During 10,000 years of cultivation, numerous forms of
wheatWheat is a worldwide cultivated grass from the Fertile Crescent region of the Near East. In 2007 world production of wheat was 607 million tons, making it the third most-produced cereal after maize and rice...
have evolved under human selection. This diversity has led to much confusion in the naming of wheats. This article explains how genetic and morphological characteristics of wheat influence its classification, and gives the most common botanical names of wheat in current use (see Table of wheat species). Information on the cultivation and uses of wheat is at the main
wheatWheat is a worldwide cultivated grass from the Fertile Crescent region of the Near East. In 2007 world production of wheat was 607 million tons, making it the third most-produced cereal after maize and rice...
page.
Aegilops and Triticum
The genus
Triticum includes the wild and domesticated species usually thought of as wheat.
In the 1950s growing awareness of the genetic similarity (including some shared genones) of the wild goatgrasses (
AegilopsAegilops is a genus of plants generally known as goatgrasses and belonging to the grass family, Poaceae. There are about 23 species and numerous sub species in the genus. Various members of the genus are classed as agricultural weeds. Growing through the winter, they resemble winter wheat...
) led some botanists 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 glumes rather than keeled glumes.
Aegilops is important in wheat evolution 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 similar to
Ae. speltoides. Hexaploid wheats (e.g.
T. aestivum 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 (also known as
Ae. squarrosa).
Early taxonomy
Botanists of the classical period, such as
ColumellaLucius Iunius Moderatus Columella was a Roman writer. After a career in the army , he took up farming...
, and in sixteenth and seventeenth century herbals, divided wheats into two groups,
Triticum corresponding to free-threshing wheats, and
Zea corresponding to hulled ('spelt') wheats.
Linneaus 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, where they grow.
The development of a modern classification depended on the discovery, in the 1920s, that wheat was divided into 3 ploidy levels.
Ploidy level
As with many grasses,
polyploidyPolyploidy occurs in cells and organisms when there are more than two paired sets of chromosomes.Most organisms are normally diploid, meaning they have two sets of chromosomes — one set inherited from each parent. Polyploidy may occur due to abnormal cell division. It is most commonly found in...
is common in wheat. Some wheats are not polyploid. There are two wild diploid 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. timopheevi.
There are no wild hexaploid wheats, although feral forms of
common wheatCommon wheat, Triticum aestivum, is a cultivated wheat species.- Nomenclature and taxonomy of the variety and its cultivars :-Evolution:...
are sometimes found. Hexaploid wheats evolved under domestication. 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,
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
meiosisIn biology, meiosis is a process of reductional division in which the number of chromosomes per cell is cut in half. In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores. As with mitosis, before meiosis begins, the DNA in the original...
, and the results of hybridisation experiments, have shown that grass genomes (complete complements of genetic matter) can be grouped into distinctive types. Each type has been given a name, e.g. B or 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 - present in wild einkorn (T. boeoticum).
- Au - 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 timopheevi group of wheats. Source not identified, but similar to Ae. speltoides.
- D - present in Ae. squarrosa, 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
- GAu T. timopheevi
- BAuD, T. aestivum
Domestication
There are four wild species, all growing in rocky habitats in the
fertile crescentThe Fertile Crescent is a region in the Near East, incorporating the Levant and Mesopotamia, and often incorrectly extended to Egypt. Mesopotamia is considered the cradle of civilization and saw the development of the earliest human civilizations and is the birthplace of writing and the wheel.The...
of the
Near EastNear East today is an ambiguous term that covers different countries for archeologists and historians, on one hand, and for political scientists, economists, and journalists, on the other...
. All the other species are
domesticatedDomestication or taming is the process whereby a population of animals or plants, through a process of selection, becomes accustomed to human provision and control. A defining characteristic of domestication is artificial selection by humans...
. 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 grain(s) is known as a spikelet. At maturity the rachis (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 PPNB 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.
For more information, see Wheat: Hulled vs. free-threshing wheat
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 printed 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)
http://www.k-state.edu/wgrc/Taxonomy/taxbow.html. He, and subsequent proponents (usually geneticists), argued that forms that were interfertile should be treated as one species (the biological species 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 BA
u. Van Slageren's 1994 classification
http://www.k-state.edu/wgrc/Taxonomy/taxvsltr.html 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.
Triticum aestivum 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
varietyIn botanical nomenclature, variety is a taxonomic rank below that of species: As such, it gets a ternary name .A variety will have an appearance distinct from other varieties, but will hybridize freely with those other varieties...
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. turgidum L. var.
mirabile Körn.
The term
cultivar (abbreviated as
cv.)is often confused with
species 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.
Advice for users
Anyone wishing to use a botanical name for wheat is best advised to follow an existing classification, such as those listed as
current at the Wheat Classification Tables Site
https://www.ksu.edu/wgrc/Taxonomy/taxintro.html. 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
taxoboxWheat is a worldwide cultivated grass from the Fertile Crescent region of the Near East. In 2007 world production of wheat was 607 million tons, making it the third most-produced cereal after maize and rice...
on the Wheat page.
The most critical point 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
| |
|
Wheat taxonomy - two schemes |
| Common name |
Genome(s) |
Genetic (GRIN Taxonomy for Plants http://www.ars-grin.gov/cgi-bin/npgs/html/index.pl) |
Traditional (Dorofeev et al. 1979 https://www.ksu.edu/wgrc/Taxonomy/taxdor.html) |
| Diploid (2x), Wild, Hulled |
| Wild einkorn |
Am |
Triticum monococcum L. subsp. aegilopoides (LinkJohann Heinrich Friedrich Link was a German naturalist and botanist.Link was born at Hildesheim as a son of the minister August Heinrich Link , who taught him the love for nature through collection of 'natural objects'...
) Thell.-Works:*Illustrierte Flora von Mittel-Europa 1906*Die Gattung Lepidium R. Br. : eine monographische Studie 1906 *Die Entstehung der Kulturpflanzen 1930 *Flora der Schweiz - Exkursionsflora, 1923...
|
Triticum boeoticum Boiss. |
| |
Au |
Triticum urartu Tumanian ex Gandilyan |
Triticum urartu Tumanian ex Gandilyan |
| Diploid (2x), Domesticated, Hulled |
| Einkorn |
Am |
Triticum monococcum L. subsp. monococcum |
Triticum monococcum L. |
| Tetraploid (4x), Wild, Hulled |
| Wild emmer |
BAu |
Triticum turgidum L. subsp. dicoccoides (Korn. ex Asch. & Graebn.) Thell. -Works:*Illustrierte Flora von Mittel-Europa 1906*Die Gattung Lepidium R. Br. : eine monographische Studie 1906 *Die Entstehung der Kulturpflanzen 1930 *Flora der Schweiz - Exkursionsflora, 1923...
|
Triticum dicoccoidesEmmer wheat , also known as farro especially in Italy, is a low yielding, awned wheat. It was one of the first crops domesticated in the Near East...
(Körn. ex Asch. & Graebner) Schweinf. |
| Tetraploid (4x), Domesticated, Hulled |
| Emmer |
BAu |
Triticum turgidum L. subsp. dicoccum (Schrank Franz von Paula Schrank was a German botanist and entomologist.Schrank was the first director of the botanical gardens in Munich from 1809 to 1832.Shrank was the first author to use the genus name Triops...
ex Schübl.Gustav Schübler was a German naturalist, and the founder of applied meteorology in Germany.In 1817 Schübler became professor of botany, natural history and agricultural chemistry at the University of Tübingen, Germany...
) Thell.-Works:*Illustrierte Flora von Mittel-Europa 1906*Die Gattung Lepidium R. Br. : eine monographische Studie 1906 *Die Entstehung der Kulturpflanzen 1930 *Flora der Schweiz - Exkursionsflora, 1923...
|
Triticum dicoccumEmmer wheat , also known as farro especially in Italy, is a low yielding, awned wheat. It was one of the first crops domesticated in the Near East...
SchrankFranz von Paula Schrank was a German botanist and entomologist.Schrank was the first director of the botanical gardens in Munich from 1809 to 1832.Shrank was the first author to use the genus name Triops...
ex SchüblerGustav Schübler was a German naturalist, and the founder of applied meteorology in Germany.In 1817 Schübler became professor of botany, natural history and agricultural chemistry at the University of Tübingen, Germany...
|
| |
BAu |
Triticum ispahanicum Heslot |
Triticum ispahanicum Heslot |
| |
BAu |
Triticum turgidum L. subsp. paleocolchicum A. Love & D. Love |
Triticum karamyschevii Nevski |
| Tetraploid (4x), Domesticated, Free-threshing |
| Durum or macaroni wheat |
BAu |
Triticum turgidum L. subsp. durum (Desf.) Husn. |
Triticum durumDurum wheat or macaroni wheat is the only tetraploid species of wheat of commercial importance that is widely cultivated today...
Desf. |
| Rivet or cone wheat |
BAu |
Triticum turgidum L. subsp. turgidum |
Triticum turgidum L. |
| Polish wheat |
BAu |
Triticum turgidum L. subsp. polonicum (L.) Thell. -Works:*Illustrierte Flora von Mittel-Europa 1906*Die Gattung Lepidium R. Br. : eine monographische Studie 1906 *Die Entstehung der Kulturpflanzen 1930 *Flora der Schweiz - Exkursionsflora, 1923...
|
Triticum polonicumTriticum polonicum, also known as polish wheat, is an ordinary variant of wheat. It's a tetraploid species with 14 chromosomes. It can be found in small areas of the Mediterranean region, Ethiopia, Russia and in other regions of Asia. It was first described by Carl Linnaeus in 1762....
L. |
| Khorasan wheat |
BAu |
Triticum turgidum L. subsp. turanicum (Jakubz.) A. Love & D. Love |
Triticum turanicum Jakubz. |
| Persian wheat |
BAu |
Triticum turgidum L. subsp. carthlicum (Nevski) A. Love & D. Love |
Triticum carthlicum Nevski in Kom. |
| Tetraploid (4x) - timopheevi group |
| Wild, Hulled |
| |
GAu |
Triticum timopheevii (Zhuk.) Zhuk. subsp. armeniacum (Jakubz.) Slageren |
Triticum araraticum Jakubz. |
| Domesticated, Hulled |
| |
GAu |
Triticum timopheevii (Zhuk.) Zhuk. subsp. timopheevii |
Triticum timopheevii Timopheev's Wheat is a tetraploid wheat that has both cultivated and wild forms. The domesticated form is restricted to western Georgia, while the wild form Timopheev's Wheat (Triticum timopheevii, also called Zanduri Wheat) is a tetraploid wheat that has both cultivated and wild forms. The...
(Zhuk.) Zhuk. |
| Hexaploid (6x), Domesticated, Hulled |
| Spelt wheat |
BAuD |
Triticum aestivum L. subsp. spelta (L.) Thell. -Works:*Illustrierte Flora von Mittel-Europa 1906*Die Gattung Lepidium R. Br. : eine monographische Studie 1906 *Die Entstehung der Kulturpflanzen 1930 *Flora der Schweiz - Exkursionsflora, 1923...
|
Triticum spelta L. |
| |
BAuD |
Triticum aestivum L. subsp. macha (Dekapr. & A. M. Menabde) Mackey |
Triticum macha Dekapr. & Menabde |
| |
BAuD |
Triticum vavilovii Jakubz. |
Triticum vavilovii (Tumanian) Jakubz. |
| Hexaploid (6x), Domesticated, Free-threshing |
| Common or bread wheat |
BAuD |
Triticum aestivum L. subsp. aestivum |
Triticum aestivumCommon wheat, Triticum aestivum, is a cultivated wheat species.- Nomenclature and taxonomy of the variety and its cultivars :-Evolution:...
L. |
| Club wheat |
BAuD |
Triticum aestivum L. subsp. compactum (Host) Mackey |
Triticum compactumCommon wheat, Triticum aestivum, is a cultivated wheat species.- Nomenclature and taxonomy of the variety and its cultivars :-Evolution:...
Host |
| Indian dwarf or shot wheat |
BAuD |
Triticum aestivum L. subsp. sphaerococcum (Percival) Mackey |
Triticum sphaerococcumCommon wheat, Triticum aestivum, is a cultivated wheat species.- Nomenclature and taxonomy of the variety and its cultivars :-Evolution:...
Percival |
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
Franz von Paula Schrank was a German botanist and entomologist.Schrank was the first director of the botanical gardens in Munich from 1809 to 1832.Shrank was the first author to use the genus name Triops...
ex SchüblerGustav Schübler was a German naturalist, and the founder of applied meteorology in Germany.In 1817 Schübler became professor of botany, natural history and agricultural chemistry at the University of Tübingen, Germany...
is also known as Triticum dicoccon SchrankFranz von Paula Schrank was a German botanist and entomologist.Schrank was the first director of the botanical gardens in Munich from 1809 to 1832.Shrank was the first author to use the genus name Triops...
.
- 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 genetical 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 Udaczin & Schachm.
- Triticum militinae Zhuk. & Migush. - mutant form of T. timopheevi.
- Triticum petropavlovskyi Udaczin & Migush.
- Triticum sinskajae A.A.Filatenko & U.K.Kurkiev - mutant, free-threshing form of T. monococcum.
- Triticum ×timococcum Kostov
- Triticum timonovum Heslot - Hexaploid, artificial cross.
- Triticum zhukovskyi Menabde & Ericzjan (T. timopheevi × T. monococcum)
Germplasm
Taxonomy
Genetics
Morphology