Taxonomy Of Wheat
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During 10,000 years of cultivation, numerous forms of
wheat Wheat is a group of wild and crop domestication, domesticated Poaceae, grasses of the genus ''Triticum'' (). They are Agriculture, cultivated for their cereal grains, which are staple foods around the world. Well-known Taxonomy of wheat, whe ...
, many of them hybrids, have developed under a combination of artificial and
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the Heredity, heritable traits characteristic of a population over generation ...
. This diversity has led to much confusion in the naming of wheats. Genetic and morphological characteristics of wheat influence its classification; many common and botanical names of wheat are in current use.


''Aegilops'' and ''Triticum''


Similarities and differences

The genus '' Triticum'' includes the wild and domesticated species usually thought of as wheat. In the 1950s growing awareness of the genetic similarity of the wild goatgrasses ('' Aegilops'') led botanists such as Bowden to amalgamate ''Aegilops'' and ''Triticum'' as one genus, ''Triticum''. This approach is still followed by some (mainly geneticists), but has not been widely adopted by taxonomists. ''Aegilops'' is morphologically highly distinct from ''Triticum'', with rounded rather than keeled
glume In botany, a glume is a bract (leaf-like structure) below a spikelet in the inflorescence (flower cluster) of grass Poaceae ( ), also called Gramineae ( ), is a large and nearly ubiquitous family (biology), family of monocotyledonous flow ...
s.


Hybridisation and polyploidy

''Aegilops'' is important in wheat
evolution Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...
because of its role in two important hybridisation events. Wild emmer ('' T. dicoccoides'' and '' T. araraticum'') resulted from the hybridisation of a wild wheat, '' T. urartu'', and an as yet unidentified goatgrass, probably closely related to '' Ae. speltoides''. Hexaploid wheats (e.g. '' T. aestivum'' the most common and '' T. spelta'') are the result of a hybridisation between a domesticated tetraploid wheat, probably '' T. dicoccum'' or '' T. durum'', and another goatgrass, '' Ae. tauschii'' or '' Ae. squarrosa''. The hexaploid genome is an allohexaploid composed of two copies each of three subgenomes, AABBDD. The is from ''T. urartu'' (AA). The is a descendant of the of an unidentified species related to ''Aegilops'' section '' Sitopsis'' (SS). This natural hybridization event happened ~3–0.8 MYA, yielding the tetraploid '' T. dicoccoides''. In time this tetraploid gave rise to '' T. turgidum'', which gave rise to modern durum. Then ~0.4 MYA ''T. diccocoides'' naturally crossed with ''Aegilops tauschii'' (DD), adding the and yielding the hexaploid.


Early taxonomy

Botanists of the classical period, such as Columella, and in sixteenth and seventeenth century
herbal A herbal is a book containing the names and descriptions of plants, usually with information on their medicinal, Herbal tonic, tonic, culinary, toxic, hallucinatory, aromatic, or Magic (paranormal), magical powers, and the legends associated wi ...
s, divided wheats into two groups, ''Triticum'' corresponding to free-threshing wheats, and ''Zea'' corresponding to hulled ('spelt') wheats.
Carl Linnaeus Carl Linnaeus (23 May 1707 – 10 January 1778), also known after ennoblement in 1761 as Carl von Linné,#Blunt, Blunt (2004), p. 171. was a Swedish biologist and physician who formalised binomial nomenclature, the modern system of naming o ...
recognised five species, all domesticated: * '' T. aestivum'' Bearded spring wheat * '' T. hybernum'' Beardless winter wheat * '' T. turgidum'' Rivet wheat * '' T. spelta'' Spelt wheat * '' T. monococcum'' Einkorn wheat Later classifications added to the number of species described, but continued to give species status to relatively minor variants, such as winter- vs. spring- forms. The wild wheats were not described until the mid-19th century because of the poor state of botanical exploration in the
Near East The Near East () is a transcontinental region around the Eastern Mediterranean encompassing the historical Fertile Crescent, the Levant, Anatolia, Egypt, Mesopotamia, and coastal areas of the Arabian Peninsula. The term was invented in the 20th ...
, where they grow. The development of a modern classification depended on the discovery, in the 1920s, that wheat was divided into 3
ploidy Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Here ''sets of chromosomes'' refers to the number of maternal and paternal chromosome copies, ...
levels.


Important characters in wheat


Ploidy level

As with many grasses, polyploidy is common in wheat. There are two wild diploid (non-polyploid) wheats, '' T. boeoticum'' and '' T. urartu''. ''T. boeoticum'' is the wild ancestor of domesticated einkorn, '' T. monococcum''. Cells of the diploid wheats each contain 2 complements of 7 chromosomes, one from the mother and one from the father (2n=2x=14, where 2n is the number of chromosomes in each somatic cell, and x is the basic chromosome number). The polyploid wheats are tetraploid (4 sets of chromosomes, 2n=4x=28), or hexaploid (6 sets of chromosomes, 2n=6x=42). The tetraploid wild wheats are wild emmer, '' T. dicoccoides'', and '' T. araraticum''. Wild emmer is the ancestor of all the domesticated tetraploid wheats, with one exception: ''T. araraticum'' is the wild ancestor of '' T. timopheevii''. There are no wild hexaploid wheats, although feral forms of common wheat are sometimes found. Hexaploid wheats developed under
domestication Domestication is a multi-generational Mutualism (biology), mutualistic relationship in which an animal species, such as humans or leafcutter ants, takes over control and care of another species, such as sheep or fungi, to obtain from them a st ...
. Genetic analysis has shown that the original hexaploid wheats were the result of a cross between a tetraploid domesticated wheat, such as ''T. dicoccum'' or ''T. durum'', and a wild goatgrass, such as ''Ae. tauschii''. Polyploidy is important to wheat classification for three reasons: *Wheats within one ploidy level will be more closely related to each other. *Ploidy level influences some plant characteristics. For example, higher levels of ploidy tend to be linked to larger cell size. *Polyploidy brings new genomes into a species. For example, ''Aegilops tauschii'' brought the D genome into hexaploid wheats, with enhanced cold-hardiness and some distinctive morphological features.


Genome

Observation of chromosome behaviour during
meiosis Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...
, and the results of hybridisation experiments, have shown that wheat genomes (complete complements of genetic matter) can be grouped into distinctive types. Each type has been given a name, A, B, and D. Grasses sharing the same genome will be more-or-less interfertile, and might be treated by botanists as one species. Identification of genome types is obviously a valuable tool in investigating hybridisation. For example, if two diploid plants hybridise to form a new polyploid form (an allopolyploid), the two original genomes will be present in the new form. Many thousands of years after the original hybridisation event, identification of the component genomes will allow identification of the original parent species. In ''Triticum'', five genomes, all originally found in diploid species, have been identified: * Am, also called Ab – present in wild einkorn (''T. boeoticum''). * A – present in ''T. urartu'' (closely related to ''T. boeoticum'' but not interfertile). * B – present in most tetraploid wheats. Source not identified, but similar to ''Ae. speltoides''. * G – present in ''timopheevii'' group of wheats. Source not identified, but similar to ''Ae. speltoides''. * D – present in ''Ae. tauschii'', and thus in all hexaploid wheats. The genetic approach to wheat taxonomy (see below) takes the genome composition as defining each species. As there are five known combinations in ''Triticum'' this translates into five ''super'' species: * Am ''T. monococcum'' * Au ''T. urartu'' * BAu ''T. turgidum'' * GAm ''T. timopheevii'' * BAuD, ''T. aestivum'' For a larger list of genome names, see .


Domestication

There are four wild species, all growing in rocky habitats in the
fertile crescent The Fertile Crescent () is a crescent-shaped region in the Middle East, spanning modern-day Iraq, Israel, Jordan, Lebanon, Palestine, and Syria, together with northern Kuwait, south-eastern Turkey, and western Iran. Some authors also include ...
of the
Near East The Near East () is a transcontinental region around the Eastern Mediterranean encompassing the historical Fertile Crescent, the Levant, Anatolia, Egypt, Mesopotamia, and coastal areas of the Arabian Peninsula. The term was invented in the 20th ...
. All the other species are domesticated. Although relatively few genes control domestication, and wild and domesticated forms are interfertile, wild and domesticated wheats occupy entirely separate habitats. Traditional classification gives more weight to domesticated status.


Hulled ''vs.'' free-threshing

All wild wheats are hulled: they have tough glumes (husks) that tightly enclose the grains. Each ''package'' of glumes, lemma and palaea, and grains is known as a spikelet. At maturity the
rachis In biology, a rachis (from the [], "backbone, spine") is a main axis or "shaft". In zoology and microbiology In vertebrates, ''rachis'' can refer to the series of articulated vertebrae, which encase the spinal cord. In this case the ''rachi ...
(central stalk of the cereal ear) disarticulates, allowing the spikelets to disperse. The first domesticated wheats, einkorn and emmer, were hulled like their wild ancestors, but with rachises that (while not entirely tough) did not disarticulate at maturity. During the
Pre-Pottery Neolithic B Pre-Pottery Neolithic B (PPNB) is part of the Pre-Pottery Neolithic, a Neolithic culture centered in upper Mesopotamia and the Levant, dating to years ago, that is, 8800–6500 BC. It was Type site, typed by British archaeologist Kathleen Kenyon ...
period, at about 8000 BC, free-threshing forms of wheat evolved, with light glumes and fully tough rachis. Hulled or free-threshing status is important in traditional classification because the different forms are usually grown separately, and have very different post-harvesting processing. Hulled wheats need substantial extra pounding or milling to remove the tough glumes.


Morphology

In addition to hulled/free-threshing status, other morphological criteria, e.g. spike laxness or glume wingedness, are important in defining wheat forms. Some of these are covered in the individual species accounts linked from this page, but Floras must be consulted for full descriptions and identification keys.


Traditional ''vs.'' genetic classifications

Although the range of recognised types of wheat has been reasonably stable since the 1930s, there are now sharply differing views as to whether these should be recognised at species level (traditional approach) or at subspecific level (genetic approach). The first advocate of the genetic approach was Bowden, in a 1959 classification (now historic rather than current). He, and subsequent proponents (usually geneticists), argued that forms that were interfertile should be treated as one species (the
biological species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of classification an ...
concept). Thus emmer and hard wheat should both be treated as subspecies (or at other infraspecific ranks) of a single tetraploid species defined by the genome BAu. Van Slageren's 1994 classification is probably the most widely used genetic-based classification at present. Users of traditional classifications give more weight to the separate habitats of the traditional species, which means that species that could hybridise do not, and to morphological characters. There are also pragmatic arguments for this type of classification: it means that most species can be described in Latin binomials, e.g. '' Triticum aestivum'', rather than the trinomials necessary in the genetic system, e.g. ''T. a.'' subsp. ''aestivum''. Both approaches are widely used.


Infraspecific classification

In the nineteenth century, elaborate schemes of classification were developed in which wheat ears were classified to botanical variety on the basis of morphological criteria such as glume hairiness and colour or grain colour. These variety names are now largely abandoned, but are still sometimes used for distinctive types of wheat such as miracle wheat, a form of '' T. turgidum'' with branched ears, known as ''T. t.'' L. var. ''mirabile'' Körn. The term "
cultivar A cultivar is a kind of Horticulture, cultivated plant that people have selected for desired phenotypic trait, traits and which retains those traits when Plant propagation, propagated. Methods used to propagate cultivars include division, root a ...
" (abbreviated as ''cv.'') is often confused with "
species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
" or "domesticate". In fact, it has a precise meaning in botany: it is the term for a distinct population of a crop, usually commercial and resulting from deliberate plant-breeding. Cultivar names are always capitalised, often placed between apostrophes, and not italicised. An example of a cultivar name is ''T. aestivum'' cv. 'Pioneer 2163'. A cultivar is often referred to by farmers as a variety, but this is best avoided in print, because of the risk of confusion with botanical varieties. The term "
landrace A landrace is a Domestication, domesticated, locally adapted, often traditional variety of a species of animal or plant that has developed over time, through adaptation to its natural and cultural Environment (biophysical), environment of agric ...
" is applied to informal, farmer-maintained populations of crop plants.


Naming

Botanical names for wheat are generally expected to follow an existing classification, such as those listed as ''current'' by the . The classifications given in the following table are among those suitable for use. If a genetic classification is favoured, the GRIN classification is comprehensive, based on van Slageren's work but with some extra taxa recognised. If the traditional classification is favoured, Dorofeev's work is a comprehensive scheme that meshes well with other less complete treatments. Wikipedia's wheat pages generally follow a version of the Dorofeev scheme – see the taxobox on the Wheat page. A general rule is that different taxonomic schemes should not be mixed in one context. In a given article, book or web page, only one scheme should be used at a time. Otherwise, it will be unclear to others how the botanical name is being used.


Table of wheat species

Note: Blank common name indicates that no common name is in use in the English language.


Explanatory notes on selected names

*''Triticum boeoticum'' Boiss. is sometimes divided into two subspecies: **''T. boeoticum'' Boiss. subsp. ''thaoudar'' (Reut. ex Hausskn.) E. Schiem. – with two grains in each spikelet, distributed to east of fertile crescent. **''T. boeoticum'' Boiss. subsp. ''boeoticum'' – one grain in each spikelet, in Balkans. *''Triticum dicoccum'' Schrank ex Schübler is also known as ''Triticum dicoccon'' Schrank. *''Triticum aethiopicum'' Jakubz. is a variant form of ''T. durum'' found in Ethiopia. It is not usually regarded as a separate species. *''Triticum karamyschevii'' Nevsky was previously known as ''Triticum paleocolchicum'' A. M. Menabde.


Artificial species and mutants

Russian botanists have given botanical names to hybrids developed during genetic experiments. As these only occur in the laboratory environment, it is questionable whether botanical names (rather than lab. numbers) are justified. Botanical names have also been given to rare mutant forms. Examples include: *'' Triticum × borisovii'' Zhebrak – ('' T. aestivum'' × '' T. timopheevi'') *'' Triticum × fungicidum'' Zhuk. – Hexaploid, artificial cross ('' T. carthlicum'' × ''T. timopheevi'') *'' Triticum jakubzineri'' Udachin & Shakhm. *'' Triticum militinae'' Zhuk. & Migush. – mutant form of ''T. timopheevi''. *'' Triticum petropavlovskyi'' Udachin & Migush. *'' Triticum sinskajae'' Filat. & Kurkiev – mutant, free-threshing form of '' T. monococcum''. *'' Triticum × timococcum'' Kostov *'' Triticum timonovum'' Heslot & Ferrary – Hexaploid, artificial cross. *'' Triticum zhukovskyi'' Menabde & Ericzjan (''T. timopheevi'' × ''T. monococcum''): 6N


See also

* Winter wheat vs. spring wheat * Cultivated plant taxonomy * List of Canadian Heritage Wheat Varieties


References


Sources

* * * * Lists of ''Triticum'' names. An essential tool. * Includes links to USDA germplasm collection, and public domain images
Germplasm Resources Information Network Germplasm Resources Information Network or GRIN is an online USDA National Genetic Resources Program software project to comprehensively manage the computer database for the holdings of all plant germplasm collected by the National Plant Germpla ...
(GRIN) *


External links


Taxonomy


Les meilleurs blés (1880 and 1909)
Also o
Pl@ntUse
Beautifully illustrated French book on wheats then in cultivation and studied by the French breeders family Vilmorin.


Genetics



Mainly concerned with the International Triticeae Meeting. Site includes genome tables for Triticeae.
GrainGenes: Triticeae TaxonomyAnnual Wheat Newsletter


Morphology


Wheat: the big picture
Illustrated guide to life cycle of wheat plant {{DEFAULTSORT:Wheat taxonomy .
wheat Wheat is a group of wild and crop domestication, domesticated Poaceae, grasses of the genus ''Triticum'' (). They are Agriculture, cultivated for their cereal grains, which are staple foods around the world. Well-known Taxonomy of wheat, whe ...