extensively mapped fitness landscapes within their selection to find that chance emergence of active sequences was more important that sequence optimization.
Early functional peptides
The first proteins would have had to arise without a fully-fledged system of protein biosynthesis. As discussed above, numerous mechanisms for the prebiotic synthesis of polypeptides exist. However, these random sequence peptides would not have likely had biological function. Thus, significant study has gone into exploring how early functional proteins could have arisen from random sequences. First, some evidence on hydrolysis rates shows that abiotically plausible peptides likely contained significant "nearest-neighbor" biases. This could have had some effect on early protein sequence diversity. In other work by Anthony Keefe and Jack Szostak, mRNA display selection on a library of 6*1012 80-mers was used to search for sequences with ATP binding activity. They concluded that approximately 1 in 1011 random sequences had ATP binding function. While this is a single example of functional frequency in the random sequence space, the methodology can serve as a powerful simulation tool for understanding early protein evolution.
Phylogeny and LUCA
Starting with the work of Carl Woese from 1977, genomics studies have placed the last universal common ancestor (LUCA) of all modern life-forms between Bacteria and a clade formed by Archaea and Eukaryota in the phylogenetic tree of life. It lived over 4 Gya. A minority of studies have placed the LUCA in Bacteria, proposing that Archaea and Eukaryota are evolutionarily derived from within Eubacteria; Thomas Cavalier-Smith suggested in 2006 that the phenotypically diverse bacterial phylum Chloroflexota contained the LUCA.
In 2016, a set of 355 genes likely present in the LUCA was identified. A total of 6.1 million prokaryotic genes from Bacteria and Archaea were sequenced, identifying 355 protein clusters from among 286,514 protein clusters that were probably common to the LUCA. The results suggest that the LUCA was anaerobic with a Wood–Ljungdahl (reductive Acetyl-CoA) pathway, nitrogen- and carbon-fixing, thermophilic. Its cofactors suggest dependence upon an environment rich in hydrogen, carbon dioxide, iron, and transition metals. Its genetic material was probably DNA, requiring the 4-nucleotide genetic code, messenger RNA, transfer RNA, and ribosomes to translate the code into proteins such as enzymes. LUCA likely inhabited an anaerobic hydrothermal vent setting in a geochemically active environment. It was evidently already a complex organism, and must have had precursors; it was not the first living thing. The physiology of LUCA has been in dispute. Previous research identified 60 proteins common to all life.
Leslie Orgel argued that early translation machinery for the genetic code would be susceptible to error catastrophe. Geoffrey Hoffmann however showed that such machinery can be stable in function against "Orgel's paradox". Metabolic reactions that have also been inferred in LUCA are the incomplete reverse Krebs cycle, gluconeogenesis, the pentose phosphate pathway, glycolysis, reductive amination, and transamination.
Suitable geological environments
A variety of geologic and environmental settings have been proposed for an origin of life. These theories are often in competition with one another as there are many differing views of prebiotic compound availability, geophysical setting, and early life characteristics. The first organism on Earth likely looked different from LUCA. Between the first appearance of life and where all modern phylogenies began branching, an unknown amount of time passed, with unknown gene transfers, extinctions, and evolutionary adaptation to various environmental niches. One major shift is believed to be from the RNA world to an RNA-DNA-protein world. Modern phylogenies provide more pertinent genetic evidence about LUCA than about its precursors.
The most popular hypotheses for settings for the origin of life are deep sea hydrothermal vents and surface bodies of water. Surface waters can be classified into hot springs, moderate temperature lakes and ponds, and cold settings.
Deep sea hydrothermal vents
Hot fluids
Early micro-fossils may have come from a hot world of gases such as methane, ammonia, carbon dioxide, and hydrogen sulfide, toxic to much current life. Analysis of the tree of life places thermophilic and hyperthermophilic bacteria and archaea closest to the root, suggesting that life may have evolved in a hot environment. The deep sea or alkaline hydrothermal vent theory posits that life began at submarine hydrothermal vents. William Martin and Michael Russell have suggested "that life evolved in structured iron monosulphide precipitates in a seepage site hydrothermal mound at a redox, pH, and temperature gradient between sulphide-rich hydrothermal fluid and iron(II)-containing waters of the Hadean ocean floor. The naturally arising, three-dimensional compartmentation observed within fossilized seepage-site metal sulphide precipitates indicates that these inorganic compartments were the precursors of cell walls and membranes found in free-living prokaryotes. The known capability of FeS and NiS to catalyze the synthesis of the acetyl-methylsulphide from carbon monoxide and methylsulphide, constituents of hydrothermal fluid, indicates that pre-biotic syntheses occurred at the inner surfaces of these metal-sulphide-walled compartments".
These form where hydrogen-rich fluids emerge from below the sea floor, as a result of serpentinization of ultra-mafic olivine with seawater and a pH interface with carbon dioxide-rich ocean water. The vents form a sustained chemical energy source derived from redox reactions, in which electron donors (molecular hydrogen) react with electron acceptors (carbon dioxide); see iron–sulfur world theory. These are exothermic reactions.
Chemiosmotic gradient
Russell demonstrated that alkaline vents created an abiogenic proton motive force chemiosmotic gradient, ideal for abiogenesis. Their microscopic compartments "provide a natural means of concentrating organic molecules," composed of iron-sulfur minerals such as mackinawite, endowed these mineral cells with the catalytic properties envisaged by Günter Wächtershäuser. This movement of ions across the membrane depends on a combination of two factors:
Diffusion force caused by concentration gradient—all particles including ions tend to diffuse from higher concentration to lower.
Electrostatic force caused by electrical potential gradient—cations like protons H+ tend to diffuse down the electrical potential, anions in the opposite direction.
These two gradients taken together can be expressed as an electrochemical gradient, providing energy for abiogenic synthesis. The proton motive force can be described as the measure of the potential energy stored as a combination of proton and voltage gradients across a membrane (differences in proton concentration and electrical potential).
The surfaces of mineral particles inside deep-ocean hydrothermal vents have catalytic properties similar to those of enzymes and can create simple organic molecules, such as methanol (CH3OH) and formic, acetic, and pyruvic acids out of the dissolved CO2 in the water, if driven by an applied voltage or by reaction with H2 or H2S.
Starting in 1985, researchers proposed that life arose at hydrothermal vents, that spontaneous chemistry in the Earth's crust driven by rock–water interactions at disequilibrium thermodynamically underpinned life's origin and that the founding lineages of the archaea and bacteria were H2-dependent autotrophs that used CO2 as their terminal acceptor in energy metabolism. In 2016, Martin suggested, based upon this evidence, that the LUCA "may have depended heavily on the geothermal energy of the vent to survive". Pores at deep sea hydrothermal vents are suggested to have been occupied by membrane-bound compartments which promoted biochemical reactions. Metabolic intermediates in the Krebs cycle, gluconeogenesis, amino acid bio-synthetic pathways, glycolysis, the pentose phosphate pathway, and including sugars like ribose, and lipid precursors can occur non-enzymatically at conditions relevant to deep-sea alkaline hydrothermal vents.
If the deep marine hydrothermal setting was the site for the origin of life, then abiogenesis could have happened as early as 4.0-4.2 Gya. If life evolved in the ocean at depths of more than ten meters, it would have been shielded both from impacts and the then high levels of ultraviolet radiation from the sun. The available energy in hydrothermal vents is maximized at 100–150 °C, the temperatures at which hyperthermophilic bacteria and thermoacidophilic archaea live. Arguments against a hydrothermal origin of life state that hyperthermophily was a result of convergent evolution in bacteria and archaea, and that a mesophilic environment would have been more likely. This hypothesis, suggested in 1999 by Galtier, was proposed one year before the discovery of the Lost City Hydrothermal Field, where white-smoker hydrothermal vents average ~45-90 °C. Moderate temperatures and alkaline seawater such as that at Lost City are now the favoured hydrothermal vent setting in contrast to acidic, high temperature (~350 °C) black-smokers.
Arguments against a vent setting
Production of prebiotic organic compounds at hydrothermal vents is estimated to be 1x108 kg yr−1. While a large amount of key prebiotic compounds, such as methane, are found at vents, they are in far lower concentrations than estimates of a Miller-Urey Experiment environment. In the case of methane, the production rate at vents is around 2-4 orders of magnitude lower than predicted amounts in a Miller-Urey Experiment surface atmosphere.
Other arguments against an oceanic vent setting for the origin of life include the inability to concentrate prebiotic materials due to strong dilution from seawater. This open-system cycles compounds through minerals that make up vents, leaving little residence time to accumulate. All modern cells rely on phosphates and potassium for nucleotide backbone and protein formation respectively, making it likely that the first life forms also shared these functions. These elements were not available in high quantities in the Archaean oceans as both primarily come from the weathering of continental rocks on land, far from vent settings. Submarine hydrothermal vents are not conducive to condensation reactions needed for polymerisation to form macromolecules.
An older argument was that key polymers were encapsulated in vesicles after condensation, which supposedly would not happen in saltwater because of the high concentrations of ions. However, while it is true that salinity inhibits vesicle formation from low-diversity mixtures of fatty acids, vesicle formation from a broader, more realistic mix of fatty-acid and 1-alkanol species is more resilient.
Surface bodies of water
Surface bodies of water provide environments able to dry out and be rewetted. Continued wet-dry cycles allow the concentration of prebiotic compounds and condensation reactions to polymerise macromolecules. Moreover, lake and ponds on land allow for detrital input from the weathering of continental rocks which contain apatite, the most common source of phosphates needed for nucleotide backbones. The amount of exposed continental crust in the Hadean is unknown, but models of early ocean depths and rates of ocean island and continental crust growth make it plausible that there was exposed land. Another line of evidence for a surface start to life is the requirement for UV for organism function. UV is necessary for the formation of the U+C nucleotide base pair by partial hydrolysis and nucleobase loss. Simultaneously, UV can be harmful and sterilising to life, especially for simple early lifeforms with little ability to repair radiation damage. Radiation levels from a young Sun were likely greater, and, with no ozone layer, harmful shortwave UV rays would reach the surface of Earth. For life to begin, a shielded environment with influx from UV-exposed sources is necessary to both benefit and protect from UV. Shielding under ice, liquid water, mineral surfaces (e.g. clay) or regolith is possible in a range of surface water settings. While deep sea vents may have input from raining down of surface exposed materials, the likelihood of concentration is lessened by the ocean's open system.
Hot springs
Most branching phylogenies are thermophilic or hyperthermophilic, making it possible that the Last universal common ancestor (LUCA) and preceding lifeforms were similarly thermophilic. Hot springs are formed from the heating of groundwater by geothermal activity. This intersection allows for influxes of material from deep penetrating waters and from surface runoff that transports eroded continental sediments. Interconnected groundwater systems create a mechanism for distribution of life to wider area.
Mulkidjanian and co-authors argue that marine environments did not provide the ionic balance and composition universally found in cells, or the ions required by essential proteins and ribozymes, especially with respect to high K+/Na+ ratio, Mn2+, Zn2+ and phosphate concentrations. They argue that the only environments that mimic the needed conditions on Earth are hot springs similar to ones at Kamchatka. Mineral deposits in these environments under an anoxic atmosphere would have suitable pH (while current pools in an oxygenated atmosphere would not), contain precipitates of photocatalytic sulfide minerals that absorb harmful ultraviolet radiation, have wet-dry cycles that concentrate substrate solutions to concentrations amenable to spontaneous formation of biopolymers created both by chemical reactions in the hydrothermal environment, and by exposure to UV light during transport from vents to adjacent pools that would promote the formation of biomolecules. The hypothesized pre-biotic environments are similar to hydrothermal vents, with additional components that help explain peculiarities of the LUCA.
A phylogenomic and geochemical analysis of proteins plausibly traced to the LUCA shows that the ionic composition of its intracellular fluid is identical to that of hot springs. The LUCA likely was dependent upon synthesized organic matter for its growth. Experiments show that RNA-like polymers can be synthesized in wet-dry cycling and UV light exposure. These polymers were encapsulated in vesicles after condensation. Potential sources of organics at hot springs might have been transport by interplanetary dust particles, extraterrestrial projectiles, or atmospheric or geochemical synthesis. Hot springs could have been abundant in volcanic landmasses during the Hadean.
Temperate surface bodies of water
A mesophilic start in surface bodies of waters hypothesis has evolved from Darwin's concept of a 'warm little pond' and the Oparin-Haldane hypothesis. Freshwater bodies under temperate climates can accumulate prebiotic materials while providing suitable environmental conditions conducive to simple life forms. The climate during the Archaean is still a highly debated topic, as there is uncertainty about what continents, oceans, and the atmosphere looked like then. Atmospheric reconstructions of the Archaean from geochemical proxies and models state that sufficient greenhouse gases were present to maintain surface temperatures between 0-40 °C. Under this assumption, there is a greater abundance of moderate temperature niches in which life could begin.
Strong lines of evidence for mesophily from biomolecular studies include Galtier's G+C nucleotide thermometer. G+C are more abundant in thermophiles due to the added stability of an additional hydrogen bond not present between A+T nucleotides. rRNA sequencing on a diverse range of modern lifeforms show that LUCA's reconstructed G+C content was likely representative of moderate temperatures.
Although most modern phylogenies are thermophilic or hyperthermophilic, it is possible that their widespread diversity today is a product of convergent evolution and horizontal gene transfer rather than an inherited trait from LUCA. The reverse gyrase topoisomerase is found exclusively in thermophiles and hyperthermophiles as it allows for coiling of DNA. The reverse gyrase enzyme requires ATP to function, both of which are complex biomolecules. If an origin of life is hypothesised to involve a simple organism that had not yet evolved a membrane, let alone ATP, this would make the existence of reverse gyrase improbable. Moreover, phylogenetic studies show that reverse gyrase had an archaeal origin, and that it was transferred to bacteria by horizontal gene transfer. This implies that reverse gyrase was not present in the LUCA.
Icy surface bodies of water
Cold-start origin of life theories stem from the idea there may have been cold enough regions on the early Earth that large ice cover could be found. Stellar evolution models predict that the Sun's luminosity was ~25% weaker than it is today. Fuelner states that although this significant decrease in solar energy would have formed an icy planet, there is strong evidence for liquid water to be present, possibly driven by a greenhouse effect. This would create an early Earth with both liquid oceans and icy poles.
Ice melts that form from ice sheets or glaciers melts create freshwater pools, another niche capable of experiencing wet-dry cycles. While these pools that exist on the surface would be exposed to intense UV radiation, bodies of water within and under ice are sufficiently shielded while remaining connected to UV exposed areas through ice cracks. Suggestions of impact melting of ice allow freshwater paired with meteoritic input, a popular vessel for prebiotic components. Near-seawater levels of sodium chloride are found to destabilize fatty acid membrane self-assembly, making freshwater settings appealing for early membranous life.
Icy environments would trade the faster reaction rates that occur in warm environments for increased stability and accumulation of larger polymers. Experiments simulating Europa-like conditions of ~20 °C have synthesised amino acids and adenine, showing that Miller-Urey type syntheses can still occur at cold temperatures. In an RNA world, the ribozyme would have had even more functions than in a later DNA-RNA-protein-world. For RNA to function, it must be able to fold, a process that is hindered by temperatures above 30 °C. While RNA folding in psychrophilic organisms is slower, the process is more successful as hydrolysis is also slower. Shorter nucleotides would not suffer from higher temperatures.
Inside the continental crust
An alternative geological environment has been proposed by the geologist Ulrich Schreiber and the physical chemist Christian Mayer: the continental crust. Tectonic fault zones could present a stable and well-protected environment for long-term prebiotic evolution. Inside these systems of cracks and cavities, water and carbon dioxide present the bulk solvents. Their phase state would depend on the local temperature and pressure conditions and could vary between liquid, gaseous and supercritical. When forming two separate phases (e.g., liquid water and supercritical carbon dioxide in depths of little more than 1 km), the system provides optimal conditions for phase transfer reactions. Concurrently, the contents of the tectonic fault zones are being supplied by a multitude of inorganic educts (e.g., carbon monoxide, hydrogen, ammonia, hydrogen cyanide, nitrogen, and even phosphate from dissolved apatite) and simple organic molecules formed by hydrothermal chemistry (e.g. amino acids, long-chain amines, fatty acids, long-chain aldehydes). Finally, the abundant mineral surfaces provide a rich choice of catalytic activity.
An especially interesting section of the tectonic fault zones is located at a depth of approximately 1000 m. For the carbon dioxide part of the bulk solvent, it provides temperature and pressure conditions near the phase transition point between the supercritical and the gaseous state. This leads to a natural accumulation zone for lipophilic organic molecules that dissolve well in supercritical CO2, but not in its gaseous state, leading to their local precipitation. Periodic pressure variations such as caused by geyser activity or tidal influences result in periodic phase transitions, keeping the local reaction environment in a constant non-equilibrium state. In presence of amphiphilic compounds (such as the long chain amines and fatty acids mentioned above), subsequent generations of vesicles are being formed that are constantly and efficiently being selected for their stability. The resulting structures could provide hydrothermal vents as well as hot springs with raw material for further development.
Homochirality
Homochirality is the geometric uniformity of materials composed of chiral (non-mirror-symmetric) units. Living organisms use molecules that have the same chirality (handedness): with almost no exceptions, amino acids are left-handed while nucleotides and sugars are right-handed. Chiral molecules can be synthesized, but in the absence of a chiral source or a chiral catalyst, they are formed in a 50/50 (racemic) mixture of both forms. Known mechanisms for the production of non-racemic mixtures from racemic starting materials include: asymmetric physical laws, such as the electroweak interaction; asymmetric environments, such as those caused by circularly polarized light, quartz crystals, or the Earth's rotation, statistical fluctuations during racemic synthesis, and spontaneous symmetry breaking.
Once established, chirality would be selected for. A small bias (enantiomeric excess) in the population can be amplified into a large one by asymmetric autocatalysis, such as in the Soai reaction. In asymmetric autocatalysis, the catalyst is a chiral molecule, which means that a chiral molecule is catalyzing its own production. An initial enantiomeric excess, such as can be produced by polarized light, then allows the more abundant enantiomer to outcompete the other.
Homochirality may have started in outer space, as on the Murchison meteorite the amino acid L-alanine (left-handed) is more than twice as frequent as its D (right-handed) form, and L-glutamic acid is more than three times as abundant as its D counterpart. Amino acids from meteorites show a left-handed bias, whereas sugars show a predominantly right-handed bias: this is the same preference found in living organisms, suggesting an abiogenic origin of these compounds.
In a 2010 experiment by Robert Root-Bernstein, "two D-RNA-oligonucleotides having inverse base sequences (D-CGUA and D-AUGC) and their corresponding L-RNA-oligonucleotides (L-CGUA and L-AUGC) were synthesized and their affinity determined for Gly and eleven pairs of L- and D-amino acids". The results suggest that homochirality, including codon directionality, might have "emerged as a function of the origin of the genetic code".
| Biology and health sciences | Biology | null |
19180096 | https://en.wikipedia.org/wiki/Worm | Worm | Worms are many different distantly related bilateral animals that typically have a long cylindrical tube-like body, no limbs, and usually no eyes.
Worms vary in size from microscopic to over in length for marine polychaete worms (bristle worms); for the African giant earthworm, Microchaetus rappi; and for the marine nemertean worm (bootlace worm), Lineus longissimus. Various types of worm occupy a small variety of parasitic niches, living inside the bodies of other animals. Free-living worm species do not live on land but instead live in marine or freshwater environments or underground by burrowing.
In biology, "worm" refers to an obsolete taxon, Vermes, used by Carolus Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, now seen to be paraphyletic. The name stems from the Old English word wyrm. Most animals called "worms" are invertebrates, but the term is also used for the amphibian caecilians and the slowworm Anguis, a legless burrowing lizard. Invertebrate animals commonly called "worms" include annelids, nematodes, flatworms, nemerteans, chaetognaths, priapulids, and insect larvae such as grubs and maggots.
The term "helminth" is sometimes used to refer to parasitic worms. The term is more commonly used in medicine, and usually refers to roundworms and tapeworms.
History
In taxonomy, "worm" refers to an obsolete grouping, Vermes, used by Carl Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, now seen to be polyphyletic. In 1758, Linnaeus created the first hierarchical classification in his Systema Naturae. In his original scheme, the animals were one of three kingdoms, divided into the classes of Vermes, Insecta, Pisces, Amphibia, Aves, and Mammalia. Since then the last four have all been subsumed into a single phylum, the Chordata, while his Insecta (which included the crustaceans and arachnids) and Vermes have been renamed or broken up. The process was begun in 1793 by Lamarck, who called the Vermes une espèce de chaos (a sort of chaos) and split the group into three new phyla, worms, echinoderms, and polyps (which contained corals and jellyfish). By 1809, in his Philosophie Zoologique, Lamarck had created 9 phyla apart from vertebrates (where he still had 4 phyla: mammals, birds, reptiles, and fish) and molluscs, namely cirripedes, annelids, crustaceans, arachnids, insects, worms, radiates, polyps, and infusorians. Chordates are remarkably wormlike by ancestry.
Informal grouping
In the 13th century, worms were recognized in Europe as part of the category of reptiles that consisted of a miscellany of egg-laying creatures, including "snakes, various fantastic monsters, lizards, assorted amphibians", as recorded by Vincent of Beauvais in his Mirror of Nature. In everyday language, the term worm is also applied to various other living forms such as larvae, insects, millipedes, centipedes, shipworms (teredo worms), or even some vertebrates (creatures with a backbone) such as blindworms and caecilians. Worms include several groups. The three main phyla are:
Platyhelminthes, includes the flatworms, tapeworms, and flukes. They have a flat, ribbon- or leaf-shaped body with a pair of eyes at the front. Some are parasites.
Nematoda, contains the threadworms, hookworms and other roundworms. Threadworms may be microscopic, such as the vinegar eelworm, or more than 1-metre (3 feet) long. They are found in damp earth, moss, decaying substances, fresh water, or salt water. Some roundworms are also parasites: the Guinea worm, for example, gets under the skin of the feet and legs of people living in tropical countries.
Annelida, consists of the segmented worms, with bodies divided into segments or rings. Among these worms are the earthworms and the bristle worms of the sea.
Familiar worms include the earthworms, members of phylum Annelida. Other invertebrate groups may be called worms, especially colloquially. In particular, many unrelated insect larvae are called "worms", such as the railroad worm, woodworm, glowworm, bloodworm, butterworm, inchworm, mealworm, silkworm, and woolly bear worm.
Worms may also be called helminths, particularly in medical terminology when referring to parasitic worms, especially the Nematoda (roundworms) and Cestoda (tapeworms). Hence, "helminthology" is the study of parasitic worms. When a human or an animal, such as a dog or horse, is said to "have worms", it means that it is infested with parasitic worms, typically roundworms or tapeworms. Deworming is a method to kill off the worms that have infected a human or animal by giving anthelmintic drugs.
"Ringworm" is not a worm at all, but a skin fungus.
Lobopodians are an informal grouping of extinct panarthropods from the Cambrian to the Carboniferous that are often called worms or "worm-like animals" despite having had legs in the form of stubby lobopods. Likewise, the extant Onychophora are sometimes called velvet worms despite possessing stubby legs.
Society and culture
Wyrm was the Old English term for carnivorous reptiles ("serpents") and mythical dragons. "Worm" has also been used as a pejorative epithet to describe a cowardly, weak or pitiable person.
Worms can also be farmed for the production of nutrient-rich vermicompost.
| Biology and health sciences | General classifications | Animals |
219544 | https://en.wikipedia.org/wiki/Junk%20%28ship%29 | Junk (ship) | A junk () is a type of Chinese sailing ship characterized by a central rudder, an overhanging flat transom, watertight bulkheads, and a flat-bottomed design. They are also characteristically built using iron nails and clamps. The term applies to many types of small coastal or river ships, usually serving as cargo ships, pleasure boats, or houseboats, but also going up in size up to large ocean-going vessels. There can be significant regional variations in the type of rig and the layout of the vessel.
Chinese junks were originally only fluvial and had square sails, but by the Song dynasty (), they adopted ocean-going technologies acquired from Southeast Asian k'un-lun po trade ships. Tanja sails and fully-battened junk rigs were introduced to Chinese junks by the 12th century CE.
Similar designs to the Chinese junk were also adopted by other East Asian countries, most notably Japan, where junks were used as merchant ships to trade goods with China and Southeast Asia.
Etymology and history of the term
The English word "junk" comes from Portuguese from Malay . The word originally referred to the Javanese djong, very large trading ships that the Portuguese first encountered in Southeast Asia. It later also included the smaller flat-bottomed Chinese chuán, even though the two were markedly different vessels. After the disappearance of the jong in the 17th century, the meaning of "junk" (and other similar words in European languages) came to refer exclusively to the Chinese ship.
The Chinese chuán and the Southeast Asian djong are frequently confused with each other and share some characteristics, including large cargo capacities, multiple (two to three) superimposed layers of hull planks, and multiple masts and sails. However the two are readily distinguishable from each other by two major differences. The first is that Southeast Asian (Austronesian) ships are built exclusively with lugs, dowels, and fiber lashings (lashed lug), in contrast to Chinese ships which are always built with iron nails and clamps. The second is that Chinese ships since the first century AD are all built with a central rudder. In contrast, Southeast Asian ships use double lateral rudders.
The development of the sea-going Chinese chuán (the "junk" in modern usage) in the Song dynasty () is believed to have been influenced by regular contacts with sea-going Southeast Asian ships (the k'un-lun po of Chinese records) in trading ports in southern China from the 1st millennium CE onward, particularly in terms of the rigging, multiple sails, and the multiple hull sheaths. However, the chuán also incorporates distinctly Chinese innovations from their indigenous river and coastal vessels (namely watertight compartments and the central rudders). "Hybrid" ships (referred to as the "South China Sea tradition") integrating technologies from both the chuán and the djong also started to appear by the 15th century.
Construction
Sails
Iconographic remains show that Chinese ships before the 12th century used square sails. A ship carving from a stone Buddhist stele shows a ship with square sail from the Liu Sung dynasty or the Liang dynasty (c. 5th or 6th century). Dunhuang cave temple no. 45 (from the 8th or 9th century) features large sailboats and sampans with inflated square sails. A wide ship with a single sail is depicted in the Xi'an mirror (after the 9th or 12th century). Eastern lug sail, which used battens and is commonly known as "junk rig", was likely not Chinese in origin: The oldest depiction of a battened junk sail comes from the Bayon temple at Angkor Thom, Cambodia. From its characteristics and location, it is likely that the ship depicted in Bayon was a Southeast Asian ship. The Chinese themselves may have adopted them around the 12th century CE.
The full-length battens of the junk sail keep the sail flatter than ideal in all wind conditions. Consequently, their ability to sail close to the wind is poorer than other fore-and-aft rigs.
Hull
Unlike other major shipbuilding traditions which developed from dugout canoes, the junk evolved from tapering rafts. It is the reason for the unique characteristics of early Chinese junks, like the absence of keels, very low decks, and solid transverse bulkheads rather than ribs or internal frames.
Classic junks were built of softwoods (although after the 17th century teak was used in Guangdong) with the outside shape built first. Then multiple internal compartment/bulkheads accessed by separate hatches and ladders, reminiscent of the interior structure of bamboo, were built in. Traditionally, the hull has a horseshoe-shaped stern supporting a high poop deck. The bottom is flat in a river junk with no keel (similar to a sampan), so that the boat relies on a daggerboard, leeboard or very large rudder to prevent the boat from slipping sideways in the water.
The internal bulkheads are characteristic of junks, providing interior compartments and strengthening the ship. They also controlled flooding in case of holing. Ships built in this manner were written of in Zhu Yu's book Pingzhou Table Talks, published by 1119 during the Song dynasty. Again, this type of construction for Chinese ship hulls was attested to by the Moroccan Muslim Berber traveler Ibn Battuta (1304–1377 CE), who described it in great detail (refer to Technology of the Song dynasty).
Benjamin Franklin wrote in a 1787 letter on the project of mail packets between the United States and France:
Similar wet wells were also apparent in Roman small craft of the 5th century CE.
Leeboards and centerboards
Other innovations included the square-pallet bilge pump, which was adopted by the West during the 16th century for work ashore, the western chain pump, which was adopted for shipboard use, being of a different derivation. Junks also relied on the compass for navigational purposes. However, as with almost all vessels of any culture before the late 19th century, the accuracy of magnetic compasses aboard ship, whether from a failure to understand deviation (the magnetism of the ship's iron fastenings) or poor design of the compass card (the standard drypoint compasses were extremely unstable), meant that they did little to contribute to the accuracy of navigation by dead reckoning. Review of the evidence shows that the Chinese embarked magnetic pointer was only sometimes used for navigation or reorientation. The reasoning is simple. Chinese mariners were as capable as any, having undertaken the journey safely for hundreds of years, had they needed a compass as an essential tool to navigate, they would have been aware of the almost random directional qualities when used at sea of the water bowl compass they used. Yet that design remained unchanged for some half a millennium. Western sailors, coming upon a similar water bowl design (no evidence as to how has yet emerged) very rapidly adapted it in a series of significant changes such that within roughly a century the water bowl had given way to the dry pivot, a rotating compass card a century later, a lubberline a generation later and gimbals seventy or eighty years after that.
Steering
Junks employed stern-mounted rudders centuries before their adoption in the West for the simple reason that Western hull forms, with their pointed sterns, obviated a centreline steering system until technical developments in Scandinavia created the first, iron mounted, pintle and gudgeon 'barn door' western examples in the early 12th century CE. A second reason for this slow development was that the side rudders in use were still extremely efficient. Thus the junk rudder's origin, form and construction was completely different in that it was the development of a centrally mounted stern steering oar, examples of which can also be seen in Middle Kingdom (c. 2050–1800 BCE) Egyptian river vessels. It was an innovation which permitted the steering of large ships and due to its design allowed height adjustment according to the depth of the water and to avoid serious damage should the junk ground. A sizable junk can have a rudder that needed up to twenty members of the crew to control in strong weather. In addition to using the sail plan to balance the junk and take the strain off the hard to operate and mechanically weakly attached rudder, some junks were also equipped with leeboards or dagger boards. The world's oldest known depiction of a stern-mounted rudder can be seen on a pottery model of a junk dating from before the 1st century CE.
History
Han to Northern and southern dynasties era (2nd–6th century)
Chinese ships at this time were essentially fluvial (riverine) in nature and operation. Chinese ships weren't used for maritime voyages to Southeast Asia and beyond until the 9th century CE. Heng suggests an even later date (11th century CE) for the beginning of Chinese maritime shipping, when the first actual records of Chinese ships (mostly from Fujian and Guangdong) leaving for foreign trade appear.
Large Austronesian trading ships docking in Chinese seaports with as many as four sails were recorded by scholars as early as the 3rd century CE. They called them the kunlun bo or kunlun po (). They were described as being capable of sailing against strong winds and violent waves, implying that Chinese ships at that time did not have that capacity. These ships were booked by Chinese Buddhist pilgrims for passage to Southern India and Sri Lanka. In the 3rd century CE, Chinese envoys were also sent to Southeast Asia ("Nanhai"), all of them explicitly used foreign ships for passage.
Sui to Tang dynasty (7th century–9th century)
In 683 CE, Tang court sent an envoy to Srivijaya, which does not mention a ship or even a mission, implying that like in previous cases, the envoy booked passage in a foreign ship. Wang (1958) stated that there are no Tang dynasty records that mentioned Chinese junks being used for trading with Southeast Asia. Kunlun bo trade increased by the 9th century, and were described as arriving regularly in trading ports in southern China in Chinese records.
Around 770 CE, there was great activity in canal and river boat construction, attributed to Liu Yen, who created 10 shipwright yards and provided competitive rewards. Chu LingYiin, for example, deployed many-decked naval vessels in the Wu Tai Battle of 934 AD.
Rise of Song dynasty (10th–13th century)
The state of Wuyue established diplomatic and maritime trade relations with Japan and the Korean states since at least 935 CE until Wuyue was absorbed by the Song dynasty in 978 CE. The relations of Wuyue with Japan and Korea were primarily motivated by Buddhism.
In 989 CE, the Song court permitted private Chinese ships to trade overseas, due to the loss of access to the northern trading routes along the Silk Road. However regulations required ships to depart and return at specific ports that they were registered to, which stifled early trade. This regulation was modified in 1090, when the Song court decreed that ships could freely register and depart from any port. The first records of Chinese ships leaving for trade abroad appear in the 11th century, mostly to Southeast Asia, but also included records of trade with Japan and the Korean states. A stipulation requiring ships to return within 9 months was added by the second half of the 11th century, which limited the range of Chinese vessels.
Needham's Science and Civilisation in China provided some descriptions of the large junk ship during the Song dynasty. Chin scholar in 1190 described the ships in the form of a poem:
"Through the streets carts and horses are rumbling and thronging-We are back in a year of the Hsüan-Ho reign-period. One day a Han-Lin scholar presented this painting, Worthy of handing down the ways and works of a peaceful time. Going east from the Water-gate one comes to the Canal of the Sui, The streets and the fields are alike incomparable (But Lao Tzu formerly warned against prosperity And today we know it has all become waste-land). Yet the vessels that sail ten thousand li on their voyages. With rudders of timber from Chhu and their masts from Wu, Fine scenery north of the bridge and south of the bridge, Recall for a time the dream of halcyon days, One can hear the flutes and drums; the towers seem close at hand."
A decade before, in 1178, the Guangzhou customs officer Zhou Qufei wrote in Lingwai Daida about the sea-going ships of Southern China again:
"The ships which sail the southern sea and south of it are like giant houses. When their sails are spread they are like great clouds in the sky. Their rudders are several tens of feet long. A single ship carries several hundred men, and has in the stores a year's supply of grain. Pigs are fed and wine is fermented on board. There is no account of dead or living, no going back to the mainland when once the people have set forth upon the cerulean sea. At daybreak, when the gong sounds aboard the ship, the animals can drink their fill, and crew and passengers alike forget all dangers. To those on board, everything is hidden and lost in space, mountains, landmarks, and the countries of foreigners. The shipmaster may say "To make such and such a country, with a favorable wind, in so many days, we should sight such and such a mountain, (then) the ship must steer in such and such a direction". But suddenly the wind may fall, and may not be strong enough to allow for the sighting of the mountain on the given day; in such a case, bearings may have to be changed. And the ship (on the other hand) may be carried far beyond (the landmark) and may lose its bearings. A gale may spring up, the ship may be blown hither and thither, it may meet with shoals or be driven upon hidden rocks, then it may be broken to the very roofs (of its deckhouses). A great ship with heavy cargo has nothing to fear from the high seas, but rather in shallow water it will come to grief."
In 1274 CE, according to a resident of Hangzhou, the large Song junks were of 5,000 liao, around , and could fit up to 600 passengers; the middle sized ships were between 1,000- 2,000 liao and could carry up to 300 passengers. Smaller ships were known as "wind-piercing" and carried up to a hundred passengers. However, historical descriptions (often second-hand) in early Chinese sources tend to greatly exaggerate dimensions, usually to twice or more of the actual lengths. Shipwrecks of large junks of the period, the Nanhai one and Quanzhou ship, measured and in length, respectively.
Yuan dynasty (14th century)
The Mongol Yuan dynasty initially lifted the 9-month restriction on maritime shipping at around 1279, resulting in Chinese trade ships displacing Southeast Asian ships in their traditional Indian Ocean routes. But by 1284, the Yuan court revoked the private trade policy of the Song dynasty, and much of the Chinese maritime trade during this period was monopolized by the state via ortogh partnerships. Most trade expeditions were controlled by foreign merchants, mainly Muslims living in trading cities in southern China, partnered with government officials and the Mongol imperial family. This ban on private trade was intermittently lifted for brief periods until 1323, when it was lifted permanently until the overthrow of the Yuan.
Chinese ships were also described by Western travelers to the east, such as Ibn Battuta. According to Ibn Battuta, who visited China in 1347:…We stopped in the port of Calicut, in which there were at the time thirteen Chinese vessels, and disembarked. On the China Sea traveling is done in Chinese ships only, so we shall describe their arrangements. The Chinese vessels are of three kinds; large ships called chunks (junks), middle sized ones called zaws (dhows) and the small ones kakams. The large ships have anything from twelve down to three sails, which are made of bamboo rods plaited into mats. They are never lowered, but turned according to the direction of the wind; at anchor they are left floating in the wind. A ship carries a complement of a thousand men, six hundred of whom are sailors and four hundred men-at-arms, including archers, men with shields and crossbows, who throw naphtha. Three smaller ones, the "half", the "third" and the "quarter", accompany each large vessel. These vessels are built in the towns of Zaytun (Quanzhou) and Sin-Kalan (Guangzhou). The vessel has four decks and contains rooms, cabins, and saloons for merchants; a cabin has chambers and a lavatory, and can be locked by its occupants. This is the manner after which they are made; two (parallel) walls of very thick wooden (planking) are raised and across the space between them are placed very thick planks (the bulkheads) secured longitudinally and transversely by means of large nails, each three ells in length. When these walls have thus been built the lower deck is fitted in and the ship is launched before the upper works are finished. —Ibn Battuta
Yuan dynasty ships carry on the tradition of Song; the Yuan navy is essentially Song navy. Both Song and Yuan employed large trading junks. Unlike Ming treasure ships, Song and Yuan great junks are propelled by oars, and have with them smaller junks, probably for maneuvering aids. The largest junks (5,000 liao) may have a hull length twice that of Quanzhou ship (1,000 liao), that is . However, the norm size for trading junks pre-1500 was most likely around long, with the length of only becoming the norm after 1500 CE. Large size could be a disadvantage for shallow harbors and many reefs of southeast asian.
The ships of the previous Song, both mercantile and military, became the backbone of the Yuan navy. In particular the failed Mongol invasions of Japan (1274–1281), as well as the failed Mongol invasion of Java (1293), essentially relied on recently acquired Song naval capabilities. Worcester estimates that the large Yuan junks were in width and over long. In general, they had no keel, stempost, or sternpost. They did have centreboards, and a watertight bulkhead to strengthen the hull, which added great weight. This type of vessel may have been common in the 13th century. The kind of ships the Mongols used for the invasion wasn't recorded but it was large as they commissioned smaller boats for rivers of Java. David Bade estimated around 50 soldiers each on 400-500 ships with their supplies, weapons and diplomats during the Java campaign, while John Man estimated around 29–44 soldiers each.
Ming dynasty (15th–17th century)
Expedition of Zheng He
The largest junks ever built were possibly those of Admiral Zheng He, for his expeditions in the Indian Ocean (1405 to 1433), although this is disputed as no contemporary records of the sizes of Zheng He's ships are known. Instead the dimensions are based on Sanbao Taijian Xia Xiyang Ji Tongsu Yanyi (Eunuch Sanbao Western Records Popular Romance, published 1597), a romanticized version of the voyages written by nearly two centuries later. Maodeng's novel describes Zheng He's ships as follows:
"Treasure ships" () nine-masted, 44.4 by 18 zhang, about long and wide.
Equine ships (), carrying horses and tribute goods and repair material for the fleet, eight-masted, 37 by 15 zhang, about long and wide.
Supply ships (), containing staple for the crew, seven-masted, 28 by 12 zhang, about long and wide.
Transport ships (), six-masted, 24 by 9.4 zhang, about long and wide.
Warships (), five-masted, 18 by 6.8 zhang, about long.
Louise Levathes suggests that the actual length of the biggest treasure ships may have been between long and wide. Modern scholars have argued on engineering grounds that it is highly unlikely that Zheng He's ship was 450 ft in length, Guan Jincheng (1947) proposed a much more modest size of 20 zhang long by 2.4 zhang wide (204 ft by 25.5 ft or 62.2 m by 7.8 m) while Xin Yuan'ou (2002) put them as 61–76 m (200–250 feet) in length. Zhao Zhigang claimed that he has solved the debate of the size difference, and stated that Zheng He's largest ship was about in length.
Comparing to other Ming records, the Chinese seem to have exaggerated their dimensions. European East Indiamen and galleons were said to be 30, 40, 50, and 60 zhang (90, 120, 150, and 180 m) in length. It was not until the mid to late 19th century that the length of the largest western wooden ship began to exceed 100 meters, even this was done using modern industrial tools and iron parts.
International Commerce
In Livro de Duarte Barbosa (), the Portuguese writer Duarte Barbosa described the Chinese as "great navigators in very large ships which they call jungos, of two masts, of a different make from ours, the sails are of matting, and so also the cordage. There are great corsairs and robbers amongst those islands and ports of China. They go with all these goods to Malacca, where they also carry much iron, saltpetre and many other things, and for the return voyage they ship there Sumatra and Malabar pepper, of which they use a great deal in China, and drugs of Cambay, much anfiam, which we call opium, and wormwood, Levant gall nuts, saffron, coral wrought and unwrought, stuffs from Cambay, Palecate, and Bengal, vermilion, quicksilver, scarlet cloth, and many other things... Many of these Chinese take their wives and children continually on the ships in which they live without possessing any other dwellings."
Sea ban
Private trade was banned in 1371 by the Hongwu Emperor, though official state-sponsored trade under the guise of "tribute" missions continued. The ban on private trade was lifted in 1405 during the Zheng He expeditions, but reinstated again in 1479. From the mid-15th to early 16th century, all Chinese maritime trading was banned under the Ming dynasty in what were known as the hai jin laws. The Zheng He expeditions had drained imperial funds and there was increasing threat of invasion from the north, leading the Xuande Emperor to order the immediate cessation of all overseas exploration. The shipping and shipbuilding knowledge acquired during the Song and Yuan dynasties gradually declined during this period.
Capture of Taiwan
In 1661, a naval fleet of 400 junks and 25,000 men led by the Ming loyalist Koxinga (), arrived in Taiwan to oust the Dutch from Zeelandia. Following a nine-month siege, Cheng captured the Dutch fortress Fort Zeelandia. A peace treaty between Koxinga and the Dutch Government was signed at Castle Zeelandia on February 1, 1662, and Taiwan became Koxinga's base for the Kingdom of Tungning.
Qing dynasty (17th-19th century)
Large, ocean-going junks played a key role in Asian trade until the 19th century. One of these junks, Keying, sailed from China around the Cape of Good Hope to the United States and England between 1846 and 1848. Many junks were fitted out with carronades and other weapons for naval or piratical uses. These vessels were typically called "war junks" or "armed junks" by Western navies which began entering the region more frequently in the 18th century. The British, Americans and French fought several naval battles with war junks in the 19th century, during the First Opium War, Second Opium War and in between.
At sea, junk sailors co-operated with their Western counterparts. For example, in 1870 survivors of the English barque Humberstone shipwrecked off Formosa, were rescued by a junk and landed safely in Macao.
Modern period (20th century)
In 1938, E. Allen Petersen escaped the advancing Japanese armies by sailing a junk, Hummel Hummel, from Shanghai to California with his wife Tani and two White Russians (Tsar loyalists).
In 1955, six young men sailed a Ming dynasty-style junk from Taiwan to San Francisco. The four-month journey aboard the Free China was captured on film and their arrival into San Francisco made international front-page news. The five Chinese-born friends saw an advertisement for an international trans-Atlantic yacht race, and jumped at the opportunity for adventure. They were joined by the then US Vice-Consul to China, who was tasked with capturing the journey on film. Enduring typhoons and mishaps, the crew, having never sailed a century-old junk before, learned along the way. The crew included Reno Chen, Paul Chow, Loo-chi Hu, Benny Hsu, Calvin Mehlert and were led by skipper Marco Chung. After a journey of , the Free China and her crew arrived in San Francisco Bay in fog on August 8, 1955. Shortly afterward the footage was featured on ABC television's Bold Journey travelogue. Hosted by John Stephenson and narrated by ship's navigator Paul Chow, the program highlighted the adventures and challenges of the junk's sailing across the Pacific, as well as some humorous moments aboard ship.
In 1959 a group of Catalan men, led by Jose Maria Tey, sailed from Hong Kong to Barcelona on a junk named Rubia. After their successful journey this junk was anchored as a tourist attraction at one end of Barcelona harbor, close to where La Rambla meets the sea. Permanently moored along with it was a reproduction of Columbus' caravel Santa Maria during the 1960s and part of the 1970s.
In 1981, Christoph Swoboda had a 65 feet (LoA) Bedar built by the boatyard of Che Ali bin Ngah on Duyong island in the estuary of the Terengganu river on the east coast of Malaysia. The Bedar is one of the two types of Malay junk schooners traditionally built there. He sailed this junk with his family and one friend to the Mediterranean and then continued with changing crew to finally finish a circumnavigation in 1998. He sold this vessel in 2000 and in 2004 he started to build a new junk in Duyong with the same craftsmen, the Pinas (or Pinis) Naga Pelangi, in order to help keep this ancient boat building tradition alive. This boat finished to be fitted out in 2010 and is working as a charter boat in the Andaman and the South China Sea.
| Technology | Naval transport | null |
219545 | https://en.wikipedia.org/wiki/Rapeseed | Rapeseed | Rapeseed (Brassica napus subsp. napus), also known as rape and oilseed rape, is a bright-yellow flowering member of the family Brassicaceae (mustard or cabbage family), cultivated mainly for its oil-rich seed, which naturally contains appreciable amounts of mildly toxic erucic acid. The term "canola" denotes a group of rapeseed cultivars that were bred to have very low levels of erucic acid and which are especially prized for use as human and animal food. Rapeseed is the third-largest source of vegetable oil and the second-largest source of protein meal in the world.
Description
Brassica napus grows to in height with hairless, fleshy, pinnatifid and glaucous lower leaves which are stalked whereas the upper leaves have no petioles.
Rapeseed flowers are bright yellow and about across. They are radial and consist of four petals in a typical cross-form, alternating with four sepals. They have indeterminate racemose flowering starting at the lowest bud and growing upward in the following days. The flowers have two lateral stamens with short filaments, and four median stamens with longer filaments whose anthers split away from the flower's center upon flowering.
The rapeseed pods are green and elongated siliquae during development that eventually ripen to brown. They grow on pedicels long, and can range from in length. Each pod has two compartments separated by an inner central wall within which a row of seeds develops. The seeds are round and have a diameter of . They have a reticulate surface texture, and are black and hard at maturity.
Similar species
B. napus can be distinguished from B. nigra by the upper leaves which do not clasp the stem, and from B. rapa by its smaller petals which are less than across.
Taxonomy
The species Brassica napus belongs to the flowering plant family Brassicaceae. Rapeseed is a subspecies with the autonym B. napus subsp. napus. It encompasses winter and spring oilseed, vegetable and fodder rape. Siberian kale is a distinct leaf rape form variety (B. napus var. pabularia) which used to be common as a winter-annual vegetable. The second subspecies of B. napus is B. napus subsp. rapifera (also subsp. napobrassica; the rutabaga, swede, or yellow turnip).
B. napus is a digenomic amphidiploid that occurred due to the interspecific hybridization between B. oleracea and B. rapa. It is a self-compatible pollinating species like the other amphidiploid Brassica species.
Etymology
The term "rape" derives from the Latin word for turnip, or , cognate with the Greek word , .
Ecology
In Northern Ireland, B. napus and B. rapa are recorded as escapes in roadside verges and waste ground.
Cultivation
Crops from the genus Brassica, including rapeseed, were among the earliest plants to be widely cultivated by humankind as early as 10,000 years ago. Rapeseed was being cultivated in India as early as 4000 B.C. and it spread to China and Japan 2000 years ago.
Rapeseed oil is predominantly cultivated in its winter form in most of Europe and Asia due to the requirement of vernalization to start the process of flowering. It is sown in autumn and remains in a leaf rosette on the soil surface during the winter. The plant grows a long vertical stem in the next spring followed by lateral branch development. It generally flowers in late spring with the process of pod development and ripening occurring over a period of 6–8 weeks until midsummer.
In Europe, winter rapeseed is grown as an annual break crop in three to four-year rotations with cereals such as wheat and barley, and break crops such as peas and beans. This is done to reduce the possibility of pests and diseases being carried over from one crop to another. Winter rape is less susceptible to crop failure as it is more vigorous than the summer variety and can compensate for damage done by pests.
Spring rapeseed is cultivated in Canada, northern Europe and Australia as it is not winter-hardy and does not require vernalization. The crop is sown in spring with stem development happening immediately after germination.
Rapeseed can be cultivated on a wide variety of well-drained soils, prefers a pH between 5.5 and 8.3 and has a moderate tolerance of soil salinity. It is predominantly a wind-pollinated plant but shows significantly increased grain yields when bee-pollinated, almost double the final yield but the effect is cultivar dependent. It is currently grown with high levels of nitrogen-containing fertilisers, and the manufacture of these generates N2O. An estimated 3–5% of nitrogen provided as fertilizer for rapeseed is converted to N2O.
Rapeseed has a high demand for nutrients - in particular, its sulphur demand is the highest among all arable crops. Since the decrease of atmospheric sulphur inputs during the 1980s sulphur fertilization has become a standard measure in oilseed rape production. Among the micronutrients, special attention in rapeseed cultivation has to be given to boron, manganese and molybdenum.
Climate change
The cultivatable range for rapeseed is expected to decrease due to climate change. The quality of the crop, in both yield and volume of oil, is also expected to decrease substantially. Some researchers recommend finding alternative varieties of Brassica for cultivation.
Diseases
The main diseases of the winter rapeseed crop are canker, light leaf spot (Pyrenopeziza brassicae), alternaria- and sclerotinia- stem rots. Canker causes leaf spotting, and premature ripening and weakening of the stem during the autumn-winter (fall-winter) period. A conazole- or triazole- fungicide treatment is required in late autumn (fall) and in spring against canker while broad-spectrum fungicides are used during the spring-summer period for alternaria and sclerotinia control. Oilseed rape cannot be planted in close rotation with itself due to soil-borne diseases such as sclerotinia, verticillium wilt and clubroot.
Transgenic rapeseed shows great promise for . Transexpression of a class II chitinase from barley (Hordeum vulgare) and a type I ribosome inactivating protein into B. juncea produces a large . Chhikara et al., 2012 finds that this combination of transgenes reduces hyphal growth by 44% and delays disease presentation in Alternaria brassicicola of juncea.
Blackleg (Leptosphaeria maculans/Phoma lingam) is a major disease. Yu et al., 2005 uses restriction fragment length polymorphism analysis on two doubled haploid populations DHP95 and DHP96. They find one resistance genes in each, and LepR1.
Pests
Rapeseed is attacked by a wide variety of insects, , as well as wood pigeons. The brassica pod midge (Dasineura brassicae), cabbage seed weevil (Ceutorhynchus assimilis), cabbage stem weevil (Ceutorhynchus pallidactylus), cabbage stem flea beetle (Psylliodes chrysocephala), rape stem weevil (Ceutorhynchus napi) and pollen beetles are the primary insect pests that prey on the oilseed rape crop in Europe. The insect pests can feed on developing pods to lay eggs inside and eat the developing seeds, bore into the plant's stem and feed on pollen, leaves and flowers. Synthetic pyrethroid insecticides are the main attack vector against insect pests though there is a large-scale use of prophylactic insecticides in many countries. Molluscicide pellets are used either before or after sowing of the rapeseed crop to protect against slugs.
Genetics and breeding
In 2014 an SNP array was released for B. napus by Dalton-Morgan et al., and another by Clarke et al., in 2016, both of which have since become widely used in molecular breeding. In a demonstration of the importance of epigenetics, Hauben et al., 2009 found that isogenic lines did not have identical energy use efficiencies in actual growing conditions, due to epigenetic differences. Specific locus amplified fragment sequencing (SLAF-seq) was applied to B. napus by Geng et al., in 2016, revealing the genetics of the past domestication process, providing data for genome-wide association studies (GWAS), and being used to construct a high-density linkage map.
History of the cultivars
In 1973, Canadian agricultural scientists launched a marketing campaign to promote canola consumption. Seed, oil, and protein meal derived from rapeseed cultivars which are low in erucic acid and low in glucosinolates was originally registered as a trademark, in 1978, of the Canola Council of Canada, as "canola". Canola is now a generic term for edible varieties of rapeseed, but is still officially defined in Canada as rapeseed oil that "must contain less than 2% erucic acid and less than 30 μmol of glucosinolates per gram of air-dried oil-free meal."
In the 1980s decreasing atmospheric sulphur inputs to Northern European soils in connection with a less efficient internal use of sulphur in the metabolism of the newly bred low-glucosinolate varieties (00-varieties) resulted in an increased appearance of white flowering, a highly specific symptom of sulphur deficiency, in rapeseed crops which during the official variety assessment procedures was wrongly attributed to a genetic inhomogeneity ("Canadian blood").
The anticipated damages of wild animals caused by foraging on 00-oilseed rape crops was caused by a shift of the animals diet towards increased uptake protein and sulphur containing metabolites at the expense of fibers, but not to specific features of the genetically altered 00-varieties.
Following the European Parliament's Transport Biofuels Directive in 2003 promoting the use of biofuels, the cultivation of winter rapeseed increased dramatically in Europe.
Bayer Cropscience, in collaboration with BGI-Shenzhen, China, KeyGene, the Netherlands, and the University of Queensland, Australia, announced it had sequenced the entire genome of B. napus and its constituent genomes present in B. rapa and B. oleracea in 2009. The "A" genome component of the amphidiploid rapeseed species B. napus has been sequenced by the Multinational Brassica Genome Project.
A genetically modified variety of rapeseed was developed in 1998, engineered for glyphosate tolerance, and is considered to be the most disease- and drought-resistant canola. By 2009, 90% of the rapeseed crops planted in Canada were of this sort.
GMO cultivars
The Monsanto company genetically engineered new cultivars of rapeseed to be resistant to the effects of its herbicide, Roundup. In 1998, they brought this to the Canadian market. Monsanto sought compensation from farmers found to have crops of this cultivar in their fields without paying a license fee. However, these farmers claimed that the pollen containing the Roundup Ready gene was blown into their fields and crossed with unaltered canola. Other farmers claimed that after spraying Roundup in non-canola fields to kill weeds before planting, Roundup Ready volunteers were left behind, causing extra expense to rid their fields of the weeds.
In a closely followed legal battle, the Supreme Court of Canada found in favor of Monsanto's patent infringement claim for unlicensed growing of Roundup Ready in its 2004 ruling on Monsanto Canada Inc. v. Schmeiser, but also ruled that Schmeiser was not required to pay any damages. The case garnered international controversy, as a court-sanctioned legitimization for the global patent protection of genetically modified crops. In March 2008, an out-of-court settlement between Monsanto and Schmeiser agreed that Monsanto would clean up the entire GMO-canola crop on Schmeiser's farm, at a cost of about CAN$660.
Production
The Food and Agriculture Organization reports global production of in the 2003–2004 season, and an estimated in the 2010–2011 season.
Worldwide production of rapeseed (including canola) has increased sixfold between 1975 and 2007. The production of canola and rapeseed since 1975 has opened up the edible oil market for rapeseed oil. Since 2002, production of biodiesel has been steadily increasing in EU and U.S. to in 2006. Rapeseed oil is positioned to supply a good portion of the vegetable oils needed to produce that fuel. World production was thus expected to trend further upward between 2005 and 2015 as biodiesel content requirements in Europe go into effect.
Uses
Rapeseed is grown for the production of edible vegetable oils, animal feed, and biodiesel. Rapeseed was the third-leading source of vegetable oil in the world in 2000, after soybean and palm oil. It is the world's second-leading source of protein meal after soybean.
Vegetable oil
Rapeseed oil is one of the oldest known vegetable oils, but historically was used in limited quantities due to high levels of erucic acid, which is damaging to cardiac muscle of animals, and glucosinolates, which made it less nutritious in animal feed. Rapeseed oil can contain up to 54% erucic acid. Food-grade oil derived from rapeseed cultivars, known as canola oil or low-erucic-acid rapeseed oil (LEAR oil), has been generally recognized as safe by the United States Food and Drug Administration. Canola oil is limited by government regulation to a maximum of 2% erucic acid by weight in the US and 2% in the EU, with special regulations for infant food. These low levels of erucic acid are not believed to cause harm in human infants.
Animal feed
Processing of rapeseed for oil production produces rapeseed meal as a byproduct. The byproduct is a high-protein animal feed, competitive with soybean. Rapeseed is an excellent silage crop (fermented and stored in air-tight conditions for later use as a winterfeed). The feed is employed mostly for cattle feeding, but is also used for pigs and poultry. However, the high levels of glucosinolates, sinapine, and phytic acid in the seed cake of rapeseed cause detrimental health effects to animals, reduce digestibility of certain nutrients, reduce palatability, and contribute to eutrophication of waterways. In China, rapeseed meal is mostly used as a soil fertilizer rather than for animal feed.
Biodiesel
Rapeseed oil is used as diesel fuel, either as biodiesel, straight in heated fuel systems, or blended with petroleum distillates for powering motor vehicles. Biodiesel may be used in pure form in newer engines without engine damage and is frequently combined with fossil-fuel diesel in ratios varying from 2% to 20% biodiesel. Owing to the costs of growing, crushing, and refining rapeseed biodiesel, rapeseed-derived biodiesel from new oil costs more to produce than standard diesel fuel, so diesel fuels are commonly made from the used oil. Rapeseed oil is the preferred oil stock for biodiesel production in most of Europe, accounting for about 80% of the feedstock, partly because rapeseed produces more oil per unit of land area compared to other oil sources, such as soybeans, but primarily because canola oil has a significantly lower gel point than most other vegetable oils.
Because of the changes to the environment caused by climate change, a 2018 study predicted that rapeseed would become an unreliable source of oil for biofuels.
Other
Rapeseed is also used as a cover crop in the US during the winter as it prevents soil erosion, produces large amounts of biomass, suppresses weeds and can improve soil tilth with its root system. Some cultivars of rapeseed are also used as annual forage and are ready for grazing livestock 80 to 90 days after planting.
Rapeseed has a high melliferous potential (produces substances that can be collected by insects) and is a main forage crop for honeybees. Monofloral rapeseed honey has a whitish or milky yellow color, peppery taste and, due to its fast crystallization time, a soft-solid texture. It crystallizes within 3 to 4 weeks and can ferment over time if stored improperly. The low fructose-to-glucose ratio in monofloral rapeseed honey causes it to quickly granulate in the honeycomb, forcing beekeepers to extract the honey within 24 hours of it being capped.
As a biolubricant, rapeseed has possible uses for bio-medical applications (e.g., lubricants for artificial joints) and the use of personal lubricant for sexual purposes. Biolubricant containing 70% or more canola/rapeseed oil has replaced petroleum-based chainsaw oil in Austria although it is typically more expensive.
Rapeseed has been researched as a means of containing radionuclides that contaminated the soil after the Chernobyl disaster as it has a rate of uptake up to three times more than other grains, and only about 3 to 6% of the radionuclides go into the oilseeds.
Rapeseed meal can be incorporated into the soil as a biofumigant. It suppresses such fungal crop pathogens as Rhizoctonia solani and Pratylenchus penetr.
| Biology and health sciences | Brassicales | null |
219640 | https://en.wikipedia.org/wiki/Animal%20husbandry | Animal husbandry | Animal husbandry is the branch of agriculture concerned with animals that are raised for meat, fibre, milk, or other products. It includes day-to-day care, management, production, nutrition, selective breeding, and the raising of livestock. Husbandry has a long history, starting with the Neolithic Revolution when animals were first domesticated, from around 13,000 BC onwards, predating farming of the first crops. During the period of ancient societies like ancient Egypt, cattle, sheep, goats, and pigs were being raised on farms.
Major changes took place in the Columbian exchange, when Old World livestock were brought to the New World, and then in the British Agricultural Revolution of the 18th century, when livestock breeds like the Dishley Longhorn cattle and Lincoln Longwool sheep were rapidly improved by agriculturalists, such as Robert Bakewell, to yield more meat, milk, and wool. A wide range of other species, such as horse, water buffalo, llama, rabbit, and guinea pig, are used as livestock in some parts of the world. Insect farming, as well as aquaculture of fish, molluscs, and crustaceans, is widespread. Modern animal husbandry relies on production systems adapted to the type of land available. Subsistence farming is being superseded by intensive animal farming in the more developed parts of the world, where, for example, beef cattle are kept in high-density feedlots, and thousands of chickens may be raised in broiler houses or batteries. On poorer soil, such as in uplands, animals are often kept more extensively and may be allowed to roam widely, foraging for themselves. Animal agriculture at modern scale drives climate change, ocean acidification, and biodiversity loss.
Most livestock are herbivores, except (among the most commonly-kept species) for pigs and chickens which are omnivores. Ruminants like cattle and sheep are adapted to feed on grass; they can forage outdoors or may be fed entirely or in part on rations richer in energy and protein, such as pelleted cereals. Pigs and poultry cannot digest the cellulose in forage and require other high-protein foods.
Etymology
The verb to husband, meaning "to manage carefully", derives from an older meaning of husband, which in the 14th century referred to the ownership and care of a household or farm, but today means the "control or judicious use of resources", and in agriculture, the cultivation of plants or animals. Farmers and ranchers who raise livestock are considered to practice animal husbandry.
History
Birth of husbandry
The domestication of livestock was driven by the need to have food on hand when hunting was unproductive. The desirable characteristics of a domestic animal are that it should be useful to the domesticator, should be able to thrive in his or her company, should breed freely, and be easy to tend. Domestication was not a single event, but a process repeated at various periods in different places. Sheep and goats were the animals that accompanied the nomads in the Middle East, while cattle and pigs were associated with more settled communities. The first wild animal to be domesticated was the dog. Half-wild dogs, perhaps starting with young individuals, may have been tolerated as scavengers and killers of vermin, and being naturally pack hunters, were predisposed to become part of the human pack and join in the hunt. Prey animals, sheep, goats, pigs and cattle, were progressively domesticated early in the history of agriculture. Pigs were domesticated in the Near East between 8,500 and 8000 BC, sheep and goats in or near the Fertile Crescent about 8,500 BC, and cattle from wild aurochs in the areas of modern Turkey and Pakistan around 8,500 BC. A cow was a great advantage to a villager as she produced more milk than her calf needed, and her strength could be put to use as a working animal, pulling a plough to increase production of crops, and drawing a sledge, and later a cart, to bring the produce home from the field. Draught animals were first used about 4,000 BC in the Middle East, increasing agricultural production immeasurably.
In southern Asia, the elephant was domesticated by 6,000 BC. Fossilised chicken bones dated to 5040 BC have been found in northeastern China, far from where their wild ancestors lived in the jungles of tropical Asia, but archaeologists believe that the original purpose of domestication was for the sport of cockfighting. Meanwhile, in South America, the llama and the alpaca had been domesticated, probably before 3,000 BC, as beasts of burden and for their wool. Neither was strong enough to pull a plough which limited the development of agriculture in the New World. Horses occur naturally on the steppes of Central Asia and their domestication began around 3,000 BC in the Black Sea and Caspian Sea region. Although horses were originally seen as a source of meat, their use as pack animals and for riding followed. Around the same time, the wild ass was being tamed in Egypt. Camels were domesticated soon after this, with the Bactrian camel in Mongolia and the Arabian camel becoming beasts of burden. By 1000 BC, caravans of Arabian camels were linking India with Mesopotamia and the Mediterranean.
Ancient civilisations
In ancient Egypt, cattle were the most important livestock, and sheep, goats, and pigs were also kept; poultry including ducks, geese, and pigeons were captured in nets and bred on farms, where they were force-fed with dough to fatten them. The Nile provided a plentiful source of fish. Honey bees were domesticated from at least the Old Kingdom, providing both honey and wax. In ancient Rome, all the livestock known in ancient Egypt were available. In addition, rabbits were domesticated for food by the first century BC. To help flush them out from their burrows, the polecat was domesticated as the ferret, its use described by Pliny the Elder.
Medieval husbandry
In northern Europe, agriculture including animal husbandry went into decline when the Roman empire collapsed. Some aspects such as the herding of animals continued throughout the period. By the 11th century, the economy had recovered and the countryside was again productive. The Domesday Book recorded every parcel of land and every animal in England: "there was not one single hide, nor a yard of land, nay, moreover ... not even an ox, nor a cow, nor a swine was there left, that was not set down in [the king's] writ." For example, the royal manor of Earley in Berkshire, one of thousands of villages recorded in the book, had in 1086 "2 fisheries worth [paying tax of] 7s and 6d [each year] and 20 acres of meadow [for livestock]. Woodland for [feeding] 70 pigs." The improvements of animal husbandry in the medieval period in Europe went hand in hand with other developments. Improvements to the plough allowed the soil to be tilled to a greater depth. Horses took over from oxen as the main providers of traction, new ideas on crop rotation were developed and the growing of crops for winter fodder gained ground. Peas, beans and vetches became common; they increased soil fertility through nitrogen fixation, allowing more livestock to be kept.
Columbian exchange
Exploration and colonisation of North and South America resulted in the introduction into Europe of such crops as maize, potatoes, sweet potatoes and manioc, while the principal Old World livestock – cattle, horses, sheep and goats – were introduced into the New World for the first time along with wheat, barley, rice and turnips.
Agricultural Revolution
Selective breeding for desired traits was established as a scientific practice by Robert Bakewell during the British Agricultural Revolution in the 18th century. One of his most important breeding programs was with sheep. Using native stock, he was able to quickly select for large, yet fine-boned sheep, with long, lustrous wool. The Lincoln Longwool was improved by Bakewell and in turn the Lincoln was used to develop the subsequent breed, named the New (or Dishley) Leicester. It was hornless and had a square, meaty body with straight top lines. These sheep were exported widely and have contributed to numerous modern breeds. Under his influence, English farmers began to breed cattle for use primarily as beef. Long-horned heifers were crossed with the Westmoreland bull to create the Dishley Longhorn.
The semi-natural, unfertilised pastures formed by traditional agricultural methods in Europe were managed by grazing and mowing. As the ecological impact of this land management strategy is similar to the impact of such natural disturbances as grazing and wildfire, this agricultural system shares many beneficial characteristics with a natural habitat, including the promotion of biodiversity. This strategy is declining in Europe today due to the intensification of agriculture. The mechanized and chemical methods used are causing biodiversity to decline.
Practices
Systems
Traditionally, animal husbandry was part of the subsistence farmer's way of life, producing not only the food needed by the family but also the fuel, fertiliser, clothing, transport and draught power. Killing the animal for food was a secondary consideration, and wherever possible its products such as wool, eggs, milk and blood (by the Maasai) were harvested while the animal was still alive. In the traditional system of transhumance, people and livestock moved seasonally between fixed summer and winter pastures; in montane regions the summer pasture was up in the mountains, the winter pasture in the valleys.
Animals can be kept extensively or intensively. Extensive systems involve animals roaming at will, or under the supervision of a herdsman, often for their protection from predators. Ranching in the Western United States involves large herds of cattle grazing widely over public and private lands. Similar cattle stations are found in South America, Australia and other places with large areas of land and low rainfall. Ranching systems have been used for sheep, deer, ostrich, emu, llama and alpaca.
In the uplands of the United Kingdom, sheep are turned out on the fells in spring and graze the abundant mountain grasses untended, being brought to lower altitudes late in the year, with supplementary feeding being provided in winter. In rural locations, pigs and poultry can obtain much of their nutrition from scavenging, and in African communities, hens may live for months without being fed, and still produce one or two eggs a week.
At the other extreme, in the more developed parts of the world, animals are often intensively managed; dairy cows may be kept in zero-grazing conditions with all their forage brought to them; beef cattle may be kept in high density feedlots; pigs may be housed in climate-controlled buildings and never go outdoors; poultry may be reared in barns and kept in cages as laying birds under lighting-controlled conditions. In between these two extremes are semi-intensive, often family-run farms where livestock graze outside for much of the year, silage or hay is made to cover the times of year when the grass stops growing, and fertiliser, feed, and other inputs are brought onto the farm from outside.
Feeding
Animals used as livestock are predominantly herbivorous, the main exceptions being the pig and the chicken which are omnivorous. The herbivores can be divided into "concentrate selectors" which selectively feed on seeds, fruits and highly nutritious young foliage, "grazers" which mainly feed on grass, and "intermediate feeders" which choose their diet from the whole range of available plant material. Cattle, sheep, goats, deer and antelopes are ruminants; they digest food in two steps, chewing and swallowing in the normal way, and then regurgitating the semidigested cud to chew it again and thus extract the maximum possible food value.
The dietary needs of these animals is mostly met by eating grass. Grasses grow from the base of the leaf-blade, enabling it to thrive even when heavily grazed or cut.
In many climates grass growth is seasonal, for example in the temperate summer or tropical rainy season, so some areas of the crop are set aside to be cut and preserved, either as hay (dried grass), or as silage (fermented grass). Other forage crops are also grown and many of these, as well as crop residues, can be ensiled to fill the gap in the nutritional needs of livestock in the lean season.
Extensively reared animals may subsist entirely on forage, but more intensively kept livestock will require energy and protein-rich foods in addition. Energy is mainly derived from cereals and cereal by-products, fats and oils and sugar-rich foods, while protein may come from fish or meat meal, milk products, legumes and other plant foods, often the by-products of vegetable oil extraction.
Pigs and poultry are non-ruminants and unable to digest the cellulose in grass and other forages, so they are fed entirely on cereals and other high-energy foodstuffs. The ingredients for the animals' rations can be grown on the farm or can be bought, in the form of pelleted or cubed, compound foodstuffs specially formulated for the different classes of livestock, their growth stages and their specific nutritional requirements. Vitamins and minerals are added to balance the diet. Farmed fish are usually fed pelleted food.
Breeding
The breeding of farm animals seldom occurs spontaneously but is managed by farmers with a view to encouraging traits seen as desirable. These include hardiness, fertility, docility, mothering abilities, fast growth rates, low feed consumption per unit of growth, better body proportions, higher yields, and better fibre qualities. Undesirable traits such as health defects and aggressiveness are selected against.
Selective breeding has been responsible for large increases in productivity. For example, in 2007, a typical broiler chicken at eight weeks old was 4.8 times as heavy as a bird of similar age in 1957, while in the thirty years to 2007, the average milk yield of a dairy cow in the United States nearly doubled.
Animal health
Good husbandry, proper feeding, and hygiene are the main contributors to animal health on the farm, bringing economic benefits through maximised production. When, despite these precautions, animals still become sick, they are treated with veterinary medicines, by the farmer and the veterinarian. In the European Union, when farmers treat their own animals, they are required to follow the guidelines for treatment and to record the treatments given. Animals are susceptible to a number of diseases and conditions that may affect their health. Some, like classical swine fever and scrapie are specific to one type of stock, while others, like foot-and-mouth disease affect all cloven-hoofed animals. Animals living under intensive conditions are prone to internal and external parasites; increasing numbers of sea lice are affecting farmed salmon in Scotland. Reducing the parasite burdens of livestock results in increased productivity and profitability.
Where the condition is serious, governments impose regulations on import and export, on the movement of stock, quarantine restrictions and the reporting of suspected cases. Vaccines are available against certain diseases, and antibiotics are widely used where appropriate. At one time, antibiotics were routinely added to certain compound foodstuffs to promote growth, but this practice is now frowned on in many countries because of the risk that it may lead to antimicrobial resistance in livestock and in humans.
Governments are concerned with zoonoses, diseases that humans may acquire from animals. Wild animal populations may harbour diseases that can affect domestic animals which may acquire them as a result of insufficient biosecurity. An outbreak of Nipah virus in Malaysia in 1999 was traced back to pigs becoming ill after contact with fruit-eating flying foxes, their faeces and urine. The pigs in turn passed the infection to humans. Avian flu H5N1 is present in wild bird populations and can be carried large distances by migrating birds. This virus is easily transmissible to domestic poultry, and to humans living in close proximity with them. Other infectious diseases affecting wild animals, farm animals and humans include rabies, leptospirosis, brucellosis, tuberculosis and trichinosis.
Range of species
There is no single universally agreed definition of which species are livestock. Widely agreed types of livestock include cattle for beef and dairy, sheep, goats, pigs, and poultry. Various other species are sometimes considered livestock, such as horses, while poultry birds are sometimes excluded. In some parts of the world, livestock includes species such as buffalo, and the South American camelids, the alpaca and llama. Some authorities use much broader definitions to include fish in aquaculture, micro-livestock such as rabbits and rodents like guinea pigs, as well as insects from honey bees to crickets raised for human consumption.
Products
Animals are raised for a wide variety of products, principally meat, wool, milk, and eggs, but also including tallow, isinglass and rennet. Animals are also kept for more specialised purposes, such as to produce vaccines and antiserum (containing antibodies) for medical use. Where fodder or other crops are grown alongside animals, manure can serve as a fertiliser, returning minerals and organic matter to the soil in a semi-closed organic system.
Branches
Dairy
Although all mammals produce milk to nourish their young, the cow is predominantly used throughout the world to produce milk and milk products for human consumption. Other animals used to a lesser extent for this purpose include sheep, goats, camels, buffaloes, yaks, reindeer, horses and donkeys.
All these animals have been domesticated over the centuries, being bred for such desirable characteristics as fecundity, productivity, docility and the ability to thrive under the prevailing conditions. Whereas in the past cattle had multiple functions, modern dairy cow breeding has resulted in specialised Holstein Friesian-type animals that produce large quantities of milk economically. Artificial insemination is widely available to allow farmers to select for the particular traits that suit their circumstances.
Whereas in the past cows were kept in small herds on family farms, grazing pastures and being fed hay in winter, nowadays there is a trend towards larger herds, more intensive systems, the feeding of silage and "zero grazing", a system where grass is cut and brought to the cow, which is housed year-round.
In many communities, milk production is only part of the purpose of keeping an animal which may also be used as a beast of burden or to draw a plough, or for the production of fibre, meat and leather, with the dung being used for fuel or for the improvement of soil fertility. Sheep and goats may be favoured for dairy production in climates and conditions that do not suit dairy cows.
Meat
Meat, mainly from farmed animals, is a major source of dietary protein and essential nutrients around the world, averaging about 8% of man's energy intake. The actual types eaten depend on local preferences, availability, cost and other factors, with cattle, sheep, pigs and goats being the main species involved. Cattle generally produce a single offspring annually which takes more than a year to mature; sheep and goats often have twins and these are ready for slaughter in less than a year; pigs are more prolific, producing more than one litter of up to about 11 piglets each year. Horses, donkeys, deer, buffalo, llamas, alpacas, guanacos and vicunas are farmed for meat in various regions. Some desirable traits of animals raised for meat include fecundity, hardiness, fast growth rate, ease of management and high food conversion efficiency. About half of the world's meat is produced from animals grazing on open ranges or on enclosed pastures, the other half being produced intensively in various factory-farming systems; these are mostly cows, pigs or poultry, and often reared indoors, typically at high densities.
Poultry
Poultry, kept for their eggs and for their meat, include chickens, turkeys, geese and ducks. The great majority of laying birds used for egg production are chickens. Methods for keeping layers range from free-range systems, where the birds can roam as they will but are housed at night for their own protection, through semi-intensive systems where they are housed in barns and have perches, litter and some freedom of movement, to intensive systems where they are kept in cages. The battery cages are arranged in long rows in multiple tiers, with external feeders, drinkers, and egg collection facilities. This is the most labour saving and economical method of egg production but has been criticised on animal welfare grounds as the birds are unable to exhibit their normal behaviours.
In the developed world, the majority of the poultry reared for meat is raised indoors in big sheds, with automated equipment under environmentally controlled conditions. Chickens raised in this way are known as broilers, and genetic improvements have meant that they can be grown to slaughter weight within six or seven weeks of hatching. Newly hatched chicks are restricted to a small area and given supplementary heating. Litter on the floor absorbs the droppings and the area occupied is expanded as they grow. Feed and water is supplied automatically and the lighting is controlled. The birds may be harvested on several occasions or the whole shed may be cleared at one time.
A similar rearing system is usually used for turkeys, which are less hardy than chickens, but they take longer to grow and are often moved on to separate fattening units to finish. Ducks are particularly popular in Asia and Australia and can be killed at seven weeks under commercial conditions.
Aquaculture
Aquaculture has been defined as "the farming of aquatic organisms including fish, molluscs, crustaceans and aquatic plants and implies some form of intervention in the rearing process to enhance production, such as regular stocking, feeding, protection from predators, etc. Farming also implies individual or corporate ownership of the stock being cultivated." In practice it can take place in the sea or in freshwater, and be extensive or intensive. Whole bays, lakes or ponds may be devoted to aquaculture, or the farmed animal may be retained in cages (fish), artificial reefs, racks or strings (shellfish). Fish and prawns can be cultivated in rice paddies, either arriving naturally or being introduced, and both crops can be harvested together.
Fish hatcheries provide larval and juvenile fish, crustaceans and shellfish, for use in aquaculture systems. When large enough these are transferred to growing-on tanks and sold to fish farms to reach harvest size. Some species that are commonly raised in hatcheries include shrimps, prawns, salmon, tilapia, oysters and scallops. Similar facilities can be used to raise species with conservation needs to be released into the wild, or game fish for restocking waterways. Important aspects of husbandry at these early stages include selection of breeding stock, control of water quality and nutrition. In the wild, there is a massive amount of mortality at the nursery stage; farmers seek to minimise this while at the same time maximising growth rates.
Insects
Bees have been kept in hives since at least the First Dynasty of Egypt, five thousand years ago, and man had been harvesting honey from the wild long before that. Fixed comb hives are used in many parts of the world and are made from any locally available material. In more advanced economies, where modern strains of domestic bee have been selected for docility and productiveness, various designs of hive are used which enable the combs to be removed for processing and extraction of honey. Quite apart from the honey and wax they produce, honey bees are important pollinators of crops and wild plants, and in many places hives are transported around the countryside to assist in pollination.
Sericulture, the rearing of silkworms, was first adopted by the Chinese during the Shang dynasty. The only species farmed commercially is the domesticated silkmoth. When it spins its cocoon, each larva produces an exceedingly long, slender thread of silk. The larvae feed on mulberry leaves and in Europe, only one generation is normally raised each year as this is a deciduous tree. In China, Korea and Japan however, two generations are normal, and in the tropics, multiple generations are expected. Most production of silk occurs in the Far East, with a synthetic diet being used to rear the silkworms in Japan.
Insects form part of the human diet in many cultures. In Thailand, crickets are farmed for this purpose in the north of the country, and palm weevil larvae in the south. The crickets are kept in pens, boxes or drawers and fed on commercial pelleted poultry food, while the palm weevil larvae live on cabbage palm and sago palm trees, which limits their production to areas where these trees grow. Another delicacy of this region is the bamboo caterpillar, and the best rearing and harvesting techniques in semi-natural habitats are being studied.
Effects
Environmental impact
Animal husbandry has a significant impact on the world environment. Both production and consumption of animal products have increased rapidly. Since 1950, meat production has tripled, whereas the production of dairy products doubled and that of eggs almost increased fourfold. Meanwhile, meat consumption has nearly doubled worldwide. Developing countries had a surge in meat consumption, particularly of monogastric livestock. Animal husbandry drives climate change, ocean acidification, and biodiversity loss, and kills 60 billion animals annually. It uses between 20 and 33% of the world's fresh water, Livestock, and the production of feed for them, occupy about a third of the Earth's ice-free land. Livestock production contributes to species extinction, desertification, and habitat destruction. and is the primary driver of the Holocene extinction. Some 70% of the agricultural land and 30% of Earth's land surface is involved directly or indirectly in animal husbandry. Habitat is destroyed by clearing forests and converting land to grow feed crops and for grazing, while predators and herbivores are frequently targeted because of a perceived threat to livestock profits; for example, animal husbandry causes up to 91% of the deforestation in the Amazon region. In addition, livestock produce greenhouse gases. Cows produce some 570 million cubic metres of methane per day, that accounts for 35 to 40% of the overall methane emissions of the planet. Further, livestock production is responsible for 65% of all human-related emissions of nitrous oxide.
Animal welfare
Since the 18th century, people have become increasingly concerned about the welfare of farm animals. Possible measures of welfare include longevity, behavior, physiology, reproduction, freedom from disease, and freedom from immunosuppression. Standards and laws for animal welfare have been created worldwide, broadly in line with the most widely held position in the western world, a form of utilitarianism: that it is morally acceptable for humans to use non-human animals, provided that no unnecessary suffering is caused, and that the benefits to humans outweigh the costs to the livestock. An opposing view is that animals have rights, should not be regarded as property, are not necessary to use, and should never be used by humans. Live export of animals has risen to meet increased global demand for livestock such as in the Middle East. Animal rights activists have objected to long-distance transport of animals; one result was the banning of live exports from New Zealand in 2003.
In culture
Since the 18th century, the farmer John Bull has represented English national identity, first in John Arbuthnot's political satires, and soon afterwards in cartoons by James Gillray and others including John Tenniel. He likes food, beer, dogs, horses, and country sports; he is practical and down to earth, and anti-intellectual.
Farm animals are widespread in books and songs for children; the reality of animal husbandry is often distorted, softened, or idealized, giving children an almost entirely fictitious account of farm life. The books often depict happy animals free to roam in attractive countryside, a picture completely at odds with the realities of the impersonal, mechanized activities involved in modern intensive farming.
Pigs, for example, appear in several of Beatrix Potter's "little books", as Piglet in A.A. Milne's Winnie the Pooh stories, and somewhat more darkly (with a hint of animals going to slaughter) as Babe in Dick King-Smith's The Sheep-Pig, and as Wilbur in E. B. White's Charlotte's Web. Pigs tend to be "bearers of cheerfulness, good humour and innocence". Many of these books are completely anthropomorphic, dressing farm animals in clothes and having them walk on two legs, live in houses, and perform human activities. The children's song "Old MacDonald Had a Farm" describes a farmer named MacDonald and the various animals he keeps, celebrating the noises they each make.
Many urban children experience animal husbandry for the first time at a petting farm; in Britain, some five million people a year visit a farm of some kind. This presents some risk of infection, especially if children handle animals and then fail to wash their hands; a strain of E. coli infected 93 people who had visited a British interactive farm in an outbreak in 2009. Historic farms such as those in the United States offer farmstays and "a carefully curated version of farming to those willing to pay for it", sometimes giving visitors a romanticised image of a pastoral idyll from an unspecified time in the pre-industrial past.
| Technology | Food and health | null |
219664 | https://en.wikipedia.org/wiki/Titania%20%28moon%29 | Titania (moon) | Titania (), also designated Uranus III, is the largest moon of Uranus. At a diameter of it is the eighth largest moon in the Solar System, with a surface area comparable to that of Australia. Discovered by William Herschel in 1787, it is named after the queen of the fairies in Shakespeare's A Midsummer Night's Dream. Its orbit lies inside Uranus's magnetosphere.
Titania consists of approximately equal amounts of ice and rock, and is probably differentiated into a rocky core and an icy mantle. A layer of liquid water may be present at the core–mantle boundary. Its surface, which is relatively dark and slightly red in color, appears to have been shaped by both impacts and endogenic processes. It is covered with numerous impact craters reaching up to in diameter, but is less heavily cratered than Oberon, outermost of the five large moons of Uranus. It may have undergone an early endogenic resurfacing event which obliterated its older, heavily cratered surface. Its surface is cut by a system of enormous canyons and scarps, the result of the expansion of its interior during the later stages of its evolution. Like all major moons of Uranus, Titania probably formed from an accretion disk which surrounded the planet just after its formation.
Infrared spectroscopy conducted from 2001 to 2005 revealed the presence of water ice as well as frozen carbon dioxide on Titania's surface, suggesting it may have a tenuous carbon dioxide atmosphere with a surface pressure of about 10 nanopascals (10−13 bar). Measurements during Titania's occultation of a star put an upper limit on the surface pressure of any possible atmosphere at 1–2 mPa (10–20 nbar). The Uranian system has been studied up close only once, by the spacecraft Voyager 2 in January 1986. It took several images of Titania, which allowed mapping of about 40% of its surface.
Discovery and naming
Titania was discovered by William Herschel on January 11, 1787, the same day he discovered Uranus's second largest moon, Oberon. He later reported the discoveries of four more satellites, although they were subsequently revealed as spurious. For nearly the next 50 years, Titania and Oberon would not be observed by any instrument other than William Herschel's, although the moon can be seen from Earth with a present-day high-end amateur telescope.
All of Uranus's moons are named after characters created by William Shakespeare or Alexander Pope. The name Titania was taken from the Queen of the Fairies in A Midsummer Night's Dream. The names of all four satellites of Uranus then known were suggested by Herschel's son John in 1852, at the request of William Lassell, who had discovered the other two moons, Ariel and Umbriel, the year before. It is uncertain if Herschel devised the names, or if Lassell did so and then sought Herschel's permission.
Titania was initially referred to as "the first satellite of Uranus", and in 1848 was given the designation by William Lassell, although he sometimes used William Herschel's numbering (where Titania and Oberon are II and IV). In 1851 Lassell eventually numbered all four known satellites in order of their distance from the planet by Roman numerals, and since then Titania has been designated .
Shakespeare's character's name is pronounced , but the moon is often pronounced , by analogy with the familiar chemical element titanium. The adjectival form, Titanian, is homonymous with that of Saturn's moon Titan. The name Titania is ancient Greek for "Daughter of the Titans".
Orbit
Titania orbits Uranus at the distance of about , being the second farthest from the planet among its five major moons after Oberon. Titania's orbit has a small eccentricity and is inclined very little relative to the equator of Uranus. Its orbital period is around 8.7 days, coincident with its rotational period. In other words, Titania is a synchronous or tidally locked satellite, with one face always pointing toward the planet.
Titania's orbit lies completely inside the Uranian magnetosphere. This is important, because the trailing hemispheres of satellites orbiting inside a magnetosphere are struck by magnetospheric plasma, which co-rotates with the planet. This bombardment may lead to the darkening of the trailing hemispheres, which is actually observed for all Uranian moons except Oberon (see below).
Because Uranus orbits the Sun almost on its side, and its moons orbit in the planet's equatorial plane, they (including Titania) are subject to an extreme seasonal cycle. Both northern and southern poles spend 42 years in a complete darkness, and another 42 years in continuous sunlight, with the sun rising close to the zenith over one of the poles at each solstice. The Voyager 2 flyby coincided with the southern hemisphere's 1986 summer solstice, when nearly the entire southern hemisphere was illuminated. Once every 42 years, when Uranus has an equinox and its equatorial plane intersects the Earth, mutual occultations of Uranus's moons become possible. In 2007–2008 a number of such events were observed including two occultations of Titania by Umbriel on August 15 and December 8, 2007.
Composition and internal structure
[[File:PIA00039 Titania.jpg|thumb|left|Voyager 2'''s highest-resolution image of Titania shows moderately cratered plains, enormous rifts and long scarps. Near the bottom, a region of smoother plains including the crater Ursula is split by the graben Belmont Chasma.|alt=A round spherical body with its left half illuminated. The surface has a mottled appearance with bright patches among relatively dark terrain. The terminator is slightly to the right from the center and runs from the top to bottom. A large crater with a central pit can be seen at the terminator in the upper half of the image. Another bright crater can be seen at the bottom intersected by a canyon. The second large canyon runs from the darkness at the lower-right side to visible center of the body.]]
Titania is the largest and most massive Uranian moon, the eighth most massive moon in the Solar System, and the 20th largest object in the Solar System. Its density of 1.68 g/cm3, which is much higher than the typical density of Saturn's satellites, indicates that it consists of roughly equal proportions of water ice and dense non-ice components; the latter could be made of rock and carbonaceous material including heavy organic compounds. The presence of water ice is supported by infrared spectroscopic observations made in 2001–2005, which have revealed crystalline water ice on the surface of the moon. Water ice absorption bands are slightly stronger on Titania's leading hemisphere than on the trailing hemisphere. This is the opposite of what is observed on Oberon, where the trailing hemisphere exhibits stronger water ice signatures. The cause of this asymmetry is not known, but it may be related to the bombardment by charged particles from the magnetosphere of Uranus, which is stronger on the trailing hemisphere (due to the plasma's co-rotation). The energetic particles tend to sputter water ice, decompose methane trapped in ice as clathrate hydrate and darken other organics, leaving a dark, carbon-rich residue behind.
Except for water, the only other compound identified on the surface of Titania by infrared spectroscopy is carbon dioxide, which is concentrated mainly on the trailing hemisphere. The origin of the carbon dioxide is not completely clear. It might be produced locally from carbonates or organic materials under the influence of the solar ultraviolet radiation or energetic charged particles coming from the magnetosphere of Uranus. The latter process would explain the asymmetry in its distribution, because the trailing hemisphere is subject to a more intense magnetospheric influence than the leading hemisphere. Another possible source is the outgassing of the primordial CO2 trapped by water ice in Titania's interior. The escape of CO2 from the interior may be related to the past geological activity on this moon.
Titania may be differentiated into a rocky core surrounded by an icy mantle. If this is the case, the radius of the core is about 66% of the radius of the moon, and its mass is around 58% of the moon's mass—the proportions are dictated by moon's composition. The pressure in the center of Titania is about 0.58 GPa (5.8 kbar). The current state of the icy mantle is unclear. If the ice contains enough ammonia or other antifreeze, Titania may have a subsurface ocean at the core–mantle boundary. The thickness of this ocean, if it exists, is up to and its temperature is around 190 K (close to the water–ammonia eutectic temperature of 176 K). However the present internal structure of Titania depends heavily on its thermal history, which is poorly known. Recent studies suggest, contrary to earlier theories, that Uranus largest moons like Titania in fact could have active subsurface oceans.
Surface features
Among Uranus's moons, Titania is intermediate in brightness between the dark Oberon and Umbriel and the bright Ariel and Miranda. Its surface shows a strong opposition surge: its reflectivity decreases from 35% at a phase angle of 0° (geometrical albedo) to 25% at an angle of about 1°. Titania has a relatively low Bond albedo of about 17%. Its surface is generally slightly red in color, but less red than that of Oberon. However, fresh impact deposits are bluer, while the smooth plains situated on the leading hemisphere near Ursula crater and along some grabens are somewhat redder. There may be an asymmetry between the leading and trailing hemispheres; the former appears to be redder than the latter by 8%. However, this difference is related to the smooth plains and may be accidental. The reddening of the surfaces probably results from space weathering caused by bombardment by charged particles and micrometeorites over the age of the Solar System. However, the color asymmetry of Titania is more likely related to accretion of a reddish material coming from outer parts of the Uranian system, possibly, from irregular satellites, which would be deposited predominately on the leading hemisphere.
Scientists have recognized three classes of geological feature on Titania: craters, chasmata (canyons) and rupes (scarps). The surface of Titania is less heavily cratered than the surfaces of either Oberon or Umbriel, which means that the surface is much younger. The crater diameters reach 326 kilometers for the largest known crater, Gertrude (there can be also a degraded basin of approximately the same size). Some craters (for instance, Ursula and Jessica) are surrounded by bright impact ejecta (rays) consisting of relatively fresh ice. All large craters on Titania have flat floors and central peaks. The only exception is Ursula, which has a pit in the center. To the west of Gertrude there is an area with irregular topography, the so-called "unnamed basin", which may be another highly degraded impact basin with the diameter of about .
Titania's surface is intersected by a system of enormous faults, or scarps. In some places, two parallel scarps mark depressions in the satellite's crust, forming grabens, which are sometimes called canyons. The most prominent among Titania's canyons is Messina Chasma, which runs for about from the equator almost to the south pole. The grabens on Titania are wide and have a relief of about 2–5 km. The scarps that are not related to canyons are called rupes, such as Rousillon Rupes near Ursula crater. The regions along some scarps and near Ursula appear smooth at Voyager's image resolution. These smooth plains were probably resurfaced later in Titania's geological history, after the majority of craters formed. The resurfacing may have been either endogenic in nature, involving the eruption of fluid material from the interior (cryovolcanism), or, alternatively it may be due to blanking by the impact ejecta from nearby large craters. The grabens are probably the youngest geological features on Titania—they cut all craters and even smooth plains.
The geology of Titania was influenced by two competing forces: impact crater formation and endogenic resurfacing. The former acted over the moon's entire history and influenced all surfaces. The latter processes were also global in nature, but active mainly for a period following the moon's formation. They obliterated the original heavily cratered terrain, explaining the relatively low number of impact craters on the moon's present-day surface. Additional episodes of resurfacing may have occurred later and led to the formation of smooth plains. Alternatively smooth plains may be ejecta blankets of the nearby impact craters. The most recent endogenous processes were mainly tectonic in nature and caused the formation of the canyons, which are actually giant cracks in the ice crust. The cracking of the crust was caused by the global expansion of Titania by about 0.7%.
Atmosphere
The presence of carbon dioxide on the surface suggests that Titania may have a tenuous seasonal atmosphere of CO2, much like that of the Jovian moon Callisto. Other gases, like nitrogen or methane, are unlikely to be present, because Titania's weak gravity could not prevent them from escaping into space. At the maximum temperature attainable during Titania's summer solstice (89 K), the vapor pressure of carbon dioxide is about 300 μPa (3 nbar).
On September 8, 2001, Titania occulted a bright star (HIP 106829) with a visible magnitude of 7.2; this was an opportunity to both refine Titania's diameter and ephemeris, and to detect any extant atmosphere. The data revealed no atmosphere to a surface pressure of 1–2 mPa (10–20 nbar); if it exists, it would have to be far thinner than that of Triton or Pluto. This upper limit is still several times higher than the maximum possible surface pressure of the carbon dioxide, meaning that the measurements place essentially no constraints on parameters of the atmosphere.
The peculiar geometry of the Uranian system causes the moons' poles to receive more solar energy than their equatorial regions. Because the vapor pressure of CO2 is a steep function of temperature, this may lead to the accumulation of carbon dioxide in the low-latitude regions of Titania, where it can stably exist on high albedo patches and shaded regions of the surface in the form of ice. During the summer, when the polar temperatures reach as high as 85–90 K, carbon dioxide sublimates and migrates to the opposite pole and to the equatorial regions, giving rise to a type of carbon cycle. The accumulated carbon dioxide ice can be removed from cold traps by magnetospheric particles, which sputter it from the surface. Titania is thought to have lost a significant amount of carbon dioxide since its formation 4.6 billion years ago.
Origin and evolution
Titania is thought to have formed from an accretion disc or subnebula; a disc of gas and dust that either existed around Uranus for some time after its formation or was created by the giant impact that most likely gave Uranus its large obliquity. The precise composition of the subnebula is not known; however, the relatively high density of Titania and other Uranian moons compared to the moons of Saturn indicates that it may have been relatively water-poor. Significant amounts of nitrogen and carbon may have been present in the form of carbon monoxide and N2 instead of ammonia and methane. The moons that formed in such a subnebula would contain less water ice (with CO and N2 trapped as a clathrate) and more rock, explaining their higher density.
Titania's accretion probably lasted for several thousand years. The impacts that accompanied accretion caused heating of the moon's outer layer. The maximum temperature of around was reached at a depth of about . After the end of formation, the subsurface layer cooled, while the interior of Titania heated due to decay of radioactive elements present in its rocks. The cooling near-surface layer contracted, while the interior expanded. This caused strong extensional stresses in the moon's crust leading to cracking. Some of the present-day canyons may be a result of this. The process lasted for about 200 million years, implying that any endogenous activity ceased billions of years ago.
The initial accretional heating together with continued decay of radioactive elements were probably strong enough to melt the ice if some antifreeze like ammonia (in the form of ammonia hydrate) or salt was present. Further melting may have led to the separation of ice from rocks and formation of a rocky core surrounded by an icy mantle. A layer of liquid water (ocean) rich in dissolved ammonia may have formed at the core–mantle boundary. The eutectic temperature of this mixture is . If the temperature dropped below this value, the ocean would have subsequently frozen. The freezing of the water would have caused the interior to expand, which may have been responsible for the formation of the majority of the canyons. However, the present knowledge of Titania's geological evolution is quite limited. Whereas more up to date analysis suggest that larger moons of Uranus are not only capable of having active subsurface oceans; but in fact; presumed to have subterranean oceans beneath them.
Exploration
So far the only close-up images of Titania have been from the Voyager 2 probe, which photographed the moon during its flyby of Uranus in January 1986. Since the closest distance between Voyager 2 and Titania was only , the best images of this moon have a spatial resolution of about 3.4 km (only Miranda and Ariel were imaged with a better resolution). The images cover about 40% of the surface, but only 24% was photographed with the precision required for geological mapping. At the time of the flyby, the southern hemisphere of Titania (like those of the other moons) was pointed towards the Sun, so the northern (dark) hemisphere could not be studied.
No other spacecraft has ever visited the Uranian system or Titania. One possibility, now discarded, was to send Cassini on from Saturn to Uranus in an extended mission. Another mission concept proposed was the Uranus orbiter and probe concept, evaluated around 2010. Uranus was also examined as part of one trajectory for a precursor interstellar probe concept, Innovative Interstellar Explorer.
The Uranus Orbiter and Probe mission architecture was identified as the highest priority for a NASA Flagship mission by the 2023–2032 Planetary Science Decadal Survey. The science questions motivating this prioritization include questions about the Uranian satellites' bulk properties, internal structure, and geologic history. A Uranus orbiter had previously been listed as the third priority for a NASA Flagship mission by the 2013–2022 Planetary Science Decadal Survey.
| Physical sciences | Solar System | Astronomy |
219713 | https://en.wikipedia.org/wiki/Titanium%20dioxide | Titanium dioxide | Titanium dioxide, also known as titanium(IV) oxide or titania , is the inorganic compound derived from titanium with the chemical formula . When used as a pigment, it is called titanium white, Pigment White 6 (PW6), or CI 77891. It is a white solid that is insoluble in water, although mineral forms can appear black. As a pigment, it has a wide range of applications, including paint, sunscreen, and food coloring. When used as a food coloring, it has E number E171. World production in 2014 exceeded 9 million tonnes. It has been estimated that titanium dioxide is used in two-thirds of all pigments, and pigments based on the oxide have been valued at a price of $13.2 billion.
Structure
In all three of its main dioxides, titanium exhibits octahedral geometry, being bonded to six oxide anions. The oxides in turn are bonded to three Ti centers. The overall crystal structures of rutile and anatase are tetragonal in symmetry whereas brookite is orthorhombic. The oxygen substructures are all slight distortions of close packing: in rutile, the oxide anions are arranged in distorted hexagonal close-packing, whereas they are close to cubic close-packing in anatase and to "double hexagonal close-packing" for brookite. The rutile structure is widespread for other metal dioxides and difluorides, e.g. RuO2 and ZnF2.
Molten titanium dioxide has a local structure in which each Ti is coordinated to, on average, about 5 oxygen atoms. This is distinct from the crystalline forms in which Ti coordinates to 6 oxygen atoms.
Synthetic and geologic occurrence
Synthetic TiO2 is mainly produced from the mineral ilmenite. Rutile, and anatase, naturally occurring TiO2, occur widely also, e.g. rutile as a 'heavy mineral' in beach sand. Leucoxene, fine-grained anatase formed by natural alteration of ilmenite, is yet another ore. Star sapphires and rubies get their asterism from oriented inclusions of rutile needles.
Mineralogy and uncommon polymorphs
Titanium dioxide occurs in nature as the minerals rutile and anatase. Additionally two high-pressure forms are known minerals: a monoclinic baddeleyite-like form known as akaogiite, and the other has a slight monoclinic distortion of the orthorhombic α-PbO2 structure and is known as riesite. Both of which can be found at the Ries crater in Bavaria. It is mainly sourced from ilmenite, which is the most widespread titanium dioxide-bearing ore around the world. Rutile is the next most abundant and contains around 98% titanium dioxide in the ore. The metastable anatase and brookite phases convert irreversibly to the equilibrium rutile phase upon heating above temperatures in the range .
Titanium dioxide has twelve known polymorphs – in addition to rutile, anatase, brookite, akaogiite and riesite, three metastable phases can be produced synthetically (monoclinic, tetragonal, and orthorhombic ramsdellite-like), and four high-pressure forms (α-PbO2-like, cotunnite-like, orthorhombic OI, and cubic phases) also exist:
The cotunnite-type phase was claimed to be the hardest known oxide with the Vickers hardness of 38 GPa and the bulk modulus of 431 GPa (i.e. close to diamond's value of 446 GPa) at atmospheric pressure. However, later studies came to different conclusions with much lower values for both the hardness (7–20 GPa, which makes it softer than common oxides like corundum Al2O3 and rutile TiO2) and bulk modulus (~300 GPa).
Titanium dioxide (B) is found as a mineral in magmatic rocks and hydrothermal veins, as well as weathering rims on perovskite. TiO2 also forms lamellae in other minerals.
Production
The largest pigment processors are Chemours, Venator, , and Tronox. Major paint and coating company end users for pigment grade titanium dioxide include Akzo Nobel, PPG Industries, Sherwin Williams, BASF, Kansai Paints and Valspar. Global pigment demand for 2010 was 5.3 Mt with annual growth expected to be about 3–4%.
The production method depends on the feedstock. In addition to ores, other feedstocks include upgraded slag. Both the chloride process and the sulfate process (both described below) produce titanium dioxide pigment in the rutile crystal form, but the sulfate process can be adjusted to produce the anatase form. Anatase, being softer, is used in fiber and paper applications. The sulfate process is run as a batch process; the chloride process is run as a continuous process.
Chloride process
In chloride process, the ore is treated with chlorine and carbon to give titanium tetrachloride, a volatile liquid that is further purified by distillation. The TiCl4 is treated with oxygen to regenerate chlorine and produce the titanium dioxide.
Sulfate process
In the sulfate process, ilmenite is treated with sulfuric acid to extract iron(II) sulfate pentahydrate. This process requires concentrated ilmenite (45–60% TiO2) or pretreated feedstocks as a suitable source of titanium. The resulting synthetic rutile is further processed according to the specifications of the end user, i.e. pigment grade or otherwise.
Examples of plants using the sulfate process are the Sorel-Tracy plant of QIT-Fer et Titane and the Eramet Titanium & Iron smelter in Tyssedal Norway.
Becher process
The Becher process is another method for the production of synthetic rutile from ilmenite. It first oxidizes the ilmenite as a means to separate the iron component.
Specialized methods
For specialty applications, TiO2 films are prepared by various specialized chemistries. Sol-gel routes involve the hydrolysis of titanium alkoxides such as titanium ethoxide:
Ti(OEt)4 + 2 H2O → TiO2 + 4 EtOH
A related approach that also relies on molecular precursors involves chemical vapor deposition. In this method, the alkoxide is volatilized and then decomposed on contact with a hot surface:
Ti(OEt)4 → TiO2 + 2 Et2O
Applications
Pigment
First mass-produced in 1916, titanium dioxide is the most widely used white pigment because of its brightness and very high refractive index, in which it is surpassed only by a few other materials (see list of indices of refraction). Titanium dioxide crystal size is ideally around 220 nm (measured by electron microscope) to optimize the maximum reflection of visible light. However, abnormal grain growth is often observed in titanium dioxide, particularly in its rutile phase. The occurrence of abnormal grain growth brings about a deviation of a small number of crystallites from the mean crystal size and modifies the physical behaviour of TiO2. The optical properties of the finished pigment are highly sensitive to purity. As little as a few parts per million (ppm) of certain metals (Cr, V, Cu, Fe, Nb) can disturb the crystal lattice so much that the effect can be detected in quality control. Approximately 4.6 million tons of pigmentary TiO2 are used annually worldwide, and this number is expected to increase as use continues to rise.
TiO2 is also an effective opacifier in powder form, where it is employed as a pigment to provide whiteness and opacity to products such as paints, coatings, plastics, papers, inks, foods, supplements, medicines (i.e. pills and tablets), and most toothpastes; in 2019 it was present in two-thirds of toothpastes on the French market. In paint, it is often referred to offhandedly as "brilliant white", "the perfect white", "the whitest white", or other similar terms. Opacity is improved by optimal sizing of the titanium dioxide particles.
Food additive
In food, it is commonly found in ice creams, chocolates, all types of candy, creamers, desserts, marshmallows, chewing gum, pastries, spreads, dressings, cakes, some cheeses, and many other foods.
Thin films
When deposited as a thin film, its refractive index and colour make it an excellent reflective optical coating for dielectric mirrors; it is also used in generating decorative thin films such as found in "mystic fire topaz".
Some grades of modified titanium based pigments as used in sparkly paints, plastics, finishes and cosmetics – these are man-made pigments whose particles have two or more layers of various oxides – often titanium dioxide, iron oxide or alumina – in order to have glittering, iridescent and or pearlescent effects similar to crushed mica or guanine-based products. In addition to these effects a limited colour change is possible in certain formulations depending on how and at which angle the finished product is illuminated and the thickness of the oxide layer in the pigment particle; one or more colours appear by reflection while the other tones appear due to interference of the transparent titanium dioxide layers. In some products, the layer of titanium dioxide is grown in conjunction with iron oxide by calcination of titanium salts (sulfates, chlorates) around 800 °C One example of a pearlescent pigment is Iriodin, based on mica coated with titanium dioxide or iron (III) oxide.
The iridescent effect in these titanium oxide particles is unlike the opaque effect obtained with usual ground titanium oxide pigment obtained by mining, in which case only a certain diameter of the particle is considered and the effect is due only to scattering.
Sunscreen and UV blocking pigments
In cosmetic and skin care products, titanium dioxide is used as a pigment, sunscreen and a thickener. As a sunscreen, ultrafine TiO2 is used, which is notable in that combined with ultrafine zinc oxide, it is considered to be an effective sunscreen that lowers the incidence of sun burns and minimizes the premature photoaging, photocarcinogenesis and immunosuppression associated with long term excess sun exposure. Sometimes these UV blockers are combined with iron oxide pigments in sunscreen to increase visible light protection.
Titanium dioxide and zinc oxide are generally considered to be less harmful to coral reefs than sunscreens that include chemicals such as oxybenzone, octocrylene and octinoxate.
Nanosized titanium dioxide is found in the majority of physical sunscreens because of its strong UV light absorbing capabilities and its resistance to discolouration under ultraviolet light. This advantage enhances its stability and ability to protect the skin from ultraviolet light. Nano-scaled (particle size of 20–40 nm) titanium dioxide particles are primarily used in sunscreen lotion because they scatter visible light much less than titanium dioxide pigments, and can give UV protection. Sunscreens designed for infants or people with sensitive skin are often based on titanium dioxide and/or zinc oxide, as these mineral UV blockers are believed to cause less skin irritation than other UV absorbing chemicals. Nano-TiO2, which blocks both UV-A and UV-B radiation, is used in sunscreens and other cosmetic products.
The EU Scientific Committee on Consumer Safety considered nano sized titanium dioxide to be safe for skin applications, in concentrations of up to 25 percent based on animal testing. The risk assessment of different titanium dioxide nanomaterials in sunscreen is currently evolving since nano-sized TiO2 is different from the well-known micronized form. The rutile form is generally used in cosmetic and sunscreen products due to it not possessing any observed ability to damage the skin under normal conditions and having a higher UV absorption. In 2016 Scientific Committee on Consumer Safety (SCCS) tests concluded that the use of nano titanium dioxide (95–100% rutile, ≦5% anatase) as a UV filter can be considered to not pose any risk of adverse effects in humans post-application on healthy skin, except in the case the application method would lead to substantial risk of inhalation (ie; powder or spray formulations). This safety opinion applied to nano TiO2 in concentrations of up to 25%.
Initial studies indicated that nano-TiO2 particles could penetrate the skin, causing concern over its use. These studies were later refuted, when it was discovered that the testing methodology couldn't differentiate between penetrated particles and particles simply trapped in hair follicles and that having a diseased or physically damaged dermis could be the true cause of insufficient barrier protection.
SCCS research found that when nanoparticles had certain photostable coatings (e.g., alumina, silica, cetyl phosphate, triethoxycaprylylsilane, manganese dioxide), the photocatalytic activity was attenuated and no notable skin penetration was observed; the sunscreen in this research was applied at amounts of 10 mg/cm2 for exposure periods of 24 hours. Coating TiO2 with alumina, silica, zircon or various polymers can minimize avobenzone degradation and enhance UV absorption by adding an additional light diffraction mechanism.
is used extensively in plastics and other applications as a white pigment or an opacifier and for its UV resistant properties where the powder disperses light – unlike organic UV absorbers – and reduces UV damage, due mostly to the particle's high refractive index.
Other uses of titanium dioxide
In ceramic glazes, titanium dioxide acts as an opacifier and seeds crystal formation.
It is used as a tattoo pigment and in styptic pencils. Titanium dioxide is produced in varying particle sizes which are both oil and water dispersible, and in certain grades for the cosmetic industry. It is also a common ingredient in toothpaste.
The exterior of the Saturn V rocket was painted with titanium dioxide; this later allowed astronomers to determine that J002E3 was likely the S-IVB stage from Apollo 12 and not an asteroid.
Titanium dioxide is an n-type semiconductor and is used in dye-sensitized solar cells. It is also used in other electronics components such as electrodes in batteries.
Research
Patenting activities
Between 2002 and 2022, there were 459 patent families that describe the production of titanium dioxide from ilmenite. The majority of these patents describe pre-treatment processes, such as using smelting and magnetic separation to increase titanium concentration in low-grade ores, leading to titanium concentrates or slags. Other patents describe processes to obtain titanium dioxide, either by a direct hydrometallurgical process or through the main industrial production processes, the sulfate process and the chloride process. The sulfate process represents 40% of the world’s titanium dioxide production and is protected in 23% of patent families. The chloride process is only mentioned in 8% of patent families, although it provides 60% of the worldwide industrial production of titanium dioxide.
Key contributors to patents on the production of titanium dioxide are companies from China, Australia and the United States, reflecting the major contribution of these countries to industrial production. Chinese companies Pangang and Lomon Billions Groups hold major patent portfolios.
Photocatalyst
Nanosized titanium dioxide, particularly in the anatase form, exhibits photocatalytic activity under ultraviolet (UV) irradiation. This photoactivity is reportedly most pronounced at the {001} planes of anatase, although the {101} planes are thermodynamically more stable and thus more prominent in most synthesised and natural anatase, as evident by the often observed tetragonal dipyramidal growth habit. Interfaces between rutile and anatase are further considered to improve photocatalytic activity by facilitating charge carrier separation and as a result, biphasic titanium dioxide is often considered to possess enhanced functionality as a photocatalyst. It has been reported that titanium dioxide, when doped with nitrogen ions or doped with metal oxide like tungsten trioxide, exhibits excitation also under visible light. The strong oxidative potential of the positive holes oxidizes water to create hydroxyl radicals. It can also oxidize oxygen or organic materials directly. Hence, in addition to its use as a pigment, titanium dioxide can be added to paints, cements, windows, tiles, or other products for its sterilizing, deodorizing, and anti-fouling properties, and is used as a hydrolysis catalyst. It is also used in dye-sensitized solar cells, which are a type of chemical solar cell (also known as a Graetzel cell).
The photocatalytic properties of nanosized titanium dioxide were discovered by Akira Fujishima in 1967 and published in 1972. The process on the surface of the titanium dioxide was called the . In thin film and nanoparticle form, titanium dioxide has the potential for use in energy production: As a photocatalyst, it can break water into hydrogen and oxygen. With the hydrogen collected, it could be used as a fuel. The efficiency of this process can be greatly improved by doping the oxide with carbon. Further efficiency and durability has been obtained by introducing disorder to the lattice structure of the surface layer of titanium dioxide nanocrystals, permitting infrared absorption. Visible-light-active nanosized anatase and rutile has been developed for photocatalytic applications.
In 1995 Fujishima and his group discovered the superhydrophilicity phenomenon for titanium dioxide coated glass exposed to sun light. This resulted in the development of self-cleaning glass and anti-fogging coatings.
Nanosized TiO2 incorporated into outdoor building materials, such as paving stones in noxer blocks or paints, could reduce concentrations of airborne pollutants such as volatile organic compounds and nitrogen oxides. A TiO2-containing cement has been produced.
Using TiO2 as a photocatalyst, attempts have been made to mineralize pollutants (to convert into CO2 and H2O) in waste water. The photocatalytic destruction of organic matter could also be exploited in coatings with antimicrobial applications.
Hydroxyl radical formation
Although nanosized anatase TiO2 does not absorb visible light, it does strongly absorb ultraviolet (UV) radiation (hv), leading to the formation of hydroxyl radicals. This occurs when photo-induced valence bond holes (h+vb) are trapped at the surface of TiO2 leading to the formation of trapped holes (h+tr) that cannot oxidize water.
TiO2 + hv → e− + h+vb
h+vb → h+tr
O2 + e− → O2•−
O2•− + O2•−+ 2 → H2O2 + O2
O2•− + h+vb → O2
O2•− + h+tr → O2
+ h+vb → HO•
e− + h+tr → recombination
Note: Wavelength (λ)= 387 nm This reaction has been found to mineralize and decompose undesirable compounds in the environment, specifically the air and in wastewater.
Nanotubes
Anatase can be converted into non-carbon nanotubes and nanowires. Hollow TiO2 nanofibers can be also prepared by coating carbon nanofibers by first applying titanium butoxide.
Solubility
Titanium dioxide is insoluble in water, organic solvents, and inorganic acids. It is slightly soluble in alkali, soluble in saturated potassium acid carbonate, and can be completely dissolved in strong sulfuric acid and hydrofluoric acid after boiling for a long time.
Health and safety
Widely-occurring minerals and even gemstones are composed of TiO2. All natural titanium, comprising more than 0.5% of the Earth's crust, exists as oxides.
Food additive
In 2006, titanium dioxide was, according to one chemical encyclopedia, regarded as "completely nontoxic when orally administered". However, this is now seriously disputed.
Government policies
TiO2 whitener in food was banned in France from 2020, due to uncertainty about safe quantities for human consumption.
In 2021, the European Food Safety Authority (EFSA) ruled that as a consequence of new understandings of nanoparticles, titanium dioxide could "no longer be considered safe as a food additive", and the EU health commissioner announced plans to ban its use across the EU, with discussions beginning in June 2021. EFSA concluded that genotoxicity—which could lead to carcinogenic effects—could not be ruled out, and that a "safe level for daily intake of the food additive could not be established". In 2022, the UK Food Standards Agency and Food Standards Scotland announced their disagreement with the EFSA ruling, and did not follow the EU in banning titanium dioxide as a food additive. Health Canada similarly reviewed the available evidence in 2022 and decided not to change their position on titanium dioxide as a food additive.
The European Union removed the authorization to use titanium dioxide (E 171) in foods, effective 7 February 2022, with a six months grace period.
As of May 2023, following the European Union 2022 ban, the U.S. states California and New York were considering banning the use of titanium dioxide in foods.
As of 2024, the Food and Drug Administration (FDA) in the United States permits titanium dioxide as a food additive. It may be used to increase whiteness and opacity in dairy products (some cheeses, ice cream, and yogurt), candies, frostings, fillings, and many other foods. The FDA regulates the labeling of products containing titanium dioxide, alllowing the product's ingredients list to identify titanium dioxide either as "color added" or "artificial colors" or "titanium dioxide;" it does not require that titanium dioxide be explicitly named despite growing scientific concerns. In 2023, the Consumer Healthcare Products Association, a manufacturer's trade group, defended the substance as safe at certain limits while allowing that additional studies could provide further insight, saying an immediate ban would be a "knee-jerk" reaction.
Industry response
Dunkin' Donuts dropped titanium dioxide from their merchandise in 2015 after public pressure.
Research as an ingestible nanomaterial
Due to the potential that long-term ingestion of titanium dioxide may be toxic, particularly to cells and functions of the gastrointestinal tract, preliminary research as of 2021 was assessing its possible role in disease development, such as inflammatory bowel disease and colorectal cancer.
Size distribution analyses showed that batches of food-grade TiO₂, which is produced with a target particle size in the 200300nm range for optimal pigmentation qualities, always include a nanoparticle-sized fraction as inevitable byproduct of the manufacturing processes.
Andrew Maynard, director of Risk Science Center at the University of Michigan, rejected the supposed danger from use of titanium dioxide in food. He says that the titanium dioxide used by Dunkin' Brands and many other food producers is not a new material, and it is not a nanomaterial either. Nanoparticles are typically smaller than 100 nanometres in diameter, yet most of the particles in food-grade titanium dioxide are much larger.
Inhalation
Titanium dioxide dust, when inhaled, has been classified by the International Agency for Research on Cancer (IARC) as an IARC Group 2B carcinogen, meaning it is possibly carcinogenic to humans.
The US National Institute for Occupational Safety and Health recommends two separate exposure limits. NIOSH recommends that fine particles be set at an exposure limit of 2.4 mg/m3, while ultrafine be set at an exposure limit of 0.3 mg/m3, as time-weighted average concentrations up to 10 hours a day for a 40-hour work week.
Although no evidence points to acute toxicity, recurring concerns have been expressed about nanophase forms of these materials. Studies of workers with high exposure to TiO2 particles indicate that even at high exposure there is no adverse effect to human health.
Environmental waste introduction
Titanium dioxide (TiO₂) is mostly introduced into the environment as nanoparticles via wastewater treatment plants. Cosmetic pigments including titanium dioxide enter the wastewater when the product is washed off into sinks after cosmetic use. Once in the sewage treatment plants, pigments separate into sewage sludge which can then be released into the soil when injected into the soil or distributed on its surface. 99% of these nanoparticles wind up on land rather than in aquatic environments due to their retention in sewage sludge. In the environment, titanium dioxide nanoparticles have low to negligible solubility and have been shown to be stable once particle aggregates are formed in soil and water surroundings. In the process of dissolution, water-soluble ions typically dissociate from the nanoparticle into solution when thermodynamically unstable. TiO2 dissolution increases when there are higher levels of dissolved organic matter and clay in the soil. However, aggregation is promoted by pH at the isoelectric point of TiO2 (pH= 5.8) which renders it neutral and solution ion concentrations above 4.5 mM.
| Physical sciences | Inorganic compounds | null |
219741 | https://en.wikipedia.org/wiki/Tit%20%28bird%29 | Tit (bird) | The tits, chickadees, and titmice constitute the Paridae, a large family of small passerine birds which occur mainly in the Northern Hemisphere and Africa. Most were formerly classified in the genus Parus.
Eurasian and African members of this family are referred to as "tits", while North American species are called either "chickadees" (onomatopoeic, derived from their distinctive "chick-a dee dee dee" alarm call) or "titmice". The name titmouse is recorded from the 14th century, composed of the Old English name for the bird, mase (Proto-Germanic *maison, Dutch mees, German Meise), and tit, denoting something small. The former spelling, "titmose", was influenced by mouse in the 16th century. Emigrants to New Zealand presumably identified some of the superficially similar birds of the genus Petroica of the family Petroicidae, the Australian robins, as members of the tit family, giving them the title tomtit, although, in fact, they are not related.
These birds are mainly small, stocky, woodland species with short, stout bills. Some have crests. They range in length from . They are adaptable birds, with a mixed diet including seeds and insects. Many species live around human habitation and come readily to bird feeders for nuts or seed, and learn to take other foods.
Description
With the exception of the three monotypic genera Sylviparus, Melanochlora, and Pseudopodoces, the tits are extremely similar in appearance, and have been described as "one of the most conservative avian families in terms of general morphology". The typical body length of adult members of the family is between in length; when the monotypic genera are added, this range is from . In weight, the family ranges from ; this contracts to when the three atypical genera are removed. The majority of the variation within the family is in plumage, and particularly colour.
The bills of the tits are generally short, varying between stout and fine, depending on diet. The more insectivorous species have finer bills, whereas those that consume more seeds have stouter bills. It is said that tits are evolving longer beaks to reach into bird feeders. The most aberrant bill of the family is possessed by Hume's ground tit of Tibet and the Himalayas, which is long and decurved.
Distribution and habitat
The tits are a widespread family of birds, occurring over most of Europe, Asia, North America, and Africa. The genus Poecile occurs from Europe through Asia into North America, as far south as southern Mexico. American species in this genus are known as chickadees. Some species in this genus have quite large natural distributions; one, the grey-headed chickadee, is distributed from Scandinavia to Alaska and Canada. The majority of the tits in the genus Periparus are found in the southeastern portion of Asia. This includes two species endemic to the Philippines. The coal tit, also in this genus, is a much more widespread species, ranging from the British Isles and North Africa to Japan. The two crested tits of the genus Lophophanes have a disjunct distribution, with one species occurring in Europe and the other in central Asia.
The genus Baeolophus is endemic to North America. The genus Parus includes the great tit that ranges from Western Europe to Indonesia. Cyanistes has a European and Asian distribution (also into northern Africa), and the three remaining genera, Pseudopodoces, Sylviparus, and Melanochlora, are all restricted to Asia.
Behaviour
Tits are active, noisy, and social birds. They are territorial during the breeding season and often join mixed-species feeding flocks during the nonbreeding season. The tits are highly adaptable, and after the corvids (crows and jays) and parrots, amongst the most intelligent of all birds. Tits recognize the difference between species that are dangerous or harmless to them, by this they can protect each other or their families. These birds do this by mobbing or escaping, however they also avoid the nest when the predators are present in order to avoid their families to be seen.
Fission–fusion society
Fission–fusion society has been documented in a number of avian taxa including this one. In brief, that means flocks can split into smaller groups or individuals, and subsequently reunite.
Vocalisations
The tits make a variety of calls and songs. They are amongst the most vocal of all birds, calling continuously in most situations, so much so that they are only ever silent for specific reasons such as avoiding predators or when intruding on a rival's territory. Quiet contact calls are made while feeding to facilitate cohesion with others in their social group. Other calls are used for signalling alarm—a well-known example being the "chic-a-dee-dee" of North American species in the genus Poecile, the call which gives them their local common name, the chickadee. The call also serves as a rallying call to summon others to mob and harass the predator. The number of "dee" syllables at the end of the call increases with the level of danger the predator poses.
Diet and feeding
The tits are generalist insectivores that consume a wide range of small insects and other invertebrates, particularly small defoliating caterpillars. They also consume seeds and nuts, particularly in the winter. One characteristic method of foraging in the family is hanging, where they inspect a branch or twig and leaves from all angles while hanging upside down to feed. In areas where numerous species of tit coexist, different species forage in different parts of the tree, their niche determined in no small way by their morphology; larger species forage on the ground, medium-sized species foraging on larger branches, and the smallest species on the ends of branches. Having obtained larger prey items or seeds, tits engage in hold-hammering, where they hold the item between their feet and hammer it with their bill until it opens. In this fashion, they can even open hazelnuts in around 20 minutes. A number of genera engage in food caching, hoarding supplies of food during the winter.
Breeding
Tits are cavity-nesting birds, typically using trees, although Pseudopodoces builds a nest on the ground. Most tree-nesting tits excavate their nests, and clutch sizes are generally large for altricial birds, ranging from usually two eggs in the rufous-vented tit of the Himalayas to as many as 10 to 14 in the blue tit of Europe. In favourable conditions, this species had laid as many as 19 eggs, which is the largest clutch of any altricial bird. Most tits are multibrooded, a necessary strategy to cope with either the harsh winters in which they reside in the Holarctic or the extremely erratic conditions of tropical Africa, where typically a single pair cannot find enough food to rear even one nestling and in drought years breeding is likely to be futile.
Many African tit species, along with Pseudopodoces, are cooperative breeders, and even pair-breeding parids are often highly social and maintain stable flocks throughout the nonbreeding season.
Tits also have a variety of methods for attracting mates, primarily through their intricate, bouncing mating dance. Only the blue tit is typically polygynous; all other species are generally monogamous. Courtship feeding is typical of pair-breeding tits to deal with the cost of rearing their large broods.
Systematics
Recently, the large Parus group has been gradually split into several genera (as indicated below), initially by North American ornithological authorities and later elsewhere. Whereas in the mid-1990s, only Pseudopodoces, Baeolophus, Melanochlora, and Sylviparus were considered well-supported by the available data as distinct from Parus. Today, this arrangement is considered paraphyletic as indicated by mtDNA cytochrome b sequence analysis, and Parus is best restricted to the Parus major—Parus fasciiventer clade, and even the latter species' closest relatives might be considered a distinct genus.
In the Sibley-Ahlquist taxonomy, the family Paridae is much enlarged to include related groups such as the penduline tits and long-tailed tits, but while the former are quite close to the tits and could conceivably be included in that family together with the stenostirid "warblers", the long-tailed tits are not. Indeed, the yellow-browed tit and the sultan tit are possibly more distant to the tits than the penduline tits are. If the two current families are lumped into the Paridae, the tits would be a subfamily Parinae.
Alternatively, all tits—save the two monotypic genera discussed in the preceding section and possibly Cyanistes, but including Hume's ground tit—could be lumped in Parus. In any case, four major clades of "typical" tits can be recognized: the dark-capped chickadees and their relatives (Poecile including Sittiparus), the long-crested Baeolophus and Lophophanes species, the usually tufted, white-cheeked Periparus (including Pardaliparus) with more subdued coloration and finally Parus sensu stricto (including Melaniparus and Machlolophus). Still, the interrelationship of these, as well as the relationships of many species within the clades, are not well-resolved at all; analysis of morphology and biogeography probably gives a more robust picture than the available molecular data.
Tits have settled North America twice, probably at some time during the Early-Mid Pliocene. The first were the ancestors of Baeolophus, with chickadees arriving somewhat later.
Species in taxonomic order
Family: PARIDAE
| Biology and health sciences | Passerida | null |
219810 | https://en.wikipedia.org/wiki/Bow%20and%20arrow | Bow and arrow | The bow and arrow is a ranged weapon system consisting of an elastic launching device (bow) and long-shafted projectiles (arrows). Humans used bows and arrows for hunting and aggression long before recorded history, and the practice was common to many prehistoric cultures. They were important weapons of war from ancient history until the early modern period, when they were rendered increasingly obsolete by the development of the more powerful and accurate firearms. Today, bows and arrows are mostly used for hunting and sports.
Archery is the art, practice, or skill of using bows to shoot arrows. A person who shoots arrows with a bow is called a bowman or an archer. Someone who makes bows is known as a bowyer, someone who makes arrows is a fletcher, and someone who manufactures metal arrowheads is an arrowsmith.
Basic design and use
A bow consists of a semi-rigid but elastic arc with a high-tensile bowstring joining the ends of the two limbs of the bow. An arrow is a projectile with a pointed tip and a long shaft with stabilizer fins (fletching) towards the back, with a narrow notch (nock) at the very end to contact the bowstring.
To load an arrow for shooting (nocking an arrow), the archer places an arrow across the middle of the bow with the bowstring in the arrow's nock. To shoot, the archer holds the bow at its center with one hand and pulls back (draws) the arrow and the bowstring with the other (typically the dominant hand). This flexes the two limbs of the bow rearwards, which perform the function of a pair of cantilever springs to store elastic energy.
While maintaining the draw, the archer typically aims the shot intuitively or by sighting along the arrow. Then the archer releases (looses) the draw, allowing the limbs' stored energy to convert into kinetic energy transmitted via the bowstring to the arrow, propelling it to fly forward with high velocity.
A container or bag for additional arrows for quick reloading is called a quiver.
When not in use, bows are generally kept unstrung, meaning one or both ends of the bowstring are detached from the bow. This removes all residual tension on the bow and can help prevent it from losing strength or elasticity over time. Many bow designs also let it straighten out more completely, reducing the space needed to store the bow. Returning the bowstring to its ready-to-use position is called stringing the bow.
History
The oldest known evidence of the bow and arrow comes from South African sites such as Sibudu Cave, where likely arrowheads have been found, dating from approximately 72,000–60,000 years ago.
The earliest probable arrowheads found outside of Africa were discovered in 2020 in Fa Hien Cave, Sri Lanka. They have been dated to 48,000 years ago. "Bow-and-arrow hunting at the Sri Lankan site likely focused on monkeys and smaller animals, such as squirrels, Langley says. Remains of these creatures were found in the same sediment as the bone points."
Small stone points from the Grotte Mandrin in Southern France, used some 54,000 years ago, have damage from use that indicates their use as projectile weapons, and some are too small (less than 10mm across as the base) for any practical use other than as arrowheads. They are associated with possibly the first groups of modern humans to leave Africa.
After the end of the last glacial period, some 12,000 years ago, the use of the bow seems to have spread to every inhabited region, except for Australasia and most of Oceania.
The earliest definite remains of bow and arrow from Europe are possible fragments from Germany found at Mannheim-Vogelstang dated 17,500–18,000 years ago, and at Stellmoor dated 11,000 years ago. Azilian points found in Grotte du Bichon, Switzerland, alongside the remains of both a bear and a hunter, with flint fragments found in the bear's third vertebra, suggest the use of arrows at 13,500 years ago.
At the site of Nataruk in Turkana County, Kenya, obsidian bladelets found embedded in a skull and within the thoracic cavity of another skeleton, suggest the use of stone-tipped arrows as weapons about 10,000 years ago.
The oldest extant bows in one piece are the elm Holmegaard bows from Denmark, which were dated to 9,000 BCE. Several bows from Holmegaard, Denmark, date 8,000 years ago. High-performance wooden bows are currently made following the Holmegaard design. The Stellmoor bow fragments from northern Germany were dated to about 8,000 BCE, but they were destroyed in Hamburg during the Second World War, before carbon 14 dating was available; their age is attributed by archaeological association.
The bow was an important weapon for both hunting and warfare from prehistoric times until the widespread use of gunpowder weapons in the 16th century. It was also common in ancient warfare, although certain cultures would not favor them. Greek poet Archilocus expressed scorn for fighting with bows and slings.
The skill of Nubian archers was renowned in ancient Egypt and beyond. Their mastery of the bow gained their land the name Ta-Seti, "Land of the Bow" in Ancient Egyptian.
Beginning with the reign of William the Conqueror, the longbow was England's principal weapon of war until the end of the Middle Ages. Genghis Khan and his Mongol hordes conquered much of the Eurasian steppe using short bows. Native Americans used archery to hunt and defend themselves during the days of English and later American colonization.
Organised warfare with bows ended in the early to mid-17th century in Western Europe, but it persisted into the 19th century in Eastern cultures, including hunting and warfare in the New World. In the Canadian Arctic, bows were made until the end of the 20th century for hunting caribou, for instance at Igloolik. The bow has more recently been used as a weapon of tribal warfare in some parts of Sub-Saharan Africa; an example was documented in 2009 in Kenya when Kisii people and Kalenjin people clashed, resulting in four deaths.
The British upper class led a revival of archery as a sport in the late 18th century. Sir Ashton Lever, an antiquarian and collector, formed the Toxophilite Society in London in 1781, under the patronage of George IV, then Prince of Wales.
Bows and arrows have been rarely used by modern special forces for survival and clandestine operations.
Construction
Parts of the bow
The basic elements of a modern bow are a pair of curved elastic limbs, traditionally made from wood, joined by a riser. However self bows such as the English longbow are made of a single piece of wood comprising both limbs and the grip. The ends of each limb are connected by a string known as the bow string. By pulling the string backwards the archer exerts compression force on the string-facing section, or belly, of the limbs as well as placing the outer section, or back, under tension. While the string is held, this stores the energy later released in putting the arrow to flight. The force required to hold the string stationary at full draw is often used to express the power of a bow, and is known as its draw weight, or weight. Other things being equal, a higher draw weight means a more powerful bow, which is able to project heavier arrows at the same velocity or the same arrow at a greater velocity.
The various parts of the bow can be subdivided into further sections. The topmost limb is known as the upper limb, while the bottom limb is the lower limb. At the tip of each limb is a nock, which is used to attach the bowstring to the limbs. The riser is usually divided into the grip, which is held by the archer, as well as the arrow rest and the bow window. The arrow rest is a small ledge or extension above the grip which the arrow rests upon while being aimed. The bow window is that part of the riser above the grip, which contains the arrow rest.
In bows drawn and held by hand, the maximum draw weight is determined by the strength of the archer. The maximum distance the string could be displaced and thus the longest arrow that could be loosed from it, a bow's draw length, is determined by the size of the archer.
A composite bow uses a combination of materials to create the limbs, allowing the use of materials specialized for the different functions of a bow limb. The classic composite bow uses wood for lightness and dimensional stability in the core, horn to store compression energy, and sinew for its ability to store energy in tension. Such bows, typically Asian, would often use a stiff end on the limb end, having the effect of a recurve. In this type of bow, this is known by the Arabic name 'siyah'.
Modern construction materials for bows include laminated wood, fiberglass, metals, and carbon fiber components.
Arrows
An arrow usually consists of a shaft with an arrowhead attached to the front end, with fletchings and a nock at the other. Modern arrows are usually made from carbon fibre, aluminum, fiberglass, and wood shafts. Carbon shafts have the advantage that they do not bend or warp, but they can often be too light weight to shoot from some bows and are expensive. Aluminum shafts are less expensive than carbon shafts, but they can bend and warp from use. Wood shafts are the least expensive option but often will not be identical in weight and size to each other and break more often than the other types of shafts. Arrow sizes vary greatly across cultures and range from very short ones that require the use of special equipment to be shot to ones in use in the Amazon River jungles that are long. Most modern arrows are in length.
Arrows come in many types, among which are breasted, bob-tailed, barreled, clout, and target. A breasted arrow is thickest at the area right behind the fletchings, and tapers towards the (nock) and head. A bob-tailed arrow is thickest right behind the head, and tapers to the nock. A barrelled arrow is thickest in the centre of the arrow. Target arrows are those arrows used for target shooting rather than warfare or hunting, and usually have simple arrowheads.
For safety reasons, a bow should never be shot without an arrow nocked; without an arrow, the energy that is normally transferred into the projectile is instead directed back into the bow itself, which will cause damage to the bow's limbs.
Arrowheads
The end of the arrow that is designed to hit the target is called the arrowhead. Usually, these are separate items that are attached to the arrow shaft by either tangs or sockets. Materials used in the past for arrowheads include flint, bone, horn, or metal. Most modern arrowheads are made of steel, but wood and other traditional materials are still used occasionally. A number of different types of arrowheads are known, with the most common being bodkins, broadheads, and piles. Bodkin heads are simple spikes made of metal of various shapes, designed to pierce armour. A broadhead arrowhead is usually triangular or leaf-shaped and has a sharpened edge or edges. Broadheads are commonly used for hunting. A pile arrowhead is a simple metal cone, either sharpened to a point or somewhat blunt, that is used mainly for target shooting. A pile head is the same diameter as the arrow shaft and is usually just fitted over the tip of the arrow. Other heads are known, including the blunt head, which is flat at the end and is used for hunting small game or birds, and is designed to not pierce the target nor embed itself in trees or other objects and make recovery difficult. Another type of arrowhead is a barbed head, usually used in warfare or hunting.
Bowstrings
Bowstrings may have a nocking point marked on them, which serves to mark where the arrow is fitted to the bowstring before shooting. The area around the nocking point is usually bound with thread to protect the area around the nocking point from wear by the archer's hands. This section is called the serving. At one end of the bowstring a loop is formed, which is permanent. The other end of the bowstring also has a loop, but this is not permanently formed into the bowstring but is constructed by tying a knot into the string to form a loop. Traditionally this knot is known as the archer's knot, but is a form of the timber hitch. The knot can be adjusted to lengthen or shorten the bowstring. The adjustable loop is known as the "tail". The string is often twisted (this being called the "flemish twist").
Bowstrings have been constructed of many materials throughout history, including fibres such as flax, silk, and hemp. Other materials used were animal guts, animal sinews, and rawhide. Modern fibres such as Dacron or Kevlar are now used in commercial bowstring construction, as well as steel wires in some compound bows. Compound bows have a mechanical system of pulley cams over which the bowstring is wound. Nylon is useful only in emergency situations, as it stretches too much.
Types of bow
There is no single accepted system of classification of bows. Bows may be described by various characteristics including the materials used, the length of the draw that they permit, the shape of the bow in sideways view, and the shape of the limb in cross-section.
Commonly-used descriptors for bows include:
By side profile
Straight bow: a bow approximately straight in side-view profile. These bows are referred to as straight, although there may be minor curves in the natural wood, and the bow may have a "set" or curvature that a wooden bow takes after use.
Recurve bow: a bow with the tips curving away from the archer. The curves straighten out as the bow is drawn and the return of the tip to its curved state after release of the arrow adds extra velocity to the arrow.
Reflex bow: a bow whose entire limbs curve away from the archer when unstrung. The curves are opposite to the direction in which the bow flexes while drawn.
By material
Self bow: a bow made from one piece of wood.
Composite bow: a bow made of more than one material.
By cross-section of limb
Longbow: a self bow with limbs rounded in cross-section, about the same height as the archer so as to allow a full draw, usually over long. The traditional English longbow was made of yew wood, but other woods are also used.
Flatbow: the limbs are approximately rectangular in cross-section. This was traditional in many Native American societies and was found to be the most efficient shape for bow limbs by American engineers in the 20th century
Other characteristics
Takedown bow: a bow that can be disassembled for transportation, usually consisting of three parts: two limbs and a riser, in addition to the string.
Compound bow: a bow with mechanical amplifiers to aid with drawing the bowstring. Usually, these amplifiers are asymmetric pulleys called cams (though they are not actually cams) at the ends of the limbs, which provide a mechanical advantage (known as the let-off) while holding the bow in full draw. Such bows typically have high draw weights and are usually drawn with a release aid with a trigger mechanism for a consistently clean release.
Crossbow: a bow mounted horizontally on a frame similar to a firearm stock, which has a locking mechanism for holding the bowstring at full draw. Crossbows typically shoot arrow-like darts called bolts or "quarrels", rather than normal arrows.
Footbow: a bow meant to be used with the legs and arms while lying down and used in the current distance record for the furthest arrow shot.
| Technology | Projectile weapons | null |
219826 | https://en.wikipedia.org/wiki/Bacillota | Bacillota | Bacillota (synonym Firmicutes) is a phylum of bacteria, most of which have gram-positive cell wall structure. The renaming of phyla such as Firmicutes in 2021 remains controversial among microbiologists, many of whom continue to use the earlier names of long standing in the literature.
The name "Firmicutes" was derived from the Latin words for "tough skin," referring to the thick cell wall typical of bacteria in this phylum. Scientists once classified the Firmicutes to include all gram-positive bacteria, but have recently defined them to be of a core group of related forms called the low-G+C group, in contrast to the Actinomycetota. They have round cells, called cocci (singular coccus), or rod-like forms (bacillus). A few Firmicutes, such as Megasphaera, Pectinatus, Selenomonas and Zymophilus, have a porous pseudo-outer membrane that causes them to stain gram-negative.
Many Bacillota (Firmicutes) produce endospores, which are resistant to desiccation and can survive extreme conditions. They are found in various environments, and the group includes some notable pathogens. Those in one family, the heliobacteria, produce energy through anoxygenic photosynthesis. Bacillota play an important role in beer, wine, and cider spoilage.
Classes
The group is typically divided into the Clostridia, which are anaerobic, and the Bacilli, which are obligate or optional aerobes.
On phylogenetic trees, the first two groups show up as paraphyletic or polyphyletic, as do their main genera, Clostridium and Bacillus. However, Bacillota as a whole is generally believed to be monophyletic, or paraphyletic with the exclusion of Mollicutes.
Phylogeny
The currently accepted taxonomy based on the List of Prokaryotic names with Standing in Nomenclature (LPSN) and the National Center for Biotechnology Information (NCBI).
The Firmicutes are thought by some to be the source of the archaea, by models there the archaea branched relatively late from bacteria, rather than forming an independently originating early lineage (domain of life) from the last universal common ancestor of cellular life (LUCA).
Genera
More than 274 genera were considered to be within the Bacillota phylum, notable genera of Bacillota include:
Bacilli, order Bacillales
Bacillus
Listeria
Staphylococcus
Bacilli, order Lactobacillales
Enterococcus
Lactobacillus
Leuconostoc
Streptococcus
Clostridia
Clostridioides
Clostridium
Selenomonas
Erysipelotrichia
Erysipelothrix
Clinical significance
Bacillota make up ~30% of the mouse and human gut microbiome. The phylum Bacillota as part of the gut microbiota has been shown to be involved in energy resorption, and potentially related to the development of diabetes and obesity. Within the gut of healthy human adults, the most abundant bacterium: Faecalibacterium prausnitzii (F. prausnitzii), which makes up 5% of the total gut microbiome, is a member of the Bacillota phylum. This species is directly associated with reduced low-grade inflammation in obesity. F. prausnitzii has been found in higher levels within the guts of obese children than in non-obese children.
In multiple studies a higher abundance of Bacillota has been found in obese individuals than in lean controls. A higher level of Lactobacillus (of the Bacillota phylum) has been found in obese patients and in one study, obese patients put on weight loss diets showed a reduced amount of Bacillota within their guts.
Diet changes in mice have also been shown to promote changes in Bacillota abundance. A higher relative abundance of Bacillota was seen in mice fed a western diet (high fat/high sugar) than in mice fed a standard low fat/ high polysaccharide diet. The higher amount of Bacillota was also linked to more adiposity and body weight within mice. Specifically, within obese mice, the class Mollicutes (within the Bacillota phylum) was the most common. When the microbiota of obese mice with this higher Bacillota abundance was transplanted into the guts of germ-free mice, the germ-free mice gained a significant amount of fat as compared to those transplanted with the microbiota of lean mice with lower Bacillota abundance.
The presence of Christensenella (Bacillota, in class Clostridia), isolated from human faeces, has been found to correlate with lower body mass index.
| Biology and health sciences | Gram-positive bacteria | Plants |
219831 | https://en.wikipedia.org/wiki/Neisseria | Neisseria | Neisseria is a large genus of bacteria that colonize the mucosal surfaces of many animals. Of the 11 species that colonize humans, only two are pathogens, N. meningitidis and N. gonorrhoeae.
Neisseria species are Gram-negative bacteria included among the Pseudomonadota, a large group of Gram-negative forms. Neisseria diplococci resemble coffee beans when viewed microscopically.
Pathogenesis and classification
Pathogens
Species of this genus (family Neisseriaceae) of parasitic bacteria grow in pairs and occasionally fours, and thrive best at 98.6 °F (37 °C) in the animal body or serum media.
The genus includes:
N. gonorrhoeae (also called the gonococcus) causes gonorrhea.
N. meningitidis (also called the meningococcus) is one of the most common causes of bacterial meningitis and the causative agent of meningococcal septicaemia.
The immune system's neutrophils are restricted in function due to the ability of Neisseria to evade opsonization by antibodies, and to replicate within neutrophils despite phagocytosis. Neisseria species are also able to alter their antigens to avoid being engulfed by a process called antigenic variation, which is observed primarily in surface-located molecules. The pathogenic species along with some commensal species, have type IV pili which serve multiple functions for this organism. Some functions of the type IV pili include: mediating attachment to various cells and tissues, twitching motility, natural competence, microcolony formation, extensive intrastrain phase, and antigenic variation.
Neisseria bacteria have also been shown to be an important factor in the early stages of canine plaque development.
Nonpathogens
This genus also contains several, believed to be commensal, or nonpathogenic, species:
Neisseria bacilliformis
Neisseria cinerea
Neisseria elongata
Neisseria flavescens
Neisseria lactamica
Neisseria macacae
Neisseria mucosa
Neisseria oralis
Neisseria polysaccharea
Neisseria sicca
Neisseria subflava
Neisseria flava
However, some of these can be associated with disease.
Biochemical identification
All the medically significant species of Neisseria are positive for both catalase and oxidase. Different Neisseria species can be identified by the sets of sugars from which they will produce acid. For example, N. gonorrhoeae makes acid from only glucose, but N. meningitidis produces acid from both glucose and maltose.
Polysaccharide capsule. N. meningitidis has a polysaccharide capsule that surrounds the outer membrane of the bacterium and protects against soluble immune effector mechanisms within the serum. It is considered to be an essential virulence factor for the bacteria. N. gonorrhoeae possesses no such capsule.
Unlike most other Gram-negative bacteria, which possess lipopolysaccharide (LPS), both pathogenic and commensal species of Neisseria have a lipooligosaccharide (LOS) which consists of a core polysaccharide and lipid A. It functions as an endotoxin, protects against antimicrobial peptides, and adheres to the asialoglycoprotein receptor on urethral epithelium. LOS is highly stimulatory to the human immune system. LOS sialylation (by the enzyme Lst) prevents phagocytosis by neutrophils and complement deposition. LOS modification by phosphoethanolamine (by the enzyme LptA) provides resistance to antimicrobial peptides and complement. Strains of the same species have the ability to produce different LOS glycoforms.
History
The genus Neisseria is named after the German bacteriologist Albert Neisser, who in 1879 discovered its first example, Neisseria gonorrhoeae, the pathogen which causes the human disease gonorrhea. Neisser also co-discovered the pathogen that causes leprosy, Mycobacterium leprae. These discoveries were made possible by the development of new staining techniques which he helped to develop.
Genomes
The genomes of at least 10 Neisseria species have been completely sequenced. The best-studied species are N. meningitidis with more than 70 strains and N. gonorrhoeae with at least 10 strains completely sequenced. Other complete genomes are available for N. elongata, N. lactamica, and N. weaveri. Whole genome shotgun sequences are available for hundreds of other species and strains. N. meningitidis encodes 2,440 to 2,854 proteins while N. gonorrhoeae encodes from 2,603 to 2,871 proteins. N. weaveri (strain NCTC 13585) has the smallest known genome with only 2,060 encoded proteins although N. meningitidis MC58 has been reported to have only 2049 genes. The genomes are generally quite similar. For example, when the genome of N. gonorrhoeae (strain FA1090) is compared to that of N. meningitidis (strain H44/76) 68% of their genes are shared.
Vaccine
Diseases caused by N. meningitidis and N. gonorrhoeae are significant health problems worldwide, the control of which is largely dependent on the availability and widespread use of comprehensive meningococcal vaccines. Development of neisserial vaccines has been challenging due to the nature of these organisms, in particular the heterogeneity, variability and/or poor immunogenicity of their outer surface components. As strictly human pathogens, they are highly adapted to the host environment, but have evolved several mechanisms to remain adaptable to changing microenvironments and avoid elimination by the host immune system. Currently, serogroup A, B, C, Y, and W-135 meningococcal infections can be prevented by vaccines. However, the prospect of developing a gonococcal vaccine is remote.
Antibiotic resistance
The acquisition of cephalosporin resistance in N. gonorrhoeae, particularly ceftriaxone resistance, has greatly complicated the treatment of gonorrhea, with the gonococcus now being classified as a "superbug".
Genetic transformation
Genetic transformation is the process by which a recipient bacterial cell takes up DNA from a neighboring cell and integrates this DNA into the recipient’s genome by recombination. In N. meningitidis and N. gonorrhoeae, DNA transformation requires the presence of short DNA sequences (9-10 monomers residing in coding regions) of the donor DNA. These sequences are called DNA uptake sequences (DUSs). Specific recognition of DUSs is mediated by a type IV pilin. Davidsen et al. reported that in N. meningitidis and N. gonorrhoeae, DUSs occur at a significantly higher density in genes involved in DNA repair and recombination (as well as in restriction-modification and replication) than in other annotated gene groups. These authors proposed that the over-representation of DUS in DNA repair and recombination genes may reflect the benefit of maintaining the integrity of the DNA repair and recombination machinery by preferentially taking up genome maintenance genes that could replace their damaged counterparts in the recipient cell. Caugant and Maiden noted that the distribution of DUS is consistent with recombination being primarily a mechanism for genome repair that can occasionally result in generation of diversity, which even more occasionally, is adaptive. It was also suggested by Michod et al. that an important benefit of transformation in N. gonorrhoeae is recombinational repair of oxidative DNA damages caused by oxidative attack by the host’s phagocytic cells.
International Pathogenic Neisseria Conference
The International Pathogenic Neisseria Conference (IPNC), occurring every two years, is a forum for the presentation of cutting-edge research on all aspects of the genus Neisseria. This includes immunology, vaccinology, and physiology and metabolism of N. meningitidis, N. gonorrhoeae and the commensal species. The first IPNC took place in 1978, and the most recent one was in September 2016. Normally, the location of the conference switches between North America and Europe, but it took place in Australia for the first time in 2006, where the venue was located in Cairns.
| Biology and health sciences | Gram-negative bacteria | Plants |
219865 | https://en.wikipedia.org/wiki/Actinomycetota | Actinomycetota | The Actinomycetota (or Actinobacteria) are a diverse phylum of Gram-positive bacteria with high GC content. They can be terrestrial or aquatic. They are of great importance to land flora because of their contributions to soil systems. In soil they help to decompose the organic matter of dead organisms so the molecules can be taken up anew by plants. While this role is also played by fungi, Actinomycetota are much smaller and likely do not occupy the same ecological niche. In this role the colonies often grow extensive mycelia, as fungi do, and the name of an important order of the phylum, Actinomycetales (the actinomycetes), reflects that they were long believed to be fungi. Some soil actinomycetota (such as Frankia) live symbiotically with the plants whose roots pervade the soil, fixing nitrogen for the plants in exchange for access to some of the plant's saccharides. Other species, such as many members of the genus Mycobacterium, are important pathogens.
Beyond the great interest in Actinomycetota for their soil role, much is yet to be learned about them. Although currently understood primarily as soil bacteria, they might be more abundant in fresh waters. Actinomycetota is one of the dominant bacterial phyla and contains one of the largest of bacterial genera, Streptomyces. Streptomyces and other actinomycetota are major contributors to biological buffering of soils. They are also the source of many antibiotics.
The Actinomycetota genus Bifidobacterium is the most common bacteria in the microbiome of human infants. Although adults have fewer bifidobacteria, intestinal bifidobacteria help maintain the mucosal barrier and reduce lipopolysaccharide in the intestine.
Although some of the largest and most complex bacterial cells belong to the Actinomycetota, the group of marine Actinomarinales has been described as possessing the smallest free-living prokaryotic cells.
Some Siberian or Antarctic Actinomycetota are said to be the oldest living organism on Earth, frozen in permafrost at around half a million years ago. The symptoms of life were detected by release from permafrost samples 640 kya or younger.
General
Most Actinomycetota of medical or economic significance are in class Actinomycetia, and belong to the order Actinomycetales. While many of these cause disease in humans, Streptomyces is notable as a source of antibiotics.
Of those Actinomycetota not in the Actinomycetales, Gardnerella is one of the most researched. Classification of Gardnerella is controversial, and MeSH catalogues it as both a Gram-positive and Gram-negative organism.
Actinomycetota, especially Streptomyces spp., are recognized as the producers of many bioactive metabolites that are useful to humans in medicine, such as antibacterials, antifungals, antivirals, antithrombotics, immunomodifiers, antitumor drugs, and enzyme inhibitors; and in agriculture, including insecticides, herbicides, fungicides, and growth-promoting substances for plants and animals. Actinomycetota-derived antibiotics that are important in medicine include aminoglycosides, anthracyclines, chloramphenicol, macrolide, tetracyclines, etc.
Actinomycetota have high guanine and cytosine content in their DNA. The G+C content of Actinomycetota can be as high as 70%, though some may have a low G+C content.
Analysis of glutamine synthetase sequence has been suggested for phylogenetic analysis of the Actinomycetota.
Phylogeny
Taxonomy
The currently accepted taxonomy is based on the List of Prokaryotic names with Standing in Nomenclature (LPSN) and National Center for Biotechnology Information (NCBI).
Class ?"Syntrophaliphaticia" corrig. Liu et al. 2020
Class "Aquicultoria" Jiao et al. 2021
Class "Geothermincolia" Jiao et al. 2021
Class "Humimicrobiia" Jiao et al. 2021
Class Acidimicrobiia Norris 2013
Class Actinomycetia (Stackebrandt et al. 1997) Salam et al. 2020 (Nitriliruptoria Ludwig et al. 2013)
Subclass Actinobacteridae Stackebrandt, Rainey & Ward-Rainey 1997
Subclass Nitriliruptoridae Kurahashi et al. 2010
Class Coriobacteriia König 2013
Class Rubrobacteria Suzuki 2013
Class Thermoleophilia Suzuki and Whitman 2013
| Biology and health sciences | Gram-positive bacteria | Plants |
219939 | https://en.wikipedia.org/wiki/Mycoplasma | Mycoplasma | Mycoplasma is a genus of bacteria that, like the other members of the class Mollicutes, lack a cell wall, and its peptidoglycan, around their cell membrane. The absence of peptidoglycan makes them naturally resistant to antibiotics such as the beta-lactam antibiotics that target cell wall synthesis. They can be parasitic or saprotrophic. Several species are pathogenic in humans, including M. pneumoniae, which is an important cause of "walking" pneumonia and other respiratory disorders, and M. genitalium, which is believed to be involved in pelvic inflammatory diseases. Mycoplasma species (like the other species of the class Mollicutes) are among the smallest organisms yet discovered, can survive without oxygen, and come in various shapes. For example, M. genitalium is flask-shaped (about 300 x 600 nm), while M. pneumoniae is more elongated (about 100 x 1000 nm), many Mycoplasma species are coccoid. Hundreds of Mycoplasma species infect animals.
In casual speech, the name "mycoplasma" (plural mycoplasmas or mycoplasms) generally refers to all members of the class Mollicutes. In formal scientific classification, the designation Mycoplasma refers exclusively to the genus, a member of the Mycoplasmataceae, the only family in the order Mycoplasmatales (see "scientific classification").
Etymology
The term "mycoplasma", from the Greek μύκης, (fungus) and πλάσμα, (formed), was first used by Albert Bernhard Frank in 1889 to describe an altered state of plant cell cytoplasm resulting from infiltration by fungus-like microorganisms. Julian Nowak later proposed the name mycoplasma for certain filamentous microorganisms imagined to have both cellular and acellular stages in their lifecycles, which could explain how they were visible with a microscope, but passed through filters impermeable to other bacteria. Later, the name for these mycoplasmas was pleuropneumonia-like organisms (PPLO), broadly referring to organisms similar in colonial morphology and filterability to the causative agent (a Mycoplasma species) of contagious bovine pleuropneumonia. At present, all these organisms are classified as Mollicutes, and the term Mycoplasma solely refers to the genus.
Species which infect humans
Species of Mycoplasma, other than those listed below, have been recovered from humans, but are assumed to have been contracted from a non-human host. The following species use humans as the primary host:
M. amphoriforme
M. buccale
M. faucium
M. fermentans
M. genitalium
M. hominis
M. incognitus
M. lipophilum
M. orale
M. penetrans
M. pirum
M. pneumoniae
M. primatum
M. salivarium
M. spermatophilum
Characteristics
Over 100 species have been included in the genus Mycoplasma, a member of the class Mollicutes. They are parasites or commensals of humans, animals, and plants. The genus Mycoplasma uses vertebrate and arthropod hosts. Dietary nitrogen availability has been shown to alter codon bias and genome evolution in Mycoplasma and the plant parasites Phytoplasma.
Mycoplasma species are among the smallest free-living organisms (about 0.2 - 0.3 μm in diameter). They have been found in the pleural cavities of cattle suffering from pleuropneumonia. These organisms are often called MLO (mycoplasma-like organisms) or, formerly, PPLO (pleuropneumonia-like organisms).
Important characteristics
Cell wall is absent and plasma membrane forms the outer boundary of the cell.
Due to the absence of cell walls these organisms can change their shape and leads to pleomorphism.
Lack of nucleus and other membrane-bound organelles.
Genetic material is a single DNA duplex and is naked.
Ribosomes are 70S type.
Possess a replicating disc at one end which assists replication process and also the separation of the genetic materials.
Heterotrophic nutrition. Some live as saprophytes but the majority are parasites of plants and animals. The parasitic nature is due to the inability of mycoplasmal bacteria to synthesise the required growth factor.
Cell and colony morphology
Due to the lack of a rigid cell wall, Mycoplasma species (like all Mollicutes) can contort into a broad range of shapes, from round to oblong. They are pleomorphic and therefore cannot be identified as rods, cocci or spirochetes.
Colonies show the typical "fried egg" appearance (about 0.5 mm in diameter).
Reproduction
In 1954, using phase-contrast microscopy, continual observations of live cells have shown that Mycoplasma species ("mycoplasmas", formerly called pleuropneumonia-like organisms, PPLO, now classified as Mollicutes) and L-form bacteria (previously also called L-phase bacteria) do not proliferate by binary fission, but by a uni- or multi-polar budding mechanism. Microphotograph series of growing microcultures of different strains of PPLOs, L-form bacteria and, as a control, a Micrococcus species (dividing by binary fission) have been presented. Additionally, electron microscopic studies have been performed.
Taxonomy
History of taxonomy
Previously, Mycoplasma species (often commonly called "mycoplasmas", now classified as Mollicutes) were sometimes considered stable L-form bacteria or even viruses, but phylogenetic analysis has identified them as bacteria that have lost their cell walls in the course of evolution.
The medical and agricultural importance of members of the genus Mycoplasma and related genera have led to the extensive cataloging of many of these organisms by culture, serology, and small sub-unit rRNA gene and whole-genome sequencing. A recent focus in the sub-discipline of molecular phylogenetics has both clarified and confused certain aspects of the organization of the class Mollicutes.
Originally, the trivial name "mycoplasmas" commonly denoted all members of the class Mollicutes (from Latin mollis "soft" and cutis "skin"), which lack cell walls due to their genetic inability to synthesize peptidoglycan.
Taxonomists once classified Mycoplasma and relatives in the phylum Firmicutes, consisting of low G+C Gram-positive bacteria such as Clostridium, Lactobacillus, and Streptococcus; but modern polyphasic analyses situate them in the phylum Tenericutes.
Historically, the description of a bacterium lacking a cell wall was sufficient to classify it to the genus Mycoplasma and as such it is the oldest and largest genus of the class with about half of the class' species (107 validly described), each usually limited to a specific host and with many hosts harboring more than one species, some pathogenic and some commensal. In later studies, many of these species were found to be phylogenetically distributed among at least three separate orders. A limiting criterion for inclusion within the genus Mycoplasma was that the organism has a vertebrate host.
By the 1990s, it had become readily apparent that this approach was problematic: the type species, M. mycoides, along with other significant mycoplasma species like M. capricolum, is evolutionarily more closely related to the genus Spiroplasma in the order Entomoplasmatales than to the other members of the genus Mycoplasma. As a result, if the group was to be rearranged to match phylogeny, a number of medically important species (e.g. M. pneumoniae, M. genitalium) would have to be put in a different genus, causing widespread confusion in medical and agricultural communities. The genus was discussed multiple times by the International Committee on Systematic Bacteriology's (ICSB) subcommittee on Mollicutes between 1992 and 2011, to no effect.
Regardless of taxonomy, by 2007 it was solidly known that Molicutes could be divided into four nontaxonomic lineages.
An "Acholeplasma" group consisting of Acholeplasmatales. This group is non-problematic, as it contains no species classified in what was then "Mycoplasma".
A "Spiroplasma" or mycoides group containing M. mycoides and the aforementioned closely-related species in "Spiroplasma" and Entomoplasmatales.
A pneumoniae group containing M. pneumoniae and closely-related species (M. muris, M. fastidiosum, U. urealyticum), the currently unculturable haemotrophic mollicutes, informally referred to as haemoplasmas (recently transferred from the genera Haemobartonella and Eperythrozoon), and Ureaplasma. This medically important group contains M. alvi (bovine), M. amphoriforme (human), M. gallisepticum (avian), M. genitalium (human), M. imitans (avian), M. pirum (uncertain/human), M. testudinis (tortoises), and M. pneumoniae (human). Most, if not all, of these species share some otherwise unique characteristics including an attachment organelle, homologs of the M. pneumoniae cytadherence-accessory proteins, and specialized modifications of the cell division apparatus.
A hominis group containing M. hominis, M. bovis, and M. pulmonis among others.
As of 2018
In 2018, Gupta et al. re-circumscribed the genus Mycoplasma around M. mycoides. A total of 78 species were removed from Mycoplasma, creating five new genera and a number of higher taxonomic levels. Under this new scheme, a new family Mycoplasmoidaceae was created to correspond to the "pneumoniae" group, with M. pneumoniae and related species transferred to a new genus Mycoplasmoides. Another new family Metamycoplasmataceae was created to correspond to the "hominis" group. Both families belong to a new order Mycoplasmoitales, distinct from the Mycoplasmatales of Mycoplasma. The taxonomy was accepted by the ICSB with validation list 184 in 2018 and became the correct name. Both List of Prokaryotic names with Standing in Nomenclature (LPSN) and National Center for Biotechnology Information (NCBI) now use the new nomenclature.
Gupta's proposed taxonomy, as expected, moved the medically important "pneumoniae" group out of Mycoplasma into its own genus. As a result, a number of mycoplasmologists petitioned to the ICSB to reject the name in 2019. They argue that although Gupta's phylogenetic methods were likely solid, the proposed name changes are too sweeping to be practically adopted, citing some principles of the Code such as "name stability". Gupta and Oren wrote a rebuttal in 2020, further detailing the pre-existing taxonomic problems. In 2022, the ICSP's Judicial Opinion 122 ruled in favor of the name changes proposed by Gupta, meaning they remain valid under the Prokaryotic Code (and for the purpose of the LPSN, they remain the "correct names"). However, the older names also remain valid; their use remains acceptable under the Code.
Gupta et al. 2019 performed some uncontroversial sorting of the order Mycoplasmatales.
Unassigned species:
"Ca. M. aoti" Barker et al. 2011
"M. bradburyae" Ramírez et al. 2023
"Ca. M. corallicola" Neulinger et al. 2009
"Ca. M. coregoni" corrig. Rasmussen et al. 2021
"Ca. M. didelphidis" corrig. Pontarolo et al. 2021
"Ca. M. erythrocervae" Watanabe et al. 2010
"Ca. M. haematocervi" corrig. Watanabe et al. 2010
"Ca. M. haematodidelphidis" corrig. Messick et al. 2002
"Ca. M. haematohydrochoeri" corrig. Vieira et al. 2021
"Ca. M. haematomacacae" corrig. Maggi et al. 2013
"Ca. M. haematominiopteri" corrig. Millán et al. 2015
"M. haematomyotis" Volokhov et al. 2023
"M. haematophyllostomi" Volokhov et al. 2023
"Ca. M. haematonasuae" corrig. Collere et al. 2021
"Ca. M. haematoparvum" Sykes et al. 2005
"Ca. M. haematosphigguri" corrig. Valente et al. 2021
"Ca. M. haematotapirus" Mongruel et al. 2022
"Ca. M. haematoterrestris" Mongruel et al. 2022
"Ca. M. haematovis" corrig. Hornok et al. 2009
"Ca. M. haemoalbiventris" Pontarolo et al. 2021
"Ca. M. haemobovis" Meli et al. 2010
"Ca. M. haemomeles" Harasawa, Orusa & Giangaspero 2014
"Ca. M. haemomuris" (Mayer 1921) Neimark et al. 2002
"Ca. M. haemoparvum" Kenny et al. 2004
M. hafezii Ziegler et al. 2019
"M. incognitus" Lo et al. 1989
"M. insons" May et al. 2007
"Ca. M. kahanei" Neimark et al. 2002
"Ca. M. mahonii" Aroh, Liles & Halanych 2023
"M. monodon" Ghadersohi & Owens 1998
M. phocimorsus Skafte-Holm et al. 2023
"M. pneumophila" Lyerova et al. 2008
"Ca. M. ravipulmonis" Neimark, Mitchelmore & Leach 1998
"Ca. M. salmoniarum" corrig. Rasmussen et al. 2021
M. seminis Fischer et al. 2021
"M. sphenisci" Frasca et al. 2005
"M. timone" Greub & Raoult 2001
"Ca. M. tructae" Sanchez et al. 2020
"Ca. M. turicense" corrig. Willi et al. 2006
"M. volis" Dillehay et al. 1995
"M. vulturii" Oaks et al. 2004
Laboratory contaminant
Mycoplasma species are often found in research laboratories as contaminants in cell culture. Mycoplasmal cell culture contamination occurs due to contamination from individuals or contaminated cell culture medium ingredients. Mycoplasma cells are physically small – less than 1 μm, so are difficult to detect with a conventional microscope.
Mycoplasmae may induce cellular changes, including chromosome aberrations, changes in metabolism and cell growth. Severe Mycoplasma infections may destroy a cell line. Detection techniques include DNA probe, enzyme immunoassays, PCR, plating on sensitive agar and staining with a DNA stain including DAPI or Hoechst.
An estimated 11 to 15% of U.S. laboratory cell cultures are contaminated with mycoplasma.
A Corning study showed that half of U.S. scientists did not test for Mycoplasma contamination in their cell cultures. The study also stated that, in former Czechoslovakia, 100% of cell cultures that were not routinely tested were contaminated while only 2% of those routinely tested were contaminated (study p. 6). Since the U.S. contamination rate was based on a study of companies that routinely checked for Mycoplasma, the actual contamination rate may be higher. European contamination rates are higher and that of other countries are higher still (up to 80% of Japanese cell cultures).
About 1% of published Gene Expression Omnibus data may have been compromised. Several antibiotic-containing formulations of antimycoplasmal reagents have been developed over the years.
Synthetic mycoplasma genome
A chemically synthesized genome of a mycoplasmal cell based entirely on synthetic DNA which can self-replicate has been referred to as Mycoplasma laboratorium.
Pathogenicity
Several Mycoplasma species can cause disease, including M. pneumoniae, which is an important cause of atypical pneumonia (formerly known as "walking pneumonia"), and M. genitalium, which has been associated with pelvic inflammatory diseases. Mycoplasma infections in humans are associated with skin eruptions in 17% of cases.
P1 antigen
The P1 antigen is the primary virulence factor of mycoplasma, specifically the Pneumoniae group. P1 is a membrane associated protein that allows adhesion to epithelial cells. The P1 receptor is also expressed on erythrocytes which can lead to autoantibody agglutination from mycobacteria infection.
Sexually transmitted infections
Mycoplasma and Ureaplasma species are not part of the normal vaginal flora. Some Mollicutes species are spread through sexual contact. These species have a negative effect on fertility. Mollicutes species colonizing the human genital tract are:
U. urealyticum
M. hominis
M. genitalium
M. penetrans
M. primatum (considered nonpathogenic)
M. spermatophilum (considered nonpathogenic)
M. hominis causes male sterility/Genitals inflammation in humans.
Mycoplasma species have been isolated from women with bacterial vaginosis. M. genitalium is found in women with pelvic inflammatory disease. In addition, infection is associated with increased risk of cervicitis, infertility, preterm birth and spontaneous abortion. Mycoplasma genitalium has developed resistance to some antibiotics.
Infant disease
Low birth-weight, preterm infants are susceptible to Mycoplasma and Ureaplasma infections. Mycoplasma species are associated with infant respiratory distress syndrome, bronchopulmonary dysplasia, and intraventricular hemorrhage in preterm infants.
Links to cancer
Several species of Mycoplasma are frequently detected in different types of cancer cells. These species are:
M. fermentans
M. genitalium
M. hyorhinis
M. penetrans
U. urealyticum
The majority of these Mycoplasma species have shown a strong correlation to malignant transformation in mammalian cells in vitro.
infection and host cell transformation
The presence of Mycoplasma was first reported in samples of cancer tissue in the 1960s. Since then, several studies tried to find and prove the connection between Mycoplasma and cancer, as well as how the bacterium might be involved in the formation of cancer. Several studies have shown that cells that are chronically infected with the bacteria go through a multistep transformation. The changes caused by chronic mycoplasmal infections occur gradually and are both morphological and genetic. The first visual sign of infection is when the cells gradually shift from their normal form to sickle-shaped. They also become hyperchromatic due to an increase of DNA in the nucleus of the cells. In later stages, the cells lose the need for solid support to grow and proliferate, as well as the normal contact-dependent inhibition cells.
Possible intracellular mechanisms
Karyotypic changes related to infections
Cells infected with Mycoplasma for an extended period of time show significant chromosomal abnormalities. These include the addition of chromosomes, the loss of entire chromosomes, partial loss of chromosomes, and chromosomal translocation. All of these genetic abnormalities may contribute to the process of malignant transformation. Chromosomal translocation and extra chromosomes help create abnormally high activity of certain proto-oncogenes, which caused by these genetic abnormalities and include those encoding c-myc, HRAS, and vav. The activity of proto-oncogenes is not the only cellular function that is affected; tumour suppressor genes are affected by the chromosomal changes induced by mycoplasma, as well. Partial or complete loss of chromosomes causes the loss of important genes involved in the regulation of cell proliferation. Two genes whose activities are markedly decreased during chronic infections with mycoplasma are the Rb and the p53 tumour suppressor genes. Another possible mechanism of carcinogenesis is RAC1 activation by a small GTPase-like protein fragment of Mycoplasma. A major feature that differentiates mycoplasmas from other carcinogenic pathogens is that the mycoplasmas do not cause the cellular changes by insertion of their own genetic material into the host cell. The exact mechanism by which the bacterium causes the changes is not yet known.
Partial reversibility of malignant transformations
The malignant transformation induced by Mycoplasma species is also different from that caused by other pathogens in that the process is reversible. The state of reversal is, however, only possible up to a certain point during the infection. The window of time when reversibility is possible varies greatly; it depends primarily on the Mycoplasma involved. In the case of M. fermentans, the transformation is reversible until around week 11 of infection and starts to become irreversible between weeks 11 and 18. If the bacteria are killed using antibiotics (i.e. ciprofloxacin or Clarithromycin) before the irreversible stage, the infected cells should return to normal.
Connections to cancer in vivo and future research
Epidemiologic, genetic, and molecular studies suggest infection and inflammation initiate certain cancers, including those of the prostate. M. genitalium and M. hyorhinis induce malignant phenotype in benign human prostate cells (BPH-1) that were not tumorigenic after 19 weeks of exposure.
Types of cancer associated
Colon cancer: In a study to understand the effects of Mycoplasma contamination on the quality of cultured human colon cancer cells, a positive correlation was found between the number of M. hyorhinis cells present in the sample and the percentage of CD133-positive cells (a glycoprotein with an unknown function).
Gastric cancer: Strong evidence indicates the infection of M. hyorhinis contributes to the development of cancer within the stomach and increases the likelihood of malignant cancer cell development.
Lung cancer: Studies on lung cancer have supported the belief that more than a coincidental positive correlation exists between the appearance of Mycoplasma strains in patients and the infection with tumorigenesis.
Prostate cancer: p37, a protein encoded for by M. hyorhinis, has been found to promote the invasiveness of prostate cancer cells. The protein also causes the growth, morphology, and gene expression of the cells to change, causing them to become a more aggressive phenotype.
Renal cancer: Patients with renal cell carcinoma (RCC) exhibited a significantly high amount of Mycoplasma sp. compared with the healthy control group. This suggests Mycoplasma may play a role in the development of RCC.
| Biology and health sciences | Gram-positive bacteria | Plants |
219940 | https://en.wikipedia.org/wiki/Vibrio | Vibrio | Vibrio is a genus of Gram-negative bacteria, possessing a curved-rod (comma) shape, several species of which can cause foodborne infection or soft-tissue infection called Vibriosis. Infection is commonly associated with eating undercooked seafood. Being highly salt tolerant and unable to survive in freshwater, Vibrio spp. are commonly found in various salt water environments. Vibrio spp. are facultative anaerobes that test positive for oxidase and do not form spores. All members of the genus are motile. They are able to have polar or lateral flagellum with or without sheaths. Vibrio species typically possess two chromosomes, which is unusual for bacteria. Each chromosome has a distinct and independent origin of replication, and are conserved together over time in the genus. Recent phylogenies have been constructed based on a suite of genes (multilocus sequence analysis).
O. F. Müller (1773, 1786) described eight species of the genus Vibrio (included in Infusoria), three of which were spirilliforms. Some of the other species are today assigned to eukaryote taxa, e.g., to the euglenoid Peranema or to the diatom Bacillaria. However, Vibrio Müller, 1773 became regarded as the name of a zoological genus, and the name of the bacterial genus became Vibrio Pacini, 1854. Filippo Pacini isolated micro-organisms he called "vibrions" from cholera patients in 1854, because of their motility. In Latin "vibrio" means "to quiver".
Biochemical characteristics of Vibrio spp.
The genus Vibrio contains a large number of species, and these vary somewhat in their biochemical characteristics. Colony, morphological, physiological, and biochemical characteristics of the genus Vibrio are shown in the Table below.
Note: Group-1: Vibrio alginolyticus; Group-2: Vibrio natriegens, Vibrio pelagius, Vibrio azureus; + = Positive; – =Negative; V =Variable (+/–)
Pathogenic strains
Several species of Vibrio are pathogens. Most disease-causing strains are associated with gastroenteritis, but can also infect open wounds and cause sepsis. They can be carried by numerous marine animals, such as crabs or prawns, and have been known to cause fatal infections in humans after exposure. Risk of clinical disease and death increases with certain factors, such as uncontrolled diabetes, elevated iron levels (cirrhosis, sickle cell disease, hemochromatosis), and cancer or other immunocompromised states. Pathogenic Vibrio species include V. cholerae (the causative agent of cholera), V. parahaemolyticus, and V. vulnificus. V. cholerae is generally transmitted by contaminated water. Pathogenic Vibrio species can cause foodborne illness (infection), usually associated with eating undercooked seafood. When ingested Vibrio bacteria can primarily result in watery diarrhea along with other secondary symptoms. The pathogenic features can be linked to quorum sensing, where bacteria are able to express their virulence factor via their signaling molecules.
V. vulnificus outbreaks commonly occur in warm climates and small, generally lethal, outbreaks occur regularly. An outbreak occurred in New Orleans after Hurricane Katrina, and several lethal cases occur most years in Florida. As of 2013 in the United States, Vibrio infections as a whole were up 43% when compared with the rates observed in 2006–2008. V. vulnificus, the most severe strain, has not increased. Foodborne Vibrio infections are most often associated with eating raw shellfish.
V. parahaemolyticus is also associated with the Kanagawa phenomenon, in which strains isolated from human hosts (clinical isolates) are hemolytic on blood agar plates, while those isolated from nonhuman sources are not hemolytic.
Many Vibrio species are also zoonotic. They cause disease in fish and shellfish, and are common causes of mortality among domestic marine life.
Diagnosis
Cholera
A common sign of Vibrio infection is cholera. Cholera primarily presents with rapid water loss by watery diarrhea. Other symptoms include vomiting and muscle cramps. Water loss can lead to dehydration which can be mild to moderate to severe. Moderate to severe dehydration requires immediate treatment. V. cholerae is the most common pathogen that causes cholera. The gold standard for detecting cholera is through cultures of stool samples or rectal swabs. Identification is then done through microscopy or by agglutination of antibodies. Cultures are done in thiosulfate citrate bile-salts sucrose agar. V cholerae will form yellow colonies.
Vibriosis
Vibriosis is a sign of a more severe Vibrio infection. Common causes of vibriosis include consumption of raw or undercooked seafood, primarily oysters, or wound exposure to sea water. The majority of V. parahaemolyticus infections can be self-limiting and symptoms include diarrhea, nausea, headaches, fever and chills. V. vulnificus can lead to a more serious disease, particularly in wound infection which can turn into necrotizing fasciitis. V. parahaemolyticus is the most common pathogen in vibriosis, however V. vulnificus is more common in people who have certain risk factors like older age, liver disease or diabetes mellitus. Like all vibrio diagnosis, vibriosis can also be determined in stool cultures. V. parahaemolyticus and V. vulnificus will form green colonies.
Treatment
Medical care depends on the clinical presentation and the presence of underlying medical conditions.
Vibrio gastroenteritis
Because Vibrio gastroenteritis is self-limited in most patients, no specific medical therapy is required. Patients who cannot tolerate oral fluid replacement may require intravenous fluid therapy.
Although most Vibrio species are sensitive to antibiotics, such as doxycycline or ciprofloxacin, antibiotic therapy does not shorten the course of the illness or the duration of pathogen excretion. However, if the patient is ill and has a high fever or an underlying medical condition, oral antibiotic therapy with doxycycline or ciprofloxacin can be initiated.
Non-cholera Vibrio infections
Patients with non-cholera Vibrio wound infection or sepsis are much more ill and frequently have other medical conditions. Medical therapy consists of:
Prompt initiation of effective antibiotic therapy (doxycycline or a quinolone)
Intensive medical therapy with aggressive fluid replacement and vasopressors for hypotension and septic shock to correct acid-base and electrolytes abnormalities that may be associated with severe sepsis
Early fasciotomy within 24 hours after development of clinical symptoms can be life-saving in patients with necrotizing fasciitis.
Early debridement of the infected wound has an important role in successful therapy and is especially indicated to avoid amputation of fingers, toes, or limbs.
Expeditious and serial surgical evaluation and intervention are required because patients may deteriorate rapidly, especially those with necrotizing fasciitis or compartment syndrome.
Reconstructive surgery, such as skin grafts, are used in the recovery phase.
Prevention
Cholera
The most effective method to prevent cholera is the improvement of water and food safety. This includes the sanitation of water, proper preparation of food and community awareness of outbreaks. Prevention has been most effective in countries where cholera is endemic.
Another method is cholera vaccines. Examples of cholera vaccines include Dukoral and Vaxchora.
Vibriosis
Prevention of vibriosis is mostly effective in food processing. Food items, mostly seafood, that commonly contain vibrio organisms are regularly controlled. The water that seafood is fished or farmed from is analyzed to determine microorganism content. Food processing methods like pasteurization and high pressure are used to eliminate microorganisms and pathogens.
Other strains
V. harveyi is a pathogen of several aquatic animals, and is notable as a cause of luminous vibriosis in shrimp (prawns). Aliivibrio fischeri (or V. fischeri) is known for its mutualistic symbiosis with the Hawaiian bobtail squid, which is dependent on microbial luminescence.
Flagella
The "typical", early-discovered Vibrio species, such as V. cholerae, have a single polar flagellum (monotrichous) with sheath. Some species, such as V. parahaemolyticus and V. alginolyticus, have both a single polar flagellum with sheath and thin flagella projecting in all directions (peritrichous), and the other species, such as V. fischeri, have tufts of polar flagella with sheath (lophotrichous).
Structure
Typical bacterial flagellum structure contains three components: the basal body, the hook and the filament. Like typical bacteria, Vibrio spp, have these three components, but with increased complexity in the basal body. In addition, Vibrio spp. use five or six distinct flagellum subunits to construct the flagellar filament, rather than the single flagellin found in many other bacteria. In Vibrio spp, most have a single flagellum located on one pole of the bacterium, although some species have additional flagella in peritrichous or lophotrichous arrangements. Another difference is that the gradient used to power the flagellar motor is sodium driven rather than proton driven; this creates greater torque, and Vibrio flagella have been shown to rotate over five times faster than the -driven flagella of E. coli. The flagellum is also surrounded by a sheath extending from the membrane. The purpose of this sheath has yet to be determined.
Effect on Virulence
Motility is very important for Vibrio spp for infection. Research has shown that a variety of Vibrios mutants that are defective in flagella synthesis or non-motile are defective in infection. Loss of motility in Vibrio has shown impaired colonization and adherence to host's intestines.
Natural transformation
Natural transformation is a common bacterial adaptation for DNA transfer that employs numerous bacterial gene products. For a recipient bacterium to bind, take up, and recombine exogenous DNA into its chromosome, it must become competent, that is, enter a special physiologic state. The DNA-uptake process of naturally competent V. cholerae involves an extended competence-induced pilus and a DNA-binding protein that acts as a ratchet and reels DNA into the periplasm. Natural transformation has also been described for V. fischeri, V. vulnificus and V. parahaemolyticus.
Small RNA
V. cholerae has been used in discoveries of many bacterial small RNAs. Using sRNA-Seq and Northern blot candidate sRNAs were identified and characterised as IGR-sRNA (intragenic region), AS-sRNAs (transcribed from the antisense strand of the open reading frame (ORF) and ORF-derived. One of the candidates from this study, IGR 7, was shown to be involved in carbon metabolism and later renamed MtlS RNA. Other sRNAs identified in V. cholerae through genetic screens and computational methods include Qrr RNA, Vibrio regulatory RNA of OmpA, MiX sRNA, Vibrio cholerae ToxT activated RNAs, foR RNA, and VqmR sRNA.
| Biology and health sciences | Gram-negative bacteria | Plants |
220009 | https://en.wikipedia.org/wiki/Bittern | Bittern | Bitterns are birds belonging to the subfamily Botaurinae of the heron family Ardeidae. Bitterns tend to be shorter-necked and more secretive than other members of the family. They were called hæferblæte and various iterations of raredumla in Old English; the word "bittern" came to English from Old French butor, itself from Gallo-Roman butitaurus, a compound of Latin būtiō (buzzard) and taurus (bull).
Bitterns usually frequent reed beds and similar marshy areas and feed on amphibians, reptiles, insects, and fish.
Bitterns, like herons, egrets, and pelicans, fly with their necks retracted, unlike the cranes, storks, ibises and spoonbills, and geese which fly with necks extended and outstretched. The genus Ixobrychus was recently found to be paraphyletic with the Botaurus genus, and Ixobrychus was then merged into Botaurus.
Species
There are currently 14 extant species divided into two genera within Botaurinae:
| Biology and health sciences | Pelecanimorphae | Animals |
220118 | https://en.wikipedia.org/wiki/Railroad%20car | Railroad car | A railroad car, railcar (American and Canadian English), railway wagon, railway carriage, railway truck, railwagon, railcarriage or railtruck (British English and UIC), also called a train car, train wagon, train carriage or train truck, is a vehicle used for the carrying of cargo or passengers on a rail transport network (a railroad/railway). Such cars, when coupled together and hauled by one or more locomotives, form a train. Alternatively, some passenger cars are self-propelled in which case they may be either single railcars or make up multiple units.
The term "car" is commonly used by itself in American English when a rail context is implicit. Indian English sometimes uses "bogie" in the same manner, though the term has other meanings in other variants of English. In American English, "railcar" is a generic term for a railway vehicle; in other countries "railcar" refers specifically to a self-propelled, powered, railway vehicle.
Although some cars exist for the railroad's own use – for track maintenance purposes, for example – most carry a revenue-earning load of passengers or freight, and may be classified accordingly as passenger cars or coaches on the one hand or freight cars (or wagons) on the other.
Passenger cars
Passenger cars, or coaches, vary in their internal fittings:
In standard-gauge railway cars, seating is usually configured into ranges from three to five seats across the width of the car, with an aisle in between (resulting in arrangements of 2+1, 2+2 or 3+2 seats) or at the side. Tables may be provided between seats facing one another. Alternatively, seats facing in the same direction may have access to a fold-down ledge on the back of the seat in front.
If the aisle is located between seats, seat rows may face the same direction, or be grouped, with twin rows facing each other.
In some vehicles intended for commuter services, seats are positioned with their backs to the side walls, either on one side or more commonly on both, facing each other across the aisle. This gives a wide accessway and allows room for standing passengers at peak times, as well as improving loading and unloading speeds.
If the aisle is at the side, the car is usually divided into small compartments. These usually contain six seats, although sometimes in second class they contain eight, and sometimes in first class they contain four.
Passenger cars can take the electricity supply for heating and lighting equipment from either of two main sources: directly from a head-end power generator on the locomotive via bus cables, or by an axle-powered generator which continuously charges batteries whenever the train is in motion.
Modern cars usually have either air conditioning or windows that can be opened (sometimes, for safety, not so far that one can hang out), or sometimes both. Various types of onboard train toilet facilities may also be provided.
Other types of passenger car exist, especially for long journeys, such as the dining car, parlor car, disco car, and in rare cases theater and movie theater car. In some cases another type of car is temporarily converted to one of these for an event.
Observation cars were built for the rear of many famous trains to allow the passengers to view the scenery. These proved popular, leading to the development of dome cars multiple units of which could be placed mid-train, and featured a glass-enclosed upper level extending above the normal roof to provide passengers with a better view.
Sleeping cars outfitted with (generally) small bedrooms allow passengers to sleep through their night-time trips, while couchette cars provide more basic sleeping accommodation. Long-distance trains often require baggage cars for the passengers' luggage. In European practice it used to be common for day coaches to be formed of compartments seating 6 or 8 passengers, with access from a side corridor. In the UK, Corridor coaches fell into disfavor in the 1960s and 1970s partially because open coaches are considered more secure by women traveling alone.
Another distinction is between single- and double deck train cars. An example of a double decker is the Amtrak superliner.
A "trainset" (or "set") is a semi-permanently arranged formation of cars, rather than one created "ad hoc" out of whatever cars are available. These are only broken up and reshuffled 'on shed' (in the maintenance depot). Trains are then built of one or more of these 'sets' coupled together as needed for the capacity of that train.
Often, but not always, passenger cars in a train are linked together with enclosed, flexible gangway connections through which passengers and crewmen can walk. Some designs incorporate semi-permanent connections between cars and may have a full-width connection, effectively making them one long, articulated 'car'. In North America, passenger cars also employ tightlock couplings to keep a train together in the event of a derailment or other accident.
Many multiple unit trains consist of cars which are semi-permanently coupled into sets: these sets may be joined together to form larger trains, but generally passengers can only move around between cars within a set. This "closed" arrangement keeps parties of travellers and their luggage together, and hence allows the separate sets to be easily split to go separate ways. Some multiple-unit trainsets are designed so that corridor connections can be easily opened between coupled sets; this generally requires driving cabs either set to the side or (as in the Dutch Koploper or the Japanese 285 series) above the passenger compartment. These cabs or driving trailers are also useful for quickly reversing the train.
First- and second-class carriages
It has been common in some systems to differentiate between first- and second-class carriages, with a premium being paid for first-class tickets, and fines imposed for non-compliance. Facilities and appurtenances applying to first-class carriages may include
Lounge-type seats, improved upholstery and additional hip- and leg-room
Reading lamps, double-glazing, sound treatment
Removable tables and seating amenable for card games
Choice of smoking and non-smoking compartments
More recently, mains power outlets and Wi-fi facilities have been offered.
Passenger car gallery
Freight cars
Freight cars (US/Canada), goods wagons (UIC), or trucks (UK) exist in a wide variety of types, adapted to carry a host of goods. Originally there were very few types of cars; the flat car or wagon, and the boxcar (US/Canada), covered wagon (UIC) or van (UK), were among the first.
Types of freight cars
Freight cars or goods wagons are generally categorized as follows:
Boxcar (US and Canada), covered wagon (UIC) or van (UK): fully enclosed car with side or end doors. Standard boxcars have about 3.5 times the capacity of a standard Semi-trailer.
Covered wagon (UIC), van (UK) or boxcar (US/Canada): fully enclosed wagon for moisture-susceptible goods.
Hicube boxcars: high-capacity high-clearance boxcar
Refrigerator car or reefer (US/Canada): refrigerated boxcar for fruits and vegetables.
CargoBeamer
Coil car: specialized flat or gondola for heavy sheet metal rolls
Combine car: combined passenger car and boxcar in one wagon
Flatcar (or flat): for larger bulky loads. Specialized flat cars include:
Aircraft Parts Car: with fixtures for large aircraft parts.
Autorack (also called auto carriers): multi-level flat for automobiles.
Centerbeam cars (US): specialized flat for building materials.
Conflat (UK): specialized flat for containers.
CargoSprinter: self-propelled container flat.
Container flatcar
Depressed-center flatcar or Wellcar or Lowmac (UK): for high-clearance loads (e.g. transformers and boilers)
Semi-trailer flatcar
Rolling highway: a train designed to carry trucks and/or semi-trailers
Single container car; Spine car, a center sill and side sill only car with lateral arms to support intermodal containers. | Technology | Rail and cable transport | null |
220200 | https://en.wikipedia.org/wiki/Oystercatcher | Oystercatcher | The oystercatchers are a group of waders forming the family Haematopodidae, which has a single genus, Haematopus. They are found on coasts worldwide apart from the polar regions and some tropical regions of Africa and South East Asia. The exceptions to this are the Eurasian oystercatcher, the South Island oystercatcher, and the Magellanic oystercatcher, which also breed inland, far inland in some cases. In the past there has been a great deal of confusion as to the species limits, with discrete populations of all black oystercatchers being afforded specific status but pied oystercatchers being considered one single species.
Taxonomy
The genus Haematopus was introduced in 1758 by the Swedish naturalist Carl Linnaeus in 1758 in the tenth edition of his Systema Naturae to accommodate a single species, the Eurasian oystercatcher Haematopus ostralegus. The genus name Haematopus comes from the Ancient Greek words haima αἳμα meaning blood, and pous πούς meaning foot, referring to the red legs of the Eurasian oystercatcher; it had been in use since Pierre Belon in 1555. The family Haematopodidae was introduced (as the subfamily Haematopodinae) by the French naturalist Charles Bonaparte in 1838.
The common name oystercatcher was coined by Mark Catesby in 1731 for the North American species H. palliatus, which he described as eating oysters. The English zoologist William Yarrell in 1843 established this as the preferred term, replacing the older name sea pie, although the term had earlier been used by the Welsh Naturalist Thomas Pennant in 1776 in his British Zoology.
Description
The different species of oystercatcher show little variation in shape or appearance. They range from in length and in wingspan. The Eurasian oystercatcher is the lightest on average, at , while the sooty oystercatcher is the heaviest, at . The plumage of all species is either all-black, or black (or dark brown) on top and white underneath.
The variable oystercatcher is slightly exceptional in being either all-black or pied. They are large, obvious, and noisy plover-like birds, with massive long orange or red bills used for smashing or prying open molluscs. The bill shape varies between species, according to the diet. Those birds with blade-like bill tips pry open or smash mollusc shells, and those with pointed bill tips tend to probe for annelid worms. They show sexual dimorphism, with females being longer-billed and heavier than males.
Feeding
The diet of oystercatchers varies with location. Species occurring inland feed upon earthworms and insect larvae. The diet of coastal oystercatchers is more varied, although dependent upon coast type; on estuaries, bivalves, the ivygastropods and polychaete worms are the most important part of the diet, whereas rocky shore oystercatchers prey upon limpets, mussels, gastropods, and chitons. Other prey items include echinoderms, fish, and crabs.
Breeding
Nearly all species of oystercatcher are monogamous, although there are reports of polygamy in the Eurasian oystercatcher. They are territorial during the breeding season (with a few species defending territories year round). There is strong mate and site fidelity in the species that have been studied, with one record of a pair defending the same site for 20 years. A single nesting attempt is made per breeding season, which is timed over the summer months. The nests of oystercatchers are simple affairs, scrapes in the ground which may be lined, and placed in a spot with good visibility.
The eggs of oystercatchers are spotted and cryptic. Between one and four eggs are laid, with three being typical in the Northern Hemisphere and two in the south. Incubation is shared but not proportionally, females tend to take more incubation and males engage in more territory defence. Incubation varies by species, lasting between 24–39 days. Oystercatchers are also known to practice "egg dumping." Like the cuckoo, they sometimes lay their eggs in the nests of other species such as seagulls, abandoning them to be raised by those birds.
Conservation
The Canary Islands oystercatcher became extinct during the 20th century. The Chatham oystercatcher is endemic to the Chatham Islands of New Zealand and is listed as endangered by the IUCN, while both the African and Eurasian oystercatchers are considered near threatened. There has been conflict with commercial shellfish farmers, but studies have found that the impact of oystercatchers is much smaller than that of shore crabs.
Species
The genus contains twelve species. Species in taxonomic order:
Several fossil species are known, including Haematopus sulcatus (Brodkorb, 1955) from the Barstovian of Florida and Zanclean of North Carolina, and which is evidently considered a synonym of the extant species Haematopus palliatus.
| Biology and health sciences | Charadriiformes | Animals |
220241 | https://en.wikipedia.org/wiki/Pratincole | Pratincole | The pratincoles or greywaders are a subfamily (Glareolinae) of birds which together with the coursers make up the family Glareolidae. They have short legs, very long pointed wings and long forked tails.
Description
Their most unusual feature for birds classed as waders is that they typically hunt their insect prey on the wing like swallows, although they can also feed on the ground. Their short bills are an adaptation to aerial feeding.
Their flight is fast and graceful like a swallow or a tern, with many twists and turns to pursue their prey. They are most active at dawn and dusk, resting in the warmest part of the day.
Like the coursers, the pratincoles are found in warmer parts of the Old World, from southern Europe and Africa east through Asia to Australia. Species breeding in temperate regions are long-distance migrants.
Their two to four eggs are laid on the ground in a bare scrape.
The downy pratincole chicks are able to run as soon as they are hatched.
The Australian pratincole, the only species not in the genus Glareola, is more terrestrial than the other pratincoles, and may be intermediate between this group and the coursers.
The name "pratincole" comes from the term pratincola coined by German naturalist Wilhelm Heinrich Kramer from the Latin words prātum meadow and incola resident.
Species list
Genus Stiltia
Australian pratincole Stiltia isabella
Genus Glareola
Collared pratincole Glareola pratincola
Oriental pratincole Glareola maldivarum
Black-winged pratincole Glareola nordmanni
Madagascar pratincole Glareola ocularis
Rock pratincole Glareola nuchalis
Grey pratincole Glareola cinerea
Small pratincole Glareola lactea
| Biology and health sciences | Charadriiformes | Animals |
220243 | https://en.wikipedia.org/wiki/Courser | Courser | The coursers are a subfamily (Cursoriinae) of birds which together with the pratincoles make up the family Glareolidae. They have long legs, short wings and long pointed bills which curve downwards. Their most unusual feature for birds classed as waders is that they inhabit deserts and similar arid regions.
They have cryptic plumage and crouch down when alarmed to avoid detection by predators.
Like the pratincoles, the coursers are found in warmer parts of the Old World. They hunt insects by running.
Their 2–3 eggs are laid on the ground.
Species in taxonomic order
Cream-colored courser Cursorius cursor
Somali courser Cursorius somalensis
Temminck's courser Cursorius temminckii
Indian courser Cursorius coromandelicus
Burchell's courser Cursorius rufus
Double-banded courser or two-banded courser, Rhinoptilus africanus
Three-banded courser or Heuglin's courser, Rhinoptilus cinctus
Bronze-winged courser or violet-tipped courser, Rhinoptilus chalcopterus
Jerdon's courser Rhinoptilus bitorquatus
| Biology and health sciences | Charadriiformes | Animals |
220432 | https://en.wikipedia.org/wiki/Xylitol | Xylitol | Xylitol is a chemical compound with the formula , or HO(CH2)(CHOH)3(CH2)OH; specifically, one particular stereoisomer with that structural formula. It is a colorless or white crystalline solid. It is classified as a polyalcohol and a sugar alcohol, specifically an alditol. Of the common sugar alcohols, only sorbitol is more soluble in water.
The name derives from , xyl[on] 'wood', with the suffix -itol used to denote it being a sugar alcohol.
Xylitol is used as a food additive and sugar substitute. Its European Union code number is E967. Replacing sugar with xylitol in food products may promote better dental health, but evidence is lacking on whether xylitol itself prevents dental cavities. In the United States, xylitol is used as a common sugar substitute, and is considered to be safe for humans.
Xylitol can be toxic to dogs.
History
Emil Fischer, a German chemist, and his assistant Rudolf Stahel isolated a new compound from beech wood chips in September 1890 and named it , after the Greek word for wood. The following year, the French chemist M. G. Bertrand isolated xylitol syrup by processing wheat and oat straw. Sugar rationing during World War II led to an interest in sugar substitutes. Interest in xylitol and other polyols became intense, leading to their characterization and manufacturing methods.
Structure, production, commerce
Xylitol is one of three 5-carbon sugar alcohols. The others are arabitol and ribitol. These three compounds differ in the stereochemistry of the three secondary alcohol groups.
Xylitol occurs naturally in small amounts in plums, strawberries, cauliflower, and pumpkin; humans and many other animals make trace amounts during metabolism of carbohydrates. Unlike most sugar alcohols, xylitol is achiral. Most other isomers of pentane-1,2,3,4,5-pentol are chiral, but xylitol has a plane of symmetry.
Industrial production starts with lignocellulosic biomass from which xylan is extracted; raw biomass materials include hardwoods, softwoods, and agricultural waste from processing maize, wheat, or rice. The mixture is hydrolyzed with acid to give xylose. The xylose is purified by chromatography. Purified xylose is catalytically hydrogenated into xylitol using a Raney nickel catalyst. The conversion changes the sugar (xylose, an aldehyde) into the primary alcohol, xylitol.
Xylitol can also be obtained by industrial fermentation, but this methodology are not as economical as the acid hydrolysis/chromatography route described above. Fermentation is effected by bacteria, fungi, or yeast, especially Candida tropicalis. According to the US Department of Energy, xylitol production by fermentation from discarded biomass is one of the most valuable renewable chemicals for commerce, forecast to be a US $1.41 billion industry by 2025.
Uses
Xylitol is used as a sugar substitute in such manufactured products as drugs, dietary supplements, confections, toothpaste, and chewing gum, but is not a common household sweetener. Xylitol has negligible effects on blood sugar because its assimilation and metabolism are independent of insulin. It is approved as a food additive and sugar substitute in the United States.
Xylitol is also found as an additive to saline solution for nasal irrigation and has been reported to be effective in improving symptoms of chronic sinusitis.
Xylitol can also be incorporated into fabrics to produce a cooling fabric. When moisture, such as sweat, comes into contact with the xylitol embedded in the fabric, it produces a cooling sensation.
Food properties
Nutrition, taste, and cooking
Humans absorb xylitol more slowly than sucrose, and xylitol supplies 40% fewer calories than an equal mass of sucrose.
Xylitol has about the same sweetness as sucrose, but is sweeter than similar compounds like sorbitol and mannitol.
Xylitol is stable enough to be used in baking, but because xylitol and other polyols are more heat-stable, they do not caramelise as sugars do. When used in foods, they lower the freezing point of the mixture.
Food risks
No serious health risk exists in most humans for normal levels of consumption. The European Food Safety Authority has not set a limit on daily intake of xylitol. Due to the adverse laxative effect that all polyols have on the digestive system in high doses, xylitol is banned from soft drinks in the European Union. Similarly, due to a 1985 report by the E.U. Scientific Committee on Food which states that "ingesting 50 g a day of xylitol can cause diarrhea", tabletop sweeteners (as well as other products containing xylitol) are required to display the warning "Excessive consumption may induce laxative effects".
Metabolism
Xylitol has 2.4 kilocalories of food energy per gram of xylitol (10 kilojoules per gram) according to U.S. and E.U. food-labeling regulations. The real value can vary, depending on metabolic factors.
Primarily, the liver metabolizes absorbed xylitol. The main metabolic route in humans occurs in cytoplasm, via nonspecific NAD-dependent dehydrogenase (polyol dehydrogenase), which transforms xylitol to -xylulose. Specific xylulokinase phosphorylates it to -xylulose-5-phosphate. This then goes to pentose phosphate pathway for further processing.
About 50% of eaten xylitol is absorbed via the intestines. Of the remaining 50% that is not absorbed by the intestines, in humans, 50–75% of the xylitol remaining in the gut is fermented by gut bacteria into short-chain organic acids and gases, which may produce flatulence. The remnant unabsorbed xylitol that escapes fermentation is excreted unchanged, mostly in feces; less than 2 g of xylitol out of every 100 g ingested is excreted via urine.
Xylitol ingestion also increases motilin secretion, which may be related to xylitol's ability to cause diarrhea. The less-digestible but fermentable nature of xylitol also contributes to constipation relieving effects.
Health effects
Dental care
A 2015 Cochrane review of ten studies between 1991 and 2014 suggested a positive effect in reducing tooth decay of xylitol-containing fluoride toothpastes when compared to fluoride-only toothpaste, but there was insufficient evidence to determine whether other xylitol-containing products can prevent tooth decay in infants, children or adults. Subsequent reviews support the belief that xylitol can suppress the growth of pathogenic Streptococcus in the mouth, thereby reducing dental caries and gingivitis, although there is concern that swallowed xylitol may cause intestinal dysbiosis. A 2022 review suggested that xylitol-containing chewing gum decreases plaque, but not xylitol-containing candy.
Earache
In 2011 EFSA "concluded that there was not enough evidence to support" the claim that xylitol-sweetened gum could prevent middle-ear infections, also known as acute otitis media (AOM). A 2016 review indicated that xylitol in chewing gum or a syrup may have a moderate effect in preventing AOM in healthy children. It may be an alternative to conventional therapies (such as antibiotics) to lower risk of earache in healthy children – reducing risk of occurrence by 25% – although there is no definitive proof that it could be used as a therapy for earache.
Diabetes
In 2011, EFSA approved a marketing claim that foods or beverages containing xylitol or similar sugar replacers cause lower blood glucose and lower insulin responses compared to sugar-containing foods or drinks. Xylitol products are used as sucrose substitutes for weight control, as xylitol has 40% fewer calories than sucrose (2.4 kcal/g compared to 4.0 kcal/g for sucrose). The glycemic index (GI) of xylitol is only 7% of the GI for glucose.
Adverse effects
Humans
When ingested at high doses, xylitol and other polyols may cause gastrointestinal discomfort, including flatulence, diarrhea, and irritable bowel syndrome (see Metabolism above); some people experience the adverse effects at lower doses. Xylitol has a lower laxation threshold than some sugar alcohols but is more easily tolerated than mannitol and sorbitol.
Increased xylitol consumption can increase oxalate, calcium, and phosphate excretion to urine (termed oxaluria, calciuria, and phosphaturia, respectively). These are known risk factors for kidney stone disease, but despite that, xylitol has not been linked to kidney disease in humans.
A 2024 study suggests that xylitol is prothrombotic (increases clotting) and is associated with cardiovascular risk when consumed at "typical dietary amounts".
Dogs and other animals
Xylitol is poisonous to dogs. Ingesting 100 milligrams of xylitol per kilogram of body weight (mg/kg bw) causes dogs to experience a dose-dependent insulin release; depending on the dose it can result in life-threatening hypoglycemia. Hypoglycemic symptoms of xylitol toxicity may arise as quickly as 30 to 60 minutes after ingestion. Vomiting is a common first symptom, which can be followed by tiredness and ataxia. At doses above 500 mg/kg bw, liver failure is likely and may result in coagulopathies like disseminated intravascular coagulation.
Xylitol is safe for rhesus macaques, horses, and rats.
A 2018 study suggests that xylitol is safe for cats in doses of up to 1000 mg/kg; however, this study was performed on only 6 cats and should not be considered definitive.
| Physical sciences | Sugar alcohols | Chemistry |
220443 | https://en.wikipedia.org/wiki/Choanoflagellate | Choanoflagellate | Choanoflagellates are a group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of animals. The name refers to the characteristic funnel-shaped "collar" of interconnected microvilli and the presence of a flagellum. Choanoflagellates are found globally in aquatic environments, and they are of particular interest to evolutionary biologists studying the origins of multicellularity in animals.
The flagellum of choanoflagellates is surrounded by microvilli at its base. Movement of the flagellum creates water currents that can propel free-swimming choanoflagellates through the water column and trap bacteria and detritus against the microvilli, where these foodstuffs are engulfed. This feeding plays an ecological role in the carbon cycle by linking different trophic levels.
Choanoflagellates bear morphological similarities to the choanocyte, a type of cell in sponges, and this provided an early clue to their close relationship with animals, which was later upheld by genetic analyses. As the sister group to Animalia, choanoflagellates serve as a useful model for reconstructions of the last unicellular ancestor of animals. According to a 2021 study, crown group craspedids (and perhaps crown group choanoflagellates if Acanthoecida arose within Craspedida) appeared 422.78 million years ago, Although a previous study from 2017 recovered the divergence of the crown group choanoflagellates (craspedids) at 786.62 million years.
Etymology
Choanoflagellate is a hybrid word from Greek meaning "funnel" (due to the shape of the collar) and the Latin word (whence English flagellum).
Appearance
Each choanoflagellate has a single flagellum, surrounded by a ring of actin-filled protrusions called microvilli, forming a cylindrical or conical "collar" ( in Greek). Movement of the flagellum draws water through the collar, and bacteria and detritus are captured by the microvilli and ingested. Water currents generated by the flagellum also push free-swimming cells along, as in animal sperm. In contrast, most other flagellates are pulled by their flagella.
In addition to the single apical flagellum surrounded by actin-filled microvilli that characterizes choanoflagellates, the internal organization of organelles in the cytoplasm is constant. A flagellar basal body sits at the base of the apical flagellum, and a second, non-flagellar basal body rests at a right angle to the flagellar base. The nucleus occupies an apical-to-central position in the cell, and food vacuoles are positioned in the basal region of the cytoplasm. Additionally, the cell body of many choanoflagellates is surrounded by a distinguishing extracellular matrix or periplast. These cell coverings vary greatly in structure and composition and are used by taxonomists for classification purposes. Many choanoflagellates build complex basket-shaped "houses", called lorica, from several silica strips cemented together. The functional significance of the periplast is unknown, but in sessile organisms, it is thought to aid attachment to the substrate. In planktonic organisms, there is speculation that the periplast increases drag, thereby counteracting the force generated by the flagellum and increasing feeding efficiency.
Choanoflagellates are either free-swimming in the water column or sessile, adhering to the substrate directly or through either the periplast or a thin pedicel. Although choanoflagellates are thought to be strictly free-living and heterotrophic, a number of choanoflagellate relatives, such as members of Ichthyosporea or Mesomycetozoa, follow a parasitic or pathogenic lifestyle. The life histories of choanoflagellates are poorly understood. Many species are thought to be solitary; however, coloniality seems to have arisen independently several times within the group, and colonial species still retain a solitary stage.
Ecology
Over 125 extant species of choanoflagellates are known, distributed globally in marine, brackish and freshwater environments from the Arctic to the tropics, occupying both pelagic and benthic zones. Although most sampling of choanoflagellates has occurred between , they have been recovered from as deep as in open water and under Antarctic ice sheets. Many species are hypothesized to be cosmopolitan on a global scale [e.g., Diaphanoeca grandis has been reported from North America, Europe and Australia (OBIS)], while other species are reported to have restricted regional distributions. Co-distributed choanoflagellate species can occupy quite different microenvironments, but in general, the factors that influence the distribution and dispersion of choanoflagellates remain to be elucidated.
A number of species, such as those in the genus Proterospongia, form simple colonies, planktonic clumps that resemble a miniature cluster of grapes in which each cell in the colony is flagellated or clusters of cells on a single stalk. A colonial species from Mono Lake, Barroeca monosierra, forms spheres filled with a branched network of an extracellular matrix where a microbiome of different species of symbiotic bacteria live. In October 2019, scientists found a new band behaviour of choanoflagellates: they apparently can coordinate to respond to light.
The choanoflagellates feed on bacteria and link otherwise inaccessible forms of carbon to organisms higher in the trophic chain. Even today, they are important in the carbon cycle and microbial food web. There is some evidence that choanoflagellates feast on viruses as well.
Life cycle
Choanoflagellates grow vegetatively, with multiple species undergoing longitudinal fission; however, the reproductive life cycle of choanoflagellates remains to be elucidated. A paper released in August 2017 showed that environmental changes, including the presence of certain bacteria, trigger the swarming and subsequent sexual reproduction of choanoflagellates. The ploidy level is unknown; however, the discovery of both retrotransposons and key genes involved in meiosis previously suggested that they used sexual reproduction as part of their life cycle. Some choanoflagellates can undergo encystment, which involves the retraction of the flagellum and collar and encasement in an electron dense fibrillar wall. On transfer to fresh media, excystment occurs; though it remains to be directly observed.
Evidence for sexual reproduction has been reported in the choanoflagellate species Salpingoeca rosetta. Evidence has also been reported for the presence of conserved meiotic genes in the choanoflagellates Monosiga brevicollis and Monosiga ovata.
Silicon biomineralization
The Acanthoecid choanoflagellates produce an extracellular basket structure known as a lorica. The lorica is composed of individual costal strips, made of a silica-protein biocomposite. Each costal strip is formed within the choanoflagellate cell and is then secreted to the cell surface. In nudiform choanoflagellates, lorica assembly takes place using a number of tentacles once sufficient costal strips have been produced to comprise a full lorica. In tectiform choanoflagellates, costal strips are accumulated in a set arrangement below the collar. During cell division, the new cell takes these costal strips as part of cytokinesis and assembles its own lorica using only these previously produced strips.
Choanoflagellate biosilicification requires the concentration of silicic acid within the cell. This is carried out by silicon transporter (SiT) proteins. Analysis of choanoflagellate SiTs shows that they are similar to the SiT-type silicon transporters of diatoms and other silica-forming stramenopiles. The SiT gene family shows little or no homology to any other genes, even to genes in non-siliceous choanoflagellates or stramenopiles. This suggests that the SiT gene family evolved via a lateral gene transfer event between Acanthoecids and Stramenopiles. This is a remarkable case of horizontal gene transfer between two distantly related eukaryotic groups, and has provided clues to the biochemistry and silicon-protein interactions of the unique SiT gene family.
Classification
Relationship to metazoans
Félix Dujardin, a French biologist interested in protozoan evolution, recorded the morphological similarities of choanoflagellates and sponge choanocytes and proposed the possibility of a close relationship as early as 1841. Over the past decade, this hypothesized relationship between choanoflagellates and animals has been upheld by independent analyses of multiple unlinked genetic sequences: 18S rDNA, nuclear protein-coding genes, and mitochondrial genomes (Steenkamp, et al., 2006; Burger, et al., 2003; Wainright, et al., 1993). Importantly, comparisons of mitochondrial genome sequences from a choanoflagellate and three sponges confirm the placement of choanoflagellates as an outgroup to Metazoa (animals, also known as Animalia) and negate the possibility that choanoflagellates evolved from metazoans (Lavrov, et al., 2005). Finally, a 2001 study of genes expressed in choanoflagellates has revealed that choanoflagellates synthesize homologues of metazoan cell signaling and adhesion genes. Genome sequencing shows that, among living organisms, the choanoflagellates are most closely related to animals.
Because choanoflagellates and metazoans are closely related, comparisons between the two groups promise to provide insights into the biology of their last common ancestor and the earliest events in metazoan evolution. The choanocytes (also known as "collared cells") of sponges (considered among the most basal metazoa) have the same basic structure as choanoflagellates. Collared cells are found in other animal groups, such as ribbon worms, suggesting this was the morphology of their last common ancestor. The last common ancestor of animals and choanoflagellates was unicellular, perhaps forming simple colonies; in contrast, the last common ancestor of all eumetazoan animals was a multicellular organism, with differentiated tissues, a definite "body plan", and embryonic development (including gastrulation). The timing of the splitting of these lineages is difficult to constrain, but was probably in the late Precambrian, >.
External relationships of Choanoflagellatea.
Phylogenetic relationships
The choanoflagellates were included in Chrysophyceae until Hibberd, 1975. Recent molecular phylogenetic reconstruction of the internal relationships of choanoflagellates allows the polarization of character evolution within the clade. Large fragments of the nuclear SSU and LSU ribosomal RNA, alpha tubulin, and heat-shock protein 90 coding genes were used to resolve the internal relationships and character polarity within choanoflagellates. Each of the four genes showed similar results independently and analysis of the combined data set (concatenated) along with sequences from other closely related species (animals and fungi) demonstrate that choanoflagellates are strongly supported as monophyletic and confirm their position as the closest known unicellular living relative of animals.
Previously, Choanoflagellida was divided into these three families based on the composition and structure of their periplast: Codonosigidae, Salpingoecidae and Acanthoecidae. Members of the family Codonosigidae appear to lack a periplast when examined by light microscopy, but may have a fine outer coat visible only by electron microscopy. The family Salpingoecidae consists of species whose cells are encased in a firm theca that is visible by both light and electron microscopy. The theca is a secreted covering predominately composed of cellulose or other polysaccharides. These divisions are now known to be paraphyletic, with convergent evolution of these forms widespread. The third family of choanoflagellates, the Acanthoecidae, has been supported as a monophyletic group. This clade possess a synapomorphy of the cells being found within a basket-like lorica, providing the alternative name of "Loricate Choanoflagellates". The Acanthoecid lorica is composed of a series of siliceous costal strips arranged into a species-specific lorica pattern."
The choanoflagellate tree based on molecular phylogenetics divides into three well supported clades. Clade 1 and Clade 2 each consist of a combination of species traditionally attributed to the Codonosigidae and Salpingoecidae, while Clade 3 comprises species from the group taxonomically classified as Acanthoecidae. The mapping of character traits on to this phylogeny indicates that the last common ancestor of choanoflagellates was a marine organism with a differentiated life cycle with sedentary and motile stages.
Taxonomy
Choanoflagellates;
Order Craspedida Cavalier-Smith 1997 em. Nitsche et al. 2011
Family Salpingoecidae Kent 1880-1882
?Dicraspedella Ellis 1930
?Diploeca Ellis 1930
?Diplosigopsis Francé 1897
?Pachysoeca Ellis 1930
?Piropsis Meunier 1910
?Salpingorhiza Klug 1936
?Sphaerodendron Zhukov, Mylnikov & Moiseev 1976 non Seemann 1865
?Stelexomonas Lackey 1942
Astrosiga Kent 1880-1882
Aulomonas Lackey 1942
Choanoeca Ellis 1930
Cladospongia Iyengar & Ramathan 1940
Codonosigopsis Senn 1900
Diplosiga Frenzel 1891
Hartaetosiga Carr, Richter & Nitsche 2017
Mylnosiga Carr, Richter & Nitsche 2017
Lagenoeca Kent 1881
Microstomoeca Carr, Richter & Nitsche 2017
Paramonosiga Jeuck, Arndt & Nitsche 2014
Salpingoeca James-Clark 1868 non Ellis 1933
Stagondoeca Carr, Richter & Nitsche 2017
Family Codonosigaceae Kent 1880-1882
Codosiga James-Clark 1866
Desmarella Kent 1880-1882
Kentrosiga Schiller 1953
Monosiga Kent 1880-1882
Proterospongia Kent 1882
Sphaeroeca Lauterborn 1894 non Meyrick 1895
Stylochromonas Lackey 1940
Order Acanthoecida Norris 1965 em. Nitsche et al. 2011 (Loricate choanoflagellates)
Conioeca Thomsen & Ostergaard 2019
Family Acanthoecidae Norris 1965 em. Nitsche et al. 2011 (Nudiform choanoflagellates)
Acanthoeca Ellis 1930
Enibas Schiwitza, Arndt & Nitsche 2019
Helgoeca Leadbeater 2008
Polyoeca Kent 1880
Savillea Loeblich III 1967
Family Stephanoecidae Leadbeater 2011 (Tectiform choanoflagellates)
?Conion Thomsen 1982
?Spiraloecion Marchant & Perrin 1986
Acanthocorbis Hara & Takahashi 1984
Amoenoscopa Hara & Takahashi 1987
Apheloecion Thomsen 1983
Bicosta Leadbeater 1978
Calliacantha Leadbeater 1978
Calotheca Thomsen & Moestrup 1983 non Desv. 1810 non Spreng. 1817 non Heyden 1887
Cosmoeca Thomsen 1984
Crinolina Thomsen 1976 non Smetana 1982
Crucispina Espeland & Throndsen 1986
Diaphanoeca Ellis 1930
Didymoeca Doweld 2003
Kakoeca Buck & Marchant 1991
Monocosta Thomsen 1979 non Monocostus Schumann 1904
Nannoeca Thomsen 1988
Parvicorbicula Deflandre 1960
Pleurasiga Schiller 1925
Polyfibula Manton 1981
Saepicula Leadbeater 1980
Saroeca Thomsen 1979
Spinoeca Thomsen, Ostergaard & Hansen 1995 non Poulsen 1973
Stephanacantha Thomsen 1983
Stephanoeca Ellis 1930
Syndetophyllum Thomsen & Moestrup 1983
Thomsenella Özdikmen 2009
Genomes and transcriptomes
Monosiga brevicollis genome
The genome of Monosiga brevicollis, with 41.6 million base pairs, is similar in size to filamentous fungi and other free-living unicellular eukaryotes, but far smaller than that of typical animals. In 2010, a phylogenomic study revealed that several algal genes are present in the genome of Monosiga brevicollis. This could be due to the fact that, in early evolutionary history, choanoflagellates consumed algae as food through phagocytosis. Carr et al. (2010) screened the M. brevicollis genome for known eukaryotic meiosis genes. Of 19 known eukaryotic meiotic genes tested (including 8 that function in no other process than meiosis), 18 were identified in M. brevicollis. The presence of meiotic genes, including meiosis specific genes, indicates that meiosis, and by implication, sex is present within the choanoflagellates.
Salpingoeca rosetta genome
The genome of Salpingoeca rosetta is 55 megabases in size. Homologs of cell adhesion, neuropeptide and glycosphingolipid metabolism genes are present in the genome.
S. rosetta has a sexual life cycle and transitions between haploid and diploid stages. In response to nutrient limitation, haploid cultures of S. rosetta become diploid. This ploidy shift coincides with mating during which small, flagellated cells fuse with larger flagellated cells. There is also evidence of historical mating and recombination in S. rosetta.
S. rosetta is induced to undergo sexual reproduction by the marine bacterium Vibrio fischeri. A single V. fischeri protein, EroS fully recapitulates the aphrodisiac-like activity of live V. fisheri.
Other genomes
The single-cell amplified genomes of four uncultured marine choanoflagellates, tentatively called UC1–UC4, were sequenced in 2019. The genomes of UC1 and UC4 are relatively complete.
Transcriptomes
An EST dataset from Monosiga ovata was published in 2006. The major finding of this transcriptome was the choanoflagellate Hoglet domain and shed light on the role of domain shuffling in the evolution of the Hedgehog signaling pathway. M. ovata has at least four eukaryotic meiotic genes.
The transcriptome of Stephanoeca diplocostata was published in 2013. This first transcriptome of a loricate choanoflagellate led to the discovery of choanoflagellate silicon transporters. Subsequently, similar genes were identified in a second loricate species, Diaphanoeca grandis. Analysis of these genes found that the choanoflagellate silicon transporters show homology to the SIT-type silicon transporters of diatoms and have evolved through horizontal gene transfer.
An additional 19 transcriptomes were published in 2018. A large number of gene families previously thought to be animal-only were found.
| Biology and health sciences | Eukaryotes | Plants |
220444 | https://en.wikipedia.org/wiki/Morchella | Morchella | Morchella, the true morels, is a genus of edible sac fungi closely related to anatomically simpler cup fungi in the order Pezizales (division Ascomycota). These distinctive fungi have a honeycomb appearance due to the network of ridges with pits composing their caps. Morels are prized by gourmet cooks, particularly in Catalan and French cuisine, but can be toxic if consumed raw or undercooked. Due to difficulties in cultivation, commercial harvesting of wild morels has become a multimillion-dollar industry in the temperate Northern Hemisphere, in particular North America, Turkey, China, the Himalayas, India, and Pakistan where these highly prized fungi are found in abundance.
Typified by Morchella esculenta in 1794, the genus has been the source of considerable taxonomical controversy throughout the years, mostly with regard to the number of species involved, with some mycologists recognising as few as three species and others over thirty. Current molecular phylogenetics suggest there might be over seventy species of Morchella worldwide, most of them exhibiting high continental endemism and provincialism.
The genus is currently the focus of extensive phylogenetic, biogeographical, taxonomical and nomenclatural studies, and several new species have been described from Australia, Canada, Cyprus, Israel, Spain, and Turkey.
Description
Morels resemble a honeycomb due to the network of ridges with pits composing their caps. Morels have a convoluted head/cap, and are varied in shape and habitat.
Similar species
When gathering morels for the table, care must be taken to distinguish them from the poisonous "false morels", a term loosely applied to describe Gyromitra esculenta, Verpa bohemica, and other morel lookalikes. Although false morels are sometimes eaten without ill effect, they can cause severe gastrointestinal upset, loss of muscular coordination (including cardiac muscle), or even death. Incidents of poisoning usually occur when they are eaten in large quantities, inadequately cooked, or over several days in a row. False morels contain gyromitrin, an organic carcinogenic poison, hydrolyzed in the body into monomethylhydrazine (MMH). Gyromitra esculenta in particular, has been reported to be responsible for up to 23% of mushroom fatalities each year in Poland.
The key morphological features distinguishing false morels from true morels are as follows:
Gyromitra species often have a "wrinkled" or "cerebral" (brain-like) appearance to the cap due to multiple wrinkles and folds, rather than the honeycomb appearance of true morels due to ridges and pits.
Gyromitra esculenta has a cap that is usually reddish-brown in colour, but sometimes also chestnut, purplish-brown, or dark brown.
Gyromitra species are typically chambered in longitudinal section, while Verpa species contain a cottony substance inside their stem, in contrast to true morels which are always hollow.
The caps of Verpa species (V. bohemica, V. conica and others) are attached to the stem only at the apex (top of the cap), unlike true morels which have caps that are attached to the stem at, or near the base of the cap. The easiest way to distinguish Verpa species from Morchella species is to slice them longitudinally.
Taxonomy
The fruit bodies of Morchella species are highly polymorphic, varying in shape, color, and size. While in many cases they do not exhibit clear-cut distinguishing features microscopically, this has historically contributed to uncertainties in taxonomy. Discriminating between the various taxa described is further hindered by uncertainty over which of these are truly biologically distinct. Remarkably, some authors in the past had suggested that the genus contains as few as 3 to 6 species, while others recognised as many as 34. Efforts to clarify the situation and re-evaluate old classical names (such as Morchella elata and others) in accordance to current phylogenetic data have been challenging, due to vague or ambiguous original descriptions and loss of holotype material. In 2012, the simultaneous description of several new taxa from Europe by Clowez and North America by Kuo and colleagues resulted in several synonymities further complicating matters, until a transatlantic study by Richard and colleagues resolved many of these issues in 2014. The genus is currently undergoing extensive re-evaluation with regard to the taxonomic status of several species.
Early taxonomic history
Morchella Dill. ex Pers. : Fr. was typified by Christiaan Hendrik Persoon in 1794, with Morchella esculenta designated as the type species for the genus. Among early pioneers who took an interest in the genus, were mycologists Julius Vincenz von Krombholz and Émile Boudier, who, in 1834 and 1897 respectively, published several species and varieties, accompanied by meticulously illustrated iconographic plates. The seminal taxon Morchella elata, whose true identity still remains unresolved, was described by Elias Fries in 1822, from a fir forest in Sweden. Other classical, early-proposed names include Morchella deliciosa, also described by Fries in 1822, Morchella semilibera, the half-free morel, originally described by de Candolle and sanctioned by Fries in 1822, Morchella vulgaris, which was recombined by Samuel Gray as a distinct species in 1821 following a forma of M. esculenta previously proposed by Persoon, and Morchella angusticeps, a large-spored species described by American mycologist Charles Peck in 1887. Morchella purpurascens, the purple morel, was first described by Boudier as a variety of M. elata in 1897 based on an 1834 plate by Krombholz, and was recombined as a distinct species in 1985 by Emile Jacquetant. Morchella eximia, a globally-occurring fire-associated species was also described by Boudier in 1910. The old, widely applied name Morchella conica, featuring in many field guides and literature across several countries, has been shown by Richard and colleagues to be illegitimate.
Classification
About 80 species of Morchella were described until the turn of the 21st century (per the Index Fungorum), a number of which were later shown to be illegitimate or synonyms. As molecular tools became widely available in the new millennium, a revived interest in the genus commenced and several new species were proposed. In 2008 Kuo described Morchella tomentosa from burned coniferous forests in western North America. In 2010 Işiloğlu and colleagues described Morchella anatolica, a basal species from Turkey later shown to be sister to Morchella rufobrunnea. A study by Clowez described over 20 new species in 2012, while later in the same year, another study by Kuo and colleagues described 19 species from North America. However, several of these newly proposed names later turned out to be synonyms. An extensive taxonomical and nomenclatural revision of the genus provided by Richard and colleagues in 2014, applied names to 30 of the genealogical lineages recognized so far and clarified several synonymities. Also in 2014, Elliott and colleagues described Morchella australiana from sclerophyll forests in Australia, while Clowez and colleagues described Morchella fluvialis from riparian forests in Spain.
In 2015, Loizides and colleagues clarified the taxonomy of Morchella tridentina, a cosmopolitan species described under many names, and recombined Morchella kakiicolor as a distinct species. Later in the same year, Clowez and colleagues described Morchella palazonii from Spain, while Voitk and colleagues described Morchella laurentiana from Canada and Morchella eohespera, a cosmopolitan species present in several continents. In an extensive phylogenetic and morphological study from Cyprus in 2016, Loizides and colleagues added two more Mediterranean species, Morchella arbutiphila and Morchella disparilis, and resurrected Morchella dunensis as an autonomous species. In the same year, Taşkın and colleagues described four of the previously unnamed phylospecies from Turkey: Morchella conifericola, Morchella feekensis, Morchella magnispora and Morchella mediteterraneensis.
Section Rufobrunnea
Morchella anatolica
synonym: Morchella lanceolata
Morchella rufobrunnea
Section Morchella
Morchella americana
synonyms: Morchella californica, Morchella claviformis, Morchella esculentoides, Morchella populina
Morchella castaneae
synonyms: Morchella brunneorosea, Morchella brunneorosea var. sordida
Morchella diminutiva
Morchella dunensis
synonyms: Morchella esculenta f. dunensis, Morchella andalusiae
Morchella esculenta
synonyms: Morchella pseudoumbrina, Morchella pseudoviridis
Morchella fluvialis
Morchella galilaea
Morchella palazonii
Morchella prava
Morchella sceptriformis
synonym: Morchella virginiana
Morchella steppicola
Morchella ulmaria
synonym: Morchella cryptica
Morchella vulgaris
synonyms: Morchella acerina, Morchella anthracina, Morchella lepida, Morchella robiniae, Morchella spongiola
Section Distantes
Morchella angusticeps
Morchella arbutiphila
Morchella australiana
Morchella brunnea
Morchella conifericola
Morchella deliciosa
synonym: Morchella conica
Morchella disparilis
Morchella dunalii
synonym: Morchella fallax
Morchella elata
Morchella eohespera
Morchella eximia
synonyms: Morchella anthracophila, Morchella carbonaria, Morchella septimelata
Morchella eximioides
Morchella exuberans
synonym: Morchella capitata
Morchella feekensis
Morchella iberica
Morchella importuna
Morchella kakiicolor
synonym: Morchella quercus-ilicis f. kakiicolor
Morchella laurentiana
Morchella magnispora
Morchella mediterraneensis
Morchella populiphila
Morchella pulchella
Morchella punctipes
Morchella purpurascens
synonyms: Morchella elata var. purpurascens, Morchella conica, Morchella conica var. purpurascens, Morchella conica var. crassa
Morchella semilibera
synonyms: Morchella gigas, Morchella gigas var. tintinnabulum, Morchella hybrida, Morchella undosa, Morchella varisiensis, Morchella esculenta var. crassipes, Phallus gigas, Eromitra gigas, Phallus undosus, Phallus crassipes, Mitrophora hybrida, Mitrophora hybrida var. crassipes, Ptychoverpa gigas, Helvella hybrida
Morchella septentrionalis
Morchella sextelata
Morchella snyderi
Morchella tomentosa
Morchella tridentina
synonyms: Morchella quercus-ilicis, Morchella frustrata, Morchella elatoides, Morchella elatoides var. elagans, Morchella conica var. pseudoeximia
Unresolved classification
Morchella anteridiformis
Morchella apicata
Morchella bicostata
Morchella conicopapyracea
Morchella crassipes
Morchella deqinensis
Morchella distans
Morchella guatemalensis
Morchella herediana
Morchella hetieri
Morchella hortensis
Morchella hotsonii
Morchella hungarica
Morchella inamoena
Morchella intermedia
Morchella meiliensis
Morchella miyabeana
Morchella neuwirthii
Morchella norvegiensis
Morchella patagonica
Morchella patula
Morchella pragensis
Morchella procera
Morchella pseudovulgaris
Morchella rielana
Morchella rigida
Morchella rigidoides
Morchella smithiana
Morchella sulcata
Morchella tasmanica
Morchella tatari
Morchella tibetica
Morchella umbrina
Morchella umbrinovelutipes
Morchella vaporaria
Phylogeny
Early phylogenetic analyses supported the hypothesis that the genus comprises only a few species with considerable phenotypic variation. Subsequent multigenic DNA studies, however, have revealed more than a dozen genealogically distinct species in North America and at least as many in Europe. DNA studies revealed three discrete clades, or genetic groups, consisting of the "white morels" (Morchella rufobrunnea and M. anatolica), the "yellow morels" (M. esculenta and others), and the "black morels" (M. elata and others). The fire-associated species Morchella tomentosa, commonly known as the "gray morel", is distinct for its fine hairs on the cap ridges and sclerotia-like underground structures, and may also deserve its own clade based on DNA evidence. Within the yellow and black clades, there are dozens of distinct species, many endemic to individual continents or regions. This species-rich view is supported by studies in Western Europe, Turkey, Cyprus, Israel, China, Patagonia, and the Himalayas.
Early ancestral reconstruction tests by O'Donnell and collaborators postulated a western North American origin of morels and the genus was estimated to have diverged from its closest genealogical relatives Verpa and Disciotis in the early Cretaceous, approximately 129 million years ago (Mya). This date was later revised by Du and collaborators, placing the divergence of the genus in the late Jurassic, approximately 154 Mya. However, neither of these reconstructions had included Morchella anatolica in the analyses, whose phylogenetic placement remained at the time unresolved. Following genetic testing of isotype collection of M. anatolica by Taşkın and colleagues, this species was shown to nest in the ancestral /Rufobrunnea clade, together with the transcontinental M. rufobrunnea. This cast doubts over the accuracy of the original reconstructions, since both species of the ancestral /Rufobrunnea clade are present in the Mediterranean, while M. anatolica is altogether absent from North America. Updated ancestral area reconstructions by Loizides and colleagues using an expanded 79-species data set, have in 2021 refuted the previous hypothesis and designated the Mediterranean basin as the most probable place of origin of morels.
Distribution and habitat
Morels can be found in the temperate Northern Hemisphere, in particular North America, Turkey, China, the Himalayas, India, and Pakistan.
Yellow morels (Morchella esculenta and related species) are more commonly found under deciduous trees rather than conifers, while black morels (M. elata and related species) are mostly found in coniferous forests, disturbed ground and recently burned areas. Morchella galilaea, and occasionally M. rufobrunnea, appear to fruit in the autumn or winter months rather than spring, which is the typical fruiting season for morels. In the American Pacific Northwest, they can be found from April to August.
Efforts to cultivate morels at a large scale have rarely been successful and the commercial morel industry relies on the harvest of wild mushrooms.
Transcontinental species
Although many species within Morchella exhibit continental endemism and provincialism, several species have been phylogenetically shown to be present in more than one continent. So far, the list of transcontinental species includes M. americana, M. eohespera, M. eximia, M. exuberans, M. galilaea, M. importuna, M. populiphila, M. pulchella, M. rufobrunnea, M. semilibera, M. sextelata, M. steppicola, and M. tridentina. The reasons behind the widespread, cosmopolitan distribution of these species, are still puzzling. Some authors have hypothesized that such transcontinental occurrences are the result of accidental anthropogenic introductions, but this view has been disputed by others, who suggested an old and natural distribution, at least for some of these species which appear to be linked to indigenous flora. Long-distance spore dispersal has also been suggested as a possible dispersal mechanism for some species, especially those belonging to fire-adapted lineages. It has been suggested that the widespread but disjunct distribution of some morel species, especially early diverging lineages like M. rufobrunnea and M. tridentina, may be the result of climatic refugia from the Quaternary glaciation.
Ecology
The ecology of Morchella species is not well understood. Many species appear to form symbiotic or endophytic relationships with trees, while others appear to act as saprotrophs.
Tree species associated with Morchella vary greatly depending on the individual species, continent, or region. Trees commonly associated with morels in Europe and across the Mediterranean include Abies (fir), Pinus (pine), Populus (poplar), Ulmus (elm), Quercus (oak), Arbutus (strawberry trees), Castanea (chestnut), Alnus (alder), Olea (olive trees), Malus (apple trees), and Fraxinus (ash). In western North America morels are often found in coniferous forests, including species of Pinus (pine), Abies (fir), Larix (larch), and Pseudotsuga (Douglas-fir), as well as in Populus (cottonwood) riparian forests. Deciduous trees commonly associated with morels in the northern hemisphere include Fraxinus (ash), Platanus (sycamore), Liriodendron (tulip tree), dead and dying elms, cottonwoods, and old apple trees (remnants of orchards). Due to their springtime phenology (March–May), morels are hardly ever found in the vicinity of common poisonous mushrooms such as the death cap (Amanita phalloides), the sulphur tuft (Hypholoma fasciculare), or the fly agaric (Amanita muscaria). They can, however, occur alongside false morels (Gyromitra and Verpa species) and elfin saddles (Helvella species), which also appear in spring.
Association with wildfire
Certain Morchella species (M. eximia, M. importuna, M. tomentosa and others) exhibit a pyrophilic behaviour and may grow abundantly in forests which have been recently burned by a fire. Moderate-intensity fires are reported to produce higher abundances of morels than low- or high-intensity fires. This is caused by the soil becoming more alkaline as the result of wood ash combining with water and being absorbed into the soil which triggers the morels to fruit. Alkaline soil conditions which trigger fruiting have been observed and exploited with small-scale commercial cultivation of morels. Where fire suppression is practiced, morels often grow in small numbers in the same spot, year after year. If these areas are overrun by wildfire they often produce a bumper crop of black morels the following spring. Commercial pickers and buyers in North America target recently burned areas for this reason. These spots may be closely guarded by mushroom pickers, as morels are widely regarded as a delicacy and often a cash crop.
Cultivation
Due to the mushroom's prized fruit bodies, several attempts have been made to grow the fungus in culture. In 1901, Repin reported successfully obtaining fruit bodies in a cave in which cultures had been established in flower pots nine years previously in 1892.
More recently, small-scale commercial growers have had success growing morels by using partially shaded rows of mulched wood. The rows of mulch piles are inoculated with morel mushroom spores in a solution of water and molasses which are poured over the piles of mulch and then they are allowed to grow undisturbed for several weeks. A solution of wood ashes mixed in water and diluted is subsequently poured over the rows of wood mulch which triggers fruiting of the morels. Morels are known to appear after fires and the alkalinity produced by wood ash mixed with water initiate fruit body formation for most species of morels.
In 2021 it was announced that indoor cultivation of black morels had been successfully achieved after decades of research and experimentation with methods by The Danish Morel Project. The project has been able to cultivate 20 lbs of morels per square yard or around 10 kg per square metre with cost estimates expected to be similar to producing white button mushrooms (Agaricus bisporus). Previous attempts at cultivation had managed to produce sclerotia but encountered issues in getting them to reliably fruit. One of the breakthroughs with this project was growing them in a climate controlled environment in conjunction with grass which is involved in stimulating fruiting in the morel mycelium. Cultivation in this manner has been noted to produce superior morels for culinary uses since they can be assured to be insect, slug and dirt free and therefore do not need to be washed and cleaned like foraged morels. Since washing morels can negatively impact the texture, reliable cultivation may result in more versatility with this ingredient in the kitchen as well as making the delicacy more affordable and accessible.
Toxicity
The consumption of Morchella species can have adverse effects. In 2023, a Montana sushi restaurant serving them was linked to 51 people who experienced gastrointestinal illness, with two reported deaths and three other hospitalizations. The consumption of raw morels in particular is advised against. An unknown toxin can be neutralized via cooking. Additionally, cooked morels can reportedly cause symptoms of upset stomach when consumed with alcohol.
When eating this fungus for the first time, it is advised to consume a small amount to minimize any allergic reaction. As with all fungi, morels for consumption must be clean and free of decay. Morels growing in old apple orchards previously treated with the deprecated insecticide lead arsenate may accumulate levels of toxic lead and arsenic that are unsuitable for human consumption.
Uses
Morels, "almost universally associated with spring," can be found in many habitats. Morel may be more likely to fruit during a period of increasing heat following a chilly period, a preference which is credited for their abundance in areas with cold winters.
Black morels (Morchella elata) are often found on land that has been disturbed by logging burning.
Nutrition
Raw morel mushrooms are 90% water, 5% carbohydrates, 3% protein, and 1% fat. A 100 gram reference amount supplies 31 calories, and is a rich source of iron (94% of the Daily Value, DV), manganese, phosphorus, zinc, and vitamin D (34% DV, if having been exposed to sunlight or artificial ultraviolet light). Raw morels contain moderate levels of several B vitamins (table).
Gastronomical value and culinary uses
They have been called "prized delicacies...they are so esteemed in Europe that people used to set fire to their own forests in hopes of eliciting a bountiful morel crop the next spring!"
Morels are a feature of many cuisines, including Provençal. Their flavor is prized by chefs worldwide, with recipes and preparation methods designed to highlight and preserve it. As with most edible fungi, they are best when collected or bought fresh. They are sometimes added to meat and poultry dishes and soups, and can be used as pasta fillings. As morels are known to contain thermolabile toxins, they must always be cooked before eating.
Morels can be preserved in several ways: They can be 'flash frozen' by simply running under cold water or putting them in a bucket to soak for a few minutes, then spread on a baking tray and placed into a freezer. After freezing, they keep very well with the frozen glaze for a long time in airtight containers. However, when thawed they can sometimes turn slightly mushy, so they are best frozen after steaming or frying. Due to their natural porosity, morels may contain trace amounts of soil which cannot be easily washed out. Any visible soil should be removed with a brush, after cutting the body in half lengthwise, if needed. Mushroom hunters sometimes recommend soaking morels in a bowl of salt water briefly prior to cooking, although many chefs would disagree.
Drying is a popular and effective method for long-term storage, and morels are widely available commercially in this form. Any insect larvae which might be present in the fruit bodies usually drop out during the drying process. Dried morels can then be reconstituted by soaking for 10–20 minutes in warm water or milk, and the soaking liquid can be used as stock.
The flavor of morels is not just appreciated by humans; in Yellowstone National Park, black morels are also known to be consumed by grizzly bears (Ursus arctos horribilis).
In popular culture
Morel hunting is a common springtime activity. Mushroom collectors may carry a mesh collecting bag, so the spores can scatter as one carries the harvest.
Every spring, hundreds of morel enthusiasts gather in Boyne City, Michigan for the National Morel Mushroom Festival, a century-old event. As one observer stated, "if there is a modern, North American reenactment of Geoffrey Chaucer's Canterbury Tales this is it." Other festivals and hunting competitions in North America include the Illinois State Morel Mushroom Hunting Championship, the Ottawa Midwest Morel Fest and the Mesick Michigan Mushroom Festival.
Vernacular names
Morchella species have been called by many local names; some of the more colorful include dryland fish, because when sliced lengthwise then breaded and fried, their outline resembles the shape of a fish; hickory chickens, as they are known in many parts of Kentucky; and merkels or miracles, based on folklore, of how a mountain family was saved from starvation by eating morels. In parts of West Virginia, they are known as molly moochers, muggins, or muggles. Due to the partial structural and textural similarity to some species of Porifera (sponges), other common names for any true morel are sponge mushroom and waffle mushroom. In the Appalachian woodlands, morels have also been called haystacks, or snakeheads. The Finnish vernacular name huhtasieni, refers to huhta, area cleared for agriculture by the slash and burn method.
The scientific name of the genus Morchella itself, is thought to have derived from morchel, an old German word close to "Möhre", carrot or beet, due to similarity in shape.
| Biology and health sciences | Edible fungi | Plants |
220446 | https://en.wikipedia.org/wiki/Ascus | Ascus | An ascus (; : asci) is the sexual spore-bearing cell produced in ascomycete fungi. Each ascus usually contains eight ascospores (or octad), produced by meiosis followed, in most species, by a mitotic cell division. However, asci in some genera or species can occur in numbers of one (e.g. Monosporascus cannonballus), two, four, or multiples of four. In a few cases, the ascospores can bud off conidia that may fill the asci (e.g. Tympanis) with hundreds of conidia, or the ascospores may fragment, e.g. some Cordyceps, also filling the asci with smaller cells. Ascospores are nonmotile, usually single celled, but not infrequently may be coenocytic (lacking a septum), and in some cases coenocytic in multiple planes. Mitotic divisions within the developing spores populate each resulting cell in septate ascospores with nuclei. The term ocular chamber, or oculus, refers to the epiplasm (the portion of cytoplasm not used in ascospore formation) that is surrounded by the "bourrelet" (the thickened tissue near the top of the ascus).
Typically, a single ascus will contain eight ascospores (or octad). The eight spores are produced by meiosis followed by a mitotic division. Two meiotic divisions turn the original diploid zygote nucleus into four haploid ones. That is, the single original diploid cell from which the whole process begins contains two complete sets of chromosomes. In preparation for meiosis, all the DNA of both sets is duplicated, to make a total of four sets. The nucleus that contains the four sets divides twice, separating into four new nuclei – each of which has one complete set of chromosomes. Following this process, each of the four new nuclei duplicates its DNA and undergoes a division by mitosis. As a result, the ascus will contain four pairs of spores. Then the ascospores are released from the ascus.
In many cases the asci are formed in a regular layer, the hymenium, in a fruiting body which is visible to the naked eye, here called an ascocarp or ascoma. In other cases, such as single-celled yeasts, no such structures are found. In rare cases asci of some genera can regularly develop inside older discharged asci one after another, e.g. Dipodascus.
Asci normally release their spores by bursting at the tip, but they may also digest themselves, passively releasing the ascospores either in a liquid or as a dry powder. Discharging asci usually have a specially differentiated tip, either a pore or an operculum. In some hymenium forming genera, when one ascus bursts, it can trigger the bursting of many other asci in the ascocarp resulting in a massive discharge visible as a cloud of spores – the phenomenon called "puffing". This is an example of positive feedback. A faint hissing sound can also be heard for species of Peziza and other cup fungi.
Asci, notably those of Neurospora crassa, have been used in laboratories for studying the process of meiosis, because the four cells produced by meiosis line up in regular order. By modifying genes coding for spore color and nutritional requirements, the biologist can study crossing over and other phenomena. The formation of asci and their use in genetic analysis are described in detail in Neurospora crassa.
Asci of most Pezizomycotina develop after the formation of croziers at their base. The croziers help maintain a brief dikaryon. The compatible nuclei of the dikaryon merge forming a diploid nucleus that then undergoes meiosis and ultimately internal ascospore formation. Members of the Taphrinomycotina and Saccharomycotina do not form croziers.
Classification
The form of the ascus, the capsule which contains the sexual spores, is important for classification of the Ascomycota. There are four basic types of ascus.
A unitunicate-operculate ascus has a "lid", the Operculum, which breaks open when the spores are mature and allows the spores to escape. Unitunicate-operculate asci only occur in those ascocarps which have apothecia, for instance the morels. 'Unitunicate' means 'single-walled'.
Instead of an operculum, a unitunicate-inoperculate ascus has an elastic ring that functions like a pressure valve. Once mature the elastic ring briefly expands and lets the spores shoot out. This type appears both in apothecia and in perithecia; an example is the illustrated Hypomyces chrysospermus.
A bitunicate ascus is enclosed in a double wall. This consists of a thin, brittle outer shell and a thick elastic inner wall. When the spores are mature, the shell splits open so that the inner wall can take up water. As a consequence this begins to extend with its spores until it protrudes above the rest of the ascocarp so that the spores can escape into free air without being obstructed by the bulk of the fruiting body. Bitunicate asci occur only in pseudothecia and are found only in the classes Dothideomycetes and Chaetothyriomycetes (which were formerly united in the old class Loculoascomycetes). Examples: Venturia inaequalis (apple scab) and Guignardia aesculi (Brown Leaf Mold of Horse Chestnut).
Prototunicate asci are mostly spherical in shape and have no mechanism for forcible dispersal. The mature ascus wall dissolves allowing the spores to escape, or it is broken open by other influences, such as animals. Asci of this type can be found both in perithecia and in cleistothecia, for instance with Dutch elm disease (Ophiostoma). This is something of a catch-all term for cases which do not fit into the other three ascus types, and they probably belong to several independent groups which evolved separately from unitunicate asci.
Ascospores
An ascospore is a spore contained in an ascus, or that was produced inside an ascus. This kind of spore is specific to fungi classified as ascomycetes (Ascomycota).
The ascospores of Blumeria graminis are formed and released under the humid conditions. After landing onto a suitable surface, unlike conidia, ascospores of Blumeria graminis showed a more variable developmental patterns.
The fungi Saccharomyces produces ascospores when grown on V-8 medium, acetate ascospore agar, or Gorodkowa medium. These ascospores are globose and located in asci. Each ascus contains one to four ascospores. The asci do not rupture at maturity. Ascospores are stained with Kinyoun stain and ascospore stain. When stained with Gram stain, ascospores are gram-negative while vegetative cells are gram-positive.
The fission yeast Schizosaccharomyces pombe is a single-celled haploid organism that reproduces asexually by mitosis and fission. However, exposure to the DNA damaging agent hydrogen peroxide induces pair-wise mating of haploid cells of opposite mating type to form transient diploid cells that then undergo meiosis to form asci, each with four ascospores. The production of viable ascospores depends on successful recombinational repair during meiosis. When this repair is defective a quality control mechanism prevents germination of damaged ascospores. These findings suggest that mating followed by meiosis is an adaptation for repairing DNA damage in the parental haploid cells in order to allow production of viable progeny ascospores.
| Biology and health sciences | Fungal morphology and anatomy | Biology |
220457 | https://en.wikipedia.org/wiki/Cycad | Cycad | Cycads are seed plants that typically have a stout and woody (ligneous) trunk with a crown of large, hard, stiff, evergreen and (usually) pinnate leaves. The species are dioecious, that is, individual plants of a species are either male or female. Cycads vary in size from having trunks only a few centimeters to several meters tall. They typically grow slowly and have long lifespans. Because of their superficial resemblance to palms or ferns, they are sometimes mistaken for them, but they are not closely related to either group.
Cycads are gymnosperms (naked-seeded), meaning their unfertilized seeds are open to the air to be directly fertilized by pollination, as contrasted with angiosperms, which have enclosed seeds with more complex fertilization arrangements. Cycads have very specialized pollinators, usually a specific species of beetle. Both male and female cycads bear cones (strobili), somewhat similar to conifer cones.
Cycads have been reported to fix nitrogen in association with various cyanobacteria living in the roots (the "coralloid" roots). These photosynthetic bacteria produce a neurotoxin called BMAA that is found in the seeds of cycads. This neurotoxin may enter a human food chain as the cycad seeds may be eaten directly as a source of flour by humans or by wild or feral animals such as bats, and humans may eat these animals. It is hypothesized that this is a source of some neurological diseases in humans. Another defence mechanism against herbivores is the accumulation of toxins in seeds and vegetative tissues; through horizontal gene transfer, cycads have acquired a family of genes (fitD) from a microbial organism, most likely a fungus, which gives them the ability to produce an insecticidal toxin.
Cycads all over the world are in decline, with four species on the brink of extinction and seven species having fewer than 100 plants left in the wild.
Description
Cycads have a cylindrical trunk which usually does not branch. However, some types of cycads, such as Cycas zeylanica, can branch their trunks. The apex of the stem is protected by modified leaves called cataphylls. Leaves grow directly from the trunk, and typically fall when older, leaving a crown of leaves at the top. The leaves grow in a rosette, with new foliage emerging from the top and center of the crown. The trunk may be buried, so the leaves appear to be emerging from the ground, so the plant appears to be a basal rosette. The leaves are generally large in proportion to the trunk size, and sometimes even larger than the trunk.
The leaves are pinnate (in the form of bird feathers, pinnae), with a central leaf stalk from which parallel "ribs" emerge from each side of the stalk, perpendicular to it. The leaves are typically either compound (with leaflets emerging from the leaf stalk as "ribs"), or have edges (margins) so deeply cut (incised) so as to appear compound. The Australian genus Bowenia and some Asian species of Cycas, like Cycas multipinnata, C. micholitzii and C. debaoensis, have leaves that are bipinnate, the leaflets each having their own subleaflets, growing in the same form on the leaflet as the leaflets do on the stalk.
Confusion with palms
Due to superficial similarities in foliage and plant structure, cycads and palms are often mistaken for each other. They also can occur in similar climates. However, they belong to different phyla and as such are not closely related. The similar structure is the product of convergent evolution.
Beyond those superficial resemblances, there are a number of differences between cycads and palms. For one, both male and female cycads are gymnosperms and bear cones (strobili), while palms are angiosperms and so flower and bear fruit. The mature foliage looks very similar between both groups, but the young emerging leaves of a cycad resemble a fiddlehead fern before they unfold and take their place in the rosette, while the leaves of palms are just small versions of the mature frond. Another difference is in the stem. Both plants leave some scars on the stem below the rosette where there used to be leaves, but the scars of a cycad are helically arranged and small, while the scars of palms are a circle that wraps around the whole stem. The stems of cycads are also in general rougher and shorter than those of palms.
Taxonomy
The two extant families of cycads all belong to the order Cycadales, and are the Cycadaceae and Zamiaceae (including Stangeriaceae). These cycads have changed little since the Jurassic in comparison to some other plant divisions. Five additional families belonging to the Medullosales became extinct by the end of the Paleozoic Era.
Based on genetic studies, cycads are thought to be more closely related to Ginkgo than to other living gymnosperms. Both are thought to have diverged from each other during the early Carboniferous.
Classification of the Cycadophyta to the rank of family.
Class Cycadopsida Brongniart 1843
Order Cycadales Persoon ex von Berchtold & Presl 1820
Suborder Cycadineae Stevenson 1992
Family Cycadaceae Persoon 1807
Genus Cycas
Suborder Zamiineae Stevenson 1992
Family Zamiaceae Horaninow 1834
subfamily Diooideae Pilg. 1926
Tribe Diooeae Schuster
Genus Dioon
subfamily Zamioideae Stevenson 1992
Tribe Encephalarteae Miquel 1861
Genus Macrozamia
Genus Lepidozamia
Genus Encephalartos
Tribe Zamieae Miquel 1861
Genus Bowenia
Genus Ceratozamia
Genus Stangeria
Genus Zamia
Genus Microcycas
Fossil genera
The following extinct cycad genera are known:
Amuriella Late Jurassic, Russian Far East (leaf fragments)
Androstrobus Triassic to Cretaceous, worldwide (leaf form genus)
Antarcticycas Middle Triassic, Antarctica (known from the whole plant)
?Anthrophyopsis Late Triassic, worldwide (leaf form genus, possibly a pteridospermatophyte)
Apoldia Triassic-Jurassic, Europe
Archaeocycas Early Permian, Texas (leaf with sporophylls)
Aricycas Late Triassic, Arizona (leaf form genus)
Beania (=Sphaereda), Triassic to Jurassic, Europe & Central Asia (leaf form genus)
Behuninia Late Jurassic, Colorado & Utah (fruiting structures)
Bucklandia Middle Jurassic to Early Cretaceous, Europe and India (leaf form genus)
Bureja Late Jurassic, Russia
Cavamonocolpites Early Cretaceous, Brazil (pollen)
Crossozamia Early to Late Permian, China (leaf form genus)
Ctenis Mesozoic-Paleogene, Worldwide (leaf form genus)
Ctenozamites Triassic-Cretaceous, worldwide (leaf form genus)
Cycadenia Triassic, Pennsylvania (trunks)
Cycadinorachis Late Jurassic, India (rachis)
Fascisvarioxylon Late Jurassic, India (petrified wood)
Gymnovulites, Latest Cretaceous/earliest Paleocene, India (seed)
Heilungia, Late Jurassic to early Cretaceous, Russia & Alaska (leaf form genus)
Leptocycas Late Triassic, North Carolina & China (known from the whole plant)
Mesosingeria, Jurassic to Early Cretaceous, Antarctica & Argentina (leaf form genus)
Michelilloa, Late Triassic, Argentina (stem)
?Nikania, Early Cretaceous, Russia (leaf fragments)
?Nilssonia, Middle Permian to Late Cretaceous, worldwide (leaf form genus) (possibly not a cycad)
?Nilssoniocladus, Early to Late Cretaceous, United States & Russia (stems, likely associated with Nilssonia, possibly deciduous)
Palaeozamia, Middle Jurassic, England
Paracycas, Middle Jurassic to Late Jurassic, Europe and Central Asia
?Phasmatocycas, Late Carboniferous to Early Permian, Kansas, Texas & New Mexico (leaf with sporophylls)
Pleiotrichium, Late Cretaceous, Germany (leaf)
Pseudoctenis, Late Permian to Late Cretaceous, worldwide (leaf form genus)
Sarmatiella, Late Triassic, Ukraine
Stangerites, Late Triassic to Early Jurassic, Virginia and Mexico (leaf form genus)
Sueria, Early Cretaceous, Argentina (leaf)
Taeniopteris, Carboniferous to Cretaceous, worldwide (polyphyletic leaf form genus, also includes bennettitales and marattialean ferns)
Fossil record
The oldest probable cycad foliage is known from the latest Carboniferous-Early Permian of South Korea and China, such as Crossozamia. Unambiguous fossils of cycads are known from the Early-Middle Permian onwards. Cycads were generally uncommon during the Permian. The two living cycad families are thought to have split from each other sometime between the Jurassic and Carboniferous. Cycads are thought to have reached their apex of diversity during the Mesozoic. Although the Mesozoic is sometimes called the "Age of Cycads," some other groups of extinct seed plants with similar foliage, such as Bennettitales and Nilssoniales, that are not closely related, may have been more abundant. The oldest records of the modern genus Cycas are from the Paleogene of East Asia. Fossils assignable to Zamiaceae are known from the Cretaceous, with fossils assignable to living genera of the family known from the Cenozoic.
Distribution
The living cycads are found across much of the subtropical and tropical parts of the world, with a few in temperate regions such as in Australia. The greatest diversity occurs in South and Central America. They are also found in Mexico, the Antilles, southeastern United States, Australia, Melanesia, Micronesia, Japan, China, Southeast Asia, Bangladesh, India, Sri Lanka, Madagascar, and southern and tropical Africa, where at least 65 species occur. Some can survive in harsh desert or semi-desert climates (xerophytic), others in wet rain forest conditions, and some in both. Some can grow in sand or even on rock, some in oxygen-poor, swampy, bog-like soils rich in organic material. Some are able to grow in full sun, some in full shade, and some in both. Some are salt tolerant (halophytes).
Species diversity of the extant cycads peaks at 17˚ 15"N and 28˚ 12"S, with a minor peak at the equator. There is therefore not a latitudinal diversity gradient towards the equator but towards the Tropic of Cancer and the Tropic of Capricorn. However, the peak near the northern tropic is largely due to Cycas in Asia and Zamia in the New World, whereas the peak near the southern tropic is due to Cycas again, and also to the diverse genus Encephalartos in southern and central Africa, and Macrozamia in Australia. Thus, the distribution pattern of cycad species with latitude appears to be an artifact of the geographical isolation of the remaining cycad genera and their species, and perhaps because they are partly xerophytic rather than simply tropical.
Cultural significance
Nuts of the Cycas orientis (nyathu) are coveted by the Yolngu in Australia's Arnhem Land as a source of food. They are harvested on their dry season to leach its poison under water overnight before ground into a paste, wrapped under bark and cooked on open fire until done. Roots of Zamia integrifolia were used by the Seminole and other native peoples to produce Florida arrowroot by a similar process.
In Vanuatu, the cycad is known as namele and is an important symbol of traditional culture. It serves as a powerful taboo sign, and a pair of namele leaves appears on the national flag and coat of arms. Together with the nanggaria plant, another symbol of Vanuatu culture, the namele also gives its name to Nagriamel, an indigenous political movement.
| Biology and health sciences | Gymnosperms | null |
220704 | https://en.wikipedia.org/wiki/Avocet | Avocet | The four species of avocets are a genus, Recurvirostra, of waders in the same avian family as the stilts. The genus name comes from Latin , 'curved backwards' and , 'bill'. The common name is thought to derive from the Italian (Ferrarese) word . Francis Willughby in 1678 noted it as the "Avosetta of the Italians".
Biology
Avocets have long legs and long, thin, upcurved bills which they sweep from side to side when feeding in the brackish or saline wetlands they prefer. Their plumage is pied, sometimes also with some red.
Members of this genus have webbed feet and readily swim. Their diet consists of aquatic insects and other small creatures.
Avocets nest on the ground in loose colonies. In estuarine settings, they may feed on exposed bay muds or mudflats. The nest is simply a lining of grass in a hollow in the ground. They lay three or four eggs of a dark greenish or brownish buff color, boldly marked with brown and black.
The pied avocet is the emblem of the Royal Society for the Protection of Birds.
Taxonomy
The genus Recurvirostra was introduced in 1758 by Swedish naturalist Carl Linnaeus in the 10th edition of his to contain a single species, the pied avocet, Recurvirostra avosetta. The genus name combines the Latin meaning 'bent' or 'curved backwards' with meaning 'bill'.
Species
The genus contains four species.
One fossil species, R. sanctaneboulae Mourer-Chauviré, 1978, dates from the late Eocene of France.
Range and habitat
In a large colony, they are aggressively defensive and chase off any other species of birds that try to nest among or near them. That causes the annoyed remark "Avocet: Exocet from some British birdwatchers.
They had been extirpated in Britain for a long time because of land reclamation of their habitat and persecution by skin and egg collectors, but during or soon after World War II, they started breeding on reclaimed land near the Wash, which was returned to salt marsh to make difficulties for any landing German invaders. Avocets use Titchfield Haven National Nature Reserve as a summer breeding ground.
| Biology and health sciences | Charadriiformes | Animals |
220705 | https://en.wikipedia.org/wiki/Stilt | Stilt | Stilt is a common name for several species of birds in the family Recurvirostridae, which also includes those known as avocets. They are found in brackish or saline wetlands in warm or hot climates.
They have extremely long legs, hence the group name, and long thin bills. Stilts typically feed on aquatic insects and other small creatures and nest on the ground surface in loose colonies.
Most sources recognize 6 species in 2 genera, although the white-backed and Hawaiian stilts are occasionally considered subspecies of the black-necked stilt.
The genus Charadrius was introduced by the French zoologist Mathurin Jacques Brisson in 1760 with the black-winged stilt (Himantopus himantopus) as the type species. The generic name Himantopus comes from the Ancient Greek meaning "strap-leg".
Species
The genus Himantopus contains five species:
Black-winged stilt, Himantopus himantopus
White-backed stilt, Himantopus melanurus
Pied stilt, Himantopus leucocephalus
Black-necked stilt, Himantopus mexicanus
Hawaiian stilt or aeʻo, Himantopus mexicanus knudseni
Black stilt, Himantopus novaezelandiae
The genus Cladorhynchus is monotypic and contains a single species:
Banded stilt, Cladorhynchus leucocephalus
A fossil stilt has been described by Bickart, 1990, as Himantopus olsoni, based on remains recovered in the Late Miocene Big Sandy Formation of Mohave County, Arizona, United States.
| Biology and health sciences | Charadriiformes | Animals |
220776 | https://en.wikipedia.org/wiki/Sturgeon | Sturgeon | {{Automatic taxobox
| name = Sturgeon
| image = Acipenser oxyrhynchus (edit).png
| image_caption = Atlantic sturgeon(Acipenser oxyrinchus oxyrinchus)
| fossil_range =
| taxon = Acipenseridae
| authority = Bonaparte, 1831
| subdivision_ranks = Genera
| subdivision =
†Boreiosturion
†Protoscaphirhynchus
†Engdahlichthys
†Anchiacipenser
†Priscosturion
Acipenser (paraphyletic)
Huso
ScaphirhynchusPseudoscaphirhynchus}}
Sturgeon (from Old English ultimately from Proto-Indo-European *str̥(Hx)yón-) is the common name for the 28 species of fish belonging to the family Acipenseridae. The earliest sturgeon fossils date to the Late Cretaceous, and are descended from other, earlier acipenseriform fish, which date back to the Early Jurassic period, some 174 to 201 million years ago. They are one of two living families of the Acipenseriformes alongside paddlefish (Polyodontidae). The family is grouped into four genera: Acipenser (which is paraphyletic, containing many distantly related sturgeon species), Huso, Scaphirhynchus, and Pseudoscaphirhynchus. Two species (A. naccarii and A. dabryanus) may be extinct in the wild, and one (P. fedtschenkoi) may be entirely extinct. Sturgeons are native to subtropical, temperate and sub-Arctic rivers, lakes and coastlines of Eurasia and North America. A Maastrichtian-age fossil found in Morocco shows that they also once lived in Africa.
Sturgeons are long-lived, late-maturing fishes with distinctive characteristics, such as a heterocercal caudal fin similar to those of sharks, and an elongated, spindle-like body that is smooth-skinned, scaleless, and armored with five lateral rows of bony plates called scutes. Several species can grow quite large, typically ranging in length. The largest sturgeon on record was a beluga female captured in the Volga Delta in 1827, measuring long and weighing . Most sturgeons are anadromous bottom-feeders, migrating upstream to spawn but spending most of their lives feeding in river deltas and estuaries. Some species inhabit freshwater environments exclusively, while others primarily inhabit marine environments near coastal areas, and are known to venture into open ocean.
Several species of sturgeon are harvested for their roe, which is processed into the luxury food caviar. This has led to serious overexploitation, which combined with other conservation threats, has brought most of the species to critically endangered status, at the edge of extinction.
Evolution
Fossil history
Acipenseriform fishes appeared in the fossil record some 174 to 201 million years ago, during the Early Jurassic, making them some of the earliest extant actinopterygian fishes. True sturgeons appear in the fossil record during the Upper Cretaceous, with amongst the oldest known remains being a partial skull from the Cenomanian (100–94 million years ago) of Alberta, Canada. In that time, sturgeons have undergone remarkably little morphological change, indicating their evolution has been exceptionally slow and earning them informal status as living fossils. This is explained in part by the long generation interval, tolerance for wide ranges of temperature and salinity, lack of predators due to size and bony plated armor, or scutes, and the abundance of prey items in the benthic environment. They do, however, still share several primitive characteristics, such as heterocercal tail, reduced squamation, more fin rays than supporting bony elements, and unique jaw suspension.
Phylogeny and taxonomy
Despite the existence of a fossil record, full classification and phylogeny of the sturgeon species has been difficult to determine, in part due to the high individual and ontogenic variation, including geographical clines in certain features, such as rostrum shape, number of scutes, and body length. A further confounding factor is the peculiar ability of sturgeons to produce reproductively viable hybrids, even between species assigned to different genera. While ray-finned fishes (Actinopterygii) have a long evolutionary history culminating in the most familiar fishes, past adaptive evolutionary radiations have left only a few survivors, such as sturgeons and gars.
The wide range of the acipenserids and their endangered status have made collection of systematic materials difficult. The factors have led researchers in the past to identify over 40 additional species that were rejected by later scientists. Whether the species in the Acipenser and Huso genera are monophyletic (descended from one ancestor) or paraphyletic (descended from many ancestors) is still unclear, though the morphologically motivated division between these two genera clearly is not supported by the genetic evidence. An effort is ongoing to resolve the taxonomic confusion using a continuing synthesis of systematic data and molecular techniques.
The phylogeny of Acipenseridae, as in the cladogram, shows that they evolved from the bony fishes. Approximate dates are from Near et al., 2012.
In currently accepted taxonomy, the class Actinopterygii and the order Acipenseriformes are both clades. The family Acipenseridae is subdivided into 2 subfamilies; Acipenserinae, including the genera Acipenser and Huso, and Scaphirhynchinae, including the genera Scaphirhynchus and Pseudoscaphirhynchus. However, multiple recent studies have recovered this arrangement as paraphyletic, instead finding A. oxyrhinchus and A. sturio to form the most basal clade among sturgeons, and all other species being in a separate clade, with the various other species of Acipenser, Scaphirhynchus, Pseudoscaphirhynchus, and Huso to have varying levels of relationship with one another.
A potential taxonomy of Acipenseridae is shown here, based on Luo et al. 2019, Nedoluzhko et al. 2020, and Shen et al. 2020. Note the paraphyletic relationships among genera:
The exact placement of Scaphirhynchus varies depending on the study and the methods used, with some placing it within the second-most basal clade comprising primarily Pacific species (shown above), whereas others place it in its own clade that is more derived than the secondmost basal clade but less derived than the most derived Atlantic and Central Asian clade. No studies have yet delineated a relationship between it and Pseudoscaphirhynchus. In addition, the exact relationships of the members of the most derived, primarily Atlantic clade vary, although most analyses at least find all the species in it to form a monophyletic clade. The placement of A. sinensis also varies by the study, with some placing it as the only Pacific member of the otherwise Atlantic-based most-derived clade, whereas others place it with the rest of the Pacific sturgeons as a sister to A. dabryanus.
Species
The family contains 8 extinct fossil species and 28 extant species/subspecies (include 1 species of Sterlet and 2 species of living fossils), in 4 genera. This list uses the original classification scheme:
Family Acipenseridae
Genus Acipenser Linnaeus, 1758
†Acipenser albertensis Lambe 1902 >
Acipenser baerii J. F. Brandt, 1869 (Siberian sturgeon)
Acipenser baerii baicalensis A. M. Nikolskii, 1896 (Baikal sturgeon)
Acipenser baerii stenorrhynchus A. M. Nikolskii, 1896
Acipenser brevirostrum Lesueur, 1818 (Shortnose sturgeon)
Acipenser dabryanus A. H. A. Duméril, 1869 (Yangtze sturgeon)
†Acipenser cruciferus (Cope 1876)
†Acipenser eruciferus Cope 1876
Acipenser fulvescens Rafinesque, 1817 (Lake sturgeon)
†Acipenser gigantissimus Nessov 1997
Acipenser gueldenstaedtii J. F. Brandt & Ratzeburg, 1833 (Russian sturgeon)
Acipenser medirostris Ayres, 1854 (Green sturgeon)
Acipenser mikadoi Hilgendorf, 1892 (Sakhalin sturgeon)
†Acipenser molassicus Probst 1882
Acipenser naccarii Bonaparte, 1836 (Adriatic sturgeon)
Acipenser nudiventris Lovetsky, 1828 (Fringebarbel sturgeon)
†Acipenser ornatus Leidy 1873
Acipenser oxyrinchus Mitchill, 1815
Acipenser oxyrinchus desotoi Vladykov, 1955 (Gulf sturgeon)
Acipenser oxyrinchus oxyrinchus Mitchill, 1815 (Atlantic sturgeon)
Acipenser persicus Borodin, 1897 (Persian sturgeon)
Acipenser ruthenus Linnaeus, 1758 (Sterlet)
Acipenser schrenckii J. F. Brandt, 1869 (Japanese sturgeon)
Acipenser sinensis J. E. Gray, 1835 (Chinese sturgeon)
Acipenser stellatus Pallas, 1771 (Starry sturgeon)
Acipenser sturio Linnaeus, 1758 (European sea sturgeon)
†Acipenser toliapicus Agassiz 1844 ex Woodward 1889
Acipenser transmontanus J. Richardson, 1836 (White sturgeon)
†Acipenser tuberculosus Probst 1882
Genus Huso J. F. Brandt & Ratzeburg, 1833
Huso dauricus (Georgi, 1775) (kaluga)
Huso huso (Linnaeus, 1758) (beluga)
Genus Scaphirhynchus Heckel, 1835 (native to North America)
Scaphirhynchus albus (Forbes & R. E. Richardson, 1905) (Pallid sturgeon)
Scaphirhynchus platorynchus (Rafinesque, 1820) (Shovelnose sturgeon)
Scaphirhynchus suttkusi J. D. Williams & Clemmer, 1991 (Alabama sturgeon)
Genus Pseudoscaphirhynchus Nikolskii, 1900 (native to Central Asia)
Pseudoscaphirhynchus fedtschenkoi (Kessler, 1872) (Syr Darya sturgeon)
Pseudoscaphirhynchus hermanni (Kessler, 1877) (Dwarf sturgeon)
Pseudoscaphirhynchus kaufmanni (Kessler, 1877) (Amu Darya sturgeon)
Range and habitat
Sturgeon range from subtropical to subarctic waters in North America and Eurasia. In North America, they range along the Atlantic Coast from the Gulf of Mexico to Newfoundland, including the Great Lakes and the St. Lawrence, Missouri, and Mississippi Rivers, as well as along the West Coast in major rivers from California and Idaho to British Columbia. They occur along the European Atlantic coast, including the Mediterranean basin, especially in the Adriatic Sea and the rivers of North Italy; in the rivers that flow into the Black, Azov, and Caspian Seas (Danube, Dnepr, Volga, Ural and Don); the north-flowing rivers of Russia that feed the Arctic Ocean (Ob, Yenisei, Lena, Kolyma); in the rivers of Central Asia (Amu Darya and Syr Darya) and Lake Baikal. In the Pacific Ocean, they are found in the Amur River along the Russian-Chinese border, on Sakhalin Island, and some rivers in northeast China.
Throughout this extensive range, almost all species are highly threatened or vulnerable to extinction due to a combination of habitat destruction, overfishing, and pollution.
No species is known to naturally occur south of the equator, though attempts at sturgeon aquaculture are being made in Uruguay, South Africa, and other places.
Most species are at least partially anadromous, spawning in fresh water and feeding in nutrient-rich, brackish waters of estuaries or undergoing significant migrations along coastlines. However, some species have evolved purely freshwater existences, such as the lake sturgeon (Acipenser fulvescens) and the Baikal sturgeon (A. baerii baicalensis), or have been forced into them by human or natural impoundment of their native rivers, as in the case of some subpopulations of white sturgeon (A. transmontanus) in the Columbia River and Siberian sturgeon (A. baerii) in the Ob basin.
Physical characteristics
Sturgeons retain several primitive characteristics from the bony fishes. Along with other members of the subclass Chondrostei, they are unique among bony fishes because their skeletons are almost entirely cartilaginous. To maintain structure, sturgeons are one of few organisms to retain a post-embryonic notochord that acts like a soft spine running through the body. Notably, however, the cartilaginous skeleton is not a primitive character, but a derived one; sturgeon ancestors had bony skeletons. They also lack vertebral centra, and are partially covered with five lateral rows of scutes rather than scales. They also have four barbels—sensory organs that precede their wide, toothless mouths. They navigate their riverine habitats traveling just off the bottom with their barbels dragging along gravel, or murky substrate. Sturgeon are recognizable for their elongated bodies, flattened rostra, distinctive scutes and barbels, and elongated upper tail lobes. The skeletal support for the paired fins of ray-finned fish is inside the body wall, although the ray-like structures in the webbing of the fins can be seen externally.
Sturgeons are among the largest fish: some beluga (Huso huso) in the Caspian Sea reportedly attain over and while for kaluga (H. dauricus) in the Amur River, similar lengths and over weights have been reported. They are also among the longest-lived of the fishes, some living well over 100 years and attaining sexual maturity at 20 years or more. The combination of slow growth and reproductive rates and the extremely high value placed on mature, egg-bearing females make sturgeon particularly vulnerable to overfishing.
Sturgeons are polyploid; some species have four, eight, or 16 sets of chromosomes.
Life cycle
Sturgeons are long-lived, late maturing fishes. Their average lifespan is 50 to 60 years, and their first spawn does not occur until they are around 15 to 20 years old. Sturgeons are broadcast spawners, and do not spawn every year because they require specific conditions. Those requirements may or may not be met every year due to varying environmental conditions, such as the proper photoperiod in spring, clear water with shallow rock or gravel substrate, where the eggs can adhere, and proper water temperature and flow for oxygenation of the eggs. A single female may release 100,000 to 3 million eggs, but not all will be fertilized. The fertilized eggs become sticky and adhere to the bottom substrate upon contact. Eight to 15 days are needed for the embryos to mature into larval fish. During that time, they are dependent on their yolk sacs for nourishment. River currents carry the larvae downstream into backwater areas, such as oxbows and sloughs, where the free-swimming fry spend their first year feeding on insect larvae and crustacea. During their first year of growth, they reach in length and migrate back into the swift-flowing currents in the main stem river.
Behavior
Sturgeons are primarily benthic feeders, with a diet of shellfish, crustaceans, and small fish. Exceptionally, both Huso'' species, the white sturgeon and the pallid sturgeon feed primarily on other fish as adults. They feed by extending their siphon-like mouths to suck food from the benthos. Having no teeth, they are unable to seize prey, though larger individuals and more predatory species can swallow very large prey items, including whole salmon. Sturgeons feed non-visually. They are believed to use a combination of sensors, including olfactory, tactile, and chemosensory cues detected by the four barbels, and electroreception using their ampullae of Lorenzini.
The sturgeons' electroreceptors are located on the head and are sensitive to weak electric fields generated by other animals or geoelectric sources. The electroreceptors are thought to be used in various behaviors such as feeding, mating and migration.
Many sturgeons leap completely out of the water, usually making a loud splash which can be heard half a mile away on the surface and probably farther under water. Why they do this is not known, but suggested functions include group communication to maintain group cohesion, catching airborne prey, courtship display, or to help shed eggs during spawning. Other plausible explanations include escape from predators, shedding parasites, or to gulp or expel air. Another explanation is that it "simply feels good". There have been some incidents of leaping sturgeon landing in boats and causing injuries to humans; in 2015, a 5-year-old girl was fatally injured after a sturgeon leapt from the Suwannee River and struck her.
Interactions with humans
Caviar
Globally, sturgeon fisheries are of great value, primarily as a source for caviar, but also for flesh. Several species of sturgeon are harvested for their roe which is processed into caviar—a delicacy, and the reason why caviar-producing sturgeons are among the most valuable and endangered of all wildlife resources.
During the 19th century, the US was the global leader in caviar production, having cornered 90% of the world's caviar trade. Atlantic sturgeon once thrived along the east coast from Canada down to Florida. They were in such abundance in the Hudson River that they were humorously called "Albany beef" and sturgeon eggs were given away at local bars as an accompaniment to 5¢ beer. White sturgeon populations along the US west coast declined simultaneously under the pressure of commercial fishing and human encroachment. Within the course of a century, the once abundant sturgeon fisheries in the US and Canada had drastically declined, and in some areas had been extirpated under the pressure of commercial overharvesting, pollution, human encroachment, habitat loss, and the damming of rivers that blocked their ancestral migration to spawning grounds.
By the turn of the century, commercial production of sturgeon caviar in the US and Canada had come to an end. Regulatory protections and conservation efforts were put in place by state and federal resource agencies in the US and Canada, such as the 1998 US federal moratorium that closed all commercial fishing for Atlantic sturgeon. It was during the 20th century that Russia grew to become the global leader as the largest producer and exporter of caviar. As with the decline in sturgeon populations in the US and Canada, the same occurred with sturgeon populations in the Caspian Sea.
Beginning with the 1979 US embargo on Iran, poaching and smuggling sturgeon caviar was big business but an illegal and dangerous one. Officers with the Washington Department of Fish and Wildlife (WDFW) busted a poaching ring that was based in Vancouver, Washington. The poachers had harvested 1.65 tons of caviar from nearly 2,000 white sturgeon that were poached from the Columbia River. The caviar was estimated to be worth around $2 million. WDFW busted another ring in 2003, and conducted an undercover sting operation in 2006–2007 that resulted in 17 successful attempts out of a total of 19.
In response to concerns over the future of sturgeons and associated commercial products, international trade for all species of sturgeons has been regulated under CITES since 1998.
Conservation
Sturgeons are threatened by the negative impacts of overfishing, poaching, habitat destruction, and the construction of dams that have altered or blocked their annual migration to ancestral spawning grounds. Some species of sturgeon are extinct, and several are on the verge of extinction, including the Chinese sturgeon, the highly prized beluga sturgeon, and the Alabama sturgeon. Many species are classified as threatened or endangered, with noticeable declines in sturgeon populations as the demand for caviar increases. IUCN data indicates that over 85% of sturgeon species are at risk of extinction, making them more critically endangered than any other group of animal species.
In addition to global restocking efforts, the monitoring of populations and habitat, and various other conservation efforts by national and state resource agencies as applicable to their respective countries, several conservation organizations have been formed to assist in the preservation of sturgeons around the world. On a global scale, one such organization is the World Sturgeon Conservation Society (WSCS) whose primary objectives include fostering the "conservation of sturgeon species and restoration of sturgeon stocks world-wide", and supporting the "information exchange among all persons interested in sturgeons." The North American Sturgeon and Paddlefish Society (NASPS) and Gesellschaft zur Rettung des Störs e.V. are WSCS affiliates. WSCS has been instrumental in organizing global conferences where scientists and researchers can exchange information and address the various conservation challenges that threaten the future of sturgeons. Conservation efforts at the grass roots level are also instrumental in helping to preserve sturgeon populations, such as Sturgeon For Tomorrow which was founded in 1977, consists of volunteers and a sturgeon guarding program to monitor known spawning sites. The organization has grown exponentially over the years and has become "the largest citizen advocacy group for sturgeon in the world", and has expanded with affiliate chapters in other states that have sturgeon populations. Other projects focus on specific local issues, such as the We Pass project, seeking a solution to the migratory impasse represented by the Iron Gates in the Danube River Basin. For example, currently all anadromous Danube sturgeon (all species except the predominantly freshwater sterlet) are now classed as Critically Endangered or extirpated from the upper and middle reaches of the Danube River above the dams.
Other uses
Before 1800, swim bladders of sturgeon (primarily Beluga sturgeon from Russia) were used as a source of isinglass, a form of collagen used historically for the clarification of wine and beer, as a predecessor for gelatin, and to preserve parchments.
The Jewish laws of kashrut, which only permit the consumption of fish with both scales and fins, forbids sturgeon, as they have ganoid scales instead of the permitted ctenoid and cycloid scales. While all Orthodox groups forbid the consumption of sturgeon, some Conservative groups do allow it. The theological debate over its kosher status can be traced back to such 19th-century reformers as Aron Chorin, though its consumption was already common in European Jewish communities.
Sturgeons were declared to be a royal fish under a statute dating back to 1324 by King Edward II of England. Technically, the British monarchy still owns all sturgeons, whales, and dolphins that inhabit the waters around England and Wales. Under the law of the United Kingdom, any sturgeons captured within the realm are personal property of the monarch.
Similar laws reserving sturgeon for the king were enforced in late medieval Denmark. An archaeological example of sturgeon in a royal context comes from the wreck of the Danish-Norwegian flagship, Gribshunden, which sank in June 1495 while King Hans sailed from Copenhagen to Kalmar, Sweden for a diplomatic summit. Archaeologists recovered from the wreck a cask containing a butchered sturgeon, probably intended for the king's table during feasts in Kalmar.
In heraldry, a sturgeon is the symbol on the coat of arms for Saint Amalberga of Temse.
| Biology and health sciences | Chondrosteans | null |
220825 | https://en.wikipedia.org/wiki/Freezing%20rain | Freezing rain | Freezing rain is rain maintained at temperatures below freezing by the ambient air mass that causes freezing on contact with surfaces. Unlike a mixture of rain and snow or ice pellets, freezing rain is made entirely of liquid droplets. The raindrops become supercooled while passing through a sub-freezing layer of air hundreds of meters above the ground, and then freeze upon impact with any surface they encounter, including the ground, trees, electrical wires, aircraft, and automobiles. The resulting ice, called glaze ice, can accumulate to a thickness of several centimeters and cover all exposed surfaces. The METAR code for freezing rain is FZRA.
A storm that produces a significant thickness of glaze ice from freezing rain is often referred to as an ice storm. Although these storms are not particularly violent, freezing rain is notorious for causing travel problems on roadways, breaking tree limbs, and downing power lines from the weight of accumulating ice. Downed power lines cause power outages in affected areas while accumulated ice can also pose significant overhead hazards. It is also known for being extremely dangerous to aircraft since the ice can effectively 'remould' the shape of the airfoil and flight control surfaces. (See atmospheric icing.)
Mechanism
Freezing rain is often associated with the approach of a warm front, when subfreezing air (temperatures at or below freezing) is trapped in the lowest levels of the atmosphere while warm air is advected aloft. This happens, for instance, when a low pressure system moves from the Mississippi River Valley toward the Appalachian Mountains and the Saint Lawrence River Valley of North America during the cold season, with a strong high pressure system sitting further east. This setup is known as cold-air damming, and is characterized by very cold and dry air at the surface within the region of high pressure. The warm air from the Gulf of Mexico is often the fuel for freezing precipitation.
Freezing rain develops when falling snow encounters a layer of warm air aloft, typically around the level, causing the snow to melt and become rain. As the rain continues to fall, it passes through a layer of subfreezing air just above the surface and cools to a temperature below freezing (). If this layer of subfreezing air is sufficiently deep, the raindrops may have time to freeze into ice pellets (sleet) before reaching the ground. However, if the subfreezing layer of air at the surface is very shallow, the rain drops falling through it will not have time to freeze and they will hit the ground as supercooled rain. When these supercooled drops make contact with the ground, power lines, tree branches, aircraft, or anything else below 0 °C, a portion of the drops instantly freezes, forming a thin film of ice, hence freezing rain. The specific physical process by which this occurs is called nucleation.
Observations
Surface observations by staffed or automatic stations are the only direct confirmation of freezing rain. One can never see directly freezing rain, rain, or snow on any type of weather radar, whether Doppler or conventional. It is possible, however, to estimate the area covered by freezing rain with radar indirectly.
The intensity of the radar echoes (reflectivity) is proportional to the form (water or ice) of the precipitation and its diameter. In fact, rain has much stronger reflective power than snow, but its diameter is much smaller. So, the reflectivity of rain coming from melted snow is only slightly higher. In the layer where the snow is melting, however, the wet flakes still have a large diameter and are coated with water, so the radar returns are much stronger.
The presence of this brightband indicates the presence of a warm layer above ground where snow can melt. This could be producing rain on the ground or the possibility of freezing rain if the temperature is below freezing. The accompanying image shows how such an artifact can be located with a cross-section through radar data. The height and slope of the brightband will give clues to the extent of the region where melting is occurring. Then, it is possible to associate this clue with surface observations and numerical prediction models to produce output such as the ones seen on television weather programs, where radar echoes are shown distinctly as rain, mixed, and snow precipitations.
Effects
At ground level
Freezing rain often causes major power outages by forming glaze ice. When the freezing rain or drizzle is light and not prolonged, the ice formed is thin and usually causes only minor damage (relieving trees of their dead branches, etc.). When large quantities accumulate, however, it is one of the most dangerous types of winter hazard. When the ice layer exceeds approximately , tree limbs with branches heavily coated in ice can break off under the enormous weight and fall onto power lines. Windy conditions and lightning, when present, will exacerbate the damage. Power lines coated with ice become extremely heavy, causing support poles, insulators and lines to break. The ice that forms on roadways makes vehicle travel dangerous. Unlike snow, wet ice provides almost no traction, and vehicles will slide even on gentle slopes. Because freezing rain does not hit the ground as an ice pellet (called "sleet") but still as a rain droplet, it conforms to the shape of the ground, or object such as a tree branch or car. This makes one thick layer of ice, often called "glaze".
Freezing rain and glaze ice on a large scale is called an ice storm. Effects on plants can be severe, as they cannot support the weight of the ice. Trees may snap as they are dormant and fragile during winter weather. Pine trees are also victims of ice storms as their needles will catch the ice, but not be able to support the weight. In February 1994, a severe ice storm caused over $1 billion in damage in the Southern United States, primarily in Mississippi, Tennessee, Alabama, and Western North Carolina, especially the Appalachians. One particularly severe ice storm struck eastern Canada and northern parts of New York and New England in the North American ice storm of 1998.
Aircraft
Freezing rain is considered to be an extreme hazard to aircraft, as it causes very rapid structural icing, freezing necessary components. Most helicopters and small airplanes lack the necessary deicing equipment to fly in freezing rain of any intensity, and heavy freezing rain can overwhelm even the most sophisticated deicing systems on large airplanes. Icing can increase an aircraft's weight but not typically enough to cause a hazard. The main danger comes from the ice changing the shape of its airfoils.
This will reduce lift and increase drag. All three factors increase stalling speed and reduce aircraft performance, making it very difficult to climb or even maintain altitude.
An aircraft can most easily avoid freezing rain by moving into warmer air. Under most conditions, this would require aircraft to descend, which it can usually do safely and easily even with a moderate accumulation of structural ice. However, freezing rain is accompanied by a temperature inversion aloft, meaning that aircraft are required to climb to move into warmer air, which is a potentially difficult and dangerous task with even a small amount of ice accumulation.
For example, in 1994, American Eagle Flight 4184 encountered heavy air traffic and poor weather that postponed the arrival of this flight at Chicago's O'Hare International Airport, where it was to have landed en route from Indianapolis, Indiana. The ATR-72, a twin-engine turboprop carrying 68 people, entered a holding pattern southeast of O'Hare. As the plane circled, supercooled cloud droplets, freezing rain or freezing drizzle formed a ridge of ice on the upper surface of its wings, eventually causing the aircraft's autopilot to suddenly disconnect and the pilots to lose control. The ATR disintegrated on impact with a field below; all passengers and crew were killed.
Ghost apples
On one occasion, freezing rain was observed to settle on hanging rotting apples and icing over them immediately, creating a glaze coating. Because of apples' lower freezing point than water, under warmer temperature the apples defrosted before the ice did, then the rotting apple mush slipped out of the bottom, leaving icy shells. These icy shells in the form of apples were called ghost apples and were observed in Michigan, United States in February 2019. A similar phenomenon was observed in February 2015 in Greenville, North Carolina, when a Jeep that had backed out of the parking lot left behind an ice shell of its lower front part (with grille and bumper) standing on icicles.
| Physical sciences | Precipitation | Earth science |
221047 | https://en.wikipedia.org/wiki/Flow%20measurement | Flow measurement | Flow measurement is the quantification of bulk fluid movement. Flow can be measured using devices called flowmeters in various ways. The common types of flowmeters with industrial applications are listed below:
Obstruction type (differential pressure or variable area)
Inferential (turbine type)
Electromagnetic
Positive-displacement flowmeters, which accumulate a fixed volume of fluid and then count the number of times the volume is filled to measure flow.
Fluid dynamic (vortex shedding)
Anemometer
Ultrasonic flow meter
Mass flow meter (Coriolis force).
Flow measurement methods other than positive-displacement flowmeters rely on forces produced by the flowing stream as it overcomes a known constriction, to indirectly calculate flow. Flow may be measured by measuring the velocity of fluid over a known area. For very large flows, tracer methods may be used to deduce the flow rate from the change in concentration of a dye or radioisotope.
Kinds and units of measurement
Both gas and liquid flow can be measured in physical quantities of kind volumetric flow rate or mass flow rates, with respective SI units such as cubic meters per second or kilograms per second, respectively. These measurements are related by the material's density. The density of a liquid is almost independent of conditions. This is not the case for gases, the densities of which depend greatly upon pressure, temperature and to a lesser extent, composition.
When gases or liquids are transferred for their energy content, as in the sale of natural gas, the flow rate may also be expressed in terms of energy flow, such as gigajoule per hour or BTU per day. The energy flow rate is the volumetric flow rate multiplied by the energy content per unit volume or mass flow rate multiplied by the energy content per unit mass. Energy flow rate is usually derived from mass or volumetric flow rate by the use of a flow computer.
In engineering contexts, the volumetric flow rate is usually given the symbol , and the mass flow rate, the symbol .
For a fluid having density , mass and volumetric flow rates may be related by .
Gas
Gases are compressible and change volume when placed under pressure, are heated or are cooled. A volume of gas under one set of pressure and temperature conditions is not equivalent to the same gas under different conditions. | Physical sciences | Fluid mechanics | Physics |
221081 | https://en.wikipedia.org/wiki/Hand%20%28unit%29 | Hand (unit) | The hand is a non-SI unit of measurement of length standardized to . It is used to measure the height of horses in many English-speaking countries, including Australia, Canada, Ireland, the United Kingdom, and the United States. It was originally based on the breadth of a human hand. The adoption of the international inch in 1959 allowed for a standardized imperial form and a metric conversion. It may be abbreviated to "h" or "hh". Although measurements between whole hands are usually expressed in what appears to be decimal format, the subdivision of the hand is not decimal but is in base 4, so subdivisions after the radix point are in quarters of a hand, which are inches. Thus, 62 inches is fifteen and a half hands, or 15.2 hh (normally said as "fifteen-two", or occasionally in full as "fifteen hands two inches").
Terminology
"Hands" may be abbreviated to "h", or "hh". The "hh" form is sometimes interpreted as standing for "hands high". When spoken aloud, hands are stated by numbers, 15.0 is "fifteen hands", 15.2 is alternately "fifteen-two" or "fifteen hands, two inches", and so on.
To convert inches to hands, the number in inches is divided by four, then the remainder is added after the radix point. Thus, a horse that measures 60 inches is 15 hands high (15 × 4 = 60) and a horse halfway between 15 and 16 hands is 15.2 hands, or 62 inches tall (15 × 4 + 2 = 62) Because the subdivision of a hand is a base 4 system, a horse 64 inches high is 16.0 hands high, not 15.4. A designation of "15.5 hands" is not halfway between 15 and 16 hands, but rather reads 15 hands and five inches, an impossibility in a base 4 radix numbering system, where a hand is four inches.
History
Ancient Egypt
The hand, sometimes also called a handbreadth or handsbreadth, is an anthropic unit, originally based on the breadth of a male human hand, either with or without the thumb, or on the height of a clenched fist.
On surviving Ancient Egyptian cubit-rods, the royal cubit is divided into seven palms of four digits or fingers each. Five digits are equal to a hand, with thumb; and six to a closed fist. The royal cubit measured approximately 525 mm, so the width of the ancient Egyptian hand was about 94 mm.
Biblical use
In Biblical exegesis the hand measurement, as for example in the Vision of the Temple, Authorized Version , is usually taken to be palm or handbreadth, and in modern translations may be rendered as "handbreadth" or "three inches".
United Kingdom
The hand is a traditional unit in the UK. It was standardised at four inches by a statute of King Henry VIII, the Horses Act 1540 (32 Hen. 8. c. 13), but some confusion between the various types of hand measurement, and particularly between the hand and the handsbreadth, appears to have persisted. Phillips's dictionary of 1706 gives four inches for the length of the handful or hand, and three inches for the handsbreadth; Mortimer gives the same, three inches for the Hand's-breadth, and four for the "Handful, or simply, Hand", but adds "The hand among horse-dealers, &c. is four-fingers' breadth, being the fist clenched, whereby the height of a horse is measured", thus equating "hand" with both the palm and the fist. Similarly, Wright's 1831 translation of Buffon mentions "A hand breadth (palmus), the breadth of the four fingers of the hand, or three inches", but the Encyclopædia Perthensis of 1816 gives under Palm (4): "A hand, or measure of lengths comprising three inches".
Use in measuring horses
Today the hand is used to measure the height of horses, ponies, and other equines. It is used in the US and also in some other nations that use the metric system, such as Australia, New Zealand, Canada, Ireland and the UK. In other parts of the world, including continental Europe and in FEI-regulated international competition, horses are measured in metric units, usually metres or centimetres. In South Africa, measurements may be given in both hands and centimetres, while in Australia, the equestrian regulations stipulate that both measurements are to be given.
In those countries where hands are the usual unit for measuring horse height, inches rather than hands are commonly used in the measurement of smaller equines including miniature horses/ponies, miniature mules, donkeys, and Shetland ponies.
A horse is measured from the ground to the top of the highest non-variable point of the skeleton, the withers. For official measurement, the spinous process of the fifth thoracic vertebra may be identified by palpation, and marked if necessary. Some varieties of Miniature horses are measured at the base of the last true hairs of the mane rather than at the withers.
For international competition regulated by the Fédération Équestre Internationale (FEI) and for USEF competition in the US, a horse can be measured with shoes on or off. In the United Kingdom, official measurement of horses is overseen by the Joint Measurement Board (JMB). For JMB purposes, the shoes must be removed and the hooves correctly prepared for shoeing prior to measurement.
| Physical sciences | English | Basics and measurement |
221151 | https://en.wikipedia.org/wiki/Bengal%20tiger | Bengal tiger | The Bengal tiger or Royal Bengal tiger is a population of the Panthera tigris tigris subspecies and the nominate tiger subspecies. It ranks among the biggest wild cats alive today. It is estimated to have been present in the Indian subcontinent since the Late Pleistocene for about 12,000 to 16,500 years. Its historical range covered the Indus River valley until the early 19th century, almost all of India, western Pakistan, southern Nepal, Bangladesh, Bhutan and southwestern China. Today, it inhabits India, Bangladesh, Nepal, Bhutan, and southwestern China. It is threatened by poaching, habitat loss and habitat fragmentation.
As of 2022, the Bengal tiger population was estimated at 3,167–3,682 individuals in India, 316–355 individuals in Nepal, 131 individuals in Bhutan and around 114 individuals in Bangladesh.
Taxonomy
Felis tigris was the scientific name used by Carl Linnaeus in 1758 for the tiger. It was subordinated to the genus Panthera by Reginald Innes Pocock in 1929. Bengal is the traditional type locality of the species and the nominate subspecies Panthera tigris tigris.
The validity of several tiger subspecies in continental Asia was questioned in 1999. Morphologically, tigers from different regions vary little, and gene flow between populations in those regions is considered to have been possible during the Pleistocene. Therefore, it was proposed to recognise only two subspecies as valid, namely P. t. tigris in mainland Asia, and P. t. sondaica in the Greater Sunda Islands and possibly in Sundaland. The nominate subspecies P. t. tigris constitutes two clades: the northern clade comprises the Siberian and Caspian tiger populations, and the southern clade all remaining continental tiger populations. The extinct and living tiger populations in continental Asia have been subsumed to P. t. tigris since the revision of felid taxonomy in 2017.
Phylogeny
Results of a genetic analysis of 32 tiger samples indicate that the Bengal tiger samples grouped into a different clade than the Siberian tiger samples.
Genetic ancestry
The Bengal tiger is defined by three distinct mitochondrial nucleotide sites and 12 unique microsatellite alleles. The pattern of genetic variation in the Bengal tiger corresponds to the premise that it arrived in India approximately 12,000 years ago. This is consistent with the lack of tiger fossils from the Indian subcontinent prior to the late Pleistocene, and the absence of tigers from Sri Lanka, which was separated from the subcontinent by rising sea levels in the early Holocene.
Characteristics
The Bengal tiger's coat is yellow to light orange, with stripes ranging from dark brown to black; the belly and the interior parts of the limbs are white, and the tail is orange with black rings. The white tiger is a recessive mutant, which is reported in the wild from time to time in Assam, Bengal, Bihar and especially in the former State of Rewa. However, it is not an occurrence of albinism. In fact, there is only one fully authenticated case of a true albino tiger, and none of black tigers, with the possible exception of one dead specimen examined in Chittagong in 1846. Fourteen Bengal tiger skins in the collection of the Natural History Museum, London have 21–29 stripes.
Another recessive mutant is the golden tiger that has a pale golden fur with red-brown stripes. The mutants are very rare in nature.
The greatest skull length of a tiger is in males and in females. It has exceptionally stout teeth. Its canines are long and thus the longest among all cats.
Body weight and size
The Bengal tiger ranks among the biggest wild cats alive today.
Males and female Bengal tigers in Panna Tiger Reserve reach a head-to-body length of and respectively, including a tail about long. Total length ranges from for male tigers and for female tigers. They typically range from in shoulder height.
Subadult males weigh between and reach when adult; subadult females weigh and reach between when adult. In central India, 42 adult male Bengal tigers weighed on average with a range of ; their total length was with a range of , and their average shoulder height was ; 39 adult female Bengal tigers weighed an average of with a maximum of and an average total length of ranging from .
Several scientists indicated that adult male Bengal tigers in the Terai consistently attain more than of body weight. Seven adult males captured in Chitwan National Park in the early 1970s had an average weight of ranging from , and that of the females was ranging from . Two male tigers captured in Chitwan National Park in the 1980s exceeded weights of and are the largest free ranging tigers reported to date.
The smallest recorded weights for Bengal tigers are from the Bangladesh Sundarbans, where adult females weigh .
Three tigresses from the Bangladesh Sundarbans had a mean weight of . The oldest female weighed and was in a relatively poor condition at the time of capture. Their skulls and body weights were distinct from those of tigers in other habitats, indicating that they may have adapted to the unique conditions of the mangrove habitat. Their small sizes are probably due to a combination of intense intraspecific competition and small size of prey available to tigers in the Sundarbans, compared to the larger deer and other prey available to tigers in other parts.
The very large "Leeds Tiger" on display at Leeds City Museum, shot in 1860 near Mussoorie, had a body length of at death. Two tigers shot in Kumaon District and near Oude at the end of the 19th century allegedly measured more than . But at the time, sportsmen had not yet adopted a standard system of measurement; some measured 'between the pegs' while others measured 'over the curves'. The greatest length of a tiger skull measured "over the bone"; this one was shot in the vicinity of Nagina in northern India.
In the beginning of the 20th century, a male tiger was shot in central India with a head and body length of between pegs, a chest girth of , a shoulder height of and a tail length of , which was perhaps bitten off by a rival male. This specimen could not be weighed, but it was estimated to weigh about . A male weighing was shot in northern India in the 1930s. A male tiger shot in Nepal weighed and measured 'over the curves'.
The heaviest wild tiger was possibly a male killed in 1967 in the foothills of the Himalayas that measured between pegs and over curves; it weighed after eating a buffalo calf. Without eating the calf, it would have likely weighed about . In the Central Provinces of India, a male tiger shot weighed and measured .
The Bengal tiger rivals the Siberian tiger in average weight.
Distribution and habitat
The Bengal tiger's historical range covered the Indus River valley until the early 19th century, almost all of India, western Pakistan, southern Nepal, Bangladesh, Bhutan, and southwestern China. Today, it inhabits India, Bangladesh, Nepal, Bhutan, and southwestern China.
It is estimated to have been present in the Indian subcontinent since the Late Pleistocene, for about 12,000 to 16,500 years.
In 1982, a sub-fossil right middle phalanx was found in a prehistoric midden near Kuruwita in Sri Lanka, which is dated to about 16,500 years ago and tentatively considered to be of a tiger. Tigers appear to have arrived in Sri Lanka during a pluvial period, during which sea levels were depressed, evidently prior to the last glacial maximum about 20,000 years ago. The tiger probably arrived too late in southern India to colonise Sri Lanka, which earlier had been connected to India by a land bridge.
Results of a phylogeographic study using 134 samples from tigers across the global range suggest that the historical northeastern distribution limit of the Bengal tiger is the region in the Chittagong Hills and Brahmaputra River basin, bordering the historical range of the Indochinese tiger.
In the Indian subcontinent, Bengal tigers inhabit tropical moist evergreen forests, tropical dry forests, tropical and subtropical moist deciduous forests, mangroves, subtropical and temperate upland forests, and alluvial grasslands. The latter habitat once covered a huge swath of grassland, riverine and moist semi-deciduous forests along the major river system of the Gangetic and Brahmaputra plains, but has now been largely converted to agricultural land or severely degraded. Today, the best examples of this habitat type are limited to a few blocks at the base of the outer foothills of the Himalayas including the Tiger Conservation Units (TCUs) Rajaji-Corbett, Bardia-Banke, and the transboundary TCUs Chitwan-Parsa-Valmiki, Dudhwa-Kailali and Shuklaphanta-Kishanpur. Tiger densities in these TCUs are high, in part because of the extraordinary biomass of ungulate prey.
In Pakistan, Khairpur was the last stronghold of the tiger by the late 19th century; the last individuals were shot in 1906 in Bahawalpur in the Indus Riverine jungles.
India
In the 20th century, Indian censuses of wild tigers relied on the individual identification of footprints known as pug marks – a method that has been criticised as deficient and inaccurate. Camera traps are now being used in many sites.
Good tiger habitats in subtropical and temperate forests include the Tiger Conservation Units (TCUs) Manas-Namdapha. TCUs in tropical dry forest include Hazaribag Wildlife Sanctuary, Nagarjunsagar-Srisailam Tiger Reserve, Kanha-Indravati corridor, Orissa dry forests, Panna National Park, Melghat Tiger Reserve and Ratapani Tiger Reserve. The TCUs in tropical moist deciduous forest are probably some of the most productive habitats for tigers and their prey, and include Kaziranga-Meghalaya, Kanha-Pench, Simlipal and Indravati Tiger Reserves. The TCUs in tropical moist evergreen forests represent the less common tiger habitats, being largely limited to the upland areas and wetter parts of the Western Ghats, and include the tiger reserves of Periyar, Kalakad-Mundathurai, Bandipur and Parambikulam Wildlife Sanctuary.
During a tiger census in 2008, camera trap and sign surveys using GIS were employed to estimate site-specific densities of tiger, co-predators and prey. Based on the result of these surveys, the total tiger population was estimated at 1,411 individuals ranging from 1,165 to 1,657 adult and sub-adult tigers of more than 1.5 years of age. Across India, six landscape complexes were surveyed that host tigers and have the potential to be connected. These landscapes comprise the following:
in the Sivaliks–Gangetic flood plain landscape there are six populations with an estimated population size of 259 to 335 individuals in an area of of forested habitats, which are located in Rajaji and Corbett National Parks, in the connected habitats of Dudhwa-Kheri-Pilibhit, in Suhelwa Tiger Reserve, in Sohagi Barwa Sanctuary and in Valmiki National Park;
in the Central Indian highlands there are 17 populations with an estimated population size of 437 to 661 individuals in an area of of forested habitats, which are located in the landscapes of Kanha-Pench, Satpura-Melghat, Sanjay-Palamau, Navegaon-Indravati; isolated populations are supported in the tiger reserves of Bandhavgarh, Tadoba, Simlipal and the national parks of Panna, Ranthambore–Kuno–Palpur–Madhav and Saranda;
in the Eastern Ghats landscape there is a single population with an estimated population size of 49 to 57 individuals in a habitat in three separate forest blocks located in the Srivenkateshwara National Park, Nagarjunasagar Tiger Reserve and the adjacent proposed Gundla Brahmeshwara National Park, and forest patches in the tehsils of Kanigiri, Badvel, Udayagiri and Giddalur;
in the Western Ghats landscape there are seven populations with an estimated population size of 336 to 487 individuals in a forested area of in three major landscape units Periyar-Kalakad-Mundathurai, Bandipur-Parambikulam-Sathyamangalam-Mudumalai-Anamalai-Mukurthi and Anshi-Kudremukh-Dandeli;
in the Brahmaputra flood plains and northeastern hills tigers live in an area of in several patchy and fragmented forests;
in the Sundarbans National Park tigers live in about of mangrove forest.
Manas-Namdapha, Orang-Laokhowa and Kaziranga-Meghalaya are Tiger Conservation Units in northeastern India, stretching over at least across several protected areas. Tigers are also present in Pakke Tiger Reserve. In the Mishmi Hills, tigers were recorded in 2017 up to an elevation of in snow.
Ranthambore National Park hosts India's westernmost tiger population. The Dangs' Forest in southeastern Gujarat is potential tiger habitat.
In May 2018, a tiger was recorded in Sahyadri Tiger Reserve for the first time in eight years.
In February 2019, a tiger was sighted in Gujarat's Lunavada area in Mahisagar district, and found dead shortly afterwards. Officials assumed that it originated in Ratapani Tiger Reserve and travelled about over two years. It probably died of starvation. In May 2019, camera traps recorded tigers in Mhadei Wildlife Sanctuary and Bhagwan Mahaveer Sanctuary and Mollem National Park, the first records in Goa since 2013.
The tigers in the Sundarbans in India and Bangladesh are the only ones in the world inhabiting mangrove forests. The population in the Indian Sundarbans was estimated as 86–90 individuals in 2018.
As of 2014, the Indian tiger population was estimated to range over an area of and number 2,226 adult and subadult tigers older than one year. About 585 tigers were present in the Western Ghats, where Radhanagari and Sahyadri Tiger Reserves were newly established. The largest population resided in Corbett Tiger Reserve with about 215 tigers. The Central Indian tiger population is fragmented and depends on wildlife corridors that facilitate connectivity between protected areas. By 2018, the population had increased to an estimated 2,603–3,346 individuals. As of 2022, the Indian population was estimated to comprise 3,167–3,682 individuals.
Bangladesh
In Bangladesh, tigers are now relegated to the forests of the Sundarbans and the Chittagong Hill Tracts. The Chittagong forest is contiguous with tiger habitat in India and Myanmar, but the tiger population is of unknown status.
As of 2004, population estimates in Bangladesh ranged from 200 to 419 individuals, most of them in the Sundarbans. This region is the only mangrove habitat in this bioregion, where tigers survive, swimming between islands in the delta to hunt prey. Bangladesh's Forest Department is raising mangrove plantations supplying forage for spotted deer. Since 2001, afforestation has continued on a small scale in the Sundarbans.
From October 2005 to January 2007, the first camera trap survey was conducted across six sites in the Bangladesh Sundarbans to estimate tiger population density. The average of these six sites provided an estimate of 3.7 tigers per . Since the Bangladesh Sundarbans is an area of , it was inferred that the total tiger population comprised approximately 200 individuals.
Home ranges of adult female tigers were recorded comprising between , which would indicate an approximate carrying capacity of 150 adult females. The small home range of adult female tigers and consequent high density of tigers in this habitat type relative to other areas may be related to both the high density of prey and the small size of the Sundarban tigers.
Since 2007, tiger monitoring surveys have been carried out every year by WildTeam in the Bangladesh Sundarbans to monitor changes in the Bangladesh tiger population and assess the effectiveness of conservation actions. This survey measures changes in the frequency of tiger track sets along the sides of tidal waterways as an index of relative tiger abundance across the Sundarbans landscape.
By 2009, the tiger population in the Bangladesh Sundarbans was estimated as 100–150 adult females or 335–500 tigers overall. Female home ranges, recorded using Global Positioning System collars, were some of the smallest recorded for tigers, indicating that the Bangladesh Sundarbans could have one of the highest densities and largest populations of tigers anywhere in the world. They are isolated from the next tiger population by a distance of up to . Information is lacking on many aspects of Sundarbans tiger ecology, including relative abundance, population status, spatial dynamics, habitat selection, life history characteristics, taxonomy, genetics, and disease. There is also no monitoring program in place to track changes in the tiger population over time, and therefore no way of measuring the response of the population to conservation activities or threats. Most studies have focused on the tiger-human conflict in the area, but two studies in the Sundarbans East Wildlife sanctuary documented habitat-use patterns of tigers, and abundances of tiger prey, and another study investigated tiger parasite load. Some major threats to tigers have been identified. The tigers living in the Sundarbans are threatened by habitat destruction, prey depletion, highly aggressive and rampant intraspecific competition, tiger-human conflict, and direct tiger loss.
By 2017, this population was estimated at 84–158 individuals.
As of 2018, 114 individuals were estimated to live in the country.
A rising sea-level due to climate change is projected to cause a severe loss of suitable habitat for this population in the following decades, around 50% by 2050 and 100% by 2070.
Nepal
The tiger population in the Terai of Nepal is split into three isolated subpopulations that are separated by cultivation and densely settled habitat. The largest population lives in Chitwan National Park and in the adjacent Parsa National Park encompassing an area of of prime lowland forest. To the west, the Chitwan population is isolated from the one in Bardiya National Park and adjacent unprotected habitat farther west, extending to within of the Shuklaphanta Wildlife Reserve, which harbours the smallest population.
From February to June 2013, a camera trapping survey was carried out in the Terai Arc Landscape, across an area of in 14 districts. The country's tiger population was estimated at 163–235 breeding adults comprising 102–152 tigers in the Chitwan-Parsa protected areas, 48–62 in Bardiya-Banke National Parks and 13–21 in Shuklaphanta National Park.
Between November 2017 and April 2018, the third nationwide survey for tiger and prey was conducted in the Terai Arc Landscape; the country's population was estimated at 220–274 tigers.
As of 2022, 316–355 individuals were estimated to live in the country.
Bhutan
In Bhutan, tigers have been documented in 17 of 18 districts. They inhabit the subtropical Himalayan foothills at an elevation of in the south to over in the temperate forests in the north. Their stronghold appears to be the country's central belt between the Mo River in the west and the Kulong River in the east ranging in elevation from . By 2015, Bhutan's tiger population was estimated at 103 individuals.
Royal Manas and Jigme Singye Wangchuck National Parks form the largest contiguous tiger conservation area in Bhutan representing subtropical to alpine habitat types.
In 2010, camera traps recorded a tiger pair at elevations of . As of 2015, the tiger population in Bhutan was estimated at 89 to 124 individuals in a survey area of .
In 2008, a tiger was recorded at an elevation of in Jigme Dorji National Park, which is the highest elevation record of a tiger known to date. In 2017, a tiger was recorded for the time in Bumdeling Wildlife Sanctuary. It probably used a wildlife corridor to reach northeastern Bhutan.
Bhutan's tiger population was estimated at 90 individuals comprising 60 females and 30 males with a population density estimate of 0.19–0.31 tigers per by March 2015. As of 2022, the population was estimated at 131 individuals.
China
The presence of the Bengal tiger in southeastern Tibet Autonomous Region, China was investigated in 1995 when the loss of livestock was high in Mêdog County due to a large predator. Tiger paw prints were found on pastures around several villages.
One tiger was shot in 1996, and about 4–5 tigers were reported by officials in the area by 1999.
About 8–12 tigers were thought to remain in this area a decade later.
A camera trapping and interview survey during 2013–2018 in nine potential sites in Mêdog County revealed that only 1–3 non-resident individuals might be entering the area south of the Yarlung Tsangpo river, but only during the dry season from October to March.
In early 2019, a Bengal tiger was photographed twice at an elevation of in a broadleaved forest in Yarlung Tsangpo Grand Canyon National Nature Reserve.
Ecology and behaviour
The basic social unit of the tiger is composed of a female and her offspring. Adult animals congregate only temporarily when special conditions permit, such as plentiful supplies of food. Otherwise, they lead solitary lives, hunting individually for the forest and grassland animals upon which they prey. Resident adults of either sex maintain home ranges, confining their movements to definite habitats within which they satisfy their needs and those of their cubs, which include prey, water and shelter. In this site, they also maintain contact with other tigers, especially those of the opposite sex. Those sharing the same ground are well aware of each other's movements and activities. In Chitwan National Park, radio-collared subadult tigers started dispersing from their natal areas earliest at the age of 19 months. Of the 14 subadults studied, the four females stayed closer to their mother's home range than the 10 males. The latter dispersed between . None of them crossed open cultivated areas that were more than wide, but moved through prime alluvial and forested habitat.
In the Panna Tiger Reserve, an adult radio-collared male tiger moved between locations on successive days in winter, and in summer. His home range was about in summer and in winter. Included in his home range were the much smaller home ranges of two females, a tigress with cubs and a subadult tigress. They occupied home ranges of .
The home ranges occupied by adult male residents tend to be mutually exclusive, even though one of these residents may tolerate a transient or sub-adult male at least for a time. A male tiger keeps a large territory in order to include the home ranges of several females within its bounds, so that he may maintain mating rights with them. Spacing among females is less complete. Typically there is partial overlap with neighbouring female residents. They tend to have core areas, which are more exclusive, at least for most of the time. Home ranges of both males and females are not stable. The shift or alteration of a home range by one animal is correlated with a shift of another. Shifts from less suitable habitat to better ones are made by animals that are already resident. New animals become residents only as vacancies occur when a former resident moves out or dies. There are more places for resident females than for resident males.
During seven years of camera trapping, tracking, and observational data in Chitwan National Park, six to nine breeding tigers, two to sixteen non-breeding tigers, and six to twenty young tigers of less than one year of age were detected in the study area of . One of the resident females left her territory to one of her female offspring and took over an adjoining area by displacing another female; and a displaced female managed to re-establish herself in a neighbouring territory made vacant by the death of the resident. Of 11 resident females, 7 were still alive at the end of the study period, two disappeared after losing their territories to rivals, and two died. The initial loss of two resident males and subsequent take over of their home ranges by new males caused social instability for two years. Of four resident males, one was still alive and three were displaced by rivals. Five litters of cubs were killed by infanticide, two litters died because they were too young to fend for themselves when their mothers died. One juvenile tiger was presumed dead after being photographed with severe injuries from a deer snare. The remaining young lived long enough to reach dispersal age, two of them becoming residents in the study area.
Hunting and diet
The Bengal tiger is a carnivore and prefers hunting large ungulates such as gaur, sambar, chital, barasingha, water buffalo, nilgai, serow and takin. Medium-sized prey includes wild boar, Indian hog deer, Indian muntjac and northern plains gray langur. Small prey such as porcupine, hare and peafowl form a small part of its diet. Because of the encroachment of humans into tiger habitat, it also preys on domestic livestock.
Bengal tigers occasionally hunt and kill predators such as Indian leopard, mugger crocodile, Asian black bear, sloth bear, and dhole. They generally do not attack adult Indian elephant and Indian rhinoceros, but such extraordinarily rare events have been recorded. In Kaziranga National Park, tigers killed 20 rhinoceros in 2007. In 2011 and 2014, two instances of Bengal tigers killing adult elephants were recorded; in Jim Corbett National Park on a 20-year-old elephant cow, and another on a 28-year-old sick elephant in Kaziranga National Park; the latter was eaten by several tigers at once. A king cobra (Ophiophagus hannah), an Indian cobra (Naja naja), Asian water monitor, rhesus macaque, fish, crabs, and very rarely fishing cats and turtles were found in the stomachs and scat of tigers in the Sundarbans. One in Chitwan National Park has been reported to have hunted three gharials.
Results of scat analyses indicate that the Bengal tigers in Nagarahole National Park preferred prey weighing more than and that on average tiger prey weighed . The prey species included chital, sambar, wild pig and gaur. Gaur remains were found in 44.8% of all tiger scat samples, sambar remains in 28.6%, wild pig remains in 14.3% and chital remains in 10.4% of all scat samples. In Bandipur National Park, gaur and sambar together also constituted 73% of tiger diet.
In most cases, Bengal tigers approach their victim from the side or behind from as close a distance as possible and grasp the prey's throat to kill it. Then they drag the carcass into cover, occasionally over several hundred metres, to consume it. The nature of the tiger's hunting method and prey availability results in a "feast or famine" feeding style: they often consume of meat at one time. In one study, tigresses from Nepal made about 40–50 kills a year and ate a minimum of of meat a day. Two males made about 40–50 kills in a year and ate of meat a day at the least.
If injured, old or weak, or when its regular prey species become scarce, Bengal tigers often attack humans and become man-eaters.
Reproduction and lifecycle
The tiger in India has no definite mating and birth seasons. Most young are born in December and April. Young have also been found in March, May, October and November. In the 1960s, certain aspects of tiger behaviour at Kanha National Park indicated that the peak of sexual activity was from November to about February, with some mating probably occurring throughout the year.
Males reach maturity at 4–5 years of age, and females at 3–4 years. A Bengal comes into estrus (commonly known as "heat") at intervals of about 3–9 weeks, and is receptive for 3–6 days. After a gestation period of 104–106 days, 1–4 cubs are born in a shelter situated in tall grass, thick bush or in caves. Newborn cubs weigh and they have a thick woolly fur that is shed after 3.5–5 months. Their eyes and ears are closed. Their milk teeth start to erupt at about 2–3 weeks after birth, and are slowly replaced by permanent dentition from 8.5 to 9.5 weeks of age onwards. They suckle for 3–6 months, and begin to eat small amounts of solid food at about 2 months of age. At this time, they follow their mother on her hunting expeditions and begin to take part in hunting at 5–6 months of age. At the age of 2–3 years, they slowly start to separate from the family group and become transient, looking out for an area where they can establish their own home range. Young males move farther away from their native home range than young females. Once the family group has split, the mother comes into heat again.
Threats
None of the Tiger Conservation Landscapes within the Bengal tiger range is large enough to support an effective population size of 250 individuals. Habitat losses and the extremely large-scale incidences of poaching are serious threats to the species' survival.
The Forest Rights Act passed by the Indian government in 2006 grants some of India's most impoverished communities the right to own and live in the forests, which likely brings them into conflict with wildlife and under-resourced, under-trained, ill-equipped forest department staff. In the past, evidence showed that humans and tigers cannot co-exist.
Poaching
The most significant immediate threat to the existence of wild tiger populations is the illegal wildlife trade in poached skins and body parts between India, Nepal and China. The governments of these countries have failed to implement adequate enforcement response, and wildlife crime remained a low priority in terms of political commitment and investment for years. There are well-organised gangs of professional poachers, who move from place to place and set up camp in vulnerable areas. Skins are rough-cured in the field and handed over to dealers, who send them for further treatment to Indian tanning centres. Buyers choose the skins from dealers or tanneries and smuggle them through a complex interlinking network to markets outside India, mainly in China. Other factors contributing to their loss are urbanisation and revenge killing. Farmers blame tigers for killing cattle and shoot them. Their skins and body parts may however become a part of the illegal trade. In Bangladesh, tigers are killed by professional poachers, local hunters, trappers and villagers. Each group of people has different motives for killing tigers, ranging from profit and excitement to safety concerns; and has access to the illegal wildlife trade in body parts.
The illicit demand for bones and body parts from wild tigers for use in Traditional Chinese medicine is the reason for the unrelenting poaching pressure on tigers on the Indian subcontinent. For at least a thousand years, tiger bones have been an ingredient in traditional medicines that are prescribed as a muscle strengthener and treatment for rheumatism and body pain.
Between 1994 and 2009, the Wildlife Protection Society of India has documented 893 cases of tigers killed in India, which is just a fraction of the actual poaching and trade in tiger parts during those years.
In 2004, all the tigers in India's Sariska Tiger Reserve were killed by poachers. In 2007, police in Allahabad raided a meeting of suspected poachers, traders and couriers. One of the arrested persons was the biggest buyer of Indian tiger parts who sold them to Chinese buyers, using women from a nomadic tribe as couriers. In 2009, none of the 24 tigers residing in the Panna Tiger Reserve were left because of excessive poaching.
In November 2011, two tigers were found dead in Maharashtra: a male tiger was trapped and killed in a wire snare; a tigress died of electrocution after chewing at an electric cable supplying power to a water pump; another dead tigress found in Kanha Tiger Reserve landscape was suspected to have been poisoned. In 2021, Bangladeshi police arrested a poacher suspected of killing 70 Bengal tigers during a period of 20 years.
Human–tiger conflict
The Indian subcontinent has served as a stage for intense confrontations between tigers and people. At the beginning of the 19th century tigers were so numerous, that the killing of tigers was officially rewarded in many localities. The Terai region supported large numbers of tigers that were pushed into marginal habitat after the 1950s, when the conversion of natural habitat for paddy fields increased. Marauding tigers began to take a toll of human life in areas bordering cultivation. They are thought to have followed domestic livestock that wintered in the plains when they returned to the hills in the spring, and then being left without prey when the herds dispersed back to their respective villages. These tigers were the old, the young and the disabled. All suffered from some disability, mainly caused either by gunshot wounds or porcupine quills.
In the Sundarbans, 10 out of 13-man-eaters recorded in the 1970s were males, and they accounted for 86% of the victims. These man-eaters have been grouped into the confirmed or dedicated ones who go hunting especially for human prey; and the opportunistic ones, who do not search for humans but will, if they encounter a man, attack, kill and devour him. In areas where opportunistic man-eaters were found, the killing of humans was correlated with their availability, most victims being claimed during the honey-gathering season. Tigers in the Sunderbans presumably attacked humans who entered their territories in search of wood, honey or fish, thus causing them to defend their territories. The number of tiger attacks on humans may be higher outside suitable areas for tigers, where numerous people are present, but with little wild prey for tigers.
In Nepal, the incidence of man-eating tigers has been only sporadic. In Chitwan National Park no cases were recorded before 1980. In the following few years, 13 people have been killed and eaten in the park and its environs. In the majority of cases, man-eating appeared to have been related to an intra-specific competition among male tigers.
An interview survey with 499 local people in Chitwan revealed that lower caste Hindus and people with less than eight years of formal education had negative attitudes to the tiger; most of them owned livestock and had heard about tigers attacking people and livestock.
In December 2012, a tiger was shot by the Kerala Forest Department on a coffee plantation on the fringes of the Wayanad Wildlife Sanctuary. Chief Wildlife Warden of Kerala ordered the hunt for the animal after mass protests erupted as the tiger had been carrying away livestock. The Forest Department had constituted a special task force to capture the animal with the assistance of a 10-member Special Tiger Protection Force and two trained Indian elephants from the Bandipur Tiger Reserve in Karnataka.
Conservation efforts
An area of special interest lies in the "Terai Arc Landscape" in the Himalayan foothills of northern India and southern Nepal, where 11 protected areas composed of dry forest foothills and tall-grass savannas harbour tigers in a landscape. The goals are to manage tigers as a single metapopulation, the dispersal of which between core refuges can help maintain genetic, demographic, and ecological integrity, and to ensure that species and habitat conservation becomes mainstreamed into the rural development agenda. In Nepal a community-based tourism model has been developed with a strong emphasis on sharing benefits with local people and on the regeneration of degraded forests. The approach has been successful in reducing poaching, restoring habitats, and creating a local constituency for conservation.
The WWF partnered with Leonardo DiCaprio to form a global campaign, "Save Tigers Now", with the ambitious goal of building political, financial and public support to double the wild tiger population by 2022. Save Tigers Now started its campaign in 12 different WWF Tiger priority landscapes, since May 2010.
This population of tigers has been assessed at the local level in several countries. It is listed as Endangered in Nepal, India, and Bhutan, While Bangladesh and China list it as Critically Endangered.
In April 2023, India signed a memorandum of understanding with Cambodia to assist the country with the tiger's reintroduction. At least of the Cardamom Mountains of Tatai Wildlife Sanctuary could be used to host tigers that are imported from India. The last tiger in Cambodia was photographed in 2007 by a camera trap. In 2016, the Cambodian government declared that the Indochinese tiger population was "functionally extinct".
In India
In 1973, Project Tiger was launched aiming at ensuring a viable tiger population in the country and preserving areas of biological importance as a natural heritage for the people. The project's task force visualised these tiger reserves as breeding nuclei, from which surplus animals would disperse to adjacent forests. The selection of areas for the reserves represented as close as possible the diversity of ecosystems across the tiger's distribution in the country. Funds and commitment were mustered to support the intensive program of habitat protection and rehabilitation under the project. By the late 1980s, the initial nine reserves covering an area of had been increased to 15 reserves covering an area of . More than 1100 tigers were estimated to inhabit the reserves by 1984.
Through this initiative the population decline was reversed initially, but has resumed in recent years; India's tiger population decreased from 3,642 in the 1990s to just over 1,400 from 2002 to 2008.
The Indian Wildlife Protection Act of 1972 enables government agencies to take strict measures so as to ensure the conservation of the Bengal tigers. The Wildlife Institute of India estimates showed that tiger numbers had fallen in Madhya Pradesh by 61%, Maharashtra by 57%, and Rajasthan by 40%. The government's first tiger census, conducted under the Project Tiger initiative begun in 1973, counted 1,827 tigers in the country that year. Using that methodology, the government observed a steady population increase, reaching 3,700 tigers in 2002. However, the use of more reliable and independent censusing technology including camera traps for the 2007–2008 all-India census has shown that the numbers were in fact less than half than originally claimed by the Forest Department.
Following the revelation that only 1,411 Bengal tigers existed in the wild in India, down from 3,600 in 2003, the Indian government set up eight new tiger reserves. Because of dwindling tiger numbers, the Indian government has pledged US$153 million to further fund the Project Tiger initiative, set up a Tiger Protection Force to combat poachers, and fund the relocation of up to 200,000 villagers to minimise human-tiger interaction. Indian tiger scientists have called for use of technology in the conservation efforts.
In 2022, Ranipur Wildlife Sanctuary was declared as the 54th tiger reserve.
In January 2008, the Government of India launched a dedicated anti-poaching force composed of experts from Indian police, forest officials and various other environmental agencies. Ranthambore National Park is often cited as a major success by Indian officials against poaching.
Kuno-Palpur in Madhya Pradesh was supposed to receive Asiatic lions from Gujarat. Since no lions have been transferred from Gujarat to Madhya Pradesh so far, it may be used as a sanctuary for the tiger instead where it could help keep down the leopard population that targets the main prey of reintroduced cheetahs.
In captivity
Bengal tigers have been captive bred since 1880 and widely crossed with tigers from other range countries.
In July 1976, Billy Arjan Singh acquired a hand-reared tigress from Twycross Zoo in the United Kingdom, and reintroduced her to the wild in Dudhwa National Park with the permission of India's then Prime Minister Indira Gandhi. In the 1990s, some tigers from this area were observed to have the typical appearance of Siberian tigers, namely a large head, pale fur, white complexion, and wide stripes, and were suspected to be Siberian–Bengal tiger hybrids. Tiger hair samples from the national park were analysed using mitochondrial sequence analysis. Results revealed that the tigers in question had a Bengal tiger mitochondrial haplotype indicating that their mother was a Bengal tiger. Skin, hair and blood samples from 71 tigers collected in Indian zoos, in the Indian Museum, Kolkata and including two samples from Dudhwa National Park were used for a microsatellite analysis that revealed that two tigers had alleles in two loci contributed by Bengal and Siberian tigers.
However, samples of two hybrid specimens constituted a too small sample base to conclusively assume that Tara was the source of the Siberian tiger genes.
Indian zoos bred tigers for the first time at the Alipore Zoo in Kolkata. The 1997 International Tiger Studbook lists the global captive population of Bengal tigers at 210 individuals that are all kept in Indian zoos, except for one female in North America. Completion of the Indian Bengal Tiger Studbook is a necessary prerequisite to establishing a captive management program for tigers in India.
In Bangladesh
WildTeam is working with local communities and the Bangladesh Forest Department to reduce human-tiger conflict in the Bangladesh Sundarbans. For over 100 years people, tigers, and livestock have been injured and killed in the conflict; in recent decades up to 50 people, 80 livestock, and 3 tigers have been killed in a year. Now, through WildTeam's work, there is a boat-based Tiger Response team that provides first aid, transport, and body retrieval support for people being killed in the forest by tigers. WildTeam has also set up 49 volunteer Village Response Teams that are trained to save tigers that have strayed into the village areas and would be otherwise killed. These village teams are made up of over 350 volunteers, who are also now supporting anti-poaching work and conservation education/awareness activities. WildTeam also works to empower local communities to access the government funds for compensating the loss/injury of livestock and people from the conflict. To monitor the conflict and assess the effectiveness of actions, WildTeam have also set up a human-tiger conflict data collection and reporting system.
In Nepal
In May 2010, Banke National Park was established with an area of .
The government aimed at doubling the country's tiger population by 2022 at the Global Tiger Summit in 2010. The tiger population reached 355 in 2022, almost tripling the population of 121 in 2009.
"Re-wilding" project in South Africa
In 2000, the Bengal tiger re-wilding project Tiger Canyons was started by John Varty, who together with the zoologist Dave Salmoni trained captive-bred tiger cubs how to stalk, hunt, associate hunting with food and regain their predatory instincts. They claimed that once the tigers proved that they can sustain themselves in the wild, they would be released into a free-range sanctuary of South Africa to fend for themselves.
The project has received controversy after accusations by their investors and conservationists of manipulating the behaviour of the tigers for the purpose of a film production, Living with Tigers, with the tigers believed to be unable to hunt. Stuart Bray, who had originally invested a large sum of money in the project, claimed that he and his wife, Li Quan, watched the film crew "[chase] the prey up against the fence and into the path of the tigers just for the sake of dramatic footage."
The four tigers involved in this project have been confirmed to be crossbred Siberian–Bengal tigers, which should neither be used for breeding nor being released into the Karoo. Tigers that are not genetically pure will not be able to participate in the tiger Species Survival Plan, as they are not used for breeding, and are not allowed to be released into the wild.
In culture
The tiger is one of the animals displayed on the Pashupati seal of the Indus Valley Civilisation. The tiger crest is the emblem on Chola coins. The presence on many coins of a tiger, a fish, and a bow suggests they are Chola issues. The latter two emblems are of the Pandya and Chera dynasties respectively, and Chola copper plate inscriptions showing the three together indicate that the Cholas had achieved political supremacy over them.
Today, the tiger is the national animal of Bangladesh and India. Bangladeshi banknotes feature a tiger. The political party Muslim League of Pakistan uses the tiger as its election symbol.
Tipu Sultan, who ruled Mysore in late 18th-century India, was also a great admirer of the animal. The famed 18th-century automaton, Tipu's Tiger was also created for him. The tiger was the dynastic symbol of this dynasty.
During the Indian Rebellion of 1857, Punch ran a political cartoon showing the Indian rebels as a tiger attacking a victim.
In arts
The main antagonist of The Jungle Book, Shere Khan, is a Bengal tiger.
The Man-Eaters of Kumaon is based on man-eating tigers and leopards in Kumaon Division.
In the fantasy adventure novel Life of Pi and in its 2012 film adaptation, a Bengal tiger named Richard Parker is the lead character.
The Bengal Tiger at the Baghdad Zoo is based on a real story of a tiger that escaped from Baghdad Zoo in 2003 and haunts the streets of Baghdad seeking the meaning of life.
The Lost Land of the Tiger is a documentary by the BBC on tigers in Bhutan.
The 2014 Indian film Roar – Tigers of the Sundarbans is about a white Bengal tigress in the Sundarbans.
Notable individuals
Notable Bengal tigers include the man-eating Tigers of Chowgarh, Chuka man-eating tiger, the Bachelor of Powalgarh and Thak man-eater, Tiger of Segur, Tiger of Mundachipallam, and the Wily Tiger of Mundachipallam.
| Biology and health sciences | Felines | Animals |
221198 | https://en.wikipedia.org/wiki/Clamp%20%28tool%29 | Clamp (tool) | A clamp is a fastening device used to hold or secure objects tightly together to prevent movement or separation through the application of inward pressure. In the United Kingdom the term cramp is often used instead when the tool is for temporary use for positioning components during construction and woodworking; thus a G cramp or a sash clamp but a wheel clamp or a surgical clamp.
There are many types of clamps available for many different purposes. Some are temporary, as used to position components while fixing them together, others are intended to be permanent. In the field of animal husbandry, using a clamp to attach an animal to a stationary object is known as "rounded clamping." A physical clamp of this type is also used to refer to an obscure investment banking term, "fund clamps." Anything that performs the action of clamping may be called a clamp, so this gives rise to a wide variety of terms across many fields.
Types
Temporary
These clamps (or cramps) are used to position components temporarily for various tasks:
Band clamp or web clamp
Bar clamp, F-clamp or sliding clamp (upper left in the top photo)
Cardellini clamp – jaw-style clamp that clamps onto round, square, or rectangular tubing; or onto flat objects, such as dimensional lumber or plywood sheets—to mount motion picture lights, or grip equipment such as gobo heads
C-clamp (also G-clamp or G-cramp) (lower centre in the top photo)
Flooring clamp A carpenter's clamp used to cramp up floorboards prior to fixing.
Forked clamp stainless steel for ST ground glass joints with/without setscrew. Sizes for: ST 14, 19, 24, 29 and 45.
Gripe (a specialized clamp, tightened with a wedge, for holding strakes in position when building a clinker boat)
Hand clamp
Handscrew (upper right in the top photo)
Holdfast, a bench clamp for holding things to a bench top or side The bench forms the fixed jaw.
Magnetic clamp (see Magnetic base)
Mitre clamp
Pipe clamp (top of the top photo)
Sash clamp (a specialized, long form of bar clamp)
Set screw
Spring clamp (first item of third row in photo)
Speed clamp
Step clamp, a type of serrated-edged clamp used in conjunction with step blocks when machining or milling parts in metalworking
Toggle clamp
Toolmakers' clamp (a smaller, precision version of the handscrew, all in steel)
Pinch Dog (a small "staple" shaped device, designed to straddle a joint, and pull the joint tightly together during the glue up process)
Clip hangers are a subset of clothes hangers
Utility clamp laboratory apparatus
Permanent
Hose clamp
Marman clamp
Wire rope clamp
Joiner's dog
Medical
There are various kinds of surgical clamps:
Foerster clamp
Hemostatic clamp
Pennington clamp
Gomco clamp
Mogen clamp
Bone clamp
Serrefine
Other
Castration clamp
Wheel clamp
Pandrol clip
Tube clamp (name for the different clamps used in a tube and clamp scaffold)
Nipple clamp
Voltage clamp
Gallery
| Technology | Hand tools | null |
221244 | https://en.wikipedia.org/wiki/Bellman%E2%80%93Ford%20algorithm | Bellman–Ford algorithm | The Bellman–Ford algorithm is an algorithm that computes shortest paths from a single source vertex to all of the other vertices in a weighted digraph.
It is slower than Dijkstra's algorithm for the same problem, but more versatile, as it is capable of handling graphs in which some of the edge weights are negative numbers. The algorithm was first proposed by , but is instead named after Richard Bellman and Lester Ford Jr., who published it in 1958 and 1956, respectively. Edward F. Moore also published a variation of the algorithm in 1959, and for this reason it is also sometimes called the Bellman–Ford–Moore algorithm.
Negative edge weights are found in various applications of graphs. This is why this algorithm is useful.
If a graph contains a "negative cycle" (i.e. a cycle whose edges sum to a negative value) that is reachable from the source, then there is no cheapest path: any path that has a point on the negative cycle can be made cheaper by one more walk around the negative cycle. In such a case, the Bellman–Ford algorithm can detect and report the negative cycle.
Algorithm
Like Dijkstra's algorithm, Bellman–Ford proceeds by relaxation, in which approximations to the correct distance are replaced by better ones until they eventually reach the solution. In both algorithms, the approximate distance to each vertex is always an overestimate of the true distance, and is replaced by the minimum of its old value and the length of a newly found path.
However, Dijkstra's algorithm uses a priority queue to greedily select the closest vertex that has not yet been processed, and performs this relaxation process on all of its outgoing edges; by contrast, the Bellman–Ford algorithm simply relaxes all the edges, and does this times, where is the number of vertices in the graph.
In each of these repetitions, the number of vertices with correctly calculated distances grows, from which it follows that eventually all vertices will have their correct distances. This method allows the Bellman–Ford algorithm to be applied to a wider class of inputs than Dijkstra's algorithm. The intermediate answers depend on the order of edges relaxed, but the final answer remains the same.
Bellman–Ford runs in time, where and are the number of vertices and edges respectively.
function BellmanFord(list vertices, list edges, vertex source) is
// This implementation takes in a graph, represented as
// lists of vertices (represented as integers [0..n-1]) and edges,
// and fills two arrays (distance and predecessor) holding
// the shortest path from the source to each vertex
distance := list of size n
predecessor := list of size n
// Step 1: initialize graph
for each vertex v in vertices do
// Initialize the distance to all vertices to infinity
distance[v] := inf
// And having a null predecessor
predecessor[v] := null
// The distance from the source to itself is zero
distance[source] := 0
// Step 2: relax edges repeatedly
repeat |V|−1 times:
for each edge (u, v) with weight w in edges do
if distance[u] + w < distance[v] then
distance[v] := distance[u] + w
predecessor[v] := u
// Step 3: check for negative-weight cycles
for each edge (u, v) with weight w in edges do
if distance[u] + w < distance[v] then
predecessor[v] := u
// A negative cycle exists; find a vertex on the cycle
visited := list of size n initialized with false
visited[v] := true
while not visited[u] do
visited[u] := true
u := predecessor[u]
// u is a vertex in a negative cycle, find the cycle itself
ncycle := [u]
v := predecessor[u]
while v != u do
ncycle := concatenate([v], ncycle)
v := predecessor[v]
error "Graph contains a negative-weight cycle", ncycle
return distance, predecessor
Simply put, the algorithm initializes the distance to the source to 0 and all other nodes to infinity. Then for all edges, if the distance to the destination can be shortened by taking the edge, the distance is updated to the new lower value.
The core of the algorithm is a loop that scans across all edges at every loop. For every , at the end of the -th iteration, from any vertex , following the predecessor trail recorded in yields a path that has a total weight that is at most , and further, is a lower bound to the length of any path from source to that uses at most edges.
Since the longest possible path without a cycle can be edges, the edges must be scanned times to ensure the shortest path has been found for all nodes. A final scan of all the edges is performed and if any distance is updated, then a path of length edges has been found which can only occur if at least one negative cycle exists in the graph.
The edge (u, v) that is found in step 3 must be reachable from a negative cycle, but it isn't necessarily part of the cycle itself, which is why it's necessary to follow the path of predecessors backwards until a cycle is detected. The above pseudo-code uses a Boolean array (visited) to find a vertex on the cycle, but any cycle finding algorithm can be used to find a vertex on the cycle.
A common improvement when implementing the algorithm is to return early when an iteration of step 2 fails to relax any edges, which implies all shortest paths have been found, and therefore there are no negative cycles. In that case, the complexity of the algorithm is reduced from to where is the maximum length of a shortest path in the graph.
Proof of correctness
The correctness of the algorithm can be shown by induction:
Lemma. After i repetitions of for loop,
if Distance(u) is not infinity, it is equal to the length of some path from s to u; and
if there is a path from s to u with at most i edges, then Distance(u) is at most the length of the shortest path from s to u with at most i edges.
Proof. For the base case of induction, consider i=0 and the moment before for loop is executed for the first time. Then, for the source vertex, source.distance = 0, which is correct. For other vertices u, u.distance = infinity, which is also correct because there is no path from source to u with 0 edges.
For the inductive case, we first prove the first part. Consider a moment when a vertex's distance is updated by
v.distance := u.distance + uv.weight. By inductive assumption, u.distance is the length of some path from source to u. Then u.distance + uv.weight is the length of the path from source to v that follows the path from source to u and then goes to v.
For the second part, consider a shortest path P (there may be more than one) from source to v with at most i edges. Let u be the last vertex before v on this path. Then, the part of the path from source to u is a shortest path from source to u with at most i-1 edges, since if it were not, then there must be some strictly shorter path from source to u with at most i-1 edges, and we could then append the edge uv to this path to obtain a path with at most i edges that is strictly shorter than P—a contradiction. By inductive assumption, u.distance after i−1 iterations is at most the length of this path from source to u. Therefore, uv.weight + u.distance is at most the length of P. In the ith iteration, v.distance gets compared with uv.weight + u.distance, and is set equal to it if uv.weight + u.distance is smaller. Therefore, after i iterations, v.distance is at most the length of P, i.e., the length of the shortest path from source to v that uses at most i edges.
If there are no negative-weight cycles, then every shortest path visits each vertex at most once, so at step 3 no further improvements can be made. Conversely, suppose no improvement can be made. Then for any cycle with vertices v[0], ..., v[k−1],
v[i].distance <= v[i-1 (mod k)].distance + v[i-1 (mod k)]v[i].weight
Summing around the cycle, the v[i].distance and v[i−1 (mod k)].distance terms cancel, leaving
0 <= sum from 1 to k of v[i-1 (mod k)]v[i].weight
I.e., every cycle has nonnegative weight.
Finding negative cycles
When the algorithm is used to find shortest paths, the existence of negative cycles is a problem, preventing the algorithm from finding a correct answer. However, since it terminates upon finding a negative cycle, the Bellman–Ford algorithm can be used for applications in which this is the target to be sought – for example in cycle-cancelling techniques in network flow analysis.
Applications in routing
A distributed variant of the Bellman–Ford algorithm is used in distance-vector routing protocols, for example the Routing Information Protocol (RIP). The algorithm is distributed because it involves a number of nodes (routers) within an Autonomous system (AS), a collection of IP networks typically owned by an ISP.
It consists of the following steps:
Each node calculates the distances between itself and all other nodes within the AS and stores this information as a table.
Each node sends its table to all neighboring nodes.
When a node receives distance tables from its neighbors, it calculates the shortest routes to all other nodes and updates its own table to reflect any changes.
The main disadvantages of the Bellman–Ford algorithm in this setting are as follows:
It does not scale well.
Changes in network topology are not reflected quickly since updates are spread node-by-node.
Count to infinity if link or node failures render a node unreachable from some set of other nodes, those nodes may spend forever gradually increasing their estimates of the distance to it, and in the meantime there may be routing loops.
Improvements
The Bellman–Ford algorithm may be improved in practice (although not in the worst case) by the observation that, if an iteration of the main loop of the algorithm terminates without making any changes, the algorithm can be immediately terminated, as subsequent iterations will not make any more changes. With this early termination condition, the main loop may in some cases use many fewer than iterations, even though the worst case of the algorithm remains unchanged. The following improvements all maintain the worst-case time complexity.
A variation of the Bellman–Ford algorithm described by , reduces the number of relaxation steps that need to be performed within each iteration of the algorithm. If a vertex v has a distance value that has not changed since the last time the edges out of v were relaxed, then there is no need to relax the edges out of v a second time. In this way, as the number of vertices with correct distance values grows, the number whose outgoing edges that need to be relaxed in each iteration shrinks, leading to a constant-factor savings in time for dense graphs. This variation can be implemented by keeping a collection of vertices whose outgoing edges need to be relaxed, removing a vertex from this collection when its edges are relaxed, and adding to the collection any vertex whose distance value is changed by a relaxation step. In China, this algorithm was popularized by Fanding Duan, who rediscovered it in 1994, as the "shortest path faster algorithm".
described another improvement to the Bellman–Ford algorithm. His improvement first assigns some arbitrary linear order on all vertices and then partitions the set of all edges into two subsets. The first subset, Ef, contains all edges (vi, vj) such that i < j; the second, Eb, contains edges (vi, vj) such that i > j. Each vertex is visited in the order , relaxing each outgoing edge from that vertex in Ef. Each vertex is then visited in the order , relaxing each outgoing edge from that vertex in Eb. Each iteration of the main loop of the algorithm, after the first one, adds at least two edges to the set of edges whose relaxed distances match the correct shortest path distances: one from Ef and one from Eb. This modification reduces the worst-case number of iterations of the main loop of the algorithm from to .
Another improvement, by , replaces the arbitrary linear order of the vertices used in Yen's second improvement by a random permutation. This change makes the worst case for Yen's improvement (in which the edges of a shortest path strictly alternate between the two subsets Ef and Eb) very unlikely to happen. With a randomly permuted vertex ordering, the expected number of iterations needed in the main loop is at most .
, at Georgetown University, created an improved algorithm that with high probability, runs in time.
| Mathematics | Graph theory | null |
221381 | https://en.wikipedia.org/wiki/Araucariaceae | Araucariaceae | Araucariaceae is a family of conifers with three living genera, Araucaria, Agathis, and Wollemia. While the family's native distribution is now largely confined to the Southern Hemisphere, except for a few species of Agathis in Malesia, it was formerly widespread in the Northern Hemisphere during the Jurassic and Cretaceous periods.
Description
Members of Araucariaceae are typically extremely tall evergreen trees, reaching heights of or more. They can also grow very large stem diameters; a New Zealand kauri tree (Agathis australis) named Tāne Mahuta ("The Lord of the Forest") has been measured at tall with a diameter at breast height of . Its total wood volume is calculated to be , making it the third-largest conifer after Sequoia and Sequoiadendron (both from the Cupressaceae subfamily Sequoioideae).
The trunks are columnar and have relatively large piths with resinous cortices. The branching is usually horizontal and tiered, arising regularly in whorls of three to seven branches or alternating in widely separated pairs.
The leaves can be small, needle-like, and curved, or they can be large, broadly ovate, and flattened. They are spirally arranged, persistent, and usually have parallel venation.
Like other conifers, they produce cones. Each tree can have both male and female cones (monoecious) or they can have only male or female cones (dioecious).
Male cones are among the largest among all conifer cones, on average. They are cylindrical and drooping, somewhat resembling catkins. They are borne singly on the tips of branches or the axils of leaves. They contain numerous sporophylls arranged in whorls or spirals. Each has four to 20 elongated pollen sacs attached to the lower surface at one end. The pollen grains are round and do not possess wings or air sacs.
Female cones are also very large. They are spherical to ovoid in shape and borne erect on thick, short shoots at branch tips. The numerous bracts and scales are either fused to each other or separate for half of their lengths. The scales almost always bear only one seed on its upper surface, in contrast to two in true pines (family Pinaceae). They are very large, among the largest seeds among conifers. They are dispersed by wind, usually using wing-like structures. On maturity, the female cones detach and fall to the ground. Due to their size, they can cause serious injuries if they hit a person. The cones of the bunya bunya, Araucaria bidwillii, for example, weigh up to , about the size and weight of a large pineapple. They can drop from heights of .
Classification and genera
Araucariaceae is classified under the order Pinales, class Pinopsida of the division Pinophyta. The division includes all living conifers. Recently however, some authorities treat Araucariaceae as a separate order, Araucariales.
Araucariaceae contains three extant genera and about 41 species.
Phylogeny
Below is the phylogeny of the Pinophyta based on cladistic analysis of molecular data. It shows the position of Araucariaceae within the division.
Relationships between living members of Araucariaceae.
Molecular evidence supports Araucariaceae and Podocarpaceae having diverged from each other during the late Permian.
Distribution and habitat
Today, 41 species are known, in three genera: Agathis, Araucaria and Wollemia, distributed largely in the Southern Hemisphere.
By far the greatest diversity is in New Caledonia (18 species), with others in Australia, Argentina, New Zealand, Chile, southern Brazil, and Malesia. In Malesia, Agathis extends a short distance into the Northern Hemisphere, reaching 18°N in the Philippines.
Uses
Several species are very popular ornamental trees in gardens in subtropical regions, and some are also very important timber trees, producing wood of high quality. Several have edible seeds similar to pine nuts, and others produce valuable resin and amber. In the forests where they occur, they are usually dominant trees, often the largest species in the forest; the largest is Araucaria hunsteinii, reported to 89 m tall in New Guinea, with several other species reaching 50–65 m tall. A. heterophylla, the Norfolk Island pine, is a well-known landscaping and house plant from this taxon.
Skillful artisans in the Erzurum Province, Turkey, have used fossilized wood of Araucariaceae for centuries to manufacture jewelry and decorative items. It is known as "Oltustone", the name deriving from the town of Oltu, where it is most commonly excavated. Despite the fact that this semiprecious gemstone is classified as "stone", wood anatomy reveals it was fossilized pieces of trunks of Araucariacea. Oltustone, also called ‘Black Amber’ is unique to Turkey. It is dull and black, but when polished, acquires an attractive black sheen.
Fossil record
Fossils widely believed to belong to Araucariaceae include the form genera Araucarites (various), Agathoxylon and Araucarioxylon (wood), Brachyphyllum (leaves), Araucariacites and Dilwynites (pollen), and Protodammara (cones).
The oldest definitive records of Araucariaceae are from the Early Jurassic, though there are potential earlier Late Triassic records. Early representatives of Araucaria are widespread across both hemispheres by the Middle Jurassic, such as Araucaria mirabilis and Araucaria sphaerocarpa from the Middle Jurassic of Argentina and England respectively. The oldest records of the Wollemia-Agathis lineage from the Cretaceous, including Emwadea microcarpa from the Albian aged Winton Formation of Australia and Wairarapaia mildenhallii from the Albian-Cenomanian of New Zealand. The oldest fossils currently confidently assignable to Agathis are those of Agathis immortalis from the Salamanca Formation of Patagonia, which dates to the Paleocene, approximately 64.67–63.49 million years ago. Agathis-like leaves are also known from the slightly older Lefipán Formation of the same region, which date to the very end of the Cretaceous. Araucariaceae fossils are also known from the latest Oligocene or earliest Miocene of the southwesternmost tip of Africa. Claimed records of Agathis from the Eocene of Canada based on chemical analysis of amber are questionable.
| Biology and health sciences | Pinophyta (Conifers) | Plants |
221399 | https://en.wikipedia.org/wiki/Buttonquail | Buttonquail | Buttonquail or hemipodes are members of a small family of birds, Turnicidae, which resemble, but are not closely related to, the quails of Phasianidae. They inhabit warm grasslands in Asia, Africa, Europe, and Australia. There are 18 species in two genera, with most species placed in the genus Turnix and a single species in the genus Ortyxelos.
Buttonquails are small, drab, running birds, which avoid flying. The female is the more richly colored of the sexes. While the quail-plover is thought to be monogamous, Turnix buttonquails are sequentially polyandrous; both sexes cooperate in building a nest in the earth, but normally only the male incubates the eggs and tends the young, while the female may go on to mate with other males.
Taxonomy
The genus Turnix was introduced in 1791 by French naturalist in Pierre Bonnaterre. The genus name is an abbreviation of the genus Coturnix. The type species was subsequently designated as the common buttonquail.
The buttonquail family, Turnicidae, was introduced in 1840 by the English zoologist George Robert Gray. The buttonquails were traditionally placed in Gruiformes or Galliformes (the crane and pheasant orders). The Sibley-Ahlquist taxonomy elevated them to ordinal status as the Turniciformes and basal to other Neoaves either because their accelerated rate of molecular evolution exceeded the limits of sensitivity of DNA-DNA hybridization or because the authors did not perform the appropriate pairwise comparisons or both. Morphological, DNA-DNA hybridization and sequence data indicate that turnicids correctly belong to the shorebirds (Charadriiformes). They seem to be an ancient group among these, as indicated by the buttonquail-like Early Oligocene fossil Turnipax and the collected molecular data.
Description
The buttonquails are a group of small terrestrial birds. The smallest species is the quail-plover, the only species in the genus Ortyxelos, which is in length and weighs only . The buttonquails in the genus Turnix range from in length and weigh between . They superficially resemble the true quails of the genus Coturnix, but differ from them in lacking a hind toe and a crop. The females of this family also possess a unique vocal organ created by an enlarged trachea and inflatable bulb in the esophagus, which they use to produce a booming call.
Breeding
Buttonquails are unusual in that females are serially polyandrous. The nest is a scape on the ground often near overhanging vegetation. The female lays a clutch of 4 or 5 eggs and then looks for a new mate. The male incubates the eggs which hatch synchronously after 12 to 15 days. The precocial chicks leave the nest soon after hatching and are cared for by the male. They can fly at two weeks of age and become independent at four weeks. For the smaller species sexual maturity is reached at three months.
Species
Family: Turnicidae
Genus: Ortyxelos
Quail-plover, Ortyxelos meiffrenii
Genus: Turnix
Common buttonquail, Turnix sylvaticus
Tawitawi common buttonquail, Turnix sylvaticus suluensis (extinct: mid-20th century)
Andalusian common buttonquail, Turnix sylvaticus sylvaticus (possibly extinct: late 20th century?)
Red-backed buttonquail, Turnix maculosus
Fynbos buttonquail, Turnix hottentottus
Black-rumped buttonquail, Turnix nanus
Yellow-legged buttonquail, Turnix tanki
Spotted buttonquail, Turnix ocellatus
Barred buttonquail, Turnix suscitator
Madagascar buttonquail, Turnix nigricollis
Black-breasted buttonquail, Turnix melanogaster
Chestnut-backed buttonquail, Turnix castanotus
Buff-breasted buttonquail, Turnix olivii
Painted buttonquail, Turnix varius
Abrolhos painted buttonquail, Turnix varius scintillans
New Caledonian buttonquail, Turnix novaecaledoniae (possibly extinct: early 20th century)
Worcester's buttonquail, Turnix worcesteri
Sumba buttonquail, Turnix everetti
Red-chested buttonquail, Turnix pyrrhothorax
Little buttonquail, Turnix velox
Gallery
| Biology and health sciences | Charadriiformes | Animals |
221430 | https://en.wikipedia.org/wiki/Bearing%20%28mechanical%29 | Bearing (mechanical) | A bearing is a machine element that constrains relative motion to only the desired motion and reduces friction between moving parts. The design of the bearing may, for example, provide for free linear movement of the moving part or for free rotation around a fixed axis; or, it may prevent a motion by controlling the vectors of normal forces that bear on the moving parts. Most bearings facilitate the desired motion by minimizing friction. Bearings are classified broadly according to the type of operation, the motions allowed, or the directions of the loads (forces) applied to the parts.
The term "bearing" is derived from the verb "to bear"; a bearing being a machine element that allows one part to bear (i.e., to support) another. The simplest bearings are bearing surfaces, cut or formed into a part, with varying degrees of control over the form, size, roughness, and location of the surface. Other bearings are separate devices installed into a machine or machine part. The most sophisticated bearings for the most demanding applications are very precise components; their manufacture requires some of the highest standards of current technology.
Types of bearings
Rotary bearings hold rotating components such as shafts or axles within mechanical systems and transfer axial and radial loads from the source of the load to the structure supporting it. The simplest form of bearing, the plain bearing, consists of a shaft rotating in a hole. Lubrication is used to reduce friction. Lubricants come in different forms, including liquids, solids, and gases. The choice of lubricant depends on the specific application and factors such as temperature, load, and speed. In the ball bearing and roller bearing, to reduce sliding friction, rolling elements such as rollers or balls with a circular cross-section are located between the races or journals of the bearing assembly. A wide variety of bearing designs exists to allow the demands of the application to be correctly met for maximum efficiency, reliability, durability, and performance.
History
It is sometimes assumed that the invention of the rolling bearing, in the form of wooden rollers supporting– or bearing –an object being moved, predates the invention of a wheel rotating on a plain bearing; this underlies speculation that cultures such as the Ancient Egyptians used roller bearings in the form of tree trunks under sleds. There is no evidence for this sequence of technological development. The Egyptians' own drawings in the tomb of Djehutihotep show the process of moving massive stone blocks on sledges as using liquid-lubricated runners which would constitute plain bearings. There are also Egyptian drawings of plain bearings used with hand drills.
Wheeled vehicles using plain bearings emerged between about 5000 BC and 3000 BC.
A recovered example of an early rolling-element bearing is a wooden ball bearing supporting a rotating table from the remains of the Roman Nemi ships in Lake Nemi, Italy. The wrecks were dated to 40 BC.
Leonardo da Vinci incorporated drawings of ball bearings in his design for a helicopter around the year 1500; this is the first recorded use of bearings in an aerospace design. However, Agostino Ramelli is the first to have published roller and thrust bearings sketches. An issue with the ball and roller bearings is that the balls or rollers rub against each other, causing additional friction. This can be reduced by enclosing each individual ball or roller within a cage. The captured, or caged, ball bearing was originally described by Galileo in the 17th century.
The first practical caged-roller bearing was invented in the mid-1740s by horologist John Harrison for his H3 marine timekeeper. In this timepiece, the caged bearing was only used for a very limited oscillating motion, but later on, Harrison applied a similar bearing design with a true rotational movement in a contemporaneous regulator clock.
The first patent on ball bearings was awarded to Philip Vaughan, a British inventor and ironmaster in Carmarthen in 1794. His was the first modern ball-bearing design, with the ball running along a groove in the axle assembly.
Bearings played a pivotal role in the nascent Industrial Revolution, allowing the new industrial machinery to operate efficiently. For example, they were used for holding wheel and axle assemblies to greatly reduce friction compared to prior non-bearing designs.
The first patent for a radial-style ball bearing was awarded to Jules Suriray, a Parisian bicycle mechanic, on 3 August 1869. The bearings were then fitted to the winning bicycle ridden by James Moore in the world's first bicycle road race, Paris-Rouen, in November 1869.
In 1883, Friedrich Fischer, founder of FAG, developed an approach for milling and grinding balls of equal size and exact roundness by means of a suitable production machine, which set the stage for the creation of an independent bearing industry. His hometown Schweinfurt later became a world-leading center for ball bearing production.
The modern, self-aligning design of ball bearing is attributed to Sven Wingquist of the SKF ball-bearing manufacturer in 1907 when he was awarded Swedish patent No. 25406 on its design.
Henry Timken, a 19th-century visionary and innovator in carriage manufacturing, patented the tapered roller bearing in 1898. The following year he formed a company to produce his innovation. Over a century, the company grew to make bearings of all types, including specialty steel bearings and an array of related products and services.
Erich Franke invented and patented the wire race bearing in 1934. His focus was on a bearing design with a cross-section as small as possible and which could be integrated into the enclosing design. After World War II, he founded with Gerhard Heydrich the company Franke & Heydrich KG (today Franke GmbH) to push the development and production of wire race bearings.
Richard Stribeck's extensive research on ball bearing steels identified the metallurgy of the commonly used 100Cr6 (AISI 52100), showing coefficient of friction as a function of pressure.
Designed in 1968 and later patented in 1972, Bishop-Wisecarver's co-founder Bud Wisecarver created vee groove bearing guide wheels, a type of linear motion bearing consisting of both an external and internal 90-degree vee angle.
In the early 1980s, Pacific Bearing's founder, Robert Schroeder, invented the first bi-material plain bearing that was interchangeable with linear ball bearings. This bearing had a metal shell (aluminum, steel or stainless steel) and a layer of Teflon-based material connected by a thin adhesive layer.
Today's ball and roller bearings are used in many applications, which include a rotating component. Examples include ultra high-speed bearings in dental drills, aerospace bearings in the Mars Rover, gearbox and wheel bearings on automobiles, flexure bearings in optical alignment systems, and air bearings used in coordinate-measuring machines.
Design
Motions
Common motions permitted by bearings are:
Radial rotation, e.g. shaft rotation;
Linear motion, e.g. drawer;
Spherical rotation, e.g. ball and socket joint;
Hinge motion, e.g. door, elbow, knee.
Materials
The first plain and rolling-element bearings were wood, closely followed by bronze. Over their history, bearings have been made of many materials, including ceramic, sapphire, glass, steel, bronze, and other metals. Plastic bearings made of nylon, polyoxymethylene, polytetrafluoroethylene, and UHMWPE, among other materials, are also in use today.
Watchmakers produce "jeweled" watches using sapphire plain bearings to reduce friction, thus allowing more precise timekeeping.
Even basic materials can have impressive durability. Wooden bearings, for instance, can still be seen today in old clocks or in water mills where the water provides cooling and lubrication.
Types
By far, the most common bearing is the plain bearing, a bearing that uses surfaces in rubbing contact, often with a lubricant such as oil or graphite. A plain bearing may or may not be a discrete device. It may be nothing more than the bearing surface of a hole with a shaft passing through it, or of a planar surface that bears another (in these cases, not a discrete device); or it may be a layer of bearing metal either fused to the substrate (semi-discrete) or in the form of a separable sleeve (discrete). With suitable lubrication, plain bearings often give acceptable accuracy, life, and friction at minimal cost. Therefore, they are very widely used.
However, there are many applications where a more suitable bearing can improve efficiency, accuracy, service intervals, reliability, speed of operation, size, weight, and costs of purchasing and operating machinery.
Thus, many types of bearings have varying shapes, materials, lubrication, principle of operation, and so on.
There are at least 6 common types of bearing, each of which operates on a different principle:
Plain bearing, consisting of a shaft rotating in a hole. There are several specific styles: bushing, journal bearing, sleeve bearing, rifle bearing, composite bearing;
Rolling-element bearings, whose performance does not depend on avoiding or reducing friction between two surfaces but employs a different principle to achieve low external friction: the rolling motion of an intermediate element in between the surfaces which bear the axial or radial load. Classified as either:
Ball bearing, in which the rolling elements are spherical balls;
Roller bearing, in which the rolling elements are cylindrical rollers, linearly-tapered (conical) rollers, rollers with a curved taper (such as so-called spherical rollers), or gears;
Jewel bearing, a plain bearing in which one of the bearing surfaces is made of an ultrahard glassy jewel material such as sapphire to reduce friction and wear;
Fluid bearing, a noncontact bearing in which the load is supported by a gas or liquid (i.e. air bearing);
Magnetic bearing, in which the load is supported by a magnetic field;
Flexure bearing, in which the motion is supported by a load element which bends.
The following table summarizes the notable characteristics of each of these bearing types.
Characteristics
Friction
Reducing friction in bearings is often important for efficiency, to reduce wear and to facilitate extended use at high speeds and to avoid overheating and premature failure of the bearing. Essentially, a bearing can reduce friction by virtue of its shape, by its material, or by introducing and containing a fluid between surfaces or by separating the surfaces with an electromagnetic field.
Shape: gains advantage usually by using spheres or rollers, or by forming flexure bearings.
Material: exploits the nature of the bearing material used. (An example would be using plastics that have low surface friction.)
Fluid: exploits the low viscosity of a layer of fluid, such as a lubricant or as a pressurized medium to keep the two solid parts from touching, or by reducing the normal force between them.
Fields: exploits electromagnetic fields, such as magnetic fields, to keep solid parts from touching.
Air pressure: exploits air pressure to keep solid parts from touching.
Combinations of these can even be employed within the same bearing. An example is where the cage is made of plastic, and it separates the rollers/balls, which reduce friction by their shape and finish.
Loads
Bearing design varies depending on the size and directions of the forces required to support. Forces can be predominately radial, axial (thrust bearings), or bending moments perpendicular to the main axis.
Speeds
Different bearing types have different operating speed limits. Speed is typically specified as maximum relative surface speeds, often specified ft/s or m/s. Rotational bearings typically describe performance in terms of the product DN where D is the mean diameter (often in mm) of the bearing and N is the rotation rate in revolutions per minute.
Generally, there is considerable speed range overlap between bearing types. Plain bearings typically handle only lower speeds, rolling element bearings are faster, followed by fluid bearings and finally magnetic bearings which are limited ultimately by centripetal force overcoming material strength.
Play
Some applications apply bearing loads from varying directions and accept only limited play or "slop" as the applied load changes. One source of motion is gaps or "play" in the bearing. For example, a 10 mm shaft in a 12 mm hole has 2 mm play.
Allowable play varies greatly depending on the use. As an example, a wheelbarrow wheel supports radial and axial loads. Axial loads may be hundreds of newtons force left or right, and it is typically acceptable for the wheel to wobble by as much as 10 mm under the varying load. In contrast, a lathe may position a cutting tool to ±0.002 mm using a ball lead screw held by rotating bearings. The bearings support axial loads of thousands of newtons in either direction and must hold the ball lead screw to ±0.002 mm across that range of loads
Stiffness
Stiffness is the amount that the gap varies when the load on the bearing changes, distinct from the friction of the bearing.
A second source of motion is elasticity in the bearing itself. For example, the balls in a ball bearing are like stiff rubber and under load deform from a round to a slightly flattened shape. The race is also elastic and develops a slight dent where the ball presses on it.
The stiffness of a bearing is how the distance between the parts separated by the bearing varies with the applied load. With rolling element bearings, this is due to the strain of the ball and race. With fluid bearings, it is due to how the pressure of the fluid varies with the gap (when correctly loaded, fluid bearings are typically stiffer than rolling element bearings).
Lubrication
Some bearings use a thick grease for lubrication, which is pushed into the gaps between the bearing surfaces, also known as packing. The grease is held in place by a plastic, leather, or rubber gasket (also called a gland) that covers the inside and outside edges of the bearing race to keep the grease from escaping. Bearings may also be packed with other materials. Historically, the wheels on railroad cars used sleeve bearings packed with waste or loose scraps of cotton or wool fiber soaked in oil, then later used solid pads of cotton.
Bearings can be lubricated by a ring oiler, a metal ring that rides loosely on the central rotating shaft of the bearing. The ring hangs down into a chamber containing lubricating oil. As the bearing rotates, viscous adhesion draws oil up the ring and onto the shaft, where the oil migrates into the bearing to lubricate it. Excess oil is flung off and collects in the pool again.
A rudimentary form of lubrication is splash lubrication. Some machines contain a pool of lubricant in the bottom, with gears partially immersed in the liquid, or crank rods that can swing down into the pool as the device operates. The spinning wheels fling oil into the air around them, while the crank rods slap at the surface of the oil, splashing it randomly on the engine's interior surfaces. Some small internal combustion engines specifically contain special plastic flinger wheels which randomly scatter oil around the interior of the mechanism.
For high-speed and high-power machines, a loss of lubricant can result in rapid bearing heating and damage due to friction. Also, in dirty environments, the oil can become contaminated with dust or debris, increasing friction. In these applications, a fresh supply of lubricant can be continuously supplied to the bearing and all other contact surfaces, and the excess can be collected for filtration, cooling, and possibly reuse. Pressure oiling is commonly used in large and complex internal combustion engines in parts of the engine where directly splashed oil cannot reach, such as up into overhead valve assemblies. High-speed turbochargers also typically require a pressurized oil system to cool the bearings and keep them from burning up due to the heat from the turbine.
Composite bearings are designed with a self-lubricating polytetrafluorethylene (PTFE) liner with a laminated metal backing. The PTFE liner offers consistent, controlled friction as well as durability, whilst the metal backing ensures the composite bearing is robust and capable of withstanding high loads and stresses throughout its long life. Its design also makes it lightweight-one tenth the weight of a traditional rolling element bearing.
Mounting
There are many methods of mounting bearings, usually involving an interference fit. When press fitting or shrink fitting a bearing into a bore or onto a shaft, it's important to keep the housing bore and shaft outer diameter to very close limits, which can involve one or more counterboring operations, several facing operations, and drilling, tapping, and threading operations. Alternatively, an interference fit can also be achieved with the addition of a tolerance ring.
Service life
The service life of the bearing is affected by many factors not controlled by the bearing manufacturers. For example, bearing mounting, temperature, exposure to external environment, lubricant cleanliness, and electrical currents through bearings. High frequency PWM inverters can induce electric currents in a bearing, which can be suppressed by the use of ferrite chokes. The temperature and terrain of the micro-surface will determine the amount of friction by touching solid parts. Certain elements and fields reduce friction while increasing speeds. Strength and mobility help determine the load the bearing type can carry. Alignment factors can play a damaging role in wear and tear, yet overcome by computer aid signaling and non-rubbing bearing types, such as magnetic levitation or air field pressure.
Fluid and magnetic bearings can have practically indefinite service lives. In practice, fluid bearings support high loads in hydroelectric plants that have been in nearly continuous service since about 1900 and show no signs of wear.
Rolling element bearing life is determined by load, temperature, maintenance, lubrication, material defects, contamination, handling, installation and other factors. These factors can all have a significant effect on bearing life. For example, the service life of bearings in one application was extended dramatically by changing how the bearings were stored before installation and use, as vibrations during storage caused lubricant failure even when the only load on the bearing was its own weight; the resulting damage is often false brinelling. Bearing life is statistical: several samples of a given bearing will often exhibit a bell curve of service life, with a few samples showing significantly better or worse life. Bearing life varies because microscopic structure and contamination vary greatly even where macroscopically they seem identical.
Bearings are often specified to give an "L10" (US) or "B10" (elsewhere) life, the duration by which ten percent of the bearings in that application can be expected to have failed due to classical fatigue failure (and not any other mode of failure such as lubrication starvation, wrong mounting etc.), or, alternatively, the duration at which ninety percent will still be operating. The L10/B10 life of the bearing is theoretical, and may not represent service life of the bearing. Bearings are also rated using the C0 (static loading) value. This is the basic load rating as a reference, and not an actual load value.
For plain bearings, some materials give a much longer life than others. Some of the John Harrison clocks still operate after hundreds of years because of the lignum vitae wood employed in their construction, whereas his metal clocks are seldom run due to potential wear.
Flexure bearings rely on elastic properties of a material. Flexure bearings bend a piece of material repeatedly. Some materials fail after repeated bending, even at low loads, but careful material selection and bearing design can make flexure bearing life indefinite.
Although long bearing life is often desirable, it is sometimes not necessary. describes a bearing for a rocket motor oxygen pump that gave several hours life, far in excess of the several tens of minutes needed.
Depending on the customized specifications (backing material and PTFE compounds), composite bearings can operate up to 30 years without maintenance.
For bearings which are used in oscillating applications, customized approaches to calculate L10/B10 are used.
Many bearings require periodic maintenance to prevent premature failure, but others require little maintenance. The latter include various kinds of polymer, fluid and magnetic bearings, as well as rolling-element bearings that are described with terms including sealed bearing and sealed for life. These contain seals to keep the dirt out and the grease in. They work successfully in many applications, providing maintenance-free operation. Some applications cannot use them effectively.
Nonsealed bearings often have a grease fitting, for periodic lubrication with a grease gun, or an oil cup for periodic filling with oil. Before the 1970s, sealed bearings were not encountered on most machinery, and oiling and greasing were a more common activity than they are today. For example, automotive chassis used to require "lube jobs" nearly as often as engine oil changes, but today's car chassis are mostly sealed for life. From the late 1700s through the mid-1900s, industry relied on many workers called oilers to lubricate machinery frequently with oil cans.
Factory machines today usually have lube systems, in which a central pump serves periodic charges of oil or grease from a reservoir through lube lines to the various lube points in the machine's bearing surfaces, bearing journals, pillow blocks, and so on. The timing and number of such lube cycles is controlled by the machine's computerized control, such as PLC or CNC, as well as by manual override functions when occasionally needed. This automated process is how all modern CNC machine tools and many other factory machines are lubricated. Similar lube systems are also used on nonautomated machines, in which case there is a hand pump that a machine operator is supposed to pump once daily (for machines in constant use) or once weekly. These are called one-shot systems from their chief selling point: one pull on one handle to lube the whole machine, instead of a dozen pumps of an alemite gun or oil can in a dozen different positions around the machine.
The oiling system inside a modern automotive or truck engine is similar in concept to the lube systems mentioned above, except that oil is pumped continuously. Much of this oil flows through passages drilled or cast into the engine block and cylinder heads, escaping through ports directly onto bearings and squirting elsewhere to provide an oil bath. The oil pump simply pumps constantly, and any excess pumped oil continuously escapes through a relief valve back into the sump.
Many bearings in high-cycle industrial operations need periodic lubrication and cleaning, and many require occasional adjustment, such as pre-load adjustment, to minimize the effects of wear.
Bearing life is often much better when the bearing is kept clean and well-lubricated. However, many applications make good maintenance difficult. One example is bearings in the conveyor of a rock crusher are exposed continually to hard abrasive particles. Cleaning is of little use because cleaning is expensive, yet the bearing is contaminated again as soon as the conveyor resumes operation. Thus, a good maintenance program might lubricate the bearings frequently but not include any disassembly for cleaning. The frequent lubrication, by its nature, provides a limited kind of cleaning action by displacing older (grit-filled) oil or grease with a fresh charge, which itself collects grit before being displaced by the next cycle. Another example are bearings in wind turbines, which makes maintenance difficult since the nacelle is placed high up in the air in strong wind areas. In addition, the turbine does not always run and is subjected to different operating behavior in different weather conditions, which makes proper lubrication a challenge.
| Technology | Components_2 | null |
221519 | https://en.wikipedia.org/wiki/Differential%20form | Differential form | In mathematics, differential forms provide a unified approach to define integrands over curves, surfaces, solids, and higher-dimensional manifolds. The modern notion of differential forms was pioneered by Élie Cartan. It has many applications, especially in geometry, topology and physics.
For instance, the expression is an example of a -form, and can be integrated over an interval contained in the domain of :
Similarly, the expression is a -form that can be integrated over a surface :
The symbol denotes the exterior product, sometimes called the wedge product, of two differential forms. Likewise, a -form represents a volume element that can be integrated over a region of space. In general, a -form is an object that may be integrated over a -dimensional manifold, and is homogeneous of degree in the coordinate differentials
On an -dimensional manifold, a top-dimensional form (-form) is called a volume form.
The differential forms form an alternating algebra. This implies that and This alternating property reflects the orientation of the domain of integration.
The exterior derivative is an operation on differential forms that, given a -form , produces a -form This operation extends the differential of a function (a function can be considered as a -form, and its differential is ). This allows expressing the fundamental theorem of calculus, the divergence theorem, Green's theorem, and Stokes' theorem as special cases of a single general result, the generalized Stokes theorem.
Differential -forms are naturally dual to vector fields on a differentiable manifold, and the pairing between vector fields and -forms is extended to arbitrary differential forms by the interior product. The algebra of differential forms along with the exterior derivative defined on it is preserved by the pullback under smooth functions between two manifolds. This feature allows geometrically invariant information to be moved from one space to another via the pullback, provided that the information is expressed in terms of differential forms. As an example, the change of variables formula for integration becomes a simple statement that an integral is preserved under pullback.
History
Differential forms are part of the field of differential geometry, influenced by linear algebra. Although the notion of a differential is quite old, the initial attempt at an algebraic organization of differential forms is usually credited to Élie Cartan with reference to his 1899 paper. Some aspects of the exterior algebra of differential forms appears in Hermann Grassmann's 1844 work, Die Lineale Ausdehnungslehre, ein neuer Zweig der Mathematik (The Theory of Linear Extension, a New Branch of Mathematics).
Concept
Differential forms provide an approach to multivariable calculus that is independent of coordinates.
Integration and orientation
A differential -form can be integrated over an oriented manifold of dimension . A differential -form can be thought of as measuring an infinitesimal oriented length, or 1-dimensional oriented density. A differential -form can be thought of as measuring an infinitesimal oriented area, or 2-dimensional oriented density. And so on.
Integration of differential forms is well-defined only on oriented manifolds. An example of a 1-dimensional manifold is an interval , and intervals can be given an orientation: they are positively oriented if , and negatively oriented otherwise. If then the integral of the differential -form over the interval (with its natural positive orientation) is
which is the negative of the integral of the same differential form over the same interval, when equipped with the opposite orientation. That is:
This gives a geometrical context to the conventions for one-dimensional integrals, that the sign changes when the orientation of the interval is reversed. A standard explanation of this in one-variable integration theory is that, when the limits of integration are in the opposite order (), the increment is negative in the direction of integration.
More generally, an -form is an oriented density that can be integrated over an -dimensional oriented manifold. (For example, a -form can be integrated over an oriented curve, a -form can be integrated over an oriented surface, etc.) If is an oriented -dimensional manifold, and is the same manifold with opposite orientation and is an -form, then one has:
These conventions correspond to interpreting the integrand as a differential form, integrated over a chain. In measure theory, by contrast, one interprets the integrand as a function with respect to a measure and integrates over a subset , without any notion of orientation; one writes to indicate integration over a subset . This is a minor distinction in one dimension, but becomes subtler on higher-dimensional manifolds; see below for details.
Making the notion of an oriented density precise, and thus of a differential form, involves the exterior algebra. The differentials of a set of coordinates, , ..., can be used as a basis for all -forms. Each of these represents a covector at each point on the manifold that may be thought of as measuring a small displacement in the corresponding coordinate direction. A general -form is a linear combination of these differentials at every point on the manifold:
where the are functions of all the coordinates. A differential -form is integrated along an oriented curve as a line integral.
The expressions , where can be used as a basis at every point on the manifold for all -forms. This may be thought of as an infinitesimal oriented square parallel to the –-plane. A general -form is a linear combination of these at every point on the manifold: and it is integrated just like a surface integral.
A fundamental operation defined on differential forms is the exterior product (the symbol is the wedge ). This is similar to the cross product from vector calculus, in that it is an alternating product. For instance,
because the square whose first side is and second side is is to be regarded as having the opposite orientation as the square whose first side is and whose second side is . This is why we only need to sum over expressions , with ; for example: . The exterior product allows higher-degree differential forms to be built out of lower-degree ones, in much the same way that the cross product in vector calculus allows one to compute the area vector of a parallelogram from vectors pointing up the two sides. Alternating also implies that , in the same way that the cross product of parallel vectors, whose magnitude is the area of the parallelogram spanned by those vectors, is zero. In higher dimensions, if any two of the indices , ..., are equal, in the same way that the "volume" enclosed by a parallelotope whose edge vectors are linearly dependent is zero.
Multi-index notation
A common notation for the wedge product of elementary -forms is so called multi-index notation: in an -dimensional context, for we define Another useful notation is obtained by defining the set of all strictly increasing multi-indices of length , in a space of dimension , denoted Then locally (wherever the coordinates apply), spans the space of differential -forms in a manifold of dimension , when viewed as a module over the ring of smooth functions on . By calculating the size of combinatorially, the module of -forms on an -dimensional manifold, and in general space of -covectors on an -dimensional vector space, is choose : This also demonstrates that there are no nonzero differential forms of degree greater than the dimension of the underlying manifold.
The exterior derivative
In addition to the exterior product, there is also the exterior derivative operator . The exterior derivative of a differential form is a generalization of the differential of a function, in the sense that the exterior derivative of is exactly the differential of . When generalized to higher forms, if is a simple -form, then its exterior derivative is a -form defined by taking the differential of the coefficient functions:
with extension to general -forms through linearity: if then its exterior derivative is
In , with the Hodge star operator, the exterior derivative corresponds to gradient, curl, and divergence, although this correspondence, like the cross product, does not generalize to higher dimensions, and should be treated with some caution.
The exterior derivative itself applies in an arbitrary finite number of dimensions, and is a flexible and powerful tool with wide application in differential geometry, differential topology, and many areas in physics. Of note, although the above definition of the exterior derivative was defined with respect to local coordinates, it can be defined in an entirely coordinate-free manner, as an antiderivation of degree 1 on the exterior algebra of differential forms. The benefit of this more general approach is that it allows for a natural coordinate-free approach to integrate on manifolds. It also allows for a natural generalization of the fundamental theorem of calculus, called the (generalized) Stokes' theorem, which is a central result in the theory of integration on manifolds.
Differential calculus
Let be an open set in . A differential -form ("zero-form") is defined to be a smooth function on – the set of which is denoted . If is any vector in , then has a directional derivative , which is another function on whose value at a point is the rate of change (at ) of in the direction:
(This notion can be extended pointwise to the case that is a vector field on by evaluating at the point in the definition.)
In particular, if is the th coordinate vector then is the partial derivative of with respect to the th coordinate vector, i.e., , where , , ..., are the coordinate vectors in . By their very definition, partial derivatives depend upon the choice of coordinates: if new coordinates , , ..., are introduced, then
The first idea leading to differential forms is the observation that is a linear function of :
for any vectors , and any real number . At each point p, this linear map from to is denoted and called the derivative or differential of at . Thus . Extended over the whole set, the object can be viewed as a function that takes a vector field on , and returns a real-valued function whose value at each point is the derivative along the vector field of the function . Note that at each , the differential is not a real number, but a linear functional on tangent vectors, and a prototypical example of a differential -form.
Since any vector is a linear combination of its components, is uniquely determined by for each and each , which are just the partial derivatives of on . Thus provides a way of encoding the partial derivatives of . It can be decoded by noticing that the coordinates , , ..., are themselves functions on , and so define differential -forms , , ..., . Let . Since , the Kronecker delta function, it follows that
The meaning of this expression is given by evaluating both sides at an arbitrary point : on the right hand side, the sum is defined "pointwise", so that
Applying both sides to , the result on each side is the th partial derivative of at . Since and were arbitrary, this proves the formula .
More generally, for any smooth functions and on , we define the differential -form pointwise by
for each . Any differential -form arises this way, and by using it follows that any differential -form on may be expressed in coordinates as
for some smooth functions on .
The second idea leading to differential forms arises from the following question: given a differential -form on , when does there exist a function on such that ? The above expansion reduces this question to the search for a function whose partial derivatives are equal to given functions . For , such a function does not always exist: any smooth function satisfies
so it will be impossible to find such an unless
for all and .
The skew-symmetry of the left hand side in and suggests introducing an antisymmetric product on differential -forms, the exterior product, so that these equations can be combined into a single condition
where is defined so that:
This is an example of a differential -form. This -form is called the exterior derivative of . It is given by
To summarize: is a necessary condition for the existence of a function with .
Differential -forms, -forms, and -forms are special cases of differential forms. For each , there is a space of differential -forms, which can be expressed in terms of the coordinates as
for a collection of functions . Antisymmetry, which was already present for -forms, makes it possible to restrict the sum to those sets of indices for which .
Differential forms can be multiplied together using the exterior product, and for any differential -form , there is a differential -form called the exterior derivative of .
Differential forms, the exterior product and the exterior derivative are independent of a choice of coordinates. Consequently, they may be defined on any smooth manifold . One way to do this is cover with coordinate charts and define a differential -form on to be a family of differential -forms on each chart which agree on the overlaps. However, there are more intrinsic definitions which make the independence of coordinates manifest.
Intrinsic definitions
Let be a smooth manifold. A smooth differential form of degree is a smooth section of the th exterior power of the cotangent bundle of . The set of all differential -forms on a manifold is a vector space, often denoted .
The definition of a differential form may be restated as follows. At any point , a -form defines an element
where is the tangent space to at and is its dual space. This space is to the fiber at of the dual bundle of the th exterior power of the tangent bundle of . That is, is also a linear functional , i.e. the dual of the th exterior power is isomorphic to the th exterior power of the dual:
By the universal property of exterior powers, this is equivalently an alternating multilinear map:
Consequently, a differential -form may be evaluated against any -tuple of tangent vectors to the same point of . For example, a differential -form assigns to each point a linear functional on . In the presence of an inner product on (induced by a Riemannian metric on ), may be represented as the inner product with a tangent vector . Differential -forms are sometimes called covariant vector fields, covector fields, or "dual vector fields", particularly within physics.
The exterior algebra may be embedded in the tensor algebra by means of the alternation map. The alternation map is defined as a mapping
For a tensor at a point ,
where is the symmetric group on elements. The alternation map is constant on the cosets of the ideal in the tensor algebra generated by the symmetric 2-forms, and therefore descends to an embedding
This map exhibits as a totally antisymmetric covariant tensor field of rank . The differential forms on are in one-to-one correspondence with such tensor fields.
Operations
As well as the addition and multiplication by scalar operations which arise from the vector space structure, there are several other standard operations defined on differential forms. The most important operations are the exterior product of two differential forms, the exterior derivative of a single differential form, the interior product of a differential form and a vector field, the Lie derivative of a differential form with respect to a vector field and the covariant derivative of a differential form with respect to a vector field on a manifold with a defined connection.
Exterior product
The exterior product of a -form and an -form , denoted , is a ()-form. At each point of the manifold , the forms and are elements of an exterior power of the cotangent space at . When the exterior algebra is viewed as a quotient of the tensor algebra, the exterior product corresponds to the tensor product (modulo the equivalence relation defining the exterior algebra).
The antisymmetry inherent in the exterior algebra means that when is viewed as a multilinear functional, it is alternating. However, when the exterior algebra is embedded as a subspace of the tensor algebra by means of the alternation map, the tensor product is not alternating. There is an explicit formula which describes the exterior product in this situation. The exterior product is
If the embedding of into is done via the map instead of , the exterior product is
This description is useful for explicit computations. For example, if , then is the -form whose value at a point is the alternating bilinear form defined by
for .
The exterior product is bilinear: If , , and are any differential forms, and if is any smooth function, then
It is skew commutative (also known as graded commutative), meaning that it satisfies a variant of anticommutativity that depends on the degrees of the forms: if is a -form and is an -form, then
One also has the graded Leibniz rule:
Riemannian manifold
On a Riemannian manifold, or more generally a pseudo-Riemannian manifold, the metric defines a fibre-wise isomorphism of the tangent and cotangent bundles. This makes it possible to convert vector fields to covector fields and vice versa. It also enables the definition of additional operations such as the Hodge star operator and the codifferential , which has degree and is adjoint to the exterior differential .
Vector field structures
On a pseudo-Riemannian manifold, -forms can be identified with vector fields; vector fields have additional distinct algebraic structures, which are listed here for context and to avoid confusion.
Firstly, each (co)tangent space generates a Clifford algebra, where the product of a (co)vector with itself is given by the value of a quadratic form – in this case, the natural one induced by the metric. This algebra is distinct from the exterior algebra of differential forms, which can be viewed as a Clifford algebra where the quadratic form vanishes (since the exterior product of any vector with itself is zero). Clifford algebras are thus non-anticommutative ("quantum") deformations of the exterior algebra. They are studied in geometric algebra.
Another alternative is to consider vector fields as derivations. The (noncommutative) algebra of differential operators they generate is the Weyl algebra and is a noncommutative ("quantum") deformation of the symmetric algebra in the vector fields.
Exterior differential complex
One important property of the exterior derivative is that . This means that the exterior derivative defines a cochain complex:
This complex is called the de Rham complex, and its cohomology is by definition the de Rham cohomology of . By the Poincaré lemma, the de Rham complex is locally exact except at . The kernel at is the space of locally constant functions on . Therefore, the complex is a resolution of the constant sheaf , which in turn implies a form of de Rham's theorem: de Rham cohomology computes the sheaf cohomology of .
Pullback
Suppose that is smooth. The differential of is a smooth map between the tangent bundles of and . This map is also denoted and called the pushforward. For any point and any tangent vector , there is a well-defined pushforward vector in . However, the same is not true of a vector field. If is not injective, say because has two or more preimages, then the vector field may determine two or more distinct vectors in . If is not surjective, then there will be a point at which does not determine any tangent vector at all. Since a vector field on determines, by definition, a unique tangent vector at every point of , the pushforward of a vector field does not always exist.
By contrast, it is always possible to pull back a differential form. A differential form on may be viewed as a linear functional on each tangent space. Precomposing this functional with the differential defines a linear functional on each tangent space of and therefore a differential form on . The existence of pullbacks is one of the key features of the theory of differential forms. It leads to the existence of pullback maps in other situations, such as pullback homomorphisms in de Rham cohomology.
Formally, let be smooth, and let be a smooth -form on . Then there is a differential form on , called the pullback of , which captures the behavior of as seen relative to . To define the pullback, fix a point of and tangent vectors , ..., to at . The pullback of is defined by the formula
There are several more abstract ways to view this definition. If is a -form on , then it may be viewed as a section of the cotangent bundle of . Using to denote a dual map, the dual to the differential of is . The pullback of may be defined to be the composite
This is a section of the cotangent bundle of and hence a differential -form on . In full generality, let denote the th exterior power of the dual map to the differential. Then the pullback of a -form is the composite
Another abstract way to view the pullback comes from viewing a -form as a linear functional on tangent spaces. From this point of view, is a morphism of vector bundles
where is the trivial rank one bundle on . The composite map
defines a linear functional on each tangent space of , and therefore it factors through the trivial bundle . The vector bundle morphism defined in this way is .
Pullback respects all of the basic operations on forms. If and are forms and is a real number, then
The pullback of a form can also be written in coordinates. Assume that , ..., are coordinates on , that , ..., are coordinates on , and that these coordinate systems are related by the formulas for all . Locally on , can be written as
where, for each choice of , ..., , is a real-valued function of , ..., . Using the linearity of pullback and its compatibility with exterior product, the pullback of has the formula
Each exterior derivative can be expanded in terms of , ..., . The resulting -form can be written using Jacobian matrices:
Here, denotes the determinant of the matrix whose entries are , .
Integration
A differential -form can be integrated over an oriented -dimensional manifold. When the -form is defined on an -dimensional manifold with , then the -form can be integrated over oriented -dimensional submanifolds. If , integration over oriented 0-dimensional submanifolds is just the summation of the integrand evaluated at points, according to the orientation of those points. Other values of correspond to line integrals, surface integrals, volume integrals, and so on. There are several equivalent ways to formally define the integral of a differential form, all of which depend on reducing to the case of Euclidean space.
Integration on Euclidean space
Let be an open subset of . Give its standard orientation and the restriction of that orientation. Every smooth -form on has the form
for some smooth function . Such a function has an integral in the usual Riemann or Lebesgue sense. This allows us to define the integral of to be the integral of :
Fixing an orientation is necessary for this to be well-defined. The skew-symmetry of differential forms means that the integral of, say, must be the negative of the integral of . Riemann and Lebesgue integrals cannot see this dependence on the ordering of the coordinates, so they leave the sign of the integral undetermined. The orientation resolves this ambiguity.
Integration over chains
Let be an -manifold and an -form on . First, assume that there is a parametrization of by an open subset of Euclidean space. That is, assume that there exists a diffeomorphism
where . Give the orientation induced by . Then defines the integral of over to be the integral of over . In coordinates, this has the following expression. Fix an embedding of in with coordinates . Then
Suppose that is defined by
Then the integral may be written in coordinates as
where
is the determinant of the Jacobian. The Jacobian exists because is differentiable.
In general, an -manifold cannot be parametrized by an open subset of . But such a parametrization is always possible locally, so it is possible to define integrals over arbitrary manifolds by defining them as sums of integrals over collections of local parametrizations. Moreover, it is also possible to define parametrizations of -dimensional subsets for , and this makes it possible to define integrals of -forms. To make this precise, it is convenient to fix a standard domain in , usually a cube or a simplex. A -chain is a formal sum of smooth embeddings . That is, it is a collection of smooth embeddings, each of which is assigned an integer multiplicity. Each smooth embedding determines a -dimensional submanifold of . If the chain is
then the integral of a -form over is defined to be the sum of the integrals over the terms of :
This approach to defining integration does not assign a direct meaning to integration over the whole manifold . However, it is still possible to assign such a meaning indirectly because every smooth manifold may be smoothly triangulated in an essentially unique way, and the integral over may be defined to be the integral over the chain determined by a triangulation.
Integration using partitions of unity
There is another approach, expounded in , which does directly assign a meaning to integration over , but this approach requires fixing an orientation of . The integral of an -form on an -dimensional manifold is defined by working in charts. Suppose first that is supported on a single positively oriented chart. On this chart, it may be pulled back to an -form on an open subset of . Here, the form has a well-defined Riemann or Lebesgue integral as before. The change of variables formula and the assumption that the chart is positively oriented together ensure that the integral of is independent of the chosen chart. In the general case, use a partition of unity to write as a sum of -forms, each of which is supported in a single positively oriented chart, and define the integral of to be the sum of the integrals of each term in the partition of unity.
It is also possible to integrate -forms on oriented -dimensional submanifolds using this more intrinsic approach. The form is pulled back to the submanifold, where the integral is defined using charts as before. For example, given a path , integrating a -form on the path is simply pulling back the form to a form on , and this integral is the integral of the function on the interval.
Integration along fibers
Fubini's theorem states that the integral over a set that is a product may be computed as an iterated integral over the two factors in the product. This suggests that the integral of a differential form over a product ought to be computable as an iterated integral as well. The geometric flexibility of differential forms ensures that this is possible not just for products, but in more general situations as well. Under some hypotheses, it is possible to integrate along the fibers of a smooth map, and the analog of Fubini's theorem is the case where this map is the projection from a product to one of its factors.
Because integrating a differential form over a submanifold requires fixing an orientation, a prerequisite to integration along fibers is the existence of a well-defined orientation on those fibers. Let and be two orientable manifolds of pure dimensions and , respectively. Suppose that is a surjective submersion. This implies that each fiber is -dimensional and that, around each point of , there is a chart on which looks like the projection from a product onto one of its factors. Fix and set . Suppose that
and that does not vanish. Following , there is a unique
which may be thought of as the fibral part of with respect to . More precisely, define to be the inclusion. Then is defined by the property that
where
is any -covector for which
The form may also be notated .
Moreover, for fixed , varies smoothly with respect to . That is, suppose that
is a smooth section of the projection map; we say that is a smooth differential -form on along . Then there is a smooth differential -form on such that, at each ,
This form is denoted . The same construction works if is an -form in a neighborhood of the fiber, and the same notation is used. A consequence is that each fiber is orientable. In particular, a choice of orientation forms on and defines an orientation of every fiber of .
The analog of Fubini's theorem is as follows. As before, and are two orientable manifolds of pure dimensions and , and is a surjective submersion. Fix orientations of and , and give each fiber of the induced orientation. Let be an -form on , and let be an -form on that is almost everywhere positive with respect to the orientation of . Then, for almost every , the form is a well-defined integrable form on . Moreover, there is an integrable -form on defined by
Denote this form by
Then proves the generalized Fubini formula
It is also possible to integrate forms of other degrees along the fibers of a submersion. Assume the same hypotheses as before, and let be a compactly supported -form on . Then there is a -form on which is the result of integrating along the fibers of . The form is defined by specifying, at each , how pairs with each -vector at , and the value of that pairing is an integral over that depends only on , , and the orientations of and . More precisely, at each , there is an isomorphism
defined by the interior product
for any choice of volume form in the orientation of . If , then a -vector at determines an -covector at by pullback:
Each of these covectors has an exterior product against , so there is an -form on along defined by
This form depends on the orientation of but not the choice of . Then the -form is uniquely defined by the property
and is smooth . This form also denoted and called the integral of along the fibers of . Integration along fibers is important for the construction of Gysin maps in de Rham cohomology.
Integration along fibers satisfies the projection formula . If is any -form on , then
Stokes's theorem
The fundamental relationship between the exterior derivative and integration is given by the Stokes' theorem: If is an ()-form with compact support on and denotes the boundary of with its induced orientation, then
A key consequence of this is that "the integral of a closed form over homologous chains is equal": If is a closed -form and and are -chains that are homologous (such that is the boundary of a -chain ), then , since the difference is the integral .
For example, if is the derivative of a potential function on the plane or , then the integral of over a path from to does not depend on the choice of path (the integral is ), since different paths with given endpoints are homotopic, hence homologous (a weaker condition). This case is called the gradient theorem, and generalizes the fundamental theorem of calculus. This path independence is very useful in contour integration.
This theorem also underlies the duality between de Rham cohomology and the homology of chains.
Relation with measures
On a general differentiable manifold (without additional structure), differential forms cannot be integrated over subsets of the manifold; this distinction is key to the distinction between differential forms, which are integrated over chains or oriented submanifolds, and measures, which are integrated over subsets. The simplest example is attempting to integrate the -form over the interval . Assuming the usual distance (and thus measure) on the real line, this integral is either or , depending on orientation: , while . By contrast, the integral of the measure on the interval is unambiguously (i.e. the integral of the constant function with respect to this measure is ). Similarly, under a change of coordinates a differential -form changes by the Jacobian determinant , while a measure changes by the absolute value of the Jacobian determinant, , which further reflects the issue of orientation. For example, under the map on the line, the differential form pulls back to ; orientation has reversed; while the Lebesgue measure, which here we denote , pulls back to ; it does not change.
In the presence of the additional data of an orientation, it is possible to integrate -forms (top-dimensional forms) over the entire manifold or over compact subsets; integration over the entire manifold corresponds to integrating the form over the fundamental class of the manifold, . Formally, in the presence of an orientation, one may identify -forms with densities on a manifold; densities in turn define a measure, and thus can be integrated .
On an orientable but not oriented manifold, there are two choices of orientation; either choice allows one to integrate -forms over compact subsets, with the two choices differing by a sign. On a non-orientable manifold, -forms and densities cannot be identified —notably, any top-dimensional form must vanish somewhere (there are no volume forms on non-orientable manifolds), but there are nowhere-vanishing densities— thus while one can integrate densities over compact subsets, one cannot integrate -forms. One can instead identify densities with top-dimensional pseudoforms.
Even in the presence of an orientation, there is in general no meaningful way to integrate -forms over subsets for because there is no consistent way to use the ambient orientation to orient -dimensional subsets. Geometrically, a -dimensional subset can be turned around in place, yielding the same subset with the opposite orientation; for example, the horizontal axis in a plane can be rotated by 180 degrees. Compare the Gram determinant of a set of vectors in an -dimensional space, which, unlike the determinant of vectors, is always positive, corresponding to a squared number. An orientation of a -submanifold is therefore extra data not derivable from the ambient manifold.
On a Riemannian manifold, one may define a -dimensional Hausdorff measure for any (integer or real), which may be integrated over -dimensional subsets of the manifold. A function times this Hausdorff measure can then be integrated over -dimensional subsets, providing a measure-theoretic analog to integration of -forms. The -dimensional Hausdorff measure yields a density, as above.
Currents
The differential form analog of a distribution or generalized function is called a current. The space of -currents on is the dual space to an appropriate space of differential -forms. Currents play the role of generalized domains of integration, similar to but even more flexible than chains.
Applications in physics
Differential forms arise in some important physical contexts. For example, in Maxwell's theory of electromagnetism, the Faraday 2-form, or electromagnetic field strength, is
where the are formed from the electromagnetic fields and ; e.g., , , or equivalent definitions.
This form is a special case of the curvature form on the principal bundle on which both electromagnetism and general gauge theories may be described. The connection form for the principal bundle is the vector potential, typically denoted by , when represented in some gauge. One then has
The current -form is
where are the four components of the current density. (Here it is a matter of convention to write instead of , i.e. to use capital letters, and to write instead of . However, the vector rsp. tensor components and the above-mentioned forms have different physical dimensions. Moreover, by decision of an international commission of the International Union of Pure and Applied Physics, the magnetic polarization vector has been called for several decades, and by some publishers ; i.e., the same name is used for different quantities.)
Using the above-mentioned definitions, Maxwell's equations can be written very compactly in geometrized units as
where denotes the Hodge star operator. Similar considerations describe the geometry of gauge theories in general.
The -form , which is dual to the Faraday form, is also called Maxwell 2-form.
Electromagnetism is an example of a gauge theory. Here the Lie group is , the one-dimensional unitary group, which is in particular abelian. There are gauge theories, such as Yang–Mills theory, in which the Lie group is not abelian. In that case, one gets relations which are similar to those described here. The analog of the field in such theories is the curvature form of the connection, which is represented in a gauge by a Lie algebra-valued one-form . The Yang–Mills field is then defined by
In the abelian case, such as electromagnetism, , but this does not hold in general. Likewise the field equations are modified by additional terms involving exterior products of and , owing to the structure equations of the gauge group.
Applications in geometric measure theory
Numerous minimality results for complex analytic manifolds are based on the Wirtinger inequality for 2-forms. A succinct proof may be found in Herbert Federer's classic text Geometric Measure Theory. The Wirtinger inequality is also a key ingredient in Gromov's inequality for complex projective space in systolic geometry.
| Mathematics | Multivariable and vector calculus | null |
221536 | https://en.wikipedia.org/wiki/Right-hand%20rule | Right-hand rule | In mathematics and physics, the right-hand rule is a convention and a mnemonic, utilized to define the orientation of axes in three-dimensional space and to determine the direction of the cross product of two vectors, as well as to establish the direction of the force on a current-carrying conductor in a magnetic field.
The various right- and left-hand rules arise from the fact that the three axes of three-dimensional space have two possible orientations. This can be seen by holding your hands together with palms up and fingers curled. If the curl of the fingers represents a movement from the first or x-axis to the second or y-axis, then the third or z-axis can point along either right thumb or left thumb.
History
The right-hand rule dates back to the 19th century when it was implemented as a way for identifying the positive direction of coordinate axes in three dimensions. William Rowan Hamilton, recognized for his development of quaternions, a mathematical system for representing three-dimensional rotations, is often attributed with the introduction of this convention. In the context of quaternions, the Hamiltonian product of two vector quaternions yields a quaternion comprising both scalar and vector components. Josiah Willard Gibbs recognized that treating these components separately, as dot and cross product, simplifies vector formalism. Following a substantial debate, the mainstream shifted from Hamilton's quaternionic system to Gibbs' three-vectors system. This transition led to the prevalent adoption of the right-hand rule in the contemporary contexts.
The cross product of vectors and is a vector perpendicular to the plane spanned by and with the direction given by the right-hand rule: If you put the index of your right hand on and the middle finger on , then the thumb points in the direction of .
The right-hand rule in physics was introduced in the late 19th century by John Fleming in his book Magnets and Electric Currents. Fleming described the orientation of the induced electromotive force by referencing the motion of the conductor and the direction of the magnetic field in the following depiction: “If a conductor, represented by the middle finger, be moved in a field of magnetic flux, the direction of which is represented by the direction of the forefinger, the direction of this motion, being in the direction of the thumb, then the electromotive force set up in it will be indicated by the direction in which the middle finger points."
Coordinates
For right-handed coordinates, if the thumb of a person's right hand points along the z-axis in the positive direction (third coordinate vector), then the fingers curl from the positive x-axis (first coordinate vector) toward the positive y-axis (second coordinate vector). When viewed at a position along the positive z-axis, the ¼ turn from the positive x- to the positive y-axis is counter-clockwise.
For left-handed coordinates, the above description of the axes is the same, except using the left hand; and the ¼ turn is clockwise.
Interchanging the labels of any two axes reverses the handedness. Reversing the direction of one axis (or three axes) also reverses the handedness. Reversing two axes amounts to a 180° rotation around the remaining axis, also preserving the handedness. These operations can be composed to give repeated changes of handedness. (If the axes do not have a positive or negative direction, then handedness has no meaning.)
Rotations
A rotating body
In mathematics, a rotating body is commonly represented by a pseudovector along the axis of rotation. The length of the vector gives the speed of rotation and the direction of the axis gives the direction of rotation according to the right-hand rule: right fingers curled in the direction of rotation and the right thumb pointing in the positive direction of the axis. This allows some simple calculations using the vector cross-product. No part of the body is moving in the direction of the axis arrow. If the thumb is pointing north, Earth rotates according to the right-hand rule (prograde motion). This causes the Sun, Moon, and stars to appear to revolve westward according to the left-hand rule.
Helixes and screws
A helix is a curved line formed by a point rotating around a center while the center moves up or down the z-axis. Helices are either right or left handed with curled fingers giving the direction of rotation and thumb giving the direction of advance along the z-axis.
The threads of a screw are helical and therefore screws can be right- or left-handed. To properly fasten or unfasten a screw, one applies the above rules: if a screw is right-handed, pointing one's right thumb in the direction of the hole and turning in the direction of the right hand's curled fingers (i.e. clockwise) will fasten the screw, while pointing away from the hole and turning in the new direction (i.e. counterclockwise) will unfasten the screw.
Curve orientation and normal vectors
In vector calculus, it is necessary to relate a normal vector of a surface to the boundary curve of the surface. Given a surface with a specified normal direction (a choice of "upward direction" with respect to ), the boundary curve around is defined to be positively oriented provided that the right thumb points in the direction of and the fingers curl along the orientation of the bounding curve .
Electromagnetism
When electricity flows (with direction given by conventional current) in a long straight wire, it creates a cylindrical magnetic field around the wire according to the right-hand rule. The conventional direction of a magnetic line is given by a compass needle.
Electromagnet: The magnetic field around a wire is relatively weak. If the wire is coiled into a helix, all the field lines inside the helix point in the same direction and each successive coil reinforces the others. The advance of the helix, the non-circular part of the current, and the field lines all point in the positive z direction. Since there is no magnetic monopole, the field lines exit the +z end, loop around outside the helix, and re-enter at the −z end. The +z end where the lines exit is defined as the north pole. If the fingers of the right hand are curled in the direction of the circular component of the current, the right thumb points to the north pole.
Lorentz force: If an electric charge moves across a magnetic field, it experiences a force according to the Lorentz force, with the direction given by the right-hand rule. If the index finger represents the direction of flow of charge (i.e. the current) and the middle finger represents the direction of the magnetic field in space, the direction of the force on the charge is represented by the thumb. Because the charge is moving, the force causes the particle path to bend. The bending force is computed by the vector cross-product. This means that the bending force increases with the velocity of the particle and the strength of the magnetic field. The force is maximum when the particle direction and magnetic fields are perpendicular, is less at any other angle, and is zero when the particle moves parallel to the field.
Ampère's right-hand grip rule
Ampère's right-hand grip rule, also called the right-hand screw rule, coffee-mug rule or the corkscrew-rule; is used either when a vector (such as the Euler vector) must be defined to represent the rotation of a body, a magnetic field, or a fluid, or vice versa, when it is necessary to define a rotation vector to understand how rotation occurs. It reveals a connection between the current and the magnetic field lines in the magnetic field that the current created. Ampère was inspired by fellow physicist Hans Christian Ørsted, who observed that needles swirled when in the proximity of an electric current-carrying wire and concluded that electricity could create magnetic fields.
Application
This rule is used in two different applications of Ampère's circuital law:
An electric current passes through a straight wire. When the thumb is pointed in the direction of conventional current (from positive to negative), the curled fingers will then point in the direction of the magnetic flux lines around the conductor. The direction of the magnetic field (counterclockwise rotation instead of clockwise rotation of coordinates when viewing the tip of the thumb) is a result of this convention and not an underlying physical phenomenon.
An electric current passes through a solenoid, resulting in a magnetic field. When wrapping the right hand around the solenoid with the fingers in the direction of the conventional current, the thumb points in the direction of the magnetic north pole.
Cross products
The cross product of two vectors is often taken in physics and engineering. For example, as discussed above, the force exerted on a moving charged particle when moving in a magnetic field B is given by the magnetic term of Lorentz force:
(vector cross product)
The direction of the cross product may be found by application of the right-hand rule as follows:
The index finger points in the direction of the velocity vector v.
The middle finger points in the direction of the magnetic field vector B.
The thumb points in the direction of the cross product F.
For example, for a positively charged particle moving to the north, in a region where the magnetic field points west, the resultant force points up.
Applications
The right-hand rule has widespread use in physics. A list of physical quantities whose directions are related by the right-hand rule is given below. (Some of these are related only indirectly to cross products, and use the second form.)
For a rotating object, if the right-hand fingers follow the curve of a point on the object, then the thumb points along the axis of rotation in the direction of the angular velocity vector.
A torque, the force that causes it, and the position of the point of application of the force.
A magnetic field, the position of the point where it is determined, and the electric current (or change in electric flux) that causes it.
A magnetic field in a coil of wire and the electric current in the wire.
The force of a magnetic field on a charged particle, the magnetic field itself, and the velocity of the object.
The vorticity at any point in the field of the flow of a fluid
The induced current from motion in a magnetic field (known as Fleming's right-hand rule).
The x, y and z unit vectors in a Cartesian coordinate system can be chosen to follow the right-hand rule. Right-handed coordinate systems are often used in rigid body and kinematics.
Meta-mathematical issues
Unlike most mathematical concepts, the meaning of a right-handed coordinate system cannot be expressed in terms of any mathematical axioms. Rather, the definition depends on chiral phenomena in the physical world, for example the culturally transmitted meaning of right and left hands, a majority human population with dominant right hand, or certain phenomena involving the weak force.
| Physical sciences | Electrodynamics | Physics |
221537 | https://en.wikipedia.org/wiki/Exterior%20algebra | Exterior algebra | In mathematics, the exterior algebra or Grassmann algebra of a vector space is an associative algebra that contains which has a product, called exterior product or wedge product and denoted with , such that for every vector in The exterior algebra is named after Hermann Grassmann, and the names of the product come from the "wedge" symbol and the fact that the product of two elements of is "outside"
The wedge product of vectors is called a blade of degree or -blade. The wedge product was introduced originally as an algebraic construction used in geometry to study areas, volumes, and their higher-dimensional analogues: The magnitude of a -blade is the area of the parallelogram defined by and and, more generally, the magnitude of a -blade is the (hyper)volume of the parallelotope defined by the constituent vectors. The alternating property that implies a skew-symmetric property that and more generally any blade flips sign whenever two of its constituent vectors are exchanged, corresponding to a parallelotope of opposite orientation.
The full exterior algebra contains objects that are not themselves blades, but linear combinations of blades; a sum of blades of homogeneous degree is called a -vector, while a more general sum of blades of arbitrary degree is called a multivector. The linear span of the -blades is called the -th exterior power of The exterior algebra is the direct sum of the -th exterior powers of and this makes the exterior algebra a graded algebra.
The exterior algebra is universal in the sense that every equation that relates elements of in the exterior algebra is also valid in every associative algebra that contains and in which the square of every element of is zero.
The definition of the exterior algebra can be extended for spaces built from vector spaces, such as vector fields and functions whose domain is a vector space. Moreover, the field of scalars may be any field. More generally, the exterior algebra can be defined for modules over a commutative ring. In particular, the algebra of differential forms in variables is an exterior algebra over the ring of the smooth functions in variables.
Motivating examples
Areas in the plane
The two-dimensional Euclidean vector space is a real vector space equipped with a basis consisting of a pair of orthogonal unit vectors
Suppose that
are a pair of given vectors in , written in components. There is a unique parallelogram having and as two of its sides. The area of this parallelogram is given by the standard determinant formula:
Consider now the exterior product of and :
where the first step uses the distributive law for the exterior product, and the last uses the fact that the exterior product is an alternating map, and in particular (The fact that the exterior product is an alternating map also forces ) Note that the coefficient in this last expression is precisely the determinant of the matrix . The fact that this may be positive or negative has the intuitive meaning that v and w may be oriented in a counterclockwise or clockwise sense as the vertices of the parallelogram they define. Such an area is called the signed area of the parallelogram: the absolute value of the signed area is the ordinary area, and the sign determines its orientation.
The fact that this coefficient is the signed area is not an accident. In fact, it is relatively easy to see that the exterior product should be related to the signed area if one tries to axiomatize this area as an algebraic construct. In detail, if denotes the signed area of the parallelogram of which the pair of vectors v and w form two adjacent sides, then A must satisfy the following properties:
for any real numbers r and s, since rescaling either of the sides rescales the area by the same amount (and reversing the direction of one of the sides reverses the orientation of the parallelogram).
, since the area of the degenerate parallelogram determined by v (i.e., a line segment) is zero.
, since interchanging the roles of v and w reverses the orientation of the parallelogram.
for any real number r, since adding a multiple of w to v affects neither the base nor the height of the parallelogram and consequently preserves its area.
, since the area of the unit square is one.
With the exception of the last property, the exterior product of two vectors satisfies the same properties as the area. In a certain sense, the exterior product generalizes the final property by allowing the area of a parallelogram to be compared to that of any chosen parallelogram in a parallel plane (here, the one with sides e1 and e2). In other words, the exterior product provides a basis-independent formulation of area.
Cross and triple products
For vectors in R3, the exterior algebra is closely related to the cross product and triple product. Using the standard basis , the exterior product of a pair of vectors
and
is
where is the basis for the three-dimensional space ⋀2(R3). The coefficients above are the same as those in the usual definition of the cross product of vectors in three dimensions, the only difference being that the exterior product is not an ordinary vector, but instead is a bivector.
Bringing in a third vector
the exterior product of three vectors is
where e1 ∧ e2 ∧ e3 is the basis vector for the one-dimensional space ⋀3(R3). The scalar coefficient is the triple product of the three vectors.
The cross product and triple product in three dimensions each admit both geometric and algebraic interpretations. The cross product can be interpreted as a vector which is perpendicular to both u and v and whose magnitude is equal to the area of the parallelogram determined by the two vectors. It can also be interpreted as the vector consisting of the minors of the matrix with columns u and v. The triple product of u, v, and w is geometrically a (signed) volume. Algebraically, it is the determinant of the matrix with columns u, v, and w. The exterior product in three dimensions allows for similar interpretations. In fact, in the presence of a positively oriented orthonormal basis, the exterior product generalizes these notions to higher dimensions.
Formal definition
The exterior algebra of a vector space over a field is defined as the quotient algebra of the tensor algebra T(V), where
by the two-sided ideal generated by all elements of the form such that . Symbolically,
The exterior product of two elements of is defined by
Algebraic properties
Alternating product
The exterior product is by construction alternating on elements of , which means that for all by the above construction. It follows that the product is also anticommutative on elements of , for supposing that ,
hence
More generally, if is a permutation of the integers , and , , ..., are elements of , it follows that
where is the signature of the permutation .
In particular, if for some , then the following generalization of the alternating property also holds:
Together with the distributive property of the exterior product, one further generalization is that a necessary and sufficient condition for to be a linearly dependent set of vectors is that
Exterior power
The th exterior power of , denoted , is the vector subspace of spanned by elements of the form
If , then is said to be a -vector. If, furthermore, can be expressed as an exterior product of elements of , then is said to be decomposable (or simple, by some authors; or a blade, by others). Although decomposable -vectors span , not every element of is decomposable. For example, given with a basis , the following 2-vector is not decomposable:
Basis and dimension
If the dimension of is and is a basis for , then the set
is a basis for . The reason is the following: given any exterior product of the form
every vector can be written as a linear combination of the basis vectors ; using the bilinearity of the exterior product, this can be expanded to a linear combination of exterior products of those basis vectors. Any exterior product in which the same basis vector appears more than once is zero; any exterior product in which the basis vectors do not appear in the proper order can be reordered, changing the sign whenever two basis vectors change places. In general, the resulting coefficients of the basis -vectors can be computed as the minors of the matrix that describes the vectors in terms of the basis .
By counting the basis elements, the dimension of is equal to a binomial coefficient:
where is the dimension of the vectors, and is the number of vectors in the product. The binomial coefficient produces the correct result, even for exceptional cases; in particular, for .
Any element of the exterior algebra can be written as a sum of -vectors. Hence, as a vector space the exterior algebra is a direct sum
(where, by convention, , the field underlying , and ), and therefore its dimension is equal to the sum of the binomial coefficients, which is .
Rank of a k-vector
If , then it is possible to express as a linear combination of decomposable -vectors:
where each is decomposable, say
The rank of the -vector is the minimal number of decomposable -vectors in such an expansion of . This is similar to the notion of tensor rank.
Rank is particularly important in the study of 2-vectors . The rank of a 2-vector can be identified with half the rank of the matrix of coefficients of in a basis. Thus if is a basis for , then can be expressed uniquely as
where (the matrix of coefficients is skew-symmetric). The rank of the matrix is therefore even, and is twice the rank of the form .
In characteristic 0, the 2-vector has rank if and only if
and
Graded structure
The exterior product of a -vector with a -vector is a -vector, once again invoking bilinearity. As a consequence, the direct sum decomposition of the preceding section
gives the exterior algebra the additional structure of a graded algebra, that is
Moreover, if is the base field, we have
and
The exterior product is graded anticommutative, meaning that if and , then
In addition to studying the graded structure on the exterior algebra, studies additional graded structures on exterior algebras, such as those on the exterior algebra of a graded module (a module that already carries its own gradation).
Universal property
Let be a vector space over the field . Informally, multiplication in is performed by manipulating symbols and imposing a distributive law, an associative law, and using the identity for . Formally, is the "most general" algebra in which these rules hold for the multiplication, in the sense that any unital associative -algebra containing with alternating multiplication on must contain a homomorphic image of . In other words, the exterior algebra has the following universal property:
Given any unital associative -algebra and any -linear map such that for every in , then there exists precisely one unital algebra homomorphism such that for all in (here is the natural inclusion of in , see above).
To construct the most general algebra that contains and whose multiplication is alternating on , it is natural to start with the most general associative algebra that contains , the tensor algebra , and then enforce the alternating property by taking a suitable quotient. We thus take the two-sided ideal in generated by all elements of the form for in , and define as the quotient
(and use as the symbol for multiplication in ). It is then straightforward to show that contains and satisfies the above universal property.
As a consequence of this construction, the operation of assigning to a vector space its exterior algebra is a functor from the category of vector spaces to the category of algebras.
Rather than defining first and then identifying the exterior powers as certain subspaces, one may alternatively define the spaces first and then combine them to form the algebra . This approach is often used in differential geometry and is described in the next section.
Generalizations
Given a commutative ring and an -module , we can define the exterior algebra just as above, as a suitable quotient of the tensor algebra . It will satisfy the analogous universal property. Many of the properties of also require that be a projective module. Where finite dimensionality is used, the properties further require that be finitely generated and projective. Generalizations to the most common situations can be found in .
Exterior algebras of vector bundles are frequently considered in geometry and topology. There are no essential differences between the algebraic properties of the exterior algebra of finite-dimensional vector bundles and those of the exterior algebra of finitely generated projective modules, by the Serre–Swan theorem. More general exterior algebras can be defined for sheaves of modules.
Alternating tensor algebra
For a field of characteristic not 2, the exterior algebra of a vector space over can be canonically identified with the vector subspace of that consists of antisymmetric tensors. For characteristic 0 (or higher than ), the vector space of -linear antisymmetric tensors is transversal to the ideal , hence, a good choice to represent the quotient. But for nonzero characteristic, the vector space of -linear antisymmetric tensors could be not transversal to the ideal (actually, for , the vector space of -linear antisymmetric tensors is contained in ); nevertheless, transversal or not, a product can be defined on this space such that the resulting algebra is isomorphic to the exterior algebra: in the first case the natural choice for the product is just the quotient product (using the available projection), in the second case, this product must be slightly modified as given below (along Arnold setting), but such that the algebra stays isomorphic with the exterior algebra, i.e. the quotient of by the ideal generated by elements of the form . Of course, for characteristic (or higher than the dimension of the vector space), one or the other definition of the product could be used, as the two algebras are isomorphic (see V. I. Arnold or Kobayashi-Nomizu).
Let be the space of homogeneous tensors of degree . This is spanned by decomposable tensors
The antisymmetrization (or sometimes the skew-symmetrization) of a decomposable tensor is defined by
and, when (for nonzero characteristic field might be 0):
where the sum is taken over the symmetric group of permutations on the symbols . This extends by linearity and homogeneity to an operation, also denoted by and , on the full tensor algebra .
Note that
Such that, when defined, is the projection for the exterior (quotient) algebra onto the r-homogeneous alternating tensor subspace.
On the other hand, the image is always the alternating tensor graded subspace (not yet an algebra, as product is not yet defined), denoted . This is a vector subspace of , and it inherits the structure of a graded vector space from that on . Moreover, the kernel of is precisely , the homogeneous subset of the ideal , or the kernel of is . When is defined, carries an associative graded product defined by (the same as the wedge product)
Assuming has characteristic 0, as is a supplement of in , with the above given product, there is a canonical isomorphism
When the characteristic of the field is nonzero, will do what did before, but the product cannot be defined as above. In such a case, isomorphism still holds, in spite of not being a supplement of the ideal , but then, the product should be modified as given below ( product, Arnold setting).
Finally, we always get isomorphic with , but the product could (or should) be chosen in two ways (or only one). Actually, the product could be chosen in many ways, rescaling it on homogeneous spaces as for an arbitrary sequence in the field, as long as the division makes sense (this is such that the redefined product is also associative, i.e. defines an algebra on ). Also note, the interior product definition should be changed accordingly, in order to keep its skew derivation property.
Index notation
Suppose that V has finite dimension n, and that a basis of V is given. Then any alternating tensor can be written in index notation with the Einstein summation convention as
where ti1⋅⋅⋅ir is completely antisymmetric in its indices.
The exterior product of two alternating tensors t and s of ranks r and p is given by
The components of this tensor are precisely the skew part of the components of the tensor product , denoted by square brackets on the indices:
The interior product may also be described in index notation as follows. Let be an antisymmetric tensor of rank . Then, for , is an alternating tensor of rank , given by
where n is the dimension of V.
Duality
Alternating operators
Given two vector spaces V and X and a natural number k, an alternating operator from Vk to X is a multilinear map
such that whenever v1, ..., vk are linearly dependent vectors in V, then
The map
which associates to vectors from their exterior product, i.e. their corresponding -vector, is also alternating. In fact, this map is the "most general" alternating operator defined on given any other alternating operator there exists a unique linear map with This universal property characterizes the space of alternating operators on and can serve as its definition.
Alternating multilinear forms
The above discussion specializes to the case when , the base field. In this case an alternating multilinear function
is called an alternating multilinear form. The set of all alternating multilinear forms is a vector space, as the sum of two such maps, or the product of such a map with a scalar, is again alternating. By the universal property of the exterior power, the space of alternating forms of degree on is naturally isomorphic with the dual vector space . If is finite-dimensional, then the latter is to . In particular, if is -dimensional, the dimension of the space of alternating maps from to is the binomial coefficient .
Under such identification, the exterior product takes a concrete form: it produces a new anti-symmetric map from two given ones. Suppose and are two anti-symmetric maps. As in the case of tensor products of multilinear maps, the number of variables of their exterior product is the sum of the numbers of their variables. Depending on the choice of identification of elements of exterior power with multilinear forms, the exterior product is defined as
or as
where, if the characteristic of the base field is 0, the alternation Alt of a multilinear map is defined to be the average of the sign-adjusted values over all the permutations of its variables:
When the field has finite characteristic, an equivalent version of the second expression without any factorials or any constants is well-defined:
where here is the subset of shuffles: permutations σ of the set such that , and . As this might look very specific and fine tuned, an equivalent raw version is to sum in the above formula over permutations in left cosets of .
Interior product
Suppose that is finite-dimensional. If denotes the dual space to the vector space , then for each , it is possible to define an antiderivation on the algebra ,
This derivation is called the interior product with , or sometimes the insertion operator, or contraction by .
Suppose that . Then is a multilinear mapping of to , so it is defined by its values on the -fold Cartesian product . If u1, u2, ..., uk−1 are elements of , then define
Additionally, let whenever is a pure scalar (i.e., belonging to ).
Axiomatic characterization and properties
The interior product satisfies the following properties:
For each and each (where by convention ),
If is an element of (), then is the dual pairing between elements of and elements of .
For each , is a graded derivation of degree −1:
These three properties are sufficient to characterize the interior product as well as define it in the general infinite-dimensional case.
Further properties of the interior product include:
Hodge duality
Suppose that has finite dimension . Then the interior product induces a canonical isomorphism of vector spaces
by the recursive definition
In the geometrical setting, a non-zero element of the top exterior power (which is a one-dimensional vector space) is sometimes called a volume form (or orientation form, although this term may sometimes lead to ambiguity). The name orientation form comes from the fact that a choice of preferred top element determines an orientation of the whole exterior algebra, since it is tantamount to fixing an ordered basis of the vector space. Relative to the preferred volume form , the isomorphism is given explicitly by
If, in addition to a volume form, the vector space V is equipped with an inner product identifying with , then the resulting isomorphism is called the Hodge star operator, which maps an element to its Hodge dual:
The composition of with itself maps and is always a scalar multiple of the identity map. In most applications, the volume form is compatible with the inner product in the sense that it is an exterior product of an orthonormal basis of . In this case,
where id is the identity mapping, and the inner product has metric signature — p pluses and q minuses.
Inner product
For a finite-dimensional space, an inner product (or a pseudo-Euclidean inner product) on defines an isomorphism of with , and so also an isomorphism of with . The pairing between these two spaces also takes the form of an inner product. On decomposable -vectors,
the determinant of the matrix of inner products. In the special case , the inner product is the square norm of the k-vector, given by the determinant of the Gramian matrix . This is then extended bilinearly (or sesquilinearly in the complex case) to a non-degenerate inner product on If ei, , form an orthonormal basis of , then the vectors of the form
constitute an orthonormal basis for , a statement equivalent to the Cauchy–Binet formula.
With respect to the inner product, exterior multiplication and the interior product are mutually adjoint. Specifically, for , , and ,
where is the musical isomorphism, the linear functional defined by
for all . This property completely characterizes the inner product on the exterior algebra.
Indeed, more generally for , , and , iteration of the above adjoint properties gives
where now is the dual -vector defined by
for all .
Bialgebra structure
There is a correspondence between the graded dual of the graded algebra and alternating multilinear forms on . The exterior algebra (as well as the symmetric algebra) inherits a bialgebra structure, and, indeed, a Hopf algebra structure, from the tensor algebra. See the article on tensor algebras for a detailed treatment of the topic.
The exterior product of multilinear forms defined above is dual to a coproduct defined on , giving the structure of a coalgebra. The coproduct is a linear function , which is given by
on elements . The symbol stands for the unit element of the field . Recall that , so that the above really does lie in . This definition of the coproduct is lifted to the full space by (linear) homomorphism.
The correct form of this homomorphism is not what one might naively write, but has to be the one carefully defined in the coalgebra article. In this case, one obtains
Expanding this out in detail, one obtains the following expression on decomposable elements:
where the second summation is taken over all -shuffles. By convention, one takes that Sh(k,0) and Sh(0,k) equals {id: {1, ..., k} → {1, ..., k}}. It is also convenient to take the pure wedge products and
to equal 1 for p = 0 and p = k, respectively (the empty product in ). The shuffle follows directly from the first axiom of a co-algebra: the relative order of the elements is preserved in the riffle shuffle: the riffle shuffle merely splits the ordered sequence into two ordered sequences, one on the left, and one on the right.
Observe that the coproduct preserves the grading of the algebra. Extending to the full space one has
The tensor symbol ⊗ used in this section should be understood with some caution: it is not the same tensor symbol as the one being used in the definition of the alternating product. Intuitively, it is perhaps easiest to think it as just another, but different, tensor product: it is still (bi-)linear, as tensor products should be, but it is the product that is appropriate for the definition of a bialgebra, that is, for creating the object . Any lingering doubt can be shaken by pondering the equalities and , which follow from the definition of the coalgebra, as opposed to naive manipulations involving the tensor and wedge symbols. This distinction is developed in greater detail in the article on tensor algebras. Here, there is much less of a problem, in that the alternating product clearly corresponds to multiplication in the exterior algebra, leaving the symbol free for use in the definition of the bialgebra. In practice, this presents no particular problem, as long as one avoids the fatal trap of replacing alternating sums of by the wedge symbol, with one exception. One can construct an alternating product from , with the understanding that it works in a different space. Immediately below, an example is given: the alternating product for the dual space can be given in terms of the coproduct. The construction of the bialgebra here parallels the construction in the tensor algebra article almost exactly, except for the need to correctly track the alternating signs for the exterior algebra.
In terms of the coproduct, the exterior product on the dual space is just the graded dual of the coproduct:
where the tensor product on the right-hand side is of multilinear linear maps (extended by zero on elements of incompatible homogeneous degree: more precisely, , where is the counit, as defined presently).
The counit is the homomorphism that returns the 0-graded component of its argument. The coproduct and counit, along with the exterior product, define the structure of a bialgebra on the exterior algebra.
With an antipode defined on homogeneous elements by , the exterior algebra is furthermore a Hopf algebra.
Functoriality
Suppose that and are a pair of vector spaces and is a linear map. Then, by the universal property, there exists a unique homomorphism of graded algebras
such that
In particular, preserves homogeneous degree. The -graded components of are given on decomposable elements by
Let
The components of the transformation relative to a basis of and is the matrix of minors of . In particular, if and is of finite dimension , then is a mapping of a one-dimensional vector space to itself, and is therefore given by a scalar: the determinant of .
Exactness
If is a short exact sequence of vector spaces, then
is an exact sequence of graded vector spaces, as is
Direct sums
In particular, the exterior algebra of a direct sum is isomorphic to the tensor product of the exterior algebras:
This is a graded isomorphism; i.e.,
In greater generality, for a short exact sequence of vector spaces there is a natural filtration
where for is spanned by elements of the form for and
The corresponding quotients admit a natural isomorphism
given by
In particular, if U is 1-dimensional then
is exact, and if W is 1-dimensional then
is exact.
Applications
Oriented volume in affine space
The natural setting for (oriented) -dimensional volume and exterior algebra is affine space. This is also the intimate connection between exterior algebra and differential forms, as to integrate we need a 'differential' object to measure infinitesimal volume. If is an affine space over the vector space , and a (simplex) collection of ordered points , we can define its oriented -dimensional volume as the exterior product of vectors (using concatenation to mean the displacement vector from point to ); if the order of the points is changed, the oriented volume changes by a sign, according to the parity of the permutation. In -dimensional space, the volume of any -dimensional simplex is a scalar multiple of any other.
The sum of the -dimensional oriented areas of the boundary simplexes of a -dimensional simplex is zero, as for the sum of vectors around a triangle or the oriented triangles bounding the tetrahedron in the previous section.
The vector space structure on generalises addition of vectors in : we have and similarly a -blade is linear in each factor.
Linear algebra
In applications to linear algebra, the exterior product provides an abstract algebraic manner for describing the determinant and the minors of a matrix. For instance, it is well known that the determinant of a square matrix is equal to the volume of the parallelotope whose sides are the columns of the matrix (with a sign to track orientation). This suggests that the determinant can be defined in terms of the exterior product of the column vectors. Likewise, the minors of a matrix can be defined by looking at the exterior products of column vectors chosen at a time. These ideas can be extended not just to matrices but to linear transformations as well: the determinant of a linear transformation is the factor by which it scales the oriented volume of any given reference parallelotope. So the determinant of a linear transformation can be defined in terms of what the transformation does to the top exterior power. The action of a transformation on the lesser exterior powers gives a basis-independent way to talk about the minors of the transformation.
Physics
In physics, many quantities are naturally represented by alternating operators. For example, if the motion of a charged particle is described by velocity and acceleration vectors in four-dimensional spacetime, then normalization of the velocity vector requires that the electromagnetic force must be an alternating operator on the velocity. Its six degrees of freedom are identified with the electric and magnetic fields.
Electromagnetic field
In Einstein's theories of relativity, the electromagnetic field is generally given as a differential 2-form in 4-space or as the equivalent alternating tensor field the electromagnetic tensor. Then or the equivalent Bianchi identity
None of this requires a metric.
Adding the Lorentz metric and an orientation provides the Hodge star operator and thus makes it possible to define or the equivalent tensor divergence where
Linear geometry
The decomposable -vectors have geometric interpretations: the bivector represents the plane spanned by the vectors, "weighted" with a number, given by the area of the oriented parallelogram with sides and . Analogously, the 3-vector represents the spanned 3-space weighted by the volume of the oriented parallelepiped with edges , , and .
Projective geometry
Decomposable -vectors in correspond to weighted -dimensional linear subspaces of . In particular, the Grassmannian of -dimensional subspaces of , denoted , can be naturally identified with an algebraic subvariety of the projective space . This is called the Plücker embedding, and the image of the embedding can be characterized by the Plücker relations.
Differential geometry
The exterior algebra has notable applications in differential geometry, where it is used to define differential forms. Differential forms are mathematical objects that evaluate the length of vectors, areas of parallelograms, and volumes of higher-dimensional bodies, so they can be integrated over curves, surfaces and higher dimensional manifolds in a way that generalizes the line integrals and surface integrals from calculus. A differential form at a point of a differentiable manifold is an alternating multilinear form on the tangent space at the point. Equivalently, a differential form of degree is a linear functional on the th exterior power of the tangent space. As a consequence, the exterior product of multilinear forms defines a natural exterior product for differential forms. Differential forms play a major role in diverse areas of differential geometry.
An alternate approach defines differential forms in terms of germs of functions.
In particular, the exterior derivative gives the exterior algebra of differential forms on a manifold the structure of a differential graded algebra. The exterior derivative commutes with pullback along smooth mappings between manifolds, and it is therefore a natural differential operator. The exterior algebra of differential forms, equipped with the exterior derivative, is a cochain complex whose cohomology is called the de Rham cohomology of the underlying manifold and plays a vital role in the algebraic topology of differentiable manifolds.
Representation theory
In representation theory, the exterior algebra is one of the two fundamental Schur functors on the category of vector spaces, the other being the symmetric algebra. Together, these constructions are used to generate the irreducible representations of the general linear group (see Fundamental representation).
Superspace
The exterior algebra over the complex numbers is the archetypal example of a superalgebra, which plays a fundamental role in physical theories pertaining to fermions and supersymmetry. A single element of the exterior algebra is called a supernumber or Grassmann number. The exterior algebra itself is then just a one-dimensional superspace: it is just the set of all of the points in the exterior algebra. The topology on this space is essentially the weak topology, the open sets being the cylinder sets. An -dimensional superspace is just the -fold product of exterior algebras.
Lie algebra homology
Let be a Lie algebra over a field , then it is possible to define the structure of a chain complex on the exterior algebra of . This is a -linear mapping
defined on decomposable elements by
The Jacobi identity holds if and only if , and so this is a necessary and sufficient condition for an anticommutative nonassociative algebra to be a Lie algebra. Moreover, in that case is a chain complex with boundary operator . The homology associated to this complex is the Lie algebra homology.
Homological algebra
The exterior algebra is the main ingredient in the construction of the Koszul complex, a fundamental object in homological algebra.
History
The exterior algebra was first introduced by Hermann Grassmann in 1844 under the blanket term of Ausdehnungslehre, or Theory of Extension.
This referred more generally to an algebraic (or axiomatic) theory of extended quantities and was one of the early precursors to the modern notion of a vector space. Saint-Venant also published similar ideas of exterior calculus for which he claimed priority over Grassmann.
The algebra itself was built from a set of rules, or axioms, capturing the formal aspects of Cayley and Sylvester's theory of multivectors. It was thus a calculus, much like the propositional calculus, except focused exclusively on the task of formal reasoning in geometrical terms.
In particular, this new development allowed for an axiomatic characterization of dimension, a property that had previously only been examined from the coordinate point of view.
The import of this new theory of vectors and multivectors was lost to mid-19th-century mathematicians,
until being thoroughly vetted by Giuseppe Peano in 1888. Peano's work also remained somewhat obscure until the turn of the century, when the subject was unified by members of the French geometry school (notably Henri Poincaré, Élie Cartan, and Gaston Darboux) who applied Grassmann's ideas to the calculus of differential forms.
A short while later, Alfred North Whitehead, borrowing from the ideas of Peano and Grassmann, introduced his universal algebra. This then paved the way for the 20th-century developments of abstract algebra by placing the axiomatic notion of an algebraic system on a firm logical footing.
| Mathematics | Other algebra topics | null |
221636 | https://en.wikipedia.org/wiki/Marsupial%20mole | Marsupial mole | Marsupial moles, the Notoryctidae family, are two species of highly specialized marsupial mammals that are found in the Australian interior.
They are small burrowing marsupials that anatomically converge on fossorial placental mammals, such as extant golden moles (Chrysochloridae) and extinct epoicotheres (Pholidota). The species are:
Notoryctes typhlops (southern marsupial mole, known as the itjaritjari by the Pitjantjatjara and Yankunytjatjara people in Central Australia)
Notoryctes caurinus (northern marsupial mole, also known as the kakarratul)
Characteristics
In an example of convergent evolution, notoryctids resemble (and fill the ecological niche of) the talpid or true moles from North America and Eurasia and the Chrysochloridae or golden moles from Southern Africa. Like chrysochlorids and epoicotheres, notoryctids use their forelimbs and enlarged central claws to dig in a parasagittal (i.e., up and down) plane, as opposed to the "lateral scratch" style of digging that characterizes talpid moles.
Marsupial moles spend most of their time underground, coming to the surface only occasionally, probably mostly after rains. They are functionally blind, their eyes having become reduced to vestigial lenses under the skin that lack a pupil. They have no external ears, just a pair of tiny holes hidden under thick hair. The head is cone-shaped, with a leathery shield over the muzzle, the body is tubular, and the tail is a short, bald stub encased in leathery skin. They are between long, weigh , and are uniformly covered in fairly short, very fine pale cream to white hair with an iridescent golden sheen. Their pouch is small but well developed and has evolved to face backwards so it does not fill with sand. It contains just two teats, so the animal cannot support more than two young at a time. They are the only marsupials with a true cloaca.
The limbs are very short, with reduced digits. The forefeet have two greatly enlarged, spade shaped, flat claws on the third and fourth digits, which are used to excavate soil in front of the animal. The hindfeet are flattened, and bear three small claws; these feet are used to push soil behind the animal as it digs. Epipubic bones are present but small and as in some other fossorial mammals (e.g., armadillos), the last five cervical vertebrae are fused, giving the head greater rigidity during digging. The animal swims through the soil and does not leave behind a permanent burrow.
The teeth of the marsupial moles are degenerate and bear no resemblance to polyprotodont or diprotodont teeth. Their dental formula varies, but is usually somewhere near . The upper molar teeth are triangular and zalambdodont, i.e., resembling an inverted Greek letter lambda in occlusal view, and the lower molars appear to have lost their talonid basins. Marsupial moles are the only marsupials that are testicond.
Fossil record
Notoryctids are represented by early Miocene fossils of Naraboryctes from Riversleigh in Queensland, Australia, which show the mosaic acquisition of dental and skeletal features of the living Notoryctes from a more surface-dwelling ancestor.
The notoryctid fossil record demonstrates that the primary cusp of the molars is the metacone, distinct from the paracone characteristic of zalambdodont tenrecs, golden moles, and Solenodon. Regarding the number of teeth in each dental quadrant (or dental formula), the fossil record demonstrates polymorphism of tooth number, both between specimens and within the same specimen. Nonetheless, older studies concluded there are four molars (typical for marsupials) in each quadrant both in living Notoryctes and the fossil notoryctid Naraboryctes.
Evolutionary affinities
American paleontologist William King Gregory wrote that "Notoryctes is a true marsupial" and this view has been repeatedly verified by phylogenetic analyses of comparative anatomy, mitochondrial DNA, nuclear DNA, rare genomic events, and combined datasets of nuclear and mitochondrial DNA and morphology and DNA. The largest phylogenetic datasets strongly support the placement of Notoryctes as the sister taxon to a dasyuromorph-peramelian clade, within the Australidelphian radiation.
| Biology and health sciences | Marsupials | Animals |
221839 | https://en.wikipedia.org/wiki/Stalagmite | Stalagmite | A stalagmite (, ; ; )
is a type of rock formation that rises from the floor of a cave due to the accumulation of material deposited on the floor from ceiling drippings. Stalagmites are typically composed of calcium carbonate, but may consist of lava, mud, peat, pitch, sand, sinter, and amberat (crystallized urine of pack rats).
The corresponding formation hanging down from the ceiling of a cave is a stalactite.
Formation and type
Limestone stalagmites
The most common stalagmites are speleothems, which usually form in limestone caves. Stalagmite formation occurs only under certain pH conditions within the cavern. They form through deposition of calcium carbonate and other minerals, which is precipitated from mineralized water solutions. Limestone is the chief form of calcium carbonate rock, which is dissolved by water that contains carbon dioxide, forming a calcium bicarbonate solution in caverns. The partial pressure of carbon dioxide in the water must be greater than the partial pressure of carbon dioxide in the cave chamber for conventional stalagmite growth.
If stalactites – the ceiling formations – grow long enough to connect with stalagmites on the floor, they form a column.
To preserve Stalagmites, it should normally not be touched, since the rock buildup is formed by minerals precipitating out of the water solution onto the existing surface; skin oils can alter the surface tension where the mineral water clings or flows, thus affecting the growth of the formation. Oils and dirt (mud, clay) from human contact can also stain the formation and change its color permanently.
Lava stalagmites
Another type of stalagmite is formed in lava tubes while molten and fluid lava is still active inside. Their mineralogical composition, close to that of siliceous minerals commonly found in basalt (for example, obsidian), the main constituent of volcanic glass, is different. Their mechanism of formation/crystallization is also notably different from that of limestone stalagmites () but the common point is that it remains driven by gravity. Drops of molten lava (siliceous material, ) solidify onto the floor of the already emptied lava tube, when the lava temperature sufficiently decreases after the passage and the complete purge of the main lava flow. Essentially, it is still the gravity deposition of material onto the floor of a cave (or a void).
However the difference from calcareous stalagmites is that the transport of siliceous material occurs in the molten state and not dissolved in aqueous solution; degassing does not play any significant role. With lava stalagmites, their formation also happens very quickly in only a matter of hours, days, or weeks, whereas limestone stalagmites may take up to thousands or hundred thousands of years. A key difference with lava stalagmites is that once the molten lava has ceased flowing, so too will the stalagmites cease to grow. This means that if the lava stalagmites were to be broken, they would never grow back. Stalagmites in lava tubes are rarer than their stalactite counterparts because during their formation, the dripping molten material most often falls onto still-moving lava flow which absorbs or carries the material away.
The generic term "lavacicle" has been applied to lava stalactites and stalagmites indiscriminately, and evolved from the word "icicle".
Ice stalagmites
A common stalagmite found seasonally or year round in many caves is the ice stalagmite, commonly referred to as icicles, especially in above-ground contexts. Water seepage from the surface will penetrate into a cave and if temperatures are below freezing temperature, the water will collect on the floor into stalagmites. Deposition may also occur directly from the freezing of water vapor. Similar to lava stalagmites, ice stalagmites form very quickly within hours or days. Unlike lava stalagmites however, they may grow back as long as water and temperatures are suitable. Ice stalagmites are more common than their stalactite counterparts because warmer air rises to the ceilings of caves and may raise temperatures to above freezing.
Ice stalactites may also form corresponding stalagmites below them, and given time, may grow together to form an ice column.
Concrete derived stalagmites
Stalactites and stalagmites can also form on concrete ceilings and floors, although they form much more rapidly there than in the natural cave environment.
The secondary deposits derived from concrete are the result of concrete degradation, where calcium ions are leached out of the concrete in solution and redeposited on the underside of a concrete structure to form stalactites and stalagmites. Calcium carbonate deposition as a stalagmite occurs when the solution carries the calcium laden leachate solution to the ground under the concrete structure. Carbon dioxide is absorbed into the alkaline leachate solution, which facilitates the chemical reactions to deposit calcium carbonate as a stalagmite. These stalagmites rarely grow taller than a few centimetres.
Secondary deposits, which create stalagmites, stalactites, flowstone etc., outside the natural cave environment, are referred to as "calthemites". These concrete derived secondary deposits cannot be referred to as "speleothems" due to the definition of the word.
Records
The largest known stalagmite in the world exceeds in height and is in Sơn Đoòng Cave, Vietnam.
In the Zagros Mountains of south Iran, approximately from the ancient city of Bishapur, in the Shapur cave on the fourth of five terraces stands the 3rd-century colossal statue of Shapur I, second ruler of the Sassanid Empire. The statue, carved from one stalagmite, is nearly high.
Photo gallery
| Physical sciences | Caves | Earth science |
221847 | https://en.wikipedia.org/wiki/Kinglet | Kinglet | A kinglet is a small bird in the family Regulidae. Species in this family were formerly classified with the Old World warblers. "Regulidae" is derived from the Latin word regulus for "petty king" or prince, and refers to the coloured crowns of adult birds. This family has representatives in North America and Eurasia. There are six species in this family; one, the Madeira firecrest, Regulus madeirensis, was only recently split from common firecrest as a separate species. One species, the ruby-crowned kinglet, differs sufficiently in its voice and plumage to be afforded its own genus, Corthylio.
Description
Kinglets are among the smallest of all passerines, ranging in size from and weighing ; the sexes are the same size. They have medium-length wings and tails, and small needle-like bills. The plumage is overall grey-green, offset by pale wingbars, and the tail tip is incised. Five species have a single stiff feather covering the nostrils, but in the ruby-crowned kinglet this is replaced by several short, stiff bristles. Most kinglets have distinctive head markings, and the males possess a colourful crown patch. In the females, the crown is duller and yellower. The long feathers forming the central crown stripe can be erected; they are inconspicuous most of the time, but are used in courtship and territorial displays when the raised crest is very striking.
There are two species of different genera in North America with largely overlapping distributions, and two in Eurasia that also have a considerable shared range. In each continent, one species (goldcrest in Eurasia and golden-crowned kinglet in North America) is a conifer specialist; these have deeply grooved pads on their feet for perching on conifer twigs and a long hind toe and claw for clinging vertically. The two generalists, ruby-crowned kinglet and common firecrest, hunt more in flight and have smoother soles, shorter hind claws and a longer tail.
Taxonomy
The kinglets are a small group of birds sometimes included in the Old World warblers but frequently given family status, especially as recent research showed that, despite superficial similarities, the crests are phylogenetically remote from the warblers. The name of the family derives from the Latin regulus, a diminutive of rex, "a king", and refer to the characteristic orange or yellow crests of adult kinglets (aside from the red crest of Corthylio). The kinglets were allocated to the warbler genus Sylvia by English naturalist John Latham in 1790, but moved to their current genus by French zoologist Georges Cuvier in 1800.
Most members of the genus Regulus are similar in size and colour pattern. The exception is the ruby-crowned kinglet, the largest species, which has a strongly red crest and no black crown stripes. It has distinctive vocalisations, and is different enough from the Old World kinglets and the other American species, the golden-crowned kinglet, to be assigned to a separate genus, Corthylio.
Species in taxonomic order
Genus Corthylio:
Ruby-crowned kinglet (C. calendula)
Genus Regulus:
Common firecrest (R. ignicapilla)
Madeira firecrest (R. madeirensis)
Golden-crowned kinglet (R. satrapa)
Flamecrest (R. goodfellowi)
Goldcrest (R. regulus)
Distribution and habitat
Kinglets are birds of the Nearctic and Palearctic realms, with representatives in temperate North America, Europe and Asia, northernmost Africa, Macaronesia and the Himalayas. They are adapted to conifer forests, although there is a certain amount of adaptability and most species will use other habitats, particularly during migration. In Macaronesia, they are adapted to laurisilva and tree heaths.
Behaviour
Diet and feeding
The tiny size and rapid metabolism of kinglets means that they must constantly forage in order to provide their energy needs. They will continue feeding even when nest building. Kinglets prevented from feeding may lose a third of their body weight in twenty minutes and may starve to death in an hour. Kinglets are insectivores, preferentially feeding on prey such as aphids and springtails that have soft cuticles. Prey is generally gleaned from the branches and leaves of trees, although in some circumstances prey may be taken on the wing or from the leaf litter on the ground.
Life cycle
Kinglet nests are small, very neat cups, almost spherical in shape, made of moss and lichen held together with spiderwebs and hung from twigs near the end of a high branch of a conifer. They are lined with hair and feathers, and a few feathers are placed over the opening. These characteristics provide good insulation against the cold environment. The female lays 7 to 12 eggs, which are white or pale buff, some having fine dark brown spots. Because the nest is small, they are stacked in layers. The female incubates; she pushes her legs (which are well supplied with blood vessels, hence warm) down among the eggs. A unique feature of kinglets is the "size hierarchy" among eggs, with early-laid eggs being smaller than later ones.
Eggs hatch asynchronously after 15 to 17 days. The young stay in the nest for 19 to 24 days. After being fed, nestlings make their way down to the bottom of the nest, pushing their still-hungry siblings up to be fed in their turn (but also to be cold).
Kinglets are the most fecund and shortest-living of all altricial birds, and probably the shortest-lived apart from a few smaller galliform species. Adult mortality for the goldcrest is estimated at over 80 percent per year and the maximum lifespan is only six years.
| Biology and health sciences | Passerida | Animals |
221923 | https://en.wikipedia.org/wiki/Starling | Starling | Starlings are small to medium-sized passerine birds in the family Sturnidae, common name of Sturnid. The Sturnidae are named for the genus Sturnus, which in turn comes from the Latin word for starling, sturnus. The family contains 128 species which are divided into 36 genera. Many Asian species, particularly the larger ones, are called mynas, and many African species are known as glossy starlings because of their iridescent plumage. Starlings are native to Europe, Asia, and Africa, as well as northern Australia and the islands of the tropical Pacific. Several European and Asian species have been introduced to these areas, as well as North America, Hawaii, and New Zealand, where they generally compete for habitats with native birds and are considered to be invasive species. The starling species familiar to most people in Europe and North America is the common starling, and throughout much of Asia and the Pacific, the common myna is indeed common.
Starlings have strong feet, their flight is strong and direct, and they are very gregarious. Their preferred habitat is fairly open country, and they eat insects and fruit. Several species live around human habitation and are effectively omnivores. Many species search for prey such as grubs by "open-bill probing", that is, forcefully opening the bill after inserting it into a crevice, thus expanding the hole and exposing the prey; this behaviour is referred to by the German verb zirkeln (pronounced ).
Plumage of many species is typically dark with a metallic sheen. Most species nest in holes and lay blue or white eggs.
Starlings have diverse and complex vocalizations and have been known to embed sounds from their surroundings into their own calls, including car alarms and human speech patterns. The birds can recognize particular individuals by their calls and are the subject of research into the evolution of human language.
Description
Starlings are medium-sized passerines. The shortest-bodied species is Kenrick's starling (Poeoptera kenricki), at , but the lightest-weight species is Abbott's starling (Poeoptera femoralis), which is . The largest starling, going on standard measurements and perhaps weight, is the Nias hill myna (Gracula robusta). This species can measure up to , and in domestication they can weigh up to . Rivaling the prior species in bulk if not dimensions, the mynas of the genus Mino are also large, especially the yellow-faced (M. dumontii) and long-tailed mynas (M. kreffti). The longest species in the family is the white-necked myna (Streptocitta albicollis), which can measure up to , although around 60% in this magpie-like species is comprised by its very long tail.
Less sexual dimorphism is seen in plumage, but with only 25 species showing such differences between the two sexes. The plumage of the starling is often brightly coloured due to iridescence; this colour is derived from the structure of the feathers, not from any pigment. Some species of Asian starling have crests or erectile feathers on the crest. Other ornamentation includes elongated tail feathers and brightly coloured bare areas on the face. These colours can be derived from pigments, or as in the Bali myna, structural colour, caused by light scattering off parallel collagen fibers. The irises of many species are red and yellow, although those of younger birds are much darker.
Distribution, habitat and movements
Starlings inhabit a wide range of habitats from the Arctic Circle to the Equator. The only habitats they do not typically occupy are very dry sandy deserts. The family is naturally absent from the Americas and from large parts of Australia, but it is present over the majority of Europe, Africa, and Asia. The genus Aplonis has also spread widely across the islands of the Pacific, reaching Polynesia, Melanesia, and Micronesia (in addition one species in the genus Mino has reached the Solomon Islands). Also, a species of this genus is the only starling found in northern Australia.
Asian species are most common in evergreen forests; 39 species found in Asia are predominantly forest birds as opposed to 24 found in more open or human modified environments. In contrast to this, African species are more likely to be found in open woodlands and savannah; 33 species are open-area specialists compared to 13 true forest species. The high diversity of species found in Asia and Africa is not matched by Europe, which has one widespread (and very common) species and two more restricted species. The European starling is both highly widespread and extremely eclectic in its habitat, occupying most types of open habitat. Like many other starling species, it has also adapted readily to human-modified habitat, including farmland, orchards, plantations, and urban areas.
Some species of starlings are migratory, either entirely, like Shelley's starling, which breeds in Ethiopia and Somaliland and migrates to Kenya, Tanzania, and Somalia, or like the white-shouldered starling, which is migratory in part of its range, but is resident in others.
The European starling was purposely introduced to North America in the 1870s through the 1890s by multiple acclimatisation societies, organizations dedicated to introducing European flora and fauna into North America for cultural and economic reasons. A persistent story alleges that Eugene Schieffelin, chairman of the American Acclimatization Society, decided all birds mentioned by William Shakespeare should be in North America, leading to the introduction of the starling to the U.S.; however, this claim is more fiction than fact. While Schieffelin and other members of the society did release starlings in Central Park in 1890, the birds had already been in the U.S. since at least the mid-1870s, and Schieffelin was not inspired to do so by Shakespeare's works.
Behaviour
The starlings are generally a highly social family. Most species associate in flocks of varying sizes throughout the year. Murmuration is the flocking of starlings, including the swarm behaviour of their large flight formations. These flocks may include other species of starlings and sometimes species from other families. This sociality is particularly evident in their roosting behaviour; in the nonbreeding season, some roosts can number in the thousands of birds.
Mimic
Starlings imitate a variety of avian species and have a repertoire of about 15–20 distinct imitations. They also imitate a few sounds other than those of wild birds. The calls of abundant species or calls that are simple in frequency structure and show little amplitude modulation are preferentially imitated. Dialects of mimicked sounds can be local.
Diet and feeding
The diets of the starlings are usually dominated by fruits and insects. Many species are important dispersers of seeds, in Asia and Africa, for example, white sandalwood and Indian banyan. In addition to trees, they are also important dispersers of parasitic mistletoes. In South Africa, the red-winged starling is an important disperser of the introduced Acacia cyclops. Starlings have been observed feeding on fermenting over-ripe fruit, which led to the speculation that they might become intoxicated by the alcohol.
Laboratory experiments on European starlings have found that they have disposal enzymes that allow them to break down alcohol very quickly. In addition to consuming fruits, many starlings also consume nectar. The extent to which starlings are important pollinators is unknown, but at least some are, such as the slender-billed starling of alpine East Africa, which pollinates giant lobelias.
Systematics
The starling family Sturnidae was introduced (as Sturnidia) by French polymath Constantine Samuel Rafinesque in 1815.
The starlings belong to the superfamily Muscicapoidea, together with thrushes, flycatchers and chats, as well as dippers, which are quite distant relatives, and Mimidae (thrashers and mockingbirds). The latter are apparently the Sturnidae's closest living relatives, replace them in the Americas, and have a rather similar but more solitary lifestyle. They are morphologically quite similar too—a partly albinistic specimen of a mimid, mislabelled as to suggest an Old World origin, was for many decades believed to represent an extinct starling (see Rodrigues starling for details).
The oxpeckers are sometimes placed here as a subfamily, but the weight of evidence has shifted towards granting them full family status as a more basal member of the Sturnidae-Mimidae group, derived from an early expansion into Africa.
Usually, the starlings are considered a family, as is done here. Sibley & Monroe included the mimids in the family and demoted the starlings to tribe rank, as Sturnini. This treatment was used by Zuccon et al. However, the grouping of Sibley & Monroe is overly coarse due to methodological drawbacks of their DNA-DNA hybridization technique and most of their proposed revisions of taxonomic rank have not been accepted (see for example Ciconiiformes). The all-inclusive Sturnidae grouping conveys little information about biogeography, and obscures the evolutionary distinctness of the three lineages. Establishing a valid name for the clade consisting of Sibley/Monroe's "pan-Sturnidae" would nonetheless be desirable to contrast them with the other major lineages of Muscicapoidea.
Starlings probably originated in the general area of East Asia, perhaps towards the southwestern Pacific, as inferred by the number of plesiomorphic lineages to occur there. Expansion into Africa appears to have occurred later, as most derived forms are found there. An alternative scenario would be African origin for the entire "sturnoid" group, with the oxpeckers representing an ancient relict and the mimids arriving in South America. This is contradicted by the North American distribution of the most basal Mimidae.
As the fossil record is limited to quite Recent forms, the proposed Early Miocene (about 25–20 Mya) divergence dates for the "sturnoids" lineages must be considered extremely tentative. Given the overall evidence for the origin of most Passeri families in the first half of the Miocene, it appears to be not too far off the mark, however.
recent studies identified two major clades of this family, corresponding to the generally drab, often striped, largish "atypical mynas" and other mainly Asian-Pacific lineages, and the often smaller, sometimes highly apomorphic taxa which are most common in Africa and the Palearctic, usually have metallic coloration, and in a number of species also bright carotinoid plumage colors on the underside. Inside this latter group, there is a clade consisting of species which, again, are usually not too brightly colored, and which consists of the "typical" myna-Sturnus assemblage.
The Philippine creepers, a single genus of three species of treecreeper-like birds, appear to be highly apomorphic members of the more initial radiation of the Sturnidae. While this may seem odd at first glance, their placement has always been contentious. In addition, biogeography virtually rules out a close relationship of Philippine creepers and treecreepers, as neither the latter nor their close relatives seem to have ever reached Wallacea, let alone the Philippines. Nonetheless, their inclusion in the Sturnidae is not entirely final and eventually, they may remain a separate family.
Genus sequence follows traditional treatments. This is apparently not entirely correct, with Scissirostrum closer to Aplonis than to Gracula, for example, and Acridotheres among the most advanced genera. Too few taxa have yet been studied as regards their relationships, however, thus a change in the sequence has to wait for further studies.
As of 2023, the review by Lovette & Rubenstein (2008) is the most recent work on the phylogeny of the group. This taxonomy is also based on the order of the IOC.
The extinct Mascarene starlings were formerly of uncertain relationships, but are now thought to belong to the Oriental-Australasian clade, being allied with the Bali myna. However, while the two more recent species (Fregipilus and Necropsar) have been classified, the prehistoric Cryptopsar has not.
| Biology and health sciences | Passerida | null |
221926 | https://en.wikipedia.org/wiki/Myna | Myna | The mynas (; also spelled mynah) are a group of birds in the starling family (Sturnidae). This is a group of passerine birds which are native to Iran and Southern Asia, especially Afghanistan, India, Pakistan, Bangladesh, Nepal and Sri Lanka. Several species have been introduced to areas like North America, Australia, South Africa, Fiji and New Zealand, especially the common myna, which is often regarded as an invasive species. It is often known as "Selarang" and "Teck Meng" in Malay and Chinese respectively in Singapore, due to their high population there.
Mynas are not a natural group; instead, the term myna is used for any starling in the Indian subcontinent, regardless of their relationships. This range was colonized twice during the evolution of starlings, first by rather ancestral starlings related to the coleto and Aplonis lineages, and millions of years later by birds related to the common starling and wattled starling's ancestors. These two groups of mynas can be distinguished in the more terrestrial adaptions of the latter, which usually also have less glossy plumage, except on the heads, and longer tails. The Bali myna, which is critically endangered and nearly extinct in the wild, is highly distinctive.
Some mynas are considered talking birds, for their ability to reproduce sounds, including human speech, when in captivity.
Myna is derived from the Urdu language (mainā) which itself is derived from Sanskrit madanā.
Characteristics
Mynas are medium-sized passerines with strong feet. Their flight is strong and direct, and they are gregarious. Their preferred habitat is fairly open country, and they eat insects and fruit.
Plumage is typically dark, often brown, although some species have yellow head ornaments. Most species nest in holes.
Some species have become well known for their imitative skills; the common hill myna is one of these.
Species
The following are species of mynas. The coleto and the two Saroglossa starlings are included because of their position in the taxonomic list.
Jungle and hill mynas
Yellow-faced myna, Mino dumontii
Golden myna, Mino anais
Long-tailed myna, Mino kreffti
Sulawesi myna, Basilornis celebensis
Helmeted myna, Basilornis galeatus
Long-crested myna, Basilornis corythaix
Apo myna, Goodfellowia miranda
White-necked myna, Streptocitta albicollis
Bare-eyed myna, Streptocitta albertinae
Fiery-browed myna, Enodes erythrophris
Finch-billed myna, Scissirostrum dubium
Golden-crested myna, Ampeliceps coronatus
Common hill myna, Gracula religiosa
Southern hill myna, Gracula indica
Enggano hill myna, Gracula enganensis
Nias hill myna, Gracula robusta
Sri Lanka hill myna, Gracula ptilogenys
"True" mynas
Great myna, Acridotheres grandis
Crested myna, Acridotheres cristatellus
Javan myna, Acridotheres javanicus
Pale-bellied myna, Acridotheres cinereus
Jungle myna, Acridotheres fuscus
Collared myna, Acridotheres albocinctus
Bank myna, Acridotheres ginginianus
Common myna, Acridotheres tristis
Bali myna, Leucopsar rothschildi
"Gracupica" mynas
Indian pied myna, Gracupica contra
Siamese pied myna, Gracupica floweri
Javan pied myna, Gracupica jalla
The following species are often included in the Acridotheres mynas:
Vinous-breasted starling, Acridotheres burmannicus
Black-winged starling, Acridotheres melanopterus
Red-billed starling, Spodiopsar sericeus
White-cheeked starling, Spodiopsar cineraceus
| Biology and health sciences | Passerida | Animals |
221931 | https://en.wikipedia.org/wiki/Oxpecker | Oxpecker | The oxpeckers are two species of bird which make up the genus Buphagus, and family Buphagidae. The oxpeckers were formerly usually treated as a subfamily, Buphaginae, within the starling family, Sturnidae, but molecular phylogenetic studies have consistently shown that they form a separate lineage that is basal to the sister clades containing the Sturnidae and the Mimidae (mockingbirds, thrashers, and allies). Oxpeckers are endemic to the savanna of Sub-Saharan Africa.
Both the English and scientific names arise from their habit of perching on large mammals (both wild and domesticated) such as cattle, zebras, impalas, hippopotamuses, rhinoceroses, and giraffes, eating ticks, small insects, botfly larvae, and other parasites, as well as the animals' blood. The behaviour of oxpeckers towards large mammals was thought to be exclusively mutual, though recent research suggests the relationship can be parasitic in nature as well.
The Swahili name for the red-billed oxpecker is Askari wa kifaru (the rhino's guard).
Taxonomy
The genus Buphagus was introduced in 1760 by the French zoologist Mathurin Jacques Brisson with the yellow-billed oxpecker as the type species. The name combines the Ancient Greek words bous "ox" and - "eating".
According to the more recent studies of Muscicapoidea phylogeny, the oxpeckers are an ancient line related to Mimidae (mockingbirds and thrashers) and starlings but not particularly close to either. Considering the known biogeography of these groups, the most plausible explanation seems that the oxpecker lineage originated in Eastern or Southeastern Asia like the other two. This would make the two species of Buphagus something like living fossils, and demonstrates that such remnants of past evolution can possess striking and unique autapomorphic adaptations.
The genus contains two species:
Distribution and habitat
The oxpeckers are endemic to sub-Saharan Africa, where they occur in most open habitats. They are absent from the driest deserts and the rainforests. Their distribution is restricted by the presence of their preferred prey, specific species of ticks, and the animal hosts of those ticks. The two species of oxpecker are sympatric over much of East Africa and may even occur on the same host animal. The nature of the interactions between the two species is unknown.
Behaviour
Diet and feeding
Oxpeckers graze exclusively on the bodies of large mammals. Certain species are seemingly preferred, whereas others, like the Lichtenstein's hartebeest or topi are generally avoided. Smaller antelope such as lechwe, duikers and reedbuck are also avoided; the smallest regularly used species is the impala, probably because of the heavy tick load and social nature of that species. In many parts of their range they now feed on cattle, but avoid camels. They feed on ectoparasites, particularly ticks, as well as insects infesting wounds and the flesh and blood of some wounds as well. They are sometimes classified as parasites, because they open wounds on the animals' backs.
Oxpecker/mammal interactions are the subject of some debate and ongoing research. They were originally thought to be an example of mutualism, but recent evidence suggests that oxpeckers may be parasites instead. Oxpeckers do eat ticks, but often the ticks have already fed on the ungulate host, and no statistically significant link has been shown between oxpecker presence and reduced ectoparasite load. Oxpeckers have been observed to open new wounds and enhance existing ones in order to drink the blood of their perches. Oxpeckers also feed on the earwax and dandruff of mammals; less is known about the possible benefits of this to the mammal, but it is suspected that this is also a parasitic behaviour. Some oxpeckers' hosts are intolerant of their presence. Elephants and some antelope will actively dislodge the oxpeckers when they land. However there have been noted instances of elephants allowing oxpeckers to eat parasites off of them. Other species tolerate oxpeckers while they search for ticks on their faces, which one author says "appears ... to be an uncomfortable and invasive process."
Breeding
The breeding season of the oxpeckers, in at least one location, is linked to the rainy season, which affects the activity of their mammalian hosts and the tick loads of those hosts. Both courtship and copulation occur on their hosts as well. They nest in holes, usually in trees but sometimes in other types of cavity, including holes in walls. The nests are lined with grasses and often with hair plucked from their hosts and even livestock such as sheep which are not usually used. The typical clutch is between two and three eggs, but the red-billed oxpecker may lay up to five eggs.
Roosting
Red-billed oxpeckers have been known to roost in reeds and trees. Studies of large savanna herbivores using cameras at night have shown that both species of oxpecker (but more often in yellow-billed oxpecker) may also roost on the bodies of herbivores, hanging under the insides of the thighs of giraffe and on top of impala and buffalo.
| Biology and health sciences | Passerida | Animals |
221932 | https://en.wikipedia.org/wiki/Pasture | Pasture | Pasture (from the Latin pastus, past participle of pascere, "to feed") is land used for grazing.
Types of pasture
Pasture lands in the narrow sense are enclosed tracts of farmland, grazed by domesticated livestock, such as horses, cattle, sheep, or swine. The vegetation of tended pasture, forage, consists mainly of grasses, with an interspersion of legumes and other forbs (non-grass herbaceous plants). Pasture is typically grazed throughout the summer, in contrast to meadow which is ungrazed or used for grazing only after being mown to make hay for animal fodder.
Pasture in a wider sense additionally includes rangelands, other unenclosed pastoral systems, and land types used by wild animals for grazing or browsing.
Pasture lands in the narrow sense are distinguished from rangelands by being managed through more intensive agricultural practices of seeding, irrigation, and the use of fertilizers, while rangelands grow primarily native vegetation, managed with extensive practices like controlled burning and regulated intensity of grazing.
Soil type, minimum annual temperature, and rainfall are important factors in pasture management.
Sheepwalk is an area of grassland where sheep can roam freely. The productivity of sheepwalk is measured by the number of sheep per area. This is dependent, among other things, on the underlying rock. Sheepwalk is also the name of townlands in County Roscommon, Ireland, and County Fermanagh, Northern Ireland. Unlike factory farming, which entails in its most intensive form entirely trough-feeding, managed or unmanaged pasture is the main food source for ruminants. Pasture feeding dominates livestock farming where the land makes crop sowing or harvesting (or both) difficult, such as in arid or mountainous regions, where types of camel, goat, antelope, yak and other ruminants live which are well suited to the more hostile terrain and very rarely factory-farmed. In more humid regions, pasture grazing is managed across a large global area for free range and organic farming. Certain pasture types suit the diet, evolution, and metabolism of particular animals. Their fertilising and tending of the land may over generations result in the pasture combined with the ruminants in question being integral to a particular ecosystem.
Examples of pasture habitats
Bocage
Grassland
Heathland
Machair
Maquis
Moorland
Pampas
Potrero (landform)
Prairie
Rangeland
Rough pasture
Savanna
Sown biodiverse pasture
Steppe
Wood pasture
Veld
| Technology | Basics_2 | null |
221998 | https://en.wikipedia.org/wiki/Thiomersal | Thiomersal | Thiomersal (INN), or thimerosal (USAN, JAN), also sold under the name merthiolate is an organomercury compound. It is a well-established antiseptic and antifungal agent.
The pharmaceutical corporation Eli Lilly and Company named it Merthiolate. It has been used as a preservative in vaccines, immunoglobulin preparations, skin test antigens, antivenins, ophthalmic and nasal products, and tattoo inks. In spite of the scientific consensus that fears about its safety are unsubstantiated, its use as a vaccine preservative has been called into question by anti-vaccination groups.
A 1999 statement issued in CDC's Morbidity and Mortality Weekly Report announced that "the Public Health Service (PHS), the American Academy of Pediatrics (AAP), and vaccine manufacturers agree that thimerosal-containing vaccines should be removed as soon as possible" and that these groups would collaborate to replace them while manufacturers committed "to eliminate or reduce as expeditiously as possible the mercury content of their vaccines."
It remains in use as a preservative for certain annual flu vaccines, mostly those stored in multi-dose vials. Single-dose vial flu shots are an option for those who prefer vaccines with no thiomersal, although no scientific data supports claims that there is any link between thiomersal and autism.
History
Morris Kharasch, a chemist then at the University of Maryland filed a patent application for thiomersal in 1927; Eli Lilly later marketed the compound under the trade name Merthiolate. In vitro tests conducted by Lilly investigators H. M. Powell and W. A. Jamieson found that it was forty to fifty times as effective as phenol against Staphylococcus aureus. It was used to kill bacteria and prevent contamination in antiseptic ointments, creams, jellies, and sprays used by consumers and in hospitals, including nasal sprays, eye drops, contact lens solutions, immunoglobulins, and vaccines. Thiomersal was used as a preservative (bactericide) so that multidose vials of vaccines could be used instead of single-dose vials, which are more expensive. By 1938, Lilly's assistant director of research listed thiomersal as one of the five most important drugs ever developed by the company.
Structure
Thiomersal features mercury(II) with a coordination number 2, i.e. two ligands are attached to Hg, the thiolate and the ethyl group. The carboxylate group confers solubility in water. Like other two-coordinate Hg(II) compounds, the coordination geometry of Hg is linear, with a 180° S-Hg-C angle. Typically, organomercury thiolate compounds are prepared from organomercury chlorides.
Uses
Antiseptic/antifungal
Thiomersal's main use is as an antiseptic and antifungal agent, due to its oligodynamic effect. In multidose injectable drug delivery systems, it prevents serious adverse effects such as the Staphylococcus infection that, in one 1928 incident, killed 12 of 21 children vaccinated with a diphtheria vaccine that lacked a preservative. Unlike other preservatives at the time, such as phenol and cresol, thiomersal does not reduce the potency of the vaccines that it protects. Bacteriostatics such as thiomersal are not needed in single-dose injectables.
In the United States, the European Union, and a few other affluent countries, thiomersal is no longer used as a preservative in routine childhood vaccination schedules. In the U.S., all vaccines routinely recommended for children 6 years of age and younger are available in formulations that do not contain thimerosal. Two vaccines (a TD and the single-dose version of the trivalent influenza vaccine Fluvirin) may contain a trace of thiomersal from steps in manufacture, but less than 1 microgram of mercury per dose. The multi-dose versions of some trivalent and quadrivalent influenza vaccines can contain up to 25 micrograms of mercury per dose from thiomersal. Also, four rarely used treatments for pit viper, coral snake, and black widow venom contain thiomersal.
Outside North America and Europe, many vaccines contain thiomersal; the World Health Organization reported no evidence of toxicity from thiomersal in vaccines and no reason on safety grounds to change to more expensive single-dose administration. The United Nations Environment Program backed away from an earlier proposal of putting thiomersal on the list of banned vaccine compounds as part of its campaign to reduce mercury exposure. It stated that eliminating it in multi-dose vaccines, primarily used in developing countries, would lead to high cost and a refrigeration requirement that developing countries could ill afford. At the Minamata Convention on Mercury in 2013 thiomersal was excluded from the treaty.
Toxicology
General toxicity
Thiomersal is very toxic by inhalation, ingestion, and in contact with skin (EC hazard symbol T+), with a danger of cumulative effects. It is also very toxic to aquatic organisms and may cause long-term adverse effects in aquatic environments (EC hazard symbol N).
In the body, it is metabolized or degraded to ethylmercury (C2H5Hg+) and thiosalicylate.
Cases have been reported of severe mercury poisoning by accidental exposure or attempted suicide, with some fatalities. Animal experiments suggest that thiomersal rapidly dissociates to release ethylmercury after injection; that mercury's disposition patterns are similar to those after exposure to equivalent doses of ethylmercury chloride; and that the central nervous system and the kidneys are targets. Loss of motor coordination is a common sign. Similar signs and symptoms have been observed in accidental human poisonings. The mechanisms of toxic action are unknown.
Fecal excretion accounts for most of the elimination from the body. Ethylmercury clears from blood with a half-life of about 18 days in adults by breakdown into other chemicals, including inorganic mercury. The half-life of ethylmercury in the brains of infant monkeys is 14 days. Risk assessment for effects on the nervous system have been made by extrapolating from dose-response relationships for methylmercury. Methylmercury and ethylmercury distribute to all body tissues, crossing the blood–brain barrier and the placental barrier, and ethylmercury also moves freely throughout the body.
Concerns based on extrapolations from methylmercury caused thiomersal to be removed from U.S. childhood vaccines, starting in 1999. Later it was reported that ethylmercury is eliminated from the body and the brain significantly faster than methylmercury, so the late-1990s risk assessments turned out to be overly conservative. Though inorganic mercury metabolized from ethylmercury has a much longer half-life in the brain, at least 120 days, it appears to be much less toxic than the inorganic mercury produced from mercury vapor, for reasons not yet understood.
As an allergen
Thiomersal is used in patch testing for people who have dermatitis, conjunctivitis, and other potentially allergic reactions. A 2007 study in Norway found that 1.9% of adults had a positive patch test reaction to thiomersal; a higher prevalence of contact allergy (up to 6.6%) was observed in German populations. Thiomersal-sensitive individuals can receive intramuscular rather than subcutaneous immunization, though there have been no large sample sized studies regarding this matter to date. In real-world practice on vaccination of adult populations, contact allergy does not seem to elicit clinical reaction.
Thiomersal allergy has decreased in Denmark, probably because of its exclusion from vaccines there. In a recent study of Polish children and adolescents with chronic/recurrent eczema, positive reactions to thiomersal were found in 11.7% of children (7–8 y.o.) and 37.6% of adolescents (16–17 y.o.). This difference in the sensitization rates can be explained by changing exposure patterns: The adolescents received six thiomersal-preserved vaccines during their life course, with the last immunization taking place 2–3 years before the study. Younger children received only four thiomersal-preserved vaccines, with the last one applied five years before the study, while further immunizations were performed with thiomersal-free vaccines.
Removal from vaccines
The Center for Biologics Evaluation and Research (CBER) at the FDA initiated a formal risk assessment of thiomersal in vaccines beginning in 1998. After determining the levels of ethylmercury exposure from the currently recommended vaccine schedule, the CBER found these amounts exceeded new standards for methylmercury exposure recently established by the Environmental Protection Agency. On July 7, 1999, both the American Academy of Pediatrics and the US Public Health Service issued a statement calling for the removal of thiomersal-containing vaccines “as expeditiously as possible.” By March 2001, thiomersal-free versions of all the recommended childhood vaccines for children up to age 6 were available in the United States following the introduction of the new DtAP vaccine.
Disproven autism hypothesis
Following the phasing out of thiomersal from most U.S. and European vaccines, some parents saw the action to remove thiomersal—in the setting of a perceived increasing rate of autism as well as increasing number of vaccines in the childhood vaccination schedule—as indicating that the preservative was the cause of autism. The scientific consensus is that no evidence supports these claims, while the rate of autism continued to climb in children who did not take the thiomersal-preserved childhood vaccines.
Scientific and medical bodies such as the Institute of Medicine and World Health Organization, as well as governmental agencies such as the Food and Drug Administration and the CDC reject any role for thiomersal in autism or other neurodevelopmental disorders. Unconvinced parents attempted to treat their autistic children with unproven and possibly dangerous treatments, and refused to vaccinate them due to fears about thiomersal toxicity. Studying thiomersal potentially diverts resources away from research into more promising areas for autism. Thousands of lawsuits have been filed in U.S. federal court to seek damages from allegedly toxic vaccines, including those purportedly caused by thiomersal.
| Physical sciences | Organic salts | Chemistry |
222142 | https://en.wikipedia.org/wiki/Panthalassa | Panthalassa | Panthalassa, also known as the Panthalassic Ocean or Panthalassan Ocean (from Greek "all" and "sea"), was the vast superocean that encompassed planet Earth and surrounded the supercontinent Pangaea, the latest in a series of supercontinents in the history of Earth. During the Paleozoic–Mesozoic transition ( 250 ), the ocean occupied almost 70% of Earth's surface, with the supercontinent Pangaea taking up less than half. The original, ancient ocean floor has now completely disappeared because of the continuous subduction along the continental margins on its circumference. Panthalassa is also referred to as the Paleo-Pacific ("old Pacific") or Proto-Pacific because the Pacific Ocean is a direct continuation of Panthalassa.
Formation
The supercontinent Rodinia began to break up 870–845 probably as a consequence of a superplume caused by mantle slab avalanches along the margins of the supercontinent. In a second episode 750 the western half of Rodinia started to rift apart: western Kalahari and South China broke away from the western margins of Laurentia; and by 720 Australia and East Antarctica had also separated. In the Early Jurassic the Pacific Plate opened originating from a triple junction between the Panthalassic Farallon, Phoenix, and Izanagi plates. Panthalassa can be reconstructed based on magnetic lineations and fracture zones preserved in the western Pacific.
Reconstruction of ocean basin
Most of the oceanic plates that formed the ocean floor of Panthalassa have been subducted and so traditional plate tectonic reconstructions based on magnetic anomalies can therefore be used only for remains from the Cretaceous and later. The former margins of the ocean, however, contain allochthonous terranes with preserved Triassic–Jurassic intra-Panthalassic volcanic arcs, including Kolyma–Omolon (northeast Asia), Anadyr–Koryak (east Asia), Oku–Niikappu (Japan), and Wrangellia and Stikinia (western North America).
Furthermore, seismic tomography is being used to identify subducted slabs in the mantle from which the location of former Panthalassic subduction zones can be derived. A series of such subduction zones, called Telkhinia, defines two separate oceans or systems of oceanic plates—the Pontus and Thalassa oceans.
Named marginal oceans or oceanic plates include (clockwise) Mongol-Okhotsk (now a suture between Mongolia and Sea of Okhotsk), Oimyakon (between Asian craton and Kolyma-Omolon), Slide Mountain Ocean (British Columbia), and Mezcalera (western Mexico).
Eastern margin
The western margin (modern coordinates) of Laurentia originated during the Neoproterozoic break-up of Rodinia. The North American Cordillera is an accretionary orogen, which grew by the progressive addition of allochthonous terranes along this margin from the Late Palaeozoic. Devonian back-arc volcanism reveals how this eastern Panthalassic margin developed into the active margin it still is in the mid-Palaeozoic.
Most of the continental fragments, volcanic arcs, and ocean basins added to Laurentia this way contained faunas of Tethyan or Asian affinity. Similar terranes added to the northern Laurentia, in contrast, have affinities with Baltica, Siberia, and the northern Caledonies. The latter terranes were probably accreted along the eastern Panthalassa margin by a Caribbean–Scotia-style subduction system.
Western margin
The evolution of the Panthalassa–Tethys boundary is poorly known because little oceanic crust is preserved—both the Izanagi and the conjugate Pacific Ocean floor is subducted and the ocean ridge that separated them probably subducted 60–55 . Today, the region is dominated by the collision of the Australian Plate with a complex network of plate boundaries in south-east Asia, including the Sundaland block. Spreading along the Pacific-Phoenix ridge ended 83 Ma at the Osbourn Trough at the Tonga-Kermadec Trench.
During the Permian, atolls developed near the Equator on the mid-Panthalassic seamounts. As Panthalassa subducted along its western margin during the Triassic and Early Jurassic, those seamounts and palaeo-atolls were accreted as allochthonous limestone blocks and fragments along the Asian margin. One such migrating atoll complex now form a and body of limestone in central Kyushu, south-west Japan.
Fusuline foraminifera, a now extinct order of single-celled organisms, diversified extensively and developed gigantism—the genus Eopolydiexodina, for example, reached up to in size—and structural sophistication, including symbiont relationships with photosynthesising algae, during the Late Carboniferous and Permian, in what is known as the Carboniferous-Earliest Permian Biodiversification Event. The Capitanian mass extinction event 260 , however, put an end to that development, with only dwarf taxa persisting throughout the Permian until the final fusuline extinction in the Great Dying 252 . Permian fusulines also developed a remarkable provincialism by which fusulines can be grouped into six domains.
Because of the large size of Panthalassa, a hundred million years could separate the accretion of different groups of fusulines. Assuming a minimum accretion rate of , the seamount chains on which those groups evolved would be separated by at least . Those groups apparently evolved in completely different environments.
A significant sea-level drop at the end of the Permian led to the end-Capitanian extinction event. The cause for the extinction is disputed, but a likely candidate is an episode of global cooling, which transformed a large amount of sea-water into continental ice.
Seamounts accreted in eastern Australia as parts of the New England orogen reveal the hotspot history of Panthalassa. From the Late Devonian to the Carboniferous, Gondwana and Panthalassa converged along the eastern margin of Australia along a west-dipping subduction system, which produced (west to east) a magmatic arc, a forearc basin, and an accretionary wedge. Subduction ceased along that margin in the Late Carboniferous and jumped eastward. From the Late Carboniferous to the Early Permian the New England orogen was dominated by an extensional setting related to a subduction to strike-slip transition. Subduction was re-initiated in the Permian and the granitic rocks of the New England Batholith were produced by a magmatic arc, indicating the presence of an active plate margin along most of the orogen. Permian to Cretaceous remains of the convergent margin, preserved as fragments in Zealandia (New Zealand, New Caledonia, and the Lord Howe Rise), were rifted off Australia during the Late Cretaceous to Early Tertiary break-up of eastern Gondwana and the opening of the Tasman Sea.
The Cretaceous Junction Plate, located north of Australia, separated the eastern Tethys from Panthalassa.
Palaeo-oceanography
Panthalassa was a hemisphere-sized ocean, much larger than the modern Pacific. It could be expected that the large size would result in relatively simple ocean current circulation patterns, such as a single gyre in each hemisphere, and a mostly stagnant and stratified ocean. Modelling studies, however, suggest that an east–west sea surface temperature (SST) gradient was present in which the coldest water was brought to the surface by upwelling in the east while the warmest water extended west into the Tethys Ocean.
Subtropical gyres dominated the circulation pattern. The two hemispherical belts were separated by the undulating Intertropical Convergence Zone (ITCZ).
In northern Panthalassa, there were mid-latitude westerlies north of 60°N with easterlies between 60°N and the Equator. Atmospheric circulation north of 30°N is associated with the North Panthalassa High, which created Ekman convergence between 15°N and 50°N and Ekman divergence between 5°N and 10°N. A pattern developed that resulted in Sverdrup transport that went northward in divergence regions and southward in convergence regions. Western boundary currents resulted in an anti-cyclonic subtropical North Panthalassa gyre at mid-latitudes and a meridional anti-cyclonic circulation centred on 20°N.
In tropical northern Panthalassa, trade winds created westward flows while equatorward flows were created by westerlies at higher latitudes. Consequently, trade winds moved water away from Gondwana towards Laurasia in the northern Panthalassa Equatorial Current. When the western margins of Panthalassa were reached, intense western boundary currents would form the Eastern Laurasia Current. At mid-latitudes, the North Panthalassa Current would bring the water back east where a weak Northwestern Gondwana Current would finally close the gyre. The accumulation of water along the western margin, coupled with the Coriolis effect, would have created a Panthalassa Equatorial Counter Current.
In the southern Panthalassa, the four currents of the subtropical gyre, the South Panthalassa Gyre, rotated counterclockwise. The South Equatorial Panthalassa Current flowed westward between the Equator and 10°S into the western, intense South Panthalassa Current. The South Polar Current then completed the gyre as the Southwestern Gondwana Current. Near the poles easterlies created a subpolar gyre that rotated clockwise.
| Physical sciences | Paleogeography | Earth science |
222154 | https://en.wikipedia.org/wiki/Blastulation | Blastulation | Blastulation is the stage in early animal embryonic development that produces the blastula. In mammalian development, the blastula develops into the blastocyst with a differentiated inner cell mass and an outer trophectoderm. The blastula (from Greek βλαστός ( meaning sprout)) is a hollow sphere of cells known as blastomeres surrounding an inner fluid-filled cavity called the blastocoel. Embryonic development begins with a sperm fertilizing an egg cell to become a zygote, which undergoes many cleavages to develop into a ball of cells called a morula. Only when the blastocoel is formed does the early embryo become a blastula. The blastula precedes the formation of the gastrula in which the germ layers of the embryo form.
A common feature of a vertebrate blastula is that it consists of a layer of blastomeres, known as the blastoderm, which surrounds the blastocoel. In mammals, the blastocyst contains an embryoblast (or inner cell mass) that will eventually give rise to the definitive structures of the fetus, and a trophoblast which goes on to form the extra-embryonic tissues.
During blastulation, a significant amount of activity occurs within the early embryo to establish cell polarity, cell specification, axis formation, and to regulate gene expression. In many animals, such as Drosophila and Xenopus, the mid blastula transition (MBT) is a crucial step in development during which the maternal mRNA is degraded and control over development is passed to the embryo. Many of the interactions between blastomeres are dependent on cadherin expression, particularly E-cadherin in mammals and EP-cadherin in amphibians.
The study of the blastula, and of cell specification has many implications in stem cell research, and assisted reproductive technology. In Xenopus, blastomeres behave as pluripotent stem cells which can migrate down several pathways, depending on cell signaling. By manipulating the cell signals during the blastula stage of development, various tissues can be formed. This potential can be instrumental in regenerative medicine for disease and injury cases. In vitro fertilisation involves the transfer of an embryo into a uterus for implantation.
Development
The blastula stage of early embryo development begins with the appearance of the blastocoel. The origin of the blastocoel in Xenopus has been shown to be from the first cleavage furrow, which is widened and sealed with tight junctions to create a cavity.
In many organisms the development of the embryo up to this point and for the early part of the blastula stage is controlled by maternal mRNA, so called because it was produced in the egg prior to fertilization and is therefore exclusively from the mother.
Midblastula transition
In many organisms including Xenopus and Drosophila, the midblastula transition usually occurs after a particular number of cell divisions for a given species, and is defined by the ending of the synchronous cell division cycles of the early blastula development, and the lengthening of the cell cycles by the addition of the G1 and G2 phases. Prior to this transition, cleavage occurs with only the synthesis and mitosis phases of the cell cycle. The addition of the two growth phases into the cell cycle allows for the cells to increase in size, as up to this point the blastomeres undergo reductive divisions in which the overall size of the embryo does not increase, but more cells are created. This transition begins the growth in size of the organism.
The mid-blastula transition is also characterized by a marked increase in transcription of new, non-maternal mRNA transcribed from the genome of the organism. Large amounts of the maternal mRNA are destroyed at this point, either by proteins such as SMAUG in Drosophila or by microRNA. These two processes shift the control of the embryo from the maternal mRNA to the nuclei.
Structure
A blastula (blastocyst in mammals), is a sphere of cells surrounding a fluid-filled cavity called the blastocoel. The blastocoel contains amino acids, proteins, growth factors, sugars, ions and other components which are necessary for cellular differentiation. The blastocoel also allows blastomeres to move during the process of gastrulation.
In Xenopus embryos, the blastula is composed of three different regions. The animal cap forms the roof of the blastocoel and goes on primarily to form ectodermal derivatives. The equatorial or marginal zone, which compose the walls of the blastocoel differentiate primarily into mesodermal tissue. The vegetal mass is composed of the blastocoel floor and primarily develops into endodermal tissue.
In the mammalian blastocyst there are three lineages that give rise to later tissue development. The epiblast gives rise to the fetus itself while the trophoblast develops into part of the placenta and the primitive endoderm becomes the yolk sac. In the mouse embryo, blastocoel formation begins at the 32-cell stage. During this process, water enters the embryo, aided by an osmotic gradient which is the result of sodium–potassium pumps that produce a high sodium gradient on the basolateral side of the trophectoderm. This movement of water is facilitated by aquaporins. A seal is created by tight junctions of the epithelial cells that line the blastocoel.
Cellular adhesion
Tight junctions are very important in embryo development. In the blastula, these cadherin mediated cell interactions are essential to development of epithelium which are most important to paracellular transport, maintenance of cell polarity and the creation of a permeability seal to regulate blastocoel formation. These tight junctions arise after the polarity of epithelial cells is established which sets the foundation for further development and specification. Within the blastula, inner blastomeres are generally non-polar while epithelial cells demonstrate polarity.
Mammalian embryos undergo compaction around the 8-cell stage where E-cadherins as well as alpha and beta catenins are expressed. This process makes a ball of embryonic cells which are capable of interacting, rather than a group of diffuse and undifferentiated cells. E-cadherin adhesion defines the apico-basal axis in the developing embryo and turns the embryo from an indistinct ball of cells to a more polarized phenotype which sets the stage for further development into a fully formed blastocyst.
Xenopus membrane polarity is established with the first cell cleavage. Amphibian EP-cadherin and XB/U cadherin perform a similar role as E-cadherin in mammals establishing blastomere polarity and solidifying cell-cell interactions which are crucial for further development.
Clinical implications
Fertilization technologies
Experiments with implantation in mice show that hormonal induction, superovulation and artificial insemination successfully produce preimplantation mouse embryos. In the mice, ninety percent of the females were induced by mechanical stimulation to undergo pregnancy and implant at least one embryo. These results prove to be encouraging because they provide a basis for potential implantation in other mammalian species, such as humans.
Stem cells
Blastula-stage cells can behave as pluripotent stem cells in many species. Pluripotent stem cells are the starting point to produce organ specific cells that can potentially aid in repair and prevention of injury and degeneration. Combining the expression of transcription factors and locational positioning of the blastula cells can lead to the development of induced functional organs and tissues. Pluripotent Xenopus cells, when used in an in vivo strategy, were able to form into functional retinas. By transplanting them to the eye field on the neural plate, and by inducing several mis-expressions of transcription factors, the cells were committed to the retinal lineage and could guide vision based behavior in the Xenopus.
| Biology and health sciences | Animal reproduction | Biology |
222177 | https://en.wikipedia.org/wiki/Gastrulation | Gastrulation | Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of cells), or in mammals the blastocyst, is reorganized into a two-layered or three-layered embryo known as the gastrula. Before gastrulation, the embryo is a continuous epithelial sheet of cells; by the end of gastrulation, the embryo has begun differentiation to establish distinct cell lineages, set up the basic axes of the body (e.g. dorsal–ventral, anterior–posterior), and internalized one or more cell types including the prospective gut.
Gastrula layers
In triploblastic organisms, the gastrula is trilaminar (three-layered). These three germ layers are the ectoderm (outer layer), mesoderm (middle layer), and endoderm (inner layer). In diploblastic organisms, such as Cnidaria and Ctenophora, the gastrula has only ectoderm and endoderm. The two layers are also sometimes referred to as the hypoblast and epiblast. Sponges do not go through the gastrula stage.
Gastrulation takes place after cleavage and the formation of the blastula, or blastocyst. Gastrulation is followed by organogenesis, when individual organs develop within the newly formed germ layers. Each layer gives rise to specific tissues and organs in the developing embryo.
The ectoderm gives rise to epidermis, the nervous system, and to the neural crest in vertebrates.
The endoderm gives rise to the epithelium of the digestive system and respiratory system, and organs associated with the digestive system, such as the liver and pancreas.
The mesoderm gives rise to many cell types such as muscle, bone, and connective tissue. In vertebrates, mesoderm derivatives include the notochord, the heart, blood and blood vessels, the cartilage of the ribs and vertebrae, and the dermis.
Following gastrulation, cells in the body are either organized into sheets of connected cells (as in epithelia), or as a mesh of isolated cells, such as mesenchyme.
Basic cell movements
Although gastrulation patterns exhibit enormous variation throughout the animal kingdom, they are unified by the five basic types of cell movements that occur during gastrulation:
Invagination
Involution
Ingression
Delamination
Epiboly
Etymology
The terms "gastrula" and "gastrulation" were coined by Ernst Haeckel, in his 1872 work "Biology of Calcareous Sponges".
Gastrula (literally, "little belly") is a neo-Latin diminutive based on the Ancient Greek ("a belly").
Importance
Lewis Wolpert, pioneering developmental biologist in the field, has been credited for noting that "It is not birth, marriage, or death, but gastrulation which is truly the most important time in your life."
Model systems
Gastrulation is highly variable across the animal kingdom but has underlying similarities. Gastrulation has been studied in many animals, but some models have been used for longer than others. Furthermore, it is easier to study development in animals that develop outside the mother. Model organisms whose gastrulation is understood in the greatest detail include the mollusc, sea urchin, frog, and chicken. A human model system is the gastruloid.
Protostomes versus deuterostomes
The distinction between protostomes and deuterostomes is based on the direction in which the mouth (stoma) develops in relation to the blastopore. Protostome derives from the Greek word protostoma meaning "first mouth" (πρῶτος + στόμα) whereas Deuterostome's etymology is "second mouth" from the words second and mouth (δεύτερος + στόμα).
The major distinctions between deuterostomes and protostomes are found in embryonic development:
Mouth/anus
In protostome development, the first opening in development, the blastopore, becomes the animal's mouth.
In deuterostome development, the blastopore becomes the animal's anus.
Cleavage
Protostomes have what is known as spiral cleavage which is determinate, meaning that the fate of the cells is determined as they are formed.
Deuterostomes have what is known as radial cleavage that is indeterminate.
Sea urchins
Sea urchins have been important model organisms in developmental biology since the 19th century. Their gastrulation is often considered the archetype for invertebrate deuterostomes.
Sea urchins exhibit highly stereotyped cleavage patterns and cell fates. Maternally deposited mRNAs establish the organizing center of the sea urchin embryo. Canonical Wnt and Delta-Notch signaling progressively segregate progressive endoderm and mesoderm.
The first cells to internalize are the primary mesenchyme cells (PMCs), which have a skeletogenic fate, which ingress during the blastula stage. Gastrulation – internalization of the prospective endoderm and non-skeletogenic mesoderm – begins shortly thereafter with invagination and other cell rearrangements the vegetal pole, which contribute approximately 30% to the final archenteron length. The gut's final length depends on cell rearrangements within the archenteron.
Amphibians
The frog genus Xenopus has been used as a model organism for the study of gastrulation.
Symmetry breaking
The sperm contributes one of the two mitotic asters needed to complete first cleavage. The sperm can enter anywhere in the animal half of the egg but its exact point of entry will break the egg's radial symmetry by organizing the cytoskeleton. Prior to first cleavage, the egg's cortex rotates relative to the internal cytoplasm by the coordinated action of microtubules, in a process known as cortical rotation. This displacement brings maternally loaded determinants of cell fate from the equatorial cytoplasm and vegetal cortex into contact, and together these determinants set up the organizer. Thus, the area on the vegetal side opposite the sperm entry point will become the organizer. Hilde Mangold, working in the lab of Hans Spemann, demonstrated that this special "organizer" of the embryo is necessary and sufficient to induce gastrulation.
The dorsal lip of the blastopore is the mechanical driver of gastrulation, and the first sign of invagination seen in the frog.
Germ layer differentiation
Specification of endoderm depends on rearrangement of maternally deposited determinants, leading to nuclearization of Beta-catenin. Mesoderm is induced by signaling from the presumptive endoderm to cells that would otherwise become ectoderm.
Cell signaling
In the frog, Xenopus, one of the signals is retinoic acid (RA). RA signaling in this organism can affect the formation of the endoderm and depending on the timing of the signaling, it can determine the fate whether its pancreatic, intestinal, or respiratory. Other signals such as Wnt and BMP also play a role in respiratory fate of the Xenopus by activating cell lineage tracers.
Amniotes
Overview
In amniotes (reptiles, birds and mammals), gastrulation involves the creation of the blastopore, an opening into the archenteron. Note that the blastopore is not an opening into the blastocoel, the space within the blastula, but represents a new inpocketing that pushes the existing surfaces of the blastula together. In amniotes, gastrulation occurs in the following sequence: (1) the embryo becomes asymmetric; (2) the primitive streak forms; (3) cells from the epiblast at the primitive streak undergo an epithelial to mesenchymal transition and ingress at the primitive streak to form the germ layers.
Symmetry breaking
In preparation for gastrulation, the embryo must become asymmetric along both the proximal-distal axis and the anteroposterior axis. The proximal-distal axis is formed when the cells of the embryo form the "egg cylinder", which consists of the extraembryonic tissues, which give rise to structures like the placenta, at the proximal end and the epiblast at the distal end. Many signaling pathways contribute to this reorganization, including BMP, FGF, nodal, and Wnt. Visceral endoderm surrounds the epiblast. The distal visceral endoderm (DVE) migrates to the anterior portion of the embryo, forming the anterior visceral endoderm (AVE). This breaks anterior-posterior symmetry and is regulated by nodal signaling.
Germ layer determination
The primitive streak is formed at the beginning of gastrulation and is found at the junction between the extraembryonic tissue and the epiblast on the posterior side of the embryo and the site of ingression. Formation of the primitive streak is reliant upon nodal signaling in the Koller's sickle within the cells contributing to the primitive streak and BMP4 signaling from the extraembryonic tissue. Furthermore, Cer1 and Lefty1 restrict the primitive streak to the appropriate location by antagonizing nodal signaling. The region defined as the primitive streak continues to grow towards the distal tip.
During the early stages of development, the primitive streak is the structure that will establish bilateral symmetry, determine the site of gastrulation and initiate germ layer formation. To form the streak, reptiles, birds and mammals arrange mesenchymal cells along the prospective midline, establishing the first embryonic axis, as well as the place where cells will ingress and migrate during the process of gastrulation and germ layer formation. The primitive streak extends through this midline and creates the antero-posterior body axis, becoming the first symmetry-breaking event in the embryo, and marks the beginning of gastrulation. This process involves the ingression of mesoderm and endoderm progenitors and their migration to their ultimate position, where they will differentiate into the three germ layers. The localization of the cell adhesion and signaling molecule beta-catenin is critical to the proper formation of the organizer region that is responsible for initiating gastrulation.
Cell internalization
In order for the cells to move from the epithelium of the epiblast through the primitive streak to form a new layer, the cells must undergo an epithelial to mesenchymal transition (EMT) to lose their epithelial characteristics, such as cell–cell adhesion. FGF signaling is necessary for proper EMT. FGFR1 is needed for the up regulation of SNAI1, which down regulates E-cadherin, causing a loss of cell adhesion. Following the EMT, the cells ingress through the primitive streak and spread out to form a new layer of cells or join existing layers. FGF8 is implicated in the process of this dispersal from the primitive streak.
Cell signaling driving gastrulation
During gastrulation, the cells are differentiated into the ectoderm or mesendoderm, which then separates into the mesoderm and endoderm. The endoderm and mesoderm form due to the nodal signaling. Nodal signaling uses ligands that are part of TGFβ family. These ligands will signal transmembrane serine/threonine kinase receptors, and this will then phosphorylate Smad2 and Smad3. This protein will then attach itself to Smad4 and relocate to the nucleus where the mesendoderm genes will begin to be transcribed. The Wnt pathway along with β-catenin plays a key role in nodal signaling and endoderm formation. Fibroblast growth factors (FGF), canonical Wnt pathway, bone morphogenetic protein (BMP), and retinoic acid (RA) are all important in the formation and development of the endoderm. FGF are important in producing the homeobox gene which regulates early anatomical development. BMP signaling plays a role in the liver and promotes hepatic fate. RA signaling also induce homeobox genes such as Hoxb1 and Hoxa5. In mice, if there is a lack in RA signaling the mouse will not develop lungs. RA signaling also has multiple uses in organ formation of the pharyngeal arches, the foregut, and hindgut.
Gastrulation in vitro
There have been a number of attempts to understand the processes of gastrulation using in vitro techniques in parallel and complementary to studies in embryos, usually though the use of 2D and 3D cell (Embryonic organoids) culture techniques using embryonic stem cells (ESCs) or induced pluripotent stem cells (iPSCs). These are associated with number of clear advantages in using tissue-culture based protocols, some of which include reducing the cost of associated in vivo work (thereby reducing, replacing and refining the use of animals in experiments; the 3Rs), being able to accurately apply agonists/antagonists in spatially and temporally specific manner which may be technically difficult to perform during Gastrulation. However, it is important to relate the observations in culture to the processes occurring in the embryo for context.
To illustrate this, the guided differentiation of mouse ESCs has resulted in generating primitive streak–like cells that display many of the characteristics of epiblast cells that traverse through the primitive streak (e.g. transient brachyury up regulation and the cellular changes associated with an epithelial to mesenchymal transition), and human ESCs cultured on micro patterns, treated with BMP4, can generate spatial differentiation pattern similar to the arrangement of the germ layers in the human embryo. Finally, using 3D embryoid body- and organoid-based techniques, small aggregates of mouse ESCs (Embryonic Organoids, or Gastruloids) are able to show a number of processes of early mammalian embryo development such as symmetry-breaking, polarisation of gene expression, gastrulation-like movements, axial elongation and the generation of all three embryonic axes (anteroposterior, dorsoventral and left-right axes).
In vitro fertilization occurs in a laboratory. The process of in vitro fertilization is when mature eggs are removed from the ovaries and are placed in a cultured medium where they are fertilized by sperm. In the culture the embryo will form. 14 days after fertilization the primitive streak forms. The formation of the primitive streak has been known to some countries as "human individuality". This means that the embryo is now a being itself, it is its own entity. The countries that believe this have created a 14-day rule in which it is illegal to study or experiment on a human embryo after the 14-day period in vitro. Research has been conducted on the first 14 days of an embryo, but no known studies have been done after the 14 days. With the rule in place, mice embryos are used understand the development after 14 days; however, there are differences in the development between mice and humans.
| Biology and health sciences | Animal reproduction | Biology |
222183 | https://en.wikipedia.org/wiki/Secretin | Secretin | Secretin is a hormone that regulates water homeostasis throughout the body and influences the environment of the duodenum by regulating secretions in the stomach, pancreas, and liver. It is a peptide hormone produced in the S cells of the duodenum, which are located in the intestinal glands. In humans, the secretin peptide is encoded by the SCT gene.
Secretin helps regulate the pH of the duodenum by inhibiting the secretion of gastric acid from the parietal cells of the stomach and stimulating the production of bicarbonate from the ductal cells of the pancreas. It also stimulates the secretion of bicarbonate and water by cholangiocytes in the bile duct, protecting it from bile acids by controlling the pH and promoting the flow in the duct. Meanwhile, in concert with secretin's actions, the other main hormone simultaneously issued by the duodenum, cholecystokinin (CCK), stimulates the gallbladder to contract, delivering its stored bile.
Prosecretin is a precursor to secretin, which is present in digestion. Secretin is stored in this unusable form, and is activated by gastric acid. This indirectly results in the neutralisation of duodenal pH, thus ensuring no damage is done to the small intestine by the aforementioned acid.
In 2007, secretin was discovered to play a role in osmoregulation by acting on the hypothalamus, pituitary gland, and kidney.
History
In 1902, William Bayliss and Ernest Starling were studying how the nervous system controls the process of digestion. It was known that the pancreas secreted digestive juices in response to the passage of food (chyme) through the pyloric sphincter into the duodenum. They discovered (by cutting all the nerves to the pancreas in their experimental animals) that this process was not, in fact, governed by the nervous system. They determined that a substance secreted by the intestinal lining stimulates the pancreas after being transported via the bloodstream. They named this intestinal secretion secretin. This type of 'chemical messenger' substance is now called a hormone, a term coined by Starling in 1905.
Secretin is frequently erroneously stated to have been the first hormone identified. However, British researchers George Oliver and Edward Albert Schäfer had already published their findings of an adrenal extract increasing blood pressure and heart rate in brief reports in 1894 and a full publication in 1895, making adrenaline the first discovered hormone.
Structure
Secretin is initially synthesized as a 120 amino acid precursor protein known as prosecretin. This precursor contains an N-terminal signal peptide, spacer, secretin itself (residues 28–54), and a 72-amino acid C-terminal peptide.
The mature secretin peptide is a linear peptide hormone, which is composed of 27 amino acids and has a molecular weight of 3055. A helix is formed in the amino acids between positions 5 and 13. The amino acids sequences of secretin have some similarities to that of glucagon, vasoactive intestinal peptide (VIP), and gastric inhibitory peptide (GIP). Fourteen of 27 amino acids of secretin reside in the same positions as in glucagon, 7 the same as in VIP, and 10 the same as in GIP.
Secretin also has an amidated carboxyl-terminal amino acid which is valine. The sequence of amino acids in secretin is H–His-Ser-Asp-Gly-Thr-Phe-Thr-Ser-Glu-Leu-Ser-Arg-Leu-Arg-Asp-Ser-Ala-Arg-Leu-Gln-Arg-Leu-Leu-Gln-Gly-Leu-Val–NH2.
Physiology
Production and secretion
Secretin is synthesized in cytoplasmic secretory granules of S-cells, which are found mainly in the mucosa of the duodenum, and in smaller numbers in the jejunum of the small intestine.
Secretin is released into circulation and/or intestinal lumen in response to low duodenal pH that ranges between 2 and 4.5 depending on species; the acidity is due to hydrochloric acid in the chyme that enters the duodenum from the stomach via the pyloric sphincter. Also, the secretion of secretin is increased by the products of protein digestion bathing the mucosa of the upper small intestine.
Secretin release is inhibited by H2 antagonists, which reduce gastric acid secretion. As a result, if the pH in the duodenum increases above 4.5, secretin cannot be released.
Function
pH regulation
Secretin primarily functions to neutralize the pH in the duodenum, allowing digestive enzymes from the pancreas (e.g., pancreatic amylase and pancreatic lipase) to function optimally.
Secretin targets the pancreas; pancreatic centroacinar cells have secretin receptors in their plasma membrane. As secretin binds to these receptors, it stimulates adenylate cyclase activity and converts ATP to cyclic AMP. Cyclic AMP acts as second messenger in intracellular signal transduction and causes the organ to secrete a bicarbonate-rich fluid that flows into the intestine. Bicarbonate is a base that neutralizes the acid, thus establishing a pH favorable to the action of other digestive enzymes in the small intestine.
Secretin also increases water and bicarbonate secretion from duodenal Brunner's glands to buffer the incoming protons of the acidic chyme, and also reduces acid secretion by parietal cells of the stomach. It does this through at least three mechanisms: 1) By stimulating release of somatostatin, 2) By inhibiting release of gastrin in the pyloric antrum, and 3) By direct downregulation of the parietal cell acid secretory mechanics.
It counteracts blood glucose concentration spikes by triggering increased insulin release from pancreas, following oral glucose intake.
Osmoregulation
Secretin modulates water and electrolyte transport in pancreatic duct cells, liver cholangiocytes, and epididymis epithelial cells. It is found to play a role in the vasopressin-independent regulation of renal water reabsorption.
Secretin is found in the magnocellular neurons of the paraventricular and supraoptic nuclei of the hypothalamus and along the neurohypophysial tract to neurohypophysis. During increased osmolality, it is released from the posterior pituitary. In the hypothalamus, it activates vasopressin release. It is also needed to carry out the central effects of angiotensin II. In the absence of secretin or its receptor in the gene knockout animals, central injection of angiotensin II was unable to stimulate water intake and vasopressin release.
It has been suggested that abnormalities in such secretin release could explain the abnormalities underlying type D syndrome of inappropriate antidiuretic hormone hypersecretion (SIADH). In these individuals, vasopressin release and response are normal, although abnormal renal expression, translocation of aquaporin 2, or both are found. It has been suggested that "Secretin as a neurosecretory hormone from the posterior pituitary, therefore, could be the long-sought vasopressin independent mechanism to solve the riddle that has puzzled clinicians and physiologists for decades."
Food intake
Secretin and its receptor are found in discrete nuclei of the hypothalamus, including the paraventricular nucleus and the arcuate nucleus, which are the primary brain sites for regulating body energy homeostasis. It was found that both central and peripheral injection of Sct reduce food intake in mouse, indicating an anorectic role of the peptide. This function of the peptide is mediated by the central melanocortin system.
Uses
Secretin is used in diagnostic tests for pancreatic function; secretin is injected and the pancreatic output can then be imaged with magnetic resonance imaging, a noninvasive procedure, or secretions generated as a result can gathered either through an endoscope or through tubes inserted through the mouth, down into the duodenum.
A recombinant human secretin has been available since 2004 for these diagnostic purposes. There were problems with the availability of this agent from 2012 to 2015.
Research
A wave of enthusiasm for secretin as a possible treatment for autism arose in the 1990s based on a hypothetical gut-brain connection; as a result the NIH ran a series of clinical trials that showed that secretin was not effective, which brought an end to popular interest.
A high-affinity and optimized secretin receptor antagonist (Y10,c[E16,K20],I17,Cha22,R25)sec(6-27) has been designed and developed which has allowed the structural characterization of secreting inactive conformation.
| Biology and health sciences | Animal hormones | Biology |
222299 | https://en.wikipedia.org/wiki/Vasopressin | Vasopressin | Human vasopressin, also called antidiuretic hormone (ADH), arginine vasopressin (AVP) or argipressin, is a hormone synthesized from the AVP gene as a peptide prohormone in neurons in the hypothalamus, and is converted to AVP. It then travels down the axon terminating in the posterior pituitary, and is released from vesicles into the circulation in response to extracellular fluid hypertonicity (hyperosmolality). AVP has two primary functions. First, it increases the amount of solute-free water reabsorbed back into the circulation from the filtrate in the kidney tubules of the nephrons. Second, AVP constricts arterioles, which increases peripheral vascular resistance and raises arterial blood pressure.
A third function is possible. Some AVP may be released directly into the brain from the hypothalamus, and may play an important role in social behavior, sexual motivation and pair bonding, and maternal responses to stress.
Vasopressin induces differentiation of stem cells into cardiomyocytes and promotes heart muscle homeostasis.
It has a very short half-life, between 16 and 24 minutes.
Physiology
Function
Vasopressin regulates the tonicity of body fluids. It is released from the posterior pituitary in response to hypertonicity and causes the kidneys to reabsorb solute-free water and return it to the circulation from the tubules of the nephron, thus returning the tonicity of the body fluids toward normal. An incidental consequence of this renal reabsorption of water is concentrated urine and reduced urine volume. AVP released in high concentrations may also raise blood pressure by inducing moderate vasoconstriction.
AVP also may have a variety of neurological effects on the brain. It may influence pair-bonding in voles. The high-density distributions of vasopressin receptor AVPr1a in prairie vole ventral forebrain regions have been shown to facilitate and coordinate reward circuits during partner preference formation, critical for pair bond formation.
A very similar substance, lysine vasopressin (LVP) or lypressin, has the same function in pigs and its synthetic version was used in human AVP deficiency, although it has been largely replaced by desmopressin.
Kidney
Vasopressin has three main effects which are:
Increasing the water permeability of distal convoluted tubule (DCT) and cortical collecting tubules (CCT), as well as outer and inner medullary collecting duct (OMCD & IMCD) in the kidney, thus allowing water reabsorption and excretion of more concentrated urine, i.e., antidiuresis. This occurs through increased transcription and insertion of water channels (Aquaporin-2) into the apical membrane of collecting tubule and collecting duct epithelial cells. Aquaporins allow water to move down their osmotic gradient and out of the nephron, increasing the amount of water re-absorbed from the filtrate (forming urine) back into the bloodstream. This effect is mediated by V2 receptors. Vasopressin also increases the concentration of calcium in the collecting duct cells, by episodic release from intracellular stores. Vasopressin, acting through cAMP, also increases transcription of the aquaporin-2 gene, thus increasing the total number of aquaporin-2 molecules in collecting duct cells.
Increasing permeability of the inner medullary portion of the collecting duct to urea by regulating the cell surface expression of urea transporters, which facilitates its reabsorption into the medullary interstitium as it travels down the concentration gradient created by removing water from the connecting tubule, cortical collecting duct, and outer medullary collecting duct.
Acute increase of sodium absorption across the ascending loop of Henle. This adds to the countercurrent multiplication which aids in proper water reabsorption later in the distal tubule and collecting duct.
The hormone vasopressin also stimulates the activity of NKCC2. Vasopressin stimulates sodium chloride reabsorption in the thick ascending limb of the nephron by activating signaling pathways. Vasopressin increases the traffic of NKCC2 to the membrane and phosphorylates some serine and threonine sites on the cytoplasmic N-terminal of the NKCC2 located in the membrane, increasing its activity. Increased NKCC2 activity aids in water reabsorption in the collecting duct through aquaporin 2 channels by creating a hypo-osmotic filtrate.
Central nervous system
Vasopressin released within the brain may have several actions:
Vasopressin is released into the brain in a circadian rhythm by neurons of the suprachiasmatic nucleus.
Vasopressin released from posterior pituitary is associated with nausea.
Recent evidence suggests that vasopressin may have analgesic effects. The analgesia effects of vasopressin were found to be dependent on both stress and sex.
Regulation
Gene regulation
Vasopressin is regulated by AVP gene expression which is managed by major clock controlled genes. In this circadian circuit known as the transcription-translation feedback loop (TTFL), Per2 protein accumulates and is phosphorylated by CK1E. Per2 subsequently inhibits the transcription factors Clock and BMAL1 in order to reduce Per2 protein levels in the cell. At the same time, Per2 also inhibits the transcription factors for the AVP gene in order to regulate its expression, the expression of vasopressin, and other AVP gene products.
Many factors influence the secretion of vasopressin:
Ethanol (alcohol) reduces the calcium-dependent secretion of AVP by blocking voltage-gated calcium channels in neurohypophyseal nerve terminals in rats.
Angiotensin II stimulates AVP secretion, in keeping with its general pressor and pro-volumic effects on the body.
Atrial natriuretic peptide inhibits AVP secretion, in part by inhibiting Angiotensin II-induced stimulation of AVP secretion.
Cortisol inhibits secretion of antidiuretic hormone.
Production and secretion
The physiological stimulus for secretion of vasopressin is increased osmolality of the plasma, monitored by the hypothalamus. A decreased arterial blood volume, (such as can occur in cirrhosis, nephrosis, and heart failure), stimulates secretion, even in the face of decreased osmolality of the plasma: it supersedes osmolality, but
with a milder effect. In other words, the unloading of arterial baroreceptors when the arterial blood volume is low stimulates vasopressin secretion despite the presence of hypoosmolality (hyponatremia).
The AVP that is measured in peripheral blood is almost all derived from secretion from the posterior pituitary gland (except in cases of AVP-secreting tumours). Vasopressin is produced by magnocellular neurosecretory neurons in the paraventricular nucleus of hypothalamus (PVN) and supraoptic nucleus (SON). It then travels down the axon through the infundibulum within neurosecretory granules that are found within Herring bodies, localized swellings of the axons and nerve terminals. These carry the peptide directly to the posterior pituitary gland, where it is stored until released into the blood.
There are other sources of AVP, beyond the hypothalamic magnocellular neurons. For example, AVP is also synthesized by parvocellular neurosecretory neurons of the PVN, transported and released at the median eminence, from which it travels through the hypophyseal portal system to the anterior pituitary, where it stimulates corticotropic cells synergistically with CRH to produce ACTH (by itself it is a weak secretagogue).
Vasopressin during surgery and anaesthesia
Vasopressin concentration is used to measure surgical stress for evaluation of surgical techniques. Plasma vasopressin concentration is elevated by noxious stimuli, predominantly during abdominal surgery, especially at gut manipulation, traction of viscera, as well as abdominal insufflation with carbon dioxide during laparoscopic surgery.
Receptors
Types of AVP receptors and their actions:
Structure and relation to oxytocin
The vasopressins are peptides consisting of nine amino acids (nonapeptides). The amino acid sequence of arginine vasopressin (argipressin) is Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly-NH2, with the cysteine residues forming a disulfide bond and the C-terminus of the sequence converted to a primary amide. Lysine vasopressin (lypressin) has a lysine in place of the arginine as the eighth amino acid, and is found in pigs and some related animals, whereas arginine vasopressin is found in humans.
The structure of oxytocin is very similar to that of the vasopressins: It is also a nonapeptide with a disulfide bridge and its amino acid sequence differs at only two positions. The two genes are located on the same chromosome separated by a relatively small distance of less than 15,000 bases in most species. The magnocellular neurons that secrete vasopressin are adjacent to magnocellular neurons that secrete oxytocin, and are similar in many respects. The similarity of the two peptides can cause some cross-reactions: oxytocin has a slight antidiuretic function, and high levels of AVP can cause uterine contractions.
Comparison of vasopressin and oxytocin neuropeptide families:
Medical use
Vasopressin is used to manage anti-diuretic hormone deficiency. Vasopressin is used to treat diabetes insipidus related to low levels of antidiuretic hormone. It is available as Pressyn.
Vasopressin has off-label uses and is used in the treatment of vasodilatory shock, gastrointestinal bleeding, ventricular tachycardia and ventricular fibrillation.
Vasopressin agonists are used therapeutically in various conditions, and its long-acting synthetic analogue desmopressin is used in conditions featuring low vasopressin secretion, as well as for control of bleeding (in some forms of von Willebrand disease and in mild haemophilia A) and in extreme cases of bedwetting by children. Terlipressin and related analogues are used as vasoconstrictors in certain conditions. Use of vasopressin analogues for esophageal varices commenced in 1970.
Vasopressin infusions are also used as second line therapy for septic shock patients not responding to fluid resuscitation or infusions of catecholamines (e.g., dopamine or norepinephrine) to increase the blood pressure while sparing the use of catecholamines. These argipressins have much shorter elimination half-life (around 20 minutes) comparing to synthetic non-arginine vasopresines with much longer elimination half-life of many hours. Further, argipressins act on V1a, V1b, and V2 receptors which consequently lead to higher eGFR and lower vascular resistance in the lungs. A number of injectable arginine vasopressins are currently in clinical use in the United States and in Europe.
Pharmacokinetics
Vasopressin is administered through an intravenous device, intramuscular injection or a subcutaneous injection. The duration of action depends on the mode of administration and ranges from thirty minutes to two hours. It has a half life of ten to twenty minutes. It is widely distributed throughout the body and remains in the extracellular fluid. It is degraded by the liver and excreted through the kidneys. Arginin vasopressins for use in septic shock are intended for intravenous use only.
Side effects
The most common side effects during treatment with vasopressin are dizziness, angina, chest pain, abdominal cramps, heartburn, nausea, vomiting, trembling, fever, water intoxication, pounding sensation in the head, diarrhoea, sweating, paleness, and flatulence. The most severe adverse reactions are myocardial infarction and hypersensitivity.
Contraindications
The use of lysine vasopressin is contraindicated in the presence of hypersensitivity to beef or pork proteins, increased BUN and chronic kidney failure. It is recommended that it be cautiously used in instances of perioperative polyuria, sensitivity to the drug, asthma, seizures, heart failure, a comatose state, migraine headaches, and cardiovascular disease.
Interactions
alcohol - may lower the antidiuretic effect
carbamazepine, chloropropamide, clofibrate, tricyclic antidepressants and fludrocortisone may raise the antidiuretic effect
lithium, demeclocycline, heparin or norepinephrine may lower the antidiuretic effect
vasopressor effect may be higher with the concurrent use of ganglionic blocking medications
Deficiency
Decreased AVP release (neurogenic — i.e. due to alcohol intoxication or tumour) or decreased renal sensitivity to AVP (nephrogenic, i.e. by mutation of V2 receptor or AQP) leads to diabetes insipidus, a condition featuring hypernatremia (increased blood sodium concentration), polyuria (excess urine production), and polydipsia (thirst).
Excess
Syndrome of Inappropriate Antidiuretic Hormone secretion (SIADH) in turn can be caused by a number of problems. Some forms of cancer can cause SIADH, particularly small cell lung carcinoma but also a number of other tumors. A variety of diseases affecting the brain or the lung (infections, bleeding) can be the driver behind SIADH. A number of drugs have been associated with SIADH, such as certain antidepressants (serotonin reuptake inhibitors and tricyclic antidepressants), the anticonvulsant carbamazepine, oxytocin (used to induce and stimulate labor), and the chemotherapy drug vincristine. It has also been associated with fluoroquinolones (including ciprofloxacin and moxifloxacin). Finally, it can occur without a clear explanation. Hyponatremia can be treated pharmaceutically through the use of vasopressin receptor antagonists.
History
Vasopressin was elucidated and synthesized for the first time by Vincent du Vigneaud.
Animal studies
Evidence for an effect of AVP on monogamy vs polygamy comes from experimental studies in several species, which indicate that the precise distribution of vasopressin and vasopressin receptors in the brain is associated with species-typical patterns of social behavior. In particular, there are consistent differences between monogamous species and polygamous species in the distribution of AVP receptors, and sometimes in the distribution of vasopressin-containing axons, even when closely related species are compared.
Human studies
Vasopressin has shown nootropic effects on pain perception and cognitive function. Vasopressin also plays a role in autism, major depressive disorder, bipolar disorder, and schizophrenia.
| Biology and health sciences | Animal hormones | Biology |
222300 | https://en.wikipedia.org/wiki/Oxytocin | Oxytocin | Oxytocin is a peptide hormone and neuropeptide normally produced in the hypothalamus and released by the posterior pituitary. Present in animals since early stages of evolution, in humans it plays roles in behavior that include social bonding, love, reproduction, childbirth, and the period after childbirth. Oxytocin is released into the bloodstream as a hormone in response to sexual activity and during childbirth. It is also available in pharmaceutical form. In either form, oxytocin stimulates uterine contractions to speed up the process of childbirth.
In its natural form, it also plays a role in maternal bonding and milk production. Production and secretion of oxytocin is controlled by a positive feedback mechanism, where its initial release stimulates production and release of further oxytocin. For example, when oxytocin is released during a contraction of the uterus at the start of childbirth, this stimulates production and release of more oxytocin and an increase in the intensity and frequency of contractions. This process compounds in intensity and frequency and continues until the triggering activity ceases. A similar process takes place during lactation and during sexual activity.
Oxytocin is derived by enzymatic splitting from the peptide precursor encoded by the human OXT gene. The deduced structure of the active nonapeptide is:
Cys – Tyr – Ile – Gln – Asn – Cys – Pro – Leu – Gly – NH2, or CYIQNCPLG-NH2.
Etymology
The term "oxytocin" derives from the Greek "ὠκυτόκος" (ōkutókos), based on ὀξύς (oxús), meaning "sharp" or "swift", and τόκος (tókos), meaning "childbirth". The adjective form is "oxytocic", which refers to medicines which stimulate uterine contractions, to speed up the process of childbirth. Colloquially, it has been referred to as the "cuddle hormone" or the "moral molecule" which have been considered misnomers.
History
The uterine-contracting properties of the principle that would later be named oxytocin were discovered by British pharmacologist Henry Hallett Dale in 1906, and its milk ejection property was described by Ott and Scott in 1910 and by Schafer and Mackenzie in 1911. In 1909 the first clinical use of oxytocin was performed by William Blair-Bell to induce childbirth in patients with complications.
By the 1920s, oxytocin and vasopressin had been isolated from pituitary tissue and given their current names. Oxytocin's molecular structure was determined in 1952. In the early 1950s, American biochemist Vincent du Vigneaud found that oxytocin is made up of nine amino acids, and he identified its amino acid sequence, the first polypeptide hormone to be sequenced. In 1953, du Vigneaud carried out the synthesis of oxytocin, the first polypeptide hormone to be synthesized. Du Vigneaud was awarded the Nobel Prize in Chemistry in 1955 for his work. Further work on different synthetic routes for oxytocin, as well as the preparation of analogues of the hormone (e.g. 4-deamido-oxytocin) was performed in the following decade by Iphigenia Photaki.
Biochemistry
Estrogen has been found to increase the secretion of oxytocin and to increase the expression of its receptor, the oxytocin receptor, in the brain. In women, a single dose of estradiol has been found to be sufficient to increase circulating oxytocin concentrations.
Biosynthesis
Oxytocin and vasopressin are the only known hormones released by the human posterior pituitary gland to act at a distance. However, oxytocin neurons make other peptides, including corticotropin-releasing hormone and dynorphin, for example, that act locally. The magnocellular neurons that make oxytocin are adjacent to magnocellular neurons that make vasopressin and are similar in many respects.
The oxytocin peptide is synthesized as an inactive precursor protein from the OXT gene. This precursor protein also includes the oxytocin carrier protein neurophysin I. The inactive precursor protein is progressively hydrolyzed into smaller fragments (one of which is neurophysin I) via a series of enzymes. The last hydrolysis that releases the active oxytocin nonapeptide is catalyzed by peptidylglycine alpha-amidating monooxygenase (PAM).
The activity of the PAM enzyme system is dependent upon vitamin C (ascorbate), which is a necessary vitamin cofactor. By chance, sodium ascorbate by itself was found to stimulate the production of oxytocin from ovarian tissue over a range of concentrations in a dose-dependent manner. Many of the same tissues (e.g. ovaries, testes, eyes, adrenals, placenta, thymus, pancreas) where PAM (and oxytocin by default) is found are also known to store higher concentrations of vitamin C.
Oxytocin is known to be metabolized by the oxytocinase, leucyl/cystinyl aminopeptidase. Other oxytocinases are also known to exist. Amastatin, bestatin (ubenimex), leupeptin, and puromycin have been found to inhibit the enzymatic degradation of oxytocin, though they also inhibit the degradation of various other peptides, such as vasopressin, met-enkephalin, and dynorphin A.
Neural sources
In the hypothalamus, oxytocin is made in magnocellular neurosecretory cells of the supraoptic and paraventricular nuclei, and is stored in Herring bodies at the axon terminals in the posterior pituitary. It is then released into the blood from the posterior lobe (neurohypophysis) of the pituitary gland. These axons (likely, but dendrites have not been ruled out) have collaterals that innervate neurons in the nucleus accumbens, a brain structure where oxytocin receptors are expressed. The endocrine effects of hormonal oxytocin, and the cognitive or behavioral effects of oxytocin neuropeptides are thought to be coordinated through its common release through these collaterals. Oxytocin is also produced by some neurons in the paraventricular nucleus that project to other parts of the brain and to the spinal cord. Depending on the species, oxytocin receptor-expressing cells are located in other areas, including the amygdala and bed nucleus of the stria terminalis.
In the pituitary gland, oxytocin is packaged in large, dense-core vesicles, where it is bound to neurophysin I as shown in the inset of the figure; neurophysin is a large peptide fragment of the larger precursor protein molecule from which oxytocin is derived by enzymatic cleavage.
Secretion of oxytocin from the neurosecretory nerve endings is regulated by the electrical activity of the oxytocin cells in the hypothalamus. These cells generate action potentials that propagate down axons to the nerve endings in the pituitary; the endings contain large numbers of oxytocin-containing vesicles, which are released by exocytosis when the nerve terminals are depolarised.
Non-neural sources
Endogenous oxytocin concentrations in the brain have been found to be as much as 1000-fold higher than peripheral levels. Outside the brain, oxytocin-containing cells have been identified in several diverse tissues, including in females in the corpus luteum and the placenta; in males in the testicles' interstitial cells of Leydig; and in both sexes in the retina, the adrenal medulla, the thymus and the pancreas. The finding of significant amounts of this classically "neurohypophysial" hormone outside the central nervous system raises many questions regarding its possible importance in these diverse tissues.
The Leydig cells in some species have been shown to possess the biosynthetic machinery to manufacture testicular oxytocin de novo, to be specific, in rats (which can synthesize vitamin C endogenously), and in guinea pigs, which, like humans, require an exogenous source of vitamin C in their diets. Oxytocin is synthesized by corpora lutea of several species, including ruminants and primates. Along with estrogen, it is involved in inducing the endometrial synthesis of prostaglandin F2α to cause regression of the corpus luteum.
Evolution
Virtually all vertebrates have an oxytocin-like nonapeptide hormone that supports reproductive functions and a vasopressin-like nonapeptide hormone involved in water regulation. The two genes are usually located close to each other (less than 15,000 bases apart) on the same chromosome, and are transcribed in opposite directions (however, in fugu, the homologs are further apart and transcribed in the same direction). The two genes are believed to result from a gene duplication event; the ancestral gene is estimated to be about 500 million years old and is found in cyclostomata (modern members of the Agnatha).
A 2023 study found that zebrafish utilize oxytocin in reaction to the fear of other fish. It found that zebrafish that have had oxytocin production removed by gene editing cannot respond to the fear of other fish. When oxytocin is injected back into the fish, they react again in a way that suggests they may have empathy in regards to this emotion. Furthermore, because the same regions of the brain are involved as in mammals, the study suggests oxytocin-based empathy may have evolved from a common ancestor many millions of years ago.
Biological function
Oxytocin has peripheral (hormonal) actions and also has actions in the brain. Its actions are mediated by specific oxytocin receptors. The oxytocin receptor is a G-protein-coupled receptor, OT-R, which requires magnesium and cholesterol and is expressed in myometrial cells. It belongs to the rhodopsin-type (class I) group of G-protein-coupled receptors.
Studies have looked at oxytocin's role in various behaviors, including orgasm, social recognition, pair bonding, anxiety, in-group bias, situational lack of honesty, autism, and maternal behaviors. Oxytocin is believed to have a significant role in social learning. There are indicators that oxytocin may help to decrease noise in the brain's auditory system, increase perception of social cues and support more targeted social behavior. It may also enhance reward responses. However, its effects may be influenced by context, such as the presence of familiar or unfamiliar individuals. In addition to its oxytocin receptor agonism, oxytocin has been found to act as a PAM of the μ- and κ-opioid receptors and this may be involved in its analgesic effects.
Physiological
The peripheral actions of oxytocin mainly reflect secretion from the pituitary gland. The behavioral effects of oxytocin are thought to reflect release from centrally projecting oxytocin neurons, different from those that project to the pituitary gland, or that are collaterals from them. Oxytocin receptors are expressed by neurons in many parts of the brain and spinal cord, including the amygdala, ventromedial hypothalamus, septum, nucleus accumbens, and brainstem, although the distribution differs markedly between species. Furthermore, the distribution of these receptors changes during development and has been observed to change after parturition in the montane vole.
Milk ejection reflex/Letdown reflex: in lactating (breastfeeding) mothers, oxytocin acts at the mammary glands, causing milk to be 'let down' into lactiferous ducts, from where it can be excreted via the nipple. Suckling by the infant at the nipple is relayed by spinal nerves to the hypothalamus. The stimulation causes neurons that make oxytocin to fire action potentials in intermittent bursts; these bursts result in the secretion of pulses of oxytocin from the neurosecretory nerve terminals of the pituitary gland.
Uterine contraction: important for cervical dilation before birth, oxytocin causes contractions during the second and third stages of labor. Oxytocin release during breastfeeding causes mild but often painful contractions during the first few weeks of lactation. This also serves to assist the uterus in clotting the placental attachment point postpartum. However, in knockout mice lacking the oxytocin receptor, reproductive behavior and birth are normal.
In male rats, oxytocin may induce erections. A burst of oxytocin is released during ejaculation in several species, including human males; its suggested function is to stimulate contractions of the reproductive tract, aiding sperm release.
Human sexual response: Oxytocin levels in plasma rise during sexual stimulation and orgasm. At least two uncontrolled studies have found increases in plasma oxytocin at orgasm in both men and women. Plasma oxytocin levels are increased around the time of self-stimulated orgasm and are still higher than baseline when measured five minutes after self arousal. The authors of one of these studies speculated that oxytocin's effects on muscle contractibility may facilitate sperm and egg transport.
In a study measuring oxytocin serum levels in women before and after sexual stimulation, the author suggests it serves an important role in sexual arousal. This study found genital tract stimulation resulted in increased oxytocin immediately after orgasm. Another study reported increases of oxytocin during sexual arousal could be in response to nipple/areola, genital, and/or genital tract stimulation as confirmed in other mammals. Murphy et al. (1987), studying men, found that plasma oxytocin levels remain unchanged during sexual arousal, but that levels increase sharply after ejaculation, returning to baseline levels within 30 minutes. In contrast, vasopressin was increased during arousal but returned to baseline at the time of ejaculation. The study concludes that (in males) vasopressin is secreted during arousal, while oxytocin is only secreted after ejaculation. A more recent study of men found an increase in plasma oxytocin immediately after orgasm, but only in a portion of their sample that did not reach statistical significance. The authors noted these changes "may simply reflect contractile properties on reproductive tissue".
Due to its similarity to vasopressin, it can reduce the excretion of urine slightly, and so it can be classified as an antidiuretic. In several species, oxytocin can stimulate sodium excretion from the kidneys (natriuresis), and in humans high doses can result in low sodium levels (hyponatremia).
Cardiac effects: oxytocin and oxytocin receptors are also found in the heart in some rodents, and the hormone may play a role in the embryonal development of the heart by promoting cardiomyocyte differentiation. However, the absence of either oxytocin or its receptor in knockout mice has not been reported to produce cardiac insufficiencies.
Modulation of hypothalamic-pituitary-adrenal axis activity: oxytocin, under certain circumstances, indirectly inhibits release of adrenocorticotropic hormone and cortisol and, in those situations, may be considered an antagonist of vasopressin.
Preparing fetal neurons for delivery (in rats): crossing the placenta, maternal oxytocin reaches the fetal brain and induces a switch in the action of neurotransmitter GABA from excitatory to inhibitory on fetal cortical neurons. This silences the fetal brain for the period of delivery and reduces its vulnerability to hypoxic damage.
Feeding: a 2012 paper suggested that oxytocin neurons in the para-ventricular hypothalamus in the brain may play a key role in suppressing appetite under normal conditions and that other hypothalamic neurons may trigger eating via inhibition of these oxytocin neurons. This population of oxytocin neurons is absent in Prader-Willi syndrome, a genetic disorder that leads to uncontrollable feeding and obesity, and may play a key role in its pathophysiology. Research on the oxytocin-related neuropeptide asterotocin in starfish also showed that in echinoderms, the chemical induces muscle relaxation, and in starfish specifically caused the organisms to evert their stomach and react as though feeding on prey, even when none were present.
Psychological
Autism: Oxytocin has been implicated in the etiology of autism, with one report suggesting autism is correlated to a mutation on the oxytocin receptor gene (OXTR). Studies involving Caucasian, Finnish and Chinese Han families provide support for the relationship of OXTR with autism. Autism may also be associated with an aberrant methylation of OXTR. However, evidence has shown that intranasal administration is likely insufficient to produce behavioural effects and it could be explained by publication bias and selective outcome reporting, impacting reproducibility of results. In addition, current discussion has challenged the intervention and stated that neurodivergent perspectives need to be considered, with focus being primarily on animal models, which have a lack of translational validity due to autism's complex social and communicative dimensions.
Protection of brain functions: Studies in rats have demonstrated that nasal application of oxytocin can alleviate impaired learning capabilities caused by restrained stress. The authors attributed this effect to an improved hippocampal response in Brain-Derived Neurotrophic Factor (BDNF) being observed. Accordingly, oxytocin has been shown to promote neural growth in the hippocampus in rats even during swim stress or glucocorticoid administration. In a mouse model of early onset of Alzheimer's, the administration of oxytocin by a gel particularly designed to make the peptide accessible for the brain, the cognitive decline and hippocampal atrophy of these mice were delayed. Moreover, the amyloid β-protein deposit and nerve cell apoptosis were retarded. An observed inhibitory impact by oxytocin on the inflammatory activity of the microglia was proposed to be an important factor.
Bonding
In the prairie vole, oxytocin released into the brain of the female during sexual activity is important for forming a pair bond with her sexual partner. Vasopressin appears to have a similar effect in males. Oxytocin has a role in social behaviors in many species, so it likely also does in humans. In a 2003 study, both humans and dog oxytocin levels in the blood rose after a five to 24 minute petting session. This possibly plays a role in the emotional bonding between humans and dogs.
Maternal behavior: Female rats given oxytocin antagonists after giving birth do not exhibit typical maternal behavior. By contrast, virgin female sheep show maternal behavior toward foreign lambs upon cerebrospinal fluid infusion of oxytocin, which they would not do otherwise. Oxytocin is involved in the initiation of human maternal behavior, not its maintenance; for example, it is higher in mothers after they interact with unfamiliar children rather than their own.
Human ingroup bonding: Oxytocin can increase positive attitudes, such as bonding, toward individuals classified as "in-group" members, whereas other individuals become classified as "out-group" members. Oxytocin has also been implicated in lying when lying would prove beneficial to other in-group members. In a study where such a relationship was examined, it was found that when individuals were administered oxytocin, rates of dishonesty in the participants' responses increased for their in-group members when a beneficial outcome for their group was expected. Both of these examples show the tendency of individuals to act in ways that benefit those considered to be members of their social group, or in-group.
Decreased oxytocin & receptor expression has been associated with aggressive behavior in aggressive-impulsive disorders.
Oxytocin is not only correlated with the preferences of individuals to associate with members of their own group, but it is also evident during conflicts between members of different groups. During conflict, individuals receiving nasally administered oxytocin demonstrate more frequent defense-motivated responses toward in-group members than out-group members. Further, oxytocin was correlated with participant desire to protect vulnerable in-group members, despite that individual's attachment to the conflict. Similarly, it has been demonstrated that when oxytocin is administered, individuals alter their subjective preferences in order to align with in-group ideals over out-group ideals. These studies demonstrate that oxytocin is associated with intergroup dynamics. Further, oxytocin influences the responses of individuals in a particular group to those of another group. The in-group bias is evident in smaller groups; however, it can also be extended to groups as large as one's entire country leading toward a tendency of strong national zeal. A study done in the Netherlands showed that oxytocin increased the in-group favoritism of their nation while decreasing acceptance of members of other ethnicities and foreigners. People also show more affection for their country's flag while remaining indifferent to other cultural objects when exposed to oxytocin. It has thus been hypothesized that this hormone may be a factor in xenophobic tendencies secondary to this effect. Thus, oxytocin appears to affect individuals at an international level where the in-group becomes a specific "home" country and the out-group grows to include all other countries.
Drugs
Drug interaction: According to several studies in animals, oxytocin inhibits the development of tolerance to various addictive drugs (opiates, cocaine, alcohol), and reduces withdrawal symptoms. MDMA (ecstasy) may increase feelings of love, empathy, and connection to others by stimulating oxytocin activity primarily via activation of serotonin 5-HT1A receptors, if initial studies in animals apply to humans. The anxiolytic drug buspirone may produce some of its effects via 5-HT1A receptor-induced oxytocin stimulation as well.
Addiction vulnerability: Concentrations of endogenous oxytocin can impact the effects of various drugs and one's susceptibility to substance use disorders, with higher concentrations associated with lower susceptibility. The status of the endogenous oxytocin system can enhance or reduce susceptibility to addiction through its bidirectional interaction with numerous systems, including the dopamine system, the hypothalamic–pituitary–adrenal axis and the immune system. Individual differences in the endogenous oxytocin system based on genetic predisposition, gender and environmental influences, may therefore affect addiction vulnerability. Oxytocin may be related to the place conditioning behaviors observed in habitual drug abusers.
Fear and anxiety
Oxytocin is typically remembered for the effect it has on prosocial behaviors, such as its role in facilitating trust and attachment between individuals. However, oxytocin has a more complex role than solely enhancing prosocial behaviors. There is consensus that oxytocin modulates fear and anxiety; that is, it does not directly elicit fear or anxiety. Two dominant theories explain the role of oxytocin in fear and anxiety. One theory states that oxytocin increases approach/avoidance to certain social stimuli and the second theory states that oxytocin increases the salience of certain social stimuli, causing animals (including humans) to pay closer attention to socially relevant stimuli.
Nasally administered oxytocin has been reported to reduce fear, possibly by inhibiting the amygdala (which is thought to be responsible for fear responses). Indeed, studies in rodents have shown oxytocin can efficiently inhibit fear responses by activating an inhibitory circuit within the amygdala. Some researchers have argued oxytocin has a general enhancing effect on all social emotions, since intranasal administration of oxytocin also increases envy and Schadenfreude. Individuals who receive an intranasal dose of oxytocin identify facial expressions of disgust more quickly than individuals who do not receive oxytocin. Facial expressions of disgust are evolutionarily linked to the idea of contagion. Thus, oxytocin increases the salience of cues that imply contamination, which leads to a faster response because these cues are especially relevant for survival. In another study, after administration of oxytocin, individuals displayed an enhanced ability to recognize expressions of fear compared to the individuals who received the placebo. Oxytocin modulates fear responses by enhancing the maintenance of social memories. Rats who are genetically modified to have a surplus of oxytocin receptors display a greater fear response to a previously conditioned stressor. Oxytocin enhances the aversive social memory, leading the rat to display a greater fear response when the aversive stimulus is encountered again.
Mood and depression
Oxytocin produces antidepressant-like effects in animal models of depression, and a deficit of it may be involved in the pathophysiology of depression in humans. The antidepressant-like effects of oxytocin are not blocked by a selective antagonist of the oxytocin receptor, suggesting that these effects are not mediated by the oxytocin receptor. In accordance, unlike oxytocin, the selective non-peptide oxytocin receptor agonist WAY-267,464 does not produce antidepressant-like effects, at least in the tail suspension test. In contrast to WAY-267,464, carbetocin, a close analogue of oxytocin and peptide oxytocin receptor agonist, notably does produce antidepressant-like effects in animals. As such, the antidepressant-like effects of oxytocin may be mediated by modulation of a different target, perhaps the vasopressin V1A receptor where oxytocin is known to weakly bind as an agonist.
Oxytocin mediates the antidepressant-like effects of sexual activity. A drug for sexual dysfunction, sildenafil enhances electrically evoked oxytocin release from the pituitary gland. In accordance, it may have promise as an antidepressant.
Sex differences
It has been shown that oxytocin differentially affects males and females. Females who are administered oxytocin are overall faster in responding to socially relevant stimuli than males who received oxytocin. Additionally, after the administration of oxytocin, females show increased amygdala activity in response to threatening scenes; however, males do not show increased amygdala activation. This phenomenon can be explained by looking at the role of gonadal hormones, specifically estrogen, which modulate the enhanced threat processing seen in females. Estrogen has been shown to stimulate the release of oxytocin from the hypothalamus and promote receptor binding in the amygdala.
It has also been shown that testosterone directly suppresses oxytocin in mice. This has been hypothesized to have evolutionary significance. With oxytocin suppressed, activities such as hunting and attacking invaders would be less mentally difficult as oxytocin is strongly associated with empathy.
Social
Because oxytocin plays a role in social bonding, maternal behaviors and emotional connections between people, it is also informally referred to as the "love hormone". This term is not a medical or scientific name but is often used to describe oxytocin's effects on human behavior and emotions.
Affecting generosity by increasing empathy during perspective taking: In a neuroeconomics experiment, intranasal oxytocin increased generosity in the Ultimatum Game by 80%, but had no effect in the Dictator Game that measures altruism. Perspective-taking is not required in the Dictator Game, but the researchers in this experiment explicitly induced perspective-taking in the Ultimatum Game by not identifying to participants into which role they would be placed. Serious methodological questions have arisen, however, with regard to the role of oxytocin in trust and generosity. Empathy in healthy males has been shown to be increased after intranasal oxytocin This is most likely due to the effect of oxytocin in enhancing eye gaze. There is some discussion about which aspect of empathy oxytocin might alter – for example, cognitive vs. emotional empathy. While studying wild chimpanzees, it was noted that after a chimpanzee shared food with a non-kin related chimpanzee, the subjects' levels of oxytocin increased, as measured through their urine. In comparison to other cooperative activities between chimpanzees that were monitored including grooming, food sharing generated higher levels of oxytocin. This comparatively higher level of oxytocin after food sharing parallels the increased level of oxytocin in nursing mothers, sharing nutrients with their kin.
Trust is increased by oxytocin. Study found that with the oxytocin nasal spray, people place more trust to strangers in handling their money. Disclosure of emotional events is a sign of trust in humans. When recounting a negative event, humans who receive intranasal oxytocin share more emotional details and stories with more emotional significance. Humans also find faces more trustworthy after receiving intranasal oxytocin. In a study, participants who received intranasal oxytocin viewed photographs of human faces with neutral expressions and found them to be more trustworthy than those who did not receive oxytocin. This may be because oxytocin reduces the fear of social betrayal in humans. Even after experiencing social alienation by being excluded from a conversation, humans who received oxytocin scored higher in trust on the Revised NEO Personality Inventory. Moreover, in a risky investment game, experimental subjects given nasally administered oxytocin displayed "the highest level of trust" twice as often as the control group. Subjects who were told they were interacting with a computer showed no such reaction, leading to the conclusion that oxytocin was not merely affecting risk aversion. When there is a reason to be distrustful, such as experiencing betrayal, differing reactions are associated with oxytocin receptor gene (OXTR) differences. Those with the CT haplotype experience a stronger reaction, in the form of anger, to betrayal.
Romantic attachment: In some studies, high levels of plasma oxytocin have been correlated with romantic attachment. For example, if a couple is separated for a long period of time, anxiety can increase due to the lack of physical affection. Oxytocin may aid romantically attached couples by increasing feelings of anxiety during separation.
Group-serving dishonesty/deception: In a carefully controlled study exploring the biological roots of immoral behavior, oxytocin was shown to promote dishonesty when the outcome favored the group to which an individual belonged instead of just the individual.
Oxytocin affects social distance between adult males and females, and may be responsible at least in part for romantic attraction and subsequent monogamous pair bonding. An oxytocin nasal spray caused men in a monogamous relationship, but not single men, to increase the distance between themselves and an attractive woman during a first encounter by 10 to 15 centimeters. The researchers suggested that oxytocin may help promote fidelity within monogamous relationships. For this reason, it is sometimes referred to as the "bonding hormone". There is some evidence that oxytocin promotes ethnocentric behavior, incorporating the trust and empathy of in-groups with their suspicion and rejection of outsiders. Furthermore, genetic differences in the oxytocin receptor gene (OXTR) have been associated with maladaptive social traits such as aggressive behavior.
Social behavior and wound healing: Oxytocin is also thought to modulate inflammation by decreasing certain cytokines. Thus, the increased release in oxytocin following positive social interactions has the potential to improve wound healing. A study by Marazziti and colleagues used heterosexual couples to investigate this possibility. They found increases in plasma oxytocin following a social interaction were correlated with faster wound healing. They hypothesized this was due to oxytocin reducing inflammation, thus allowing the wound to heal more quickly. This study provides preliminary evidence that positive social interactions may directly influence aspects of health.
According to a study published in 2014, silencing of oxytocin receptor interneurons in the medial prefrontal cortex (mPFC) of female mice resulted in loss of social interest in male mice during the sexually receptive phase of the estrous cycle. Oxytocin evokes feelings of contentment, reductions in anxiety, and feelings of calmness and security when in the company of the mate. This suggests oxytocin may be important for the inhibition of the brain regions associated with behavioral control, fear, and anxiety, thus allowing orgasm to occur. Research has also demonstrated that oxytocin can decrease anxiety and protect against stress, particularly in combination with social support. It is found that endocannabinoid signaling mediates oxytocin-driven social reward. During a 2008 study, a lack of oxytocin in mice was associated with abnormalities in emotional behavior. Another study conducted in 2014 saw similar results with a variation in the oxytocin receptor connected with dopamine transport and how levels of oxytocin are dependent on the levels of dopamine transporter levels. One study explored the effects of low levels of oxytocin and the other on possible explanation of what affects oxytocin receptors. As a lack of social skills and proper emotional behavior are common signs of Autism, low levels of oxytocin could become a new sign for individuals that fall into the Autism Spectrum.
Chemistry
Oxytocin is a peptide of nine amino acids (a nonapeptide) in the sequence cysteine-tyrosine-isoleucine-glutamine-asparagine-cysteine-proline-leucine-glycine-amide (Cys – Tyr – Ile – Gln – Asn – Cys – Pro – Leu – Gly – NH2, or CYIQNCPLG-NH2); its C-terminus has been converted to a primary amide and a disulfide bridge joins the cysteine moieties. Oxytocin has a molecular mass of 1007 Da, and one international unit (IU) of oxytocin is the equivalent of 1.68 μg of pure peptide.
While the structure of oxytocin is highly conserved in placental mammals, a novel structure of oxytocin was reported in 2011 in marmosets, tamarins, and other new world primates. Genomic sequencing of the gene for oxytocin revealed a single in-frame mutation (thymine for cytosine) which results in a single amino acid substitution at the 8-position (proline for leucine). Since this original Lee et al. paper, two other laboratories have confirmed Pro8-OT and documented additional oxytocin structural variants in this primate taxon. Vargas-Pinilla et al. sequenced the coding regions of the OXT gene in other genera in new world primates and identified the following variants in addition to Leu8- and Pro8-OT: Ala8-OT, Thr8-OT, and Val3/Pro8-OT. Ren et al. identified a variant further, Phe2-OT in howler monkeys.
Recent advances in analytical instrumental techniques highlighted the importance of liquid chromatography (LC) coupled with mass spectrometry (MS) for measuring oxytocin levels in various samples derived from biological sources. Most of these studies optimized the oxytocin quantification in electrospray ionization (ESI) positive mode, using [M+H]+ as the parent ion at mass-to-charge ratio (m/z) 1007.4 and the fragment ions as diagnostic peaks at m/z 991.0, m/z 723.2 and m/z 504.2. These important ion selections paved the way for the development of current methods of oxytocin quantification using MS instrumentation.
The structure of oxytocin is very similar to that of vasopressin. Both are nonapeptides with a single disulfide bridge, differing only by two substitutions in the amino acid sequence (differences from oxytocin bolded for clarity): Cys – Tyr – Phe – Gln – Asn – Cys – Pro – Arg – Gly – NH2. Oxytocin and vasopressin were isolated and their total synthesis reported in 1954, work for which Vincent du Vigneaud was awarded the 1955 Nobel Prize in Chemistry with the citation: "for his work on biochemically important sulphur compounds, especially for the first synthesis of a polypeptide hormone."
Oxytocin and vasopressin are the only known hormones released by the human posterior pituitary gland to act at a distance. However, oxytocin neurons make other peptides, including corticotropin-releasing hormone and dynorphin, for example, that act locally. The magnocellular neurosecretory cells that make oxytocin are adjacent to magnocellular neurosecretory cells that make vasopressin. These are large neuroendocrine neurons which are excitable and can generate action potentials.
In medicine
Small-molecule oxytocin receptor agonists like LIT-001 may prove to be useful in the treatment of social deficits, for instance in autism.
| Biology and health sciences | Biochemistry and molecular biology | null |
222324 | https://en.wikipedia.org/wiki/Archegonium | Archegonium | An archegonium (: archegonia), from the Ancient Greek ἀρχή ("beginning") and γόνος ("offspring"), is a multicellular structure or organ of the gametophyte phase of certain plants, producing and containing the ovum or female gamete. The corresponding male organ is called the antheridium. The archegonium has a long neck canal or venter and a swollen base. Archegonia are typically located on the surface of the plant thallus, although in the hornworts they are embedded.
Bryophytes
In bryophytes and other cryptogams, sperm reach the archegonium by swimming in water films, whereas in Pinophyta and angiosperms, the pollen are delivered by wind or animal vectors and the sperm are delivered by means of a pollen tube.
In the moss Physcomitrella patens, archegonia are not embedded but are located on top of the leafy gametophore (s. Figure). The Polycomb protein FIE is expressed in the unfertilized egg cell (right) as the blue colour after GUS staining reveals. Soon after fertilisation, the FIE gene is inactivated (the blue colour is no longer visible, left) in the young embryo.
Gymnosperms
They are much-reduced and embedded in the megagametophytes of gymnosperms. The term is not used for angiosperms or the gnetophytes Gnetum and Welwitschia because the megagametophyte is reduced to just a few cells, one of which differentiates into the egg cell. The function of surrounding the gamete is assumed in large part by diploid cells of the megasporangium (nucellus) inside the ovule. Gymnosperms have their archegonium formed after pollination inside female conifer cones (megastrobili).
| Biology and health sciences | Plant reproduction | Biology |
222367 | https://en.wikipedia.org/wiki/Multivalued%20function | Multivalued function | In mathematics, a multivalued function, multiple-valued function, many-valued function, or multifunction, is a function that has two or more values in its range for at least one point in its domain. It is a set-valued function with additional properties depending on context; some authors do not distinguish between set-valued functions and multifunctions, but English Wikipedia currently does, having a separate article for each.
A multivalued function of sets f : X → Y is a subset
Write f(x) for the set of those y ∈ Y with (x,y) ∈ Γf. If f is an ordinary function, it is a multivalued function by taking its graph
They are called single-valued functions to distinguish them.
Distinction from set-valued relations
Although other authors may distinguish them differently (or not at all), Wriggers and Panatiotopoulos (2014) distinguish multivalued functions from set-valued relations (also called set-valued functions) by the fact that multivalued functions only take multiple values at finitely (or denumerably) many points, and otherwise behave like a function. Geometrically, this means that the graph of a multivalued function is necessarily a line of zero area that doesn't loop, while the graph of a set-valued relation may contain solid filled areas or loops.
Motivation
The term multivalued function originated in complex analysis, from analytic continuation. It often occurs that one knows the value of a complex analytic function in some neighbourhood of a point . This is the case for functions defined by the implicit function theorem or by a Taylor series around . In such a situation, one may extend the domain of the single-valued function along curves in the complex plane starting at . In doing so, one finds that the value of the extended function at a point depends on the chosen curve from to ; since none of the new values is more natural than the others, all of them are incorporated into a multivalued function.
For example, let be the usual square root function on positive real numbers. One may extend its domain to a neighbourhood of in the complex plane, and then further along curves starting at , so that the values along a given curve vary continuously from . Extending to negative real numbers, one gets two opposite values for the square root—for example for —depending on whether the domain has been extended through the upper or the lower half of the complex plane. This phenomenon is very frequent, occurring for th roots, logarithms, and inverse trigonometric functions.
To define a single-valued function from a complex multivalued function, one may distinguish one of the multiple values as the principal value, producing a single-valued function on the whole plane which is discontinuous along certain boundary curves. Alternatively, dealing with the multivalued function allows having something that is everywhere continuous, at the cost of possible value changes when one follows a closed path (monodromy). These problems are resolved in the theory of Riemann surfaces: to consider a multivalued function as an ordinary function without discarding any values, one multiplies the domain into a many-layered covering space, a manifold which is the Riemann surface associated to .
Inverses of functions
If f : X → Y is an ordinary function, then its inverse is the multivalued function
defined as Γf, viewed as a subset of X × Y. When f is a differentiable function between manifolds, the inverse function theorem gives conditions for this to be single-valued locally in X.
For example, the complex logarithm log(z) is the multivalued inverse of the exponential function ez : C → C×, with graph
It is not single valued, given a single w with w = log(z), we have
Given any holomorphic function on an open subset of the complex plane C, its analytic continuation is always a multivalued function.
Concrete examples
Every real number greater than zero has two real square roots, so that square root may be considered a multivalued function. For example, we may write ; although zero has only one square root, .
Each nonzero complex number has two square roots, three cube roots, and in general n nth roots. The only nth root of 0 is 0.
The complex logarithm function is multiple-valued. The values assumed by for real numbers and are for all integers .
Inverse trigonometric functions are multiple-valued because trigonometric functions are periodic. We have As a consequence, arctan(1) is intuitively related to several values: /4, 5/4, −3/4, and so on. We can treat arctan as a single-valued function by restricting the domain of tan x to – a domain over which tan x is monotonically increasing. Thus, the range of arctan(x) becomes . These values from a restricted domain are called principal values.
The antiderivative can be considered as a multivalued function. The antiderivative of a function is the set of functions whose derivative is that function. The constant of integration follows from the fact that the derivative of a constant function is 0.
Inverse hyperbolic functions over the complex domain are multiple-valued because hyperbolic functions are periodic along the imaginary axis. Over the reals, they are single-valued, except for arcosh and arsech.
These are all examples of multivalued functions that come about from non-injective functions. Since the original functions do not preserve all the information of their inputs, they are not reversible. Often, the restriction of a multivalued function is a partial inverse of the original function.
Branch points
Multivalued functions of a complex variable have branch points. For example, for the nth root and logarithm functions, 0 is a branch point; for the arctangent function, the imaginary units i and −i are branch points. Using the branch points, these functions may be redefined to be single-valued functions, by restricting the range. A suitable interval may be found through use of a branch cut, a kind of curve that connects pairs of branch points, thus reducing the multilayered Riemann surface of the function to a single layer. As in the case with real functions, the restricted range may be called the principal branch of the function.
Applications
In physics, multivalued functions play an increasingly important role. They form the mathematical basis for Dirac's magnetic monopoles, for the theory of defects in crystals and the resulting plasticity of materials, for vortices in superfluids and superconductors, and for phase transitions in these systems, for instance melting and quark confinement. They are the origin of gauge field structures in many branches of physics.
| Mathematics | Functions: General | null |
222412 | https://en.wikipedia.org/wiki/Numbat | Numbat | The numbat (Myrmecobius fasciatus), also known as the noombat or walpurti, is an insectivorous marsupial. It is diurnal and its diet consists almost exclusively of termites.
The species was once widespread across southern Australia, but is now restricted to several small colonies in Western Australia. It is therefore considered an endangered species and protected by conservation programs. Numbats were recently re-introduced to fenced reserves in South Australia and New South Wales. The numbat is the faunal emblem of Western Australia.
Taxonomy
The numbat genus Myrmecobius is the sole member of the family Myrmecobiidae, one of four families that make up the order Dasyuromorphia, the Australian marsupial carnivores.
The species is not closely related to other extant marsupials; the current arrangement in the order Dasyuromorphia places its monotypic family with the diverse and carnivorous species of Dasyuridae. Genetic studies have shown the ancestors of the numbat diverged from other marsupials between 32 and 42 million years ago, during the late Eocene.
Two subspecies have been described, but one of these—the rusty coloured Myrmecobius fasciatus rufus Finlayson, 1933,—has been extinct since at least the 1960s, and only the nominate subspecies (M. fasciatus fasciatus) is extant. The population described by Finlayson occurred in the arid central regions of South Australia, and he thought they had once extended to the coast. The separation to subspecies was not recognised in the national census of Australian mammals, following W. D. L. Ride and others. As its name implies, M. fasciatus rufus had a more reddish coat than the surviving population. Only a very small number of fossil specimens are known, the oldest dating back to the Pleistocene, and no other species from the same family have been identified.
The following is a phylogenetic tree based on mitochondrial genome sequences:
Placement of the family within the order of dasyuromorphs may be summarised as
Order Dasyuromorphia
Family Thylacinidae
Family Dasyuridae (72 species in 20 genera)
Family Myrmecobiidae
Genus Myrmecobius
Species Myrmecobius fasciatus
Family †Malleodectidae
The common names are adopted from the extant names at the time of English colonisation, numbat, from the Nyungar language of southwest Australia, and walpurti, the name in the Pitjantjatjara dialect. The orthography and pronunciation of the Nyungar name is regularised, following a survey of published sources and contemporary consultation that resulted in the name noombat, pronounced noom'bat.
Other names include banded anteater and marsupial anteater.
Description
The numbat is a small, distinctively-striped animal between long, including the tail, with a finely pointed muzzle and a prominent, bushy tail about the same length as its body. Colour varies considerably, from soft grey to reddish-brown, often with an area of brick red on the upper back, and always with a conspicuous black stripe running from the tip of the muzzle through the eye to the base of the small, round-tipped ear. Between four and eleven white stripes cross the animal's hindquarters, which gradually become fainter towards the midback. The underside is cream or light grey, while the tail is covered with long, grey hair flecked with white. Weight varies between .
Unlike most other marsupials, the numbat is diurnal, largely because of the constraints of having a specialised diet without having the usual physical equipment for it. Most ecosystems with a generous supply of termites have a fairly large creature with powerful forelimbs bearing heavy claws. Numbats are not large, and they have five toes on the fore feet, and four on the hind feet. However, like other mammals that eat termites or ants, the numbat has a degenerate jaw with up to 50 very small, nonfunctional teeth, and although it is able to chew, rarely does so, because of the soft nature of its diet. Uniquely among terrestrial mammals, an additional cheek tooth is located between the premolars and molars; whether this represents a supernumerary molar tooth or a deciduous tooth retained into adult life is unclear. As a result, although not all individuals have the same dental formula, in general, it follows the unique pattern:
Like many ant- or termite-eating animals, the numbat has a long and narrow tongue coated with sticky saliva produced by large submandibular glands. A further adaptation to the diet is the presence of numerous ridges along the soft palate, which apparently help to scrape termites off the tongue so they can be swallowed. The digestive system is relatively simple, and lacks many of the adaptations found in other entomophagous animals, presumably because termites are easier to digest than ants, having a softer exoskeleton. Numbats are apparently able to gain a considerable amount of water from their diets, since their kidneys lack the usual specialisations for retaining water found in other animals living in their arid environment. Numbats also possess a sternal scent gland, which may be used for marking their territories.
Although the numbat finds termite mounds primarily using scent, it has the highest visual acuity of any marsupial, and, unusually for marsupials, has a high proportion of cone cells in the retina. These are both likely adaptations for its diurnal habits, and vision does appear to be the primary sense used to detect potential predators.
Distribution and habitat
Numbats were formerly widely distributed across southern Australia, from Western Australia to north-western New South Wales. However, their range has significantly decreased since the arrival of Europeans, and the species has survived only in two small patches of land in the Dryandra Woodland and the Tone-Perup Nature Reserve, both in Western Australia.
Today, numbats are naturally found only in areas of eucalypt forest, but they were once more widespread in other types of semiarid woodland, spinifex grassland, and in terrain dominated by sand dune. There are estimated to be fewer than 1,000 left in the wild.
After measures aimed at excluding feral cats, the number of numbats trapped during annual population surveys in the Dryandra Woodland had increased to 35 by November 2020, after recording just 10 in 2019 and 5 in 2018. There had not been so many numbats recorded since 36 were recorded in the 1990s.
The species has been successfully reintroduced into three fenced, feral predator-proof reserves in more varied environments; Yookamurra Sanctuary in the mallee of South Australia, Scotia Sanctuary in semi-arid NSW, and Western Australia's Mount Gibson Sanctuary. Reintroduction began at large fenced reserves in Mallee Cliffs National Park in NSW in December 2020, and on South Australia's Eyre Peninsula in 2022.
Attempted reintroductions of the species to fenced reserves in two other areas – one in the South Australian arid zone, near Roxby Downs, and the other in the northernmost part of its former range, at Newhaven Sanctuary in the Northern Territory – both failed.
There are plans to reintroduce the species to a managed and semi-fenced area of the southern Yorke Peninsula in South Australia, as part of the Marna Banggara (formerly Great Southern Ark) project.
Ecology and habits
Numbats are insectivores and subsist on a diet of termites (of the genera Heterotermes, Coptotermes, Amitermes, Microcerotermes, Termes, Paracapritermes, Nasutitermes, Tumulitermes, and Occasitermes). An adult numbat requires up to 20,000 termites each day. The only marsupial fully active by day, the numbat spends most of its time searching for termites. It digs them up from loose earth with its front claws and captures them with its long, sticky tongue. Despite its banded anteater name, it apparently does not intentionally eat ants; although the remains of ants have occasionally been found in numbat excreta, these belong to species that themselves prey on termites, so were presumably eaten accidentally, along with the main food. Known native predators include various reptiles and raptors, such as the carpet python, sand goanna, wedge-tailed eagle, collared sparrowhawk, brown goshawk, and the little eagle. They are also preyed upon by invasive red foxes and feral cats.
Adult numbats are solitary and territorial; an individual male or female establishes a territory of up to 1.5 square km (370 acres) early in life, and defends it from others of the same sex. The animal generally remains within that territory from then on; male and female territories overlap, and in the breeding season, males will venture outside their normal home ranges to find mates.
While the numbat has relatively powerful claws for its size, it is not strong enough to get at termites inside their concrete-like mounds, and so must wait until the termites are active. It uses a well-developed sense of smell to locate the shallow and unfortified underground galleries that termites construct between the nest and their feeding sites; these are usually only a short distance below the surface of the soil, and vulnerable to the numbat's digging claws.
The numbat synchronises its day with termite activity, which is temperature dependent: in winter, it feeds from midmorning to midafternoon; in summer, it rises earlier, takes shelter during the heat of the day, and feeds again in the late afternoon. Numbats are able to enter a state of torpor, which may last up to fifteen hours a day during the winter months.
At night, the numbat retreats to a nest, which can be in a log or tree hollow, or in a burrow, typically a narrow shaft long which terminates in a spherical chamber lined with soft plant material: grass, leaves, flowers, and shredded bark. The numbat is able to block the opening of its nest, with the thick hide of its rump, to prevent a predator being able to access the burrow.
Numbats have relatively few vocalisations, but have been reported to hiss, growl, or make a repetitive 'tut' sound when disturbed.
Reproduction
Numbats breed in February and March (late austral summer), normally producing one litter a year. They are able to produce a second if the first is lost. Gestation lasts 15 days, and results in the birth of four young. Unusual for marsupials, female numbats have no pouch, although the four teats are protected by a patch of crimped, golden hair and by the swelling of the surrounding abdomen and thighs during lactation.
The young are long at birth. They crawl immediately to the teats and remain attached until late July or early August, by which time they have grown to . They are long when they first develop fur, the patterning of the adult begins to appear once they reach . The young are left in a nest or carried on the mother's back after weaning, becoming fully independent by November. Females are sexually mature by the following summer, but males do not reach maturity for another year.
Conservation status
At the time of European colonisation, the numbat was found across western, central, and southern regions of Australia, extending as far east as New South Wales and Victorian state borders and as far north as the southwest corner of the Northern Territory. It was at home in a wide range of woodland and semiarid habitats. The deliberate release of the European red fox in the 19th century, however, is presumed to have wiped out the entire numbat population in Victoria, NSW, South Australia and the Northern Territory, and almost all numbats in Western Australia. By the late 1970s, the population was well under 1,000 individuals, concentrated in two small areas not far from Perth, at protected areas of the Dryandra forest and at Perup.
The population recognised and described as a subspecies by Finlayson, M. fasciatus rufus, is presumed to be extinct.
The first record of the species described it as beautiful, and its popular appeal led to its selection as the faunal emblem of the state of Western Australia and initiated efforts to conserve it from extinction.
The two small Western Australia populations apparently were able to survive because both areas have many hollow logs that may serve as refuge from predators. Being diurnal, the numbat is much more vulnerable to predation than most other marsupials of a similar size: its natural predators include the little eagle, brown goshawk, collared sparrowhawk, and carpet python. When the Western Australia government instituted an experimental program of fox baiting at Dryandra (one of the two remaining sites), numbat sightings increased by a factor of 40.
An intensive research and conservation program since 1980 has succeeded in increasing the numbat population substantially, and reintroductions to fox-free areas have begun. Perth Zoo is very closely involved in breeding this native species in captivity for release into the wild. Despite the encouraging degree of success so far, the numbat remains at considerable risk of extinction and is classified as an endangered species.
Since 2006, Project Numbat volunteers have helped to save the numbat from extinction. One of Project Numbat's main objectives is to raise funds that go towards conservation projects, and to raise awareness through presentations held by volunteers at schools, community groups and events.
Numbats can be successfully reintroduced into areas of their former range if protected from introduced predators.
Early records
The numbat first became known to Europeans in 1831. It was discovered by an exploration party exploring the Avon Valley under the leadership of Robert Dale. George Fletcher Moore, who was a member of the expedition, drew a picture in his diary on 22 September 1831, and recounted the discovery:
and the following day:
The first classification of specimens was published by George Robert Waterhouse, describing the species in 1836 and the family in 1841.
Myrmecobius fasciatus was included in the first part of John Gould's The Mammals of Australia, issued in 1845, with a plate by H. C. Richter illustrating the species.
| Biology and health sciences | Marsupials | Animals |
222422 | https://en.wikipedia.org/wiki/Dasyuromorphia | Dasyuromorphia | Dasyuromorphia (, meaning "hairy tail" in Greek) is an order comprising most of the Australian carnivorous marsupials, including quolls, dunnarts, the numbat, the Tasmanian devil, and the extinct thylacine. In Australia, the exceptions include the omnivorous bandicoots (order Peramelemorphia) and the marsupial moles (which eat meat but are very different and are now accorded an order of their own, Notoryctemorphia). Numerous South American species of marsupials (orders Didelphimorphia, Paucituberculata, and Microbiotheria) are also carnivorous, as were some extinct members of the order Diprotodontia, including extinct kangaroos (such as Ekaltadeta and Propleopus) and thylacoleonids, and some members of the partially extinct clade Metatheria and all members of the extinct superorder Sparassodonta.
The order contains four families: one with just a single living species (the numbat), two with only extinct species (including the thylacine and Malleodectes), and one, the Dasyuridae, with 73 extant species.
Characteristics
Unlike herbivores, which tend to become highly specialized for particular ecological niches and diversify greatly in form, carnivores tend to be broadly similar to one another, certainly on the level of gross external form. Just as Northern Hemisphere carnivores like cats, mongooses, foxes and weasels are much more alike in structure than, for example, camels, goats, pigs and giraffes, so too are the marsupial predators constrained to retain general-purpose, look-alike forms—forms which mirror those of placental carnivores. The names given to them by early European settlers reflect this: the thylacine was called the Tasmanian tiger or Tasmanian wolf, quolls were called native cats or native foxes, and so on.
The primary specialisation among marsupial predators is that of size: prior to the massive environmental changes that came about with the arrival of humans about 50,000 years ago, there were several very large carnivores, none of them members of the Dasyuromorphia and all of them now extinct. Those that survived into historical times ranged from the wolf-sized thylacine to the tiny long-tailed planigale which at 4 to 6 grams is less than half the size of a mouse. Most, however, tend towards the lower end of the size scale, typically between about 15 or 20 grams and about 2 kilograms, or from the size of a domestic mouse to that of a small domestic cat.
Phylogeny
| Biology and health sciences | Marsupials | null |
1757261 | https://en.wikipedia.org/wiki/Gouldian%20finch | Gouldian finch | The Gouldian finch (Chloebia gouldiae), also known as the Gould's finch or the rainbow finch, is a colourful passerine bird that is native to Australia.
Taxonomy
The Gouldian finch was described by British ornithologist John Gould in 1844 as Amadina gouldiae, in honour of his deceased wife Elizabeth. Specimens of the bird were sent to him by British naturalist Benjamin Bynoe, although they had been described some years before by French naturalists Jacques Bernard Hombron and Honoré Jacquinot. It is also known as the rainbow finch, Gould's finch, or sometimes just Gould. The Gouldian finch is sister to the parrotfinches in the genus Erythrura.
Description
Both sexes are brightly coloured with black, green, yellow, and red markings. The females tend to be less brightly coloured. One major difference between the sexes is that the male's chest is purple, while the female's is a lighter mauve.
Gouldian finches are about long. Their heads may be red, black, or yellow. Formerly considered three different kinds of finches, it is now known that these are colour variants of one species that exist in the wild. Selective breeding has also developed mutations (blue, yellow and silver instead of a green back) in both body and breast colour.
There are several "prominent rounded tubercles" with an "opalescent lustre" at the back of the gape. These tubercles are commonly (and incorrectly) described as phosphorescent in spite of much scientific evidence to the contrary. It is believed that these tubercles simply reflect light and are not luminescent.
Distribution and habitat
Gouldian finches are native to northern Australia, in particular the Kimberley and Northern Territory.
Prior to the Australian government's ban on the export of Australian fauna, Gouldian finches were exported worldwide, which has resulted in viable captive breeding populations being held in many countries.
Conservation status
This species has been considered an Endangered species by the Australian Government in the last two "Endangered Species Act"s - the Australian Endangered Species Protection Act (ESPA) of 1992 and the Environment Protection and Biodiversity Conservation (EPBC) Act, 1999. Its status on the IUCN Red List is currently Least concern, but it was considered to be endangered previously: Threatened in 1988, EN in six assessments between 1994 and 2008, and Near threatened in three assessments between 2012 and 2016.
The number of Gouldian finches in the wild decreased dramatically in the 20th century due to human-caused habitat loss. The population went from hundreds of thousands in the early 20th century to 2,500 or fewer by the 1980s. The current estimated population continues to be 2,500 or fewer birds. Early research suggested that a parasite called the air sac mite was responsible for the species' decline, but the mite is no longer considered a major factor. The primary threat to wild Gouldian finch populations is an increase in extensive wildfires in the late dry season of its native habitat, which negatively impacts the availability of both tree hollows for breeding, and the seeds that comprise the bulk of the Gouldian finch's diet. Cyclones and climate change have also negatively impacted tree hollow availability in the Northern Territory.
Behaviour
Outside the breeding season, Gouldian finches often join mixed flocks consisting of long-tailed finches and masked finches. Flocks can consist of up to 1,000–2,000 individuals. During the breeding season, they are normally found on rough scree slopes where vegetation is sparse. In the dry season, they are much more nomadic and will move to wherever their food and water can be found.
Feeding
Like other finches, the Gouldian finch is a seed eater. They eat up to 30% of their bodyweight each day. During the breeding season, Gouldian finches mainly feed on ripe and half-ripe grass seeds of sorghum. During the dry season, they mainly forage on the ground for seeds. During the wet season, spinifex grass seed (Triodia sp.) is an important part of their diet. So far Gouldians have been recorded eating six different species of grass seed, but researchers have yet to find evidence of insect consumption.
Breeding
Gouldian finches will usually make their nests in tree hollows. They usually breed in the early part of the dry season, when there is plenty of food available. When a male is courting a female, he bobs about and ruffles his feathers in an attempt to show off his bright colours. He will expand his chest and fluff out the feathers on his forehead. After mating, the female will lay a clutch of about 4–8 eggs. Both parents help brood the eggs during the daytime, and it is the female who stays on the eggs at night. When the eggs hatch, both parents care for the young. Gouldian finches leave the nest after between 19 and 25 days and are completely independent at 40 days old.
Gouldian finches have brightly coloured gapes and call loudly when the parent birds return so that they are able to find and feed their mouths in the dark nest.
It has been shown that female Gouldian finches from Northern Australia can control the sex of their offspring by choosing mates according to their head colour. A certain amount of genetic incompatibility between black and red-headed birds can result in high mortality (up to 80%) in female offspring when birds of different head colours mate. If the female mates with a finch of different head colour, this genetic incompatibility can be addressed by over-producing sons, up to a ratio of four males to one female. This is one of the first proven instances of birds biasing the sex of their offspring to overcome genetic weaknesses.
Aviculture
Gouldian finches are a popular species in aviculture because of their striking colours and low care requirements. Gouldian finches get along well with other species of grass finch and some other docile species of bird, such as waxbills and parrot finches.
Trapping for aviculture
In the Kimberley District of Western Australia, where most wild Gouldian finch were trapped for aviculture, it was often reported as one of the more common of the eleven finch species. Until 1977, it was trapped in greater numbers than any other finch. From 1897, when finch trapping started in the Kimberley, it was the most sought after finch by trappers and the most desired by fanciers. Between the years 1934 and 1939, the Gouldian finch was the most exported single finch species. The Perth Zoo exported 22,064 finches of which 12,509 were Gouldian. Private dealers exported 35,315 finches, of which 14,504 were Gouldian. The number of finches taken in the 1958 finch trapping season was the largest for one year, of the 38,649 finches taken, 11,286 were Gouldian. The last licensed trapping of Gouldian finch in Western Australia was on 15 November 1981. In that year's finch trapping season, of the 23,450 finches taken 1,054 were Gouldian. However, it is now illegal to export these birds from Australia.
In popular culture
The Gouldian finch is used as the basis of the ViewSonic logo.
Gallery
Gouldian finch mutations
| Biology and health sciences | Passerida | Animals |
2441752 | https://en.wikipedia.org/wiki/Arsenate%20mineral | Arsenate mineral | Arsenate minerals usually refer to the naturally occurring orthoarsenates, possessing the (AsO4)3− anion group and, more rarely, other arsenates with anions like AsO3(OH)2− (also written HAsO42−) (example: pharmacolite Ca(AsO3OH).2H2O) or (very rarely) [AsO2(OH)2]− (example: andyrobertsite). Arsenite minerals are much less common. Both the Dana and the Strunz mineral classifications place the arsenates in with the phosphate minerals.
Example arsenate minerals include:
Annabergite Ni3(AsO4)2·8H2O
Austinite CaZn(AsO4)(OH)
Clinoclase Cu3(AsO4)(OH)3
Conichalcite CaCu(AsO4)(OH)
Cornubite Cu5(AsO4)2(OH)4
Cornwallite Cu2+5(AsO4)2(OH)2
Erythrite Co3(AsO4)2·8H2O
Mimetite Pb5(AsO4)3Cl
Olivenite Cu2(AsO4)OH
Nickel–Strunz Classification -08- Phosphates
IMA-CNMNC proposes a new hierarchical scheme (Mills et al., 2009). This list uses it to modify the Classification of Nickel–Strunz (mindat.org, 10 ed, pending publication).
Abbreviations:
"*" - discredited (IMA/CNMNC status).
"?" - questionable/doubtful (IMA/CNMNC status).
"REE" - Rare-earth element (Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu)
"PGE" - Platinum-group element (Ru, Rh, Pd, Os, Ir, Pt)
03.C Aluminofluorides, 06 Borates, 08 Vanadates (04.H V[5,6] Vanadates), 09 Silicates:
Neso: insular (from Greek νησος nēsos, island)
Soro: grouping (from Greek σωροῦ sōros, heap, mound (especially of corn))
Cyclo: ring
Ino: chain (from Greek ις [genitive: ινος inos], fibre)
Phyllo: sheet (from Greek φύλλον phyllon, leaf)
Tekto: three-dimensional framework
Nickel–Strunz code scheme: NN.XY.##x
NN: Nickel–Strunz mineral class number
X: Nickel–Strunz mineral division letter
Y: Nickel–Strunz mineral family letter
##x: Nickel–Strunz mineral/group number, x add-on letter
Class: arsenates and vanadates
08.A Arsenates and vanadates without additional anions, without H2O
08.AA With small cations (some also with larger ones): 05 Alarsite
08.AB With medium-sized cations: 25 Xanthiosite, 30 Lammerite, 35 Mcbirneyite, 35 Stranskiite, 35 Pseudolyonsite, 40 Lyonsite
08.AC With medium-sized and large cations: 05 Howardevansite; 10 Arseniopleite, 10 Caryinite, 10 Johillerite, 10 Nickenichite, 10 Bradaczekite, 10 Yazganite, 10 Odanielite; 25 Berzeliite, 25 Manganberzeliite, 25 Palenzonaite, 25 Schäferite; 75 Ronneburgite, 80 Tillmannsite, 85 Filatovite
08.AD With only large cations: 10 Weilite, 10 Svenekite; 30 Schultenite, 35 Chernovite-(Y), 35 Dreyerite, 35 Wakefieldite-(La), 35 Wakefieldite-(Nd), 35 Wakefieldite-(Ce), 35 Wakefieldite-(Y); 40 Pucherite, 50 Gasparite-(Ce), 50 Rooseveltite; 55 Tetrarooseveltite, 60 Chursinite, 65 Clinobisvanite
08.B Arsenates and vanadates with additional anions, without H2O
08.BA With small and medium-sized cations: 10 Bergslagite
08.BB With only medium-sized cations, (OH, etc.):RO4 £1:1: 15 Sarkinite; 30 Zincolivenite, 30 Eveite, 30 Olivenite, 30 Adamite, 30 Auriacusite; 35 Paradamite, 40 Wilhelmkleinite, 50 Namibite, 60 Urusovite, 65 Theoparacelsite, 70 Turanite, 75 Stoiberite, 80 Fingerite, 85 Averievite
08.BC With only medium-sized cations, (OH, etc.):RO4 > 1:1 and < 2:1: 05 Angelellite, 15 Aerugite
08.BD With only medium-sized cations, (OH, etc.):RO4 = 2:1: 05 Cornwallite, 10 Arsenoclasite, 15 Parwelite, 20 Reppiaite, 30 Cornubite
08.BE With only medium-sized cations, (OH, etc.):RO4 > 2:1: 20 Clinoclase, 25 Gilmarite, 25 Arhbarite, 30 Allactite, 30 Flinkite, 35 Chlorophoenicite, 35 Magnesiochlorophoenicite, 40 Gerdtremmelite, 45 Arakiite, 45 Kraisslite, 45 Dixenite, 45 Hematolite, 45 Mcgovernite, 45 Turtmannite, 45 Carlfrancisite, 50 Synadelphite, 55 Holdenite, 60 Kolicite, 65 Sabelliite, 70 Jarosewichite, 75 Theisite, 80 Coparsite
08.BF With medium-sized and large cations, (OH, etc.):RO4 < 0.5:1: 20 Nabiasite
08.BG With medium-sized and large cations, (OH, etc.):RO4 = 0.5:1: 05 Arsenbrackebuschite, 05 Brackebuschite, 05 Gamagarite, 05 Arsentsumebite, 05 Feinglosite, 05 Bushmakinite, 05 Tokyoite, 05 Calderonite
08.BH With medium-sized and large cations, (OH, etc.):RO4 = 1:1: 10 Durangite, 10 Maxwellite, 10 Tilasite; 30 Carminite, 30 Sewardite; 35 Austinite, 35 Adelite, 35 Duftite, 35 Arsendescloizite, 35 Conichalcite, 35 Gabrielsonite, 35 Nickelaustinite, 35 Cobaltaustinite, 35 Tangeite, 35 Gottlobite; 40 Descloizite, 40 Cechite, 40 Pyrobelonite, 40 Mottramite; 45 Bayldonite, 45 Vesignieite; 50 Paganoite, 65 Leningradite
08.BK With medium-sized and large cations, (OH, etc.): 10 Medenbachite, 10 Cobaltneustadtelite, 10 Neustadtelite, 20 Heyite, 25 Jamesite
08.BL With medium-sized and large cations, (OH, etc.):RO4 = 3:1: 05 Beudantite, 05 Hidalgoite, 05 Gallobeudantite, 05 Kemmlitzite, 10 Segnitite, 10 Arsenogorceixite, 10 Arsenocrandallite, 10 Arsenogoyazite, 10 Dussertite, 10 Philipsbornite, 13 Arsenowaylandite, 13 Arsenoflorencite-(Ce), 13 Arsenoflorencite-(La), 13 Arsenoflorencite-(Nd), 13 Graulichite-(Ce)
08.BM With medium-sized and large cations, (OH, etc.):RO4 = 4:1: 05 Retzian-(Ce), 05 Retzian-(La), 05 Retzian-(Nd), 10 Kolitschite
08.BN With only large cations, (OH, etc.):RO4 = 0.33:1: 05 Fermorite, 05 Johnbaumite-M, 05 Johnbaumite, 05 Clinomimetite, 05 Hedyphane, 05 Mimetite-M, 05 Mimetite, 05 Morelandite, 05 Svabite, 05 Turneaureite, 05 Vanadinite
08.BO With only large cations, (OH, etc.):RO4 1:1: 10 Preisingerite, 10 Schumacherite, 15 Atelestite, 15 Hechtsbergite, 20 Kombatite, 20 Sahlinite, 35 Kuznetsovite, 45 Schlegelite
08.C Arsenates and vanadates without additional anions, with H2O
08.CA With small and large/medium cations: 30 Arsenohopeite, 35 Warikahnite, 50 Keyite, 55 Pushcharovskite, 60 Prosperite
08.CB With only medium-sized cations, RO4:H2O = 1:1: 10 Nyholmite, 10 Miguelromeroite, 10 Sainfeldite, 10 Villyaellenite; 15 Krautite, 15 Fluckite; 20 Cobaltkoritnigite, 20 Koritnigite; 25 Yvonite, 30 Geminite, 35 Schubnelite, 40 Radovanite, 45 Kazakhstanite, 50 Kolovratite, 55 Irhtemite, 60 Burgessite
08.CC With only medium-sized cations, RO4:H2O = 1:1.5: 10 Kaatialaite, 15 Leogangite
08.CD With only medium-sized cations, RO4:H2O = 1:2: 10 Mansfieldite, 10 Scorodite, 10 Yanomamite; 15 Parascorodite, 25 Sterlinghillite, 30 Rollandite
08.CE With only medium-sized cations, RO4:H2O £1:2.5: 05 Geigerite, 05 Chudobaite, 15 Brassite, 20 Rosslerite, 30 Veselovskyite, 30 Ondrušite, 30 Lindackerite, 30 Pradetite; 40 Ferrisymplesite, 40 Manganohörnesite, 40 Annabergite, 40 Erythrite, 40 Hörnesite, 40 Köttigite, 40 Parasymplesite, 45 Symplesite, 60 Kaňkite, 65 Steigerite, 70 Metaschoderite, 70 Schoderite, 85 Metaköttigite
08.CF With large and medium-sized cations, RO4:H2O > 1:1: 05 Grischunite
08.CG With large and medium-sized cations, RO4:H2O = 1:1: 05 Gaitite, 05 Parabrandtite, 05 Talmessite, 10 Roselite, 10 Rruffite, 10 Brandtite, 10 Zincroselite, 10 Wendwilsonite, 15 Ferrilotharmeyerite, 15 Cabalzarite, 15 Lotharmeyerite, 15 Cobaltlotharmeyerite, 15 Mawbyite, 15 Cobalttsumcorite, 15 Nickellotharmeyerite, 15 Schneebergite, 15 Nickelschneebergite, 15 Tsumcorite, 15 Thometzekite, 15 Manganlotharmeyerite, 15 Mounanaite, 15 Krettnichite; 20 Zincgartrellite, 20 Lukrahnite, 20 Gartrellite, 20 Helmutwinklerite, 20 Rappoldite, 20 Phosphogartrellite; 25 Pottsite, 35 Nickeltalmessite
08.CH With large and medium-sized cations, RO4:H2O < 1:1: 05 Walentaite, 15 Picropharmacolite; 55 Smolianinovite, 55 Fahleite; 60 Barahonaite-(Fe), 60 Barahonaite-(Al)
08.CJ With only large cations: 20 Haidingerite, 25 Vladimirite, 30 Ferrarisite, 35 Machatschkiite; 40 Phaunouxite, 40 Rauenthalite; 50 Pharmacolite, 55 Mcnearite, 65 Sincosite, 65 Bariosincosite, 75 Guerinite
08.D arsenates and vanadates
08.DA With small (and occasionally larger) cations: 05 Bearsite, 35 Philipsburgite, 50 Ianbruceite
08.DB With only medium-sized cations, (OH, etc.):RO4 < 1:1: 05 Pitticite, 35 Sarmientite, 40 Bukovskyite, 45 Zykaite, 75 Braithwaiteite
08.DC With only medium-sized cations, (OH, etc.):RO4 = 1:1 and < 2:1: 07 Euchroite, 10 Legrandite, 12 Strashimirite; 15 Arthurite, 15 Ojuelaite, 15 Cobaltarthurite, 15 Bendadaite; 20 Coralloite, 30 Maghrebite, 32 Tinticite, 55 Mapimite, 57 Ogdensburgite
08.DD With only medium-sized cations, (OH, etc.):RO4 = 2:1: 05 Chenevixite, 05 Luetheite; 10 Akrochordite, 10 Guanacoite
08.DE With only medium-sized cations, (OH, etc.):RO4 = 3:1: 15 Bulachite, 25 Ceruleite, 40 Juanitaite
08.DF With only medium-sized cations, (OH, etc.):RO4 > 3:1: 10 Liskeardite, 15 Rusakovite, 20 Liroconite, 30 Chalcophyllite, 35 Parnauite
08.DG With large and medium-sized cations, (OH, etc.):RO4 < 0.5:1: 05 Shubnikovite, 05 Lavendulan, 05 Lemanskiite, 05 Zdenekite
08.DH With large and medium-sized cations, (OH, etc.):RO4 < 1:1: 30 Arseniosiderite, 30 Kolfanite, 30 Sailaufite; 45 Mahnertite; 50 Andyrobertsite, 50 Calcioandyrobertsite; 60 Bouazzerite
08.DJ With large and medium-sized cations, (OH, etc.):RO4 = 1:l: 15 Camgasite, 45 Attikaite
08.DK With large and medium-sized cations, (OH, etc.):RO4 > 1:1 and < 2:1: Richelsdorfite, 10 Bariopharmacosiderite, 10 Pharmacosiderite, 10 Natropharmacosiderite, 10 Hydroniumpharmacosiderite; 12 Pharmacoalumite, 12 Natropharmacoalumite, 12 Bariopharmacoalumite
08.DL With large and medium-sized cations, (OH, etc.):RO4 = 2:1: 15 Agardite-(Ce), 15 Agardite-(La), 15 Agardite-(Nd), 15 Agardite-(Y), 15 Goudeyite, 15 Zalesiite, 15 Mixite, 15 Plumboagardite; 20 Cheremnykhite, 20 Dugganite, 20 Wallkilldellite-(Fe), 20 Wallkilldellite-(Mn)
08.DM With large and medium-sized cations, (OH, etc.):RO4 > 2:1: 05 Esperanzaite, 10 Clinotyrolite, 10 Tyrolite, 15 Betpakdalite-CaCa, 15 Betpakdalite-NaCa, 20 Phosphovanadylite-Ba, 20 Phosphovanadylite-Ca, 25 Yukonite, 40 Santafeite
08.E Uranyl arsenates
08.EA UO2:RO4 = 1:2: 05 Orthowalpurgite, 05 Walpurgite; 10 Hallimondite
08.EB UO2:RO4 = 1:1: 05 Metarauchite, 05 Heinrichite, 05 Kahlerite, 05 Novacekite, 05 Uranospinite, 05 Zeunerite; 10 Metazeunerite, 10 Metauranospinite, 10 Metaheinrichite, 10 Metakahlerite, 10 Metakirchheimerite, 10 Metalodevite, 10 Metanovacekite; 15 Uramarsite, 15 Trogerite, 15 Abernathyite, 15 Natrouranospinite; 20 Chistyakovaite, 25 Arsenuranospathite
08.EC UO2:RO4 = 3:2: 10 Arsenuranylite, 15 Hugelite, 20 Arsenovanmeersscheite, 45 Nielsbohrite
08.ED Unclassified: 10 Asselbornite
08.F Polyarsenates and [4]-polyvanadates
08.FA Polyarsenates and [4]-polyvanadates, without OH and H2O; dimers of corner-sharing RO4 tetrahedra: 05 Blossite, 10 Ziesite, 15 Chervetite, 25 Petewilliamsite
08.FC [4]-Polyvanadates, with H2O only: 05 Fianelite, 15 Pintadoite
08.FD [4]-Polyvanadates, with OH and H2O: 05 Martyite, 05 Volborthite
08.FE Ino-[4]-vanadates: 05 Ankinovichite, 05 Alvanite
08.X Unclassified Strunz arsenates and vanadates
| Physical sciences | Minerals | Earth science |
2442882 | https://en.wikipedia.org/wiki/Soil%20physics | Soil physics | Soil physics is the study of soil's physical properties and processes. It is applied to management and prediction under natural and managed ecosystems. Soil physics deals with the dynamics of physical soil components and their phases as solids, liquids, and gases. It draws on the principles of physics, physical chemistry, engineering, and meteorology. Soil physics applies these principles to address practical problems of agriculture, ecology, and engineering.
Prominent soil physicists
Edgar Buckingham (1867–1940)
The theory of gas diffusion in soil and vadose zone water flow in soil.
Willard Gardner (1883–1964)
First to use porous cups and manometers for capillary potential measurements and accurately predicted the moisture distribution above a water table.
Lorenzo A. Richards (1904–1993)
General transport of water in unsaturated soil, measurement of soil water potential using tensiometer.
John R. Philip (1927–1999)
Analytical solution to general soil water transport, Environmental Mechanics.
| Physical sciences | Soil science | Earth science |
2443027 | https://en.wikipedia.org/wiki/Supernova%20nucleosynthesis | Supernova nucleosynthesis | Supernova nucleosynthesis is the nucleosynthesis of chemical elements in supernova explosions.
In sufficiently massive stars, the nucleosynthesis by fusion of lighter elements into heavier ones occurs during sequential hydrostatic burning processes called helium burning, carbon burning, oxygen burning, and silicon burning, in which the byproducts of one nuclear fuel become, after compressional heating, the fuel for the subsequent burning stage. In this context, the word "burning" refers to nuclear fusion and not a chemical reaction.
During hydrostatic burning these fuels synthesize overwhelmingly the alpha nuclides (), nuclei composed of integer numbers of helium-4 nuclei. Initially, two helium-4 nuclei fuse into a single beryllium-8 nucleus. The addition of another helium 4 nucleus to the beryllium yields carbon-12, followed by oxygen-16, neon-20 and so on, each time adding 2 protons and 2 neutrons to the growing nucleus. A rapid final explosive burning is caused by the sudden temperature spike owing to passage of the radially moving shock wave that was launched by the gravitational collapse of the core. W. D. Arnett and his Rice University colleagues demonstrated that the final shock burning would synthesize the non-alpha-nucleus isotopes more effectively than hydrostatic burning was able to do, suggesting that the expected shock-wave nucleosynthesis is an essential component of supernova nucleosynthesis. Together, shock-wave nucleosynthesis and hydrostatic-burning processes create most of the isotopes of the elements carbon (), oxygen (), and elements with (from neon to nickel). As a result of the ejection of the newly synthesized isotopes of the chemical elements by supernova explosions, their abundances steadily increased within interstellar gas. That increase became evident to astronomers from the initial abundances in newly born stars exceeding those in earlier-born stars.
Elements heavier than nickel are comparatively rare owing to the decline with atomic weight of their nuclear binding energies per nucleon, but they too are created in part within supernovae. Of greatest interest historically has been their synthesis by rapid capture of neutrons during the r-process, reflecting the common belief that supernova cores are likely to provide the necessary conditions. However, newer research has proposed a promising alternative (see the r-process below). The r-process isotopes are approximately 100,000 times less abundant than the primary chemical elements fused in supernova shells above. Furthermore, other nucleosynthesis processes in supernovae are thought to be responsible also for some nucleosynthesis of other heavy elements, notably, the proton capture process known as the rp-process, the slow capture of neutrons (s-process) in the helium-burning shells and in the carbon-burning shells of massive stars, and a photodisintegration process known as the -process (gamma-process). The latter synthesizes the lightest, most neutron-poor, isotopes of the elements heavier than iron from preexisting heavier isotopes.
History
In 1946, Fred Hoyle proposed that elements heavier than hydrogen and helium would be produced by nucleosynthesis in the cores of massive stars. It had previously been thought that the elements we see in the modern universe had been largely produced during its formation. At this time, the nature of supernovae was unclear and Hoyle suggested that these heavy elements were distributed into space by rotational instability. In 1954, the theory of nucleosynthesis of heavy elements in massive stars was refined and combined with more understanding of supernovae to calculate the abundances of the elements from carbon to nickel. Key elements of the theory included:
the prediction of the excited state in the C nucleus that enables the triple-alpha process to burn resonantly to carbon and oxygen;
the thermonuclear sequels of carbon-burning synthesizing Ne, Mg and Na; and
oxygen-burning synthesizing silicon, aluminum, and sulphur.
The theory predicted that silicon burning would happen as the final stage of core fusion in massive stars, although nuclear science could not then calculate exactly how. Hoyle also predicted that the collapse of the evolved cores of massive stars was "inevitable" owing to their increasing rate of energy loss by neutrinos and that the resulting explosions would produce further nucleosynthesis of heavy elements and eject them into space.
In 1957, a paper by the authors E. M. Burbidge, G. R. Burbidge, W. A. Fowler, and Hoyle expanded and refined the theory and achieved widespread acclaim. It became known as the B²FH or BBFH paper, after the initials of its authors. The earlier papers fell into obscurity for decades after the more-famous B²FH paper did not attribute Hoyle's original description of nucleosynthesis in massive stars. Donald D. Clayton has attributed the obscurity also to Hoyle's 1954 paper describing its key equation only in words, and a lack of careful review by Hoyle of the B²FH draft by coauthors who had themselves not adequately studied Hoyle's paper. During his 1955 discussions in Cambridge with his co-authors in preparation of the B²FH first draft in 1956 in Pasadena, Hoyle's modesty had inhibited him from emphasizing to them the great achievements of his 1954 theory.
Thirteen years after the B²FH paper, W.D. Arnett and colleagues demonstrated that the final burning in the passing shock wave launched by collapse of the core could synthesize non-alpha-particle isotopes more effectively than hydrostatic burning could, suggesting that explosive nucleosynthesis is an essential component of supernova nucleosynthesis. A shock wave rebounded from matter collapsing onto the dense core, if strong enough to lead to mass ejection of the mantle of supernovae, would necessarily be strong enough to provide the sudden heating of the shells of massive stars needed for explosive thermonuclear burning within the mantle. Understanding how that shock wave can reach the mantle in the face of continuing infall onto the shock became the theoretical difficulty. Supernova observations assured that it must occur.
White dwarfs were proposed as possible progenitors of certain supernovae in the late 1960s, although a good understanding of the mechanism and nucleosynthesis involved did not develop until the 1980s. This showed that ejected very large amounts of radioactive nickel and lesser amounts of other iron-peak elements, with the nickel decaying rapidly to cobalt and then iron.
Era of computer models
The papers of Hoyle (1946) and Hoyle (1954) and of B²FH (1957) were written by those scientists before the advent of the age of computers. They relied on hand calculations, deep thought, physical intuition, and familiarity with details of nuclear physics. Brilliant as these founding papers were, a cultural disconnect soon emerged with a younger generation of scientists who began to construct computer programs that would eventually yield numerical answers for the advanced evolution of stars and the nucleosynthesis within them.
Cause
A supernova is a violent explosion of a star that occurs under two principal scenarios. The first is that a white dwarf star, which is the remnant of a low-mass star that has exhausted its nuclear fuel, undergoes a thermonuclear explosion after its mass is increased beyond its Chandrasekhar limit by accreting nuclear-fuel mass from a more diffuse companion star (usually a red giant) with which it is in binary orbit. The resulting runaway nucleosynthesis completely destroys the star and ejects its mass into space. The second, and about threefold more common, scenario occurs when a massive star (12–35 times more massive than the sun), usually a supergiant at the critical time, reaches nickel-56 in its core nuclear fusion (or burning) processes. Without exothermic energy from fusion, the core of the pre-supernova massive star loses heat needed for pressure support, and collapses owing to the strong gravitational pull. The energy transfer from the core collapse causes the supernova display.
The nickel-56 isotope has one of the largest binding energies per nucleon of all isotopes, and is therefore the last isotope whose synthesis during core silicon burning releases energy by nuclear fusion, exothermically. The binding energy per nucleon declines for atomic weights heavier than , ending fusion's history of supplying thermal energy to the star. The thermal energy released when the infalling supernova mantle hits the semi-solid core is very large, about 10 ergs, about a hundred times the energy released by the supernova as the kinetic energy of its ejected mass. Dozens of research papers have been published in the attempt to describe the hydrodynamics of how that small one percent of the infalling energy is transmitted to the overlying mantle in the face of continuous infall onto the core. That uncertainty remains in the full description of core-collapse supernovae.
Nuclear fusion reactions that produce elements heavier than iron absorb nuclear energy and are said to be endothermic reactions. When such reactions dominate, the internal temperature that supports the star's outer layers drops. Because the outer envelope is no longer sufficiently supported by the radiation pressure, the star's gravity pulls its mantle rapidly inward. As the star collapses, this mantle collides violently with the growing incompressible stellar core, which has a density almost as great as an atomic nucleus, producing a shockwave that rebounds outward through the unfused material of the outer shell. The increase of temperature by the passage of that shockwave is sufficient to induce fusion in that material, often called explosive nucleosynthesis. The energy deposited by the shockwave somehow leads to the star's explosion, dispersing fusing matter in the mantle above the core into interstellar space.
Silicon burning
After a star completes the oxygen burning process, its core is composed primarily of silicon and sulfur. If it has sufficiently high mass, it further contracts until its core reaches temperatures in the range of 2.7–3.5 billion K (). At these temperatures, silicon and other isotopes suffer photoejection of nucleons by energetic thermal photons () ejecting especially alpha particles (He). The nuclear process of silicon burning differs from earlier fusion stages of nucleosynthesis in that it entails a balance between alpha-particle captures and their inverse photo ejection which establishes abundances of all alpha-particle elements in the following sequence in which each alpha particle capture shown is opposed by its inverse reaction, namely, photo ejection of an alpha particle by the abundant thermal photons:
{|
|-
| Si || + || || He || || || S || + ||
|-
| S || + || || He || || || Ar || + ||
|-
| Ar || + || || He || || || Ca || + ||
|-
| Ca || + || || He || || || Ti || + ||
|-
| Ti || + || || He || || || Cr || + ||
|-
| Cr || + || || He || || || Fe || + ||
|-
| Fe || + || || He || || || Ni || + ||
|-
| Ni || + || || He || || || Zn || + ||
|}
The alpha-particle nuclei Ti and those more massive in the final five reactions listed are all radioactive, but they decay after their ejection in supernova explosions into abundant isotopes of Ca, Ti, Cr, Fe and Ni. This post-supernova radioactivity became of great importance for the emergence of gamma-ray-line astronomy.
In these physical circumstances of rapid opposing reactions, namely alpha-particle capture and photo ejection of alpha particles, the abundances are not determined by alpha-particle-capture cross sections; rather they are determined by the values that the abundances must assume in order to balance the speeds of the rapid opposing-reaction currents. Each abundance takes on a stationary value that achieves that balance. This picture is called nuclear quasiequilibrium. Many computer calculations, for example, using the numerical rates of each reaction and of their reverse reactions have demonstrated that quasiequilibrium is not exact but does characterize well the computed abundances. Thus, the quasiequilibrium picture presents a comprehensible picture of what actually happens. It also fills in an uncertainty in Hoyle's 1954 theory. The quasiequilibrium buildup shuts off after Ni because the alpha-particle captures become slower whereas the photo ejections from heavier nuclei become faster. Non-alpha-particle nuclei also participate, using a host of reactions similar to
Ar + neutron Ar + photon
and its inverse which set the stationary abundances of the non-alpha-particle isotopes, where the free densities of protons and neutrons are also established by the quasiequilibrium. However, the abundance of free neutrons is also proportional to the excess of neutrons over protons in the composition of the massive star; therefore the abundance of Ar, using it as an example, is greater in ejecta from recent massive stars than it was from those in early stars of only H and He; therefore Cl, to which Ar decays after the nucleosynthesis, is called a "secondary isotope".
In interest of brevity, the next stage, an intricate photo-disintegration rearrangement, and the nuclear quasiequilibrium that it achieves, are referred to as silicon burning.
The silicon burning in the star progresses through a temporal sequence of such nuclear quasiequilibria in which the abundance of Si slowly declines and that of Ni slowly increases. This amounts to a nuclear abundance change 2 Si ≫ Ni, which may be thought of as silicon burning into nickel ("burning" in the nuclear sense).
The entire silicon-burning sequence lasts about one day in the core of a contracting massive star and stops after Ni has become the dominant abundance. The final explosive burning caused when the supernova shock passes through the silicon-burning shell lasts only seconds, but its roughly 50% increase in the temperature causes furious nuclear burning, which becomes the major contributor to nucleosynthesis in the mass range 28–60 .
After the final Ni stage, the star can no longer release energy via nuclear fusion, because a nucleus with 56 nucleons has the lowest mass per nucleon of all the elements in the sequence. The next step up in the alpha-particle chain would be Zn. However Zn has slightly more mass per nucleon than Ni, and thus would require a thermodynamic energy loss rather than a gain as happened in all prior stages of nuclear burning.
Ni (which has 28 protons) has a half-life of 6.02 days and decays via β decay to Co (27 protons), which in turn has a half-life of 77.3 days as it decays to Fe (26 protons). However, only minutes are available for the Ni to decay within the core of a massive star.
This establishes Ni as the most abundant of the radioactive nuclei created in this way. Its radioactivity energizes the late supernova light curve and creates the pathbreaking opportunity for gamma-ray-line astronomy. See SN 1987A light curve for the aftermath of that opportunity.
Clayton and Meyer have recently generalized this process still further by what they have named the secondary supernova machine, attributing the increasing radioactivity that energizes late supernova displays to the storage of increasing Coulomb energy within the quasiequilibrium nuclei called out above as the quasiequilibria shift from primarily Si to primarily Ni. The visible displays are powered by the decay of that excess Coulomb energy.
During this phase of the core contraction, the potential energy of gravitational compression heats the interior to roughly three billion kelvins, which briefly maintains pressure support and opposes rapid core contraction. However, since no additional heat energy can be generated via new fusion reactions, the final unopposed contraction rapidly accelerates into a collapse lasting only a few seconds. At that point, the central portion of the star is crushed into either a neutron star or, if the star is massive enough, into a black hole.
The outer layers of the star are blown off in an explosion triggered by the outward moving supernova shock, known as a Type II supernova whose displays last days to months. The escaping portion of the supernova core may initially contain a large density of free neutrons, which may synthesize, in about one second while inside the star, roughly half of the elements in the universe that are heavier than iron via a rapid neutron-capture mechanism known as the r-process. See below.
Nuclides synthesized
Stars with initial masses less than about eight times the sun never develop a core large enough to collapse and they eventually lose their atmospheres to become white dwarfs, stable cooling spheres of carbon supported by the pressure of degenerate electrons. Nucleosynthesis within those lighter stars is therefore limited to nuclides that were fused in material located above the final white dwarf. This limits their modest yields returned to interstellar gas to carbon-13 and nitrogen-14, and to isotopes heavier than iron by slow capture of neutrons (the s-process).
A significant minority of white dwarfs will explode, however, either because they are in a binary orbit with a companion star that loses mass to the stronger gravitational field of the white dwarf, or because of a merger with another white dwarf. The result is a white dwarf which exceeds its Chandrasekhar limit and explodes as a synthesizing about a solar mass of radioactive Ni isotopes, together with smaller amounts of other iron peak elements. The subsequent radioactive decay of the nickel to iron keeps Type Ia optically very bright for weeks and creates more than half of all the iron in the universe.
Virtually all of the remainder of stellar nucleosynthesis occurs, however, in stars that are massive enough to end as core collapse supernovae. In the pre-supernova massive star this includes helium burning, carbon burning, oxygen burning and silicon burning. Much of that yield may never leave the star but instead disappears into its collapsed core. The yield that is ejected is substantially fused in last-second explosive burning caused by the shock wave launched by core collapse. Prior to core collapse, fusion of elements between silicon and iron occurs only in the largest of stars, and then in limited amounts. Thus, the nucleosynthesis of the abundant primary elements defined as those that could be synthesized in stars of initially only hydrogen and helium (left by the Big Bang), is substantially limited to core-collapse supernova nucleosynthesis.
r-process nucleosynthesis
During supernova nucleosynthesis, the r-process creates very neutron-rich heavy isotopes, which decay after the event to the first stable isotope, thereby creating the neutron-rich stable isotopes of all heavy elements. This neutron capture process occurs in high neutron density with high temperature conditions.
In the r-process, any heavy nuclei are bombarded with a large neutron flux to form highly unstable neutron rich nuclei which very rapidly undergo beta decay to form more stable nuclei with higher atomic number and the same atomic mass. The neutron density is extremely high, about 10 neutrons per cubic centimeter.
Initial calculations of an evolving r-process, showing the evolution of calculated results with time, also suggested that the r-process abundances are a superposition of differing neutron fluences. Small fluence produces the first r-process abundance peak near atomic weight but no actinides, whereas large fluence produces the actinides uranium and thorium but no longer contains the abundance peak. These processes occur in a fraction of a second to a few seconds, depending on details. Hundreds of subsequent papers published have utilized this time-dependent approach. The only modern nearby supernova, 1987A, has not revealed r-process enrichments. Modern thinking is that the r-process yield may be ejected from some supernovae but swallowed up in others as part of the residual neutron star or black hole.
Entirely new astronomical data about the r-process was discovered in 2017 when the LIGO and Virgo gravitational-wave observatories discovered a merger of two neutron stars that had previously been orbiting one another. That can happen when both massive stars in orbit with one another become core-collapse supernovae, leaving neutron-star remnants.
The localization on the sky of the source of those gravitational waves radiated by that orbital collapse and merger of the two neutron stars, creating a black hole, but with significant ejected mass of highly neutronized matter, enabled several teams to discover and study the remaining optical counterpart of the merger, finding spectroscopic evidence of r-process material thrown off by the merging neutron stars.
The bulk of this material seems to consist of two types: Hot blue masses of highly radioactive r-process matter of lower-mass-range heavy nuclei () and cooler red masses of higher mass-number r-process nuclei () rich in actinides (such as uranium, thorium, californium etc.). When released from the huge internal pressure of the neutron star, this neutron-rich spherical ejecta expands and radiates detected optical light for about a week. Such duration of luminosity would not be possible without heating by internal radioactive decay, which is provided by r-process nuclei near their waiting points. Two distinct mass regions ( and ) for the r-process yields have been known since the first time dependent calculations of the r-process. Because of these spectroscopic features it has been argued that r-process nucleosynthesis in the Milky Way may have been primarily ejecta from neutron-star mergers rather than from supernovae.
| Physical sciences | Stellar astronomy | Astronomy |
16969883 | https://en.wikipedia.org/wiki/Cementation%20%28geology%29 | Cementation (geology) | Minerals bond grains of sediment together by growing around them. This process is called cementation and is a part of the rock cycle.
Cementation involves ions carried in groundwater chemically precipitating to form new crystalline material between sedimentary grains. The new pore-filling minerals form "bridges" between original sediment grains, thereby binding them together. In this way, sand becomes sandstone, and gravel becomes conglomerate or breccia. Cementation occurs as part of the diagenesis or lithification of sediments. Cementation occurs primarily below the water table regardless of sedimentary grain sizes present. Large volumes of pore water must pass through sediment pores for new mineral cements to crystallize and so millions of years are generally required to complete the cementation process. Common mineral cements include calcite, quartz, and silica phases like cristobalite, iron oxides, and clay minerals; other mineral cements also occur.
Cementation is continuous in the groundwater zone, so much so that the term "zone of cementation" is sometimes used interchangeably. Cementation occurs in fissures or other openings of existing rocks and is a dynamic process more or less in equilibrium with a dissolution or dissolving process.
Cement found on the sea floor is commonly aragonite and can take different textural forms. These textural forms include pendant cement, meniscus cement, isopachous cement, needle cement, botryoidal cement, blocky cement, syntaxial rim cement, and coarse mosaic cement. The environment in which each of the cements is found depends on the pore space available. Cements that are found in phreatic zones include: isopachous, blocky, and syntaxial rim cements. As for calcite cementation, which occurs in meteoric realms (freshwater sources), the cement is produced by the dissolution of less stable aragonite and high-Mg calcite. (Boggs, 2011)
Classifying rocks while using the Folk classification depends on the matrix, which is either sparry (prominently composed of cement) or micritic (prominently composed of mud).
Types of carbonate cement
Beachrock is a type of carbonate beach sand that has been cemented together by a process called synsedimentary cementation. Beachrock may contain meniscus cements or pendant cements. As the water between the narrow spaces of grains drains from the beachrock, a small portion of it is held back by capillary forces, where meniscus cement will form. Pendant cements form on the bottom of grains where water droplets are held.
Hardgrounds are hard crusts of carbonate material that form on the bottom of the ocean floor, below the lowest tide level. Isopachous (which means equal thickness) cement forms in subaqueous conditions where the grains are completely surrounded by water (Boggs, 2006).
Carbonate cements can also be formed by biological organisms such as Sporosarcina pasteurii, which binds sand together given organic compounds and a calcium source (Chou et al., 2010).
Cementing has significant effects on the properties and stability of many soil materials. Cementation is not always easily identified and its effects cannot be easily determined quantitatively. It is known to contribute to clay tenderness and may be responsible for an apparent preconsolidation pressure. The filtration of iron compounds from a very sensitive clay from Labrador, Canada, resulted in a 30 t/m reduction in apparent preconsolidation pressure. Coop and Airey (2003) show that for carbonate soils, cementation develops immediately after deposition and allows the soil to maintain a loose structure. Non-recognition of cementation has resulted in construction disputes. For example, a land on a major Project is marked as glacier on contract drawings. It was so hard that it had to be detonated. The contractor claimed that the soil was cemented during excavation as it was formed due to the clay matrix as well as the gravel. The owner concluded that this was due to the weathering of the pebbles. Proper evaluation of the material before the award of the contract could have avoided the problem. Clay particles adhere to the surfaces of larger silt and sand particles, a process called clay bonding. Eventually, larger grains are embedded in a clay matrix and their influence on geotechnical behavior is limited. The clay confinement maintains a large void ratio even at high effective stresses, allowing the interparticle forces to spring up.
| Physical sciences | Sedimentology | Earth science |
4524914 | https://en.wikipedia.org/wiki/Lantian%20Man | Lantian Man | Lantian Man (), Homo erectus lantianensis) is a subspecies of Homo erectus known from an almost complete mandible from Chenchiawo (陈家窝) Village discovered in 1963, and a partial skull from Gongwangling (公王岭) Village discovered in 1964, situated in Lantian County on the Loess Plateau. The former dates to about 710–684 thousand years ago, and the latter 1.65–1.59 million years ago. This makes Lantian Man the second-oldest firmly dated H. erectus beyond Africa (after H. e. georgicus), and the oldest in East Asia. The fossils were first described by Woo Ju-Kan in 1964, who considered the subspecies an ancestor to Peking Man (H. e. pekinensis).
Like Peking Man, Lantian Man has a heavy brow ridge, a receding forehead, possibly a sagittal keel running across the midline of the skull, and exorbitantly thickened bone. The skull is small by absolute measure, and has narrower postorbital constriction. The teeth are proportionally large compared to other Asian H. erectus. The brain volume of the Gongwangling skull is about 780 cc, similar to contemporary archaic humans in Africa, but much smaller than later Asian H. erectus and modern humans.
Lantian Man inhabited the mild grasslands at the northern base of the Qinling Mountains. For stone tools, Lantian Man manufactured mainly heavy-duty tools including choppers, spheroids, heavy-duty scrapers, handaxes, picks, cleavers. The latter three are characteristic of the Acheulean industry, which is usually only applied to African and Western Eurasian sites. It appears the Acheulean persisted far longer in this region than elsewhere.
Taxonomy
On July 19, 1963, a team funded by the Chinese Institute of Vertebrate Palaeontology and Palaeoanthropology (IVPP) recovered a fossil human mandible (lower jawbone) outside Chenchiawo Village, Lantian County in the Shaanxi Province of Northwest China. It was found in the bottom end of a thick layer of reddish clays, atop a metre-thick (3 ft) layer of gravel. Lantian is situated on the Loess Plateau, which is geologically stratified into alternating units of loess (wind-blown sediment deposits) and paleosol (soil deposits). The mandible was formally described by Chinese palaeoanthropologist Woo Ju-Kan (吴汝康) in 1964, who noted its similarity to the Peking Man (at the time "Sinanthropus" pekinensis), and provisionally classified it as "Sinanthropus" lantianensis. This spurred further investigation of Lantian County, which recovered a human tooth by the end of the May 1964 and the rest of the skull by October, at the Gongwangling site at the foothills of the Qinling Mountains. Woo also assigned it to "S." lantianensis, but later that year, he recognised the genus was falling out of favour and was being synonymised with Homo erectus. He recommended the combination Homo erectus lantianensis. Nonetheless, the skull is too distorted to morphologically assess Lantian Man's relationship with other H. erectus populations, so it is unclear if Lantian Man and Peking Man are more closely related to each other than Java Man (H. e. erectus).
The discovery of Lantian Man was in the midst of an ever-increasing number of Chinese fossil ape sites, bringing the country to the forefront of anthropological discussions, beyond the capital's famous Peking Man. These were publicised in local site museums constructed in the 1980's and 1990's; Lantian Man became one such spectacle for the Shaanxi History Museum.
Lantian Man was early on recognised as being older than Peking Man on purely morphological grounds. In 1973, American anthropologists Jean Aigner and William S. Laughlin suggested the Chenchiawo site was deposited 300,000 years ago and the Gongwangling site 700,000 years ago based on the animal remains (biostratigraphy), constrained to the Middle Pleistocene. In 1978, Chinese palaeoanthropologist Ma Xinghua and colleagues estimated, respectively, 650,000 and 750 to 800 thousand years ago using palaeomagnetism, extending into the Early Pleistocene. Using the same methods later that year, Chinese palaeoanthropologist Cheng Gouliang and colleagues instead reported 500,000 and 1 million years ago. In 1984, Chinese palaeoanthropologists Liu Dongsheng and Ding Menglin suggested the layers date to 500 to 690 thousand and 730 to 800 thousand years ago. In 1989, Chinese palaeoanthropologists An Zhisheng and Ho Chuan Kun stratigraphically placed the Chenchiawo at Palaeosol Unit 5 and the Gongwangling skull to Loess Unit 15, and palaeomagnetically dated them to 650,000 and 1.15 million years ago. This made Lantian Man the oldest firmly dated Asian human species at the time.
An and Ho's dates became widely used, but in 2015, Chinese palaeoanthropologist Zhu-Yu Zhu and colleagues noticed a discontinuity in the stratigraphy, which put the Chenchiawo mandible in Palaeosol Unit 6, and the Gongwangling skull all the way down in Palaeosol Unit 23. This makes them 1.65–1.59 million and 710–684 thousand years old. Lantian Man is then roughly contemporaneous with the earliest humans to leave Africa: the 1.75 million year old Dmanisi humans (H. e. georgicus), the 1.6–1.5 million year old Sangiran humans (H. e. erectus), and the 1.7–1.4 million year old Yuanmou Man (H. e. yuanmouensis). In 2018, Zhu reported 2.1 million year old stone tools at the Shangchen site in Lantian. Such early dates indicate H. erectus rapidly dispersed across the Old World once out of Africa. In 2011, Indonesian palaeoanthropologist Yahdi Zaim and colleagues suggested the open habitats of China and Southeast Asia were colonised by two distinct waves of H. erectus based on dental anatomy, separated by a rainforest belt south of the Qinling Mountains.
Anatomy
The Gongwangling skull is relatively complete, and comprises the frontal bone (forehead), most of the parietal bones (top of the head), the right temporal bone (sides of the head), the bottom margins of the nasal bones (between the eyes), and pieces of the maxillae (upper jaws). It is a bit distorted, with the right orbit jutting out farther than the left, several elements are slightly flattened, the depressions and the middle of the frontal bone are craggy due to corrosion, and the left parietal flexes out a bit more than normal. Based on the size and wearing of the molars (and assuming they degrade faster than those of modern humans), Woo estimated the individual was a 30 year old female. Overall, the skull is quite archaic, according to Woo reminiscent of the contemporary Mojokerto skull from Java. Woo calculated a brain volume of about 780 cc, which is quite small for H. erectus. For comparison, later Asian H. erectus average roughly 1,000 cc, and present-day modern humans 1,270 cc for males and 1,130 cc for females. Contemporary African archaic humans (H. habilis, H. rudolfensis, and H. e? ergaster) ranged from 500–900 cc.
Like Peking Man, the brow ridge is a solid, continuous bar; the forehead is low and receding; and there may have been a sagittal keel running across the midline, but the region is too eroded to definitively tell. The two hard layers of bone (separated by spongy diploë) in the skull are extraordinarily thickened. The temporal lines arcing across the parietals are ridges. Unlike Peking Man, the brow projects more at the midpoint and does not terminate in a sulcus (a defined dip), instead extending even farther. Lantian Man also has greater post-orbital constriction. The nasal bones are rather wide. The orbits are rectangular and lack the supraorbital foramen and the lacrimal fossa. The upper second molar is longer and narrower than the third. Woo reconstructed the skull's length x breadth as , much smaller than the adult dimensions of Peking Man or Java Man.
The Chenchiawo mandible was the most complete mandible from Pleistocene of China at the time, preserving most elements except for pieces of the rami (the ascending portion which connects with the skull). Woo considered the specimen an elderly female based on size and wearing of the teeth. The mandible is mostly consistent with that of Peking Man, except the rami ascend at a smaller angle, the mental foramen is placed lower, the rows of molar teeth have significantly larger angles, and the teeth are larger than what would be expected for a female.
Pathology
The Chenchiawo mandible is missing the third molars, probably a genetic disorder, the first such case for an extinct human species. The right cheek teeth, especially the first molar, feature deterioration and abnormal thickening, which are indicators of gum disease. The first premolar was also lost, probably as a result of this. Nonetheless, none of the teeth developed cavities.
Culture
Palaeoenvironment
The Loess Plateau is a fossil-rich area; the mammal assemblage indicates it has broadly remained a mild grassland throughout the Pleistocene. Gongwangling sits at the base of the Qinling Mountains, which today is a natural barrier separating northern and southern China, forming plains to the north and forest to the south, but at the time may not have posed such an insurmountable wall. Consequently, among the 41 other mammalian species unearthed, Gongwangling also comprises several fauna typical of South China: the giant panda, the elephant Stegodon orientalis, the tapir Tapirus sinensis, the giant tapir, the chalicothere Nestoritherium sinensis, the tufted deer, the mainland serow, and the snub-nosed monkey. Other forest-going creatures (not typical of the south) are: the Etruscan bear, the pig Sus lydekker, and the deer Cervus grayi and Sinomegaceros konwanlinensis. More common were grassland and open-habitat creatures including: badgers, the giant hyena Pachycrocuta, the Zhoukoudian wolf, the tiger, the leopard, the cheetah-like Sivapanthera, the saber-toothed Megantereon, the horse Equus sanmemiensis, the rhino Dicerorhinus, the bovid Leptobos, and several northerly rodents. This assemblage indicates a mild and semi-humid climate, featuring plains adjacent to forest mountains. Chenchiawo features the dhole, the Asian badger, the tiger, the Asian elephant, S. lydekkeri, Sinomegaceros, and C. grayi, in addition to seven species of northerly rodents, which are consistent with a warm, semi-humid to semi-arid grassland to bushland environment.
Technology
The stone tool technology of China was long thought to be so distinct from contemporary western sites that they were incomparable, the former characterised as a simple chopper industry, and the latter as a handaxe industry (the Acheulean). In 1944, American archaeologist Hallam L. Movius drew the "Movius Line" partitioning west from east. As Chinese archaeology progressed through the 1980s, characteristically Acheulean tools were uncovered in China (including Lantian), and the strict Movius Line fell apart.
As of 2014, total of 27 stone tool bearing sites from the Early to Middle Pleistocene have been discovered within the vicinity of Chenchiawo and Gongwangling, in addition to two Late Pleistocene sites. In 2018, Zhu and colleagues reported 2.1 million year old stone tools at the Shangchen site on the Loess Plateau, the oldest evidence of humans out of Africa. Only 26 stone tools were excavated at Gongwangling, 20 from adjacent sites, and 10 from Chenchiawo. In total, the Early to Middle Pleistocene assemblage comprise largely heavy-duty tools including choppers, handaxes, picks, cleavers, spheroids, and heavy-duty scrapers made of predominantly local river cobble — quartzite, quartz, greywacke, and igneous pebbles — and more rarely higher quality sandstone, limestone, and chert. Handaxes, cleavers, and picks are characteristic of the Acheulean, which seems to have prevailed for some time in this region even while the west was transitioning to the Middle Stone Age/Middle Paleolithic in the Late Pleistocene. They seem to have preferred the bipolar percussion technique (smashing the core into several flakes with a hammerstone, out of which at least a few should be the correct size and shape), and less frequently used the anvil-chipping technique (hitting the core against an anvil to slowly chip away pieces into a usable edge).
| Biology and health sciences | Homo | Biology |
4528497 | https://en.wikipedia.org/wiki/Yuanmou%20Man | Yuanmou Man | Yuanmou Man (, Homo erectus yuanmouensis) is a subspecies of H. erectus which inhabited the Yuanmou Basin in Yunnan Province, southwestern China, roughly 1.7 million years ago. It is the first fossil evidence of humans in China, though they probably reached the region by at least 2 million years ago. Yuanmou Man is known only from two upper first incisors presumed to have belonged to a male, and a partial tibia presumed to have belonged to a female. The female may have stood about in life. These remains are anatomically quite similar to those contemporary early Homo in Africa, namely H. habilis and H. (e?) ergaster.
Yuanmou Man inhabited a mixed environment featuring grassland, bushland, marshland, and forest dominated by pine and alder. They lived alongside chalicotheres, deer, the elephant Stegodon, rhinos, cattle, pigs, and the giant short-faced hyaena. The site currently sits at an elevation of . They manufactured simple cores, flakes, choppers, pointed tools, and scrapers which paralleled the technology of their African contemporaries.
Taxonomy
Discovery
On May 1, 1965, geologist Qian Fang recovered two archaic human upper first incisors (catalogue number V1519) from fossiliferous deposits of the Yuanmou Basin near Shangnabang village, Yuanmou County, Yunnan Province, China. When they were formally described in 1973, they were determined to have belonged to a young male. Qian's field team was funded by the Chinese Academy of Geosciences. The Institute of Vertebrate Paleontology and Paleoanthropology funded further excavation of the site, and reported 16 stone tools, of which six were found in situ and 10 nearby.
The Yuanmou Basin sits just to the southeast of the Tibetan Plateau, and is the lowest basin on the central Yunnan–Guizhou Plateau at an elevation of . The Yuanmou Formation is divided into four members and 28 layers. The human teeth were discovered in the silty clay and sandy conglomerates of Member 4 (the uppermost member) near the bottom of layer 25.
In December 1984, a field team dispatched by the Beijing Natural History Museum to survey the Guojiabao site, just away from the original Yuanmou Man teeth, unearthed a left human tibial shaft in a layer just overlying layer 25 in Member 4. The tibia was described in 1991, and was determined to belong to a young female H. e. yuanmouensis.
Age
The Yuanmou Formation has been identified as a fossil-bearing site since the 1920s, and palaeontological work on the area suggest a Lower Pleistocene age. Because the formation is faulted (several rock masses have been displaced), biostratigraphy (dating an area based on animal remains) of the human-bearing layer is impossible.
In 1976, Li Pu and colleagues palaeomagnetically dated the incisors to the Gilsa geomagnetic polarity event (when the Earth's magnetic polarity reversed for a short interval) roughly 1.7 million years ago. In 1977, with a much larger sample size, Cheng Guoliang and colleagues instead placed the area during the Olduvai subchron, and dated it to 1.64–1.63 million years ago. In 1979, Li Renwei and Lin Daxing measured the alloisoleucine/isoleucine protein ratio in animal bones and produced a date of 0.8 million years ago for the Yuanmou Man. In 1983, Liu Dongsheng and Ding Menglin — using palaeomagnetism, biostratigraphy, and lithostratigraphy — reported a date of 0.6–0.5 million years ago during the Middle Pleistocene, despite the animal remains pointing to an older date. In 1988, Q. Z. Liang agreed with Cheng on dating it to the Olduvai subchron. In 1991, Qian and Guo Xing Zhou came to the same conclusion as Liang. In 1998, R. Grün and colleagues, using electron spin resonance dating on 14 horse and rhino teeth, calculated an interval of 1.6–1.1 million years ago. In 2002, Masayuki Hyodo and colleagues, using palaeomagnetism, reported a date of 0.7 million years ago near the Matuyama–Brunhes geomagnetic boundary during the Middle Pleistocene. Later that year, the boundary was re-dated to 0.79–0.78 million years ago by geophysicist Brad Singer and colleagues. In 2003, Ri Xiang Zhu and colleagues made note of the inconsistency among previous palaeomagnetic studies, and in 2008 palaeomagnetically dated it to roughly 1.7 million years ago. They believed Middle Pleistocene dates were probably caused by too small a sample size. The tibia was probably found somewhere in layers 25–28, and by Zhu's calculations would date to 1.7–1.4 million years ago.
The date of 1.7 million years ago is widely cited. This makes the Yuanmou Man the earliest fossil evidence of humans in China, and roughly contemporaneous with the oldest humans in Southeast Asia. It was part of a major expansion of H. erectus across Asia, the species extending from 40°N in Xiaochangliang to 7°S in Java, and inhabiting temperate grassland to tropical woodland. The Yuanmou Man is only slightly younger than the Dmanisi hominins from the Caucasus, 1.77–1.75 million years old, who are the oldest fossil evidence of human emigration out of Africa. These dates could therefore mean that humans spread rather rapidly across the Old World, over a period of less than 70,000 years. Yuanmou Man could also indicate humans dispersed from south to north across China, but there are too few other well-constrained early Chinese sites to test this hypothesis. Humans likely already settled in China at earliest 2.12 million years ago evidenced by stone tools recovered from the Loess Plateau in northwestern China.
Classification
The teeth were formally described in 1973 by Chinese palaeoanthropologist Hu Chengzhi, who identified it as a new subspecies of Homo erectus, distinct from and much more archaic than the Middle Pleistocene Peking Man, H. ("Sinanthropus") e. pekinensis, from Beijing. He named it H. ("S.") e. yuanmouensis, and believed it represents an early stage in the evolution of Chinese H. erectus. In 1985, Chinese palaeoanthropologist Wu Rukang said that few authors in the field recognise H. e. yuanmouensis as valid, with most favouring there having been only one subspecies of H. erectus which inhabited China. However, in 2011, Indonesian palaeoanthropologist Yahdi Zaim and colleagues, based on dental comparisons, concluded that the Sangiran H. erectus—which had colonised Java by 1.6 million years ago—had descended from a different dispersal event than the Peking Man who had colonised China by 780,000 years ago. The Sangiran teeth were notably more reminiscent of the early H. habilis from Africa than those of the Peking Man. This would mean that H. erectus dispersed across East Asia multiple times.
Anatomy
For the teeth, only the left and right first incisors are preserved for the Yuanmou Man. The left incisor measures in breadth and in width, and the right incisor and . The incisors are overall robust. They notably flare out in breadth from bottom to top. The labial (lip) side is mostly flat with the exception of some grooves and depressions, and the base is somewhat convex like that of the Peking Man. The lingual (tongue) side caves in like that of the Peking Man, but has a defined ridge running down the middle like H. (e?) ergaster and H. habilis. The cross section at the neck of the tooth (at the gum line) is nearly elliptical. Similar anatomy is also exhibited in Late Pleistocene archaic human specimens from Xujiayao, China; and Neanderthals from Krapina, Croatia.
The tibia is a long mid-shaft fragment. It is gracile and laterally (on the sides) flattened. The anterior (front) aspect is round and obtuse, and has a weak S-curve. The interosseous crest (which separates the muscles on the back of the leg from those of the front of the leg) is shallow. On the posterior (back) aspect, there is a ridge running down the middle, developed attachment (where the muscle used to attach to the bone) for the flexor digitorum longus muscle (which flexes the toes), and a defined soleal line. It is somewhat similar to tibiae assigned to H. habilis. Like other H. erectus, the tibia is quite thick, constricting the medullary cavity where the bone marrow is stored. At the probable midpoint of the shaft, the circumference is , the breadth from left to right is , and from front to back is . The individual may have stood roughly .
Culture
Palaeohabitat
A total of 35 other animals have been reported from layer 25. The mammals are: the chalicothere Nestoritherium; the deer Cervocerus ultimus, Procapreolus stenosis, Paracervulus attenuatus, Rusa yunnanensis, Cervus stehlini, Muntiacus lacustris, M. nanus, gazelle, and chital; cattle; pigs; the elephant Stegodon; rhino; the giant short-faced hyaena Pachycrocuta brevirostris licenti; pika; and small rodents. Various mollusks, turtles, crustaceans, and plants were also found. Fossil pollen deposits on the Yuanmou teeth and artefacts can largely be assigned to herbaceous plants, pine, and alder. Altogether, they suggest Yuanmou Man inhabited a mixed environment featuring open grassland, bushland, forest, marshland, and freshwater, not unlike what is suggested for the Dmanisi hominins.
Technology
In 1973, three retouched tools were found within of the incisors, two of them in a layer in elevation below the incisors, and an additional one about above the incisors. Three similar tools were recovered at the surface of the Shangnabang site. Within , cores, flakes, choppers, pointed tools, and scrapers were found, totaling 16 tools. They were made of quartz and quartzite probably gathered from the nearby river. Most were modified with direct hammering (using a hammerstone to flake off pieces), but a few were modified with the bipolar technique (smashing the core with the hammerstone, creating several flakes). The tools feature simple conchoidal fracture, not unlike contemporary Oldowan tools from Africa.
In 1985, Chinese palaeoanthropologist Jia Lanpo described two probably burnt mammal bones as well as considerable charcoal remnants from Yuanmou. He suggested this could represent extremely early fire usage by humans. These bones and similarly aged burnt remnants from various parts of the world are now considered to be the products of natural wildfires, making it unlikely that the Peking Man was using fire.
| Biology and health sciences | Homo | Biology |
4529793 | https://en.wikipedia.org/wiki/Inostrancevia | Inostrancevia | Inostrancevia is an extinct genus of large carnivorous therapsids which lived during the Late Permian in what is now European Russia and Southern Africa. The first-known fossils of this gorgonopsian were discovered in the context of a long series of excavations carried out from 1899 to 1914 in the Northern Dvina, Russia. Among these are two near-complete skeletons embodying the first described specimens of this genus, being also the first gorgonopsian identified in Russia. Several other fossil materials were discovered there, and the various finds led to confusion as to the exact number of valid species, before only two of them were formally recognized, namely I. alexandri and I . latifrons. A third species, I. uralensis, was erected in 1974, but the fossil remains of this taxon are very thin and could come from another genus. More recent research carried out in South Africa and Tanzania has discovered specimens identified as belonging to this genus, with the South African specimens being classified within the species I. africana. The whole genus is named in honor of Alexander Inostrantsev, professor of Vladimir P. Amalitsky, the paleontologist who described the taxon.
Possessing a skull measuring approximately long depending on the species, all for a body length reaching , Inostrancevia is the largest known gorgonopsian, being rivaled in size only by the imposing Rubidgea. It has a broad and elongated skull equipped with large oval-shaped temporal fenestrae. It also has very advanced dentition, possessing large canines, the longest of which can reach and which may have been used to shear the skin of prey. Like most other gorgonopsians, Inostrancevia had a particularly large jaw opening angle, which would have allowed it to inflict fatal bites. Gorgonopsians in general would have been relatively fast predators, killing their prey by delivering slashing bites with their saber teeth. The skeleton is robustly constructed, but new studies are necessary for a better anatomical description and understanding about its paleobiological functioning.
Gorgonopsians were a group of carnivorous stem mammals with saber teeth that disappeared at the end of the Permian. The first classifications placed Inostrancevia as close to African taxa before 1948, the year in which Friedrich von Huene erected a distinct family, Inostranceviidae. Although this model was mainly followed in the scientific literature of the 20th and early 21st centuries, phylogenetic analysis published since 2018 consider it to belong to a group of derived Russian origin gorgonopsians, now classified alongside the genera Suchogorgon, Sauroctonus and Pravoslavlevia, the latter and Inostrancevia forming the subfamily Inostranceviinae. Russian and African fossil records show that Inostrancevia lived in river ecosystems containing many tetrapods, where it appears to have been the main predator. These faunas were mainly occupied by dicynodonts and pareiasaurs, which would most likely have constituted its prey. In the Russian territory, Inostrancevia would have been the only large gorgonopsian present, while it would have been briefly contemporary with the rubidgeines in Tanzania. When the rubidgeines disappeared from South African territory, Inostrancevia would have in turn occupied the role of apex predator before disappearing in turn during the Permian-Triassic extinction.
Research history
Russian discoveries
During the 1890s, Russian paleontologist Vladimir Amalitsky discovered freshwater sediments dating from the Upper Permian in Northern Dvina, Arkhangelsk Oblast, northern European Russia. The locality, known as PIN 2005, consists of a creek with sandstone and lens-shaped exposures in a bank escarpment, containing many particularly well-preserved fossil skeletons. This type of fauna from this period, previously known only from South Africa and India, is considered as one of the greatest paleontological discoveries of the late 19th and early 20th centuries. After the preliminary reconnaissance of the place, Amalitsky conducts systematic research with his companion . The first excavations began in 1899, and several of her findings where sent to Warsaw, Poland, in order to be prepared there. The exhumations of the fossils then lasted until 1914, when the research stopped due to the start of the World War I. When this event begins, Amalitsky tries to save his collection of fossils residing in Warsaw in order to transfer it to the Nizhny Novgorod oblast. However, the arrival of the October Revolution in 1917 and the changing politics within the country indirectly led to his death in December of the same year. Subsequently, his fossil collection was transferred to Leningrad and became an integral part of the geological department of the city's university. During World War II, part of the fossils from the collection were transferred to the Paleontological Institute of the Russian Academy of Sciences in Moscow.
Among all the fossils Amalitsky described before his death are two remarkably complete skeletons of large gorgonopsians, now cataloged as PIN 1758 and PIN 2005/1578. After identification, he assigned the two specimens within a new genus and species, which he named Inostranzevia alexandri, the specimen PIN 2005/1578 being later recognized as its lectotype. These two specimens constitute the first known quasi-complete postcranial remains identified among gorgonopsians as well as the first representative of this group to have been identified in Russia. Although the taxon was not officially described posthumously until 1922, the use of this name in scientific literature dates back to the beginning of the 20th century, notably in the works of Friedrich von Huene and Edwin Ray Lankester. Taxonomic issues regarding the original naming of the genus are the subject of a study which should be published later. Although the etymology of the genus and type species is not provided in the earliest-known descriptions of the taxon, the full name of the animal is named in honor of the renowned geologist , who was one of Amalitsky's teachers. Amalitsky's article generally describes all the fossil discoveries made in the Northern Dvina, and not Inostrancevia itself, the article mentioning that further research on this gorgonopsian is subject to research.
In 1927, one of Amalitsky's colleagues, Pavel Pravoslavlev, wrote two works, including a book, which are the first in-depth descriptions of the fossils today attributed to this taxon. In his works, Pravoslavlev changed the typography of the name "Inostranzevia" to "Inostrancevia". Although the original name has been used a few times in recent scientific literature, the second term has since entered into universal usage and must be maintained according to the rule of article 33.3.1 of the ICZN. Among the many species of Inostrancevia erected and described on its part, only I. latifrons is found to be valid. The holotype of this species, cataloged PIN 2005/1857, consists of a large skull missing the lower jaw, discovered in the same locality as that of the first known specimens of I. alexandri. Another skull was also discovered in the same locality as the holotype, while an incomplete skeleton was discovered in the village of Zavrazhye, located in Vladimir Oblast. The specific name latifrons comes from the Latin latus "broad" and frōns "forehead", in reference to the size and the more robust cranial constitution than that of I. alexandri. In 1974, Leonid Petrovich Tatarinov carried out a large revision of the theriodonts then known in the USSR. In his work, he revises the validity of the taxa erected by Pravoslavlev and describes a third species of Inostrancevia, I. uralensis, on the basis of part of the skull. The holotype specimen, cataloged as PIN 2896/1, consists of a left basioccipital having been discovered in the locality of Blumental-3, located in the Orenburg Oblast. The specific name uralensis refers to the Ural River, where the holotype specimen of the taxon was found. However, due to its poor fossil preservation of this species, Tatarinov argues that it is possible that I. uralensis could belong to a new genus of large gorgonopsians.
African discoveries
In 2010, the Bloemfontein Museum sent an expedition to the farm of Nooitgedacht 68, located near the town of Bethulie, in the Karoo Basin, South Africa. It was during this same expedition that Nthaopa Ntheri discovered a partial skeleton of a large gorgonopsian which include an almost complete skull, cataloged as NMQR 4000. During a second expedition launched the following year, John Nyaphuli discovers another partial skeleton similar to that of the previously discovered specimen, cataloged as NMQR 3707. The existence of these two specimens are mentioned in 2014 in the chapter of a work listing the discoveries made at this farm, but it was not until 2023 that Christian F. Kammerer and his colleagues made the first official description concerning these latter. Their descriptions unexpectedly confirm that these specimens belong to Inostrancevia, which is a significant first given that the genus was historically only reported in Russia. The specimens nevertheless possessed some differences allowing them to be distinguished from the Russian lineages, they were then classified in the newly erected species I. africana, the specimen NMQR 4000 being designated as the holotype of this species, while NMQR 3707 was bequeathed as a paratype. The specific name, meaning "Africa" in Latin, refers to the first proven presence of its kind within that continent. However, the article officially describing this animal focuses primarily on the stratigraphic significance of the findings and is only a brief introduction to the anatomy of the new fossil material, being the subject for future study. Earlier, in June 2007, a team of paleontologists discovered an isolated left premaxilla, cataloged as NMT RB380, in the Ruhuhu Basin, southern Tanzania. The fossil bone was subsequently scanned and identified as Inostrancevia sp. in a 2024 study led by Anna J. Brant and Christian A. Sidor.
Synonyms and formerly assigned species
In his two works published in 1927, Pravoslavlev also named several additional gorgonopsian taxa,. In their broad revision of the classification of therapsids published in 1956, David Watson and Alfred Romer recognized without argumentation almost all of the taxa erected by Pravoslavlev as valid, but their opinions were never followed in subsequent works. In addition of I. latifrons, Pravoslavlev names and describes two additional species of the genus Inostrancevia: I. parva and I. proclivis. In 1940, the paleontologist Ivan Yefremov expressed doubts about this classification, and considered that the holotype specimen of I. parva should be viewed as a juvenile of the genus and not as a distinct species. It was in 1953 that Boris Pavlovich Vyuschkov completely revised the species named for Inostrancevia. For I. parva, he moves it to a new genus, which he names Pravoslavlevia, in honor of the original author who named the species. Although being a distinct and valid genus, Pravoslavlevia turns out to be a closely related taxon. Also in his article, he considers that I. proclivis is a junior synonym of I. alexandri, but remains open to the question of the existence of this species, arguing his opinion with the insufficient preservation of type specimens. This taxon will be definitively judged as being conspecific to I. alexandri in the revision of the genus carried out by Tatarinov in 1974.
In the first work published earlier the same year, Pravoslavlev erected another genus of gorgonopsians, Amalitzkia, with the type species A. annae. In his larger work published subsequently, he erected a second species under the name of A. wladimiri. The genus as well as the two species are named in honor of the couple of paleontologists who carried out the work on the first known specimens of I. alexandri. In 1953, Vjuschkov discovered that the genus Amalitzkia is a junior synonym of Inostrancevia, renaming A. wladimiri to I. wladimiri, before the latter was itself recognized as a junior synonym of I. latifrons by later publications. For some unclear reason, Vjuschkov refers A. annae as a nomen nudum, when his description is quite viable. Just like A. wladimiri, A. annae will be synonymized with I. latifrons by Tatarinov in 1974.
In 2003, Mikhail F. Ivakhnenko erected a new genus of Russian gorgonopsian under the name of Leogorgon klimovensis, on the basis of a partial braincase and a large referred canine, both discovered in the Klimovo-1 locality, in the Vologda Oblast. In his official description, Ivakhnenko classifies this taxon among the subfamily Rubidgeinae, whose fossils are exclusively known from what is now Africa. This would therefore make Leogorgon the first known representative of this group to have lived outside this continent. In 2008, however, Ivakhnenko noted that, due to its poorly known anatomy, Leogorgon could be a relative of the Russian Phthinosuchidae rather than the sole Russian representative of the Rubidgeinae. In 2016, Kammerer formally rejected Ivakhnenko's classifications, because the holotype braincase of Leogorgon likely came from a dicynodont, while the attributed canine tooth is indistinguishable from that of Inostrancevia. Since then, Leogorgon has been recognized as a nomen dubium of which part of the fossils possibly come from Inostrancevia.
Other species belonging to distinct lineages were sometimes inadvertently classified in the genus Inostrancevia. For example, in 1940, Efremov classifies a gorgonopsian of then-problematic status as I. progressus. However, in 1955, Alexey Bystrow moved this species to the separate genus Sauroctonus. A large maxilla discovered in Vladimir Oblast in the 1950s was also assigned to Inostrancevia, but the fossil would be reassigned to a large therocephalian in 1997, and later designated as the holotype of the genus Megawhaitsia in 2008.
Description
Inostrancevia is a gorgonopsian with a fairly robust morphology, the Spanish paleoartist Mauricio Antón describing it as a "scaled-up version" of Lycaenops. The numerous descriptions given to this taxon make it one of the most emblematic animals of the Permian period, mainly because of its large size among gorgonopsians, rivaled only by the South African genus Rubidgea, the latter having a roughly similar size. Gorgonopsians were skeletally robust, yet long-limbed for therapsids, with a somewhat dog-like stance, though with outwards-turned elbows. It is unknown whether non-mammaliaform therapsids such as gorgonopsians were covered in hair or not.
The specimens PIN 2005/1578 and PIN 1758, belonging to I. alexandri, are among the largest and most complete gorgonopsian fossils identified to date. Both specimens are around long, with the skulls alone measuring over . However, I. latifrons, although known from more fragmentary fossils, is estimated to have a more imposing size, the skull being long, indicating that it would have measured and weighed . The size of I. uralensis is unknown due to very incomplete fossils, but it appears to be smaller than I. latifrons. The two known specimens of I. africana are among the largest gorgonopsians to have been discovered in Africa, the holotype skull measuring , while that of the paratype reaches . These proportions are matched only by the largest known specimens of Rubidgea. Based on comparisons with various other gorgonopsians, the Tanzanian specimen of Inostrancevia would have had a skull with an estimated length of between long. However, the authors mention that it is difficult to know if the specimen would have been similar in size to those of recognized species.
Skull
The overall shape of the skull of Inostrancevia is similar to those of other gorgonopsians, i. e. long and narrow. It has a broad back skull, a raised and elongated snout, relatively small eye sockets and thin cranial arches. The pineal foramen is located near the posterior edge of the parietals and rests on a strong projection in the middle of an elongated hollow like impression. The sagittal suture is reinforced with complex curvatures. The ventral surface of the palatine bones is completely smooth, lacking traces of palatine teeth or tubercles. Just like Viatkogorgon, the top margin of the quadrate is thickened. The dentary bone appear to have a clearly visible chin-like structure. The four recognized species are distinguished by their own specific characteristics. I. alexandri is distinguished by its relatively narrow occiput, a broad and rounded oval temporal fenestra and the transverse flangues of the pterygoid with teeth. I. latifrons is distinguished by a comparatively lower and broader snout, larger parietal region, fewer teeth and a less developed palatal tuberosities. I. uralensis is characterized by a transversely elongated oval slot-like temporal fenestra. I. africana is characterized by the strong constriction of the jugal under the orbit, a proportionally longer snout, the pineal foramen located in a deep parietal depression, as well as a much more raised and massive dentary bone.
The jaws of Inostrancevia are powerfully developed, equipped with teeth able to hold and tear the skin of prey. The teeth are also devoid of cusps and can be distinguished into three types: the incisors, the canines and the postcanines. All teeth are more or less laterally compressed and have finely serrated front and rear edges. When the mouth is closed, the upper canines move into position at the sides of the mandible, reaching its lower edge. The canines of Inostrancevia measuring between and , they are among the largest identified among non-mammalian therapsids, only the anomodont Tiarajudens have similarly sized canines. In his 1927 description, Pravoslavlev describes the teeth of Inostrancevia as reminding him of those of the saber-toothed cat Machairodus. In the upper and lower jaws, these canines are roughly equal in size and are slightly curved. The incisors turn out to be very robust. A unique trait among gorgonopsians, Inostrancevia only has four incisors on the premaxilla, unlike other representatives of the group who generally have five. The postcanine teeth are present on the upper jaw, on its slightly upturned alveolar edges. In contrast, they are completely absent from the lower jaw. There are indications that the tooth replacement would have taken place by the young teeth, growing at the root of the old ones and gradually supplanting them. The capsule of the canines is very large, containing up to three capsules of replacement canines at different stages of development.
Postcranial skeleton
Although the postcranial anatomy of Inostrancevia was first described in detail in 1927 by Pravoslavlev, new discoveries and anatomical descriptions of this taxon have led authors suggesting further revisions to broaden the skeletal understanding of the animal. The skeleton of Inostrancevia is of very robust constitution, mainly at the level of the limbs. The ungual phalanges have an acute triangular shape. Inostrancevia has the most autapomorphic postcranial skeleton identified on a gorgonopsian. The scapula of this latter is unmistakable, with an enlarged plate-like blade unlike that of any other known gorgonopsians, but its anatomy is also unusual, with ridges and thickened tibiae, especially at their joint margins. The scapular blade of Inostrancevia being extremely enlarged, its morphology will most likely be subject to future study regarding its paleobiological function.
Classification and evolution
From its original description published in 1922, Inostrancevia was immediately classified in the family Gorgonopsidae after anatomical comparisons made with the type genus Gorgonops. This classification was maintained as such until 1948, when von Huene established a separate family of gorgonopsians, Inostranceviidae, to include Inostrancevia. Huene's opinion was generally shared in various studies published subsequently during the 20th century and even into the 21st century, although with some alternative classifications. In 1974, Tatarinov classified Pravoslavlevia as a sister taxon of Inostrancevia within this family. In 1989, Denise Sigogneau-Russell proposes a similar classification, but moves the taxon reuniting the two genera as a subfamily, being renamed Inostranceviinae, and is classified in the more general family Gorgonopsidae. In 2002, in his revision of the Russian gorgonopsians, Mikhail F. Ivakhnenko re-erects the family Inostranceviidae and classifies the taxon as one of the lineages of the superfamily "Rubidgeoidea", placed alongside the Rubidgeidae and Phtinosuchidae. One year later, in 2003, he reclassifies Inostrancevia in the family Inostranceviidae, similar to Tatarinov's proposal, but the latter classifies it alone, making it a monotypic taxon.
In a thesis written in 2007, German paleontologist Eva V. I. Gebauer carried out the very first phylogenetic analysis of gorgonopsians. Based on observations made on the occipital bones and canines, Gebauer moved Inostrancevia as a sister taxon to the Rubidgeinae, a lineage consisting of robust African gorgonopsians. In 2016 Christian F. Kammerer regarded Gebauer's analysis as "unsatisfactory", citing that many of the characters used by her analysis were based upon skull proportions that are variable within taxa, both individually and ontogenetically (i.e. traits that change through growth). In 2018, in their description of Nochnitsa and re-description of the skull of Viatkogorgon, Kammerer and Vladimir Masyutin propose that all Russian and African taxa should be separately grouped into two distinct clades. For Russian genera (except basal taxa), this relationship is supported by notable cranial traits, such as the close contact between the pterygoid and the vomer. The classification proposed by Kammerer and Masyutin will serve as the basis for all other subsequent phylogenetic studies of gorgonopsians. Using this model, the 2023 study by Kammerer and colleagues describing I. africana recovers it as a sister taxon to I. alexandri within the Russian origin clade. As with previous classifications, Pravoslavlevia is still considered as the sister taxon of Inostrancevia. The following cladogram shows the position of Inostrancevia within the Gorgonopsia after Kammerer and Rubidge (2022):
Gorgonopsians are a major group of carnivorous therapsids, the oldest known definitive specimen coming from the Mediterranean island of Majorca, and which probably dates to the early Middle Permian, or even earlier. During the Middle Permian, the majority of representatives of this clade were quite small and their ecosystems were mainly dominated by dinocephalians, large therapsids characterized by strong bone robustness. However, some genera, notably Phorcys, are relatively larger in size and already occupy the role of superpredator in the one of the oldest geological strata of the Karoo Supergroup. Gorgonopsians were the first group of predatory animals to develop saber teeth, long before true mammals and dinosaurs evolved. This feature later evolved independently multiple times in different predatory mammal groups, such as felids and thylacosmilids. Geographically, gorgonopsians are mainly distributed in the present territories of Africa and European Russia, with, however, an indeterminate specimen having been identified in the Turpan Depression, in north-west China, as well as a possible fragmentary specimen discovered in the Kundaram Formation, located in central India. After the Capitanian extinction, gorgonopsians began to occupy ecological niches abandoned by dinocephalians and large therocephalians, and adopted an increasingly imposing size, which very quickly gave them the role of superpredators. In Africa, it is mainly the rubidgeines who occupy this role, while in Russia, only Inostrancevia acquires as such, the only known gorgonopsian and contemporary of this latter, Pravoslalevia, being considerably smaller.
Paleobiology
Hunting strategy
One of the most recognizable characteristics of Inostrancevia (and other gorgonopsians, as well) is the presence of long, saber-like canines on the upper and lower jaws. How these animals would have used this dentition is debated. The bite force of saber-toothed predators (like Inostrancevia), using three-dimensional analysis, was determined by Stephan Lautenschlager and colleagues in 2020: their findings detailed that, despite morphological convergence among saber-toothed predators, there is a range of methods of possible killing techniques. The similarly-sized Rubidgea is capable of producing a bite force of 715 newtons; although lacking the necessary jaw strength to crush bone, the analysis found that even the most massive gorgonopsians possessed a more powerful bite than other saber-toothed predators. The study also indicated that the jaw of Inostrancevia was capable of a massive gape, perhaps enabling it to deliver a lethal bite, and in a fashion similar to the hypothesised killing technique of Smilodon (or 'saber-toothed cat').
Antón provided an overview of gorgonopsian biology in is 2013 book, writing that despite their differences from saber-toothed mammals, many features of their skeletons indicated they were not sluggish reptiles but active predators. While their brains were relatively smaller than those of mammals, and their sideways placed eyes provided limited stereoscopic vision, they had well-developed turbinals in their nasal cavity, a feature associated with an advanced sense of smell, which would have helped them track prey and carrion. The canine saber teeth were used for delivering the slashing killing-bite, while the incisors, which formed an arch in front of the saber teeth, held the prey and cut the flesh while feeding. To allow them to increase their gape when biting, gorgonopsians had several bones in their mandibles that could move in relation to each other and had a double articulation with the skull—unlike in mammals where the rear joint articular bone has become the malleus ear bone. Antón envisioned gorgonopsians would hunt by leaving their cover when prey was close enough, and use their relatively greater speed to pounce quickly on it, grab it with their forelimbs, and bite any part of the body that would fit in their jaws. Such a bite would cause a large loss of blood, but the predator would continue to try to bite vulnerable parts of the body.
Motion
Antón stated in 2013 that while the post-cranial skeletons of gorgonopsians were basically reptilian, their stance was far more upright than in more primitive synapsids, like pelycosaurs, which were more sprawling. Regular locomotion of gorgonopsians would have been similar to the "high walks" seen in crocodilians, wherein the belly is carried above the ground, with the feet pointing forwards, and the limbs carried under the trunk instead of to the sides. The forelimbs had a more horizontal posture than the hindlimbs, with the elbows pointing outwards during movement, but the gait of the hindlimbs would have resembled that of mammals. As in reptiles, the tail muscles (such as the caudofemoralis) were important in flexion of the hindlimb, whereas the tails of mammals are merely for balance. Their feet were probably plantigrade (where the soles were placed flat on the ground), though they were likely more swift and agile than their prey. Their feet were more symmetrical compared to the reptilian condition, making contact with the ground more efficient, similar to running mammals.
Palaeoecology
Paleoenvironment
Inostrancevia is currently the only formally recognized gorgonopsian genus to have had a transcontinental distribution, being present in both territories from which the group's fossils are unanimously recorded, namely in Southern Africa and European Russia. In all the geological formations concerned, Inostrancevia would have been one if not the main apexpredator of these faunas, targeting a large majority of the tetrapods living alongside it, and more probably towards dicynodonts and pareiasaurs During the Late Permian when Inostrancevia lived, the Southern Urals (close in proximity to the Sokolki assemblage) were located around latitude 28–34°N and defined as a "cold desert" dominated by fluvial deposits. The Salarevo Formation in particular (a horizon where the Russian species Inostrancevia hails from) was deposited in a seasonal, semi-arid-to-arid area with multiple shallow water lakes which was periodically flooded. The Paleoflora of much of European Russia at the time was dominated by a genus of peltaspermaceaen, Tatarina, and other related genera, followed by ginkgophytes and conifers. On the other hand, ferns were relatively rare and sphenophytes were only locally present. There are also hygrophyte and halophyte plants in coastal areas as well as conifers that are more resistant to drought and higher altitudes. The Upper Daptocephalus Assemblage Zone in South Africa would have been a well-drained floodplain. The Usili Formation in Tanzania corresponds to an alluvial plain which would have had numerous small meandering streams passing through well-vegetated floodplains. The basement of this formation would also have housed a generally high phreatic zone.
Contemporary fauna
In the Russian fossil record, Inostrancevia is currently the only large gorgonopsian to have been documented, with Pravoslavlevia being a smaller representative. In Tanzania, however, the taxon was coeval with a considerable number of other gorgonopsians, including even the large rubidgeines Dinogorgon and Rubidgea. In South Africa, Inostrancevia would probably have occupied the place of the main apexpredator after the extinction of the rubidgeines. However, it is possible that I. africana would not have been the only gorgonopsian to have been discovered on the Nooitgedacht 68 farm, because an indeterminate specimen belonging to this group is also listed there. In southern Africa, dicynodonts are the most abundant fossil tetrapods, while in the Russian archives only Vivaxosaurus is known. Apart from gorgonopsians, the genus was also contemporaneous with other theriodonts, such as therocephalians (mainly akidnognathids) and numerous basal cynodonts such as Dvinia and Procynosuchus. Exclusively in the Usili Formation, Inostrancevia would have been contemporary with biarmosuchians of the genera Burnetia and Pembacephalus. A number of other non-synapsid tetrapods were contemporaneous with Inostrancevia. Among sauropsids, pareiasaurs, notably Scutosaurus, are the tetrapods most present in the Russian fossil record, although other representatives such as Anthodon and Pareiasaurus are known in African formations. Contemporary archosauromorphs such as Aenigmastropheus and Proterosuchus have only been identified in Africa, respectively in Tanzania and South Africa. Contemporary temnospondyls include Dvinosaurus in Russia and Peltobatrachus in Tanzania. Reptiliomorphs like Chroniosuchus and Kotlassia have been identified in Russia.
Extinction
Gorgonopsians, including Inostrancevia, disappeared in the Late Lopingian during the Permian–Triassic extinction event, mainly due to volcanic activities that originated in the Siberian Traps. The resulting eruption caused a significant climatic disruption unfavorable to their survival, leading to their extinction. Their ecological niches gave way to modern terrestrial ecosystems including sauropsids, mostly archosaurs, and among the few therapsids surviving the event, mammals. However, some Russian gorgonopsians have already disappeared a little time before the event, having consequently abandoned some of their niches to large therocephalians.
| Biology and health sciences | Proto-mammals | Animals |
7842592 | https://en.wikipedia.org/wiki/Kugelblitz%20%28astrophysics%29 | Kugelblitz (astrophysics) | A kugelblitz () is a theoretical astrophysical object predicted by general relativity. It is a concentration of heat, light or radiation so intense that its energy forms an event horizon and becomes self-trapped. In other words, if enough radiation is aimed into a region of space, the concentration of energy can warp spacetime so much that it creates a black hole. This would be a black hole the original mass–energy of which was in the form of radiant energy rather than matter; however, there is currently no uniformly accepted method of distinguishing black holes by origin.
John Archibald Wheeler's 1955 Physical Review paper entitled "Geons" refers to the kugelblitz phenomenon and explores the idea of creating such particles (or toy models of particles) from spacetime curvature.
A study published in Physical Review Letters in 2024 argues that the formation of a kugelblitz is impossible due to dissipative quantum effects like vacuum polarization, which prevent sufficient energy buildup to create an event horizon. The study concludes that such a phenomenon cannot occur in any realistic scenario within our universe.
The kugelblitz phenomenon has been considered a possible basis for interstellar engines (drives) for future black hole starships.
In fiction
A kugelblitz is a major plot point in the third season of the American superhero television series The Umbrella Academy.
A kugelblitz is the home of a major faction in Frederik Pohl's "Gateway" novels.
| Physical sciences | Basics_2 | Astronomy |
7847673 | https://en.wikipedia.org/wiki/Developmental%20coordination%20disorder | Developmental coordination disorder | Developmental coordination disorder (DCD), also known as developmental motor coordination disorder, developmental dyspraxia, or simply dyspraxia (from Ancient Greek praxis 'activity'), is a neurodevelopmental disorder characterized by impaired coordination of physical movements as a result of brain messages not being accurately transmitted to the body. Deficits in fine or gross motor skills movements interfere with activities of daily living. It is often described as disorder in skill acquisition, where the learning and execution of coordinated motor skills is substantially below that expected given the individual's chronological age. Difficulties may present as clumsiness, slowness and inaccuracy of performance of motor skills (e.g., catching objects, using cutlery, handwriting, riding a bike, use of tools or participating in team sports or swimming). It is often accompanied by difficulty with organisation and/or problems with attention, working memory and time management.
A diagnosis of DCD is reached only in the absence of other neurological impairments such as cerebral palsy, multiple sclerosis, or Parkinson's disease. The condition is lifelong and its onset is in early childhood. It is thought to affect about 5% of the population. Occupational therapy can help people with dyspraxia to develop their coordination and achieve things that they might otherwise find extremely challenging to accomplish. Dyspraxia has nothing to do with intelligence but people with dyspraxia may struggle with self-esteem because their peers can easily do things they struggle with on a daily basis. Dyspraxia is not often known as a disability in the general public.
Signs and symptoms
The World Health Organization (WHO) recognizes DCD as a condition and have published their definition in the International Classification of Diseases. This describes DCD as:
The American Psychiatric Association (APA)'s Diagnostic and Statistical Manual, DSM-5 classifies Developmental Coordination Disorder (DCD) as a discrete motor disorder under the broader heading of neurodevelopmental disorders. It is often described as a disorder in skill acquisition or motor learning, where the learning and execution of coordinated motor skills is substantially below that expected given the individual's chronological age. Various areas of development can be affected by DCD and these may persist into adulthood.
In children, DCD may exhibit as delays in early development of sitting, crawling, walking; poor ability or difficulties with childhood activities such as running, jumping, hopping, catching, sports and swimming; slowness; frequent tripping and bruising; poor handwriting skills; difficulties with self care; difficulties with skills such as using cutlery or tying shoelaces; poor spatial understanding; difficulty following instructions; poor time management; and often losing objects.
In adulthood, in addition to a childhood history as above, the condition may manifest as a difficulty learning new motor skills or applying skills in a different or busy environment, poor organisation and time management skills, missed deadlines and lateness for appointments (or earliness as a coping strategy), and awkward pauses before answering in conversation. There is often a history of underachievement in education or the workplace. Although skills can be acquired, such as neat handwriting, handwriting speed will then be much lower than expected.
Evidence from research and clinical practice indicates that DCD is not just a physical disorder, and there may be deficits in executive functions, behavioural organisation and emotional regulation that extend beyond the motor impairments and which are independent of diagnoses of co-morbidities. In addition to the physical or motor impairments, developmental coordination disorder is associated with problems with memory, especially working memory. This typically results in difficulty remembering instructions, difficulty organizing one's time and remembering deadlines, increased propensity to lose things or problems carrying out tasks which require remembering several steps in sequence (such as cooking). Whilst most of the general population experience these problems to some extent, they have a much more significant impact on the lives of dyspraxic people. However, many dyspraxics have excellent long-term memories, despite poor short-term memory. Many dyspraxic people benefit from working in a structured environment, as repeating the same routine minimises difficulty with time-management and allows them to commit procedures to long-term memory.
People with developmental coordination disorder sometimes have difficulty moderating the amount of sensory information that their body is constantly sending them, so as a result these dyspraxic people may be prone to sensory overload and panic attacks.
Moderate to extreme difficulty doing physical tasks is experienced by some people with dyspraxia, and fatigue is common because so much energy is expended trying to execute physical movements correctly. Some dyspraxic people have hypotonia, low muscle tone, which can also detrimentally affect balance.
Gross motor control
Whole body movement and motor coordination issues mean that major developmental targets including walking, running, climbing and jumping can be affected. The difficulties vary from person to person and can include the following:
Poor timing.
Poor balance (sometimes even falling over in mid-step). Tripping over one's own feet is also common.
Difficulty combining movements into a controlled sequence.
Difficulty remembering the next movement in a sequence.
Problems with spatial awareness, or proprioception.
Trouble picking up and holding onto simple objects such as pencils, owing to poor muscle tone or proprioception.
Clumsiness to the point of knocking things over, causing minor injuries to oneself and bumping into people accidentally.
Difficulty in determining left from right.
Cross-laterality, ambidexterity, and a shift in the preferred hand are also common in people with developmental coordination disorder.
Problems with chewing foods.
Fine motor control
Fine-motor problems can cause difficulty with a wide variety of other tasks such as using a knife and fork, fastening buttons and shoelaces, cooking, brushing teeth, styling hair, shaving, applying cosmetics, opening jars and packets, locking and unlocking doors, and doing housework.
Difficulties with fine motor co-ordination lead to problems with handwriting,
Problems associated with this area may include:
Learning basic movement patterns.
Developing a desired writing speed.
Establishing the correct pencil grip.
Handwriting that is difficult to read and may miss words in sentences or place words in the incorrect order
The acquisition of graphemes – e.g. the letters of the Latin alphabet, as well as numbers.
Developmental verbal dyspraxia
Developmental verbal dyspraxia (DVD) is a type of ideational dyspraxia, causing speech and language impairments. This is the favoured term in the UK; however, it is also sometimes referred to as articulatory dyspraxia, and in the United States the usual term is childhood apraxia of speech (CAS).
Key problems include:
Difficulties controlling the speech organs.
Difficulties making speech sounds.
Difficulty sequencing sounds within a word, and
Difficulty sequencing sounds forming words into sentences.
Difficulty controlling breathing, suppressing salivation and phonation when talking or singing with lyrics.
Slow language development.
Associated disorders and secondary consequences
DCD is known to co-occur with other neurodevelopmental disorders. Most common is attention deficit hyperactivity disorder (ADHD), with an estimated 50% of people with ADHD also having DCD and vice versa. Other co-occurring conditions are autism spectrum disorder (ASD), Developmental Speech and Language Disorder and Developmental Learning Disorder.
People who have developmental coordination disorder may also have one or more of these other co-morbid conditions:
Fetal alcohol spectrum disorder
Dyscalculia (difficulty with numbers).
Dysgraphia (an inability to write neatly or draw).
Dyslexia (difficulty with reading and spelling).
Hypermobility
Hypotonia (low muscle tone).
Nonverbal learning disorder.
Sensory processing disorder.
Visual perception deficits.
However, a person with DCD is unlikely to have all of these conditions. The pattern of difficulty varies widely from person to person; an area of major weakness for one dyspraxic person can be an area of strength or gift for another. For example, while some dyspraxic people have difficulty with reading and spelling due to dyslexia, or with numeracy due to dyscalculia, others may have brilliant reading and spelling or mathematical abilities. Co-morbidity between ADHD and DCD is particularly high.
Sensory processing disorder
Sensory processing disorder (SPD) concerns having oversensitivity or undersensitivity to physical stimuli, such as touch, light, sound, and smell. This may manifest itself as an inability to tolerate certain textures such as sandpaper or certain fabrics such as wool, oral intolerance of excessively textured food (commonly known as picky eating), being touched by another individual (in the case of touch oversensitivity) or it may require the consistent use of sunglasses outdoors since sunlight may be intense enough to cause discomfort to a dyspraxic person (in the case of light oversensitivity). An aversion to loud music and naturally loud environments (such as clubs and bars) is typical behavior of individuals with dyspraxia who have auditory oversensitivity, while only being comfortable in unusually warm or cold environments is typical of a dyspraxic person with temperature oversensitivity. Undersensitivity to stimuli may also cause problems, as individuals do not receive the sensory input they need to understand where their bodies are in space. This can make it even more challenging to complete tasks. Dyspraxic people who are undersensitive to pain may injure themselves without realising it. Some dyspraxic people may be oversensitive to some stimuli and undersensitive to others.
Developmental Language Disorder
Developmental Language Disorder (DLD) research has found that students with developmental coordination disorder and normal language skills still experience learning difficulties despite relative strengths in language. This means that, for students with developmental coordination disorder, their working memory abilities determine their learning difficulties. Any strength in language that they have is not able to sufficiently support their learning.
Students with developmental coordination disorder struggle most in visual-spatial memory. When compared to their peers without motor difficulties, students with developmental coordination disorder are seven times more likely than typically developing students to achieve very poor scores in visual-spatial memory. As a result of this working memory impairment, students with developmental coordination disorder have learning deficits as well.
Psychological and social consequences
Psychological domain: Children with DCD may struggle with lower self-efficacy and lower self-perceived competence in peer and social relations. Some demonstrate greater aggressiveness and hyperactivity.
Social domain: Children may be more vulnerable to social rejection and bullying, possibly resulting in higher levels of loneliness.
Diagnosis
Assessments for developmental coordination disorder typically require a developmental history, detailing ages at which significant developmental milestones, such as crawling and walking, occurred. Motor skills screening includes activities designed to indicate developmental coordination disorder, including balancing, physical sequencing, touch sensitivity, and variations on walking activities.
The American Psychiatric Association has four primary inclusive diagnostic criteria for determining if a child has developmental coordination disorder.
The criteria are as follows:
Motor coordination will be greatly reduced, although the intelligence of the child is normal for the age.
The difficulties the child experiences with motor coordination or planning interfere with the child's daily life.
The difficulties with coordination are not due to any other medical condition
If the child does also experience comorbidities such as intellectual or other developmental disability; motor coordination is still disproportionally affected.
Screening tests
Currently there is no single "gold standard" assessment test for DCD. Various screening tests may be used, including the following.
{| class="wikitable" style="width:90%; border:solid 1px #999999; margin:0 0 1em 1em;"
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!Screening tests that can be used to assess developmental coordination disorder
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Movement Assessment Battery for Children (Movement-ABC – Movement-ABC 2)
Peabody Developmental Motor Scales- Second Edition (PDMS-2)
Bruininks-Oseretsky Test of Motor Proficiency (BOTMP-BOT-2)
Motoriktest für vier- bis sechsjährige Kinder (MOT 4–6)
Körperkoordinationtest für Kinder (KTK)
Test of Gross Motor Development, Second Edition (TGMD-2)
Maastrichtse Motoriek Test (MMT)
Wechsler Adult Intelligence Scale (WAIS-III and WAIS-IV)
Wechsler Individual Achievement Test (WAIT-II)
Test Of Word Reading Efficiency Second Edition (TOWRE-2)
Developmental Coordination Disorder Questionnaire (DCD-Q). The DCD-Q has been translated into many languages. For French-speaking populations, a Canadian-French version and a European-French version are available.
Children's Self-Perceptions of Adequacy in, and Predilection for Physical Activity (CSAPPA)
|}
A baseline motor assessment establishes the starting point for developmental intervention programs. Comparing children to normal rates of development may help to establish areas of significant difficulty.
However, research in the British Journal of Special Education has shown that knowledge is severely limited in many who should be trained to recognise and respond to various difficulties, including developmental coordination disorder, dyslexia and deficits in attention, motor control and perception (DAMP). The earlier that difficulties are noted and timely assessments occur, the quicker intervention can begin. A teacher or GP could miss a diagnosis if they are only applying a cursory knowledge.
A diagnosis of DCD is reached only in the absence of other neurological impairments such as cerebral palsy, multiple sclerosis, or Parkinson's disease.
Classification
Developmental coordination disorder is classified in the fifth revision of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) as a motor disorder, in the category of neurodevelopmental disorders.
Prevalence
The exact proportion of people with the disorder is unknown since the disorder can be difficult to detect due to a lack of specific laboratory tests, thus making diagnosis of the condition one of elimination of all other possible causes/diseases. Approximately 5–6% of children and adults are affected by this condition. and approximately 2% are severely affected.
DCD is a lifelong neurological condition that is expected to be as common in males as it is in females. Currently however, the diagnosis criteria favour males which results in over 80% of males being diagnosed before the age of 16 compared to only 22% for females.
Management
There is no cure for the condition. Instead, it is managed through therapy. Physical therapy or occupational therapy can help those living with the condition.
Some people with the condition find it helpful to find alternative ways of carrying out tasks or organizing themselves, such as typing on a laptop instead of writing by hand, or using diaries and calendars to keep organized. A review completed in 2017 by Cochrane of task-oriented interventions for DCD resulted in inconsistent findings and a call for further research and randomized controlled trials.
History
Collier first described developmental coordination disorder as "congenital maladroitness". A. Jean Ayres referred to developmental coordination disorder as a disorder of sensory integration in 1972, while in 1975 Sasson Gubbay, MD, called it the "clumsy child syndrome". Developmental coordination disorder has also been called "minimal brain dysfunction", although the two latter names are no longer in use.
Other names include developmental apraxia, disorder of attention and motor perception (DAMP) dyspraxia, developmental dyspraxia, "motor learning difficulties", perceptuo-motor dysfunction, and sensorimotor dysfunction.
The World Health Organization currently lists developmental coordination disorder as "Specific Developmental Disorder of Motor Function".
In popular culture
Fictional characters
Helen Burns, a character from Charlotte Brontë's Jane Eyre, is alleged to have been based on the author's dyspraxic elder sister Maria Brontë.
Ryan Sinclair, a character in the BBC science fiction television programme Doctor Who.
Public figures
People who have publicly stated they have been diagnosed with developmental coordination disorder include:
David Bailey, photographer
John "TotalBiscuit" Bain, games critic
Cara Delevingne, model
Ellis Genge, Rugby Union player
Gage Golightly, actress
Olive Gray, actor
Tom Hunt, politician
Harriet Kemsley, comedian
Emma Lewell-Buck, politician
Mel B, singer
Will Poulter, actor
Daniel Radcliffe, actor
Holly Smale, author
Florence Welch, singer
Toyah Willcox, musician
| Biology and health sciences | Disabilities | Health |
417903 | https://en.wikipedia.org/wiki/Orthonectida | Orthonectida | Orthonectida () is a small phylum of poorly known parasites of marine invertebrates that are among the simplest of multi-cellular organisms. Members of this phylum are known as orthonectids.
Biology
The adults, which are the sexual stage, are microscopic wormlike animals, consisting of a single layer of ciliated outer cells surrounding a mass of sex cells. They swim freely within the bodies of their hosts, which include flatworms, polychaete worms, bivalve molluscs, and echinoderms. Most are gonochoristic, with separate male and female individuals, but a few species are hermaphroditic.
When they are ready to reproduce, adults leave the host, and sperm from the males penetrate the bodies of the females to achieve internal fertilisation. The resulting zygote develops into a ciliated larva that escapes from the mother to seek out new hosts. Once it finds a host, the larva loses its cilia and develops into a syncytial plasmodium larva. This, in turn, breaks up into numerous individual cells called agametes (ameiotic generative cells) which grow into the next generation of adults.
Classification
The phylum consists of about 20 known species, of which Rhopalura ophiocomae is the best-known. The phylum is not divided into classes or orders, and contains just two families.
Although originally described in 1877 as a class, and later characterized as an order of the phylum Mesozoa, a 1996 study has suggested that orthonectids are quite different from the rhombozoans, the other group in Mesozoa. The genome of one orthonectid species, Intoshia linei, has been sequenced. These animals are simplified spiralians. The genome data confirm earlier findings which allocated these organisms to Spiralia based on their morphology.
Their position in the spiralian phylogenetic tree has yet to be determined. Some work appears to relate them to the Annelida and, within the Annelida, finds them most closely allied to the Clitellata. On the other hand, a 2022 study compensating for long-branch attraction has recovered the traditional grouping of Orthonectida with rhombozoans in a monophyletic Mesozoa placed close to Platyhelminthes or Gnathifera. This supports a previous study which found orthonectids and rhombozoans to make a monophyletic taxon Mesozoa and form a clade with Rouphozoa (platyhelminths and gastrotrichs).
Known species
Phylum Orthonectida
Family Rhopaluridae Stunkard, 1937
Ciliocincta
Ciliocincta akkeshiensis Tajika, 1979 – Hokkaido, Japan; in flatworms (Turbellaria)
Ciliocincta julini (Caullery and Mesnil, 1899) – E North Atlantic, in polychaetes
Ciliocincta sabellariae Kozloff, 1965 – San Juan Islands, WA (USA); in polychaete (Neosabellaria cementarium)
Intoshia
Intoshia leptoplanae Giard, 1877 – E North Atlantic, in flatworms (Leptoplana)
Intoshia linei Giard, 1877 – E North Atlantic, in nemertines (Lineus) = Rhopalura linei
Intoshia major Shtein, 1953 – Arctic Ocean; in gastropods (Lepeta, Natica, Solariella) = Rhopalura major
Intoshia metchnikovi (Caullery & Mesnil, 1899) – E North Atlantic, in polychaetes and nemertines
Intoshia paraphanostomae (Westblad, 1942) – E North Atlantic, in flatworms (Acoela)
Intoshia variabili (Alexandrov & Sljusarev, 1992) – Arctic Ocean, in flatworms (Macrorhynchus)
Rhopalura
Rhopalura elongata Shtein, 1953 – Arctic Ocean, in bivalves (Astarte)
Rhopalura gigas (Giard, 1877)
Rhopalura granosa Atkins, 1933 – E North Atlantic, in bivalves (Pododesmus)
Rhopalura intoshi Metchnikoff – Mediterranean, in nemertines
Rhopalura litoralis Shtein, 1953 – Arctic Ocean, in gastropods (Lepeta, Natica, Solariella)
Rhopalura major Shtein, 1953
Rhopalura murmanica Shtein, 1953 – Arctic Ocean, in gastropods (Rissoa, Columbella)
Rhopalura ophiocomae Giard, 1877 – E North Atlantic, in ophiuroids (usually Amphipholis)
Rhopalura pelseneeri Caullery & Mesnil, 1901 – E North Atlantic, polychaetes and nemertines
Rhopalura philinae Lang, 1954 – E North Atlantic, in gastropods
Rhopalura pterocirri de Saint-Joseph, 1896 – E North Atlantic, in polychaetes
Rhopalura vermiculicola
Stoecharthrum
Stoecharthrum burresoni Kozloff, 1993
Stoecharthrum fosterae Kozloff, 1993
Stoecharthrum giardi Caullery & Mesnil, 1899 – E North Atlantic, in polychaetes
Stoecharthrum monnati Kozloff, 1993 – E North Atlantic, in molluscs
Family Pelmatosphaeridae Stunkard, 1937
Pelmatosphaera
Pelmatosphaera polycirri Caullery and Mesnil, 1904 – E North Atlantic, in polychaetes and nemertines
| Biology and health sciences | Spiralia | Animals |
417929 | https://en.wikipedia.org/wiki/LASIK | LASIK | LASIK or Lasik (; "laser-assisted in situ keratomileusis"), commonly referred to as laser eye surgery or laser vision correction, is a type of refractive surgery for the correction of myopia, hyperopia, and astigmatism. LASIK surgery is performed by an ophthalmologist who uses a femtosecond laser or a microkeratome to create a corneal flap to expose the corneal stroma and then an excimer laser to reshape the corneal stroma in order to improve visual acuity.
LASIK is very similar to another surgical corrective procedure, photorefractive keratectomy (PRK), and LASEK. All represent advances over radial keratotomy in the surgical treatment of refractive errors of vision. For people with moderate to high myopia or thin corneas which cannot be treated with LASIK or PRK, the phakic intraocular lens is an alternative.
As of 2018, roughly 9.5 million Americans have had LASIK and, globally, between 1991 and 2016, more than 40 million procedures were performed. However, the procedure seemed to be a declining option as of 2015.
Process
In March 2009, the FDA officially recognized the new LASIK standard from The American National Standards Institute (ANSI), entitled "Laser Systems for Corneal Reshaping."
A detailed pre-operative screening will assess corneal thickness, shape, and refractive error, ensuring the patient is a suitable candidate. During the surgery, a surgeon uses a femtosecond laser or a microkeratome blade to create a thin corneal flap, which is then carefully folded back to expose the underlying tissue. An excimer laser precisely reshapes the stromal layer of the cornea, removing microscopic amounts of tissue to correct refractive errors. This step is guided by a pre-determined surgical plan tailored to the patient's specific visual needs. After the cornea is reshaped, the flap is repositioned, serving as a natural bandage that adheres without the need for stitches. The entire procedure typically takes 10–15 minutes per eye and offers minimal discomfort and rapid recovery, allowing most patients to return to normal activities within a day or two.
Preoperative procedures
Pre-operative examination and education
In the United States, the US Food and Drug Administration (FDA) has approved LASIK for people 18 years of age and older, but the American Academy of Ophthalmology recommends people wait until age 21 because vision needs to stabilize. More importantly the patient's eye prescription should be stable for at least one year prior to surgery.
The patient may be examined with pupillary dilation and education given prior to the procedure. Before the surgery, the patient's corneas are examined with a pachymeter to determine their thickness, and with a topographer, or corneal topography machine, to measure their surface contour. Using low-power lasers, a topographer creates a topographic map of the cornea. The procedure is contraindicated if the topographer finds difficulties such as keratoconus The preparatory process also detects astigmatism and other irregularities in the shape of the cornea. Using this information, the surgeon calculates the amount and the location of corneal tissue to be removed. The patient is prescribed and self-administers an antibiotic beforehand to minimize the risk of infection after the procedure and is sometimes offered a short acting oral sedative medication as a pre-medication. Prior to the procedure, anaesthetic eye drops are instilled. Factors that may rule out LASIK for some patients include large pupils, thin corneas and extremely dry eyes.
Operative procedure
LASIK permanently changes the shape of the cornea, the clear covering of the front of the eye, using an excimer laser. A mechanical microkeratome (a blade device) or a laser keratome (femtosecond laser) is used to cut a flap in the cornea. A hinge is left at one end of this flap. The flap is folded back revealing the corneal stroma, the middle section of the cornea. Pulses from a computer-controlled laser (excimer laser) vaporize a portion of the stroma and the flap is replaced.
Performing the laser ablation in the deeper corneal stroma provides for more rapid visual recovery and less pain than the earlier technique, photorefractive keratectomy (PRK).
Postoperative care
Patients are usually given a course of antibiotic and anti-inflammatory eye drops. These are continued in the weeks following surgery. Patients are told to rest and are given dark eyeglasses to protect their eyes from bright lights and occasionally protective goggles to prevent rubbing of the eyes when asleep and to reduce dry eyes. They also are required to moisturize the eyes with preservative-free tears and follow directions for prescription drops. Occasionally after the procedure a bandage contact lens is placed to aid the healing, and typically removed after 3–4 days. Patients should be adequately informed by their surgeons of the importance of proper post-operative care to minimize the risk of complications.
Wavefront-guided
Wavefront-guided LASIK is a variation of LASIK surgery in which, rather than applying a simple correction of only long/short-sightedness and astigmatism (only lower order aberrations as in traditional LASIK), an ophthalmologist applies a spatially varying correction, guiding the computer-controlled excimer laser with measurements from a wavefront sensor. The goal is to achieve a more optically perfect eye, though the result still depends on the physician's success at predicting changes that occur during healing and other factors that may have to do with the regularity/irregularity of the cornea and the axis of any residual astigmatism. Another important factor is whether the excimer laser can correctly register eye position in 3 dimensions, and to track the eye in all the possible directions of eye movement. If a wavefront guided treatment is performed with less than perfect registration and tracking, pre-existing aberrations can be worsened. In older patients, scattering from microscopic particles (cataract or incipient cataract) may play a role that outweighs any benefit from wavefront correction.
When treating a patient with preexisting astigmatism, most wavefront-guided LASIK lasers are designed to treat regular astigmatism as determined externally by corneal topography. In patients who have an element of internally induced astigmatism, therefore, the wavefront-guided astigmatism correction may leave regular astigmatism behind (a cross-cylinder effect). If the patient has preexisting irregular astigmatism, wavefront-guided approaches may leave both regular and irregular astigmatism behind. This can result in less-than-optimal visual acuity compared with a wavefront-guided approach combined with vector planning, as shown in a 2008 study.
The "leftover" astigmatism after a purely surface-guided laser correction can be calculated beforehand, and is called ocular residual astigmatism (ORA). ORA is a calculation of astigmatism due to the noncorneal surface (internal) optics. The purely refraction-based approach represented by wavefront analysis actually conflicts with corneal surgical experience developed over many years.
The pathway to "super vision" thus may require a more customized approach to corneal astigmatism than is usually attempted, and any remaining astigmatism ought to be regular (as opposed to irregular), which are both fundamental principles of vector planning overlooked by a purely wavefront-guided treatment plan. This was confirmed by the 2008 study mentioned above, which found a greater reduction in corneal astigmatism and better visual outcomes under mesopic conditions using wavefront technology combined with vector analysis than using wavefront technology alone, and also found equivalent higher-order aberrations (see below). Vector planning also proved advantageous in patients with keratoconus.
No good data can be found that compare the percentage of LASIK procedures that employ wavefront guidance versus the percentage that do not, nor the percentage of refractive surgeons who have a preference one way or the other. Wavefront technology continues to be positioned as an "advance" in LASIK with putative advantages; however, it is clear that not all LASIK procedures are performed with wavefront guidance.
Still, surgeons claim patients are generally more satisfied with this technique than with previous methods, particularly regarding lowered incidence of "halos," the visual artifact caused by spherical aberration induced in the eye by earlier methods. A meta-analysis of eight trials showed a lower incidence of these higher order aberrations in patients who had wavefront-guided LASIK compared to non-wavefront-guided LASIK. Based on their experience, the United States Air Force has described WFG-Lasik as giving "superior vision results".
Topography-assisted
Topography-assisted LASIK is intended to be an advancement in precision and reduce night-vision side effects. The first topography-assisted device received FDA approval 13 September 2013.
History
Barraquer's early work
In the 1950s, the microkeratome and keratomileusis technique were developed in Bogotá, Colombia, by the Spanish ophthalmologist José Barraquer. In his clinic, he would cut thin (one hundredth of a mm thick) flaps in the cornea to alter its shape. Barraquer also investigated how much of the cornea had to be left unaltered in order to provide stable long-term results.
Laser refractive surgery
In 1980, Rangaswamy Srinivasan, Samuel E. Blum, and James J. Wynne at the IBM Research laboratory, discovered that an ultraviolet excimer laser could etch living tissue, with precision and with no thermal damage to the surrounding area. The phenomenon was termed "ablative photo-decomposition" (APD).
Five years later, in 1985, Steven Trokel at the Edward S. Harkness Eye Institute, Columbia University in New York City, published his work using the excimer laser in radial keratotomy. He wrote,
"The central corneal flattening obtained by radial diamond knife incisions has been duplicated by radial laser incisions in 18 enucleated human eyes. The incisions, made by 193 nm far-ultraviolet light radiation emitted by the excimer laser, produced corneal flattening ranging from 0.12 to 5.35 diopters. Both the depth of the corneal incisions and the degree of central corneal flattening correlated with the laser energy applied. Histopathology revealed the remarkably smooth edges of the laser incisions."
Patent
A number of patents have been issued for several techniques related to LASIK. Rangaswamy Srinivasan and James Wynne filed a patent application on the ultraviolet excimer laser, in 1986, issued in 1988. In 1989, Gholam A. Peyman was granted a US patent for using an excimer laser to modify corneal curvature. It was,
"A method and apparatus for modifying the curvature of a live cornea via use of an excimer laser. The live cornea has a thin layer removed therefrom, leaving an exposed internal surface thereon. Then, either the surface or thin layer is exposed to the laser beam along a predetermined pattern to ablate desired portions. The thin layer is then replaced onto the surface. Ablating a central area of the surface or thin layer makes the cornea less curved, while ablating an annular area spaced from the center of the surface or layer makes the cornea more curved. The desired predetermined pattern is formed by use of a variable diaphragm, a rotating orifice of variable size, a movable mirror or a movable fiber optic cable through which the laser beam is directed towards the exposed internal surface or removed thin layer."
The patents related to so-called broad-beam LASIK and PRK technologies were granted to US companies including Visx and Summit during 1990–1995 based on the fundamental US patent issued to IBM (1988) which claimed the use of UV laser for the ablation of organic tissues.
Implementation in the United States
The LASIK technique was implemented in the US after its successful application elsewhere. The Food and Drug Administration (FDA) commenced a trial of the excimer laser in 1989. The first enterprise to receive FDA approval to use an excimer laser for photo-refractive keratectomy was Summit Technology.
In 1992, under the direction of the FDA, Greek ophthalmologist Ioannis Pallikaris introduced LASIK to ten VISX centers. In 1998, the "Kremer Excimer Laser", serial number KEA 940202, received FDA approval for its singular use for performing LASIK. Subsequently, Summit Technology was the first company to receive FDA approval to mass manufacture and distribute excimer lasers. VISX and other companies followed.
Pallikaris suggested a flap of cornea could be raised by microkeratome prior to the performing of PRK with the excimer laser. The addition of a flap to PRK became known as LASIK. The use of a femtosecond laser to raise the flap of cornea was discovered after a graduate student at the University of Michigan suffered an accidental laser eye injury in 1993. Tibor Juhasz and Ron Kurtz developed this approach and went on to found IntraLase to perform bladeless LASIK surgery.
Recent years
The procedure seems to be a declining option for many in the United States, dropping more than 50 percent, from about 1.5 million surgeries in 2007 to 604,000 in 2015, according to the eye-care data source Market Scope. A study determined the frequency with which LASIK was searched on Google from 2007 to 2011. Within this time frame, LASIK searches declined by 40% in the United States. Countries such as the U.K. and India also showed a decline, 22% and 24% respectively. Canada, however, showed an increase in LASIK searches by 8%. This decrease in interest can be attributed to several factors: the emergence of refractive cataract surgery, the economic recession in 2008, and unfavorable media coverage from the FDA's 2008 press release on LASIK.
Effectiveness
In 2006, the British National Health Service's National Institute for Health and Clinical Excellence (NICE) considered evidence of the effectiveness and the potential risks of the laser surgery, stating "current evidence suggests that photorefractive (laser) surgery for the correction of refractive errors is safe and effective for use in appropriately selected patients. Clinicians undertaking photorefractive (laser) surgery for the correction of refractive errors should ensure that patients understand the benefits and potential risks of the procedure. Risks include failure to achieve the expected improvement in unaided vision, development of new visual disturbances, corneal infection and flap complications. These risks should be weighed against those of wearing spectacles or contact lenses." The FDA reports "The safety and effectiveness of refractive procedures has not been determined in patients with some diseases."
Satisfaction
Surveys of LASIK surgery find rates of patient satisfaction between 92 and 98 percent. In March 2008, the American Society of Cataract and Refractive Surgery published a patient satisfaction meta-analysis of over 3,000 peer-reviewed articles from international clinical journals. Data from a systematic literature review conducted from 1988 to 2008, consisting of 309 peer-reviewed articles about "properly conducted, well-designed, randomized clinical trials" found a 95.4 percent patient satisfaction rate among LASIK patients.
A 2017 study claims that overall, preoperative symptoms decreased significantly, and visual acuity excelled. A meta-analysis discovered that 97% of patients achieved uncorrected visual acuity (UCVA) of 20/40, while 62% achieved 20/20.
Dissatisfaction
Some people with poor outcomes from LASIK surgical procedures report a significantly reduced quality of life because of vision problems or pain associated with the surgery. A small percentage of patients may need further surgery because their condition is over- or under-corrected. Some patients need to wear contact lenses or glasses even after treatment.
The most common reason for dissatisfaction in LASIK patients is chronic severe dry eye. Independent research indicates 95% of patients experience dry eye in the initial post-operative period. This number has been reported to up to 60% after one month. Symptoms begin to improve in the vast majority of patients in the 6 to 12 months following the surgery. However, 30% of post-LASIK referrals to tertiary ophthalmology care centers have been shown to be due to chronic dry eye.
Morris Waxler, a former FDA official who was involved in the approval of LASIK, subsequently criticized its widespread use. In 2010, Waxler made media appearances and claimed that the procedure had a failure rate greater than 50%. The FDA responded that Waxler's information was "filled with false statements, incorrect citations" and "mischaracterization of results".
Presbyopia
A type of LASIK, known as presbyLasik, may be used in presbyopia. Results are, however, more variable and some people have a decrease in visual acuity.
Risks
Dry eyes
95% of patients report dry-eye symptoms after LASIK. Although it is usually temporary, it can develop into chronic and severe dry eye syndrome. Quality of life can be severely affected by dry-eye syndrome.
Underlying conditions with dry eye such as Sjögren's syndrome are considered contraindications to Lasik.
Treatments include artificial tears, prescription tears, and punctal occlusion. Punctal occlusion is accomplished by placing a collagen or silicone plug in the tear duct, which normally drains fluid from the eye. Some patients complain of ongoing dry-eye symptoms despite such treatments and the symptoms may be permanent.
Halos
Some post-LASIK patients see halos and starbursts around bright lights at night.
Complications due to LASIK have been classified as those that occur due to preoperative, intraoperative, early postoperative, or late postoperative sources: According to the UK National Health Service, complications occur in fewer than 5% of cases.
Other complications
Flap complications – The incidence of flap complications is about 0.244%. Flap complications (such as displaced flaps or folds in the flaps that necessitate repositioning, diffuse lamellar keratitis, and epithelial ingrowth) are common in lamellar corneal surgeries but rarely lead to permanent loss of visual acuity. The incidence of these microkeratome-related complications decreases with increased physician experience.
Flap interface particles – are a finding whose clinical significance is undetermined. Particles of various sizes and reflectivity are clinically visible in about 38.7% of eyes examined via slit lamp biomicroscopy and in 100% of eyes examined by confocal microscopy.
Diffuse lamellar keratitis – an inflammatory process that involves an accumulation of white blood cells at the interface between the LASIK corneal flap and the underlying stroma. It is known colloquially as "sands of Sahara syndrome" because on slit lamp exam, the inflammatory infiltrate appears similar to waves of sand. The USAeyes organisation reports an incidence of 2.3% after LASIK. It is most commonly treated with steroid eye drops. Sometimes it is necessary for the eye surgeon to lift the flap and manually remove the accumulated cells. DLK has not been reported with photorefractive keratectomy due to the absence of flap creation.
Infection – the incidence of infection responsive to treatment has been estimated at 0.04%.
Post-LASIK corneal ectasia – a condition where the cornea starts to bulge forwards at a variable time after LASIK, causing irregular astigmatism. the condition is similar to keratoconus.
Subconjunctival hemorrhage – A report shows the incidence of subconjunctival hemorrhage has been estimated at 10.5%.
Corneal scarring – or permanent problems with cornea's shape making it impossible to wear contact lenses.
Epithelial ingrowth – estimated at 0.01%.
Traumatic flap dislocations – Cases of late traumatic flap dislocations have been reported up to thirteen years after LASIK.
Retinal detachment: estimated at 0.36 percent.
Choroidal neovascularization: estimated at 0.33 percent.
Uveitis: estimated at 0.18 percent.
For climbers – Although the cornea usually is thinner after LASIK, because of the removal of part of the stroma, refractive surgeons strive to maintain the maximum thickness to avoid structurally weakening the cornea. Decreased atmospheric pressure at higher altitudes has not been demonstrated as extremely dangerous to the eyes of LASIK patients. However, some mountain climbers have experienced a myopic shift at extreme altitudes.
Late postoperative complications – A large body of evidence on the chances of long-term complications is not yet established and may be changing due to advances in operator experience, instruments and techniques.
Potential best vision loss may occur a year after the surgery regardless of the use of eyewear.
Ocular neuropathic pain (corneal neuralgia); rare
FDA's position
In October 2009, the US FDA, the US National Eye Institute (NEI), and the US Department of Defense (DoD) launched the LASIK Quality of Life Collaboration Project (LQOLCP) to help better understand the potential risk of severe problems that can result from LASIK in response to widespread reports of problems experienced by patients after LASIK laser eye surgery. This project examined patient-reported outcomes with LASIK (PROWL). The project consisted of three phases: pilot phase, phase I, phase II (PROWL-1) and phase III (PROWL-2).
The results of the LASIK Quality of Life Study were published in October 2014.
The FDA's director of the Division of Ophthalmic Devices said about the LASIK study: "Given the large number of patients undergoing LASIK annually, dissatisfaction and disabling symptoms may occur in a significant number of patients". Also in 2014, FDA published an article highlighting the risks and a list of factors and conditions individuals should consider when choosing a doctor for their refractive surgery.
Contraindications
Not everyone is eligible to receive LASIK. Severe keratoconus or thin corneas may disqualify patients from LASIK, though other procedures may be viable options. Those with Fuchs' corneal endothelial dystrophy, corneal epithelial basement membrane dystrophy, retinal tears, autoimmune diseases, severe dry eyes, and significant blepharitis should be treated before consideration for LASIK. Women who are pregnant or nursing are generally not eligible to undergo LASIK.
People with large pupils (e.g. due to taking medications or in the younger age group) may be particularly prone to symptoms such as glare, halos, starbursts, and ghost images (double vision) in dim light after surgery. Because the laser can only work on the inner section of the cornea, the outer rim is left unaffected. In dim lighting, a patient's pupils dilate and may be predisposed to optic aberrations due to refractive asynchrony of the two regions with regards to the incoming light.
Further research
Since 1991, there have been further developments such as faster lasers; larger spot areas; bladeless flap incisions; intraoperative corneal pachymetry; and "wavefront-optimized" and "wavefront-guided" techniques which were introduced by the University of Michigan's Center for Ultrafast Optical Science. The goal of replacing standard LASIK in refractive surgery is to avoid permanently weakening the cornea with incisions and to deliver less energy to the surrounding tissues. More recently, techniques like Epi-Bowman Keratectomy have been developed that avoid touching the epithelial basement membrane or Bowman's layer.
Experimental techniques
"plain" LASIK: LASEK, Epi-LASIK,
Wavefront-guided PRK,
advanced intraocular lenses.
Femtosecond laser intrastromal vision correction: using all-femtosecond correction, for example, Femtosecond Lenticule EXtraction, FLIVC, or IntraCOR),
Keraflex: a thermobiochemical solution which has received the CE Mark for refractive correction. and is in European clinical trials for the correction of myopia and keratoconus.
Technolas FEMTEC laser: for incisionless IntraCOR ablation for presbyopia, with trials ongoing for myopia and other conditions.
LASIK with the IntraLase femtosecond laser: early trials comparing to the LASIK with microkeratomes for the correction of myopia suggest no significant differences in safety or efficacy. However, the femtosecond laser has a potential advantage in predictability, although this finding was not significant.
Comparison to photorefractive keratectomy
A systematic review that compared PRK and LASIK concluded that LASIK has shorter recovery time and less pain. The two techniques after a period of one year have similar results.
A 2017 systematic review found uncertainty in visual acuity, but found that in one study, those receiving PRK were less likely to achieve a refractive error, and were less likely to have an over-correction than compared to LASIK.
| Biology and health sciences | Surgery | Health |
418101 | https://en.wikipedia.org/wiki/Evolutionary%20biology | Evolutionary biology | Evolutionary biology is the subfield of biology that studies the evolutionary processes such as natural selection, common descent, and speciation that produced the diversity of life on Earth. In the 1930s, the discipline of evolutionary biology emerged through what Julian Huxley called the modern synthesis of understanding, from previously unrelated fields of biological research, such as genetics and ecology, systematics, and paleontology.
The investigational range of current research has widened to encompass the genetic architecture of adaptation, molecular evolution, and the different forces that contribute to evolution, such as sexual selection, genetic drift, and biogeography. The newer field of evolutionary developmental biology ("evo-devo") investigates how embryogenesis is controlled, thus yielding a wider synthesis that integrates developmental biology with the fields of study covered by the earlier evolutionary synthesis.
Subfields
Evolution is the central unifying concept in biology. Biology can be divided into various ways. One way is by the level of biological organization, from molecular to cell, organism to population. Another way is by perceived taxonomic group, with fields such as zoology, botany, and microbiology, reflecting what was once seen as the major divisions of life. A third way is by approaches, such as field biology, theoretical biology, experimental evolution, and paleontology. These alternative ways of dividing up the subject have been combined with evolutionary biology to create subfields like evolutionary ecology and evolutionary developmental biology.
More recently, the merge between biological science and applied sciences gave birth to new fields that are extensions of evolutionary biology, including evolutionary robotics, engineering, algorithms, economics, and architecture. The basic mechanisms of evolution are applied directly or indirectly to come up with novel designs or solve problems that are difficult to solve otherwise. The research generated in these applied fields, contribute towards progress, especially from work on evolution in computer science and engineering fields such as mechanical engineering.
In evolutionary developmental biology, scientists look at how the different processes in development play a role in how a specific organism reaches its current body plan. The genetic regulation of ontogeny and the phylogenetic process is what allows for this kind of understanding of biology. By looking at different processes during development, and going through the evolutionary tree, one can determine at which point a specific structure came about.
History
The idea of evolution by natural selection was proposed by Charles Darwin in 1859, but evolutionary biology, as an academic discipline in its own right, emerged during the period of the modern synthesis in the 1930s and 1940s. It was not until the 1980s that many universities had departments of evolutionary biology.
Microbiology too is becoming an evolutionary discipline now that microbial physiology and genomics are better understood. The quick generation time of bacteria and viruses such as bacteriophages makes it possible to explore evolutionary questions.
Many biologists have contributed to shaping the modern discipline of evolutionary biology. Theodosius Dobzhansky and E. B. Ford established an empirical research programme. Ronald Fisher, Sewall Wright, and J. B. S. Haldane created a sound theoretical framework. Ernst Mayr in systematics, George Gaylord Simpson in paleontology and G. Ledyard Stebbins in botany helped to form the modern synthesis. James Crow, Richard Lewontin, Dan Hartl, Marcus Feldman, and Brian Charlesworth trained a generation of evolutionary biologists.
Research topics
Research in evolutionary biology covers many topics and incorporates ideas from diverse areas, such as molecular genetics and mathematical and theoretical biology. Some fields of evolutionary research try to explain phenomena that were poorly accounted for in the modern evolutionary synthesis. These include speciation, the evolution of sexual reproduction, the evolution of cooperation, the evolution of ageing, and evolvability.
Some evolutionary biologists ask the most straightforward evolutionary question: "what happened and when?". This includes fields such as paleobiology, where paleobiologists and evolutionary biologists, including Thomas Halliday and Anjali Goswami, studied the evolution of early mammals going far back in time during the Mesozoic and Cenozoic eras (between 299 million to 12,000 years ago). Other fields related to generic exploration of evolution ("what happened and when?" ) include systematics and phylogenetics.
The modern evolutionary synthesis was devised at a time when the molecular basis of genes was unknown. Today, evolutionary biologists try to determine the genetic architecture underlying visible evolutionary phenomena such as adaptation and speciation. They seek answers to questions such as which genes are involved, how interdependent are the effects of different genes, what do the genes do, and what changes happen to them (e.g., point mutations vs. gene duplication or even genome duplication). They try to reconcile the high heritability seen in twin studies with the difficulty in finding which genes are responsible for this heritability using genome-wide association studies. The modern evolutionary synthesis involved agreement about which forces contribute to evolution, but not about their relative importance.
Journals
Some scientific journals specialise exclusively in evolutionary biology as a whole, including the journals Evolution, Journal of Evolutionary Biology, and BMC Evolutionary Biology. Some journals cover sub-specialties within evolutionary biology, such as the journals Systematic Biology, Molecular Biology and Evolution and its sister journal Genome Biology and Evolution, and Cladistics.
Other journals combine aspects of evolutionary biology with other related fields. For example, Molecular Ecology, Proceedings of the Royal Society of London Series B, The American Naturalist and Theoretical Population Biology have overlap with ecology and other aspects of organismal biology. Overlap with ecology is also prominent in the review journals Trends in Ecology and Evolution and Annual Review of Ecology, Evolution, and Systematics. The journals Genetics and PLoS Genetics overlap with molecular genetics questions that are not obviously evolutionary in nature.
| Biology and health sciences | Basics_4 | Biology |
418403 | https://en.wikipedia.org/wiki/Volumetric%20flow%20rate | Volumetric flow rate | In physics and engineering, in particular fluid dynamics, the volumetric flow rate (also known as volume flow rate, or volume velocity) is the volume of fluid which passes per unit time; usually it is represented by the symbol (sometimes ). It contrasts with mass flow rate, which is the other main type of fluid flow rate. In most contexts a mention of rate of fluid flow is likely to refer to the volumetric rate. In hydrometry, the volumetric flow rate is known as discharge.
Volumetric flow rate should not be confused with volumetric flux, as defined by Darcy's law and represented by the symbol , with units of m3/(m2·s), that is, m·s−1. The integration of a flux over an area gives the volumetric flow rate.
The SI unit is cubic metres per second (m3/s). Another unit used is standard cubic centimetres per minute (SCCM). In US customary units and imperial units, volumetric flow rate is often expressed as cubic feet per second (ft3/s) or gallons per minute (either US or imperial definitions). In oceanography, the sverdrup (symbol: Sv, not to be confused with the sievert) is a non-SI metric unit of flow, with equal to ; it is equivalent to the SI derived unit cubic hectometer per second (symbol: hm3/s or hm3⋅s−1). Named after Harald Sverdrup, it is used almost exclusively in oceanography to measure the volumetric rate of transport of ocean currents.
Fundamental definition
Volumetric flow rate is defined by the limit
that is, the flow of volume of fluid through a surface per unit time .
Since this is only the time derivative of volume, a scalar quantity, the volumetric flow rate is also a scalar quantity. The change in volume is the amount that flows after crossing the boundary for some time duration, not simply the initial amount of volume at the boundary minus the final amount at the boundary, since the change in volume flowing through the area would be zero for steady flow.
IUPAC prefers the notation and for volumetric flow and mass flow respectively, to distinguish from the notation for heat.
Alternative definition
Volumetric flow rate can also be defined by
where
= flow velocity,
= cross-sectional vector area/surface.
The above equation is only true for uniform or homogeneous flow velocity and a flat or planar cross section. In general, including spatially variable or non-homogeneous flow velocity and curved surfaces, the equation becomes a surface integral:
This is the definition used in practice. The area required to calculate the volumetric flow rate is real or imaginary, flat or curved, either as a cross-sectional area or a surface. The vector area is a combination of the magnitude of the area through which the volume passes through, , and a unit vector normal to the area, . The relation is .
Derivation
The reason for the dot product is as follows. The only volume flowing through the cross-section is the amount normal to the area, that is, parallel to the unit normal. This amount is
where is the angle between the unit normal and the velocity vector of the substance elements. The amount passing through the cross-section is reduced by the factor . As increases less volume passes through. Substance which passes tangential to the area, that is perpendicular to the unit normal, does not pass through the area. This occurs when and so this amount of the volumetric flow rate is zero:
These results are equivalent to the dot product between velocity and the normal direction to the area.
Relationship with mass flow rate
When the mass flow rate is known, and the density can be assumed constant, this is an easy way to get :
where
= mass flow rate (in kg/s),
= density (in kg/m3).
Related quantities
In internal combustion engines, the time area integral is considered over the range of valve opening. The time lift integral is given by
where is the time per revolution, is the distance from the camshaft centreline to the cam tip, is the radius of the camshaft (that is, is the maximum lift), is the angle where opening begins, and is where the valve closes (seconds, mm, radians). This has to be factored by the width (circumference) of the valve throat. The answer is usually related to the cylinder's swept volume.
Some key examples
In cardiac physiology: the cardiac output
In hydrology: discharge
List of rivers by discharge
List of waterfalls by flow rate
Weir § Flow measurement
In dust collection systems: the air-to-cloth ratio
| Physical sciences | Fluid mechanics | Physics |
418509 | https://en.wikipedia.org/wiki/Mullet%20%28fish%29 | Mullet (fish) | The mullets or grey mullets are a family (Mugilidae) of ray-finned fish found worldwide in coastal temperate and tropical waters, and some species in fresh water. Mullets have served as an important source of food in Mediterranean Europe since Roman times. The family includes about 78 species in 26 genera.
Mullets are distinguished by the presence of two separate dorsal fins, small triangular mouths, and the absence of a lateral line organ. They feed on detritus, and most species have unusually muscular stomachs and a complex pharynx to help in digestion.
Classification and naming
Taxonomically, the family is currently treated as the sole member of the order Mugiliformes, but as Nelson says, "there has been much disagreement concerning the relationships" of this family. The presence of fin spines clearly indicates membership in the superorder Acanthopterygii, and in the 1960s, they were classed as primitive perciforms, while others have grouped them in Atheriniformes. They are classified as an order, Mugiliformes, within the subseries Ovalentaria of the clade Percomorpha in the 5th Edition of Fishes of the World.
In North America, "mullet" by itself usually refers to Mugilidae. In Europe, the word "mullet" is usually qualified, the "grey mullets" being Mugilidae and the "red mullets" or "surmullets" being Mullidae, notably members of the genus Mullus. Outside Europe, the Mullidae are often called "goatfish". Fish with common names including the word "mullet" may be a member of one family or the other, or even unrelated such as the freshwater Catostomus commersonii.
However, recent taxonomic work has reorganised the family and the following genera make up the Mugilidae:
Agonostomus Bennett, 1832
Aldrichetta Whitley, 1945
Cestraeus Valenciennes, 1836
Chaenomugil Gill, 1863
Chelon Artedi, 1763
Crenimugil Schultz, 1946
Dajaus Valenciennes, 1836
Ellochelon Whitley, 1930
Gracilimugil Whitley, 1941
Joturus Poey, 1860
Minimugil Durand, Chen, Shen, Fu & Borsa, 2012
Mugil Linnaeus, 1758
Myxus Günther, 1861
Neomyxus Steindachner, 1878
Neochelon Durand, Chen, Shen, Fu & Borsa 2012
Oedalechilus Fowler 1903
Osteomugil G. Luther, 1982
Parachelon Durand, Chen, Shen, Fu & Borsa 2012
Paramugil Ghasemzadeh, Ivantsoff & Aarn 2004
Planiliza Whitley, 1945
Plicomugil Schultz, 1953
Pseudomyxus Durand, Chen, Shen, Fu & Borsa 2012
Rhinomugil Gill, 1863
Sicamugil Fowler, 1939
Squalomugil Ogilby, 1908
Trachystoma Ogilby, 1888
Behaviour
A common noticeable behaviour in mullet is the tendency to leap out of the water. There are two distinguishable types of leaps: a straight, clean slice out of the water to escape predators and a slower, lower jump while turning to its side that results in a larger, more distinguishable, splash. The reasons for this lower jump are disputed, but have been hypothesised to be in order to gain oxygen rich air for gas exchange in a small organ above the pharynx.
Development
The ontogeny of mugilid larvae has been well studied, with the larval development of Mugil cephalus in particular being studied intensively due to its wide range of distribution and interest to aquaculture. The previously understudied osteological development of Mugil cephalus was investigated in a 2021 study, with four embryonic and six larval developmental steps being described in aquaculture-reared and wild-caught specimens. These descriptions provided clarification of questionable characters of adult mullets and revealed informative details with potential implications for phylogenetic hypotheses, as well as providing an overdue basis of comparison for aquaculture-reared mullets to enable recognition of malformations.
| Biology and health sciences | Fishes | null |
418809 | https://en.wikipedia.org/wiki/Base%20metal | Base metal | A base metal is a common and inexpensive metal, as opposed to a precious metal such as gold or silver. In numismatics, coins often derived their value from the precious metal content; however, base metals have also been used in coins in the past and today.
Specific definitions
In contrast to noble metals, base metals may be distinguished by oxidizing or corroding relatively easily and reacting variably with diluted hydrochloric acid (HCl) to form hydrogen. Examples include iron, nickel, lead and zinc. Copper is also considered a base metal because it oxidizes relatively easily, although it does not react with HCl.
In mining and economics, the term base metals refers to industrial non-ferrous metals excluding precious metals. These include copper, lead, nickel and zinc.
The U.S. Customs and Border Protection agency is more inclusive in its definition of commercial base metals. Its list includes—in addition to copper, lead, nickel, and zinc—the following metals: iron and steel (an alloy), aluminium, tin, tungsten, molybdenum, tantalum, cobalt, bismuth, cadmium, titanium, zirconium, antimony, manganese, beryllium, chromium, germanium, vanadium, gallium, hafnium, indium, niobium, rhenium, and thallium, and their alloys.
Other uses
In the context of plated metal products, the base metal underlies the plating metal, as copper underlies silver in Sheffield plate.
| Physical sciences | d-Block | Chemistry |
419094 | https://en.wikipedia.org/wiki/Adipose%20tissue | Adipose tissue | Adipose tissue (also known as body fat or simply fat) is a loose connective tissue composed mostly of adipocytes. It also contains the stromal vascular fraction (SVF) of cells including preadipocytes, fibroblasts, vascular endothelial cells and a variety of immune cells such as adipose tissue macrophages. Its main role is to store energy in the form of lipids, although it also cushions and insulates the body.
Previously treated as being hormonally inert, in recent years adipose tissue has been recognized as a major endocrine organ, as it produces hormones such as leptin, estrogen, resistin, and cytokines (especially TNFα). In obesity, adipose tissue is implicated in the chronic release of pro-inflammatory markers known as adipokines, which are responsible for the development of metabolic syndromea constellation of diseases including type 2 diabetes, cardiovascular disease and atherosclerosis.
Adipose tissue is derived from preadipocytes and its formation appears to be controlled in part by the adipose gene. The two types of adipose tissue are white adipose tissue (WAT), which stores energy, and brown adipose tissue (BAT), which generates body heat. Adipose tissuemore specifically brown adipose tissuewas first identified by the Swiss naturalist Conrad Gessner in 1551.
Anatomical features
In humans, adipose tissue is located: beneath the skin (subcutaneous fat), around internal organs (visceral fat), in bone marrow (yellow bone marrow), intermuscular (muscular system), and in the breast (breast tissue). Adipose tissue is found in specific locations, which are referred to as adipose depots. Apart from adipocytes, which comprise the highest percentage of cells within adipose tissue, other cell types are present, collectively termed stromal vascular fraction (SVF) of cells. SVF includes preadipocytes, fibroblasts, adipose tissue macrophages, and endothelial cells.
Adipose tissue contains many small blood vessels. In the integumentary system, which includes the skin, it accumulates in the deepest level, the subcutaneous layer, providing insulation from heat and cold. Around organs, it provides protective padding. However, its main function is to be a reserve of lipids, which can be oxidised to meet the energy needs of the body and to protect it from excess glucose by storing triglycerides produced by the liver from sugars, although some evidence suggests that most lipid synthesis from carbohydrates occurs in the adipose tissue itself. Adipose depots in different parts of the body have different biochemical profiles. Under normal conditions, it provides feedback for hunger and diet to the brain.
Mice
Mice have eight major adipose depots, four of which are within the abdominal cavity. The paired gonadal depots are attached to the uterus and ovaries in females and the epididymis and testes in males; the paired retroperitoneal depots are found along the dorsal wall of the abdomen, surrounding the kidney, and, when massive, extend into the pelvis. The mesenteric depot forms a glue-like web that supports the intestines and the omental depot (which originates near the stomach and spleen) and - when massive - extends into the ventral abdomen. Both the mesenteric and omental depots incorporate much lymphoid tissue as lymph nodes and milky spots, respectively.
The two superficial depots are the paired inguinal depots, which are found anterior to the upper segment of the hind limbs (underneath the skin) and the subscapular depots, paired medial mixtures of brown adipose tissue adjacent to regions of white adipose tissue, which are found under the skin between the dorsal crests of the scapulae. The layer of brown adipose tissue in this depot is often covered by a "frosting" of white adipose tissue; sometimes these two types of fat (brown and white) are hard to distinguish. The inguinal depots enclose the inguinal group of lymph nodes. Minor depots include the pericardial, which surrounds the heart, and the paired popliteal depots, between the major muscles behind the knees, each containing one large lymph node. Of all the depots in the mouse, the gonadal depots are the largest and the most easily dissected, comprising about 30% of dissectible fat.
Obesity
In an obese person, excess adipose tissue hanging downward from the abdomen is referred to as a panniculus. A panniculus complicates surgery of the morbidly obese individual. It may remain as a literal "apron of skin" if a severely obese person loses large amounts of fat (a common result of gastric bypass surgery). Obesity is treated through exercises, diet, and behavioral therapy, liposuctions. Reconstructive surgery is one aspect of treatment.
Visceral fat
Visceral fat or abdominal fat (also known as organ fat or intra-abdominal fat) is located inside the abdominal cavity, packed between the organs (stomach, liver, intestines, kidneys, etc.). Visceral fat is different from subcutaneous fat underneath the skin, and intramuscular fat interspersed in skeletal muscles. Fat in the lower body, as in thighs and buttocks, is subcutaneous and is not consistently spaced tissue, whereas fat in the abdomen is mostly visceral and semi-fluid. Visceral fat is composed of several adipose depots, including mesenteric, epididymal white adipose tissue (EWAT), and perirenal depots. Visceral fat is often expressed in terms of its area in cm2 (VFA, visceral fat area).
An excess of visceral fat is known as abdominal obesity, or "belly fat", in which the abdomen protrudes excessively. New developments such as the Body Volume Index (BVI) are specifically designed to measure abdominal volume and abdominal fat. Excess visceral fat is also linked to type 2 diabetes, insulin resistance, inflammatory diseases, and other obesity-related diseases. Likewise, the accumulation of neck fat (or cervical adipose tissue) has been shown to be associated with mortality. Several studies have suggested that visceral fat can be predicted from simple anthropometric measures, and predicts mortality more accurately than body mass index or waist circumference.
Men are more likely to have fat stored in the abdomen due to sex hormone differences. Estrogen (female sex hormone) causes fat to be stored in the buttocks, thighs, and hips in women. When women reach menopause and the estrogen produced by the ovaries declines, fat migrates from the buttocks, hips and thighs to the waist; later fat is stored in the abdomen.
Visceral fat can be caused by excess cortisol levels. At least 10 MET-hours per week of aerobic exercise leads to visceral fat reduction in those without metabolic-related disorders. Resistance training and caloric restriction also reduce visceral fat, although their effect may not be cumulative. Both exercise and hypocaloric diet cause loss of visceral fat, but exercise has a larger effect on visceral fat versus total fat. High-intensity exercise is one way to effectively reduce total abdominal fat. An energy-restricted diet combined with exercise will reduce total body fat and the ratio of visceral adipose tissue to subcutaneous adipose tissue, suggesting a preferential mobilization for visceral fat over subcutaneous fat.
Epicardial fat
Epicardial adipose tissue (EAT) is a particular form of visceral fat deposited around the heart and found to be a metabolically active organ that generates various bioactive molecules, which might significantly affect cardiac function. Marked component differences have been observed in comparing EAT with subcutaneous fat, suggesting a location-specific impact of stored fatty acids on adipocyte function and metabolism.
Subcutaneous fat
Most of the remaining nonvisceral fat is found just below the skin in a region called the hypodermis. This subcutaneous fat is not related to many of the classic obesity-related pathologies, such as heart disease, cancer, and stroke, and some evidence even suggests it might be protective. The typically female (or gynecoid) pattern of body fat distribution around the hips, thighs, and buttocks is subcutaneous fat, and therefore poses less of a health risk compared to visceral fat.
Like all other fat organs, subcutaneous fat is an active part of the endocrine system, secreting the hormones leptin and resistin.
The relationship between the subcutaneous adipose layer and total body fat in a person is often modelled by using regression equations. The most popular of these equations was formed by Durnin and Wormersley, who rigorously tested many types of skinfold, and, as a result, created two formulae to calculate the body density of both men and women. These equations present an inverse correlation between skinfolds and body density—as the sum of skinfolds increases, the body density decreases.
Factors such as sex, age, population size or other variables may make the equations invalid and unusable, and, , Durnin and Wormersley's equations remain only estimates of a person's true level of fatness. New formulae are still being created.
Marrow fat
Marrow fat, also known as marrow adipose tissue (MAT), is a poorly understood adipose depot that resides in the bone and is interspersed with hematopoietic cells as well as bony elements. The adipocytes in this depot are derived from mesenchymal stem cells (MSC) which can give rise to fat cells, bone cells as well as other cell types. The fact that MAT increases in the setting of calorie restriction/ anorexia is a feature that distinguishes this depot from other fat depots. Exercise regulates MAT, decreasing MAT quantity and diminishing the size of marrow adipocytes. The exercise regulation of marrow fat suggests that it bears some physiologic similarity to other white adipose depots. Moreover, increased MAT in obesity further suggests a similarity to white fat depots.
Ectopic fat
Ectopic fat is the storage of triglycerides in tissues other than adipose tissue, that are supposed to contain only small amounts of fat, such as the liver, skeletal muscle, heart, and pancreas. This can interfere with cellular functions and hence organ function and is associated with insulin resistance in type-2 diabetes. It is stored in relatively high amounts around the organs of the abdominal cavity, but is not to be confused with visceral fat.
The specific cause for the accumulation of ectopic fat is unknown. The cause is likely a combination of genetic, environmental, and behavioral factors that are involved in excess energy intake and decreased physical activity. Substantial weight loss can reduce ectopic fat stores in all organs and this is associated with an improvement of the function of those organs.
In the latter case, non-invasive weight loss interventions like diet or exercise can decrease ectopic fat (particularly in heart and liver) in overweight or obese children and adults.
Physiology
Free fatty acids (FFAs) are liberated from lipoproteins by lipoprotein lipase (LPL) and enter the adipocyte, where they are reassembled into triglycerides by esterifying them onto glycerol. Human fat tissue contains from 61% to 94% lipids, with obese and lean individuals tending towards the high and low ends of this range, respectively.
There is a constant flux of FFAs entering and leaving adipose tissue. The net direction of this flux is controlled by insulin and leptin—if insulin is elevated, then there is a net inward flux of FFA, and only when insulin is low can FFA leave adipose tissue. Insulin secretion is stimulated by high blood sugar, which results from consuming carbohydrates.
In humans, lipolysis (hydrolysis of triglycerides into free fatty acids) is controlled through the balanced control of lipolytic B-adrenergic receptors and a2A-adrenergic receptor-mediated antilipolysis.
Fat cells have an important physiological role in maintaining triglyceride and free fatty acid levels, as well as determining insulin resistance. Abdominal fat has a different metabolic profile—being more prone to induce insulin resistance. This explains to a large degree why central obesity is a marker of impaired glucose tolerance and is an independent risk factor for cardiovascular disease (even in the absence of diabetes mellitus and hypertension). Studies of female monkeys at Wake Forest University (2009) discovered that individuals with higher stress have higher levels of visceral fat in their bodies. This suggests a possible cause-and-effect link between the two, wherein stress promotes the accumulation of visceral fat, which in turn causes hormonal and metabolic changes that contribute to heart disease and other health problems.
Recent advances in biotechnology have allowed for the harvesting of adult stem cells from adipose tissue, allowing stimulation of tissue regrowth using a patient's own cells. In addition, adipose-derived stem cells from both human and animals reportedly can be efficiently reprogrammed into induced pluripotent stem cells without the need for feeder cells. The use of a patient's own cells reduces the chance of tissue rejection and avoids ethical issues associated with the use of human embryonic stem cells. A growing body of evidence also suggests that different fat depots (i.e. abdominal, omental, pericardial) yield adipose-derived stem cells with different characteristics. These depot-dependent features include proliferation rate, immunophenotype, differentiation potential, gene expression, as well as sensitivity to hypoxic culture conditions. Oxygen levels seem to play an important role on the metabolism and in general the function of adipose-derived stem cells.
Adipose tissue is a major peripheral source of aromatase in both males and females, contributing to the production of estradiol.
Adipose derived hormones include:
Adiponectin
Resistin
Plasminogen activator inhibitor-1 (PAI-1)
TNFα
IL-6
Leptin
Estradiol (E2)
Adipose tissues also secrete a type of cytokines (cell-to-cell signalling proteins) called adipokines (adipose cytokines), which play a role in obesity-associated complications. Perivascular adipose tissue releases adipokines such as adiponectin that affect the contractile function of the vessels that they surround.
Brown fat
Brown fat or brown adipose tissue (BAT) is a specialized form of adipose tissue important for adaptive thermogenesis in humans and other mammals. BAT can generate heat by "uncoupling" the respiratory chain of oxidative phosphorylation within mitochondria through tissue-specific expression of uncoupling protein 1 (UCP1). BAT is primarily located around the neck and large blood vessels of the thorax, where it may effectively act in heat exchange. BAT is robustly activated upon cold exposure by the release of catecholamines from sympathetic nerves that results in UCP1 activation.
Nearly half of the nerves present in adipose tissue are sensory neurons connected to the dorsal root ganglia.
BAT activation may also occur in response to overfeeding. UCP1 activity is stimulated by long chain fatty acids that are produced subsequent to β-adrenergic receptor activation. UCP1 is proposed to function as a fatty acid proton symporter, although the exact mechanism has yet to be elucidated. In contrast, UCP1 is inhibited by ATP, ADP, and GTP.
Attempts to simulate this process pharmacologically have so far been unsuccessful. Techniques to manipulate the differentiation of "brown fat" could become a mechanism for weight loss therapy in the future, encouraging the growth of tissue with this specialized metabolism without inducing it in other organs. A review on the eventual therapeutic targeting of brown fat to treat human obesity was published by Samuelson and Vidal-Puig in 2020.
Until recently, brown adipose tissue in humans was thought to be primarily limited to infants, but new evidence has overturned that belief. Metabolically active tissue with temperature responses similar to brown adipose was first reported in the neck and trunk of some human adults in 2007, and the presence of brown adipose in human adults was later verified histologically in the same anatomical regions.
Beige fat and WAT browning
Browning of WAT, also referred to as "beiging", occurs when adipocytes within WAT depots develop features of BAT. Beige adipocytes take on a multilocular appearance (containing several lipid droplets) and increase expression of uncoupling protein 1 (UCP1). In doing so, these normally energy-storing adipocytes become energy-releasing adipocytes.
The calorie-burning capacity of brown and beige fat has been extensively studied as research efforts focus on therapies targeted to treat obesity and diabetes. The drug 2,4-dinitrophenol, which also acts as a chemical uncoupler similarly to UCP1, was used for weight loss in the 1930s. However, it was quickly discontinued when excessive dosing led to adverse side effects including hyperthermia and death. β3-adrenergic agonists, like CL316,243, have also been developed and tested in humans. However, the use of such drugs has proven largely unsuccessful due to several challenges, including varying species receptor specificity and poor oral bioavailability.
Cold is a primary regulator of BAT processes and induces WAT browning. Browning in response to chronic cold exposure has been well documented and is a reversible process. A study in mice demonstrated that cold-induced browning can be completely reversed in 21 days, with measurable decreases in UCP1 seen within a 24-hour period. A study by Rosenwald et al. revealed that when the animals are re-exposed to a cold environment, the same adipocytes will adopt a beige phenotype, suggesting that beige adipocytes are retained.
Transcriptional regulators, as well as a growing number of other factors, regulate the induction of beige fat. Four regulators of transcription are central to WAT browning and serve as targets for many of the molecules known to influence this process. These include peroxisome proliferator-activated receptor gamma (PPARγ), PRDM16, peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC-1α), and Early B-Cell Factor-2 (EBF2).
The list of molecules that influence browning has grown in direct proportion to the popularity of this topic and is constantly evolving as more knowledge is acquired. Among these molecules are irisin and fibroblast growth factor 21 (FGF21), which have been well-studied and are believed to be important regulators of browning. Irisin is secreted from muscle in response to exercise and has been shown to increase browning by acting on beige preadipocytes. FGF21, a hormone secreted mainly by the liver, has garnered a great deal of interest after being identified as a potent stimulator of glucose uptake and a browning regulator through its effects on PGC-1α. It is increased in BAT during cold exposure and is thought to aid in resistance to diet-induced obesity FGF21 may also be secreted in response to exercise and a low protein diet, although the latter has not been thoroughly investigated. Data from these studies suggest that environmental factors like diet and exercise may be important mediators of browning. In mice, it was found that beiging can occur through the production of methionine-enkephalin peptides by type 2 innate lymphoid cells in response to interleukin 33.
Genomics and bioinformatics tools to study browning
Due to the complex nature of adipose tissue and a growing list of browning regulatory molecules, great potential exists for the use of bioinformatics tools to improve study within this field. Studies of WAT browning have greatly benefited from advances in these techniques, as beige fat is rapidly gaining popularity as a therapeutic target for the treatment of obesity and diabetes.
DNA microarray is a bioinformatics tool used to quantify expression levels of various genes simultaneously, and has been used extensively in the study of adipose tissue. One such study used microarray analysis in conjunction with Ingenuity IPA software to look at changes in WAT and BAT gene expression when mice were exposed to temperatures of 28 and 6 °C. The most significantly up- and downregulated genes were then identified and used for analysis of differentially expressed pathways. It was discovered that many of the pathways upregulated in WAT after cold exposure are also highly expressed in BAT, such as oxidative phosphorylation, fatty acid metabolism, and pyruvate metabolism. This suggests that some of the adipocytes switched to a beige phenotype at 6 °C. Mössenböck et al. also used microarray analysis to demonstrate that insulin deficiency inhibits the differentiation of beige adipocytes but does not disturb their capacity for browning. These two studies demonstrate the potential for the use of microarray in the study of WAT browning.
RNA sequencing (RNA-Seq) is a powerful computational tool that allows for the quantification of RNA expression for all genes within a sample. Incorporating RNA-Seq into browning studies is of great value, as it offers better specificity, sensitivity, and a more comprehensive overview of gene expression than other methods. RNA-Seq has been used in both human and mouse studies in an attempt characterize beige adipocytes according to their gene expression profiles and to identify potential therapeutic molecules that may induce the beige phenotype. One such study used RNA-Seq to compare gene expression profiles of WAT from wild-type (WT) mice and those overexpressing Early B-Cell Factor-2 (EBF2). WAT from the transgenic animals exhibited a brown fat gene program and had decreased WAT specific gene expression compared to the WT mice. Thus, EBF2 has been identified as a potential therapeutic molecule to induce beiging.
Chromatin immunoprecipitation with sequencing (ChIP-seq) is a method used to identify protein binding sites on DNA and assess histone modifications. This tool has enabled examination of epigenetic regulation of browning and helps elucidate the mechanisms by which protein-DNA interactions stimulate the differentiation of beige adipocytes. Studies observing the chromatin landscapes of beige adipocytes have found that adipogenesis of these cells results from the formation of cell specific chromatin landscapes, which regulate the transcriptional program and, ultimately, control differentiation. Using ChIP-seq in conjunction with other tools, recent studies have identified over 30 transcriptional and epigenetic factors that influence beige adipocyte development.
Genetics
The thrifty gene hypothesis (also called the famine hypothesis) states that in some populations the body would be more efficient at retaining fat in times of plenty, thereby endowing greater resistance to starvation in times of food scarcity. This hypothesis, originally advanced in the context of glucose metabolism and insulin resistance, has been discredited by physical anthropologists, physiologists, and the original proponent of the idea himself with respect to that context, although according to its developer it remains "as viable as when [it was] first advanced" in other contexts.
In 1995, Jeffrey Friedman, in his residency at the Rockefeller University, together with Rudolph Leibel, Douglas Coleman et al. discovered the protein leptin that the genetically obese mouse lacked. Leptin is produced in the white adipose tissue and signals to the hypothalamus. When leptin levels drop, the body interprets this as a loss of energy, and hunger increases. Mice lacking this protein eat until they are four times their normal size.
Leptin, however, plays a different role in diet-induced obesity in rodents and humans. Because adipocytes produce leptin, leptin levels are elevated in the obese. However, hunger remains, and—when leptin levels drop due to weight loss—hunger increases. The drop of leptin is better viewed as a starvation signal than the rise of leptin as a satiety signal. However, elevated leptin in obesity is known as leptin resistance. The changes that occur in the hypothalamus to result in leptin resistance in obesity are currently the focus of obesity research.
Gene defects in the leptin gene (ob) are rare in human obesity. , only 14 individuals from five families have been identified worldwide who carry a mutated ob gene (one of which was the first ever identified cause of genetic obesity in humans)—two families of Pakistani origin living in the UK, one family living in Turkey, one in Egypt, and one in Austria—and two other families have been found that carry a mutated ob receptor. Others have been identified as genetically partially deficient in leptin, and, in these individuals, leptin levels on the low end of the normal range can predict obesity.
Several mutations of genes involving the melanocortins (used in brain signaling associated with appetite) and their receptors have also been identified as causing obesity in a larger portion of the population than leptin mutations.
Physical properties
Adipose tissue has a density of ~0.9 g/ml. Thus, a person with more adipose tissue will float more easily than a person of the same weight with more muscular tissue, since muscular tissue has a density of 1.06 g/ml.
Body fat meter
A body fat meter is a tool used to measure the body fat to weight ratio in the human body. Different meters use various methods to determine the ratio. They tend to under-read body fat percentage.
In contrast with clinical tools like DXA and underwater weighing, one relatively inexpensive type of body fat meter uses the principle of bioelectrical impedance analysis (BIA) in order to determine an individual's body fat percentage. To achieve this, the meter passes a small, harmless, electric current through the body and measures the resistance, then uses information on the person's weight, height, age, and sex to calculate an approximate value for the person's body fat percentage. The calculation measures the total volume of water in the body (lean tissue and muscle contain a higher percentage of water than fat), and estimates the percentage of fat based on this information. The result can fluctuate several percentage points depending on what has been eaten and how much water has been drunk before the analysis. This method is quick and readily accessible, but imprecise. Alternative methods are: skin fold methods using calipers, underwater weighing, whole body air displacement plethysmography (ADP) and DXA.
Animal studies
Within the fat (adipose) tissue of CCR2 deficient mice, there is an increased number of eosinophils, greater alternative Macrophage activation, and a propensity towards type 2 cytokine expression. Furthermore, this effect was exaggerated when the mice became obese from a high fat diet.
Gallery
| Biology and health sciences | Tissues | Biology |
419100 | https://en.wikipedia.org/wiki/Organ%20system | Organ system | An organ system is a biological system consisting of a group of organs that work together to perform one or more functions. Each organ has a specialized role in an organism body, and is made up of distinct tissues.
Humans
Main article: List of systems of the human body
There are 11 distinct organ systems in human beings, which form the basis of human anatomy and physiology. The 11 organ systems: the respiratory system, digestive and excretory system, circulatory system, urinary system, integumentary system, skeletal system, muscular system, endocrine system, lymphatic system, nervous system, and reproductive system. There are other systems in the body that are not organ systems—for example, the immune system protects the organism from infection, but it is not an organ system since it is not composed of organs. Some organs are in more than one system—for example, the nose is in the respiratory system and also serves as a sensory organ in the nervous system; the testes and ovaries are both part of the reproductive and endocrine systems.
Other animals
Other animals have similar organ systems to humans although simpler animals may have fewer organs in an organ system or even fewer organ systems.
Plants
Plants have two major organs systems. Vascular plants have two distinct organ systems: a shoot system, and a root system. The shoot system consists stems, leaves, and the reproductive parts of the plant (flowers and fruits). The shoot system generally grows above ground, where it absorbs the light needed for photosynthesis. The root system, which supports the plants and absorbs water and minerals, is usually underground.
| Biology and health sciences | Basic anatomy | Biology |
419386 | https://en.wikipedia.org/wiki/Applied%20science | Applied science | Applied science is the application of the scientific method and scientific knowledge to attain practical goals. It includes a broad range of disciplines, such as engineering and medicine. Applied science is often contrasted with basic science, which is focused on advancing scientific theories and laws that explain and predict natural or other phenomena.
There are applied natural sciences, as well as applied formal and social sciences. Applied science examples include genetic epidemiology which applies statistics and probability theory, and applied psychology, including criminology.
Applied research
Applied research is the use of empirical methods to collect data for practical purposes. It accesses and uses accumulated theories, knowledge, methods, and techniques for a specific state, business, or client-driven purpose. In contrast to engineering, applied research does not include analyses or optimization of business, economics, and costs. Applied research can be better understood in any area when contrasting it with basic or pure research. Basic geographical research strives to create new theories and methods that aid in explaining the processes that shape the spatial structure of physical or human environments. Instead, applied research utilizes existing geographical theories and methods to comprehend and address particular empirical issues. Applied research usually has specific commercial objectives related to products, procedures, or services. The comparison of pure research and applied research provides a basic framework and direction for businesses to follow.
Applied research deals with solving practical problems and generally employs empirical methodologies. Because applied research resides in the messy real world, strict research protocols may need to be relaxed. For example, it may be impossible to use a random sample. Thus, transparency in the methodology is crucial. Implications for the interpretation of results brought about by relaxing an otherwise strict canon of methodology should also be considered.
Moreover, this type of research method applies natural sciences to human conditions:
Action research: aids firms in identifying workable solutions to issues influencing them.
Evaluation research: researchers examine available data to assist clients in making wise judgments.
Industrial research: create new goods/services that will satisfy the demands of a target market. (Industrial development would be scaling up production of the new goods/services for mass consumption to satisfy the economic demand of the customers while maximizing the ratio of the good/service output rate to resource input rate, the ratio of good/service revenue to material & energy costs, and the good/service quality. Industrial development would be considered engineering. Industrial development would fall outside the scope of applied research.)
Since applied research has a provisional close-to-the-problem and close-to-the-data orientation, it may also use a more provisional conceptual framework, such as working hypotheses or pillar questions. The OECD's Frascati Manual describes applied research as one of the three forms of research, along with basic research & experimental development.
Due to its practical focus, applied research information will be found in the literature associated with individual disciplines.
Branches
Applied research is a method of problem-solving and is also practical in areas of science, such as its presence in applied psychology. Applied psychology uses human behavior to grab information to locate a main focus in an area that can contribute to finding a resolution. More specifically, this study is applied in the area of criminal psychology. With the knowledge obtained from applied research, studies are conducted on criminals alongside their behavior to apprehend them. Moreover, the research extends to criminal investigations. Under this category, research methods demonstrate an understanding of the scientific method and social research designs used in criminological research. These reach more branches along the procedure towards the investigations, alongside laws, policy, and criminological theory.
Engineering is the practice of using natural science, mathematics, and the engineering design process to solve technical problems, increase efficiency and productivity, and improve systems. The discipline of engineering encompasses a broad range of more specialized fields of engineering, each with a more specific emphasis on particular areas of applied mathematics, applied science, and types of application. Engineering is often characterized as having four main branches: chemical engineering, civil engineering, electrical engineering, and mechanical engineering. Some scientific subfields used by engineers include thermodynamics, heat transfer, fluid mechanics, statics, dynamics, mechanics of materials, kinematics, electromagnetism, materials science, earth sciences, and engineering physics.
Medical sciences, such as medical microbiology, pharmaceutical research, and clinical virology, are applied sciences that apply biology and chemistry to medicine.
In education
In Canada, the Netherlands, and other places, the Bachelor of Applied Science (BASc) is sometimes equivalent to the Bachelor of Engineering and is classified as a professional degree. This is based on the age of the school where applied science used to include boiler making, surveying, and engineering. There are also Bachelor of Applied Science degrees in Child Studies. The BASc tends to focus more on the application of the engineering sciences. In Australia and New Zealand, this degree is awarded in various fields of study and is considered a highly specialized professional degree.
In the United Kingdom's educational system, Applied Science refers to a suite of "vocational" science qualifications that run alongside "traditional" General Certificate of Secondary Education or A-Level Sciences. Applied Science courses generally contain more coursework (also known as portfolio or internally assessed work) compared to their traditional counterparts. These are an evolution of the GNVQ qualifications offered up to 2005. These courses regularly come under scrutiny and are due for review following the Wolf Report 2011; however, their merits are argued elsewhere.
In the United States, The College of William & Mary offers an undergraduate minor as well as Master of Science and Doctor of Philosophy degrees in "applied science". Courses and research cover varied fields, including neuroscience, optics, materials science and engineering, nondestructive testing, and nuclear magnetic resonance. University of Nebraska–Lincoln offers a Bachelor of Science in applied science, an online completion Bachelor of Science in applied science, and a Master of Applied Science. Coursework is centered on science, agriculture, and natural resources with a wide range of options, including ecology, food genetics, entrepreneurship, economics, policy, animal science, and plant science. In New York City, the Bloomberg administration awarded the consortium of Cornell-Technion $100 million in City capital to construct the universities' proposed Applied Sciences campus on Roosevelt Island.
| Technology | General | null |
419444 | https://en.wikipedia.org/wiki/Solenoidal%20vector%20field | Solenoidal vector field | In vector calculus a solenoidal vector field (also known as an incompressible vector field, a divergence-free vector field, or a transverse vector field) is a vector field v with divergence zero at all points in the field:
A common way of expressing this property is to say that the field has no sources or sinks.
Properties
The divergence theorem gives an equivalent integral definition of a solenoidal field; namely that for any closed surface, the net total flux through the surface must be zero:
where is the outward normal to each surface element.
The fundamental theorem of vector calculus states that any vector field can be expressed as the sum of an irrotational and a solenoidal field. The condition of zero divergence is satisfied whenever a vector field v has only a vector potential component, because the definition of the vector potential A as:
automatically results in the identity (as can be shown, for example, using Cartesian coordinates):
The converse also holds: for any solenoidal v there exists a vector potential A such that (Strictly speaking, this holds subject to certain technical conditions on v, see Helmholtz decomposition.)
Etymology
Solenoidal has its origin in the Greek word for solenoid, which is σωληνοειδές (sōlēnoeidēs) meaning pipe-shaped, from σωλην (sōlēn) or pipe.
Examples
The magnetic field B (see Gauss's law for magnetism)
The velocity field of an incompressible fluid flow
The vorticity field
The electric field E in neutral regions ();
The current density J where the charge density is unvarying, .
The magnetic vector potential A in Coulomb gauge
| Mathematics | Multivariable and vector calculus | null |
419513 | https://en.wikipedia.org/wiki/Input%20method | Input method | An input method (or input method editor, commonly abbreviated IME) is an operating system component or program that enables users to generate characters not natively available on their input devices by using sequences of characters (or mouse operations) that are available to them. Using an input method is usually necessary for languages that have more graphemes than there are keys on the keyboard.
For instance, on the computer, this allows the user of Latin keyboards to input Chinese, Japanese, Korean and Indic characters. On hand-held devices, it enables the user to type on the numeric keypad to enter Latin alphabet characters (or any other alphabet characters) or touch a screen display to input text. On some operating systems, an input method is also used to define the behavior of the dead keys.
Implementations
Although originally coined for CJK (Chinese, Japanese and Korean) computing, the term is now sometimes used generically to refer to a program to support the input of any language. To illustrate, in the X Window System, the facility to allow the input of Latin characters with diacritics is also called an input method.
On Windows XP or later Windows, Input method, or IME, are also called Text Input Processor, which are implemented by the Text Services Framework API.
Relationship between the methodology and implementation
While the term input method editor was originally used for Microsoft Windows, its use has now gained acceptance in other operating systems, especially when it is important to distinguish between the computer interface and implementation of input methods, or among the input methods themselves, the editing functionality of the program or operating system component providing the input method, and the general support of input methods in an operating system. This term has, for example, gained general acceptance on the Linux operating system; it is also used on the Mac OS.
The term input method generally refers to a particular way to use the keyboard to input a particular language, for example the Cangjie method, the pinyin method, or the use of dead keys.
On the other hand, the term input method editor on Microsoft products refers to the program that allows an input method to be used (for example MS New Pinyin), or the editing area that allows the user to do the input. It can also refer to a character palette, which allows any Unicode character to be input individually. One might also interpret IME to refer to the editor used for creating or modifying the data files upon which an input method relies.
| Technology | Computer software | null |
419522 | https://en.wikipedia.org/wiki/Leaf%20beetle | Leaf beetle | The insects of the beetle family Chrysomelidae are commonly known as leaf beetles, and include over 37,000 (and probably at least 50,000) species in more than 2,500 genera, making up one of the largest and most commonly encountered of all beetle families. Numerous subfamilies are recognized, but the precise taxonomy and systematics are likely to change with ongoing research.
Leaf beetles are partially recognizable by their tarsal formula, which appears to be 4-4-4, but is actually 5-5-5 as the fourth tarsal segment is very small and hidden by the third. As with many taxa, no single character defines the Chrysomelidae; instead, the family is delineated by a set of characters. Some lineages are only distinguished with difficulty from longhorn beetles (family Cerambycidae), namely by the antennae not arising from frontal tubercles.
Adult and larval leaf beetles feed on all sorts of plant tissue. Many are serious pests of cultivated plants, for example the Colorado potato beetle (Leptinotarsa decemlineata), the asparagus beetle (Crioceris asparagi), the cereal leaf beetle (Oulema melanopus), the mustard beetle (Phaedon cochleariae) and various flea beetles, and a few act as vectors of plant diseases. Others are beneficial due to their use in biocontrol of invasive weeds. Some Chrysomelidae are conspicuously colored, typically in glossy yellow to red or metallic blue-green hues, and some (especially Cassidinae) have spectacularly bizarre shapes. Thus, they are highly popular among insect collectors.
Description
The imagos of leaf beetles are small to medium-sized, i.e. most species range from 1.0 to 18 mm in length, excluding appendages, with just a few larger species such as Alurnus humeralis, which reaches 35 mm. The bodies of most species are domed, and oval in dorsal view (though some are round or elongated), and they often possess a metallic luster or multiple colors. In most specimens, the antennae are notably shorter than head, thorax, and abdomen, i.e. not more than half their combined length. The second antennal segment is of normal size (which differentiates leaf beetles from the closely related longhorn beetles). In most species, the antennal segments are of a more or less equal shape, at most they gradually widen towards the tip, although some Galerucinae in particular have modified segments, mainly in males. The first segment of the antenna in most cases is larger than the following ones. The pronotum of leaf beetles varies between species. In most, it is slightly to highly domed and trapezoidal to rounded-squarish in dorsal view. In some subfamilies such as the Cassidinae and to a lesser extent the Cryptocephalinae, the head is covered by the pronotum and thus not visible from above. The first three sternites are not fused, instead being linked by mobile sutures. Most species possess wings, although the level of development and thus flight ability varies widely, including within a single species, and some are flightless with fused elytra.
Subfamilies
The family includes these subfamilies:
Bruchinae Latreille, 1802 – bean weevils or seed beetles
Cassidinae Gyllenhaal, 1813 – tortoise beetles; includes the former "Hispinae"
Chrysomelinae Latreille, 1802 – broad-bodied leaf beetles
Criocerinae Latreille, 1804 – asparagus beetles, lily beetles, etc.
Cryptocephalinae Gyllenhaal, 1813 – cylindrical leaf beetles and warty leaf beetles; includes former "Chlamisinae" and "Clytrinae"
Donaciinae Kirby, 1837 – longhorned leaf beetles
Eumolpinae Hope, 1840 – oval leaf beetles
Galerucinae Latreille, 1802 – includes the former "Alticinae" (flea beetles)
Lamprosomatinae Lacordaire, 1848
Sagrinae Leach, 1815 – frog-legged beetles or kangaroo beetles
Spilopyrinae Chapuis, 1874
Synetinae LeConte & Horn, 1883 – sometimes considered a tribe of Eumolpinae
Until recently, the subfamily Bruchinae was considered a separate family, while two former subfamilies are presently considered families (Orsodacnidae and Megalopodidae). Other commonly recognized subfamilies have recently been grouped with other subfamilies, usually reducing them to tribal rank (e.g., the former Alticinae, Chlamisinae, Clytrinae, and Hispinae). The extinct subfamily Protoscelidinae, containing fossils described from the Middle to Late Jurassic Karabastau Formation, Kazakhstan, has been transferred to the family Anthribidae.
Diet
Chrysomelidae in general are herbivorous. Adults mostly feed on leaves and flowers of angiosperm plants, while larval diets are diverse.
Bruchinae larvae are seed-borers, usually in seeds of legumes. Many adults feed on pollen, not necessarily that of the larval host. Some do not feed as adults.
Cassidinae larvae may be leaf miners (many of the former Hispinae), stem borers (e.g. Estigmena) and external leaf feeders (e.g. Leptispa, Oediopalpa).
Chrysomelinae generally feed on leaves as adults and larvae, though some species feed on flowers instead.
Criocerinae larvae are usually leaf miners or feed externally on leaves. Some species are gallers instead.
Eumolpinae larvae feed on roots.
Most Cryptocephalinae larvae live and feed in leaf litter, making them detritivores, while a few feed on green leaves. Some Cryptocephalinae have larvae that live in ant nests (myrmecophily), where they feed on dead plant or even dead animal matter.
The semi-aquatic Donaciinae have larvae feeding on the sap of roots of aquatic plants. In addition to food, they also obtain oxygen this way, from the plant's intercellular spaces. Adults feed on leaves of aquatic plants.
Galerucinae are quite varied, with larvae living in soil and feeding on rootlets (e.g. Aulacophora, Cerotoma, Diabrotica), mining leaves (some Monoxia) or feeding externally on plants (e.g. Arima, Galeruca, Galerucella).
Lamprosomatinae larvae feed on green plant parts or graze on bark.
Sagrinae larvae mostly form galls in stems of shrubs, though Mecynodera balyi instead feeds inside seed pods of Pandorea vines. Adults feed on pollen.
Spilopyrinae larvae are external leaf feeders.
Synetinae larvae feed on roots, mainly of trees in cold northern forests.
To be able to digest the plant matter, the beetles use enzymes like pectinases. This group of enzymes are either produced by the beetles themselves, due to horizontal gene transfer, or symbiotic bacteria provides them with the enzymes. But both solutions are never used simultaneously.
Natural enemies
A Finnish researcher published an exhaustive paper describing the natural enemies of the alder leaf beetle Plagiosterna aenea and other species of leaf beetles observed in the field. Predators of chrysomelid eggs include true bugs such as Anthocorus nemorum and Orthotylus marginalis. Hoverflies (e.g. Parasyrphus nigritarsis) sometimes lay eggs adjacent to beetle egg clutches and when the fly larva hatches it consumes beetle eggs and young larvae. Larval predators include A. nemorum, the bug Rhacognathus punctatus, and the wasp Symmorphus bifasciatus. Some species of wasps, such as Polistes carolina, have been known to prey upon Chrysomelidae larvae after the eggs are laid in flowers. Adult beetles are consumed by R. punctatus. More information about natural enemies can be found in the articles about the chrysomelid beetles Chrysomela aeneicollis, Phratora laticollis and Phratora vitellinae.
Gallery
| Biology and health sciences | Beetles (Coleoptera) | Animals |
419667 | https://en.wikipedia.org/wiki/Missile%20guidance | Missile guidance | Missile guidance refers to a variety of methods of guiding a missile or a guided bomb to its intended target. The missile's target accuracy is a critical factor for its effectiveness. Guidance systems improve missile accuracy by improving its Probability of Guidance (Pg).
These guidance technologies can generally be divided up into a number of categories, with the broadest categories being "active", "passive", and "preset" guidance. Missiles and guided bombs generally use similar types of guidance system, the difference between the two being that missiles are powered by an onboard engine, whereas guided bombs rely on the speed and height of the launch aircraft for propulsion.
History
The concept of unmanned guidance originated at least as early as World War I, with the idea of remotely guiding an airplane bomb onto a target, such as the systems developed for the first powered drones by Archibald Low (the father of radio guidance).
In World War II, guided missiles were first developed, as part of the German V-weapons program. Project Pigeon was American behaviorist B.F. Skinner's attempt to develop a pigeon-guided bomb.
The first U.S. ballistic missile with a highly accurate inertial guidance system was the short-range PGM-11 Redstone.
Categories of guidance systems
Guidance systems are divided into different categories according to whether they are designed to attack fixed or moving targets. The weapons can be divided into two broad categories: Go-onto-target (GOT) and go-onto-location-in-space (GOLIS) guidance systems. A GOT missile can target either a moving or fixed target, whereas a GOLIS weapon is limited to a stationary or near-stationary target. The trajectory that a missile takes while attacking a moving target is dependent upon the movement of the target. A moving target can be an immediate threat to the missile launcher. The target must be promptly eliminated in order to preserve the launcher. In GOLIS systems, the problem is simpler because the target is not moving.
GOT systems
In every go-onto-target system there are three subsystems:
Target tracker
Missile tracker
Guidance computer
The way these three subsystems are distributed between the missile and the launcher result in two different categories:
Remote control guidance: The guidance computer is on the launcher. The target tracker is also placed on the launching platform.
Homing guidance: The guidance computers are in the missile and in the target tracker.
Remote control guidance
These guidance systems usually need the use of radars and a radio or wired link between the control point and the missile; in other words, the trajectory is controlled with the information transmitted via radio or wire (see Wire-guided missile). These systems include:
Command guidance – The missile tracker is on the launching platform. These missiles are fully controlled by the launching platform that sends all control orders to the missile. The two variants are
Command to line-of-sight (CLOS)
Command off line-of-sight (COLOS)
Line-of-sight beam riding guidance (LOSBR) – The target tracker is on board the missile. The missile already has some orientation capability meant for flying inside the beam that the launching platform is using to illuminate the target. It can be manual or automatic.
Command to line-of-sight
The CLOS system uses only the angular coordinates between the missile and the target to ensure the collision. The missile is made to be in the line of sight between the launcher and the target (LOS), and any deviation of the missile from this line is corrected. Since so many types of missile use this guidance system, they are usually subdivided into four groups: A particular type of command guidance and navigation where the missile is always commanded to lie on the line of sight (LOS) between the tracking unit and the aircraft is known as command to line of sight (CLOS) or three-point guidance. That is, the missile is controlled to stay as close as possible on the LOS to the target after missile capture is used to transmit guidance signals from a ground controller to the missile. More specifically, if the beam acceleration is taken into account and added to the nominal acceleration generated by the beam-rider equations, then CLOS guidance results. Thus, the beam rider acceleration command is modified to include an extra term. The beam-riding performance described above can thus be significantly improved by taking the beam motion into account. CLOS guidance is used mostly in shortrange air defense and antitank systems.
Manual command to line-of-sight
Both target tracking and missile tracking and control are performed manually. The operator watches the missile flight, and uses a signaling system to command the missile back into the straight line between operator and target (the "line of sight"). This is typically useful only for slower targets, where significant "lead" is not required. MCLOS is a subtype of command guided systems. In the case of glide bombs or missiles against ships or the supersonic Wasserfall against slow-moving B-17 Flying Fortress bombers this system worked, but as speeds increased MCLOS was quickly rendered useless for most roles.
Semi-manual command to line-of-sight
Target tracking is automatic, while missile tracking and control is manual.
Semi-automatic command to line-of-sight
Target tracking is manual, but missile tracking and control is automatic. It is similar to MCLOS but some automatic systems position the missile in the line of sight while the operator simply tracks the target. SACLOS has the advantage of allowing the missile to start in a position invisible to the user, as well as generally being considerably easier to operate. It is the most common form of guidance against ground targets such as tanks and bunkers.
Automatic command to line-of-sight
Target tracking, missile tracking and control are automatic.
Command off line-of-sight
This guidance system was one of the first to be used and still is in service, mainly in anti-aircraft missiles. In this system, the target tracker and the missile tracker can be oriented in different directions. The guidance system ensures the interception of the target by the missile by locating both in space. This means that they will not rely on the angular coordinates like in CLOS systems. They will need another coordinate which is distance. To make it possible, both target and missile trackers have to be active. They are always automatic and the radar has been used as the only sensor in these systems. The SM-2MR Standard is inertially guided during its mid-course phase, but it is assisted by a COLOS system via radar link provided by the AN/SPY-1 radar installed in the launching platform.
Line-of-sight beam riding guidance
LOSBR uses a "beam" of some sort, typically radio, radar or laser, which is pointed at the target and detectors on the rear of the missile keep it centered in the beam. Beam riding systems are often SACLOS, but do not have to be; in other systems the beam is part of an automated radar tracking system. A case in point is the later versions of the RIM-8 Talos missile as used in Vietnam – the radar beam was used to take the missile on a high arcing flight and then gradually brought down in the vertical plane of the target aircraft, the more accurate SARH homing being used at the last moment for the actual strike. This gave the enemy pilot the least possible warning that his aircraft was being illuminated by missile guidance radar, as opposed to search radar. This is an important distinction, as the nature of the signal differs, and is used as a cue for evasive action.
LOSBR suffers from the inherent weakness of inaccuracy with increasing range as the beam spreads out. Laser beam riders are more accurate in this regard, but they are all short-range, and even the laser can be degraded by bad weather. On the other hand, SARH becomes more accurate with decreasing distance to the target, so the two systems are complementary.
Homing guidance
Proportional navigation
Proportional navigation (also known as "PN" or "Pro-Nav") is a guidance principle (analogous to proportional control) used in some form or another by most homing air target missiles. It is based on the fact that two objects are on a collision course when the direction of their direct line-of-sight does not change. PN dictates that the missile velocity vector should rotate at a rate proportional to the rotation rate of the line of sight (line-Of-sight rate or LOS-rate) and in the same direction.
Radar homing
Active homing
Active homing uses a radar system on the missile to provide a guidance signal. Typically, electronics in the missile keep the radar pointed directly at the target, and the missile then looks at this "angle" of its own centerline to guide itself. Radar resolution is based on the size of the antenna, so in a smaller missile these systems are useful for attacking only large targets, ships or large bombers for instance. Active radar systems remain in widespread use in anti-shipping missiles, and in "fire-and-forget" air-to-air missile systems such as the AIM-120 AMRAAM and R-77.
Semi-active homing
Semi-active homing systems combine a passive radar receiver on the missile with a separate targeting radar that "illuminates" the target. Since the missile is typically being launched after the target was detected using a powerful radar system, it makes sense to use that same radar system to track the target, thereby avoiding problems with resolution or power, and reducing the weight of the missile. Semi-active radar homing (SARH) is by far the most common "all weather" guidance solution for anti-aircraft systems, both ground- and air-launched.
It has the disadvantage for air-launched systems that the launch aircraft must keep moving towards the target in order to maintain radar and guidance lock. This has the potential to bring the aircraft within range of shorter-ranged IR-guided (infrared-guided) missile systems. It is an important consideration now that "all aspect" IR missiles are capable of "kills" from head on, something which did not prevail in the early days of guided missiles. For ships and mobile or fixed ground-based systems, this is irrelevant as the speed (and often size) of the launch platform precludes "running away" from the target or opening the range so as to make the enemy attack fail.
SALH is similar to SARH but uses a laser as a signal. Another difference is that most laser-guided weapons employ turret-mounted laser designators which increase the launching aircraft's ability to maneuver after launch. How much maneuvering can be done by the guiding aircraft depends on the turret field of view and the system's ability to maintain a lock-on while maneuvering. As most air-launched, laser-guided munitions are employed against surface targets the designator providing the guidance to the missile need not be the launching aircraft; designation can be provided by another aircraft or by a completely separate source (frequently troops on the ground equipped with the appropriate laser designator).
Passive homing
Infrared homing is a passive system that homes in on the heat generated by the target. Typically used in the anti-aircraft role to track the heat of jet engines, it has also been used in the anti-vehicle role with some success. This means of guidance is sometimes also referred to as "heat seeking".
Contrast seekers use a video camera, typically black and white, to image a field of view in front of the missile, which is presented to the operator. When launched, the electronics in the missile look for the spot on the image where the contrast changes the fastest, both vertically and horizontally, and then attempts to keep that spot at a constant location in its view. Contrast seekers have been used for air-to-ground missiles, including the AGM-65 Maverick, because most ground targets can be distinguished only by visual means. However they rely on there being strong contrast changes to track, and even traditional camouflage can render them unable to "lock on".
Retransmission homing
Retransmission homing, also called "track-via-missile" or "TVM", is a hybrid between command guidance, semi-active radar homing and active radar homing. The missile picks up radiation broadcast by the tracking radar which bounces off the target and relays it to the tracking station, which relays commands back to the missile.
AI guidance
In 2017, Russian weapons manufacturer Tactical Missiles Corporation announced that it was developing missiles that would use artificial intelligence to choose their own targets. In 2019, the United States Army announced it was developing a similar technology.
GOLIS systems
Whatever the mechanism used in a go-onto-location-in-space guidance system is, it must contain preset information about the target. These systems' main characteristic is the lack of a target tracker. The guidance computer and the missile tracker are located in the missile. The lack of target tracking in GOLIS necessarily implies navigational guidance.
Navigational guidance is any type of guidance executed by a system without a target tracker. The other two units are on board the missile. These systems are also known as self-contained guidance systems; however, they are not always entirely autonomous due to the missile trackers used. They are subdivided by their missile tracker's function as follows:
Entirely autonomous – Systems where the missile tracker does not depend on any external navigation source, and can be divided into:
Inertial guidance
With a gimballed gyrostabilized platform or fluid-suspended gyrostabilized platform
With strapdown inertial guidance
Preset guidance
Dependent on natural sources – Navigational guidance systems where the missile tracker depends on a natural external source:
Celestial guidance
Astro-inertial guidance
Terrestrial guidance
Topographic reconnaissance (Ex: TERCOM)
Photographic reconnaissance (Ex: DSMAC)
Magnetic guidance
Dependent on artificial sources – Navigational guidance systems where the missile tracker depends on an artificial external source:
Satellite navigation
Global positioning system (GPS)
Global navigation satellite system (GLONASS)
Hyperbolic navigation
DECCA
LORAN C
Preset guidance
Preset guidance is the simplest type of missile guidance. From the distance and direction of the target, the trajectory of the flight path is determined. Before firing, this information is programmed into the missile's guidance system, which, during flight, maneuvers the missile to follow that path. All of the guidance components (including sensors such as accelerometers or gyroscopes) are contained within the missile, and no outside information (such as radio instructions) is used. An example of a missile using preset guidance is the V-2 rocket.
Inertial guidance
Inertial guidance uses sensitive measurement devices to calculate the location of the missile due to the acceleration put on it after leaving a known position. Early mechanical systems were not very accurate, and required some sort of external adjustment to allow them to hit targets even the size of a city. Modern systems use solid state ring laser gyros that are accurate to within metres over ranges of 10,000 km, and no longer require additional inputs. Gyroscope development has culminated in the AIRS found on the MX missile, allowing for an accuracy of less than 100 m at intercontinental ranges. Many civilian aircraft use inertial guidance using a ring laser gyroscope, which is less accurate than the mechanical systems found in ICBMs, but which provide an inexpensive means of attaining a fairly accurate fix on location (when most airliners such as Boeing's 707 and 747 were designed, GPS was not the widely commercially available means of tracking that it is today). Today guided weapons can use a combination of INS, GPS and radar terrain mapping to achieve extremely high levels of accuracy such as that found in modern cruise missiles.
Inertial guidance is most favored for the initial guidance and reentry vehicles of strategic missiles, because it has no external signal and cannot be jammed. Additionally, the relatively low precision of this guidance method is less of an issue for large nuclear warheads.
Astro-inertial guidance
Astro-inertial guidance, or stellar-inertial guidance, is a sensor fusion-information fusion of inertial guidance and celestial navigation. It is usually employed on submarine-launched ballistic missiles. Unlike silo-based intercontinental ballistic missiles, whose launch point does not move and thus can serve as a reference, SLBMs are launched from moving submarines, which complicates the necessary navigational calculations and increases circular error probable. Stellar-inertial guidance is used to correct small position and velocity errors that result from launch condition uncertainties due to errors in the submarine navigation system and errors that may have accumulated in the guidance system during the flight due to imperfect instrument calibration.
The USAF sought a precision navigation system for maintaining route accuracy and target tracking at very high speeds. Nortronics, Northrop's electronics development division, had developed an astro-inertial navigation system (ANS), which could correct inertial navigation errors with celestial observations, for the SM-62 Snark missile, and a separate system for the ill-fated AGM-48 Skybolt missile, the latter of which was adapted for the SR-71.
It uses star positioning to fine-tune the accuracy of the inertial guidance system after launch. As the accuracy of a missile is dependent upon the guidance system knowing the exact position of the missile at any given moment during its flight, the fact that stars are a fixed reference point from which to calculate that position makes this a potentially very effective means of improving accuracy.
In the Trident missile system this was achieved by a single camera that was trained to spot just one star in its expected position (it is believed that the missiles from Soviet submarines would track two separate stars to achieve this), if it was not quite aligned to where it should be then this would indicate that the inertial system was not precisely on target and a correction would be made.
Terrestrial guidance
TERCOM, for "terrain contour matching", uses altitude maps of the strip of land from the launch site to the target, and compares them with information from a radar altimeter on board. More sophisticated TERCOM systems allow the missile to fly a complex route over a full 3D map, instead of flying directly to the target. TERCOM is the typical system for cruise missile guidance, but is being supplanted by GPS systems and by DSMAC, digital scene-matching area correlator, which employs a camera to view an area of land, digitizes the view, and compares it to stored scenes in an onboard computer to guide the missile to its target.
DSMAC is reputed to be so lacking in robustness that destruction of prominent buildings marked in the system's internal map (such as by a preceding cruise missile) upsets its navigation.
| Technology | Missiles | null |
419751 | https://en.wikipedia.org/wiki/Great%20Dark%20Spot | Great Dark Spot | The Great Dark Spot (also known as GDS-89, for Great Dark Spot, 1989) was one of a series of dark spots on Neptune similar in appearance to Jupiter's Great Red Spot. In 1989, GDS-89 was the first Great Dark Spot on Neptune to be observed by NASA's Voyager 2 space probe. Like Jupiter's spot, the Great Dark Spots are anticyclonic storms. However, their interiors are relatively cloud-free, and unlike Jupiter's spot, which has lasted for hundreds of years, their lifetimes appear to be shorter, forming and dissipating once every few years or so. Based on observations taken with Voyager 2 and since then with the Hubble Space Telescope, Neptune appears to spend somewhat more than half its time with a Great Dark Spot. Little is known about the origins, movement, and disappearance of the dark spots observed on the planet since 1989.
Characteristics
The Great Dark Spot was captured by NASA's Voyager 2 space probe in Neptune's southern hemisphere. The dark, elliptically shaped spot (with initial dimensions of 13,000 × 6,600 km, or 8,100 × 4,100 mi), was about the same size as Earth, and was similar in general appearance to Jupiter's Great Red Spot. One major difference compared to Jupiter's Great Red Spot is that Neptune's Great Dark Spot has shown the ability to shift north-south over time, while the Great Red Spot is held in the same latitudinal region by global east-west wind currents. Around the edges of the storm, winds were measured at up to 2,100 kilometers per hour (1,300 mph), the fastest recorded in the Solar System. The Great Dark Spot is thought to be a hole in the methane cloud deck of Neptune. The spot was observed at different times with different sizes and shapes.
The Great Dark Spot generated large white clouds at or just below the tropopause layer similar to high-altitude cirrus clouds found on Earth. Unlike the clouds on Earth, however, which are composed of crystals of water ice, Neptune's cirrus clouds are made up of crystals of frozen methane. These high altitude clouds are located somewhere between 50–100 km (30–60 miles) above the main cloud deck. While cirrus clouds usually form and then disperse within a period of a few hours, the clouds in the Great Dark Spot were still present after 36 hours, or two rotations of the planet.
Neptune's dark spots are thought to occur in the troposphere at lower altitudes than the brighter upper cloud deck features. As they are stable features that can persist for several months, they are thought to be vortex structures.
Disappearance
When the spot was to be photographed again in November 1994 by the Hubble Space Telescope, it had disappeared completely, leaving astronomers to believe that it had either been covered up or had vanished. The persistence of companion clouds shows that some former dark spots may continue to exist as cyclones even though they are no longer visible as a dark feature. Dark spots may dissipate when they migrate too close to the equator, or possibly through some other unknown mechanisms.
Other dark spots observed
Following the Great Dark Spot, several other dark spots have been observed. In 1989, when the Voyager 2 observed the Great Dark Spot (GDS), a second dark spot, Dark Spot 2 (DS2) was found. Dark Spot 2 fully dissipated prior to the year 1994. Beginning in 1994, the Hubble became the only operating facility to detect the presence and observe dark spots on Neptune and is still used to the present day. Hubble is able to view images at blue wavelength, which is the only way features are visible. In 1994, a Northern Dark Spot (NGDS-1994) formed in the northern hemisphere and disappeared between 1998 and 2000. The storm for its duration showed to be stable in latitude. In 1996, a separate Northern Dark Spot (NGDS-1996) formed and was observed until its disappearance, which occurred prior to 1998. Similarly to the prior dark spot, this one exhibited little to no meridional drift. In 2015, a Southern Dark Spot (SDS) was discovered by the Hubble Outer Planet Atmosphere Legacy (OPAL) program. The Southern Dark Spot exhibited a poleward drift before its disappearance in 2017. In 2016, an almost identical spot as the Great Dark Spot (GDS) emerged in Neptune's northern hemisphere. This new spot, called the Northern Great Dark Spot (NGDS), has remained visible for several years. It is unknown whether this spot is still present on the planet, as observations using the Hubble telescope are limited.
More recently, in 2018, a newer main dark spot and a smaller dark spot were identified and studied. This discovery of the dark spot in Neptune's northern hemisphere was monumental in that it was the first dark spot that the Hubble Telescope was able to document from birth. The storm is much smaller in comparison than the one discovered by NASA's Voyager 2, but was found to be larger in diameter than the Atlantic Ocean at approximately 4,600 miles across. In August 2020, the new Great Dark Spot suddenly stopped its southward motion and reversed direction, contrary to projections that the storm would continue to the equator, where it would have met its likely demise. It is believed that the storms remain stable in the northern hemisphere due to the effect of Coriolis forces. However, as the storms moved towards the equator, the Coriolis forces weakened, causing the storms to dissipate.
Around the same time, a smaller "Dark Spot Jr." was found near the larger storm, before disappearing later on. Dark Spot Jr. as the name suggests was smaller than the prior dark spot, only measuring 3,900 miles in diameter. The coincidental appearance of this storm led astronomers to believe that the prior storm's reversal of motion may have been related to the birth of the smaller storm.
While the formation of the storms is still under investigation, it had been concluded from observations regarding the Southern Dark Spot (SDS-2015) and Northern Great Dark Spot (NGDS-2018) that their origins are preceded by an increase in cloud activity in the given region 2–3 years prior to becoming visible. The storms from 1989–2018 have exhibited different movement patterns and are generally only visible for a few years. Furthermore, the disappearance of dark spots including the Southern Dark Spot can be linked to companion clouds reaching the center of the storm and blocking the view of the blue wavelengths that are used to track the vortex, prior to their disappearance.
Proposed missions
Two mission ideas have been proposed to NASA to visit Neptune in the coming years. Trident was proposed in 2021 as a discovery mission to visit Neptune and its moon Triton in the year, but two missions to Venus (DAVINCI+ and VERITAS) were selected over it. Neptune Odyssey is a flagship orbiter mission concept with similar goals as Trident and is targeted for a launch date of 2033. These missions have a high focus on learning more about Neptune's largest moon Triton, but also aim to gain more information about the atmosphere of Neptune. An analysis of a nuclear-electric propulsion mission to Neptune was published by the China National Space Administration.
| Physical sciences | Solar System | Astronomy |
420067 | https://en.wikipedia.org/wiki/Foot-pound%20%28energy%29 | Foot-pound (energy) | The foot-pound force (symbol: ft⋅lbf, ft⋅lbf, or ft⋅lb ) is a unit of work or energy in the engineering and gravitational systems in United States customary and imperial units of measure. It is the energy transferred upon applying a force of one pound-force (lbf) through a linear displacement of one foot. The corresponding SI unit is the joule, though in terms of energy, one joule is not equal to one foot-pound.
Usage
The term foot-pound is also used as a unit of torque (see pound-foot (torque)). In the United States this is often used to specify, for example, the tightness of a fastener (such as screws and nuts) or the output of an engine. Although they are dimensionally equivalent, energy (a scalar) and torque (a Euclidean vector) are distinct physical quantities. Both energy and torque can be expressed as a product of a force vector with a displacement vector (hence pounds and feet); energy is the scalar product of the two, and torque is the vector product.
Although calling the torque unit "pound-foot" has been academically suggested, both are still commonly called "foot-pound" in colloquial usage. To avoid confusion, it is not uncommon for people to specify each as "foot-pound of energy" or "foot-pound of torque" respectively.
In small arms ballistics and particularly in the United States, the foot-pound is often used to specify the muzzle energy of a bullet.
Conversion factors
Energy
1 foot pound-force is equivalent to:
joules
ergs
about British thermal units
calories
eV = EeV = GeV
Power
1 foot pound-force per second is equivalent to:
1.3558179483314 watts
horsepower
Related conversions:
1 watt ≈ ft⋅lbf/min = ft⋅lbf/s
1 horsepower (mechanical) = 33,000 ft⋅lbf/min = 550 ft⋅lbf/s
| Physical sciences | Energy | Basics and measurement |
11559418 | https://en.wikipedia.org/wiki/International%20System%20of%20Quantities | International System of Quantities | The International System of Quantities (ISQ) is a standard system of quantities used in physics and in modern science in general. It includes basic quantities such as length and mass and the relationships between those quantities. This system underlies the International System of Units (SI) but does not itself determine the units of measurement used for the quantities.
The system is formally described in a multi-part ISO standard ISO/IEC 80000 (which also defines many other quantities used in science and technology), first completed in 2009 and subsequently revised and expanded.
Base quantities
The base quantities of a given system of physical quantities is a subset of those quantities, where no base quantity can be expressed in terms of the others, but where every quantity in the system can be expressed in terms of the base quantities. Within this constraint, the set of base quantities is chosen by convention. There are seven ISQ base quantities. The symbols for them, as for other quantities, are written in italics.
The dimension of a physical quantity does not include magnitude or units. The conventional symbolic representation of the dimension of a base quantity is a single upper-case letter in roman (upright) sans-serif type.
Derived quantities
A derived quantity is a quantity in a system of quantities that is defined in terms of only the base quantities of that system. The ISQ defines many derived quantities and corresponding derived units.
Dimensional expression of derived quantities
The conventional symbolic representation of the dimension of a derived quantity is the product of powers of the dimensions of the base quantities according to the definition of the derived quantity. The dimension of a quantity is denoted by , where the dimensional exponents are positive, negative, or zero. The dimension symbol may be omitted if its exponent is zero. For example, in the ISQ, the quantity dimension of velocity is denoted . The following table lists some quantities defined by the ISQ.
Dimensionless quantities
A quantity of dimension one is historically known as a dimensionless quantity (a term that is still commonly used); all its dimensional exponents are zero and its dimension symbol is . Such a quantity can be regarded as a derived quantity in the form of the ratio of two quantities of the same dimension. The named dimensionless units "radian" (rad) and "steradian" (sr) are acceptable for distinguishing dimensionless quantities of different kind, respectively plane angle and solid angle.
Logarithmic quantities
Level
The level of a quantity is defined as the logarithm of the ratio of the quantity with a stated reference value of that quantity. Within the ISQ it is differently defined for a root-power quantity (also known by the deprecated term field quantity) and for a power quantity. It is not defined for ratios of quantities of other kinds. Within the ISQ, all levels are treated as derived quantities of dimension 1. Several units for levels are defined by the SI and classified as "non-SI units accepted for use with the SI units".
An example of level is sound pressure level, with the unit of decibel.
Other logarithmic quantities
Units of logarithmic frequency ratio include the octave, corresponding to a factor of 2 in frequency (precisely) and the decade, corresponding to a factor 10.
The ISQ recognizes another logarithmic quantity, information entropy, for which the coherent unit is the natural unit of information (symbol nat).
Documentation
The system is formally described in a multi-part ISO standard ISO/IEC 80000, first completed in 2009 but subsequently revised and expanded, which replaced standards published in 1992, ISO 31 and ISO 1000. Working jointly, ISO and IEC have formalized parts of the ISQ by giving information and definitions concerning quantities, systems of quantities, units, quantity and unit symbols, and coherent unit systems, with particular reference to the ISQ. ISO/IEC 80000 defines physical quantities that are measured with the SI units and also includes many other quantities in modern science and technology. The name "International System of Quantities" is used by the General Conference on Weights and Measures (CGPM) to describe the system of quantities that underlie the International System of Units.
| Physical sciences | Measurement systems | Basics and measurement |
10129055 | https://en.wikipedia.org/wiki/Common%20octopus | Common octopus | The common octopus (Octopus vulgaris) is a mollusk belonging to the class Cephalopoda. Octopus vulgaris is one of the most studied of all octopus species, and also one of the most intelligent. It ranges from the eastern Atlantic, extends from the Mediterranean Sea and the southern coast of England, to the southern coast of South Africa. It also occurs off the Azores, Canary Islands, and Cape Verde Islands. The species is also common in the Western Atlantic.
Characteristics
Octopus vulgaris grows to in mantle length with arms up to long. It lives for 1–2 years and may weigh up to . Mating may become cannibalistic. O. vulgaris is caught by bottom trawls on a huge scale off the northwestern coast of Africa. More than are harvested annually.
The common octopus hunts at dusk. Crabs, crayfish, and bivalve mollusks (such as cockles) are preferred, although the octopus eats almost anything it can catch. It is able to change colour to blend in with its surroundings, and is able to jump upon any unwary prey that strays across its path. Using its beak, it is able to break into the shells of shelled mollusks. It also possesses venom to subdue its prey.
They have evolved to have large nervous systems and brains. An individual has about 500 million neurons in its body, almost comparable to dogs. They are intelligent enough to distinguish brightness, navigate mazes, recognize individual people, learn how to unscrew a jar or raid lobster traps. They have also been observed keeping "gardens", in which they collect various marine plant life and algae, alongside collections of shells and rocks; this behavior may have inspired the 1969 Beatles title, "Octopus' Garden". O. vulgaris was the first invertebrate animal protected by the Animals (Scientific Procedures) Act 1986 in the UK. Training experiments have shown the common octopus can distinguish the brightness, size, shape, and horizontal or vertical orientation of objects.
Physiology
Habitat and demands
The common octopus has world wide distribution in tropical, subtropical and temperate waters throughout the world. They prefer the floor of relatively shallow, rocky, coastal waters, often no deeper than . Although they prefer around , salinity throughout their global habitat is found to be between roughly . They are exposed to a wide variety of temperatures in their environments, but their preferred temperature ranges from about . In especially warm seasons, the octopus can often be found deeper than usual to escape the warmer layers of water. In moving vertically throughout the water, the octopus is subjected to various pressures and temperatures, which affect the concentration of oxygen available in the water. This can be understood through Henry's law, which states that the concentration of a gas in a substance is proportional to pressure and solubility, which is influenced by temperature. These various discrepancies in oxygen availability introduce a requirement for regulation methods.
Primarily, the octopus situates itself in a shelter where a minimal amount of its body is presented to the external water. When it does move, most of the time it is along the ocean or sea floor, in which case the underside of the octopus is still obscured. This crawling increases metabolic demands greatly, requiring they increase their oxygen intake by roughly 2.4 times the amount required for a resting octopus. This increased demand is met by an increase in the stroke volume of the octopus' heart.
The octopus does sometimes swim throughout the water, exposing itself completely. In doing so, it uses a jet mechanism that involves creating a much higher pressure in its mantle cavity that allows it to propel itself through the water. As the common octopus' heart and gills are located within its mantle, this high pressure also constricts and puts constraints on the various vessels that are returning blood to the heart. Ultimately, this creates circulation issues and is not a sustainable form of transportation, as the octopus cannot attain an oxygen intake that can balance the metabolic demands of maximum exertion.
Respiration
The octopus uses gills as its respiratory surface. The gill is composed of branchial ganglia and a series of folded lamellae. Primary lamellae extend out to form demi branches and are further folded to form the secondary free folded lamellae, which are only attached at their tops and bottoms. The tertiary lamellae are formed by folding the secondary lamellae in a fan-like shape. Water moves slowly in one direction over the gills and lamellae, into the mantle cavity and out of the octopus' funnel.
The structure of the octopus' gills allows for a high amount of oxygen uptake; up to 65% in water at . The thin skin of the octopus accounted for a large portion of oxygen uptake in an in-vitro study; the estimate suggests around 41% of all oxygen absorption is through the skin when at rest. This number is affected by the activity of the animal – the oxygen uptake increases when the octopus is exercising due to its entire body being constantly exposed to water, but the total amount of oxygen absorption through skin is actually decreased to 33% as a result of the metabolic cost of swimming. When the animal is curled up after eating, its absorption through its skin can drop to 3% of its total oxygen uptake. The octopus' respiratory pigment, hemocyanin, also assists in increasing oxygen uptake. Octopuses can maintain a constant oxygen uptake even when oxygen concentrations in the water decrease to around or 31.6% saturation (standard deviation 8.3%). If oxygen saturation in sea water drops to about 1–10% it can be fatal for Octopus vulgaris depending on the weight of the animal and the water temperature. Ventilation may increase to pump more water carrying oxygen across the gills but due to receptors found on the gills the energy use and oxygen uptake remains at a stable rate. The high percent of oxygen extraction allows for energy saving and benefits for living in an area of low oxygen concentration.
Water is pumped into the mantle cavity of the octopus, where it comes into contact with the internal gills. The water has a high concentration of oxygen compared to the blood returning from the veins, so oxygen diffuses into the blood. The tissues and muscles of the octopus use oxygen and release carbon dioxide when breaking down glucose in the Krebs cycle. The carbon dioxide then dissolves into the blood or combines with water to form carbonic acid, which decreases blood pH. The Bohr effect explains why oxygen concentrations are lower in venous blood than arterial blood and why oxygen diffuses into the bloodstream. The rate of diffusion is affected by the distance the oxygen has to travel from the water to the bloodstream as indicated by Fick's laws of diffusion. Fick's laws explain why the gills of the octopus contain many small folds that are highly vascularized. They increase surface area, thus also increase the rate of diffusion. The capillaries that line the folds of the gill epithelium have a very thin tissue barrier (10 μm), which allows for fast, easy diffusion of the oxygen into the blood. In situations where the partial pressure of oxygen in the water is low, diffusion of oxygen into the blood is reduced, Henry's law can explain this phenomenon. The law states that at equilibrium, the partial pressure of oxygen in water will be equal to that in air; but the concentrations will differ due to the differing solubility. This law explains why O. vulgaris has to alter the amount of water cycled through its mantle cavity as the oxygen concentration in water changes.
The gills are in direct contact with water – carrying more oxygen than the blood – that has been brought into the mantle cavity of the octopus. Gill capillaries are quite small and abundant, which creates an increased surface area that water can come into contact with, thus resulting in enhanced diffusion of oxygen into the blood. Some evidence indicates that lamellae and vessels within the lamellae on the gills contract to aid in propelling blood through the capillaries.
Circulation
The octopus has three hearts, one main two-chambered heart charged with sending oxygenated blood to the body and two smaller branchial hearts, one next to each set of gills. The circulatory circuit sends oxygenated blood from the gills to the atrium of the systemic heart, then to its ventricle which pumps this blood to the rest of the body. Deoxygenated blood from the body goes to the branchial hearts which pump the blood across the gills to oxygenate it, and then the blood flows back to the systemic atrium for the process to begin again. Three aortae leave the systemic heart, two minor ones (the abdominal aorta and the gonadal aorta) and one major one, the dorsal aorta which services most of the body. The octopus also has large blood sinuses around its gut and behind its eyes that function as reserves in times of physiologic stress.
The octopus' heart rate does not change significantly with exercise, though temporary cardiac arrest of the systemic heart can be induced by oxygen debt, almost any sudden stimulus, or mantle pressure during jet propulsion. Its only compensation for exertion is through an increase in stroke volume of up to three times by the systemic heart, which means it suffers an oxygen debt with almost any rapid movement. The octopus is, however, able to control how much oxygen it pulls out of the water with each breath using receptors on its gills, allowing it to keep its oxygen uptake constant over a range of oxygen pressures in the surrounding water. The three hearts are also temperature and oxygen dependent and the beat rhythm of the three hearts are generally in phase with the two branchial hearts beating together followed by the systemic heart. The Frank–Starling law also contributes to overall heart function, through contractility and stroke volume, since the total volume of blood vessels must be maintained, and must be kept relatively constant within the system for the heart to function properly.
The blood of the octopus is composed of copper-rich hemocyanin, which is less efficient than the iron-rich hemoglobin of vertebrates, thus does not increase oxygen affinity to the same degree. Oxygenated hemocyanin in the arteries binds to , which is then released when the blood in the veins is deoxygenated. The release of into the blood causes it to acidify by forming carbonic acid. The Bohr effect explains that carbon dioxide concentrations affect the blood pH and the release or intake of oxygen. The Krebs cycle uses the oxygen from the blood to break down glucose in active tissues or muscles and releases carbon dioxide as a waste product, which leads to more oxygen being released. Oxygen released into the tissues or muscles creates deoxygenated blood, which returns to the gills in veins. The two brachial hearts of the octopus pump blood from the veins through the gill capillaries. The newly oxygenated blood drains from the gill capillaries into the systemic heart, where it is then pumped back throughout the body.
Blood volume in the octopus' body is about 3.5% of its body weight but the blood's oxygen-carrying capacity is only about 4 volume percent. This contributes to their susceptibility to the oxygen debt mentioned before. Shadwick and Nilsson concluded that the octopus circulatory system is "fundamentally unsuitable for high physiologic performance". Since the binding agent is found within the plasma and not the blood cells, a limit exists to the oxygen uptake that the octopus can experience. If it were to increase the hemocyanin within its blood stream, the fluid would become too viscous for the myogenic hearts to pump. Poiseuille's law explains the rate of flow of the bulk fluid throughout the entire circulatory system through the differences of blood pressure and vascular resistance.
Like those of vertebrates, octopus blood vessels are very elastic, with a resilience of 70% at physiologic pressures. They are primarily made of an elastic fiber called octopus arterial elastomer, with stiffer collagen fibers recruited at high pressure to help the vessel maintain its shape without over-stretching. Shadwick and Nilsson theorized that all octopus blood vessels may use smooth-muscle contractions to help move blood through the body, which would make sense in the context of them living under water with the attendant pressure.
The elasticity and contractile nature of the octopus aorta serves to smooth out the pulsing nature of blood flow from the heart as the pulses travel the length of the vessel, while the vena cava serves in an energy-storage capacity. Stroke volume of the systemic heart changes inversely with the difference between the input blood pressure through the vena cava and the output back pressure through the aorta.
Osmoregulation
The hemolymph, pericardial fluid and urine of cephalopods, including the common octopus, are all isosmotic with each other, as well as with the surrounding sea water. It has been suggested that cephalopods do not osmoregulate, which would indicate that they are conformers. This means that they adapt to match the osmotic pressure of their environment, and because there is no osmotic gradient, there is no net movement of water from the organism to the seawater, or from the seawater into the organism. Octopuses have an average minimum salinity requirement of , and that any disturbance introducing significant amounts of fresh water into their environment can prove fatal.
In terms of ions, however, a discrepancy does seem to occur between ionic concentrations found in the seawater and those found within cephalopods. In general, they seem to maintain hypoionic concentrations of sodium, calcium, and chloride in contrast to the salt water. Sulfate and potassium exist in a hypoionic state, as well, with the exception of the excretory systems of cephalopods, where the urine is hyperionic. These ions are free to diffuse, and because they exist in hypoionic concentrations within the organism, they would be moving into the organism from the seawater. The fact that the organism can maintain hypoionic concentrations suggests not only that a form of ionic regulation exists within cephalopods, but also that they also actively excrete certain ions such as potassium and sulfate to maintain homeostasis.
O. vulgaris has a mollusc-style kidney system, which is very different from mammals. The system is built around an appendage of each branchial heart, which is essentially an extension of its pericardium. These long, ciliated ducts filter the blood into a pair of kidney sacs, while actively reabsorbing glucose and amino acids into the bloodstream. The renal sacs actively adjust the ionic concentrations of the urine, and actively add nitrogenous compounds and other metabolic waste products to the urine. Once filtration and reabsorption are complete, the urine is emptied into O. vulgaris''' mantle cavity via a pair of renal papillae, one from each renal sac.
Temperature and body size directly affect the oxygen consumption of O. vulgaris, which alters the rate of metabolism. When oxygen consumption decreases, the amount of ammonia excretion also decreases due to the slowed metabolic rate. O. vulgaris has four different fluids found within its body: blood, pericardial fluid, urine, and renal fluid. The urine and renal fluid have high concentrations of potassium and sulphate, but low concentrations of chloride. The urine has low calcium concentrations, which suggests it has been actively removed. The renal fluid has similar calcium concentrations to the blood. Chloride concentrations are high in the blood, while sodium varies. The pericardial fluid has concentrations of sodium, potassium, chlorine and calcium similar to that of the salt water supporting the idea that O. vulgaris does not osmoregulate, but conforms. However, it has lower sulphate concentrations. The pericardial duct contains an ultrafiltrate of the blood known as the pericardial fluid, and the rate of filtration is partly controlled by the muscle- and nerve-rich branchial hearts. The renal appendages move nitrogenous and other waste products from the blood to the renal sacs, but do not add volume. The renal fluid has a higher concentration of ammonia than the urine or the blood, thus the renal sacs are kept acidic to help draw the ammonia from the renal appendages. The ammonia diffuses down its concentration gradient into the urine or into the blood, where it gets pumped through the branchial hearts and diffuses out the gills. The excretion of ammonia by O. vulgaris makes them ammonotelic organisms. Aside from ammonia, a few other nitrogenous waste products have been found to be excreted by O. vulgaris such as urea, uric acid, purines, and some free amino acids, but in smaller amounts.
Within the renal sacs, two recognized and specific cells are responsible for the regulation of ions. The two kinds of cells are the lacuna-forming cells and the epithelial cells that are typical to kidney tubules. The epithelia cells are ciliated, cylindrical, and polarized with three distinct regions. These three regions are apical, middle cytoplasmic, and basal lamina. The middle cytoplasmic region is the most active of the three due to the concentration of multiple organelles within, such as mitochondria and smooth and rough endoplasmic reticulum, among others. The increase of activity is due to the interlocking labyrinth of the basal lamina creating a crosscurrent activity similar to the mitochondrial-rich cells found in teleost marine fish. The lacuna-forming cells are characterized by contact to the basal lamina, but not reaching the apical rim of the associated epithelial cells and are located in the branchial heart epithelium. The shape varies widely and are occasionally more electron-dense than the epithelial cells, seen as a "diffused kidney" regulating ion concentrations.
One adaptation that O. vulgaris has is some direct control over its kidneys. It is able to switch at will between the right or left kidney doing the bulk of the filtration, and can also regulate the filtration rate so that the rate does not increase when the animal's blood pressure goes up due to stress or exercise. Some species of octopuses, including O. vulgaris, also have a duct that runs from the gonadal space into the branchial pericardium. Wells theorized that this duct, which is highly vascularized and innervated, may enable the reabsorption of important metabolites from the ovisac fluid of pregnant females by directing this fluid into the renal appendages.
Thermoregulation
As an oceanic organism, O. vulgaris experiences a temperature variance due to many factors, such as season, geographical location, and depth. For example, octopuses living around Naples may experience a temperature of in the summer and in the winter. These changes would occur quite gradually, however, and thus would not require any extreme regulation.
The common octopus is a poikilothermic, eurythermic ectotherm, meaning that it conforms to the ambient temperature. This implies that no real temperature gradient is seen between the organism and its environment, and the two are quickly equalized. If the octopus swims to a warmer locale, it gains heat from the surrounding water, and if it swims to colder surroundings, it loses heat in a similar fashion.O. vulgaris can apply behavioral changes to manage wide varieties of environmental temperatures. Respiration rate in octopods is temperature-sensitive – respiration increases with temperature. Its oxygen consumption increases when in water temperatures between , reaches a maximum at , and then begins to drop at . The optimum temperature for metabolism and oxygen consumption is between . Variations in temperature can also induce a change in hemolymph protein levels along oxygen consumption. As temperature increases, protein concentrations increase in order to accommodate the temperature. Also the cooperativity of hemocyanin increases, but the affinity decreases. Conversely, a decrease in temperature results in a decrease in respiratory pigment cooperativity and increase in affinity. The slight rise in P50 that occurs with temperature change allows oxygen pressure to remain high in the capillaries, allowing for elevated diffusion of oxygen into the mitochondria during periods of high oxygen consumption. The increase in temperature results in higher enzyme activity, yet the decrease in hemocyanin affinity allows enzyme activity to remain constant and maintain homeostasis. The highest hemolymph protein concentrations are seen at and then drop at temperatures above this. Oxygen affinity in the blood decreases by at a pH of 7.4. The octopod's thermal tolerance is limited by its ability to consume oxygen, and when it fails to provide enough oxygen to circulate at extreme temperatures the effects can be fatal. O. vulgaris has a pH-independent venous reserve that represents the amount of oxygen that remains bound to the respiratory pigment at constant pressure of oxygen. This reserve allows the octopus to tolerate a wide range of pH related to temperature.
As a temperature conformer, O. vulgaris does not have any specific organ or structure dedicated to heat production or heat exchange. Like all animals, they produce heat as a result of ordinary metabolic processes such as digestion of food, but take no special means to keep their body temperature within a certain range. Their preferred temperature directly reflects the temperature to which they are acclimated. They have an acceptable ambient temperature range of , with their optimum for maximum metabolic efficiency being about .
As ectothermal animals, common octopuses are highly influenced by changes in temperature. All species have a thermal preference where they can function at their basal metabolic rate. The low metabolic rate allows for rapid growth, thus these cephalopods mate as the water becomes closest to the preferential zone. Increasing temperatures cause an increase in oxygen consumption by O. vulgaris. Increased oxygen consumption can be directly related to the metabolic rate, because the breakdown of molecules such as glucose requires an input of oxygen, as explained by the Krebs cycle. The amount of ammonia excreted conversely decreases with increasing temperature. The decrease in ammonia being excreted is also related to the metabolism of the octopus due to its need to spend more energy as the temperature increases. Octopus vulgaris will reduce the amount of ammonia excreted in order to use the excess solutes that it would have otherwise excreted due to the increased metabolic rate. Octopuses do not regulate their internal temperatures until it reaches a threshold where they must begin to regulate to prevent death. The increase in metabolic rate shown with increasing temperatures is likely due to the octopus swimming to shallower or deeper depths to stay within its preferential temperature zone.
Reproduction
Spawning of O. vulgaris in this area extends from December to September with a unique peak in spring months.
| Biology and health sciences | Cephalopods | Animals |
94248 | https://en.wikipedia.org/wiki/Populus | Populus | Populus is a genus of 25–30 species of deciduous flowering plants in the family Salicaceae, native to most of the Northern Hemisphere. English names variously applied to different species include poplar (), aspen, and cottonwood.
The western balsam poplar (P. trichocarpa) was the first tree to have its full DNA code determined by DNA sequencing, in 2006.
Description
The genus has a large genetic diversity, and can grow from tall, with trunks up to in diameter.
The bark on young trees is smooth and white to greenish or dark gray, and often has conspicuous lenticels; on old trees, it remains smooth in some species, but becomes rough and deeply fissured in others. The shoots are stout, with (unlike in the related willows) the terminal bud present. The leaves are spirally arranged, and vary in shape from triangular to circular or (rarely) lobed, and with a long petiole; in species in the sections Populus and Aigeiros, the petioles are laterally flattened, so that breezes easily cause the leaves to wobble back and forth, giving the whole tree a "twinkling" appearance in a breeze. Leaf size is very variable even on a single tree, typically with small leaves on side shoots, and very large leaves on strong-growing lead shoots. The leaves often turn bright gold to yellow before they fall during autumn.
The flowers are mostly dioecious (rarely monoecious) and appear in early spring before the leaves. They are borne in long, drooping, sessile or pedunculate catkins produced from buds formed in the axils of the leaves from the previous year. The flowers are each seated in a cup-shaped disk which is borne on the base of a scale which is itself attached to the rachis of the catkin. The scales are obovate, lobed, and fringed, membranous, hairy or smooth, and usually caducous. The male flowers are without calyx or corolla, and comprise a group of four to 60 stamens inserted on a disk; filaments are short and pale yellow; anthers are oblong, purple or red, introrse, and two-celled; the cells open longitudinally. The female flower also has no calyx or corolla, and comprises a single-celled ovary seated in a cup-shaped disk. The style is short, with two to four stigmata, variously lobed, and numerous ovules. Pollination is by wind, with the female catkins lengthening considerably between pollination and maturity. The fruit is a two- to four-valved dehiscent capsule, green to reddish-brown, mature in midsummer, containing numerous minute, light-brown seeds surrounded by tufts of long, soft, white hairs aiding wind dispersal.
Classification
The genus Populus has traditionally been divided into six sections on the basis of leaf and flower characters; this classification is followed below. Recent genetic studies have largely supported this, confirming some previously suspected reticulate evolution due to past hybridisation and introgression events between the groups. Some species (noted below) had differing relationships indicated by their nuclear DNA (paternally inherited) and chloroplast DNA sequences (maternally inherited), a clear indication of likely hybrid origin. Hybridisation continues to be common in the genus, with several hybrids between species in different sections known. There are currently 57 accepted species in the genus.
Phylogeny
Some of the most easily identifiable fossils of this genus belongs to Poplus wilmattae, which come from the Late Paleocene of North America about 58 million years ago. However, fossils from the Cretaceous of this genus have been found in Tibet and Heilongjiang, China.
Selected species
Populus section Populus – aspens and white poplar (circumpolar subarctic and cool temperate, and mountains farther south, white poplar warm temperate)
Populus adenopoda – Chinese aspen (eastern Asia)
Populus alba – white poplar (southern Europe to central Asia)
Populus × canescens (P. alba × P. tremula) – grey poplar
Populus davidiana – Korean aspen (eastern Asia)
Populus grandidentata – bigtooth aspen (eastern North America)
Populus luziarum – Jalisco, Mexico
Populus primaveralepensis – Jalisco, Mexico
Populus sieboldii – Japanese aspen (eastern Asia)
Populus tremula – aspen, common aspen, Eurasian aspen, European aspen, quaking aspen (Europe, northern Asia)
Populus tremuloides – quaking aspen or trembling aspen (North America)
Populus section Aigeiros – black poplars, some of the cottonwoods (North America, Europe, western Asia; temperate)
Populus deltoides – eastern cottonwood (eastern North America)
Populus fremontii – Fremont cottonwood (western North America)
Populus nigra – black poplar (Europe), placed here by nuclear DNA; cpDNA places it in sect. Populus (including Populus afghanica)
Populus × canadensis (P. deltoides × P. nigra) – hybrid black poplar
Populus × inopina (P. nigra × P. fremontii) – hybrid black poplar
Populus section Tacamahaca – balsam poplars (North America, Asia; cool temperate)
Populus angustifolia – willow-leaved poplar or narrowleaf cottonwood (central North America)
Populus balsamifera – Balsam poplar (northern North America) (= P. candicans, P. tacamahaca)
Populus cathayana – (northeast Asia)
Populus ciliata – (Asia)
Populus koreana J.Rehnder – Korean poplar (northeast Asia)
Populus laurifolia – laurel-leaf poplar (central Asia)
Populus maximowiczii A.Henry – Maximowicz' poplar, Korean poplar, Mongolian poplar, Japanese poplar (northeast Asia)
Populus simonii – Simon's poplar (northeast Asia)
Populus suaveolens Fischer – Korean poplar, Mongolian poplar, Japanese poplar (northeast Asia)
Populus szechuanica – Sichuan poplar (northeast Asia), placed here by nuclear DNA; cpDNA places it in sect. Aigeiros
Populus trichocarpa – western balsam poplar or black cottonwood (western North America)
Populus tristis (northeast Asia), placed here by nuclear DNA; cpDNA places it in sect. Aigeiros
Populus ussuriensis – Ussuri poplar (northeast Asia)
Populus yunnanensis – Yunnan poplar (east Asia)
Populus section Leucoides – necklace poplars or bigleaf poplars (eastern North America, eastern Asia; warm temperate)
Populus heterophylla – downy poplar (southeastern North America)
Populus lasiocarpa – Chinese necklace poplar (eastern Asia)
Populus wilsonii – Wilson's poplar (eastern Asia)
Populus section Turanga – subtropical poplars (southwest Asia, east Africa; subtropical to tropical)
Populus euphratica – Euphrates poplar (North Africa, southwest and central Asia)
Populus ilicifolia – Tana River poplar (East Africa)
Populus section Abaso – Mexican poplars (Mexico; subtropical to tropical)
Populus guzmanantlensis (Mexico) (may be conspecific with Populus simaroa)
Populus mexicana – Mexico poplar (Mexico)
Intersectional hybrids
Populus × acuminata (P. angustifolia × P. deltoides) – lanceleaf cottonwood
Populus Pacific albus (North America)
Ecology
Poplars of the cottonwood section are often wetlands or riparian trees. The aspens are among the most important boreal broadleaf trees.
Poplars and aspens are important food plants for the larvae of a large number of Lepidoptera species. Pleurotus populinus, the aspen oyster mushroom, is found exclusively on dead wood of Populus trees in North America.
Several species of Populus in the United Kingdom and other parts of Europe have experienced heavy dieback; this is thought in part to be due to Sesia apiformis which bores into the trunk of the tree during its larval stage.
Cultivation
Many poplars are grown as ornamental trees, with numerous cultivars used. They have the advantage of growing to a very large size at a rapid pace. Almost all poplars take root readily from cuttings or where broken branches lie on the ground (they also often have remarkable suckering abilities, and can form huge colonies from a single original tree, such as the famous Pando forest made of thousands of Populus tremuloides clones).
Trees with fastigiate (erect, columnar) branching are particularly popular, and are widely grown across Europe and southwest Asia. However, like willows, poplars have very vigorous and invasive root systems stretching up to from the trees; planting close to houses or ceramic water pipes may result in damaged foundations and cracked walls and pipes due to their search for moisture.
A simple, reproducible, high-frequency micropropagation protocol in eastern cottonwood Populus deltoides has been reported by Yadav et al. 2009.
India
In India, the poplar is grown commercially by farmers, mainly in the Punjab region. Common poplar varieties are:
G48 (grown in the plains of Punjab, Haryana, UP)
w22 (grown in mountainous regions, e.g., Himachal Pradesh, Pathankot, Jammu)
The trees are grown from kalam or cuttings, harvested annually in January and February, and commercially available up to 15November.
Poplars are most commonly used to make plywood: Yamuna Nagar in Haryana state has a large plywood industry reliant upon poplar. It is graded according to sizes known as "over" (over ), "under" (), and "sokta" (less than ).
Pakistan
In Pakistan, poplar is grown on a commercial level by farmers in Punjab, Sindh, and Khyber Pakhtunkhwa Provinces. However, all varieties are seriously susceptible to termite attack, causing significant losses to poplar every year. Logs of poplar are therefore also used as bait in termite traps for biocontrol of termites in crops.
Uses
Although the wood from Populus is known as poplar wood, a common high-quality hardwood "poplar" with a greenish colour is actually from an unrelated genus Liriodendron. Populus wood is a lighter, more porous material.
Its flexibility and close grain make it suitable for a number of applications, similar to those of willow. The Greeks and Etruscans made shields of poplar, and Pliny the Elder also recommended poplar for this purpose. Poplar continued to be used for shield construction through the Middle Ages and was renowned for a durability similar to that of oak, but with a substantial reduction in weight.
Food
In addition to the foliage and other parts of Populus species being consumed by animals, the starchy sap layer (underneath the outer bark) is edible to humans, both raw and cooked.
Manufacturing
In many areas, fast-growing hybrid poplars are grown on plantations for pulpwood
Poplar is widely used for the manufacture of paper.
It is also sold as inexpensive hardwood timber, used for pallets and cheap plywood; more specialised uses including matches and matchboxes and the boxes for Camembert cheese.
Poplar wood is also widely used in the snowboard industry for the snowboard core, because it has exceptional flexibility, and is sometimes used in the bodies of electric guitars and drums.
Poplar wood, particularly when seasoned, makes a good hearth for a bow drill.
Because of its high tannic acid content, the bark has been used in Europe for tanning leather.
Poplar wood can be used to produce chopsticks or wooden shoes.
Baking moulds from peeled poplar may be used in the freezer, oven, or microwave oven.
Energy
Interest exists in using poplar as an energy crop for biomass, in energy forestry systems, particularly in light of its high energy-in to energy-out ratio, large carbon mitigation potential, and fast growth.
In the United Kingdom, poplar (as with fellow energy crop willow) is typically grown in a short rotation coppice system for two to five years (with single or multiple stems), then harvested and burned - the yield of some varieties can be as high as 12 oven-dry tonnes per hectare every year.
In warmer regions like Italy this crop can produce up to 13.8, 16.4 oven-dry tonnes of biomass per hectare every year for biannual and triennial cutting cycles also showing a positive energy balance and a high energy efficiency.
Fuel
Biofuel is another option for using poplar as bioenergy supply. In the United States, scientists studied converting short rotation coppice poplar into sugars for biofuel (e.g. ethanol) production.
Considering the relative cheap price, the process of making biofuel from SRC can be economically feasible, although the conversion yield from short rotation coppice (as juvenile crops) were lower than regular mature wood. Besides biochemical conversion, thermochemical conversion (e.g. fast pyrolysis) was also studied for making biofuel from short rotation coppice poplar and was found to have higher energy recovery than that from bioconversion.
Art
Poplar was the most common wood used in Italy for panel paintings; the Mona Lisa and most famous early Italian Renaissance paintings are on poplar. The wood is generally white, often with a slightly yellowish colour.
Some stringed instruments are made with one-piece poplar backs; violas made in this fashion are said to have a particularly resonant tone. Similarly, though typically it is considered to have a less attractive grain than the traditional sitka spruce, poplar is beginning to be targeted by some harp luthiers as a sustainable and even superior alternative for their sound boards: in these cases another hardwood veneer is sometimes applied to the resonant poplar base both for cosmetic reasons, and supposedly to fine-tune the acoustic properties.
Land management
Lombardy poplars are frequently used as a windbreak around agricultural fields to protect against wind erosion.
Agriculture
Logs from the poplar provide a growing medium for shiitake mushrooms.
Phytoremediation
Poplar represents a suitable candidate for phytoremediation since it has the ability to remove and store harmful pollutants in its trunk while also removing air pollution. This plant has been successfully used to target many types of pollutants including trace element (TEs) in soil and sewage sludge, Polychlorinated Biphenyl (PCBs), Trichloroethylene (TCE), Polycyclic Aromatic Hydrocarbon (PAHs).
Culture
Two notable poems in English lament the cutting down of poplars, William Cowper's "The Poplar Field" and Gerard Manley Hopkins' "Binsey Poplars felled 1879".
In Billie Holiday's "Strange Fruit", she sings "Black bodies swinging in the southern breeze/Strange fruit hanging from the poplar trees…".
The Odd Poplars Alley, in Iași, Romania, is one of the spots where Mihai Eminescu sought inspiration in his works (the poem "Down Where the Lonely Poplars Grow"). In 1973, the 15 white poplars still left (with age ranges between 233 and 371 years) were declared natural monuments.
In Ukraine, one of neighborhoods of Kyiv is named after Populus nigra as Osokorky, a local name.
| Biology and health sciences | Malpighiales | Plants |
94287 | https://en.wikipedia.org/wiki/Action%20%28firearms%29 | Action (firearms) | In firearms terminology, an action is the functional mechanism of a breechloading firearm that handles (loads, locks, fires, extracts, and ejects) the ammunition cartridges, or the method by which that mechanism works. Actions are technically not present on muzzleloaders, as all those are single-shot firearms with a closed off breech with the powder and projectile manually loaded from the muzzle. Instead, the muzzleloader ignition mechanism is referred to as the lock (e.g. matchlock, wheellock, flintlock, and caplock).
Actions can be categorized in several ways, including single action versus double action, break action versus lever-action, pump-action, bolt-action, among many other types. The term action can also include short, long, and magnum if it is in reference to the length of the rifle's receiver and the length of the bolt. The short action rifle usually can accommodate a cartridge length of or smaller. The long action rifle can accommodate a cartridge of , and the magnum action rifle can accommodate cartridges of .
Single-shot actions
Single-shot actions operate only to ignite a cartridge that is separately set up ("in battery") for firing, and are incapable of moving the cartridge, itself. As the name implies, all single-shot firearms (unless they are multi-barreled) can only hold one round of ammunition and need to be manually reloaded after every firing. Historically, these are the earliest cartridge firearm actions invented.
Breechblock
Dropping block
The dropping block are actions wherein the breechblock lowers or "drops" into the receiver to open the breech, usually actuated by an underlever. There are two principal types of dropping block: the tilting block and the falling block.
Pivoting block
In a tilting block or pivoting block action, the breechblock is hinged on a pin mounted at the rear (in contrast with tilting bolt, which is not hinged). When the lever is operated, the block tilts down and forward, exposing the chamber. The best-known pivoting block designs are the Peabody, the Peabody–Martini, and Ballard actions.
The original Peabody rifles, manufactured by the Providence Tool Company, used a manually cocked side-hammer. Swiss gunsmith Friedrich Martini developed a pivoting block action by modifying the Peabody, which incorporated a hammerless striker that was cocked by the operating lever with the same single efficient motion that also pivoted the block. The 1871 Martini–Henry which replaced the "trapdoor" Snider–Enfield was the standard British Army rifle of the later Victorian era, and the Martini is also a popular action for civilian rifles.
Charles H. Ballard's self-cocking tilting-block action was produced by the Marlin Firearms Company from 1875 and earned a superlative reputation among long-range "Creedmoor" target shooters. Surviving Marlin Ballards are today highly prized by collectors, especially those mounted in the elaborate Swiss-style Schützen stocks of the day.
Falling block
In a falling block or sliding block action, a solid metal breechblock "slides" vertically in grooves cut into the breech of the firearm and actuated by a lever. Examples of firearms using the falling-block action are the Sharps rifle and Ruger No. 1.
Rolling block
In a rolling block action, the breechblock takes the form of a part-cylinder, with a pivot pin through its axis. The operator rotates or "rolls" the block to open and close the breech; it is a simple, rugged, and reliable design. Rolling blocks are most often associated with firearms made by Remington in the late 19th century; in the Remington action the hammer serves to lock the breech closed at the moment of firing, and the block in turn prevents the hammer from falling with the breech open.
Hinged block
The hinged block used in the earliest metallic-cartridge breechloaders designed for general military issue began as conversions of muzzle-loading rifle-muskets. The upper rear portion of the barrel was filed or milled away and replaced by a hinged breechblock, which opened upward to permit loading. An internal angled firing pin allowed the re-use of the rifle's existing side-hammer. The Allin action made by Springfield Arsenal in the US hinged forward; the Snider–Enfield used by the British opened to the side. Whereas the British quickly replaced the Snider with a dropping-block Peabody-style Martini action, the US Army felt the trapdoor action to be adequate and followed its muzzleloader conversions with the new-production Springfield Model 1873, which was the principal longarm used as a weapon in the Indian Wars and was still in service with some units in the Spanish–American War.
Break-action
A break action is a type of firearm where the barrel(s) are hinged and can be "broken open" to expose the breech. Multi-barrel break action firearms are usually subdivided into over-and-under or side-by-side configurations for two barrel configurations or "combination gun" when mixed rifle and shotgun barrels are used.
Bolt-action
Although bolt-action guns are usually associated with fixed or detachable box magazines (multi-shot), some are single-shot. In fact, the first general-issue military breechloader was a single-shot bolt action: the paper-cartridge Prussian needle gun of 1841. France countered in 1866 with its superior Chassepot rifle, also a paper-cartridge bolt action. The first metallic-cartridge bolt actions in general military service were the Berdan Type II introduced by Russia in 1870, the Mauser Model 1871, and a modified Chassepot, the Gras rifle of 1874; all these were single-shots.
Today, most top-level smallbore match rifles are single-shot bolt action rifles.
Single-shot bolt actions in .22 caliber were also widely manufactured as inexpensive "boys' guns" in the earlier 20th century; and there have been a few single-shot bolt-action shotguns, usually in .410 bore.
Eccentric screw action
The eccentric screw action first seen on the M1867 Werndl–Holub and later on the Magnum Research Lone Eagle pistol, the breech closure is a rotating drum with the same axis, but offset from the bore. When locked, a firing pin aligns with the primer, the breech is otherwise solid. When rotated open, a slot in the drum is exposed for extraction and feeding of a new round. Though first used on the Werndl-Holub, this action is commonly known as a cannon breech due to its association with the French 75mm Model of 1897 cannon. The French M1897 was, itself, based on William Hubbell's .
Other actions
The Ferguson rifle: British Major Patrick Ferguson designed his rifle, considered to be the first military breechloader in the 1770s. A plug-shaped breechblock was screw-threaded so that rotating the handle underneath would lower and raise it for loading with ball and loose powder; the flintlock action still required conventional priming.
The Hall rifle: First U.S. cavalry breechloader, originally made in flint but later made-in and converted to percussion in 1830s–1840s. The breech section tilts up to accept a paper cartridge. Excellent machine-made construction, but still tended to leak gas at the breech.
The Kammerlader: A crank-operated Norwegian firearm produced around the time of the Prussian Needle-gun. Originally used a paper cartridge. Later, many were converted to rimfire; this was the first Norwegian breechloader.
The Tarpley carbine: This is categorized into falling block action, but the breech block is hinged, unlike the others.
The Morse Carbine: This mostly brass action is somewhat like the Hall rifle, except it was designed to take a special centerfire cartridge. Very few of these were actually made; all were constructed in the late 1850s.
The Joslyn rifle:
Rising Breech Carbine:
Repeating actions
Repeating actions are characterized by reciprocating/rotating components that can move cartridges in and out of battery from an ammunition-holding device (which is a magazine, cylinder, or belt), which allows the gun to hold multiple rounds and shoot repeatedly before needing a manual ammunition reload.
Manual operation
Revolver
A revolver is a multi-chamber (but single-barrelled) firearm that houses cartridges in a rotary cylinder which indexes each round into alignment with the bore (with the help of a forcing cone) prior to each shot. Revolvers are most often handguns; however, examples of revolving rifles, shotguns, and cannons have been made. The cylinder is most often rotated via linkage to a manually manipulated external hammer, although some revolvers are "double-action" and can use the manual pull of the trigger to drive both the cylinder rotation and hammer cocking. Some examples of firearms using the revolver principle are the Smith & Wesson Model 3 and Colt Model 1889.
Bolt action
In bolt-action firearms, the opening and closing of the breech is operated by direct manual manipulation of the bolt via a protruding bolt handle.
Rotating bolt action
Most bolt-actions utilize a rotating bolt ("turn-pull") design, where the bolt handle must be rotated upwards for unlocking before the bolt can be pulled back to opening the breech and eject any spent cartridge, and must be rotated back down for locking after the bolt closes the breech. The three predominant rotating bolt-action systems are the Mauser, Lee–Enfield, and Mosin–Nagant systems, with the Mauser system emerging into the mainstream as the most widely used rotating bolt-action design.
Straight-pull action
There are also straight pull bolt-action systems that use complex bolt head designs to facilitate locking instead of needing to rotate the bolt handle every time.
In the Mauser-style turn-bolt action, the bolt handle must be rotated upward, pull rearward, pushed forward, and finally rotated back downward into lock. In a straight pull bolt-action, the bolt can be cycled without rotating, hence reducing the required range of motion by the shooter from four movements to two, with the goal of increasing the rate of fire. The Ross and Schmidt–Rubin rifles load via stripper clips, albeit of an unusual paperboard and steel design in the Schmidt–Rubin rifle, while the Mannlicher uses en-bloc clips. The Schmidt–Rubin series, which culminated in the K31, are also known for being among the most accurate military service rifles ever made. Yet another variant of the straight pull bolt-action, of which the M1895 Lee Navy is an example, is a camming action in which pulling the bolt handle causes the bolt to rock, freeing a stud from the receiver and unlocking the bolt.
In 1993, the German firearms
company Blaser, introduced the Blaser R93, a new straight pull bolt-action rifle where locking is achieved by a series of concentric "claws" that protrude/retract from the bolthead, a design that is referred to as Radialbundverschluss ("radial connection"). As of 2017 the Rifle Shooter magazine listed its successor Blaser R8 as one of the three most popular straight pull bolt-action together with Merkel Helix and Browning Maral. Some other notable modern straight pull bolt-action rifles are made by Chapuis, Heym, Lynx, Rößler, Strasser, and Steel Action.
In the sport of biathlon, because shooting speed is an important performance factor and semi-automatic guns are illegal for race use, straight pull bolt-actions are quite common, and are used almost exclusively on the Biathlon World Cup. The first company to make straight pull bolt-actions for .22 caliber was J. G. Anschütz; the action is specifically the straight-pull ball bearing-lock action, which features spring-loaded ball bearings on the side of the bolt which lock into a groove inside the bolt's housing. With the new design came a new dry-fire method; instead of the bolt being turned up slightly, the action is locked back to catch the firing pin.
Pump-action
In pump action firearms, a sliding grip at the fore-end beneath the barrel is manually operated by the user to eject and chamber cartridges. Pump actions are predominantly found in shotguns. Some examples of firearms using the pump-action are the Winchester Model 1912, Remington 870, and Mossberg 500.
Lever-action
The lever-action firearms, a linked lever is manually operated to eject and chamber cartridges. Some examples of firearms using lever-action are the Henry Model 1860, Winchester Model 1876, and Marlin Model 1894.
Bolt release
The bolt release or lever release action is a hybrid repeating action that uses the physical manipulation of a bolt release lever/button to complete the cartridge chambering process. However, unlike the lever action (which demands the shooter's hand to actually provide the force needed for cycling the action), bolt release firearms eject the used cartridge automatically without involving the lever, usually via blowback or gas operation, and often uses a spring-assisted mechanism to chamber the next round. However, after moving rearwards the bolt is stopped by a bolt catch and will not move back into battery position and chamber the new round, until the user manually disengages the catch by depressing a release lever/button. Due to the fact that the action can not complete its loading cycle without manual input from the user, it is technically a manually operated action rather than a self-loading one.
Whilst the basic principle can be traced back to other self-ejecting rifles, such as the single-shot Harrington & Richardson Model 755 rifle, this action has since been popularized in the United Kingdom by Southern Gun Company, who manufacture with "Manually Actuated Release System" (MARS) action rifles/pistol-caliber carbines in .223, .308, 9mm and .45 ACP calibers, as the interrupted mechanism complies with The Firearms (Amendment) Act 1988 which bans possession of self-loading centrefire rifles. The French company Verney-Carron makes and exports the Speedline hunting rifle and the Véloce shotgun, which has caused some moral concern in the mainstream media in Australia due to lobbying by the Greens and anti-gun groups such as Gun Control Australia, with David Shoebridge quoting the term "semi-semi-automatic". Similarly, Savage Arms has introduced the A17R and A22R rimfire rifles (both modified from its new A-series rifles, with a bolt release lever in front of the trigger guard), aiming at the Australian market, but law enforcement agencies such as the Northern Territory Police has attempted to unilaterally defining these rifles as "linear repeating firearms with assisted ejection" and reclassify them as semi-automatic, and hence prohibited without at least a Category C license, which is off-limit to most urban and rural residents who do not own farms. In 2020, CZ also introduced CZ 515, a bolt-release modified version of the CZ 512, to the Australian market via its importer Winchester Australia. The Turkish manufacturer Pardus Arms also produces the 12 gauge-caliber BRS17 shotgun, which uses a bolt release button on the back of the receiver to chamber rounds before firing.
Other actions
Rotary cannon: Gatling gun, M134 Minigun
Chain gun: Hughes Chain Gun, Guycot Chain Rifle, Treeby chain gun
Kalthoff repeater
Cookson repeater
Belton flintlock
The Jennings Magazine Rifle
Meigs Sliding Guard Action Repeater
Roper repeater
The Orvill Robinson Model 2 rifle: Orvill Robinson, a New York-based firearms designer, developed two rifles. His first, patented in 1870 and commonly referred to by collectors as the "Model 1" though it has no official designation, was a precursor to straight pull bolt-actions like the Mannlicher M1886. The second rifle designed by Robinson, patented in 1872, was very different, employing a double hinged action that folded upward from the receiver to remove the spent casing and back down and forward to chamber a new round. Though hammer-fired, it is recognizable as a manually actuated ancestor of the toggle action found in firearms such as the Luger Parabellum 1908 pistol or Pedersen Rifle.
Krag-Petersson Rifle Though frequently classified as only single-shot firearms, one tilting block rifle usually falls under the category of repeating firearms. The user, upon ejecting a round from the chamber, would load a round from the underbarrel magazine onto the loading surface of the tilting block, then raise it to the mouth of the chamber where the user could then easily push it forward into the chamber. Though this would not meet most standards of "repeating" for most modern users, the classification has been in use historically.
Remington-Rider Magazine Pistol has a manually-actuated rolling block action to pull a cartridge from a tubular magazine set below the barrel and simultaneously cock the firearm. The block was rolled back into battery, loading the cartridge into the chamber, by spring pressure while the hammer remained in the cocked position.
Autoloading operation
Blowback operation
The blowback operation is a system in which semi-automatic and fully automatic firearms operate through the energy created by combustion in the chamber and bore acting directly on the bolt face through the cartridge. In blowback operation the bolt is not locked to the chamber, relying only on spring pressure and inertia from the weight of the bolt to keep the action from opening too quickly. Blowback operation is used for low-powered cartridges due to the weight of the bolt required.
Delayed blowback actions use some mechanism to slow down rearward travel of the bolt, allowing this action to handle more powerful ammunition and/or reduced weight of the bolt.
Examples of blowback operation
Simple blowback: Halcón M-1943, Uzi submachine gun, Varan PMX-80
Lever-delayed blowback: FAMAS, Sterling 7.62, AA-52, 2B-A-40, TKB-517
Roller-delayed blowback: SIG 510, HK MP5, HK P9, HK G3
Gas-delayed blowback: Volkssturmgewehr 1-5, HK P7, Steyr GB
Toggle-delayed blowback: Schwarzlose MG M.07/12, Luger rifle and Pedersen rifle
Blish Lock: early Thompson submachine guns
Hesitation locked: Remington Model 51 and R51 pistols
Chamber-ring delayed blowback: Seecamp pistol
Blow-forward operation
The blow-forward operation uses a fixed breech and moving barrel that is forced forward relative to the breech by the friction of the projectile against the bore as well as the breech recoiling away from the barrel. The barrel is spring loaded and returns automatically to chamber a fresh round from the magazine. Examples of this action are the Steyr Mannlicher M1894, Hino Komuro M1908 Pistol and the Schwarzlose Model 1908.
Recoil operation
The recoil operation is a type of locked-breech action used in semi-automatic and fully automatic firearms. It also uses energy from the combustion in the chamber acting directly on the bolt through the cartridge head, but in this case the firearm has a reciprocating barrel and breech assembly, combined with a bolt that locks to the breech. The breech remains locked as the bolt and barrel travel rearward together for some distance, allowing pressure in the chamber to drop to a safe level before the breech is opened.
Examples of recoil operation
Short-recoil: Colt M1911, MAB PA-15, Browning Hi-Power, HK USP, Glock, Mamba Pistol, M2 Browning machine gun, MG42, Vz 52 pistol, M82
Long-recoil: Browning Auto 5, Femaru STOP Pistol, Mars Automatic Pistol, Chauchat
Inertia: Sjögren Inertial, certain Benelli shotguns
Gas operation
The gas operation is a system of operation mechanism used to provide energy to semi-automatic and fully automatic firearms. In gas-operation, a portion of high pressure gas from the cartridge being fired is tapped through a hole in the barrel and diverted to operate the action. There are three basic types: long stroke gas piston (where the gas piston goes the same distance as the operating stroke of the action parts, and is often attached to the action parts), short stroke gas piston (where the gas piston travels a shorter distance than the operating stroke of the action parts), and direct impingement (AKA "direct gas", "gas impingement", where there is no piston, and the gas acts directly on the action parts). A fourth type, now considered obsolete and ineffective, are those systems based on the Bang rifle that utilize a muzzle cap to capture gas after the bullet has left the barrel. While this system is successful in boosting the operating power of recoil operated guns, it is insufficient and too susceptible to fouling for use as the primary operating system.
Examples of gas operation
Short-stroke gas piston: FN FAL, SAR-87, HK G36
Long-stroke gas piston: M1 Garand, AK-47, FN FNC
Direct impingement: MAS 49, M16, AG-42
Gas trap: Gewehr 41, Bang M1922 rifle
| Technology | Mechanisms_2 | null |
94858 | https://en.wikipedia.org/wiki/Thunder | Thunder | Thunder is the sound caused by lightning. Depending upon the distance from and nature of the lightning, it can range from a long, low rumble to a sudden, loud crack. The sudden increase in temperature and hence pressure caused by the lightning produces rapid expansion of the air in the path of a lightning bolt. In turn, this expansion of air creates a sonic shock wave, often referred to as a "thunderclap" or "peal of thunder". The scientific study of thunder is known as brontology and the irrational fear (phobia) of thunder is called brontophobia.
Etymology
The d in Modern English thunder (from earlier Old English þunor) is epenthetic, and is now found as well in Modern Dutch donder (cf. Middle Dutch donre; also Old Norse þorr, Old Frisian þuner, Old High German donar, all ultimately descended from Proto-Germanic *þunraz). In Latin the term was tonare "to thunder". The name of the Nordic god Thor comes from the Old Norse word for thunder.
The shared Proto-Indo-European root is *tón-r̥ or *, also found in Gaulish Taranis.
Cause
The cause of thunder has been the subject of centuries of speculation and scientific inquiry. Early thinking was that it was made by deities, but the ancient Greek philosophers attributed it to natural causes, such as wind striking clouds (Anaximander, Aristotle) and movement of air within clouds (Democritus). The Roman philosopher Lucretius held it was from the sound of hail colliding within clouds. By the mid-19th century, the accepted theory was that lightning produced a vacuum and that the collapse of that vacuum produced what is known as thunder.
Scientists have agreed since the 20th century that thunder must begin with a shock wave in the air due to the sudden thermal expansion of the plasma in the lightning channel. The temperature inside the lightning channel, measured by spectral analysis, varies during its 50 μs existence, rising sharply from an initial temperature of about 20,000 K to about 30,000 K, then dropping away gradually to about 10,000 K. The average is about . This heating causes a rapid outward expansion, impacting the surrounding cooler air at a speed faster than sound would otherwise travel. The resultant outward-moving pulse is a shock wave, similar in principle to the shock wave formed by an explosion, or at the front of a supersonic aircraft. Near the source, the sound pressure level of thunder is usually 165 to 180 dB, but can exceed 200 dB in some cases.
Experimental studies of simulated lightning have produced results largely consistent with this model, though there is continued debate about the precise physical mechanisms of the process. Other causes have also been proposed, relying on electrodynamic effects of the enormous current acting on the plasma in the bolt of lightning.
Consequences
The shock wave in thunder is sufficient to cause property damage and injury, such as internal contusion, to individuals nearby. Thunder can rupture the eardrums of people nearby, leading to permanently impaired hearing. Even if not, it can lead to temporary deafness.
Types
Vavrek et al. (n.d.) reported that the sounds of thunder fall into categories based on loudness, duration, and pitch. Claps are loud sounds lasting 0.2 to 2 seconds and containing higher pitches. Peals are sounds changing in loudness and pitch. Rolls are irregular mixtures of loudness and pitches. Rumbles are less loud, last for longer (up to more than 30 seconds), and are of low pitch.
Inversion thunder results when lightning strikes occur between the cloud and ground during a temperature inversion. The resulting thunder sounds have significantly greater acoustic energy than those produced from the same distance in non-inversion conditions. In a temperature inversion, the air near the ground is cooler than the air higher up. Inversions often happen when warm, moist air passes above a cold front. Within a temperature inversion, sound energy is prevented from dispersing vertically as it would in non-inversion conditions, and is thus concentrated in the near-ground layer.
Cloud-to-ground lightning (CG) typically consists of two or more return strokes, from ground to cloud. Later return strokes have greater acoustic energy than the first.
Perception
The most noticeable aspect of lightning and thunder is that the lightning is seen before the thunder is heard. This is a consequence of the speed of light being much greater than the speed of sound. The speed of sound in dry air is approximately or at .
This translates to approximately ; saying "one thousand and one... one thousand and two..." is a useful method of counting the seconds from the perception of a given lightning flash to the perception of its thunder (which can be used to gauge the proximity of lightning for the sake of safety). To estimate the distance from the lightning strike, divide the counted seconds by five for miles, or three for kilometers.
A very bright flash of lightning and an almost simultaneous sharp "crack" of thunder, a thundercrack, therefore indicates that the lightning strike was very near.
Close-in lightning has been described first as a clicking or cloth-tearing sound, then a cannon shot sound or loud crack/snap, followed by continuous rumbling. The early sounds are from the leader parts of lightning, then the near parts of the return stroke, then the distant parts of the return stroke.
| Physical sciences | Storms | Earth science |
95154 | https://en.wikipedia.org/wiki/Associative%20array | Associative array | In computer science, an associative array, map, symbol table, or dictionary is an abstract data type that stores a collection of (key, value) pairs, such that each possible key appears at most once in the collection. In mathematical terms, an associative array is a function with finite domain. It supports 'lookup', 'remove', and 'insert' operations.
The dictionary problem is the classic problem of designing efficient data structures that implement associative arrays.
The two major solutions to the dictionary problem are hash tables and search trees.
It is sometimes also possible to solve the problem using directly addressed arrays, binary search trees, or other more specialized structures.
Many programming languages include associative arrays as primitive data types, while many other languages provide software libraries that support associative arrays. Content-addressable memory is a form of direct hardware-level support for associative arrays.
Associative arrays have many applications including such fundamental programming patterns as memoization and the decorator pattern.
The name does not come from the associative property known in mathematics. Rather, it arises from the association of values with keys. It is not to be confused with associative processors.
Operations
In an associative array, the association between a key and a value is often known as a "mapping"; the same word may also be used to refer to the process of creating a new association.
The operations that are usually defined for an associative array are:
Insert or put
add a new pair to the collection, mapping the key to its new value. Any existing mapping is overwritten. The arguments to this operation are the key and the value.
Remove or delete
remove a pair from the collection, unmapping a given key from its value. The argument to this operation is the key.
Lookup, find, or get
find the value (if any) that is bound to a given key. The argument to this operation is the key, and the value is returned from the operation. If no value is found, some lookup functions raise an exception, while others return a default value (such as zero, null, or a specific value passed to the constructor).
Associative arrays may also include other operations such as determining the number of mappings or constructing an iterator to loop over all the mappings. For such operations, the order in which the mappings are returned is usually implementation-defined.
A multimap generalizes an associative array by allowing multiple values to be associated with a single key. A bidirectional map is a related abstract data type in which the mappings operate in both directions: each value must be associated with a unique key, and a second lookup operation takes a value as an argument and looks up the key associated with that value.
Properties
The operations of the associative array should satisfy various properties:
lookup(k, insert(j, v, D)) = if k == j then v else lookup(k, D)
lookup(k, new()) = fail, where fail is an exception or default value
remove(k, insert(j, v, D)) = if k == j then remove(k, D) else insert(j, v, remove(k, D))
remove(k, new()) = new()
where k and j are keys, v is a value, D is an associative array, and new() creates a new, empty associative array.
Example
Suppose that the set of loans made by a library is represented in a data structure. Each book in a library may be checked out by one patron at a time. However, a single patron may be able to check out multiple books. Therefore, the information about which books are checked out to which patrons may be represented by an associative array, in which the books are the keys and the patrons are the values. Using notation from Python or JSON, the data structure would be:
{
"Pride and Prejudice": "Alice",
"Wuthering Heights": "Alice",
"Great Expectations": "John"
}
A lookup operation on the key "Great Expectations" would return "John". If John returns his book, that would cause a deletion operation, and if Pat checks out a book, that would cause an insertion operation, leading to a different state:
{
"Pride and Prejudice": "Alice",
"The Brothers Karamazov": "Pat",
"Wuthering Heights": "Alice"
}
Implementation
For dictionaries with very few mappings, it may make sense to implement the dictionary using an association list, which is a linked list of mappings. With this implementation, the time to perform the basic dictionary operations is linear in the total number of mappings. However, it is easy to implement and the constant factors in its running time are small.
Another very simple implementation technique, usable when the keys are restricted to a narrow range, is direct addressing into an array: the value for a given key k is stored at the array cell A[k], or if there is no mapping for k then the cell stores a special sentinel value that indicates the lack of a mapping. This technique is simple and fast, with each dictionary operation taking constant time. However, the space requirement for this structure is the size of the entire keyspace, making it impractical unless the keyspace is small.
The two major approaches for implementing dictionaries are a hash table or a search tree.
Hash table implementations
The most frequently used general-purpose implementation of an associative array is with a hash table: an array combined with a hash function that separates each key into a separate "bucket" of the array. The basic idea behind a hash table is that accessing an element of an array via its index is a simple, constant-time operation. Therefore, the average overhead of an operation for a hash table is only the computation of the key's hash, combined with accessing the corresponding bucket within the array. As such, hash tables usually perform in O(1) time, and usually outperform alternative implementations.
Hash tables must be able to handle collisions: the mapping by the hash function of two different keys to the same bucket of the array. The two most widespread approaches to this problem are separate chaining and open addressing. In separate chaining, the array does not store the value itself but stores a pointer to another container, usually an association list, that stores all the values matching the hash. By contrast, in open addressing, if a hash collision is found, the table seeks an empty spot in an array to store the value in a deterministic manner, usually by looking at the next immediate position in the array.
Open addressing has a lower cache miss ratio than separate chaining when the table is mostly empty. However, as the table becomes filled with more elements, open addressing's performance degrades exponentially. Additionally, separate chaining uses less memory in most cases, unless the entries are very small (less than four times the size of a pointer).
Tree implementations
Self-balancing binary search trees
Another common approach is to implement an associative array with a self-balancing binary search tree, such as an AVL tree or a red–black tree.
Compared to hash tables, these structures have both strengths and weaknesses. The worst-case performance of self-balancing binary search trees is significantly better than that of a hash table, with a time complexity in big O notation of O(log n). This is in contrast to hash tables, whose worst-case performance involves all elements sharing a single bucket, resulting in O(n) time complexity. In addition, and like all binary search trees, self-balancing binary search trees keep their elements in order. Thus, traversing its elements follows a least-to-greatest pattern, whereas traversing a hash table can result in elements being in seemingly random order. Because they are in order, tree-based maps can also satisfy range queries (find all values between two bounds) whereas a hashmap can only find exact values. However, hash tables have a much better average-case time complexity than self-balancing binary search trees of O(1), and their worst-case performance is highly unlikely when a good hash function is used.
A self-balancing binary search tree can be used to implement the buckets for a hash table that uses separate chaining. This allows for average-case constant lookup, but assures a worst-case performance of O(log n). However, this introduces extra complexity into the implementation and may cause even worse performance for smaller hash tables, where the time spent inserting into and balancing the tree is greater than the time needed to perform a linear search on all elements of a linked list or similar data structure.
Other trees
Associative arrays may also be stored in unbalanced binary search trees or in data structures specialized to a particular type of keys such as radix trees, tries, Judy arrays, or van Emde Boas trees, though the relative performance of these implementations varies. For instance, Judy trees have been found to perform less efficiently than hash tables, while carefully selected hash tables generally perform more efficiently than adaptive radix trees, with potentially greater restrictions on the data types they can handle. The advantages of these alternative structures come from their ability to handle additional associative array operations, such as finding the mapping whose key is the closest to a queried key when the query is absent in the set of mappings.
Comparison
Ordered dictionary
The basic definition of a dictionary does not mandate an order. To guarantee a fixed order of enumeration, ordered versions of the associative array are often used. There are two senses of an ordered dictionary:
The order of enumeration is always deterministic for a given set of keys by sorting. This is the case for tree-based implementations, one representative being the container of C++.
The order of enumeration is key-independent and is instead based on the order of insertion. This is the case for the "ordered dictionary" in .NET Framework, the LinkedHashMap of Java and Python.
The latter is more common. Such ordered dictionaries can be implemented using an association list, by overlaying a doubly linked list on top of a normal dictionary, or by moving the actual data out of the sparse (unordered) array and into a dense insertion-ordered one.
Language support
Associative arrays can be implemented in any programming language as a package and many language systems provide them as part of their standard library. In some languages, they are not only built into the standard system, but have special syntax, often using array-like subscripting.
Built-in syntactic support for associative arrays was introduced in 1969 by SNOBOL4, under the name "table". TMG offered tables with string keys and integer values. MUMPS made multi-dimensional associative arrays, optionally persistent, its key data structure. SETL supported them as one possible implementation of sets and maps. Most modern scripting languages, starting with AWK and including Rexx, Perl, PHP, Tcl, JavaScript, Maple, Python, Ruby, Wolfram Language, Go, and Lua, support associative arrays as a primary container type. In many more languages, they are available as library functions without special syntax.
In Smalltalk, Objective-C, .NET, Python, REALbasic, Swift, VBA and Delphi they are called dictionaries; in Perl, Ruby and Seed7 they are called hashes; in C++, C#, Java, Go, Clojure, Scala, OCaml, Haskell they are called maps (see map (C++), unordered_map (C++), and ); in Common Lisp and Windows PowerShell, they are called hash tables (since both typically use this implementation); in Maple and Lua, they are called tables. In PHP and R, all arrays can be associative, except that the keys are limited to integers and strings. In JavaScript (see also JSON), all objects behave as associative arrays with string-valued keys, while the Map and WeakMap types take arbitrary objects as keys. In Lua, they are used as the primitive building block for all data structures. In Visual FoxPro, they are called Collections. The D language also supports associative arrays.
Permanent storage
Many programs using associative arrays will need to store that data in a more permanent form, such as a computer file. A common solution to this problem is a generalized concept known as archiving or serialization, which produces a text or binary representation of the original objects that can be written directly to a file. This is most commonly implemented in the underlying object model, like .Net or Cocoa, which includes standard functions that convert the internal data into text. The program can create a complete text representation of any group of objects by calling these methods, which are almost always already implemented in the base associative array class.
For programs that use very large data sets, this sort of individual file storage is not appropriate, and a database management system (DB) is required. Some DB systems natively store associative arrays by serializing the data and then storing that serialized data and the key. Individual arrays can then be loaded or saved from the database using the key to refer to them. These key–value stores have been used for many years and have a history as long as that of the more common relational database (RDBs), but a lack of standardization, among other reasons, limited their use to certain niche roles. RDBs were used for these roles in most cases, although saving objects to a RDB can be complicated, a problem known as object-relational impedance mismatch.
After approximately 2010, the need for high-performance databases suitable for cloud computing and more closely matching the internal structure of the programs using them led to a renaissance in the key–value store market. These systems can store and retrieve associative arrays in a native fashion, which can greatly improve performance in common web-related workflows.
| Mathematics | Data structures and types | null |
95160 | https://en.wikipedia.org/wiki/Spiny%20lobster | Spiny lobster | Spiny lobsters, also known as langustas, langouste, or rock lobsters, are a family (Palinuridae) of about 60 species of achelate crustaceans, in the Decapoda Reptantia. Spiny lobsters are also, especially in Australia, New Zealand, Ireland, South Africa, and the Bahamas, called crayfish, sea crayfish, or crawfish ("kreef" in South Africa), terms which elsewhere are reserved for freshwater crayfish.
Classification
The furry lobsters (such as Palinurellus) were previously separated into a family of their own, the Synaxidae, but they are usually considered members of the Palinuridae. The slipper lobsters (Scyllaridae) are their next-closest relatives, and these two or three families make up the Achelata. Genera of spiny lobsters include Palinurus and a number of anagrams thereof: Panulirus, Linuparus, etc. The name derives from the small Italian port of Palinuro, which was known for harvesting the European spiny lobster (Palinurus elephas) in ancient Roman times. The town itself was named for the legendary figure of Palinurus, who was a helmsman in Virgil's Æneid.
In total, 12 extant genera are recognised, containing around 60 living species:
Jasus Parker, 1883
Justitia Holthuis, 1946
Linuparus White, 1847
Nupalirus Kubo, 1955
Palibythus Davie, 1990
Palinurellus De Man, 1881
Palinurus Weber, 1795
Palinustus A. Milne-Edwards, 1880
Panulirus White, 1847
Projasus George and Grindley, 1964
Puerulus Ortmann, 1897
Sagmariasus Holthuis, 1991
Description
Although they superficially resemble true lobsters in terms of overall shape and having a hard carapace and exoskeleton, the two groups are not closely related. Spiny lobsters can be easily distinguished from true lobsters by their very long, thick, spiny antennae, by the lack of chelae (claws) on the first four pairs of walking legs, although the females of most species have a small claw on the fifth pair, and by a particularly specialized larval phase called phyllosoma. True lobsters have much smaller antennae and claws on the first three pairs of legs, with the first being particularly enlarged.
Spiny lobsters typically have a slightly compressed carapace, lacking any lateral ridges. Their antennae lack a scaphocerite, the flattened exopod of the antenna. This is fused to the epistome (a plate between the labrum and the basis of the antenna). The flagellum, at the top of the antenna, is stout, tapering, and very long. The ambulatory legs (pereopods) end in claws (chelae).
Size
The size of the adults varies from a few centimetres to 30–40 cm. In general, it is said that rarely some individuals can reach 60 cm (Panulirus argus).
Nevertheless, some reports – the authenticity of which can be questioned – are of much larger lobsters. One such source is Bernard Gorsky's travel book La derniére ile. In this, the author lists the following statements:
According to a 1956 article from the New Caledonian daily newspaper La France Australe (published in Nyoma): "Since yesterday, a so-called porcelain spiny lobster, stuffed, can be seen in the window of Balande. Its length is 2 m, (including its antennae) and it weighed 11 kg.
Inhabitants of a small island in the Coral Sea caught a 2 m 10 cm, 17 kg porcelain spiny lobster, according to an Australian publication.
Gorsky himself caught 6–7 kg lobsters with local tribesmen on the Loyalty Islands group's Mouli island and mentioned them in the article in La France Australe. However, according to the locals, even bigger crabs can live there. According to the residents, a man from the Leikigne tribe (they live nearby on the other side of the Fayawa Strait) reported the following: he once went fishing with a friend and the friend drowned. He did not come to the surface, he followed him into the depths. Two legs protruded from a hollow, and in the hollow sat a huge crayfish, and it was eating the fisherman. The crawfish was said to be as thick as the trunk of a full-grown palm tree. (At the time, the locals (the people of Leikigne) gave credence to the report and believed that the victim could not have drowned because he swam "like a dolphin" – but a shark would not have killed him either, because there are usually no sharks in the lagoon there. According to them, only a lobster could be really responsible.) Since one of Gorsky's narrators ("Guy") was 20 years old at the time of the story (1965), and the incident occurred when he was 12, the story must have been around 1957 if true.
A study was conducted regarding the effect of growth and survival when you change the frequency of feeding the Spiny Lobster and it was determined that if there is increased feed frequency from one to sixteen feeds daily then that is where growth and feed attraction are at the peak of their performance. If the lobsters are fed too much though, more than 16 feeds a day causes decreased feed intake and reduction in overall growth. It was also determined that the rapid leaching of feed suggests that there is a beneficial effect of feeding multiple frequencies on growth and intake.
Fossil record
The fossil record of spiny lobsters has been extended by the discovery in 1995 of a 110-million-year-old fossil near El Espiñal in Chiapas, Mexico. Workers from the National Autonomous University of Mexico have named the fossil Palinurus palaecosi, and report that it is closest to members of the genus Palinurus currently living off the coasts of Africa.
Ecology
Spiny lobsters are found in almost all warm seas, including the Caribbean and the Mediterranean Sea, but are particularly common in Australasia, where they are referred to commonly as crayfish or sea crayfish (Jasus edwardsii), and in South Africa (Jasus lalandii).
Spiny lobsters tend to live in crevices of rocks and coral reefs, only occasionally venturing out at night to seek snails, clams, sea-hares, crabs, or sea urchins to eat. They sometimes migrate in very large groups in long files of lobsters across the sea floor. These lines may be more than 50 lobsters long. Spiny lobsters navigate using the smell and taste of natural substances in the water that change in different parts of the ocean. It was recently discovered that spiny lobsters can also navigate by detecting the Earth's magnetic field. They keep together by contact, using their long antennae. Potential predators may be deterred from eating spiny lobsters by a loud screech made by the antennae of the spiny lobsters rubbing against a smooth part of the exoskeleton. Spiny lobsters usually exhibit the social habit of being together. However recent studies indicate that healthy lobsters move away from infected ones, leaving the diseased lobsters to fend for themselves.
Like true lobsters, spiny lobsters are edible and are an economically significant food source; they are the biggest food export of the Bahamas, for instance.
Sound
Many spiny lobsters produce rasping sounds to repel predators by rubbing the "plectrum" at the base of the spiny lobster's antennae against a "file". The noise is produced by frictional vibrations – sticking and slipping, similar to rubber materials sliding against hard surfaces. While a number of insects use frictional vibration mechanisms to generate sound, this particular acoustic mechanism is unique in the animal kingdom. Significantly, the system does not rely on the hardness of the exoskeleton, as many other arthropod sounds do, meaning that the spiny lobsters can continue to produce the deterrent noises even in the period following a moult when they are most vulnerable. The stridulating organ is present in all but three genera in the family (Jasus, Projasus, and the furry lobster Palinurellus), and its form can distinguish different species.
| Biology and health sciences | Crayfishes and lobsters | Animals |
95250 | https://en.wikipedia.org/wiki/Development%20of%20the%20human%20body | Development of the human body | Development of the human body is the process of growth to maturity. The process begins with fertilization, where an egg released from the ovary of a female is penetrated by a sperm cell from a male. The resulting zygote develops through mitosis and cell differentiation, and the resulting embryo then implants in the uterus, where the embryo continues development through a fetal stage until birth. Further growth and development continues after birth, and includes both physical and psychological development that is influenced by genetic, hormonal, environmental and other factors. This continues throughout life: through childhood and adolescence into adulthood.
Before birth
Development before birth, or prenatal development () is the process in which a zygote, and later an embryo, and then a fetus develops during gestation. Prenatal development starts with fertilization and the formation of the zygote, the first stage in embryonic development which continues in fetal development until birth.
Fertilization
Fertilization occurs when the sperm successfully enters the ovum's membrane. The chromosomes of the sperm are passed into the egg to form a unique genome. The egg becomes a zygote and the germinal stage of embryonic development begins. The germinal stage refers to the time from fertilization, through the development of the early embryo, up until implantation. The germinal stage is over at about 10 days of gestation.
The zygote contains a full complement of genetic material with all the biological characteristics of a single human being, and develops into the embryo. Embryonic development has four stages: the morula stage, the blastula stage, the gastrula stage, and the neurula stage. Prior to implantation, the embryo remains in a protein shell, the zona pellucida, and undergoes a series of rapid mitotic cell divisions called cleavage. A week after fertilization the embryo still has not grown in size, but hatches from the zona pellucida and adheres to the lining of the mother's uterus. This induces a decidual reaction, wherein the uterine cells proliferate and surround the embryo thus causing it to become embedded within the uterine tissue. The embryo, meanwhile, proliferates and develops both into embryonic and extra-embryonic tissue, the latter forming the fetal membranes and the placenta. In humans, the embryo is referred to as a fetus in the later stages of prenatal development. The transition from embryo to fetus is arbitrarily defined as occurring 8 weeks after fertilization. In comparison to the embryo, the fetus has more recognizable external features and a set of progressively developing internal organs. A nearly identical process occurs in other species.
Embryonic development
Human embryonic development refers to the development and formation of the human embryo. It is characterised by the process of cell division and cellular differentiation of the embryo that occurs during the early stages of development. In biological terms, human development entails growth from a one-celled zygote to an adult human being. Fertilization occurs when the sperm cell successfully enters and fuses with an egg cell (ovum). The genetic material of the sperm and egg then combine to form a single cell called a zygote and the germinal stage of prenatal development commences. The embryonic stage covers the first eight weeks of development; at the beginning of the ninth week the embryo is termed a fetus.
The germinal stage refers to the time from fertilization through the development of the early embryo until implantation is completed in the uterus. The germinal stage takes around 10 days. During this stage, the zygote begins to divide, in a process called cleavage. A blastocyst is then formed and implanted in the uterus. Embryonic development continues with the next stage of gastrulation, when the three germ layers of the embryo form in a process called histogenesis, and the processes of neurulation and organogenesis follow.
In comparison to the embryo, the fetus has more recognizable external features and a more complete set of developing organs. The entire process of embryonic development involves coordinated spatial and temporal changes in gene expression, cell growth and cellular differentiation. A nearly identical process occurs in other species, especially among chordates.
Fetal development
A fetus is a stage in the human development considered to begin nine weeks after fertilization. In biological terms, however, prenatal development is a continuum, with many defining features distinguishing an embryo from a fetus. A fetus is also characterized by the presence of all the major body organs, though they will not yet be fully developed and functional and some not yet situated in their final location.
Maternal influences
The fetus and embryo develop within the uterus, an organ that sits within the pelvis of the mother. The process the mother experiences whilst carrying the fetus or embryo is referred to as pregnancy. The placenta connects the developing fetus to the uterine wall to allow nutrient uptake, thermo-regulation, waste elimination, and gas exchange via the mother's blood supply; to fight against internal infection; and to produce hormones which support pregnancy. The placenta provides oxygen and nutrients to growing fetuses and removes waste products from the fetus' blood. The placenta attaches to the wall of the uterus, and the fetus' umbilical cord develops from the placenta. These organs connect the mother and the fetus. Placentas are a defining characteristic of placental mammals, but are also found in marsupials and some non-mammals with varying levels of development. The homology of such structures in various viviparous organisms is debatable, and in invertebrates such as Arthropoda, is analogous at best.
After birth
Infancy and childhood
Childhood is the age span ranging from birth to adolescence. In developmental psychology, childhood is divided up into the developmental stages of toddlerhood (learning to walk), early childhood (play age), middle childhood (school age), and adolescence (puberty through post-puberty). Various childhood factors could affect a person's attitude formation.
Prepubescence
Neonate (newborn)
Infant (baby)
Toddler
Play age
Elementary school age, may coincide with preadolescence (preteen)
The Tanner stages can be used to approximately judge a child's age based on physical development.
Puberty
Puberty is the process of physical changes through which a child's body matures into an adult body capable of sexual reproduction. It is initiated by hormonal signals from the brain to the gonads: the ovaries in a girl, the testicles in a boy. In response to the signals, the gonads produce hormones that stimulate libido and the growth, function, and transformation of the brain, bones, muscle, blood, skin, hair, breasts, and sex organs. Physical growth—height and weight—accelerates in the first half of puberty and is completed when an adult body has been developed. Until the maturation of their reproductive capabilities, the pre-pubertal physical differences between boys and girls are the external sex organs.
On average, girls begin puberty around ages 10–11 and end puberty around 15–17; boys begin around ages 11–12 and end around 16–17. The major landmark of puberty for females is menarche, the onset of menstruation, which occurs on average between ages 12 and 13; for males, it is the first ejaculation, which occurs on average at age 13. In the 21st century, the average age at which children, especially girls, reach puberty is lower compared to the 19th century, when it was 15 for girls and 16 for boys. This can be due to any number of factors, including improved nutrition resulting in rapid body growth, increased weight and fat deposition, or exposure to endocrine disruptors such as xenoestrogens, which can at times be due to food consumption or other environmental factors. Puberty which starts earlier than usual is known as precocious puberty, and puberty which starts later than usual is known as delayed puberty.
Notable among the morphologic changes in size, shape, composition, and functioning of the pubertal body, is the development of secondary sex characteristics, the "filling in" of the child's body; from girl to woman, from boy to man.
Adulthood
Biologically, an adult is a human or other organism that has reached sexual maturity. In human context, the term adult has additional meanings associated with social and legal concepts. In contrast to a legal minor, a legal adult is a person who has attained the age of majority and is therefore regarded as independent, self-sufficient, and responsible. The typical age of legal majority is 18 years in most contexts, although the definition of majority may vary by legal rights and country.
Human adulthood encompasses psychological adult development. Definitions of adulthood are often inconsistent and contradictory; an adolescent may be biologically an adult and display adult behavior but still be treated as a child if they are under the legal age of majority. Conversely, a legal adult may possess none of the maturity and responsibility that is supposed to define them; the mental and physical development and maturity of an individual has been proven to be greatly influenced by their life circumstances.
Organ systems
Human organs and organ systems develop in a process known as organogenesis. This begins in the third week of embryonic development, when the gastrula forms three distinct germ layers, the ectoderm, mesoderm and endoderm. The ectoderm will eventually develop into the outer layer of skin and nervous system. The mesoderm will form skeletal muscles, blood cells, the reproductive system, the urinary system, most of the circulatory system, and the connective tissue of the torso. The endoderm will develop into the epithelium of the respiratory and gastrointestinal tracts and several glands.
Linear growth
During childhood, the bones undergo a complex process of elongation that occurs in a specific area called the epiphyseal growth plates (EGP). This process is regulated by various hormones and factors, including the growth hormone, vitamin D, and others. These hormones promote the production of insulin-like growth factor-1 (IGF-1), which plays a key role in the formation of new bone cells. Adequate nutrient intake is essential for the production of these hormones, which are critical for proper bone growth. However, a lack of proper nutrition can hinder this process and result in stunted growth.
Linear growth takes place in the epiphyseal growth plates (EGP) of long bones. In the growth plate, chondrocytes proliferate, hypertrophy and secrete cartilage extracellular matrix. New cartilage is subsequently remodeled into bone tissue, causing bones to grow longer. Linear growth is a complex process regulated by the growth hormone (GH) – insulin-like growth factor-1 (IGF-1) axis, the thyroxine/triiodothyronine axis, androgens, estrogens, vitamin D, glucocorticoids and possibly leptin. GH is secreted by the anterior pituitary gland in response to hypothalamic, pituitary and circulating factors. It affects growth by binding to receptors in the EGP, and inducing production and release of IGF-1 by the liver. IGF-1 has six binding proteins (IGFBPs), exhibiting different effects on body tissues, where IGFBP-3 is most abundant in human circulation. IGF-1 initiates growth through differentiation and maturation of osteoblasts, and regulates release of GH from the pituitary through feedback mechanisms. The GH/IGF-1 axis is responsive to dietary intake and infections. The endocrine system seems to allow for rapid growth only when the organism is able to consume sufficient amounts of nutrients and signaling from key nutrients such as amino acids and zinc to induce production of IGF-1 is present. At the same time inflammation and increased production of pro-inflammatory cytokines may cause GH resistance and a decrease in circulating IGF-1 and IGFBP-3 which in turn reduces endochondrial ossification and growth. However, the EGP appears to conserve much growth capacity to allow for catch-up growth. Concerns have been raised about associations between catch-up growth and increased risk of non-communicable diseases in adulthood. In a large study based on 5 birth cohorts in Brazil, Guatemala, India, the Philippines and South Africa, faster linear growth at 0–2 years was associated with improvements in adult stature and school performance, but also an increased likelihood of overweight (mainly related to lean mass) and a slightly elevated blood pressure in young adulthood.
| Biology and health sciences | Development | null |
95464 | https://en.wikipedia.org/wiki/Centrosome | Centrosome | In cell biology, the centrosome (Latin centrum 'center' + Greek sōma 'body') (archaically cytocentre) is an organelle that serves as the main microtubule organizing center (MTOC) of the animal cell, as well as a regulator of cell-cycle progression. The centrosome provides structure for the cell. The centrosome is thought to have evolved only in the metazoan lineage of eukaryotic cells. Fungi and plants lack centrosomes and therefore use other structures to organize their microtubules. Although the centrosome has a key role in efficient mitosis in animal cells, it is not essential in certain fly and flatworm species.
Centrosomes are composed of two centrioles arranged at right angles to each other, and surrounded by a dense, highly structured mass of protein termed the pericentriolar material (PCM). The PCM contains proteins responsible for microtubule nucleation and anchoring — including γ-tubulin, pericentrin and ninein. In general, each centriole of the centrosome is based on a nine-triplet microtubule assembled in a cartwheel structure, and contains centrin, cenexin and tektin.
In many cell types, the centrosome is replaced by a cilium during cellular differentiation. However, once the cell starts to divide, the cilium is replaced again by the centrosome.
History
The centrosome was discovered jointly by Walther Flemming in 1875 and Edouard Van Beneden in 1876, and later described and named in 1888 by Theodor Boveri.
Functions
Centrosomes are associated with the nuclear membrane during the prophase stage of the cell cycle. During mitosis, the nuclear membrane breaks down, and the centrosome-nucleated microtubules can interact with the chromosomes to build the mitotic spindle.
The mother centriole, the older of the two in the centriole pair, also has a central role in making cilia and flagella.
The centrosome is copied only once per cell cycle, so that each daughter cell inherits one centrosome, containing two structures called centrioles. The centrosome replicates during the S phase of the cell cycle. During the prophase in the process of cell division called mitosis, the centrosomes migrate to opposite poles of the cell. The mitotic spindle then forms between the two centrosomes. Upon division, each daughter cell receives one centrosome. Aberrant numbers of centrosomes in a cell have been associated with cancer. Doubling of a centrosome is similar to DNA replication in two respects: the semiconservative nature of the process and the action of CDK2 as a regulator of the process. But the processes are essentially different in that centrosome doubling does not occur by template reading and assembly. The mother centriole just aids in the accumulation of materials required for the assembly of the daughter centriole.
Centrioles, however, are not required for the progression of mitosis. When the centrioles are irradiated by a laser, mitosis proceeds normally with a morphologically normal spindle. Moreover, development of the fruit fly Drosophila is largely normal when centrioles are absent due to a mutation in a gene required for their duplication. In the absence of the centrioles, the microtubules of the spindle are focused by motors, allowing the formation of a bipolar spindle. Many cells can completely undergo interphase without centrioles.
Unlike centrioles, centrosomes are required for survival of the organism. Cells without centrosomes lack radial arrays of astral microtubules. They are also defective in spindle positioning and in the ability to establish a central localization site in cytokinesis. The function of centrosomes in this context is hypothesized to ensure the fidelity of cell division, because it greatly increases the efficacy. Some cell types arrest in the following cell cycle when centrosomes are absent. This is not a universal phenomenon.
When the nematode C. elegans egg is fertilized, the sperm delivers a pair of centrioles. These centrioles will form the centrosomes, which will direct the first cell division of the zygote, and this will determine its polarity. It's not yet clear whether the role of the centrosome in polarity determination is microtubule-dependent or independent.
In human reproduction, the sperm supplies the centriole that creates the centrosome and microtubule system of the zygote.
Centrosome alterations in cancer cells
Theodor Boveri, in 1914, described centrosome aberrations in cancer cells. This initial observation was subsequently extended to many types of human tumors. Centrosome alterations in cancer can be divided in two subgroups — i.e., structural or numeric aberrations — yet both can be found simultaneously in a tumor.
Structural aberrations
Usually, structural aberrations appear due to uncontrolled expression of centrosome components, or due to post-translational modifications (such as phosphorylations) that are not adequate for the components. These modifications may produce variations in centrosome size (usually too large, due to an excess of pericentriolar material). In addition, because centrosomal proteins have a tendency to form aggregates, centrosome-related bodies (CRBs) are often observed in ectopic places. Both enlarged centrosomes and CRBs are similar to the centrosomal structures observed in tumors. Even more, these structures can be induced in culture cells by overexpression of specific centrosomal proteins, such as CNap-1 or Nlp. These structures may look very similar, yet detailed studies reveal that they may present very different properties, depending on their proteic composition. For instance, their capacity to incorporate γ-TuRC complexes (see also: γ-tubulin) can be very variable, and so their capacity to nucleate microtubules therefore affects the shape, polarity and motility of implicated tumor cells in different ways.
Numeric aberrations
The presence of an inadequate number of centrosomes is very often linked to the appearance of genome instability and the loss of tissue differentiation. However, the method to count the centrosome number (with two centrioles to each centrosome) is often not very precise, because it is frequently assessed using fluorescence microscopy, which does not have high enough optical resolution to resolve centrioles that are very close to each other. Nevertheless, it is clear that the presence of an excess of centrosomes is a common event in human tumors. It has been observed that loss of the tumor-suppressor p53 produces superfluous centrosomes, as well as deregulating other proteins implicated in cancer formation in humans, such as BRCA1 and BRCA2. (For references, see .) An excess of centrosomes can be generated by very different mechanisms: specific reduplication of the centrosome, cytokinesis failure during cell division (generating an increase in chromosome number), cell fusion (such as in cases of infection by specific viruses) or de novo generation of centrosomes. At this point, there is insufficient information to know how prevalent these mechanisms are in vivo, but it is possible that the increase in centrosome numbers due to a failure during cell division might be more frequent than appreciated, because many "primary" defects in one cell (deregulation of the cell cycle, defective DNA or chromatin metabolism, failure in the spindle checkpoint, etc.) would generate a failure in cell division, an increase in ploidy and an increase in centrosome numbers as a "secondary" effect.
Evolution
The evolutionary history of the centrosome and the centriole has been traced for some of the signature genes — e.g., the centrins. Centrins participate in calcium signaling and are required for centriole duplication. There exist two main subfamilies of centrins, both of which are present in the early-branching eukaryote Giardia intestinalis. Centrins have therefore been present in the common ancestor of eukaryotes. Conversely, they have no recognizable homologs in archea and bacteria and are thus part of the "eukaryotic signature genes". Although there are studies on the evolution of the centrins and centrioles, no studies have been published on the evolution of the pericentriolar material.
It is evident that some parts of the centrosome are highly diverged in the model species Drosophila melanogaster and Caenorhabditis elegans. For example, both species have lost one of the centrin subfamilies that are usually associated with centriole duplication. Drosophila melanogaster mutants that lack centrosomes can even develop to morphologically normal adult flies, which then die shortly after birth because their sensory neurons lack cilia. Thus, these flies have evolved functionally redundant machinery, which is independent of the centrosomes.
Associated nucleotides
Research in 2006 indicated that centrosomes from Atlantic surf clam eggs contain RNA sequences. The sequences identified were found in "few to no" other places in the cell, and do not appear in existing genome databases. One identified RNA sequence contains a putative RNA polymerase, leading to the hypothesis of an RNA-based genome within the centrosome. However, subsequent research has shown that centrosome do not contain their own DNA-based genomes. While it was confirmed that RNA molecules associate with centrosomes, the sequences have still been found within the nucleus. Furthermore, centrosomes can form de novo after having been removed (e.g., by laser irradiation) from normal cells.
| Biology and health sciences | Organelles | Biology |
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