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Based on the most current evidence, salmonids diverged from the rest of teleost fish no later than 88 million years ago, during the late Cretaceous. This divergence was marked by a whole-genome duplication event in the ancestral salmonid, where the diploid ancestor became tetraploid. This duplication is the fourth of its kind to happen in the evolutionary lineage of the salmonids, with two having occurred commonly to all bony vertebrates, and another specifically in the teleost fishes.
Extant salmonids all show evidence of partial tetraploidy, as studies show the genome has undergone selection to regain a diploid state. Work done in the rainbow trout (Onchorhynchus mykiss) has shown that the genome is still partially-tetraploid. Around half of the duplicated protein-coding genes have been deleted, but all apparent miRNA sequences still show full duplication, with potential to influence regulation of the rainbow trout's genome. This pattern of partial tetraploidy is thought to be reflected in the rest of extant salmonids.
The first fossil species representing a true salmonid fish (E. driftwoodensis) does not appear until the middle Eocene. This fossil already displays traits associated with extant salmonids, but as the genome of E. driftwoodensis cannot be sequenced, it cannot be confirmed if polyploidy was present in this animal at this point in time. This fossil is also significantly younger than the proposed salmonid divergence from the rest of the teleost fishes, and is the earliest confirmed salmonid currently known. This means that the salmonids have a ghost lineage of approximately 33 million years.
Given a lack of earlier transition fossils, and the inability to extract genomic data from specimens other than extant species, the dating of the whole-genome duplication event in salmonids was historically a very broad categorization of times, ranging from 25 to 100 million years in age. New advances in calibrated relaxed molecular clock analyses have allowed for a closer examination of the salmonid genome, and has allowed for a more precise dating of the whole-genome duplication of the group, that places the latest possible date for the event at 88 million years ago. | Salmonidae | Wikipedia | 458 | 48878 | https://en.wikipedia.org/wiki/Salmonidae | Biology and health sciences | Salmoniformes | null |
This more precise dating and examination of the salmonid whole-genome duplication event has allowed more speculation on the radiation of species within the group. Historically, the whole-genome duplication event was thought to be the reason for the variation within Salmonidae. Current evidence done with molecular clock analyses revealed that much of the speciation of the group occurred during periods of intense climate change associated with the last ice ages, with especially high speciation rates being observed in salmonids that developed an anadromous lifestyle.
Classification
Together with the closely related orders Esociformes (pikes and mudminnows), Osmeriformes (true smelts) and Argentiniformes (marine smelts and barreleyes), Salmoniformes comprise the superorder Protacanthopterygii.
The only extant family within Salmoniformes, Salmonidae, is divided into three subfamilies and around 10 genera containing about 220 species. The concepts of the number of species recognised vary among researchers and authorities; the numbers presented below represent the higher estimates of diversity:
Order Salmoniformes
Family: Salmonidae
Subfamily: Coregoninae
Coregonus - whitefishes (78 species)
Prosopium - round whitefishes (6 species)
Stenodus - beloribitsa and nelma (2 species)
†Beckius (1 species, Oligocene)
†Parastenodus (1 species, Eocene)
Subfamily: Thymallinae
Thymallus - graylings (14 species)
Subfamily: Salmoninae
†Eosalmo (1 species, Eocene)
Tribe: Salmonini
Salmo - Atlantic salmon and trout (47 species)
Salvelinus - Char and trout (e.g. brook trout, lake trout) (51 species)
Salvethymus - Long-finned char (1 species)
Tribe: Oncorhynchini
Brachymystax - lenoks (4 species)
Hucho - taimens (4 species)
Oncorhynchus - Pacific salmon and trout (12 species)
Parahucho - Sakhalin taimen (1 species)
Hybrid crossbreeding
The following table shows results of hybrid crossbreeding combination in Salmonidae.
note :- : The identical kind, O : (survivability), X : (Fatality) | Salmonidae | Wikipedia | 475 | 48878 | https://en.wikipedia.org/wiki/Salmonidae | Biology and health sciences | Salmoniformes | null |
The Scorpaeniformes are a diverse order of ray-finned fish, including the lionfishes and sculpins, but have also been called the Scleroparei. It is one of the five largest orders of bony fishes by number of species, with over 1,320.
They are known as "mail-cheeked" fishes due to their distinguishing characteristic, the suborbital stay: a backwards extension of the third circumorbital bone (part of the lateral head/cheek skeleton, below the eye socket) across the cheek to the preoperculum, to which it is connected in most species.
Scorpaeniform fishes are carnivorous, mostly feeding on crustaceans and on smaller fish. Most species live on the sea bottom in relatively shallow waters, although species are known from deep water, from the midwater, and even from fresh water. They typically have spiny heads, and rounded pectoral and caudal fins. Most species are less than in length, but the full size range of the order varies from the velvetfishes belonging to the family Aploactinidae, which can be just long as adults, to the skilfish (Erilepis zonifer), which can reach in total length.
One of the suborders of the Scorpaeniformes is the Scorpaenoidei. This suborder is usually found in the benthic zone, which is the lowest region of any water body like oceans or lakes.
There are two groups of the Scorpaenoidei. The sea robins is the first, which are further classified into two families: the sea robins and the armored sea robins. One significant difference between the two families of sea robins is the presence of spine-bearing plate on the armored sea robins which is absent in the sea robins family.
The second group of the Scorpaenoidei suborder is the scorpionfishes, which according to Minouri Ishida's work in 1994 and recent studies, have twelve families. The scorpionfishes are very dynamic in size with the smallest one having a range of 2–3 cm, while the largest have a length of approximately 100 cm.
Classification
The division of Scorpaeniformes into families is not settled; accounts range from 26 to 35 families. The 5th edition of Fishes of the World classifies the order as follows: | Scorpaeniformes | Wikipedia | 503 | 48885 | https://en.wikipedia.org/wiki/Scorpaeniformes | Biology and health sciences | Fishes | null |
Order Scorpaeniformes
Suborder Scorpaenoidei
Superfamily Congiopodoidea
Family Aploactinidae Jordan & Starks, 1904 (Velvetfishes)
Family Congiopodidae Gill, 1889 (Racehorses, pigfishes or horsefishes)
Superfamily Pataecoidea
Family Pataecidae Gill, 1872 (Australian prowfishes)
Family Gnathanacanthidae Gill, 1892 (Red velvetfish)
Superfamily Scorpaenoidea
Family Eschmeyeridae Mandrytsa, 2001 (the cofish)
Family Scorpaenidae Risso, 1827 (Scorpionfishes)
Suborder Platycephaloidei
Superfamily Platycephaloidea
Family Bembridae Kaup, 1873 (Deepwater flatheads)
Family Platycephalidae Swainson, 1839 (True flatheads)
Family Hoplichthyidae Kaup, 1873 (Ghost flatheads)
Superfamily Trigloidea
Family Triglidae Rafinesque, 1815 (Common searobins)
Family Peristediidae Jordan & Gilbert, 1883 (Armored searobins)
Suborder Normanichthyiodei
Family Normanichthyidae Clark, 1837 (the Barehead scorpionfish or mote sculpin)
Suborder Zoarcoidei
Superfamily Anarhichadoidea
Family Anarhichadidae Bonaparte, 1835 (Wolffishes)
Family Cryptacanthodidae Gill, 1861 (Wrymouths)
Family Stichaeidae Gill, 1864 (Pricklebacks)
Family Pholidae Gill, 1893 (Gunnels)
Superfamily Bathymasteroidea
Family Bathymasteridae Jordan & Gilbert, 1883 (Ronquils)
Family Ptilichthyidae Jordan & Gilbert, 1883 (Quillfish)
Superfamily Zoarcoidea
Family Eulophiidae H. M. Smith, 1902 (Spinous eelpouts)
Family Zoarcidae Swainson, 1839 (True Eelpouts)
Superfamily Zaproroidea
Family Scytalinidae Jordan & Starks, 1895 (Graveldivers)
Family Zaproridae Jordan, 1896 (Prowfishes)
Suborder Gasterosteoidei
Family Hypoptychidae Steindachner, 1880 (the Korean Sandlance)
Family Aulorhynchidae Gill (1861) (Tubesnouts)
Family Gasterosteidae Bonaparte, 1831 (Sticklebacks)
Suborder Cottoidei | Scorpaeniformes | Wikipedia | 505 | 48885 | https://en.wikipedia.org/wiki/Scorpaeniformes | Biology and health sciences | Fishes | null |
Superfamily Anoplopomatoidea (Quast, 1965)
Family Anoplopomatidae Jordan & Gilbert, 1883 (Blackcod)
Superfamily Zaniolepidoidea Shinohara, 1994
Family Zaniolepididae Jordan & Gilbert, 1883 (Combfishes)
Superfamily Hexagrammoidea Gill, 1889
Family Hexagrammidae Jordan, 1888 (Greenlings)
Superfamily Trichodontoidea Nazarkin & Voskoboinikova, 2000
Family Trichodontidae Bleeker, 1859 (Sandfishes)
Superfamily Cottoidea Gill, 1889
Family Jordaniidae Jordan & Evermann, 1898 (Longfin sculpins)
Family Rhamphocottidae Jordan & Gilbert, 1883 (Grunt sculpins)
Family Scorpaenichthyidae Jordan & Evermann, 1898
Family Agonidae Swainson, 1839 (Poachers and searavens)
Family Cottidae Bonaparte, 1831 (Sculpins)
Family Psychrolutidae Günther, 1861 (Bighead sculpins)
Family Bathylutichthyidae Balushkin & Voskoboinikova, 1990 (Antarctic sculpins)
Superfamily Cyclopteroidea Gill, 1873
Family Cyclopteridae Bonaparte, 1831 (lumpfishes or lumpsuckers)
Family Liparidae Gill, 1861 (Snailfishes)
This classification is not settled, however, and some authorities classify these groupings largely within the Order Perciformes as the suborders Scorpaenoidei, Platycephaloidei, Triglioidei and Cottoidei, Cottodei including the infraorders Anoplopomatales, Zoarcales, Gasterosteales, Zaniolepidoales, Hexagrammales and Cottales. These infraorders largely correspond with the superfamilies in the Cottoidei set out in the 5th edition of Fishes of the World. | Scorpaeniformes | Wikipedia | 408 | 48885 | https://en.wikipedia.org/wiki/Scorpaeniformes | Biology and health sciences | Fishes | null |
A lyrebird is either of two species of ground-dwelling Australian birds that compose the genus Menura, and the family Menuridae. They are most notable for their impressive ability to mimic natural and artificial sounds from their environment, and the striking beauty of the male bird's huge tail when it is fanned out in courtship display. Lyrebirds have unique plumes of neutral-coloured tailfeathers and are among Australia's best-known native birds.
Taxonomy
The classification of lyrebirds was the subject of much debate after the first specimens reached European scientists after 1798. Based on specimens sent from New South Wales to England, Major-General Thomas Davies illustrated and described this species as the superb lyrebird, which he called Menura superba, in an 1800 presentation to the Linnean Society of London, but this work was not published until 1802; in the intervening time period, however, the species was described and named Menura novaehollandiae by John Latham in 1801, and this is the accepted name by virtue of nomenclatural priority.
The genus name Menura refers to the pattern of repeated transparent crescents (or "lunules") on the superb lyrebird's outer tail-feathers, from the Ancient Greek words mēnē "moon" and ourá "tail".
Lyrebirds are named because their outer tail feathers are broad and curved in a S shape that together resemble the shape of a lyre.
Systematics
Lyrebirds were thought to be Galliformes like the broadly similar looking partridge, junglefowl, and pheasants familiar to Europeans, reflected in the early names given to the superb lyrebird, including native pheasant. They were also called peacock-wrens and Australian birds-of-paradise. The idea that they were related to the pheasants was abandoned when the first chicks, which are altricial, were described. They were not classed with the passerines until a paper was published in 1840, twelve years after they were assigned a discrete family, Menuridae. Within that family they compose a single genus, Menura.
It is generally accepted that the lyrebird family is most closely related to the scrub-birds (Atrichornithidae) and some authorities combine both in a single family, but evidence that they are also related to the bowerbirds remains controversial. | Lyrebird | Wikipedia | 489 | 48896 | https://en.wikipedia.org/wiki/Lyrebird | Biology and health sciences | Corvoidea | null |
Lyrebirds are ancient Australian animals: the Australian Museum has fossils of lyrebirds dating back to about 15 million years ago. The prehistoric Menura tyawanoides has been described from Early Miocene fossils found at the famous Riversleigh site.
Species
Two species of lyrebird are extant:
Description
The lyrebirds are large passerine birds, amongst the largest in the order. They are ground living birds with strong legs and feet and short rounded wings. They are poor fliers and rarely fly except for periods of downhill gliding. The superb lyrebird is the larger of the two species. Lyrebirds measure 31 to 39 inches in length, including their tail. Males tend to be slightly larger than females. Females weigh around 2 pounds, and males weigh around 2.4 pounds.
Distribution and habitat
The superb lyrebird is found in areas of rainforest in Victoria, New South Wales, and south-east Queensland. It is also found in Tasmania where it was introduced in the 19th century. Many superb lyrebirds live in the Dandenong Ranges National Park and Kinglake National Park around Melbourne, the Royal National Park and Illawarra region south of Sydney, in many other parks along the east coast of Australia, and non protected bushland. Albert's lyrebird is found only in a small area of Southern Queensland rainforest.
Behaviour and ecology
Lyrebirds are shy and difficult to approach, particularly the Albert's lyrebird, with the result that little information about its behaviour has been documented. When lyrebirds detect potential danger, they pause and scan the surroundings, sound an alarm, and either flee the area on foot, or seek cover and freeze. Firefighters sheltering in mine shafts during bushfires have been joined by lyrebirds.
Diet and feeding
Lyrebirds feed on the ground and as individuals. A range of invertebrate prey is taken, including insects such as cockroaches, beetles (both adults and larvae), earwigs, fly larvae, and the adults and larvae of moths. Other prey taken includes centipedes, spiders, earthworms. Less commonly taken prey includes stick insects, bugs, amphipods, lizards, frogs and occasionally, seeds. They find food by scratching with their feet through the leaf-litter.
Breeding | Lyrebird | Wikipedia | 463 | 48896 | https://en.wikipedia.org/wiki/Lyrebird | Biology and health sciences | Corvoidea | null |
Lyrebirds are long-lived birds that can live as long as 30 years. They have long breeding cycles and start breeding later in life than other passerine birds. Female superb lyrebirds start breeding at the age of five or six, and males at the age of six to eight. Males defend territories from other males, and those territories may contain the breeding territories of up to eight females. Within the male territories, the males create or use display platforms; for the superb lyrebird, this is a mound of bare soil; for the Albert's lyrebird, it is a pile of twigs on the forest floor.
Male lyrebirds call mostly during winter, when they construct and maintain an open arena-mound in dense bush, on which they sing and dance in an elaborate courtship display performed for potential mates, of which the male lyrebird has several. The strength, volume, and location of the nest built by the female lyrebird is dependent on the rainfall and predation during the nest building period. It is important for the nest to be water resistant and hidden in secluded areas so predators cannot attack. Once the nest is made in the preferred location, the female lyrebird lays a single egg. The egg is incubated over 50 days solely by the female, and the female also fosters the chick alone.
Vocalizations and mimicry | Lyrebird | Wikipedia | 277 | 48896 | https://en.wikipedia.org/wiki/Lyrebird | Biology and health sciences | Corvoidea | null |
A lyrebird's song is one of the more distinctive aspects of its behavioural biology. Lyrebirds sing throughout the year, but the peak of the breeding season, from June to August, is when they sing with the most intensity. During this peak males may sing for four hours of the day, almost half the hours of daylight. The song of the lyrebird is a mixture of elements of its own song and mimicry of other species. Lyrebirds render with great fidelity the individual songs of other birds and the chatter of flocks of birds, and also mimic other animals such as possums, koalas and dingoes. Lyrebirds have been recorded mimicking human sounds such as a mill whistle, a cross-cut saw, chainsaws, car engines and car alarms, fire alarms, rifle-shots, camera shutters, dogs barking, crying babies, music, mobile phone ring tones, and even the human voice. However, while the mimicry of human noises is widely reported, the extent to which it happens is exaggerated and the phenomenon is unusual. Parts of the lyrebird's own song can resemble human-made sound effects, which has given rise to the urban legend that they frequently imitate video game or film sounds.
The superb lyrebird's mimicked calls are learned from the local environment, including from other superb lyrebirds. An instructive example is the population of superb lyrebirds in Tasmania, which have retained the calls of species not native to Tasmania in their repertoire, with some local Tasmanian endemic bird songs added. The female lyrebirds of both species are also mimics capable of complex vocalisations. Superb lyrebird females are silent during courtship; however, they regularly produce sophisticated vocal displays during foraging and nest defense. A recording of a superb lyrebird mimicking sounds of an electronic shooting game, workmen and chainsaws was added to the National Film and Sound Archive's Sounds of Australia registry in 2013.
Both species of lyrebird produced elaborate lyrebird-specific vocalisations including 'whistle songs'. Males also sing songs specifically associated with their song and dance displays. | Lyrebird | Wikipedia | 443 | 48896 | https://en.wikipedia.org/wiki/Lyrebird | Biology and health sciences | Corvoidea | null |
One researcher, Sydney Curtis, has recorded flute-like lyrebird calls in the vicinity of the New England National Park. Similarly, in 1969, a park ranger, Neville Fenton, recorded a lyrebird song which resembled flute sounds in the New England National Park, near Dorrigo in northern coastal New South Wales. After much detective work by Fenton, it was discovered that in the 1930s, a flute player living on a farm adjoining the park used to play tunes near his pet lyrebird. The lyrebird adopted the tunes into his repertoire, and retained them after release into the park. Neville Fenton forwarded a tape of his recording to Norman Robinson. Because a lyrebird is able to carry two tunes at the same time, Robinson filtered out one of the tunes and put it on the phonograph for the purposes of analysis. One witness suggested that the song represents a modified version of two popular tunes in the 1930s: "The Keel Row" and "Mosquito's Dance". Musicologist David Rothenberg has endorsed this information. However, a "flute lyrebird" research group (including Curtis and Fenton) formed to investigate the veracity of this story found no evidence of "Mosquito Dance" and only remnants of "Keel Row" in contemporary and historical lyrebird recordings from this area. Neither were they able to prove that a lyrebird chick had been a pet, although they acknowledged compelling evidence on both sides of the argument.
Status and conservation
Until the 2019–2020 Australian bushfire season, superb lyrebirds were not considered threatened in the short to medium term. Concern has since grown as early analyses have shown the extent of destruction of the lyrebird's preferred wet-forest habitats, which in less intense previous bushfire seasons have been spared, in large part due to their moisture content. Albert's lyrebird has a very restricted habitat and had been listed as vulnerable by the IUCN, but because the species and its habitat were carefully managed, the species was re-assessed to near threatened in 2009. The superb lyrebird had already been seriously threatened by habitat destruction in the past. Its population had since recovered, but the 2019–2020 bushfires damaged much of its habitat, which may lead to a reclassification of its status from "common" to "threatened". Beyond this new threat are the long-term vulnerabilities to predation by cats and foxes, as well as human population pressure on its habitat.
In culture
Painting by John Gould | Lyrebird | Wikipedia | 507 | 48896 | https://en.wikipedia.org/wiki/Lyrebird | Biology and health sciences | Corvoidea | null |
The lyrebird is so called because the male bird has a spectacular tail, consisting of 16 highly modified feathers (two long slender lyrates at the centre of the plume, two broader medians on the outside edges and twelve filamentaries arrayed between them), which was originally thought to resemble a lyre. This happened when a superb lyrebird specimen (which had been taken from Australia to England during the early 19th century) was prepared for display at the British Museum by a taxidermist who had never seen a live lyrebird. The taxidermist mistakenly thought that the tail would resemble a lyre, and that the tail would be held in a similar way to that of a peacock during courtship display, and so he arranged the feathers in this way. Later, John Gould (who had also never seen a live lyrebird), painted the lyrebird from the British Museum specimen.
The male lyrebird's tail is not held as in John Gould's painting. Instead, the male lyrebird's tail is fanned over the lyrebird during courtship display, with the tail completely covering his head and back—as can be seen in the image in the "breeding" section of this page, and also the image of the 10-cent coin, where the superb lyrebird's tail (in courtship display) is portrayed accurately.
Lyrebird emblems and logos
The lyrebird has been featured as a symbol and emblem many times, especially in New South Wales and Victoria (where the superb lyrebird has its natural habitat), and in Queensland (where Albert's lyrebird has its natural habitat). | Lyrebird | Wikipedia | 342 | 48896 | https://en.wikipedia.org/wiki/Lyrebird | Biology and health sciences | Corvoidea | null |
A male superb lyrebird is featured on the reverse of the Australian 10-cent coin.
A superb lyrebird featured on the Australian one shilling postage stamp first issued in 1932.
A stylised superb lyrebird appears in the transparent window of the Australian 100 dollar note.
A silhouette of a male superb lyrebird is the logo of the Australian Film Commission.
An illustration of a male superb lyrebird, in courtship display, is the emblem of the New South Wales National Parks and Wildlife Service.
The pattern on the curtains of the Victorian State Theatre is the image of a male superb lyrebird, in courtship display, as viewed from the front.
A stylised illustration of a male Albert's lyrebird was the logo of the Queensland Conservatorium of Music, before the Conservatorium became part of Griffith University. In the logo, the top part of the lyrebird's tail became a music stave.
Australian band You Am I's 2008 album Dilettantes and its first single, "Erasmus", feature a drawing of a lyrebird by artist Ken Taylor.
A stylised illustration of part of a male superb lyrebird's tail is the logo for the Lyrebird Arts Council of Victoria.
The lyrebird is also featured atop the crest of Panhellenic Sorority Alpha Chi Omega, whose symbol is the lyre.
There are many other companies with the name of Lyrebird, and these also have lyrebird logos.
"Land of the Lyrebird" is an alternative name for the Strzelecki Ranges in the Gippsland region of Victoria.
A silhouetted male superb lyrebird in courtship display features in the masthead of The Betoota Advocate. | Lyrebird | Wikipedia | 361 | 48896 | https://en.wikipedia.org/wiki/Lyrebird | Biology and health sciences | Corvoidea | null |
The atomic radius of a chemical element is a measure of the size of its atom, usually the mean or typical distance from the center of the nucleus to the outermost isolated electron. Since the boundary is not a well-defined physical entity, there are various non-equivalent definitions of atomic radius. Four widely used definitions of atomic radius are: Van der Waals radius, ionic radius, metallic radius and covalent radius. Typically, because of the difficulty to isolate atoms in order to measure their radii separately, atomic radius is measured in a chemically bonded state; however theoretical calculations are simpler when considering atoms in isolation. The dependencies on environment, probe, and state lead to a multiplicity of definitions.
Depending on the definition, the term may apply to atoms in condensed matter, covalently bonding in molecules, or in ionized and excited states; and its value may be obtained through experimental measurements, or computed from theoretical models. The value of the radius may depend on the atom's state and context.
Electrons do not have definite orbits nor sharply defined ranges. Rather, their positions must be described as probability distributions that taper off gradually as one moves away from the nucleus, without a sharp cutoff; these are referred to as atomic orbitals or electron clouds. Moreover, in condensed matter and molecules, the electron clouds of the atoms usually overlap to some extent, and some of the electrons may roam over a large region encompassing two or more atoms.
Under most definitions the radii of isolated neutral atoms range between 30 and 300 pm (trillionths of a meter), or between 0.3 and 3 ångströms. Therefore, the radius of an atom is more than 10,000 times the radius of its nucleus (1–10 fm), and less than 1/1000 of the wavelength of visible light (400–700 nm).
For many purposes, atoms can be modeled as spheres. This is only a crude approximation, but it can provide quantitative explanations and predictions for many phenomena, such as the density of liquids and solids, the diffusion of fluids through molecular sieves, the arrangement of atoms and ions in crystals, and the size and shape of molecules.
History | Atomic radius | Wikipedia | 441 | 48900 | https://en.wikipedia.org/wiki/Atomic%20radius | Physical sciences | Periodic table | Chemistry |
The concept of atomic radius was preceded in the 19th century by the concept of atomic volume, a relative measure of how much space would on average an atom occupy in a given solid or liquid material. By the end of the century this term was also used in an absolute sense, as a molar volume divided by Avogadro constant. Such a volume is different for different crystalline forms even of the same compound, but physicists used it for rough, order-of-magnitude estimates of the atomic size, getting 10−8–10−7 cm for copper.
The earliest estimates of the atomic size was made by opticians in the 1830s, particularly Cauchy, who developed models of light dispersion assuming a lattice of connected "molecules". In 1857 Clausius developed a gas-kinetic model which included the equation for mean free path. In the 1870s it was used to estimate gas molecule sizes, as well as an aforementioned comparison with visible light wavelength and an estimate from the thickness of soap bubble film at which its contractile force rapidly diminishes. By 1900, various estimates of mercury atom diameter averaged around 275±20 pm (modern estimates give 300±10 pm, see below).
In 1920, shortly after it had become possible to determine the sizes of atoms using X-ray crystallography, it was suggested that all atoms of the same element have the same radii. However, in 1923, when more crystal data had become available, it was found that the approximation of an atom as a sphere does not necessarily hold when comparing the same atom in different crystal structures.
Definitions
Widely used definitions of atomic radius include:
Van der Waals radius: In the simplest definition, half the minimum distance between the nuclei of two atoms of the element that are not otherwise bound by covalent or metallic interactions. The Van der Waals radius may be defined even for elements (such as metals) in which Van der Waals forces are dominated by other interactions. Because Van der Waals interactions arise through quantum fluctuations of the atomic polarisation, the polarisability (which can usually be measured or calculated more easily) may be used to define the Van der Waals radius indirectly. | Atomic radius | Wikipedia | 440 | 48900 | https://en.wikipedia.org/wiki/Atomic%20radius | Physical sciences | Periodic table | Chemistry |
Ionic radius: the nominal radius of the ions of an element in a specific ionization state, deduced from the spacing of atomic nuclei in crystalline salts that include that ion. In principle, the spacing between two adjacent oppositely charged ions (the length of the ionic bond between them) should equal the sum of their ionic radii.
Covalent radius: the nominal radius of the atoms of an element when covalently bound to other atoms, as deduced from the separation between the atomic nuclei in molecules. In principle, the distance between two atoms that are bound to each other in a molecule (the length of that covalent bond) should equal the sum of their covalent radii.
Metallic radius: the nominal radius of atoms of an element when joined to other atoms by metallic bonds.
Bohr radius: the radius of the lowest-energy electron orbit predicted by Bohr model of the atom (1913). It is only applicable to atoms and ions with a single electron, such as hydrogen, singly ionized helium, and positronium. Although the model itself is now obsolete, the Bohr radius for the hydrogen atom is still regarded as an important physical constant, because it is equivalent to the quantum-mechanical most probable distance of the electron from the nucleus.
Empirically measured atomic radius
The following table shows empirically measured covalent radii for the elements, as published by J. C. Slater in 1964. The values are in picometers (pm or 1×10−12 m), with an accuracy of about 5 pm. The shade of the box ranges from red to yellow as the radius increases; gray indicates lack of data.
Explanation of the general trends
Electrons in atoms fill electron shells from the lowest available energy level. As a consequence of the Aufbau principle, each new period begins with the first two elements filling the next unoccupied s-orbital. Because an atom's s-orbital electrons are typically farthest from the nucleus, this results in a significant increase in atomic radius with the first elements of each period.
The atomic radius of each element generally decreases across each period due to an increasing number of protons, since an increase in the number of protons increases the attractive force acting on the atom's electrons. The greater attraction draws the electrons closer to the protons, decreasing the size of the atom. Down each group, the atomic radius of each element typically increases because there are more occupied
electron energy levels and therefore a greater distance between protons and electrons. | Atomic radius | Wikipedia | 512 | 48900 | https://en.wikipedia.org/wiki/Atomic%20radius | Physical sciences | Periodic table | Chemistry |
The increasing nuclear charge is partly counterbalanced by the increasing number of electrons—a phenomenon that is known as shielding—which explains why the size of atoms usually increases down each column despite an increase in attractive force from the nucleus. Electron shielding causes the attraction of an atom's nucleus on its electrons to decrease, so electrons occupying higher energy states farther from the nucleus experience reduced attractive force, increasing the size of the atom. However, elements in the 5d-block (lutetium to mercury) are much smaller than this trend predicts due to the weak shielding of the 4f-subshell. This phenomenon is known as the lanthanide contraction. A similar phenomenon exists for actinides; however, the general instability of transuranic elements makes measurements for the remainder of the 5f-block difficult and for transactinides nearly impossible. Finally, for sufficiently heavy elements, the atomic radius may be decreased by relativistic effects. This is a consequence of electrons near the strongly charged nucleus traveling at a sufficient fraction of the speed of light to gain a nontrivial amount of mass.
The following table summarizes the main phenomena that influence the atomic radius of an element:
Lanthanide contraction
The electrons in the 4f-subshell, which is progressively filled from lanthanum (Z = 57) to ytterbium (Z = 70), are not particularly effective at shielding the increasing nuclear charge from the sub-shells further out. The elements immediately following the lanthanides have atomic radii which are smaller than would be expected and which are almost identical to the atomic radii of the elements immediately above them. Hence lutetium is in fact slightly smaller than yttrium, hafnium has virtually the same atomic radius (and chemistry) as zirconium, and tantalum has an atomic radius similar to niobium, and so forth. The effect of the lanthanide contraction is noticeable up to platinum (Z = 78), after which it is masked by a relativistic effect known as the inert-pair effect.
Due to lanthanide contraction, the 5 following observations can be drawn: | Atomic radius | Wikipedia | 444 | 48900 | https://en.wikipedia.org/wiki/Atomic%20radius | Physical sciences | Periodic table | Chemistry |
The size of Ln3+ ions regularly decreases with atomic number. According to Fajans' rules, decrease in size of Ln3+ ions increases the covalent character and decreases the basic character between Ln3+ and OH− ions in Ln(OH)3, to the point that Yb(OH)3 and Lu(OH)3 can dissolve with difficulty in hot concentrated NaOH. Hence the order of size of Ln3+ is given: La3+ > Ce3+ > ..., ... > Lu3+.
There is a regular decrease in their ionic radii.
There is a regular decrease in their tendency to act as a reducing agent, with an increase in atomic number.
The second and third rows of d-block transition elements are quite close in properties.
Consequently, these elements occur together in natural minerals and are difficult to separate.
d-block contraction
The d-block contraction is less pronounced than the lanthanide contraction but arises from a similar cause. In this case, it is the poor shielding capacity of the 3d-electrons which affects the atomic radii and chemistries of the elements immediately following the first row of the transition metals, from gallium (Z = 31) to bromine (Z = 35).
Calculated atomic radius
The following table shows atomic radii computed from theoretical models, as published by Enrico Clementi and others in 1967. The values are in picometres (pm). | Atomic radius | Wikipedia | 301 | 48900 | https://en.wikipedia.org/wiki/Atomic%20radius | Physical sciences | Periodic table | Chemistry |
The kilogram per cubic metre (symbol: kg·m−3, or kg/m3) is the unit of density in the International System of Units (SI). It is defined by dividing the SI unit of mass, the kilogram, by the SI unit of volume, the cubic metre.
Conversions
1 kg/m3 = 1 g/L (exactly)
1 kg/m3 = 0.001 g/cm3 (exactly)
1 kg/m3 ≈ 0.06243 lb/ft3 (approximately)
1 kg/m3 ≈ 0.1335 oz/US gal (approximately)
1 kg/m3 ≈ 0.1604 oz/imp gal (approximately)
1 g/cm3 = 1000 kg/m3 (exactly)
1 lb/ft3 ≈ 16.02 kg/m3 (approximately)
1 oz/(US gal) ≈ 7.489 kg/m3 (approximately)
1 oz/(imp gal) ≈ 6.236 kg/m3 (approximately)
Relation to other measures
The density of water is about 1000 kg/m3 or 1 g/cm3, because the size of the gram was originally based on the mass of a cubic centimetre of water.
In chemistry, g/cm3 is more commonly used. | Kilogram per cubic metre | Wikipedia | 254 | 48902 | https://en.wikipedia.org/wiki/Kilogram%20per%20cubic%20metre | Physical sciences | Density | Basics and measurement |
Nucleosynthesis is the process that creates new atomic nuclei from pre-existing nucleons (protons and neutrons) and nuclei. According to current theories, the first nuclei were formed a few minutes after the Big Bang, through nuclear reactions in a process called Big Bang nucleosynthesis. After about 20 minutes, the universe had expanded and cooled to a point at which these high-energy collisions among nucleons ended, so only the fastest and simplest reactions occurred, leaving our universe containing hydrogen and helium. The rest is traces of other elements such as lithium and the hydrogen isotope deuterium. Nucleosynthesis in stars and their explosions later produced the variety of elements and isotopes that we have today, in a process called cosmic chemical evolution. The amounts of total mass in elements heavier than hydrogen and helium (called 'metals' by astrophysicists) remains small (few percent), so that the universe still has approximately the same composition.
Stars fuse light elements to heavier ones in their cores, giving off energy in the process known as stellar nucleosynthesis. Nuclear fusion reactions create many of the lighter elements, up to and including iron and nickel in the most massive stars. Products of stellar nucleosynthesis remain trapped in stellar cores and remnants except if ejected through stellar winds and explosions. The neutron capture reactions of the r-process and s-process create heavier elements, from iron upwards.
Supernova nucleosynthesis within exploding stars is largely responsible for the elements between oxygen and rubidium: from the ejection of elements produced during stellar nucleosynthesis; through explosive nucleosynthesis during the supernova explosion; and from the r-process (absorption of multiple neutrons) during the explosion.
Neutron star mergers are a recently discovered major source of elements produced in the r-process. When two neutron stars collide, a significant amount of neutron-rich matter may be ejected which then quickly forms heavy elements.
Cosmic ray spallation is a process wherein cosmic rays impact nuclei and fragment them. It is a significant source of the lighter nuclei, particularly 3He, 9Be and 10,11B, that are not created by stellar nucleosynthesis. Cosmic ray spallation can occur in the interstellar medium, on asteroids and meteoroids, or on Earth in the atmosphere or in the ground.
This contributes to the presence on Earth of cosmogenic nuclides. | Nucleosynthesis | Wikipedia | 497 | 48903 | https://en.wikipedia.org/wiki/Nucleosynthesis | Physical sciences | Nuclear physics | null |
On Earth new nuclei are also produced by radiogenesis, the decay of long-lived, primordial radionuclides such as uranium, thorium, and potassium-40.
History
Timeline
It is thought that the primordial nucleons themselves were formed from the quark–gluon plasma around 13.8 billion years ago during the Big Bang as it cooled below two trillion degrees. A few minutes afterwards, starting with only protons and neutrons, nuclei up to lithium and beryllium (both with mass number 7) were formed, but hardly any other elements. Some boron may have been formed at this time, but the process stopped before significant carbon could be formed, as this element requires a far higher product of helium density and time than were present in the short nucleosynthesis period of the Big Bang. That fusion process essentially shut down at about 20 minutes, due to drops in temperature and density as the universe continued to expand. This first process, Big Bang nucleosynthesis, was the first type of nucleogenesis to occur in the universe, creating the so-called primordial elements.
A star formed in the early universe produces heavier elements by combining its lighter nucleihydrogen, helium, lithium, beryllium, and boronwhich were found in the initial composition of the interstellar medium and hence the star. Interstellar gas therefore contains declining abundances of these light elements, which are present only by virtue of their nucleosynthesis during the Big Bang, and also cosmic ray spallation. These lighter elements in the present universe are therefore thought to have been produced through thousands of millions of years of cosmic ray (mostly high-energy proton) mediated breakup of heavier elements in interstellar gas and dust. The fragments of these cosmic-ray collisions include helium-3 and the stable isotopes of the light elements lithium, beryllium, and boron. Carbon was not made in the Big Bang, but was produced later in larger stars via the triple-alpha process. | Nucleosynthesis | Wikipedia | 416 | 48903 | https://en.wikipedia.org/wiki/Nucleosynthesis | Physical sciences | Nuclear physics | null |
The subsequent nucleosynthesis of heavier elements (Z ≥ 6, carbon and heavier elements) requires the extreme temperatures and pressures found within stars and supernovae. These processes began as hydrogen and helium from the Big Bang collapsed into the first stars after about 500 million years. Star formation has been occurring continuously in galaxies since that time. The primordial nuclides were created by Big Bang nucleosynthesis, stellar nucleosynthesis, supernova nucleosynthesis, and by nucleosynthesis in exotic events such as neutron star collisions. Other nuclides, such as Ar, formed later through radioactive decay. On Earth, mixing and evaporation has altered the primordial composition to what is called the natural terrestrial composition. The heavier elements produced after the Big Bang range in atomic numbers from Z = 6 (carbon) to Z = 94 (plutonium). Synthesis of these elements occurred through nuclear reactions involving the strong and weak interactions among nuclei, and called nuclear fusion (including both rapid and slow multiple neutron capture), and include also nuclear fission and radioactive decays such as beta decay. The stability of atomic nuclei of different sizes and composition (i.e. numbers of neutrons and protons) plays an important role in the possible reactions among nuclei. Cosmic nucleosynthesis, therefore, is studied among researchers of astrophysics and nuclear physics ("nuclear astrophysics").
History of nucleosynthesis theory
The first ideas on nucleosynthesis were simply that the chemical elements were created at the beginning of the universe, but no rational physical scenario for this could be identified. Gradually it became clear that hydrogen and helium are much more abundant than any of the other elements. All the rest constitute less than 2% of the mass of the Solar System, and of other star systems as well. At the same time it was clear that oxygen and carbon were the next two most common elements, and also that there was a general trend toward high abundance of the light elements, especially those with isotopes composed of whole numbers of helium-4 nuclei (alpha nuclides).
Arthur Stanley Eddington first suggested in 1920 that stars obtain their energy by fusing hydrogen into helium and raised the possibility that the heavier elements may also form in stars. This idea was not generally accepted, as the nuclear mechanism was not understood. In the years immediately before World War II, Hans Bethe first elucidated those nuclear mechanisms by which hydrogen is fused into helium. | Nucleosynthesis | Wikipedia | 509 | 48903 | https://en.wikipedia.org/wiki/Nucleosynthesis | Physical sciences | Nuclear physics | null |
Fred Hoyle's original work on nucleosynthesis of heavier elements in stars, occurred just after World War II. His work explained the production of all heavier elements, starting from hydrogen. Hoyle proposed that hydrogen is continuously created in the universe from vacuum and energy, without need for universal beginning.
Hoyle's work explained how the abundances of the elements increased with time as the galaxy aged. Subsequently, Hoyle's picture was expanded during the 1960s by contributions from William A. Fowler, Alastair G. W. Cameron, and Donald D. Clayton, followed by many others. The seminal 1957 review paper by E. M. Burbidge, G. R. Burbidge, Fowler and Hoyle is a well-known summary of the state of the field in 1957. That paper defined new processes for the transformation of one heavy nucleus into others within stars, processes that could be documented by astronomers.
The Big Bang itself had been proposed in 1931, long before this period, by Georges Lemaître, a Belgian physicist, who suggested that the evident expansion of the Universe in time required that the Universe, if contracted backwards in time, would continue to do so until it could contract no further. This would bring all the mass of the Universe to a single point, a "primeval atom", to a state before which time and space did not exist. Hoyle is credited with coining the term "Big Bang" during a 1949 BBC radio broadcast, saying that Lemaître's theory was "based on the hypothesis that all the matter in the universe was created in one big bang at a particular time in the remote past." It is popularly reported that Hoyle intended this to be pejorative, but Hoyle explicitly denied this and said it was just a striking image meant to highlight the difference between the two models. Lemaître's model was needed to explain the existence of deuterium and nuclides between helium and carbon, as well as the fundamentally high amount of helium present, not only in stars but also in interstellar space. As it happened, both Lemaître and Hoyle's models of nucleosynthesis would be needed to explain the elemental abundances in the universe. | Nucleosynthesis | Wikipedia | 461 | 48903 | https://en.wikipedia.org/wiki/Nucleosynthesis | Physical sciences | Nuclear physics | null |
The goal of the theory of nucleosynthesis is to explain the vastly differing abundances of the chemical elements and their several isotopes from the perspective of natural processes. The primary stimulus to the development of this theory was the shape of a plot of the abundances versus the atomic number of the elements. Those abundances, when plotted on a graph as a function of atomic number, have a jagged sawtooth structure that varies by factors up to ten million. A very influential stimulus to nucleosynthesis research was an abundance table created by Hans Suess and Harold Urey that was based on the unfractionated abundances of the non-volatile elements found within unevolved meteorites. Such a graph of the abundances is displayed on a logarithmic scale below, where the dramatically jagged structure is visually suppressed by the many powers of ten spanned in the vertical scale of this graph.
Processes
There are a number of astrophysical processes which are believed to be responsible for nucleosynthesis. The majority of these occur within stars, and the chain of those nuclear fusion processes are known as hydrogen burning (via the proton–proton chain or the CNO cycle), helium burning, carbon burning, neon burning, oxygen burning and silicon burning. These processes are able to create elements up to and including iron and nickel. This is the region of nucleosynthesis within which the isotopes with the highest binding energy per nucleon are created. Heavier elements can be assembled within stars by a neutron capture process known as the s-process or in explosive environments, such as supernovae and neutron star mergers, by a number of other processes. Some of those others include the r-process, which involves rapid neutron captures, the rp-process, and the p-process (sometimes known as the gamma process), which results in the photodisintegration of existing nuclei.
Major types
Big Bang nucleosynthesis | Nucleosynthesis | Wikipedia | 395 | 48903 | https://en.wikipedia.org/wiki/Nucleosynthesis | Physical sciences | Nuclear physics | null |
Big Bang nucleosynthesis occurred within the first three minutes of the beginning of the universe and is responsible for much of the abundance of (protium), (D, deuterium), (helium-3), and (helium-4). Although continues to be produced by stellar fusion and alpha decays and trace amounts of continue to be produced by spallation and certain types of radioactive decay, most of the mass of the isotopes in the universe are thought to have been produced in the Big Bang. The nuclei of these elements, along with some and are considered to have been formed between 100 and 300 seconds after the Big Bang when the primordial quark–gluon plasma froze out to form protons and neutrons. Because of the very short period in which nucleosynthesis occurred before it was stopped by expansion and cooling (about 20 minutes), no elements heavier than beryllium (or possibly boron) could be formed. Elements formed during this time were in the plasma state, and did not cool to the state of neutral atoms until much later.
Stellar nucleosynthesis
Stellar nucleosynthesis is the nuclear process by which new nuclei are produced. It occurs in stars during stellar evolution. It is responsible for the galactic abundances of elements from carbon to iron. Stars are thermonuclear furnaces in which H and He are fused into heavier nuclei by increasingly high temperatures as the composition of the core evolves. Of particular importance is carbon because its formation from He is a bottleneck in the entire process. Carbon is produced by the triple-alpha process in all stars. Carbon is also the main element that causes the release of free neutrons within stars, giving rise to the s-process, in which the slow absorption of neutrons converts iron into elements heavier than iron and nickel.
The products of stellar nucleosynthesis are generally dispersed into the interstellar gas through mass loss episodes and the stellar winds of low mass stars. The mass loss events can be witnessed today in the planetary nebulae phase of low-mass star evolution, and the explosive ending of stars, called supernovae, of those with more than eight times the mass of the Sun. | Nucleosynthesis | Wikipedia | 453 | 48903 | https://en.wikipedia.org/wiki/Nucleosynthesis | Physical sciences | Nuclear physics | null |
The first direct proof that nucleosynthesis occurs in stars was the astronomical observation that interstellar gas has become enriched with heavy elements as time passed. As a result, stars that were born from it late in the galaxy, formed with much higher initial heavy element abundances than those that had formed earlier. The detection of technetium in the atmosphere of a red giant star in 1952, by spectroscopy, provided the first evidence of nuclear activity within stars. Because technetium is radioactive, with a half-life much less than the age of the star, its abundance must reflect its recent creation within that star. Equally convincing evidence of the stellar origin of heavy elements is the large overabundances of specific stable elements found in stellar atmospheres of asymptotic giant branch stars. Observation of barium abundances some 20–50 times greater than found in unevolved stars is evidence of the operation of the s-process within such stars. Many modern proofs of stellar nucleosynthesis are provided by the isotopic compositions of stardust, solid grains that have condensed from the gases of individual stars and which have been extracted from meteorites. Stardust is one component of cosmic dust and is frequently called presolar grains. The measured isotopic compositions in stardust grains demonstrate many aspects of nucleosynthesis within the stars from which the grains condensed during the star's late-life mass-loss episodes.
Explosive nucleosynthesis | Nucleosynthesis | Wikipedia | 299 | 48903 | https://en.wikipedia.org/wiki/Nucleosynthesis | Physical sciences | Nuclear physics | null |
Supernova nucleosynthesis occurs in the energetic environment in supernovae, in which the elements between silicon and nickel are synthesized in quasiequilibrium established during fast fusion that attaches by reciprocating balanced nuclear reactions to 28Si. Quasiequilibrium can be thought of as almost equilibrium except for a high abundance of the 28Si nuclei in the feverishly burning mix. This concept was the most important discovery in nucleosynthesis theory of the intermediate-mass elements since Hoyle's 1954 paper because it provided an overarching understanding of the abundant and chemically important elements between silicon (A = 28) and nickel (A = 60). It replaced the incorrect although much cited alpha process of the B2FH paper, which inadvertently obscured Hoyle's 1954 theory. Further nucleosynthesis processes can occur, in particular the r-process (rapid process) described by the B2FH paper and first calculated by Seeger, Fowler and Clayton, in which the most neutron-rich isotopes of elements heavier than nickel are produced by rapid absorption of free neutrons. The creation of free neutrons by electron capture during the rapid compression of the supernova core along with the assembly of some neutron-rich seed nuclei makes the r-process a primary process, and one that can occur even in a star of pure H and He. This is in contrast to the B2FH designation of the process as a secondary process. This promising scenario, though generally supported by supernova experts, has yet to achieve a satisfactory calculation of r-process abundances. The primary r-process has been confirmed by astronomers who had observed old stars born when galactic metallicity was still small, that nonetheless contain their complement of r-process nuclei; thereby demonstrating that the metallicity is a product of an internal process. The r-process is responsible for our natural cohort of radioactive elements, such as uranium and thorium, as well as the most neutron-rich isotopes of each heavy element.
The rp-process (rapid proton) involves the rapid absorption of free protons as well as neutrons, but its role and its existence are less certain. | Nucleosynthesis | Wikipedia | 446 | 48903 | https://en.wikipedia.org/wiki/Nucleosynthesis | Physical sciences | Nuclear physics | null |
Explosive nucleosynthesis occurs too rapidly for radioactive decay to decrease the number of neutrons, so that many abundant isotopes with equal and even numbers of protons and neutrons are synthesized by the silicon quasi-equilibrium process. During this process, the burning of oxygen and silicon fuses nuclei that themselves have equal numbers of protons and neutrons to produce nuclides which consist of whole numbers of helium nuclei, up to 15 (representing 60Ni). Such multiple-alpha-particle nuclides are totally stable up to 40Ca (made of 10 helium nuclei), but heavier nuclei with equal and even numbers of protons and neutrons are tightly bound but unstable. The quasi-equilibrium produces radioactive isobars 44Ti, 48Cr, 52Fe, and 56Ni, which (except 44Ti) are created in abundance but decay after the explosion and leave the most stable isotope of the corresponding element at the same atomic weight. The most abundant and extant isotopes of elements produced in this way are 48Ti, 52Cr, and 56Fe. These decays are accompanied by the emission of gamma-rays (radiation from the nucleus), whose spectroscopic lines can be used to identify the isotope created by the decay. The detection of these emission lines were an important early product of gamma-ray astronomy.
The most convincing proof of explosive nucleosynthesis in supernovae occurred in 1987 when those gamma-ray lines were detected emerging from supernova 1987A. Gamma-ray lines identifying 56Co and 57Co nuclei, whose half-lives limit their age to about a year, proved that their radioactive cobalt parents created them. This nuclear astronomy observation was predicted in 1969 as a way to confirm explosive nucleosynthesis of the elements, and that prediction played an important role in the planning for NASA's Compton Gamma-Ray Observatory.
Other proofs of explosive nucleosynthesis are found within the stardust grains that condensed within the interiors of supernovae as they expanded and cooled. Stardust grains are one component of cosmic dust. In particular, radioactive 44Ti was measured to be very abundant within supernova stardust grains at the time they condensed during the supernova expansion. This confirmed a 1975 prediction of the identification of supernova stardust (SUNOCONs), which became part of the pantheon of presolar grains. Other unusual isotopic ratios within these grains reveal many specific aspects of explosive nucleosynthesis. | Nucleosynthesis | Wikipedia | 500 | 48903 | https://en.wikipedia.org/wiki/Nucleosynthesis | Physical sciences | Nuclear physics | null |
Neutron star mergers
The merger of binary neutron stars (BNSs) is now believed to be the main source of r-process elements. Being neutron-rich by definition, mergers of this type had been suspected of being a source of such elements, but definitive evidence was difficult to obtain. In 2017 strong evidence emerged, when LIGO, VIRGO, the Fermi Gamma-ray Space Telescope and INTEGRAL, along with a collaboration of many observatories around the world, detected both gravitational wave and electromagnetic signatures of a likely neutron star merger, GW170817, and subsequently detected signals of numerous heavy elements such as gold as the ejected degenerate matter decays and cools. The first detection of the merger of a neutron star and black hole (NSBHs) came in July 2021 and more after but analysis seem to favor BNSs over NSBHs as the main contributors to heavy metal production.
Black hole accretion disk nucleosynthesis
Nucleosynthesis may happen in accretion disks of black holes.
Cosmic ray spallation
Cosmic ray spallation process reduces the atomic weight of interstellar matter by the impact with cosmic rays, to produce some of the lightest elements present in the universe (though not a significant amount of deuterium). Most notably spallation is believed to be responsible for the generation of almost all of 3He and the elements lithium, beryllium, and boron, although some and are thought to have been produced in the Big Bang. The spallation process results from the impact of cosmic rays (mostly fast protons) against the interstellar medium. These impacts fragment carbon, nitrogen, and oxygen nuclei present. The process results in the light elements beryllium, boron, and lithium in the cosmos at much greater abundances than they are found within solar atmospheres. The quantities of the light elements 1H and 4He produced by spallation are negligible relative to their primordial abundance.
Beryllium and boron are not significantly produced by stellar fusion processes, since 8Be has an extremely short half-life of seconds.
Empirical evidence
Theories of nucleosynthesis are tested by calculating isotope abundances and comparing those results with observed abundances. Isotope abundances are typically calculated from the transition rates between isotopes in a network. Often these calculations can be simplified as a few key reactions control the rate of other reactions.
Minor mechanisms and processes | Nucleosynthesis | Wikipedia | 492 | 48903 | https://en.wikipedia.org/wiki/Nucleosynthesis | Physical sciences | Nuclear physics | null |
Tiny amounts of certain nuclides are produced on Earth by artificial means. Those are our primary source, for example, of technetium. However, some nuclides are also produced by a number of natural means that have continued after primordial elements were in place. These often act to create new elements in ways that can be used to date rocks or to trace the source of geological processes. Although these processes do not produce the nuclides in abundance, they are assumed to be the entire source of the existing natural supply of those nuclides. | Nucleosynthesis | Wikipedia | 113 | 48903 | https://en.wikipedia.org/wiki/Nucleosynthesis | Physical sciences | Nuclear physics | null |
These mechanisms include:
Radioactive decay may lead to radiogenic daughter nuclides. The nuclear decay of many long-lived primordial isotopes, especially uranium-235, uranium-238, and thorium-232 produce many intermediate daughter nuclides before they too finally decay to isotopes of lead. The Earth's natural supply of elements like radon and polonium is via this mechanism. The atmosphere's supply of argon-40 is due mostly to the radioactive decay of potassium-40 in the time since the formation of the Earth. Little of the atmospheric argon is primordial. Helium-4 is produced by alpha-decay, and the helium trapped in Earth's crust is also mostly non-primordial. In other types of radioactive decay, such as cluster decay, larger species of nuclei are ejected (for example, neon-20), and these eventually become newly formed stable atoms.
Radioactive decay may lead to spontaneous fission. This is not cluster decay, as the fission products may be split among nearly any type of atom. Thorium-232, uranium-235, and uranium-238 are primordial isotopes that undergo spontaneous fission. Natural technetium and promethium are produced in this manner.
Nuclear reactions. Naturally occurring nuclear reactions powered by radioactive decay give rise to so-called nucleogenic nuclides. This process happens when an energetic particle from radioactive decay, often an alpha particle, reacts with a nucleus of another atom to change the nucleus into another nuclide. This process may also cause the production of further subatomic particles, such as neutrons. Neutrons can also be produced in spontaneous fission and by neutron emission. These neutrons can then go on to produce other nuclides via neutron-induced fission, or by neutron capture. For example, some stable isotopes such as neon-21 and neon-22 are produced by several routes of nucleogenic synthesis, and thus only part of their abundance is primordial.
Nuclear reactions due to cosmic rays. By convention, these reaction-products are not termed "nucleogenic" nuclides, but rather cosmogenic nuclides. Cosmic rays continue to produce new elements on Earth by the same cosmogenic processes discussed above that produce primordial beryllium and boron. One important example is carbon-14, produced from nitrogen-14 in the atmosphere by cosmic rays. Iodine-129 is another example. | Nucleosynthesis | Wikipedia | 499 | 48903 | https://en.wikipedia.org/wiki/Nucleosynthesis | Physical sciences | Nuclear physics | null |
The zenith (, ) is the imaginary point on the celestial sphere directly "above" a particular location. "Above" means in the vertical direction (plumb line) opposite to the gravity direction at that location (nadir). The zenith is the "highest" point on the celestial sphere.
Origin
The word zenith derives from an inaccurate reading of the Arabic expression (), meaning "direction of the head" or "path above the head", by Medieval Latin scribes in the Middle Ages (during the 14th century), possibly through Old Spanish. It was reduced to samt ("direction") and miswritten as senit/cenit, the m being misread as ni. Through the Old French cenith, zenith first appeared in the 17th century.
Relevance and use
The term zenith sometimes means the highest point, way, or level reached by a celestial body on its daily apparent path around a given point of observation. This sense of the word is often used to describe the position of the Sun ("The sun reached its zenith..."), but to an astronomer, the Sun does not have its own zenith and is at the zenith only if it is directly overhead.
In a scientific context, the zenith is the direction of reference for measuring the zenith angle (or zenith angular distance), the angle between a direction of interest (e.g. a star) and the local zenith - that is, the complement of the altitude angle (or elevation angle).
The Sun reaches the observer's zenith when it is 90° above the horizon, and this only happens between the Tropic of Cancer and the Tropic of Capricorn. The point where this occurs is known as the subsolar point. In Islamic astronomy, the passing of the Sun over the zenith of Mecca becomes the basis of the qibla observation by shadows twice a year on 27/28 May and 15/16 July.
At a given location during the course of a day, the Sun reaches not only its zenith but also its nadir, at the antipode of that location 12 hours from solar noon.
In astronomy, the altitude in the horizontal coordinate system and the zenith angle are complementary angles, with the horizon perpendicular to the zenith. The astronomical meridian is also determined by the zenith, and is defined as a circle on the celestial sphere that passes through the zenith, nadir, and the celestial poles. | Zenith | Wikipedia | 492 | 48909 | https://en.wikipedia.org/wiki/Zenith | Physical sciences | Celestial sphere: General | Astronomy |
A zenith telescope is a type of telescope designed to point straight up at or near the zenith, and used for precision measurement of star positions, to simplify telescope construction, or both. The NASA Orbital Debris Observatory and the Large Zenith Telescope are both zenith telescopes, since the use of liquid mirrors meant these telescopes could only point straight up.
On the International Space Station, zenith and nadir are used instead of up and down, referring to directions within and around the station, relative to the earth.
Zenith star
Zenith stars (also "star on top", "overhead star", "latitude star") are stars whose declination equals the latitude of the observers location, and hence at some time in the day or night pass culminate (pass) through the zenith. When at the zenith the right ascension of the star equals the local sidereal time at your location. In celestial navigation this allows latitude to be determined, since the declination of the star equals the latitude of the observer. If the current time at Greenwich is known at the time of the observation, the observers longitude can also be determined from the right ascension of the star. Hence "Zenith stars" lie on or near the circle of declination equal to the latitude of the observer ("zenith circle"). Zenith stars are not to be confused with "steering stars" of a sidereal compass rose of a sidereal compass. | Zenith | Wikipedia | 284 | 48909 | https://en.wikipedia.org/wiki/Zenith | Physical sciences | Celestial sphere: General | Astronomy |
The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This curve divides all viewing directions based on whether it intersects the relevant body's surface or not.
The true horizon is a theoretical line, which can only be observed to any degree of accuracy when it lies along a relatively smooth surface such as that of Earth's oceans. At many locations, this line is obscured by terrain, and on Earth it can also be obscured by life forms such as trees and/or human constructs such as buildings. The resulting intersection of such obstructions with the sky is called the visible horizon. On Earth, when looking at a sea from a shore, the part of the sea closest to the horizon is called the offing.
The true horizon surrounds the observer and it is typically assumed to be a circle, drawn on the surface of a perfectly spherical model of the relevant celestial body, i.e., a small circle of the local osculating sphere. With respect to Earth, the center of the true horizon is below the observer and below sea level. Its radius or horizontal distance from the observer varies slightly from day to day due to atmospheric refraction, which is greatly affected by weather conditions. Also, the higher the observer's eyes are from sea level, the farther away the horizon is from the observer. For instance, in standard atmospheric conditions, for an observer with eye level above sea level by , the horizon is at a distance of about .
When observed from very high standpoints, such as a space station, the horizon is much farther away and it encompasses a much larger area of Earth's surface. In this case, the horizon would no longer be a perfect circle, not even a plane curve such as an ellipse, especially when the observer is above the equator, as the Earth's surface can be better modeled as an oblate ellipsoid than as a sphere.
Etymology
The word horizon derives from the Greek () 'separating circle', where is from the verb ὁρίζω () 'to divide, to separate', which in turn derives from () 'boundary, landmark'.
Appearance and usage | Horizon | Wikipedia | 457 | 48910 | https://en.wikipedia.org/wiki/Horizon | Physical sciences | Celestial sphere | null |
Historically, the distance to the visible horizon has long been vital to survival and successful navigation, especially at sea, because it determined an observer's maximum range of vision and thus of communication, with all the obvious consequences for safety and the transmission of information that this range implied. This importance lessened with the development of the radio and the telegraph, but even today, when flying an aircraft under visual flight rules, a technique called attitude flying is used to control the aircraft, where the pilot uses the visual relationship between the aircraft's nose and the horizon to control the aircraft. Pilots can also retain their spatial orientation by referring to the horizon.
In many contexts, especially perspective drawing, the curvature of the Earth is disregarded and the horizon is considered the theoretical line to which points on any horizontal plane converge (when projected onto the picture plane) as their distance from the observer increases. For observers near sea level, the difference between this geometrical horizon (which assumes a perfectly flat, infinite ground plane) and the true horizon (which assumes a spherical Earth surface) is imperceptible to the unaided eye. However, for someone on a hill looking out across the sea, the true horizon will be about a degree below a horizontal line.
In astronomy, the horizon is the horizontal plane through the eyes of the observer. It is the fundamental plane of the horizontal coordinate system, the locus of points that have an altitude of zero degrees. While similar in ways to the geometrical horizon, in this context a horizon may be considered to be a plane in space, rather than a line on a picture plane.
Distance to the horizon
Ignoring the effect of atmospheric refraction, distance to the true horizon from an observer close to the Earth's surface is about
where h is height above sea level and R is the Earth radius.
The expression can be simplified as:
where the constant equals k.
In this equation, Earth's surface is assumed to be perfectly spherical, with R equal to about . | Horizon | Wikipedia | 400 | 48910 | https://en.wikipedia.org/wiki/Horizon | Physical sciences | Celestial sphere | null |
Examples
Assuming no atmospheric refraction and a spherical Earth with radius R=:
For an observer standing on the ground with h = , the horizon is at a distance of .
For an observer standing on the ground with h = , the horizon is at a distance of .
For an observer standing on a hill or tower above sea level, the horizon is at a distance of .
For an observer standing on a hill or tower above sea level, the horizon is at a distance of .
For an observer standing on the roof of the Burj Khalifa, from ground, and about above sea level, the horizon is at a distance of .
For an observer atop Mount Everest ( in altitude), the horizon is at a distance of .
For an observer aboard a commercial passenger plane flying at a typical altitude of , the horizon is at a distance of .
For a U-2 pilot, whilst flying at its service ceiling , the horizon is at a distance of .
Other planets
On terrestrial planets and other solid celestial bodies with negligible atmospheric effects, the distance to the horizon for a "standard observer" varies as the square root of the planet's radius. Thus, the horizon on Mercury is 62% as far away from the observer as it is on Earth, on Mars the figure is 73%, on the Moon the figure is 52%, on Mimas the figure is 18%, and so on.
Derivation
If the Earth is assumed to be a featureless sphere (rather than an oblate spheroid) with no atmospheric refraction, then the distance to the horizon can easily be calculated.
The tangent-secant theorem states that
Make the following substitutions:
d = OC = distance to the horizon
D = AB = diameter of the Earth
h = OB = height of the observer above sea level
D+h = OA = diameter of the Earth plus height of the observer above sea level,
with d, D, and h all measured in the same units. The formula now becomes
or
where R is the radius of the Earth.
The same equation can also be derived using the Pythagorean theorem.
At the horizon, the line of sight is a tangent to the Earth and is also perpendicular to Earth's radius.
This sets up a right triangle, with the sum of the radius and the height as the hypotenuse.
With
d = distance to the horizon
h = height of the observer above sea level
R = radius of the Earth
referring to the second figure at the right leads to the following: | Horizon | Wikipedia | 509 | 48910 | https://en.wikipedia.org/wiki/Horizon | Physical sciences | Celestial sphere | null |
The exact formula above can be expanded as:
where R is the radius of the Earth (R and h must be in the same units). For example,
if a satellite is at a height of 2000 km, the distance to the horizon is ;
neglecting the second term in parentheses would give a distance of , a 7% error.
Approximation
If the observer is close to the surface of the Earth, then it is valid to disregard h in the term , and the formula becomes-
Using kilometres for d and R, and metres for h, and taking the radius of the Earth as 6371 km, the distance to the horizon is
.
Using imperial units, with d and R in statute miles (as commonly used on land), and h in feet, the distance to the horizon is
.
If d is in nautical miles, and h in feet, the constant factor is about 1.06, which is close enough to 1 that it is often ignored, giving:
These formulas may be used when h is much smaller than the radius of the Earth (6371 km or 3959 mi), including all views from any mountaintops, airplanes, or high-altitude balloons. With the constants as given, both the metric and imperial formulas are precise to within 1% (see the next section for how to obtain greater precision).
If h is significant with respect to R, as with most satellites, then the approximation is no longer valid, and the exact formula is required.
Related measures
Arc distance
Another relationship involves the great-circle distance s along the arc over the curved surface of the Earth to the horizon; this is more directly comparable to the geographical distance on a map.
It can be formulated in terms of γ in radians,
then
Solving for s gives
The distance s can also be expressed in terms of the line-of-sight distance d; from the second figure at the right,
substituting for γ and rearranging gives
The distances d and s are nearly the same when the height of the object is negligible compared to the radius (that is, h ≪ R).
Zenith angle
When the observer is elevated, the horizon zenith angle can be greater than 90°. The maximum visible zenith angle occurs when the ray is tangent to Earth's surface; from triangle OCG in the figure at right,
where is the observer's height above the surface and is the angular dip of the horizon. It is related to the horizon zenith angle by: | Horizon | Wikipedia | 501 | 48910 | https://en.wikipedia.org/wiki/Horizon | Physical sciences | Celestial sphere | null |
For a non-negative height , the angle is always ≥ 90°.
Objects above the horizon
To compute the greatest distance DBL at which an observer B can see the top of an object L above the horizon, simply add the distances to the horizon from each of the two points:
DBL = DB + DL
For example, for an observer B with a height of hB1.70 m standing on the ground, the horizon is DB4.65 km away. For a tower with a height of hL100 m, the horizon distance is DL35.7 km. Thus an observer on a beach can see the top of the tower as long as it is not more than DBL40.35 km away. Conversely, if an observer on a boat (hB1.7m) can just see the tops of trees on a nearby shore (hL10m), the trees are probably about DBL16 km away.
Referring to the figure at the right, and using the approximation above, the top of the lighthouse will be visible to a lookout in a crow's nest at the top of a mast of the boat if
where DBL is in kilometres and hB and hL are in metres.
As another example, suppose an observer, whose eyes are two metres above the level ground, uses binoculars to look at a distant building which he knows to consist of thirty storeys, each 3.5 metres high. He counts the stories he can see and finds there are only ten. So twenty stories or 70 metres of the building are hidden from him by the curvature of the Earth. From this, he can calculate his distance from the building:
which comes to about 35 kilometres.
It is similarly possible to calculate how much of a distant object is visible above the horizon. Suppose an observer's eye is 10 metres above sea level, and he is watching a ship that is 20 km away. His horizon is:
kilometres from him, which comes to about 11.3 kilometres away. The ship is a further 8.7 km away. The height of a point on the ship that is just visible to the observer is given by:
which comes to almost exactly six metres. The observer can therefore see that part of the ship that is more than six metres above the level of the water. The part of the ship that is below this height is hidden from him by the curvature of the Earth. In this situation, the ship is said to be hull-down.
Effect of atmospheric refraction | Horizon | Wikipedia | 506 | 48910 | https://en.wikipedia.org/wiki/Horizon | Physical sciences | Celestial sphere | null |
Due to atmospheric refraction the distance to the visible horizon is further than the distance based on a simple geometric calculation. If the ground (or water) surface is colder than the air above it, a cold, dense layer of air forms close to the surface, causing light to be refracted downward as it travels, and therefore, to some extent, to go around the curvature of the Earth. The reverse happens if the ground is hotter than the air above it, as often happens in deserts, producing mirages. As an approximate compensation for refraction, surveyors measuring distances longer than 100 meters subtract 14% from the calculated curvature error and ensure lines of sight are at least 1.5 metres from the ground, to reduce random errors created by refraction.
If the Earth were an airless world like the Moon, the above calculations would be accurate. However, Earth has an atmosphere of air, whose density and refractive index vary considerably depending on the temperature and pressure. This makes the air refract light to varying extents, affecting the appearance of the horizon. Usually, the density of the air just above the surface of the Earth is greater than its density at greater altitudes. This makes its refractive index greater near the surface than at higher altitudes, which causes light that is travelling roughly horizontally to be refracted downward. This makes the actual distance to the horizon greater than the distance calculated with geometrical formulas. With standard atmospheric conditions, the difference is about 8%. This changes the factor of 3.57, in the metric formulas used above, to about 3.86. For instance, if an observer is standing on seashore, with eyes 1.70 m above sea level, according to the simple geometrical formulas given above the horizon should be 4.7 km away. Actually, atmospheric refraction allows the observer to see 300 metres farther, moving the true horizon 5 km away from the observer. | Horizon | Wikipedia | 389 | 48910 | https://en.wikipedia.org/wiki/Horizon | Physical sciences | Celestial sphere | null |
This correction can be, and often is, applied as a fairly good approximation when atmospheric conditions are close to standard. When conditions are unusual, this approximation fails. Refraction is strongly affected by temperature gradients, which can vary considerably from day to day, especially over water. In extreme cases, usually in springtime, when warm air overlies cold water, refraction can allow light to follow the Earth's surface for hundreds of kilometres. Opposite conditions occur, for example, in deserts, where the surface is very hot, so hot, low-density air is below cooler air. This causes light to be refracted upward, causing mirage effects that make the concept of the horizon somewhat meaningless. Calculated values for the effects of refraction under unusual conditions are therefore only approximate. Nevertheless, attempts have been made to calculate them more accurately than the simple approximation described above.
Outside the visual wavelength range, refraction will be different. For radar (e.g. for wavelengths 300 to 3 mm i.e. frequencies between 1 and 100 GHz) the radius of the Earth may be multiplied by 4/3 to obtain an effective radius giving a factor of 4.12 in the metric formula i.e. the radar horizon will be 15% beyond the geometrical horizon or 7% beyond the visual. The 4/3 factor is not exact, as in the visual case the refraction depends on atmospheric conditions.
Integration method—Sweer
If the density profile of the atmosphere is known, the distance d to the horizon is given by
where RE is the radius of the Earth, ψ is the dip of the horizon and δ is the refraction of the horizon. The dip is determined fairly simply from
where h is the observer's height above the Earth, μ is the index of refraction of air at the observer's height, and μ0 is the index of refraction of air at Earth's surface.
The refraction must be found by integration of
where is the angle between the ray and a line through the center of the Earth. The angles ψ and are related by
Simple method—Young
A much simpler approach, which produces essentially the same results as the first-order approximation described above, uses the geometrical model but uses a radius . The distance to the horizon is then
Taking the radius of the Earth as 6371 km, with d in km and h in m,
with d in mi and h in ft,
In the case of radar one typically has resulting (with d in km and h in m) in | Horizon | Wikipedia | 510 | 48910 | https://en.wikipedia.org/wiki/Horizon | Physical sciences | Celestial sphere | null |
Results from Young's method are quite close to those from Sweer's method, and are sufficiently accurate for many purposes.
Vanishing points
The horizon is a key feature of the picture plane in the science of graphical perspective. Assuming the picture plane stands vertical to ground, and P is the perpendicular projection of the eye point O on the picture plane, the horizon is defined as the horizontal line through P. The point P is the vanishing point of lines perpendicular to the picture. If S is another point on the horizon, then it is the vanishing point for all lines parallel to OS. But Brook Taylor (1719) indicated that the horizon plane determined by O and the horizon was like any other plane:
The term of Horizontal Line, for instance, is apt to confine the Notions of a Learner to the Plane of the Horizon, and to make him imagine, that that Plane enjoys some particular Privileges, which make the Figures in it more easy and more convenient to be described, by the means of that Horizontal Line, than the Figures in any other plane;…But in this Book I make no difference between the Plane of the Horizon, and any other Plane whatsoever...
The peculiar geometry of perspective where parallel lines converge in the distance, stimulated the development of projective geometry which posits a point at infinity where parallel lines meet. In her book Geometry of an Art (2007), Kirsti Andersen described the evolution of perspective drawing and science up to 1800, noting that vanishing points need not be on the horizon. In a chapter titled "Horizon", John Stillwell recounted how projective geometry has led to incidence geometry, the modern abstract study of line intersection. Stillwell also ventured into foundations of mathematics in a section titled "What are the Laws of Algebra ?" The "algebra of points", originally given by Karl von Staudt deriving the axioms of a field was deconstructed in the twentieth century, yielding a wide variety of mathematical possibilities. Stillwell states
This discovery from 100 years ago seems capable of turning mathematics upside down, though it has not yet been fully absorbed by the mathematical community. Not only does it defy the trend of turning geometry into algebra, it suggests that both geometry and algebra have a simpler foundation than previously thought. | Horizon | Wikipedia | 456 | 48910 | https://en.wikipedia.org/wiki/Horizon | Physical sciences | Celestial sphere | null |
Characiformes is an order of ray-finned fish, comprising the characins and their allies. Grouped in 18 recognized families, more than 2000 different species are described, including the well-known piranha and tetras.
Taxonomy
The Characiformes form part of a series called the Otophysi within the superorder Ostariophysi. The Otophysi contain three other orders, Cypriniformes, Siluriformes, and Gymnotiformes. The Characiformes form a group known as the Characiphysi with the Siluriformes and Gymnotiformes. The order Characiformes is the sister group to the orders Siluriformes and Gymnotiformes, though this has been debated in light of recent molecular evidence.
Originally, the characins were all grouped within a single family, the Characidae. Since then, 18 different families have been separated out. However, classification varies somewhat, and the most recent (2011) study confirms the circumscribed Characidae as monophyletic. Currently, 18 families, about 270 genera, and at least 1674 species are known.
The suborder Citharinoidei, which contains the families Distichodontidae and Citharinidae, is considered the sister group to the rest of the characins, suborder Characoidei. This group has a very ancient divergence from the rest of the Characiformes, dating back to the Early Cretaceous or earlier, and it has been suggested that it be better treated as its own order, the Cithariniformes.
Evolution
The Characiformes likely first originated and diversified on the supercontinent of West Gondwana (composed of modern Africa and South America) during the Cretaceous period, though fossils are poorly known. During the Cretaceous Period, the rift between South America and Africa would be forming; this may explain the contrast in diversity between the two continents. Their low diversity in Africa may explain why some primitive fish families and the Cypriniformes coexist with them whereas they are absent in South America, where these fish may have been driven extinct. The characiforms had not spread into Africa soon enough to also reach the land connection between Africa and Asia. The earliest they could have spread into Central America was the late Miocene. | Characiformes | Wikipedia | 494 | 48963 | https://en.wikipedia.org/wiki/Characiformes | Biology and health sciences | Characiformes | null |
Fossils
The earliest characiform fossils date back to the Late Cretaceous, around the Santonian. Other fossil teeth date back to the Cenomanian of Morocco, but it has been suggested that these teeth may be of early ginglymodians. Previously, the oldest characiform was assumed to be Santanichthys of the Early Cretaceous (Albian Age) of Brazil. This presumably marine taxon was used as evidence of characiformes potentially having marine origins. However, more recent studies indicate that Santanaichthys is likely a basal otophysan rather than a characiform. Similarly, Salminops from Spain and Sorbinicharax from Italy, previously also considered potential marine characiforms, are now thought to have no characiform affinities and are considered indeterminate teleosts. Given this, there is no paleontological support for characiforms having marine origins.
Uniquely, Late Cretaceous characiform fossils are found significantly north of their modern distribution. Indeterminate characiform teeth are known from the Santonian of Hungary and Maastrichtian of France, which have a large, multi-cusped appearance reminiscent of African alestids. Similarly, two Campanian freshwater characiform genera, Primuluchara and Eotexachara, are known from North America, with Primuluchara having a very wide distribution across Laramidia, ranging from Texas to as far north as southern Canada (Dinosaur Park Formation). It is likely that the warmer conditions of the Late Cretaceous allowed early characins to range farther north than the present day, with African characins colonizing Europe and South American characins colonizing North America. Early characins may have had some level of salt tolerance, allowing for such colonizations to take place.
Within their modern distribution, a number of modern South American characin families have their earliest occurrences in the Maastrichtian of Bolivia, with isolated teeth and skeletal elements identifiable to Acestrorhynchidae, Characidae, and Serrasalmidae.
Phylogeny
Below is a phylogeny of living Characiformes based on Betancur-Rodriguez et al. 2017 and Nelson, Grande & Wilson 2016. | Characiformes | Wikipedia | 464 | 48963 | https://en.wikipedia.org/wiki/Characiformes | Biology and health sciences | Characiformes | null |
Description
Characins possess a Weberian apparatus, a series of bony parts connecting the swim bladder and inner ear. Superficially, the Characiformes somewhat resemble their relatives of the order Cypriniformes, but have a small, fleshy adipose fin between the dorsal fin and tail. Most species have teeth within the mouth, since they are often carnivorous. The body is almost always covered in well-defined scales. The mouth is also usually not truly protractile.
The largest characins are Hydrocynus goliath and Salminus franciscanus and Hoplias aimara, both of which are up to . The smallest in size is about in the Bolivian pygmy blue characin, Xenurobrycon polyancistrus. Many members are under .
Distribution and habitat
Characins are most diverse in the Neotropics, where they are found in lakes and rivers throughout most of South and Central America. The red-bellied piranha, a member of the family Serrasalmidae within the Characiformes, is endemic to the Neotropical realm. At least 209 species of characins are found in Africa, including the distichodontids, citharinids, alestids, and hepsetids. The rest of the characins originate from the Americas.
Relationship to humans
A few characins become quite large, and are important as food or game. Most, however, are small shoaling fish. Many species commonly called tetras are popular in aquaria because of their bright colors, general hardiness, and tolerance towards other fish in community tanks. | Characiformes | Wikipedia | 341 | 48963 | https://en.wikipedia.org/wiki/Characiformes | Biology and health sciences | Characiformes | null |
Characidae, the characids or characins, is a family of freshwater subtropical and tropical fish belonging to the order Characiformes. The name "characins" is a historical one, but scientists today tend to prefer "characids" to reflect their status as a, by and large, monophyletic group (at family rank). To arrive there, this family has undergone much systematic and taxonomic change. Among those fishes remaining in the Characidae currently are the tetras, comprising the very similar genera Hemigrammus and Hyphessobrycon, as well as a few related forms, such as the cave and neon tetras. Fish of this family are important as food in several regions, and also constitute a large percentage of captive freshwater aquarium fish species.
These fish vary in length; many are less than . One of the smallest species, Hyphessobrycon roseus, grows to a maximum length of 1.9 cm.
These fish inhabit a wide range and variety of habitats. New World fishes, they originate in the Americas, ranging from southwestern Texas and México through most of Central and South America, including such major waterways as the Amazon and Orinoco Rivers. Many of these fish come from rivers and tributaries, while the blind cave tetra, for example, inhabits flooded caves.
Systematics
This family has undergone a large amount of systematic and taxonomic change. More recent revision has moved many former members of the family into their own related but distinct families – the pencilfishes of the genus Nannostomus are a typical example, having now been moved into the Lebiasinidae, the assorted predatory species belonging to Hoplias and Hoplerythrinus have now been moved into the Erythrinidae, and the sabre-toothed fishes of the genus Hydrolycus have been moved into the Cynodontidae. The former subfamily Alestiinae was promoted to family level (Alestiidae) and the subfamilies Crenuchinae and Characidiinae were moved to the family Crenuchidae. | Characidae | Wikipedia | 428 | 48971 | https://en.wikipedia.org/wiki/Characidae | Biology and health sciences | Characiformes | Animals |
Other fish families that were formerly classified as members of the Characidae, but which were moved into separate families of their own during recent taxonomic revisions (after 1994) include Acestrorhynchidae, Anostomidae, Chilodontidae, Citharinidae, Ctenoluciidae, Curimatidae, Distichodontidae, Gasteropelecidae, Hemiodontidae, Hepsetidae, Parodontidae, Prochilodontidae, Serrasalmidae, and Triportheidae.
The larger piranhas were originally classified as belonging to the Characidae, but various revisions place them in their own related family, the Serrasalmidae. This reassignment has yet to enjoy universal acceptance, but is gaining in popularity among taxonomists working with these fishes. Given the current state of flux of the Characidae, a number of other changes will doubtless take place, reassigning once-familiar species to other families. Indeed, the entire phylogeny of the Ostariophysi – fishes possessing a Weberian apparatus – has yet to be settled conclusively. Until that phylogeny is settled, the opportunity for yet more upheavals within the taxonomy of the characoid fishes is considerable.
Classification
Phylogeny
Taxonomy
The subfamilies and tribes currently recognized by most if not all authors, and their respective genera, are:
Subfamily Spintherobolus clade
Amazonspinther
Spintherobolus
Subfamily Stethaprioninae | Characidae | Wikipedia | 316 | 48971 | https://en.wikipedia.org/wiki/Characidae | Biology and health sciences | Characiformes | Animals |
Tribe Rhoadsiini [Astyanax clade]
Astyanacinus
Astyanax
Carlana
Ctenobrycon
Inpaichthys
Nematobrycon
Oligosarcus
Parastremma
Psellogrammus
Rhoadsia
Tribe Stygichthyini [Jupiaba clade]
Coptobrycon
Erythrocharax
Jupiaba
Macropsobrycon
Parecbasis
Stygichthys
Tribe Pristellini [Hemigrammus clade; Aphyoditini]
Aphyodite
Atopomesus
Axelrodia
Brittanichthys
Bryconella
Nematocharax
Phycocharax
Tribe Stethaprionini
Brachychalcinus
Gymnocorymbus
Orthospinus
Poptella
Stethaprion
Stichonodon
Tribe Gymnocharacini
Andromakhe
Dectobrycon
Grundulus
Gymnocharacinus
Hollandichthys
Moenkhausia
Psalidodon
Pseudochalceus
Rachoviscus
Schultzites
Tribe Scissorini
Genycharax
Leptobrycon
Microschemobrycon
Mixobrycon
Oligobrycon
Oxybrycon
Scissor
Serrabrycon
Thrissobrycon
Tucanoichthys
Tyttobrycon
Subfamily Stevardiinae | Characidae | Wikipedia | 273 | 48971 | https://en.wikipedia.org/wiki/Characidae | Biology and health sciences | Characiformes | Animals |
Tribe Eretmobryconini
Eretmobrycon
Markiana
Tribe Xenurobryconini
Iotabrycon
Ptychocharax
Scopaeocharax
Tyttocharax
Xenurobrycon
Tribe Argopleura clade
Argopleura
Tribe Glandulocaudini
Glandulocauda
Lophiobrycon
Mimagoniates
Tribe Stevardiini
Chrysobrycon
Corynopoma
Gephyrocharax
Hysteronotus
Pseudocorynopoma
Pterobrycon
Tribe Hemibryconini
Acrobrycon
Boehlkea
Hemibrycon
Tribe Creagrutini
Carlastyanax
Creagrutus
Tribe Landonini
Landonia
Tribe Phenacobryconini
Phenacobrycon
Tribe Trochilocharacini
Trochilocharax
Tribe Diapomini
Attonitus
Aulixidens
Bryconacidnus
Bryconadenos
Bryconamericus
Caiapobrycon
Ceratobranchia
Cyanogaster
Diapoma
Hypobrycon
Knodus
Lepidocharax
Microgenys
Monotocheirodon
Othonocheirodus
Phallobrycon
Piabarchus
Piabina
Planaltina
Rhinobrycon
Rhinopetitia
Subfamily Characinae
Tribe Protocheirodontini
Protocheirodon
Tribe Pseudocheirodontini
Nanocheirodon
Pseudocheirodon
Tribe Aphyocharacini
Aphyocharacidium
Aphyocharax
Leptagoniates
Inpaichthys
Paragoniates
Phenagoniates
Prionobrama
Xenagoniates
Tribe Cheirodontini
Cheirodon
Heterocheirodon
Prodontocharax
Saccoderma
Tribe Compsurini
Acinocheirodon
Aphyocheirodon
Cheirodontops
Compsura
Ctenocheirodon
Kolpotocheirodon
Odontostilbe
Serrapinnus
Tribe Exodontini
Bryconexodon
Exodon
Roeboexodon
Tribe Tetragonopterini
Tetragonopterus
Tribe Characini
Acanthocharax
Acestrocephalus
Charax
Cynopotamus
Galeocharax
Phenacogaster
Priocharax
Roeboides
Subfamily Pristellinae | Characidae | Wikipedia | 474 | 48971 | https://en.wikipedia.org/wiki/Characidae | Biology and health sciences | Characiformes | Animals |
Bario
Deuterodon
Ectrepopterus
Hasemania
Hemigrammus
Hyphessobrycon
Moenkhausia
Myxiops
Paracheirodon
Parapristella
Petitella
Pristella
Probolodus
Thayeria
Former members
The Chalceidae, Iguanodectidae, Bryconidae and Heterocharacinae are the most recent clades to be removed in order to maintain a monophyletic Characidae.
Subfamily Iguanodectinae moved to Iguanodectidae
Bryconops
Iguanodectes
Piabucus
Subfamily Heterocharacinae moved to Acestrorhynchidae
Gnathocharax
Heterocharax
Hoplocharax
Lonchogenys
Subfamily Bryconinae moved to Bryconidae
Brycon
Chilobrycon
Henochilus
Subfamily Salmininae moved to Bryconidae
Salminus
Genera incertae sedis
Chalceus moved to Chalceidae
Genera incertae sedis
A large number of taxa in this family are incertae sedis. The relationships of many fish in this family – in particular species traditionally placed in the Tetragonopterinae, which had become something of a "wastebin taxon" – are poorly known, a comprehensive phylogenetic study for the entire family is needed. The genera Hyphessobrycon, Astyanax, Hemigrammus, Moenkhausia, and Bryconamericus include the largest number of currently recognized species
among characid fishes that are in need of revision; Astyanax and Hyphessobrycon in the usual delimitation are among the largest genera in this family. These genera were originally proposed between 1854 and 1908 and are still more or less defined as by Carl H. Eigenmann in 1917, though diverse species have been added to each genus since that time. The anatomical diversity within each genus, the fact that each of these generic groups at the present time cannot be well-defined, and the high number of species involved are the major reasons for the lack of phylogenetic analyses dealing with the relationships of the species within these generic "groups". | Characidae | Wikipedia | 448 | 48971 | https://en.wikipedia.org/wiki/Characidae | Biology and health sciences | Characiformes | Animals |
Basidiomycota () is one of two large divisions that, together with the Ascomycota, constitute the subkingdom Dikarya (often referred to as the "higher fungi") within the kingdom Fungi. Members are known as basidiomycetes. More specifically, Basidiomycota includes these groups: agarics, puffballs, stinkhorns, bracket fungi, other polypores, jelly fungi, boletes, chanterelles, earth stars, smuts, bunts, rusts, mirror yeasts, and Cryptococcus, the human pathogenic yeast.
Basidiomycota are filamentous fungi composed of hyphae (except for basidiomycota-yeast) and reproduce sexually via the formation of specialized club-shaped end cells called basidia that normally bear external meiospores (usually four). These specialized spores are called basidiospores. However, some Basidiomycota are obligate asexual reproducers. Basidiomycota that reproduce asexually (discussed below) can typically be recognized as members of this division by gross similarity to others, by the formation of a distinctive anatomical feature (the clamp connection), cell wall components, and definitively by phylogenetic molecular analysis of DNA sequence data.
Classification | Basidiomycota | Wikipedia | 273 | 48980 | https://en.wikipedia.org/wiki/Basidiomycota | Biology and health sciences | Fungi | null |
A 2007 classification, adopted by a coalition of 67 mycologists recognized three subphyla (Pucciniomycotina, Ustilaginomycotina, Agaricomycotina) and two other class level taxa (Wallemiomycetes, Entorrhizomycetes) outside of these, among the Basidiomycota. As now classified, the subphyla join and also cut across various obsolete taxonomic groups (see below) previously commonly used to describe Basidiomycota. According to a 2008 estimate, Basidiomycota comprise three subphyla (including six unassigned classes) 16 classes, 52 orders, 177 families, 1,589 genera, and 31,515 species.
Wijayawardene et al. 2020 produced an update that recognized 19 classes (Agaricomycetes, Agaricostilbomycetes, Atractiellomycetes, Bartheletiomycetes, Classiculomycetes, Cryptomycocolacomycetes, Cystobasidiomycetes, Dacrymycetes, Exobasidiomycetes, Malasseziomycetes, Microbotryomycetes, Mixiomycetes, Monilielliomycetes, Pucciniomycetes, Spiculogloeomycetes, Tremellomycetes, Tritirachiomycetes, Ustilaginomycetes and Wallemiomycetes) with multiple orders and genera.
Traditionally, the Basidiomycota were divided into two classes, now obsolete:
Homobasidiomycetes (alternatively called holobasidiomycetes), including true mushrooms
Heterobasidiomycetes, including the jelly, rust and smut fungi
Nonetheless these former concepts continue to be used as two types of growth habit groupings, the "mushrooms" (e.g. Schizophyllum commune) and the non-mushrooms (e.g. Mycosarcoma maydis). | Basidiomycota | Wikipedia | 417 | 48980 | https://en.wikipedia.org/wiki/Basidiomycota | Biology and health sciences | Fungi | null |
Agaricomycotina
The Agaricomycotina include what had previously been called the Hymenomycetes (an obsolete morphological based class of Basidiomycota that formed hymenial layers on their fruitbodies), the Gasteromycetes (another obsolete class that included species mostly lacking hymenia and mostly forming spores in enclosed fruitbodies), as well as most of the jelly fungi. This sub-phyla also includes the "classic" mushrooms, polypores, corals, chanterelles, crusts, puffballs and stinkhorns. The three classes in the Agaricomycotina are the Agaricomycetes, the Dacrymycetes, and the Tremellomycetes.
The class Wallemiomycetes is not yet placed in a subdivision, but recent genomic evidence suggests that it is a sister group of Agaricomycotina.
Pucciniomycotina
The Pucciniomycotina include the rust fungi, the insect parasitic/symbiotic genus Septobasidium, a former group of smut fungi (in the Microbotryomycetes, which includes mirror yeasts), and a mixture of odd, infrequently seen, or seldom recognized fungi, often parasitic on plants. The eight classes in the Pucciniomycotina are Agaricostilbomycetes, Atractiellomycetes, Classiculomycetes, Cryptomycocolacomycetes, Cystobasidiomycetes, Microbotryomycetes, Mixiomycetes, and Pucciniomycetes.
Ustilaginomycotina
The Ustilaginomycotina are most (but not all) of the former smut fungi and the Exobasidiales. The classes of the Ustilaginomycotina are the Exobasidiomycetes, the Entorrhizomycetes, and the Ustilaginomycetes.
Genera included
There are several genera classified in the Basidiomycota that are 1) poorly known, 2) have not been subjected to DNA analysis, or 3) if analysed phylogenetically do not group with as yet named or identified families, and have not been assigned to a specific family (i.e., they are incertae sedis with respect to familial placement). These include: | Basidiomycota | Wikipedia | 501 | 48980 | https://en.wikipedia.org/wiki/Basidiomycota | Biology and health sciences | Fungi | null |
Anastomyces W.P.Wu, B.Sutton & Gange (1997)
Anguillomyces Marvanová & Bärl. (2000)
Anthoseptobasidium Rick (1943)
Arcispora Marvanová & Bärl. (1998)
Arrasia Bernicchia, Gorjón & Nakasone (2011)
Brevicellopsis Hjortstam & Ryvarden (2008)
Celatogloea P.Roberts (2005)
Cleistocybe Ammirati, A.D.Parker & Matheny (2007)
Cystogloea P. Roberts (2006)
Dacryomycetopsis Rick (1958)
Eriocybe Vellinga (2011)
Hallenbergia Dhingra & Priyanka (2011)
Hymenoporus Tkalčec, Mešić & Chun Y.Deng (2015)
Kryptastrina Oberw. (1990)
Microstella K.Ando & Tubaki (1984)
Neotyphula Wakef. (1934)
Nodulospora Marvanová & Bärl. (2000)
Paraphelaria Corner (1966)
Punctulariopsis Ghob.-Nejh. (2010)
Radulodontia Hjortstam & Ryvarden (2008)
Restilago Vánky (2008)
Sinofavus W.Y.Zhuang (2008)
Zanchia Rick (1958)
Zygodesmus Corda (1837)
Zygogloea P.Roberts (1994)
Typical life cycle | Basidiomycota | Wikipedia | 328 | 48980 | https://en.wikipedia.org/wiki/Basidiomycota | Biology and health sciences | Fungi | null |
Unlike animals and plants which have readily recognizable male and female counterparts, Basidiomycota (except for the Rust (Pucciniales)) tend to have mutually indistinguishable, compatible haploids which are usually mycelia being composed of filamentous hyphae. Typically haploid Basidiomycota mycelia fuse via plasmogamy and then the compatible nuclei migrate into each other's mycelia and pair up with the resident nuclei. Karyogamy is delayed, so that the compatible nuclei remain in pairs, called a dikaryon. The hyphae are then said to be dikaryotic. Conversely, the haploid mycelia are called monokaryons. Often, the dikaryotic mycelium is more vigorous than the individual monokaryotic mycelia, and proceeds to take over the substrate in which they are growing. The dikaryons can be long-lived, lasting years, decades, or centuries. The monokaryons are male nor female. They have either a () or a () mating system. This results in the fact that following meiosis, the resulting haploid basidiospores and resultant monokaryons, have nuclei that are compatible with 50% (if bipolar) or 25% (if tetrapolar) of their sister basidiospores (and their resultant monokaryons) because the mating genes must differ for them to be compatible. However, there are sometimes more than two possible alleles for a given locus, and in such species, depending on the specifics, over 90% of monokaryons could be compatible with each other. | Basidiomycota | Wikipedia | 354 | 48980 | https://en.wikipedia.org/wiki/Basidiomycota | Biology and health sciences | Fungi | null |
The maintenance of the dikaryotic status in dikaryons in many Basidiomycota is facilitated by the formation of clamp connections that physically appear to help coordinate and re-establish pairs of compatible nuclei following synchronous mitotic nuclear divisions. Variations are frequent and multiple. In a typical Basidiomycota lifecycle the long lasting dikaryons periodically (seasonally or occasionally) produce basidia, the specialized usually club-shaped end cells, in which a pair of compatible nuclei fuse (karyogamy) to form a diploid cell. Meiosis follows shortly with the production of 4 haploid nuclei that migrate into 4 external, usually apical basidiospores. Variations occur, however. Typically the basidiospores are ballistic, hence they are sometimes also called ballistospores. In most species, the basidiospores disperse and each can start a new haploid mycelium, continuing the lifecycle. Basidia are microscopic but they are often produced on or in multicelled large fructifications called basidiocarps or basidiomes, or fruitbodies, variously called mushrooms, puffballs, etc. Ballistic basidiospores are formed on sterigmata which are tapered spine-like projections on basidia, and are typically curved, like the horns of a bull. In some Basidiomycota the spores are not ballistic, and the sterigmata may be straight, reduced to stubs, or absent. The basidiospores of these non-ballistosporic basidia may either bud off, or be released via dissolution or disintegration of the basidia. | Basidiomycota | Wikipedia | 351 | 48980 | https://en.wikipedia.org/wiki/Basidiomycota | Biology and health sciences | Fungi | null |
In summary, meiosis takes place in a diploid basidium. Each one of the four haploid nuclei migrates into its own basidiospore. The basidiospores are ballistically discharged and start new haploid mycelia called monokaryons. There are no males or females, rather there are compatible thalli with multiple compatibility factors. Plasmogamy between compatible individuals leads to delayed karyogamy leading to establishment of a dikaryon. The dikaryon is long lasting but ultimately gives rise to either fruitbodies with basidia or directly to basidia without fruitbodies. The paired dikaryon in the basidium fuse (i.e. karyogamy takes place). The diploid basidium begins the cycle again.
Meiosis
Coprinopsis cinerea is a basidiomycete mushroom. It is particularly suited to the study of meiosis because meiosis progresses synchronously in about 10 million cells within the mushroom cap, and the meiotic prophase stage is prolonged. Burns et al. studied the expression of genes involved in the 15-hour meiotic process, and found that the pattern of gene expression of C. cinerea was similar to two other fungal species, the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. These similarities in the patterns of expression led to the conclusion that the core expression program of meiosis has been conserved in these fungi for over half a billion years of evolution since these species diverged.
Cryptococcus neoformans and Mycosarcoma maydis are examples of pathogenic basidiomycota. Such pathogens must be able to overcome the oxidative defenses of their respective hosts in order to produce a successful infection. The ability to undergo meiosis may provide a survival benefit for these fungi by promoting successful infection. A characteristic central feature of meiosis is recombination between homologous chromosomes. This process is associated with repair of DNA damage, particularly double-strand breaks. The ability of C. neoformans and M. maydis to undergo meiosis may contribute to their virulence by repairing the oxidative DNA damage caused by their host's release of reactive oxygen species. | Basidiomycota | Wikipedia | 469 | 48980 | https://en.wikipedia.org/wiki/Basidiomycota | Biology and health sciences | Fungi | null |
Variations in lifecycles
Many variations occur: some variations are self-compatible and spontaneously form dikaryons without a separate compatible thallus being involved. These fungi are said to be homothallic, versus the normal heterothallic species with mating types. Others are secondarily homothallic, in that two compatible nuclei following meiosis migrate into each basidiospore, which is then dispersed as a pre-existing dikaryon. Often such species form only two spores per basidium, but that too varies. Following meiosis, mitotic divisions can occur in the basidium. Multiple numbers of basidiospores can result, including odd numbers via degeneration of nuclei, or pairing up of nuclei, or lack of migration of nuclei. For example, the chanterelle genus Craterellus often has six-spored basidia, while some corticioid Sistotrema species can have two-, four-, six-, or eight-spored basidia, and the cultivated button mushroom, Agaricus bisporus. can have one-, two-, three- or four-spored basidia under some circumstances. Occasionally, monokaryons of some taxa can form morphologically fully formed basidiomes and anatomically correct basidia and ballistic basidiospores in the absence of dikaryon formation, diploid nuclei, and meiosis. A rare few number of taxa have extended diploid lifecycles, but can be common species. Examples exist in the mushroom genera Armillaria and Xerula, both in the Physalacriaceae. Occasionally, basidiospores are not formed and parts of the "basidia" act as the dispersal agents, e.g. the peculiar mycoparasitic jelly fungus, Tetragoniomyces or the entire "basidium" acts as a "spore", e.g. in some false puffballs (Scleroderma). In the human pathogenic genus Cryptococcus, four nuclei following meiosis remain in the basidium, but continually divide mitotically, each nucleus migrating into synchronously forming nonballistic basidiospores that are then pushed upwards by another set forming below them, resulting in four parallel chains of dry "basidiospores".
Other variations occur: some as standard lifecycles (that themselves have variations within variations) within specific orders. | Basidiomycota | Wikipedia | 500 | 48980 | https://en.wikipedia.org/wiki/Basidiomycota | Biology and health sciences | Fungi | null |
Rusts (Pucciniales, previously known as Uredinales) at their greatest complexity, produce five different types of spores on two different host plants in two unrelated host families. Such rusts are heteroecious (requiring two hosts) and macrocyclic (producing all five spores types). Wheat stem rust is an example. By convention, the stages and spore states are numbered by Roman numerals. Typically, basidiospores infect host one, also known as the alternate or sexual host, and the mycelium forms pycnidia, which are miniature, flask-shaped, hollow, submicroscopic bodies embedded in the host tissue (such as a leaf). This stage, numbered "0", produces single-celled spores that ooze out in a sweet liquid and that act as nonmotile spermatia, and also protruding receptive hyphae. Insects and probably other vectors such as rain carry the spermatia from spermagonium to spermagonium, cross inoculating the mating types. Neither thallus is male or female. Once crossed, the dikaryons are established and a second spore stage is formed, numbered "I" and called aecia, which form dikaryotic aeciospores in dry chains in inverted cup-shaped bodies embedded in host tissue. These aeciospores then infect the second host, known as the primary or asexual host (in macrocyclic rusts). On the primary host a repeating spore stage is formed, numbered "II", the urediospores in dry pustules called uredinia. Urediospores are dikaryotic and can infect the same host that produced them. They repeatedly infect this host over the growing season. At the end of the season, a fourth spore type, the teliospore, is formed. It is thicker-walled and serves to overwinter or to survive other harsh conditions. It does not continue the infection process, rather it remains dormant for a period and then germinates to form basidia (stage "IV"), sometimes called a promycelium. In the Pucciniales, the basidia are cylindrical and become 3-septate after meiosis, with each of the 4 cells bearing one basidiospore each. The basidiospores disperse and start the infection process on host 1 again | Basidiomycota | Wikipedia | 507 | 48980 | https://en.wikipedia.org/wiki/Basidiomycota | Biology and health sciences | Fungi | null |
Autoecious rusts complete their life-cycles on one host instead of two, and microcyclic rusts cut out one or more stages | Basidiomycota | Wikipedia | 30 | 48980 | https://en.wikipedia.org/wiki/Basidiomycota | Biology and health sciences | Fungi | null |
Smuts
The characteristic part of the life-cycle of smuts is the thick-walled, often darkly pigmented, ornate, teliospore that serves to survive harsh conditions such as overwintering and also serves to help disperse the fungus as dry diaspores. The teliospores are initially dikaryotic but become diploid via karyogamy. Meiosis takes place at the time of germination. A promycelium is formed that consists of a short hypha (equated to a basidium). In some smuts such as Mycosarcoma maydis the nuclei migrate into the promycelium that becomes septate (i.e., divided into cellular compartments separated by cell walls called septa), and haploid yeast-like conidia/basidiospores sometimes called sporidia, bud off laterally from each cell. In various smuts, the yeast phase may proliferate, or they may fuse, or they may infect plant tissue and become hyphal. In other smuts, such as Tilletia caries, the elongated haploid basidiospores form apically, often in compatible pairs that fuse centrally resulting in H-shaped diaspores which are by then dikaryotic. Dikaryotic conidia may then form. Eventually the host is infected by infectious hyphae. Teliospores form in host tissue. Many variations on these general themes occur.
Smuts with both a yeast phase and an infectious hyphal state are examples of dimorphic Basidiomycota. In plant parasitic taxa, the saprotrophic phase is normally the yeast while the infectious stage is hyphal. However, there are examples of animal and human parasites where the species are dimorphic but it is the yeast-like state that is infectious. The genus Filobasidiella forms basidia on hyphae but the main infectious stage is more commonly known by the anamorphic yeast name Cryptococcus, e.g. Cryptococcus neoformans and Cryptococcus gattii. | Basidiomycota | Wikipedia | 434 | 48980 | https://en.wikipedia.org/wiki/Basidiomycota | Biology and health sciences | Fungi | null |
The dimorphic Basidiomycota with yeast stages and the pleiomorphic rusts are examples of fungi with anamorphs, which are the asexual stages. Some Basidiomycota are only known as anamorphs. Many are called basidiomycetous yeasts, which differentiates them from ascomycetous yeasts in the Ascomycota. Aside from yeast anamorphs and uredinia, aecia, and pycnidia, some Basidiomycota form other distinctive anamorphs as parts of their life cycles. Examples are Collybia tuberosa with its apple-seed-shaped and coloured sclerotium, Dendrocollybia racemosa with its sclerotium and its Tilachlidiopsis racemosa conidia, Armillaria with their rhizomorphs, Hohenbuehelia with their Nematoctonus nematode infectious, state and the coffee leaf parasite, Mycena citricolor, and its Decapitatus flavidus propagules called gemmae. | Basidiomycota | Wikipedia | 235 | 48980 | https://en.wikipedia.org/wiki/Basidiomycota | Biology and health sciences | Fungi | null |
Ascomycota is a phylum of the kingdom Fungi that, together with the Basidiomycota, forms the subkingdom Dikarya. Its members are commonly known as the sac fungi or ascomycetes. It is the largest phylum of Fungi, with over 64,000 species. The defining feature of this fungal group is the "ascus" (), a microscopic sexual structure in which nonmotile spores, called ascospores, are formed. However, some species of Ascomycota are asexual and thus do not form asci or ascospores. Familiar examples of sac fungi include morels, truffles, brewers' and bakers' yeast, dead man's fingers, and cup fungi. The fungal symbionts in the majority of lichens (loosely termed "ascolichens") such as Cladonia belong to the Ascomycota.
Ascomycota is a monophyletic group (containing all of the descendants of a common ancestor). Previously placed in the Basidiomycota along with asexual species from other fungal taxa, asexual (or anamorphic) ascomycetes are now identified and classified based on morphological or physiological similarities to ascus-bearing taxa, and by phylogenetic analyses of DNA sequences.
Ascomycetes are of particular use to humans as sources of medicinally important compounds such as antibiotics, as well as for fermenting bread, alcoholic beverages, and cheese. Examples of ascomycetes include Penicillium species on cheeses and those producing antibiotics for treating bacterial infectious diseases.
Many ascomycetes are pathogens, both of animals, including humans, and of plants. Examples of ascomycetes that can cause infections in humans include Candida albicans, Aspergillus niger and several tens of species that cause skin infections. The many plant-pathogenic ascomycetes include apple scab, rice blast, the ergot fungi, black knot, and the powdery mildews. The members of the genus Cordyceps are entomopathogenic fungi, meaning that they parasitise and kill insects. Other entomopathogenic ascomycetes have been used successfully in biological pest control, such as Beauveria. | Ascomycota | Wikipedia | 479 | 48981 | https://en.wikipedia.org/wiki/Ascomycota | Biology and health sciences | Fungi | null |
Several species of ascomycetes are biological model organisms in laboratory research. Most famously, Neurospora crassa, several species of yeasts, and Aspergillus species are used in many genetics and cell biology studies.
Sexual reproduction in ascomycetes
Ascomycetes are 'spore shooters'. They are fungi which produce microscopic spores inside special, elongated cells or sacs, known as 'asci', which give the group its name.
Asexual reproduction is the dominant form of propagation in the Ascomycota, and is responsible for the rapid spread of these fungi into new areas. Asexual reproduction of ascomycetes is very diverse from both structural and functional points of view. The most important and general is production of conidia, but chlamydospores are also frequently produced. Furthermore, Ascomycota also reproduce asexually through budding.
Conidia formation
Asexual reproduction may occur through vegetative reproductive spores, the conidia. The asexual, non-motile haploid spores of a fungus, which are named after the Greek word for dust (conia), are hence also known as . The conidiospores commonly contain one nucleus and are products of mitotic cell divisions and thus are sometimes called , which are genetically identical to the mycelium from which they originate. They are typically formed at the ends of specialized hyphae, the conidiophores. Depending on the species they may be dispersed by wind or water, or by animals. Conidiophores may simply branch off from the mycelia or they may be formed in fruiting bodies.
The hypha that creates the sporing (conidiating) tip can be very similar to the normal hyphal tip, or it can be differentiated. The most common differentiation is the formation of a bottle shaped cell called a , from which the spores are produced. Not all of these asexual structures are a single hypha. In some groups, the conidiophores (the structures that bear the conidia) are aggregated to form a thick structure.
E.g. In the order Moniliales, all of them are single hyphae with the exception of the aggregations, termed as coremia or synnema. These produce structures rather like corn-stokes, with many conidia being produced in a mass from the aggregated conidiophores. | Ascomycota | Wikipedia | 507 | 48981 | https://en.wikipedia.org/wiki/Ascomycota | Biology and health sciences | Fungi | null |
The diverse conidia and conidiophores sometimes develop in asexual sporocarps with different characteristics (e.g. acervulus, pycnidium, sporodochium). Some species of ascomycetes form their structures within plant tissue, either as parasite or saprophytes. These fungi have evolved more complex asexual sporing structures, probably influenced by the cultural conditions of plant tissue as a substrate. These structures are called the . This is a cushion of conidiophores created from a pseudoparenchymatous stroma in plant tissue. The is a globose to flask-shaped parenchymatous structure, lined on its inner wall with conidiophores. The is a flat saucer shaped bed of conidiophores produced under a plant cuticle, which eventually erupt through the cuticle for dispersal.
Budding
Asexual reproduction process in ascomycetes also involves the budding which we clearly observe in yeast. This is termed a "blastic process". It involves the blowing out or blebbing of the hyphal tip wall. The blastic process can involve all wall layers, or there can be a new cell wall synthesized which is extruded from within the old wall.
The initial events of budding can be seen as the development of a ring of chitin around the point where the bud is about to appear. This reinforces and stabilizes the cell wall. Enzymatic activity and turgor pressure act to weaken and extrude the cell wall. New cell wall material is incorporated during this phase. Cell contents are forced into the progeny cell, and as the final phase of mitosis ends a cell plate, the point at which a new cell wall will grow inwards from, forms.
Characteristics of ascomycetes | Ascomycota | Wikipedia | 386 | 48981 | https://en.wikipedia.org/wiki/Ascomycota | Biology and health sciences | Fungi | null |
Ascomycota are morphologically diverse. The group includes organisms from unicellular yeasts to complex cup fungi.
98% of lichens have an Ascomycota as the fungal part of the lichen.
There are 2000 identified genera and 30,000 species of Ascomycota.
The unifying characteristic among these diverse groups is the presence of a reproductive structure known as the , though in some cases it has a reduced role in the life cycle.
Many ascomycetes are of commercial importance. Some play a beneficial role, such as the yeasts used in baking, brewing, and wine fermentation, plus truffles and morels, which are held as gourmet delicacies.
Many of them cause tree diseases, such as Dutch elm disease and apple blights.
Some of the plant pathogenic ascomycetes are apple scab, rice blast, the ergot fungi, black knot, and the powdery mildews.
The yeasts are used to produce alcoholic beverages and breads. The mold Penicillium is used to produce the antibiotic penicillin.
Almost half of all members of the phylum Ascomycota form associations with algae to form lichens.
Others, such as morels (a highly prized edible fungi), form important relationships with plants, thereby providing enhanced water and nutrient uptake and, in some cases, protection from insects.
Most ascomycetes are terrestrial or parasitic. However, some have adapted to marine or freshwater environments. As of 2015, there were 805 marine fungi in the Ascomycota, distributed among 352 genera.
The cell walls of the hyphae are variably composed of chitin and β-glucans, just as in Basidiomycota. However, these fibers are set in a matrix of glycoprotein containing the sugars galactose and mannose.
The mycelium of ascomycetes is usually made up of septate hyphae. However, there is not necessarily any fixed number of nuclei in each of the divisions.
The septal walls have septal pores which provide cytoplasmic continuity throughout the individual hyphae. Under appropriate conditions, nuclei may also migrate between septal compartments through the septal pores. | Ascomycota | Wikipedia | 476 | 48981 | https://en.wikipedia.org/wiki/Ascomycota | Biology and health sciences | Fungi | null |
A unique character of the Ascomycota (but not present in all ascomycetes) is the presence of Woronin bodies on each side of the septa separating the hyphal segments which control the septal pores. If an adjoining hypha is ruptured, the Woronin bodies block the pores to prevent loss of cytoplasm into the ruptured compartment. The Woronin bodies are spherical, hexagonal, or rectangular membrane bound structures with a crystalline protein matrix. | Ascomycota | Wikipedia | 108 | 48981 | https://en.wikipedia.org/wiki/Ascomycota | Biology and health sciences | Fungi | null |
Modern classification
There are three subphyla that are described and accepted:
The Pezizomycotina are the largest subphylum and contains all ascomycetes that produce ascocarps (fruiting bodies), except for one genus, Neolecta, in the Taphrinomycotina. It is roughly equivalent to the previous taxon, Euascomycetes. The Pezizomycotina includes most macroscopic "ascos" such as truffles, ergot, ascolichens, cup fungi (discomycetes), pyrenomycetes, lorchels, and caterpillar fungus. It also contains microscopic fungi such as powdery mildews, dermatophytic fungi, and Laboulbeniales.
The Saccharomycotina comprise most of the "true" yeasts, such as baker's yeast and Candida, which are single-celled (unicellular) fungi, which reproduce vegetatively by budding. Most of these species were previously classified in a taxon called Hemiascomycetes.
The Taphrinomycotina include a disparate and basal group within the Ascomycota that was recognized following molecular (DNA) analyses. The taxon was originally named Archiascomycetes (or Archaeascomycetes). It includes hyphal fungi (Neolecta, Taphrina, Archaeorhizomyces), fission yeasts (Schizosaccharomyces), and the mammalian lung parasite Pneumocystis. | Ascomycota | Wikipedia | 332 | 48981 | https://en.wikipedia.org/wiki/Ascomycota | Biology and health sciences | Fungi | null |
Outdated taxon names
Several outdated taxon names—based on morphological features—are still occasionally used for species of the Ascomycota. These include the following sexual (teleomorphic) groups, defined by the structures of their sexual fruiting bodies: the Discomycetes, which included all species forming apothecia; the Pyrenomycetes, which included all sac fungi that formed perithecia or pseudothecia, or any structure resembling these morphological structures; and the Plectomycetes, which included those species that form cleistothecia. Hemiascomycetes included the yeasts and yeast-like fungi that have now been placed into the Saccharomycotina or Taphrinomycotina, while the Euascomycetes included the remaining species of the Ascomycota, which are now in the Pezizomycotina, and the Neolecta, which are in the Taphrinomycotina.
Some ascomycetes do not reproduce sexually or are not known to produce asci and are therefore anamorphic species. Those anamorphs that produce conidia (mitospores) were previously described as mitosporic Ascomycota. Some taxonomists placed this group into a separate artificial phylum, the Deuteromycota (or "Fungi Imperfecti"). Where recent molecular analyses have identified close relationships with ascus-bearing taxa, anamorphic species have been grouped into the Ascomycota, despite the absence of the defining ascus. Sexual and asexual isolates of the same species commonly carry different binomial species names, as, for example, Aspergillus nidulans and Emericella nidulans, for asexual and sexual isolates, respectively, of the same species.
Species of the Deuteromycota were classified as Coelomycetes if they produced their conidia in minute flask- or saucer-shaped conidiomata, known technically as pycnidia and acervuli. The Hyphomycetes were those species where the conidiophores (i.e., the hyphal structures that carry conidia-forming cells at the end) are free or loosely organized. They are mostly isolated but sometimes also appear as bundles of cells aligned in parallel (described as synnematal) or as cushion-shaped masses (described as sporodochial). | Ascomycota | Wikipedia | 512 | 48981 | https://en.wikipedia.org/wiki/Ascomycota | Biology and health sciences | Fungi | null |
Morphology
Most species grow as filamentous, microscopic structures called hyphae or as budding single cells (yeasts). Many interconnected hyphae form a thallus usually referred to as the mycelium, which—when visible to the naked eye (macroscopic)—is commonly called mold. During sexual reproduction, many Ascomycota typically produce large numbers of asci. The ascus is often contained in a multicellular, occasionally readily visible fruiting structure, the ascocarp (also called an ascoma). Ascocarps come in a very large variety of shapes: cup-shaped, club-shaped, potato-like, spongy, seed-like, oozing and pimple-like, coral-like, nit-like, golf-ball-shaped, perforated tennis ball-like, cushion-shaped, plated and feathered in miniature (Laboulbeniales), microscopic classic Greek shield-shaped, stalked or sessile. They can appear solitary or clustered. Their texture can likewise be very variable, including fleshy, like charcoal (carbonaceous), leathery, rubbery, gelatinous, slimy, powdery, or cob-web-like. Ascocarps come in multiple colors such as red, orange, yellow, brown, black, or, more rarely, green or blue. Some ascomyceous fungi, such as Saccharomyces cerevisiae, grow as single-celled yeasts, which—during sexual reproduction—develop into an ascus, and do not form fruiting bodies.
In lichenized species, the thallus of the fungus defines the shape of the symbiotic colony. Some dimorphic species, such as Candida albicans, can switch between growth as single cells and as filamentous, multicellular hyphae. Other species are pleomorphic, exhibiting asexual (anamorphic) as well as a sexual (teleomorphic) growth forms. | Ascomycota | Wikipedia | 423 | 48981 | https://en.wikipedia.org/wiki/Ascomycota | Biology and health sciences | Fungi | null |
Except for lichens, the non-reproductive (vegetative) mycelium of most ascomycetes is usually inconspicuous because it is commonly embedded in the substrate, such as soil, or grows on or inside a living host, and only the ascoma may be seen when fruiting. Pigmentation, such as melanin in hyphal walls, along with prolific growth on surfaces can result in visible mold colonies; examples include Cladosporium species, which form black spots on bathroom caulking and other moist areas. Many ascomycetes cause food spoilage, and, therefore, the pellicles or moldy layers that develop on jams, juices, and other foods are the mycelia of these species or occasionally Mucoromycotina and almost never Basidiomycota. Sooty molds that develop on plants, especially in the tropics are the thalli of many species.
Large masses of yeast cells, asci or ascus-like cells, or conidia can also form macroscopic structures. For example. Pneumocystis species can colonize lung cavities (visible in x-rays), causing a form of pneumonia. Asci of Ascosphaera fill honey bee larvae and pupae causing mummification with a chalk-like appearance, hence the name "chalkbrood". Yeasts for small colonies in vitro and in vivo, and excessive growth of Candida species in the mouth or vagina causes "thrush", a form of candidiasis.
The cell walls of the ascomycetes almost always contain chitin and β-glucans, and divisions within the hyphae, called "septa", are the internal boundaries of individual cells (or compartments). The cell wall and septa give stability and rigidity to the hyphae and may prevent loss of cytoplasm in case of local damage to cell wall and cell membrane. The septa commonly have a small opening in the center, which functions as a cytoplasmic connection between adjacent cells, also sometimes allowing cell-to-cell movement of nuclei within a hypha. Vegetative hyphae of most ascomycetes contain only one nucleus per cell (uninucleate hyphae), but multinucleate cells—especially in the apical regions of growing hyphae—can also be present. | Ascomycota | Wikipedia | 500 | 48981 | https://en.wikipedia.org/wiki/Ascomycota | Biology and health sciences | Fungi | null |
Metabolism
In common with other fungal phyla, the Ascomycota are heterotrophic organisms that require organic compounds as energy sources. These are obtained by feeding on a variety of organic substrates including dead matter, foodstuffs, or as symbionts in or on other living organisms. To obtain these nutrients from their surroundings, ascomycetous fungi secrete powerful digestive enzymes that break down organic substances into smaller molecules, which are then taken up into the cell. Many species live on dead plant material such as leaves, twigs, or logs. Several species colonize plants, animals, or other fungi as parasites or mutualistic symbionts and derive all their metabolic energy in form of nutrients from the tissues of their hosts.
Owing to their long evolutionary history, the Ascomycota have evolved the capacity to break down almost every organic substance. Unlike most organisms, they are able to use their own enzymes to digest plant biopolymers such as cellulose or lignin. Collagen, an abundant structural protein in animals, and keratin—a protein that forms hair and nails—, can also serve as food sources. Unusual examples include Aureobasidium pullulans, which feeds on wall paint, and the kerosene fungus Amorphotheca resinae, which feeds on aircraft fuel (causing occasional problems for the airline industry), and may sometimes block fuel pipes. Other species can resist high osmotic stress and grow, for example, on salted fish, and a few ascomycetes are aquatic. | Ascomycota | Wikipedia | 321 | 48981 | https://en.wikipedia.org/wiki/Ascomycota | Biology and health sciences | Fungi | null |
The Ascomycota is characterized by a high degree of specialization; for instance, certain species of Laboulbeniales attack only one particular leg of one particular insect species. Many Ascomycota engage in symbiotic relationships such as in lichens—symbiotic associations with green algae or cyanobacteria—in which the fungal symbiont directly obtains products of photosynthesis. In common with many basidiomycetes and Glomeromycota, some ascomycetes form symbioses with plants by colonizing the roots to form mycorrhizal associations. The Ascomycota also represents several carnivorous fungi, which have developed hyphal traps to capture small protists such as amoebae, as well as roundworms (Nematoda), rotifers, tardigrades, and small arthropods such as springtails (Collembola).
Distribution and living environment
The Ascomycota are represented in all land ecosystems worldwide, occurring on all continents including Antarctica. Spores and hyphal fragments are dispersed through the atmosphere and freshwater environments, as well as ocean beaches and tidal zones. The distribution of species is variable; while some are found on all continents, others, as for example the white truffle Tuber magnatum, only occur in isolated locations in Italy and Eastern Europe. The distribution of plant-parasitic species is often restricted by host distributions; for example, Cyttaria is only found on Nothofagus (Southern Beech) in the Southern Hemisphere.
Reproduction
Asexual reproduction
Asexual reproduction is the dominant form of propagation in the Ascomycota, and is responsible for the rapid spread of these fungi into new areas. It occurs through vegetative reproductive spores, the conidia. The conidiospores commonly contain one nucleus and are products of mitotic cell divisions and thus are sometimes called mitospores, which are genetically identical to the mycelium from which they originate. They are typically formed at the ends of specialized hyphae, the conidiophores. Depending on the species they may be dispersed by wind or water, or by animals. | Ascomycota | Wikipedia | 458 | 48981 | https://en.wikipedia.org/wiki/Ascomycota | Biology and health sciences | Fungi | null |
Asexual spores
Different types of asexual spores can be identified by colour, shape, and how they are released as individual spores. Spore types can be used as taxonomic characters in the classification within the Ascomycota. The most frequent types are the single-celled spores, which are designated amerospores. If the spore is divided into two by a cross-wall (septum), it is called a didymospore.
When there are two or more cross-walls, the classification depends on spore shape. If the septae are transversal, like the rungs of a ladder, it is a phragmospore, and if they possess a net-like structure it is a dictyospore. In staurospores ray-like arms radiate from a central body; in others (helicospores) the entire spore is wound up in a spiral like a spring. Very long worm-like spores with a length-to-diameter ratio of more than 15:1, are called scolecospores.
Conidiogenesis and dehiscence
Important characteristics of the anamorphs of the Ascomycota are conidiogenesis, which includes spore formation and dehiscence (separation from the parent structure). Conidiogenesis corresponds to Embryology in animals and plants and can be divided into two fundamental forms of development: blastic conidiogenesis, where the spore is already evident before it separates from the conidiogenic hypha, and thallic conidiogenesis, during which a cross-wall forms and the newly created cell develops into a spore. The spores may or may not be generated in a large-scale specialized structure that helps to spread them. | Ascomycota | Wikipedia | 369 | 48981 | https://en.wikipedia.org/wiki/Ascomycota | Biology and health sciences | Fungi | null |
These two basic types can be further classified as follows:
blastic-acropetal (repeated budding at the tip of the conidiogenic hypha, so that a chain of spores is formed with the youngest spores at the tip),
blastic-synchronous (simultaneous spore formation from a central cell, sometimes with secondary acropetal chains forming from the initial spores),
blastic-sympodial (repeated sideways spore formation from behind the leading spore, so that the oldest spore is at the main tip),
blastic-annellidic (each spore separates and leaves a ring-shaped scar inside the scar left by the previous spore),
blastic-phialidic (the spores arise and are ejected from the open ends of special conidiogenic cells called phialides, which remain constant in length),
basauxic (where a chain of conidia, in successively younger stages of development, is emitted from the mother cell),
blastic-retrogressive (spores separate by formation of crosswalls near the tip of the conidiogenic hypha, which thus becomes progressively shorter),
thallic-arthric (double cell walls split the conidiogenic hypha into cells that develop into short, cylindrical spores called arthroconidia; sometimes every second cell dies off, leaving the arthroconidia free),
thallic-solitary (a large bulging cell separates from the conidiogenic hypha, forms internal walls, and develops to a phragmospore).
Sometimes the conidia are produced in structures visible to the naked eye, which help to distribute the spores. These structures are called "conidiomata" (singular: conidioma), and may take the form of pycnidia (which are flask-shaped and arise in the fungal tissue) or acervuli (which are cushion-shaped and arise in host tissue).
Dehiscence happens in two ways. In schizolytic dehiscence, a double-dividing wall with a central lamella (layer) forms between the cells; the central layer then breaks down thereby releasing the spores. In rhexolytic dehiscence, the cell wall that joins the spores on the outside degenerates and releases the conidia. | Ascomycota | Wikipedia | 486 | 48981 | https://en.wikipedia.org/wiki/Ascomycota | Biology and health sciences | Fungi | null |
Heterokaryosis and parasexuality
Several Ascomycota species are not known to have a sexual cycle. Such asexual species may be able to undergo genetic recombination between individuals by processes involving heterokaryosis and parasexual events.
Parasexuality refers to the process of heterokaryosis, caused by merging of two hyphae belonging to different individuals, by a process called anastomosis, followed by a series of events resulting in genetically different cell nuclei in the mycelium.
The merging of nuclei is not followed by meiotic events, such as gamete formation and results in an increased number of chromosomes per nuclei. Mitotic crossover may enable recombination, i.e., an exchange of genetic material between homologous chromosomes. The chromosome number may then be restored to its haploid state by nuclear division, with each daughter nuclei being genetically different from the original parent nuclei. Alternatively, nuclei may lose some chromosomes, resulting in aneuploid cells. Candida albicans (class Saccharomycetes) is an example of a fungus that has a parasexual cycle (see Candida albicans and Parasexual cycle).
Sexual reproduction
Sexual reproduction in the Ascomycota leads to the formation of the ascus, the structure that defines this fungal group and distinguishes it from other fungal phyla. The ascus is a tube-shaped vessel, a meiosporangium, which contains the sexual spores produced by meiosis and which are called ascospores.
Apart from a few exceptions, such as Candida albicans, most ascomycetes are haploid, i.e., they contain one set of chromosomes per nucleus. During sexual reproduction there is a diploid phase, which commonly is very short, and meiosis restores the haploid state. The sexual cycle of one well-studied representative species of Ascomycota is described in greater detail in Neurospora crassa. Also, the adaptive basis for the maintenance of sexual reproduction in the Ascomycota fungi was reviewed by Wallen and Perlin. They concluded that the most plausible reason for the maintenance of this capability is the benefit of repairing DNA damage by using recombination that occurs during meiosis. DNA damage can be caused by a variety of stresses such as nutrient limitation. | Ascomycota | Wikipedia | 498 | 48981 | https://en.wikipedia.org/wiki/Ascomycota | Biology and health sciences | Fungi | null |
Formation of sexual spores
The sexual part of the life cycle commences when two hyphal structures mate. In the case of homothallic species, mating is enabled between hyphae of the same fungal clone, whereas in heterothallic species, the two hyphae must originate from fungal clones that differ genetically, i.e., those that are of a different mating type. Mating types are typical of the fungi and correspond roughly to the sexes in plants and animals; however one species may have more than two mating types, resulting in sometimes complex vegetative incompatibility systems. The adaptive function of mating type is discussed in Neurospora crassa.
Gametangia are sexual structures formed from hyphae, and are the generative cells. A very fine hypha, called trichogyne emerges from one gametangium, the ascogonium, and merges with a gametangium (the antheridium) of the other fungal isolate. The nuclei in the antheridium then migrate into the ascogonium, and plasmogamy—the mixing of the cytoplasm—occurs. Unlike in animals and plants, plasmogamy is not immediately followed by the merging of the nuclei (called karyogamy). Instead, the nuclei from the two hyphae form pairs, initiating the dikaryophase of the sexual cycle, during which time the pairs of nuclei synchronously divide. Fusion of the paired nuclei leads to mixing of the genetic material and recombination and is followed by meiosis. A similar sexual cycle is present in the red algae (Rhodophyta). A discarded hypothesis held that a second karyogamy event occurred in the ascogonium prior to ascogeny, resulting in a tetraploid nucleus which divided into four diploid nuclei by meiosis and then into eight haploid nuclei by a supposed process called brachymeiosis, but this hypothesis was disproven in the 1950s. | Ascomycota | Wikipedia | 418 | 48981 | https://en.wikipedia.org/wiki/Ascomycota | Biology and health sciences | Fungi | null |
From the fertilized ascogonium, dinucleate hyphae emerge in which each cell contains two nuclei. These hyphae are called ascogenous or fertile hyphae. They are supported by the vegetative mycelium containing uni– (or mono–) nucleate hyphae, which are sterile. The mycelium containing both sterile and fertile hyphae may grow into fruiting body, the ascocarp, which may contain millions of fertile hyphae.
An ascocarp is the fruiting body of the sexual phase in Ascomycota. There are five morphologically different types of ascocarp, namely:
Naked asci: these occur in simple ascomycetes; asci are produced on the organism's surface.
Perithecia: Asci are in flask-shaped ascoma (perithecium) with a pore (ostiole) at the top.
Cleistothecia: The ascocarp (a cleistothecium) is spherical and closed.
Apothecia: The asci are in a bowl shaped ascoma (apothecium). These are sometimes called the "cup fungi".
Pseudothecia: Asci with two layers, produced in pseudothecia that look like perithecia. The ascospores are arranged irregularly.
The sexual structures are formed in the fruiting layer of the ascocarp, the hymenium. At one end of ascogenous hyphae, characteristic U-shaped hooks develop, which curve back opposite to the growth direction of the hyphae. The two nuclei contained in the apical part of each hypha divide in such a way that the threads of their mitotic spindles run parallel, creating two pairs of genetically different nuclei. One daughter nucleus migrates close to the hook, while the other daughter nucleus locates to the basal part of the hypha. The formation of two parallel cross-walls then divides the hypha into three sections: one at the hook with one nucleus, one at the basal of the original hypha that contains one nucleus, and one that separates the U-shaped part, which contains the other two nuclei. | Ascomycota | Wikipedia | 472 | 48981 | https://en.wikipedia.org/wiki/Ascomycota | Biology and health sciences | Fungi | null |
Fusion of the nuclei (karyogamy) takes place in the U-shaped cells in the hymenium, and results in the formation of a diploid zygote. The zygote grows into the ascus, an elongated tube-shaped or cylinder-shaped capsule. Meiosis then gives rise to four haploid nuclei, usually followed by a further mitotic division that results in eight nuclei in each ascus. The nuclei along with some cytoplasma become enclosed within membranes and a cell wall to give rise to ascospores that are aligned inside the ascus like peas in a pod.
Upon opening of the ascus, ascospores may be dispersed by the wind, while in some cases the spores are forcibly ejected form the ascus; certain species have evolved spore cannons, which can eject ascospores up to 30 cm. away. When the spores reach a suitable substrate, they germinate, form new hyphae, which restarts the fungal life cycle.
The form of the ascus is important for classification and is divided into four basic types: unitunicate-operculate, unitunicate-inoperculate, bitunicate, or prototunicate. See the article on asci for further details.
Ecology
The Ascomycota fulfil a central role in most land-based ecosystems. They are important decomposers, breaking down organic materials, such as dead leaves and animals, and helping the detritivores (animals that feed on decomposing material) to obtain their nutrients. Ascomycetes, along with other fungi, can break down large molecules such as cellulose or lignin, and thus have important roles in nutrient cycling such as the carbon cycle.
The fruiting bodies of the Ascomycota provide food for many animals ranging from insects and slugs and snails (Gastropoda) to rodents and larger mammals such as deer and wild boars.
Many ascomycetes also form symbiotic relationships with other organisms, including plants and animals.
Lichens | Ascomycota | Wikipedia | 432 | 48981 | https://en.wikipedia.org/wiki/Ascomycota | Biology and health sciences | Fungi | null |
Probably since early in their evolutionary history, the Ascomycota have formed symbiotic associations with green algae (Chlorophyta), and other types of algae and cyanobacteria. These mutualistic associations are commonly known as lichens, and can grow and persist in terrestrial regions of the earth that are inhospitable to other organisms and characterized by extremes in temperature and humidity, including the Arctic, the Antarctic, deserts, and mountaintops. While the photoautotrophic algal partner generates metabolic energy through photosynthesis, the fungus offers a stable, supportive matrix and protects cells from radiation and dehydration. Around 42% of the Ascomycota (about 18,000 species) form lichens, and almost all the fungal partners of lichens belong to the Ascomycota.
Mycorrhizal fungi and endophytes
Members of the Ascomycota form two important types of relationship with plants: as mycorrhizal fungi and as endophytes. Mycorrhiza are symbiotic associations of fungi with the root systems of the plants, which can be of vital importance for growth and persistence for the plant. The fine mycelial network of the fungus enables the increased uptake of mineral salts that occur at low levels in the soil. In return, the plant provides the fungus with metabolic energy in the form of photosynthetic products.
Endophytic fungi live inside plants, and those that form mutualistic or commensal associations with their host, do not damage their hosts. The exact nature of the relationship between endophytic fungus and host depends on the species involved, and in some cases fungal colonization of plants can bestow a higher resistance against insects, roundworms (nematodes), and bacteria; in the case of grass endophytes the fungal symbiont produces poisonous alkaloids, which can affect the health of plant-eating (herbivorous) mammals and deter or kill insect herbivores. | Ascomycota | Wikipedia | 423 | 48981 | https://en.wikipedia.org/wiki/Ascomycota | Biology and health sciences | Fungi | null |
Symbiotic relationships with animals
Several ascomycetes of the genus Xylaria colonize the nests of leafcutter ants and other fungus-growing ants of the tribe Attini, and the fungal gardens of termites (Isoptera). Since they do not generate fruiting bodies until the insects have left the nests, it is suspected that, as confirmed in several cases of Basidiomycota species, they may be cultivated.
Bark beetles (family Scolytidae) are important symbiotic partners of ascomycetes. The female beetles transport fungal spores to new hosts in characteristic tucks in their skin, the mycetangia. The beetle tunnels into the wood and into large chambers in which they lay their eggs. Spores released from the mycetangia germinate into hyphae, which can break down the wood. The beetle larvae then feed on the fungal mycelium, and, on reaching maturity, carry new spores with them to renew the cycle of infection. A well-known example of this is Dutch elm disease, caused by Ophiostoma ulmi, which is carried by the European elm bark beetle, Scolytus multistriatus.
Plant disease interactions
One of their most harmful roles is as the agent of many plant diseases. For instance: | Ascomycota | Wikipedia | 268 | 48981 | https://en.wikipedia.org/wiki/Ascomycota | Biology and health sciences | Fungi | null |
Dutch elm disease, caused by the closely related species Ophiostoma ulmi and Ophiostoma novo-ulmi, has led to the death of many elms in Europe and North America.
The originally Asian Cryphonectria parasitica is responsible for attacking Sweet Chestnuts (Castanea sativa), and virtually eliminated the once-widespread American Chestnut (Castanea dentata),
A disease of maize (Zea mays), which is especially prevalent in North America, is brought about by Cochliobolus heterostrophus.
Taphrina deformans causes leaf curl of peach.
Uncinula necator is responsible for the disease powdery mildew, which attacks grapevines.
Species of Monilinia cause brown rot of stone fruit such as peaches (Prunus persica) and sour cherries (Prunus ceranus).
Members of the Ascomycota such as Stachybotrys chartarum are responsible for fading of woolen textiles, which is a common problem especially in the tropics.
Blue-green, red and brown molds attack and spoil foodstuffs – for instance Penicillium italicum rots oranges.
Cereals infected with Fusarium graminearum contain mycotoxins like deoxynivalenol (DON), which causes Fusarium ear blight and skin and mucous membrane lesions when eaten by pigs.
Human disease interactions | Ascomycota | Wikipedia | 303 | 48981 | https://en.wikipedia.org/wiki/Ascomycota | Biology and health sciences | Fungi | null |
Aspergillus fumigatus, the most common cause of fungal infection in the lungs of immune-compromised patients often resulting in death. Also the most frequent cause of Allergic bronchopulmonary aspergillosis, which often occurs in patients with Cystic fibrosis as well as Asthma.
Candida albicans, a yeast that attacks the mucous membranes, can cause an infection of the mouth or vagina called thrush or candidiasis, and is also blamed for "yeast allergies".
Fungi like Epidermophyton cause skin infections but are not very dangerous for people with healthy immune systems. However, if the immune system is damaged they can be life-threatening; for instance, Pneumocystis jirovecii is responsible for severe lung infections that occur in AIDS patients.
Ergot (Claviceps purpurea) is a direct menace to humans when it attacks wheat or rye and produces highly poisonous alkaloids, causing ergotism if consumed. Symptoms include hallucinations, stomach cramps, and a burning sensation in the limbs ("Saint Anthony's Fire").
Aspergillus flavus, which grows on peanuts and other hosts, generates aflatoxin, which damages the liver and is highly carcinogenic.
Histoplasma capsulatum causes histoplasmosis, which affects immunocompromised patients.
Blastomyces dermatitidis is the causal agent of blastomycosis, an invasive and often serious fungal infection found occasionally in humans and other animals in regions where the fungus is endemic.
Paracoccidioides brasiliensis and Paracoccidioides lutzii are the causal agents of paracoccidioidomycosis.
Coccidioides immitis and Coccidioides posadasii are the causative agent of coccidioidomycosis (valley fever).
Talaromyces marneffei, formerly called Penicillium marneffei causes talaromycosis | Ascomycota | Wikipedia | 420 | 48981 | https://en.wikipedia.org/wiki/Ascomycota | Biology and health sciences | Fungi | null |
Beneficial effects for humans
On the other hand, ascus fungi have brought some significant benefits to humanity.
The most famous case may be that of the mold Penicillium chrysogenum (formerly Penicillium notatum), which, probably to attack competing bacteria, produces an antibiotic that, under the name of penicillin, triggered a revolution in the treatment of bacterial infectious diseases in the 20th century.
The medical importance of Tolypocladium niveum as an immunosuppressor can hardly be exaggerated. It excretes Ciclosporin, which, as well as being given during Organ transplantation to prevent rejection, is also prescribed for auto-immune diseases such as multiple sclerosis. However, there is some doubt over the long-term side effects of the treatment. | Ascomycota | Wikipedia | 169 | 48981 | https://en.wikipedia.org/wiki/Ascomycota | Biology and health sciences | Fungi | null |
Some ascomycete fungi can be easily altered through genetic engineering procedures. They can then produce useful proteins such as insulin, human growth hormone, or TPa, which is employed to dissolve blood clots.
Several species are common model organisms in biology, including Saccharomyces cerevisiae, Schizosaccharomyces pombe, and Neurospora crassa. The genomes of some ascomycete fungi have been fully sequenced.
Baker's Yeast (Saccharomyces cerevisiae) is used to make bread, beer and wine, during which process sugars such as glucose or sucrose are fermented to make ethanol and carbon dioxide. Bakers use the yeast for carbon dioxide production, causing the bread to rise, with the ethanol boiling off during cooking. Most vintners use it for ethanol production, releasing carbon dioxide into the atmosphere during fermentation. Brewers and traditional producers of sparkling wine use both, with a primary fermentation for the alcohol and a secondary one to produce the carbon dioxide bubbles that provide the drinks with a "sparkling" texture in the case of wine and the desirable foam in the case of beer.
Enzymes of Penicillium camemberti play a role in the manufacture of the cheeses Camembert and Brie, while those of Penicillium roqueforti do the same for Gorgonzola, Roquefort and Stilton.
In Asia, Aspergillus oryzae is added to a pulp of soaked soya beans to make soy sauce and is used to break down starch in rice and other grains into simple sugars for fermentation into East Asian alcoholic beverages such as huangjiu and sake.
Finally, some members of the Ascomycota are choice edibles; morels (Morchella spp.), truffles (Tuber spp.), and lobster mushroom (Hypomyces lactifluorum) are some of the most sought-after fungal delicacies.
Cordyceps militaris is known for its numerous medicinal benefits, including supporting the immune system, reducing inflammation, providing antioxidant effects, enhancing metabolic health, improving athletic performance, and promoting respiratory health. It contains bioactive compounds such as cordycepin, cordycepic acid, adenosine, and polysaccharides, beta-glucans, and ergosterol. | Ascomycota | Wikipedia | 501 | 48981 | https://en.wikipedia.org/wiki/Ascomycota | Biology and health sciences | Fungi | null |
Road transport or road transportation is a type of transport using roads. Transport on roads can be roughly grouped into the transportation of goods and transportation of people. In many countries licensing requirements and safety regulations ensure a separation of the two industries. Movement along roads may be by bike, automobile, bus, truck, or by animal such as horse or oxen. Standard networks of roads were adopted by Romans, Persians, Aztec, and other early empires, and may be regarded as a feature of empires. Cargo may be transported by trucking companies, while passengers may be transported via mass transit. Commonly defined features of modern roads include defined lanes and signage. Various classes of road exist, from two-lane local roads with at-grade intersections to controlled-access highways with all cross traffic grade-separated.
The nature of road transportation of goods depends on, apart from the degree of development of the local infrastructure, the distance the goods are transported by road, the weight and volume of an individual shipment, and the type of goods transported. For short distances and light small shipments, a van or pickup truck may be used. For large shipments even if less than a full truckload a truck is more appropriate. (Also see Trucking and Hauling below). In some countries cargo is transported by road in horse-drawn carriages, donkey carts or other non-motorized mode. Delivery services are sometimes considered a separate category from cargo transport. In many places, fast food is transported on roads by various types of vehicles. For inner city delivery of small packages and documents bike couriers are quite common.
People are transported on roads. Special modes of individual transport by road such as cycle rickshaws may also be locally available. There are also specialist modes of road transport for particular situations, such as ambulances.
History
Early roads
The first methods of road transport were horses, oxen or even humans carrying goods over dirt tracks that often followed game trail. The Persians later built a network of Royal Roads across their empire.
With the advent of the Roman Empire, there was a need for armies to be able to travel quickly from one region to another, and the roads that existed were often muddy, which greatly delayed the movement of large masses of troops. To resolve this issue, the Romans built solid and lasting roads. The Roman roads used deep roadbeds of crushed stone as an underlying layer to ensure that they kept dry, as the water would flow out from the crushed stone, instead of becoming mud in clay soils. The Islamic Caliphate later built tar-paved roads in Baghdad. | Road transport | Wikipedia | 510 | 49020 | https://en.wikipedia.org/wiki/Road%20transport | Technology | Road transport | null |
New road networks
As states developed and became richer, especially with the Renaissance, new roads and bridges began to be built, often based on Roman designs. Although there were attempts to rediscover Roman methods, there was little useful innovation in road building before the 18th century.
Starting in the early 18th century, the British Parliament began to pass a series of acts that gave the local justices powers to erect toll-gates on the roads, in exchange for professional upkeep. The toll-gate erected at Wade's Mill became the first effective toll-gate in England. The first scheme that had trustees who were not justices was established through a turnpike act in 1707, for a section of the London-Chester road between Foothill and Stony Stafford. The basic principle was that the trustees would manage resources from the several parishes through which the highway passed, augment this with tolls from users from outside the parishes and apply the whole to the maintenance of the main highway. This became the pattern for the turnpiking of a growing number of highways, sought by those who wished to improve flow of commerce through their part of a county.
In 18th century West Africa, road transport throughout the Ashanti Empire was maintained via a network of well-kept roads that connected the Ashanti capital with territories within its jurisdiction and influence. After significant road construction undertaken by the kingdom of Dahomey, toll roads were established with the function of collecting yearly taxes based on the goods carried by the people of Dahomey and their occupation. The Royal Road was built in the late 18th century by King Kpengla which stretched from Abomey through Cana up to Ouidah.
The quality of early turnpike roads was varied. Although turnpiking did result in some improvement to each highway, the technologies used to deal with geological features, drainage, and the effects of weather were all in their infancy. Road construction improved slowly, initially through the efforts of individual surveyors such as John Metcalf in Yorkshire in the 1760s. British turnpike builders began to realize the importance of selecting clean stones for surfacing while excluding vegetable material and clay, resulting in more durable roads.
Industrial civil engineering
By the late 18th and early 19th centuries, new methods of highway construction had been pioneered by the work of three British engineers, John Metcalf, Thomas Telford and John Loudon McAdam, and by the French road engineer Pierre-Marie-Jérôme Trésaguet. | Road transport | Wikipedia | 491 | 49020 | https://en.wikipedia.org/wiki/Road%20transport | Technology | Road transport | null |
The first professional road builder to emerge during the Industrial Revolution was John Metcalf, who constructed about of turnpike road, mainly in the north of England, from 1765. He believed a good road should have good foundations, be well drained and have a smooth convex surface to allow rainwater to drain quickly into ditches at the side. He understood the importance of good drainage, knowing it was rain that caused most problems on the roads.
Pierre-Marie-Jérôme Trésaguet established the first scientific approach to road building in France at the same time. He wrote a memorandum on his method in 1775, which became general practice in France. It involved a layer of large rocks, covered by a layer of smaller gravel. The lower layer improved on Roman practice in that it was based on the understanding that the purpose of this layer (the sub-base or base course) is to transfer the weight of the road and its traffic to the ground, while protecting the ground from deformation by spreading the weight evenly. Therefore, the sub-base did not have to be a self-supporting structure. The upper running surface provided a smooth surface for vehicles while protecting the large stones of the sub-base.
The surveyor and engineer Thomas Telford also made substantial advances in the engineering of new roads and the construction of bridges. His method of road building involved the digging of a large trench in which a foundation of heavy rock was set. He also designed his roads so that they sloped downwards from the centre, allowing drainage to take place, a major improvement on the work of Trésaguet. The surface of his roads consisted of broken stone. He also improved on methods for the building of roads by improving the selection of stone based on thickness, taking into account traffic, alignment and slopes. During his later years, Telford was responsible for rebuilding sections of the London to Holyhead road, a task completed by his assistant of ten years, John MacNeill.
It was another Scottish engineer, John Loudon McAdam, who designed the first modern roads. He developed an inexpensive paving material of soil and stone aggregate (known as macadam). His road building method was simpler than Telford's, yet more effective at protecting roadways: he discovered that massive foundations of rock upon rock were unnecessary, and asserted that native soil alone would support the road and traffic upon it, as long as it was covered by a road crust that would protect the soil underneath from water and wear. | Road transport | Wikipedia | 495 | 49020 | https://en.wikipedia.org/wiki/Road%20transport | Technology | Road transport | null |
Also unlike Telford and other road builders, McAdam laid his roads as level as possible. His road required only a rise of three inches from the edges to the center. Cambering and elevation of the road above the water table enabled rainwater to run off into ditches on either side. Size of stones was central to the McAdam's road building theory. The lower road thickness was restricted to stones no larger than . The upper layer of stones was limited to size and stones were checked by supervisors who carried scales. A workman could check the stone size himself by seeing if the stone would fit into his mouth. The importance of the 20 mm stone size was that the stones needed to be much smaller than the 100 mm width of the iron carriage tyres that traveled on the road. Macadam roads were being built widely in the United States and Australia in the 1820s and in Europe in the 1830s and 1840s.
20th century
Macadam roads were adequate for use by horses and carriages or coaches, but they were very dusty and subject to erosion with heavy rain. The Good Roads Movement occurred in the United States between the late 1870s and the 1920s. Advocates for improved roads led by bicyclists turned local agitation into a national political movement. | Road transport | Wikipedia | 251 | 49020 | https://en.wikipedia.org/wiki/Road%20transport | Technology | Road transport | null |
Outside cities, roads were dirt or gravel; mud in the winter and dust in the summer. Early organizers cited Europe where road construction and maintenance was supported by national and local governments. In its early years, the main goal of the movement was education for road building in rural areas between cities and to help rural populations gain the social and economic benefits enjoyed by cities where citizens benefited from railroads, trolleys and paved streets. Even more than traditional vehicles, the newly invented bicycles could benefit from good country roads. Later on, they did not hold up to higher-speed motor vehicle use. Methods to stabilise macadam roads with tar date back to at least 1834 when John Henry Cassell, operating from Cassell's Patent Lava Stone Works in Millwall, patented "Pitch Macadam".
This method involved spreading tar on the subgrade, placing a typical macadam layer, and finally sealing the macadam with a mixture of tar and sand. Tar-grouted macadam was in use well before 1900 and involved scarifying the surface of an existing macadam pavement, spreading tar, and re-compacting. Although the use of tar in road construction was known in the 19th century, it was little used and was not introduced on a large scale until the motorcar arrived on the scene in the early 20th century.
Modern tarmac was patented by British civil engineer Edgar Purnell Hooley, who noticed that spilled tar on the roadway kept the dust down and created a smooth surface. He took out a patent in 1901 for tarmac. Hooley's 1901 patent involved mechanically mixing tar and aggregate prior to lay-down and then compacting the mixture with a steamroller. The tar was modified by adding small amounts of Portland cement, resin, and pitch.
The first version of modern controlled-access highways evolved during the first half of the 20th century. The Long Island Motor Parkway on Long Island, New York, opened in 1908 as a private venture, was the world's first limited-access roadway. It included many modern features, including banked turns, guard rails and reinforced concrete tarmac. Traffic could turn left between the parkway and connectors, crossing oncoming traffic, so it was not a controlled-access highway (or "freeway" as later defined by the federal government's Manual on Uniform Traffic Control Devices). | Road transport | Wikipedia | 480 | 49020 | https://en.wikipedia.org/wiki/Road%20transport | Technology | Road transport | null |
Modern controlled-access highways originated in the early 1920s in response to the rapidly increasing use of the automobile, the demand for faster movement between cities and as a consequence of improvements in paving processes, techniques and materials. These original high-speed roads were referred to as "dual highways" and have been modernized and are still in use today.
Italy was the first country in the world to build controlled-access highways reserved for fast traffic and for motor vehicles only. The Autostrada dei Laghi ("Lakes Motorway"), the first built in the world, connecting Milan to Lake Como and Lake Maggiore, and now parts of the A8 and A9 motorways, was devised by Piero Puricelli and was inaugurated in 1924. This motorway, called autostrada, contained only one lane in each direction and no interchanges. The Bronx River Parkway was the first road in North America to utilize a median strip to separate the opposing lanes, to be constructed through a park and where intersecting streets crossed over bridges. The Southern State Parkway opened in 1927, while the Long Island Motor Parkway was closed in 1937 and replaced by the Northern State Parkway (opened 1931) and the contiguous Grand Central Parkway (opened 1936). In Germany, construction of the Bonn-Cologne Autobahn began in 1929 and was opened in 1932 by Konrad Adenauer, then the mayor of Cologne.
In Canada, the first precursor with semi-controlled access was The Middle Road between Hamilton and Toronto, which featured a median divider between opposing traffic flow, as well as the nation's first cloverleaf interchange. This highway developed into the Queen Elizabeth Way, which featured a cloverleaf and trumpet interchange when it opened in 1937 and until the Second World War boasted the longest illuminated stretch of roadway built. A decade later, the first section of Highway 401 was opened, based on earlier designs. It has since become North America's busiest highway. | Road transport | Wikipedia | 385 | 49020 | https://en.wikipedia.org/wiki/Road%20transport | Technology | Road transport | null |
The word freeway was first used in February 1930 by Edward M. Bassett. Bassett argued that roads should be classified into three basic types: highways, parkways, and freeways. In Bassett's zoning and property law-based system, abutting property owners have the rights of light, air and access to highways but to not parkways and freeways; the latter two are distinguished in that the purpose of a parkway is recreation, while the purpose of a freeway is movement. Thus as originally conceived, a freeway is a strip of public land devoted to movement to which abutting property owners do not have rights of light, air or access.
Trucking and haulage
Trucking companies (in American English terminology) or haulage companies / hauliers (in British English) accept cargo for road transport. Truck drivers operate either independently – working directly for the client – or through freight carriers or shipping agents. Some big companies (e.g. grocery store chains) operate their own internal trucking operations. The market size for general freight trucking was nearly $125 billion in 2010.
In the U.S. many truckers own their truck (rig), and are known as owner-operators. Some road transportation is done on regular routes or for only one consignee per run (full truckload), while others transport goods from many different loading stations/shippers to various consignees per run (less-than-truckload). On some long runs only cargo for one leg of the route (to) is known when the cargo is loaded. Truckers may have to wait at the destination for a backhaul.
A bill of lading issued by the shipper provides the basic document for road freight. On cross-border transportation the trucker will present the cargo and documentation provided by the shipper to customs for inspection (for EC see also Schengen Agreement). This also applies to shipments that are transported out of a free port. | Road transport | Wikipedia | 398 | 49020 | https://en.wikipedia.org/wiki/Road%20transport | Technology | Road transport | null |
Hours of service
To avoid accidents caused by fatigue, truckers have to adhere to strict rules for drive time and required rest periods. In the United States and Canada, these regulations are known as hours of service, and in the European Union as drivers working hours. One such regulation is the Hours of Work and Rest Periods (Road Transport) Convention, 1979. Tachographs or Electronic on-board recorders record the times the vehicle is in motion and stopped. Some companies use two drivers per truck to ensure uninterrupted transportation; with one driver resting or sleeping in a bunk in the back of the cab while the other is driving.
Licenses
Truck drivers often need special licenses to drive, known in the U.S. as a commercial driver's license. In the U.K. a large goods vehicle licence is required. For transport of hazardous materials (see dangerous goods) truckers need a licence, which usually requires them to pass an exam (e.g. in the EU). They have to make sure they affix proper labels for the respective hazard(s) to their vehicle. Liquid goods are transported by road in tank trucks (in American English) or tanker lorries (in British English) (also road-tankers) or special tank containers for intermodal transport. For transportation of live animals special requirements have to be met in many countries to prevent cruelty to animals (see animal rights). For fresh and frozen goods refrigerator trucks or reefers are used.
Weights
Some loads are weighed at the point of origin and the driver is responsible for ensuring weights conform to maximum allowed standards. This may involve using on-board weight gauges (load pressure gauges), knowing the empty weight of the transport vehicle and the weight of the load, or using a commercial weight scale. In route weigh stations check that gross vehicle weights do not exceed the maximum weight for that particular jurisdiction and will include individual axle weights. This varies by country, states within a country, and may include federal standards. The United States uses FMCSA federal standards that include bridge law formulas. Many states, not on the national road system, use their own road and bridge standards. Enforcement scales may include portable scales, scale houses with low speed scales or weigh-in-motion (WIM) scales. | Road transport | Wikipedia | 465 | 49020 | https://en.wikipedia.org/wiki/Road%20transport | Technology | Road transport | null |
The European Union uses the International Recommendation, OIML R 134-2 (2009). The process may involve a scale house and low-speed scales or higher-speed WIM road or bridge scales with the goal of public safety, as well as road and bridge safety, according to the Bridges Act.
Modern roads
Today, roadways are primarily asphalt or concrete. Both are based on McAdam's concept of stone aggregate in a binder, asphalt cement or Portland cement respectively. Asphalt is known as a flexible pavement, one which slowly will "flow" under the pounding of traffic. Concrete is a rigid pavement, which can take heavier loads but is more expensive and requires more carefully prepared subbase. So, generally, major roads are concrete and local roads are asphalt. Concrete roads are often covered with a thin layer of asphalt to create a wearing surface.
Modern pavements are designed for heavier vehicle loads and faster speeds, requiring thicker slabs and deeper subbase. Subbase is the layer or successive layers of stone, gravel and sand supporting the pavement. It is needed to spread out the slab load bearing on the underlying soil and to conduct away any water getting under the slabs. Water will undermine a pavement over time, so much of pavement and pavement joint design are meant to minimize the amount of water getting and staying under the slabs.
Shoulders are also an integral part of highway design. They are multipurpose; they can provide a margin of side clearance, a refuge for incapacitated vehicles, an emergency lane, and parking space. They also serve a design purpose, and that is to prevent water from percolating into the soil near the main pavement's edge. Shoulder pavement is designed to a lower standard than the pavement in the traveled way and won't hold up as well to traffic, so driving on the shoulder is generally prohibited.
Pavement technology is still evolving, albeit in not easily noticed increments. For instance, chemical additives in the pavement mix make the pavement more weather resistant, grooving and other surface treatments improve resistance to skidding and hydroplaning, and joint seals which were once tar are now made of low maintenance neoprene.
Traffic control | Road transport | Wikipedia | 441 | 49020 | https://en.wikipedia.org/wiki/Road%20transport | Technology | Road transport | null |
Nearly all roadways are built with devices meant to control traffic. Most notable to the motorist are those meant to communicate directly with the driver. Broadly, these fall into three categories: signs, signals or pavement markings. They help the driver navigate; they assign the right-of-way at intersections; they indicate laws such as speed limits and parking regulations; they advise of potential hazards; they indicate passing and no passing zones; and otherwise deliver information and to assure traffic is orderly and safe.
Two hundred years ago these devices were signs, nearly all informal. In the late 19th century signals began to appear in the biggest cities at a few highly congested intersections. They were manually operated, and consisted of semaphores, flags or paddles, or in some cases colored electric lights, all modeled on railroad signals. In the 20th century signals were automated, at first with electromechanical devices and later with computers. Signals can be quite sophisticated: with vehicle sensors embedded in the pavement, the signal can control and choreograph the turning movements of heavy traffic in the most complex of intersections. In the 1920s traffic engineers learned how to coordinate signals along a thoroughfare to increase its speeds and volumes. In the 1980s, with computers, similar coordination of whole networks became possible.
In the 1920s pavement markings were introduced. Initially they were used to indicate the road's centerline. Soon after they were coded with information to aid motorists in passing safely. Later, with multi-lane roads they were used to define lanes. Other uses, such as indicating permitted turning movements and pedestrian crossings soon followed.
In the 20th century traffic control devices were standardized. Before then every locality decided on what its devices would look like and where they would be applied. This could be confusing, especially to traffic from outside the locality. In the United States standardization was first taken at the state level, and late in the century at the federal level. Each country has a Manual of Uniform Traffic Control Devices (MUTCD) and there are efforts to blend them into a worldwide standard.
Besides signals, signs, and markings, other forms of traffic control are designed and built into the roadway. For instance, curbs and rumble strips can be used to keep traffic in a given lane and median barriers can prevent left turns and even U-turns.
Toll roads | Road transport | Wikipedia | 469 | 49020 | https://en.wikipedia.org/wiki/Road%20transport | Technology | Road transport | null |
Early toll roads were usually built by private companies under a government franchise. They typically paralleled or replaced routes already with some volume of commerce, hoping the improved road would divert enough traffic to make the enterprise profitable. Plank roads were particularly attractive as they greatly reduced rolling resistance and mitigated the problem of getting mired in mud. Another improvement, better grading to lessen the steepness of the worst stretches, allowed draft animals to haul heavier loads.
A toll road in the United States is often called a turnpike. The term turnpike probably originated from the gate, often a simple pike, which blocked passage until the fare was paid at a toll house (or toll booth in current terminology). When the toll was paid the pike, which was mounted on a swivel, was turned to allow the vehicle to pass. Tolls were usually based on the type of cargo being transported, not the type of vehicle. The practice of selecting routes so as to avoid tolls is called shunpiking. This may be simply to avoid the expense, as a form of economic protest (or boycott), or simply to seek a road less traveled as a bucolic interlude.
Companies were formed to build, improve, and maintain a particular section of roadway, and tolls were collected from users to finance the enterprise. The enterprise was usually named to indicate the locale of its roadway, often including the name of one of both of the termini. The word turnpike came into common use in the names of these roadways and companies, and is essentially used interchangeably with toll road in current terminology.
In the United States, toll roads began with the Lancaster Turnpike in the 1790s, within Pennsylvania, connecting Philadelphia and Lancaster. In the state of New York, the Great Western Turnpike was started in Albany in 1799 and eventually extended, by several alternate routes, to near what is now Syracuse, New York.
Toll roads peaked in the mid 19th century, and by the turn of the twentieth century most toll roads were taken over by state highway departments. The demise of this early toll road era was due to the rise of canals and railroads, which were more efficient (and thus cheaper) in moving freight over long distances. Roads wouldn't again be competitive with rails and barges until the first half of the 20th century when the internal combustion engine replaces draft animals as the source of motive power. | Road transport | Wikipedia | 478 | 49020 | https://en.wikipedia.org/wiki/Road%20transport | Technology | Road transport | null |
With the development, mass production, and popular embrace of the automobile, faster and higher capacity roads were needed. In the 1920s limited access highways appeared. Their main characteristics were dual roadways with access points limited to (but not always) grade-separated interchanges. Their dual roadways allowed high volumes of traffic, the need for no or few traffic lights along with relatively gentle grades and curves allowed higher speeds.
The first limited access highways were Parkways, so called because of their often park-like landscaping and, in the metropolitan New York City area, they connected the region's system of parks. When the German autobahns built in the 1930s introduced higher design standards and speeds, road planners and road-builders in the United States started developing and building toll roads to similar high standards. The Pennsylvania Turnpike, which largely followed the path of a partially built railroad, was the first, opening in 1940.
After 1940 with the Pennsylvania Turnpike, toll roads saw a resurgence, this time to fund limited access highways. In the late 1940s and early 1950s, after World War II interrupted the evolution of the highway, the US resumed building toll roads. They were to still higher standards and one road, the New York State Thruway, had standards that became the prototype for the U.S. Interstate Highway System. Several other major toll-roads which connected with the Pennsylvania Turnpike were established before the creation of the Interstate Highway System. These were the Indiana Toll Road, Ohio Turnpike, and New Jersey Turnpike.
Interstate Highway System
In the United States, beginning in 1956, Dwight D. Eisenhower National System of Interstate and Defense Highways, commonly called the Interstate Highway System was built. It uses 12 foot (3.65m) lanes, wide medians, a maximum of 4% grade, and full access control, though many sections don't meet these standards due to older construction or constraints. This system created a continental-sized network meant to connect every population center of 50,000 people or more. | Road transport | Wikipedia | 399 | 49020 | https://en.wikipedia.org/wiki/Road%20transport | Technology | Road transport | null |
By 1956, most limited access highways in the eastern United States were toll roads. In that year, the Federal Aid Highway Act of 1956 was passed, funding non-toll roads with 90% federal dollars and 10% state match, giving little incentive for states to expand their turnpike system. Funding rules initially restricted collections of tolls on newly funded roadways, bridges, and tunnels. In some situations, expansion or rebuilding of a toll facility using Interstate Highway Program funding resulted in the removal of existing tolls. This occurred in Virginia on Interstate 64 at the Hampton Roads Bridge-Tunnel when a second parallel roadway to the regional 1958 bridge-tunnel was completed in 1976.
Since the completion of the initial portion of the Interstate Highway System, regulations were changed, and portions of toll facilities have been added to the system. Some states are again looking at toll financing for new roads and maintenance, to supplement limited federal funding. In some areas, new road projects have been completed with public-private partnerships funded by tolls, such as the Pocahontas Parkway (I-895) near Richmond, Virginia.
The newest policy passed by Congress and the Obama administration regarding highways is the Surface and Air Transportation Programs Extension Act of 2011.
Pneumatic tyres
As the horse-drawn carriage was replaced by the car, bus and lorry or truck, and speeds increased, the need for smoother roads and less vertical displacement became more apparent, and pneumatic tyres were developed to decrease the apparent roughness. Wagon and carriage wheels, made of wood, had a tyre in the form of an iron strip that kept the wheel from wearing out quickly. Pneumatic tyres, which had a larger footprint than iron tyres, also were less likely to get bogged down in the mud on unpaved roads. | Road transport | Wikipedia | 359 | 49020 | https://en.wikipedia.org/wiki/Road%20transport | Technology | Road transport | null |
In biology, epigenetics is the study of heritable traits, or a stable change of cell function, that happen without changes to the DNA sequence. The Greek prefix epi- ( "over, outside of, around") in epigenetics implies features that are "on top of" or "in addition to" the traditional (DNA sequence based) genetic mechanism of inheritance. Epigenetics usually involves a change that is not erased by cell division, and affects the regulation of gene expression. Such effects on cellular and physiological phenotypic traits may result from environmental factors, or be part of normal development. Epigenetic factors can also lead to cancer.
The term also refers to the mechanism of changes: functionally relevant alterations to the genome that do not involve mutation of the nucleotide sequence. Examples of mechanisms that produce such changes are DNA methylation and histone modification, each of which alters how genes are expressed without altering the underlying DNA sequence. Further, non-coding RNA sequences have been shown to play a key role in the regulation of gene expression. Gene expression can be controlled through the action of repressor proteins that attach to silencer regions of the DNA. These epigenetic changes may last through cell divisions for the duration of the cell's life, and may also last for multiple generations, even though they do not involve changes in the underlying DNA sequence of the organism; instead, non-genetic factors cause the organism's genes to behave (or "express themselves") differently.
One example of an epigenetic change in eukaryotic biology is the process of cellular differentiation. During morphogenesis, totipotent stem cells become the various pluripotent cell lines of the embryo, which in turn become fully differentiated cells. In other words, as a single fertilized egg cell – the zygote – continues to divide, the resulting daughter cells change into all the different cell types in an organism, including neurons, muscle cells, epithelium, endothelium of blood vessels, etc., by activating some genes while inhibiting the expression of others.
Definitions
The term epigenesis has a generic meaning of "extra growth" that has been used in English since the 17th century. In scientific publications, the term epigenetics started to appear in the 1930s (see Fig. on the right). However, its contemporary meaning emerged only in the 1990s. | Epigenetics | Wikipedia | 500 | 49033 | https://en.wikipedia.org/wiki/Epigenetics | Biology and health sciences | Genetics and taxonomy | null |
A definition of the concept of epigenetic trait as a "stably heritable phenotype resulting from changes in a chromosome without alterations in the DNA sequence" was formulated at a Cold Spring Harbor meeting in 2008, although alternate definitions that include non-heritable traits are still being used widely.
Waddington's canalisation, 1940s
The hypothesis of epigenetic changes affecting the expression of chromosomes was put forth by the Russian biologist Nikolai Koltsov. From the generic meaning, and the associated adjective epigenetic, British embryologist C. H. Waddington coined the term epigenetics in 1942 as pertaining to epigenesis, in parallel to Valentin Haecker's 'phenogenetics' (). Epigenesis in the context of the biology of that period referred to the differentiation of cells from their initial totipotent state during embryonic development.
When Waddington coined the term, the physical nature of genes and their role in heredity was not known. He used it instead as a conceptual model of how genetic components might interact with their surroundings to produce a phenotype; he used the phrase "epigenetic landscape" as a metaphor for biological development. Waddington held that cell fates were established during development in a process he called canalisation much as a marble rolls down to the point of lowest local elevation. Waddington suggested visualising increasing irreversibility of cell type differentiation as ridges rising between the valleys where the marbles (analogous to cells) are travelling.
In recent times, Waddington's notion of the epigenetic landscape has been rigorously formalized in the context of the systems dynamics state approach to the study of cell-fate. Cell-fate determination is predicted to exhibit certain dynamics, such as attractor-convergence (the attractor can be an equilibrium point, limit cycle or strange attractor) or oscillatory.
Contemporary
Robin Holliday defined in 1990 epigenetics as "the study of the mechanisms of temporal and spatial control of gene activity during the development of complex organisms."
More recent usage of the word in biology follows stricter definitions. As defined by Arthur Riggs and colleagues, it is "the study of mitotically and/or meiotically heritable changes in gene function that cannot be explained by changes in DNA sequence." | Epigenetics | Wikipedia | 482 | 49033 | https://en.wikipedia.org/wiki/Epigenetics | Biology and health sciences | Genetics and taxonomy | null |
The term has also been used, however, to describe processes which have not been demonstrated to be heritable, such as some forms of histone modification. Consequently, there are attempts to redefine "epigenetics" in broader terms that would avoid the constraints of requiring heritability. For example, Adrian Bird defined epigenetics as "the structural adaptation of chromosomal regions so as to register, signal or perpetuate altered activity states." This definition would be inclusive of transient modifications associated with DNA repair or cell-cycle phases as well as stable changes maintained across multiple cell generations, but exclude others such as templating of membrane architecture and prions unless they impinge on chromosome function. Such redefinitions however are not universally accepted and are still subject to debate. The NIH "Roadmap Epigenomics Project", which ran from 2008 to 2017, uses the following definition: "For purposes of this program, epigenetics refers to both heritable changes in gene activity and expression (in the progeny of cells or of individuals) and also stable, long-term alterations in the transcriptional potential of a cell that are not necessarily heritable." In 2008, a consensus definition of the epigenetic trait, a "stably heritable phenotype resulting from changes in a chromosome without alterations in the DNA sequence," was made at a Cold Spring Harbor meeting.
The similarity of the word to "genetics" has generated many parallel usages. The "epigenome" is a parallel to the word "genome", referring to the overall epigenetic state of a cell, and epigenomics refers to global analyses of epigenetic changes across the entire genome. The phrase "genetic code" has also been adapted – the "epigenetic code" has been used to describe the set of epigenetic features that create different phenotypes in different cells from the same underlying DNA sequence. Taken to its extreme, the "epigenetic code" could represent the total state of the cell, with the position of each molecule accounted for in an epigenomic map, a diagrammatic representation of the gene expression, DNA methylation and histone modification status of a particular genomic region. More typically, the term is used in reference to systematic efforts to measure specific, relevant forms of epigenetic information such as the histone code or DNA methylation patterns.
Mechanisms | Epigenetics | Wikipedia | 498 | 49033 | https://en.wikipedia.org/wiki/Epigenetics | Biology and health sciences | Genetics and taxonomy | null |
Covalent modification of either DNA (e.g. cytosine methylation and hydroxymethylation) or of histone proteins (e.g. lysine acetylation, lysine and arginine methylation, serine and threonine phosphorylation, and lysine ubiquitination and sumoylation) play central roles in many types of epigenetic inheritance. Therefore, the word "epigenetics" is sometimes used as a synonym for these processes. However, this can be misleading. Chromatin remodeling is not always inherited, and not all epigenetic inheritance involves chromatin remodeling. In 2019, a further lysine modification appeared in the scientific literature linking epigenetics modification to cell metabolism, i.e. lactylation | Epigenetics | Wikipedia | 175 | 49033 | https://en.wikipedia.org/wiki/Epigenetics | Biology and health sciences | Genetics and taxonomy | null |
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