Taxonomy of wheat

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Miracle wheat (Triticum turgidum var. mirabile) Blemiracle.jpg
Miracle wheat (Triticum turgidum var. mirabile)

During 10,000 years of cultivation, numerous forms of wheat, many of them hybrids, have developed under a combination of artificial and natural selection. [1] [2] 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.

Contents

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. [3] This approach is still followed by some (mainly geneticists), but has not been widely adopted by taxonomists. [4] Aegilops is morphologically highly distinct from Triticum, with rounded rather than keeled glumes. [5]

Hybridisation and polyploidy

Wheat origins by repeated hybridization and polyploidy (e.g. "6N" means 6 sets of chromosomes per cell rather than the usual 2). Only a few of the wheat species involved are shown. The goatgrass species involved are not known for certain. Polyploid wheat origins.svg
Wheat origins by repeated hybridization and polyploidy (e.g. "6N" means 6 sets of chromosomes per cell rather than the usual 2). Only a few of the wheat species involved are shown. The goatgrass species involved are not known for certain.

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 closely related to Ae. speltoides . [7] 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 . [6] [8] The hexaploid genome is an allohexaploid composed of two copies each of three subgenomes, AABBDD. [9] The A genome is from T. urartu (AA). [9] The B genome is a descendant of the S genome of an unidentified species related to Aegilops section Sitopsis (SS). [9] This natural hybridization event happened ~3–0.8 MYA, yielding the tetraploid T. dicoccoides . [9] In time this tetraploid gave rise to T. turgidum , which gave rise to modern durum. [9] Then ~0.4 MYA T. diccocoides naturally crossed with Aegilops tauschii (DD), adding the D genome and yielding the hexaploid. [9]

Early taxonomy

Botanists of the classical period, such as Columella, 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. [4]

Carl Linnaeus recognised five species, all domesticated: [4]

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. [4]

The development of a modern classification depended on the discovery, in the 1920s, that wheat was divided into 3 ploidy levels. [10]

Important characters in wheat

Ploidy level

As with many grasses, polyploidy is common in wheat. [11] There are two wild diploid (non-polyploid) wheats, T. boeoticum and T. urartu . T. boeoticum is the wild ancestor of domesticated einkorn, T. monococcum . [12] 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 . [13]

There are no wild hexaploid wheats, although feral forms of common wheat are sometimes found. Hexaploid wheats developed 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, such as Ae. tauschii. [8]

Polyploidy is important to wheat classification for three reasons:

Genome

Observation of chromosome behaviour during meiosis, 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. [16]

In Triticum, five genomes, all originally found in diploid species, have been identified:

The genetic approach to wheat taxonomy (see below) takes the genome composition as defining each species. [17] As there are five known combinations in Triticum this translates into five super species:

For a larger list of genome names, see Triticeae § Genetics.

Domestication

There are four wild species, all growing in rocky habitats in the fertile crescent of the Near East. [18] 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 (central stalk of the cereal ear) disarticulates, allowing the spikelets to disperse. [19]

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 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). [20] 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 BAu. Van Slageren's 1994 classification is probably the most widely used genetic-based classification at present. [21]

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" (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. The term "landrace" 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 Wheat Genetics Resource Center . [22] 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

Wheat taxonomy – two schemes
Common nameGenome(s)Genetic ( [23] )Traditional (Dorofeev et al. 1979 [24] )
Diploid (2x), wild, hulled
Wild einkornAm Triticum monococcum L. subsp. aegilopoides(Link) Thell. Triticum boeoticum Boiss.
Au Triticum urartu Tumanian ex GandilyanTriticum urartuTumanian ex Gandilyan
Diploid (2x), domesticated, hulled
EinkornAm Triticum monococcum L. subsp. monococcum Triticum monococcum L.
Tetraploid (4x), wild, hulled
Wild emmerBAu Triticum turgidum L. subsp. dicoccoides(Korn. ex Asch. & Graebn.) Thell. Triticum dicoccoides (Körn. ex Asch. & Graebner) Schweinf.
Tetraploid (4x), domesticated, hulled
EmmerBAu Triticum turgidum L. subsp. dicoccum(Schrank ex Schübl.) Thell. Triticum dicoccum Schrank ex Schübler
BAu Triticum ispahanicum HeslotTriticum ispahanicumHeslot
BAu Triticum turgidum L. subsp. paleocolchicum Á. & D. Löve Triticum karamyschevii Nevski
Tetraploid (4x), domesticated, free-threshing
Durum or macaroni wheatBAu Triticum turgidum L. subsp. durum(Desf.) Husn. Triticum durum Desf.
Rivet, cone or English wheatBAu Triticum turgidum L. subsp. turgidum Triticum turgidum L.
Polish wheatBAu Triticum turgidum L. subsp. polonicum(L.) Thell. Triticum polonicum L.
Khorasan wheatBAu Triticum turgidum L. subsp. turanicum(Jakubz.) Á. & D. Löve Triticum turanicum Jakubz.
Persian wheatBAu Triticum turgidum L. subsp. carthlicum(Nevski) Á. & D. Löve Triticum carthlicum Nevski in Kom.
Tetraploid (4x) – timopheevi group
Wild, hulled
GAm Triticum timopheevii (Zhuk.) Zhuk. subsp. armeniacum(Jakubz.) Slageren Triticum araraticum Jakubz.
Domesticated, hulled
GAm Triticum timopheevii(Zhuk.) Zhuk. subsp. timopheevii Triticum timopheevii (Zhuk.) Zhuk.
Hexaploid (6x), domesticated, hulled
Spelt wheatBAuD Triticum aestivum L. subsp. spelta(L.) Thell. 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 wheatBAuD Triticum aestivum L. subsp. aestivumTriticum aestivum L.
Club wheatBAuD Triticum aestivum L. subsp. compactum(Host) Mackey Triticum compactum Host
Indian dwarf or shot wheatBAuD Triticum aestivum L. subsp. sphaerococcum(Percival) Mackey Triticum sphaerococcum Percival

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

Explanatory notes on selected names

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:

See also

Related Research Articles

<span class="mw-page-title-main">Wheat</span> Genus of grass cultivated for the grain

Wheat is a grass widely cultivated for its seed, a cereal grain that is a worldwide staple food. The many species of wheat together make up the genus Triticum ; the most widely grown is common wheat. The archaeological record suggests that wheat was first cultivated in the regions of the Fertile Crescent around 9600 BC. Botanically, the wheat kernel is a caryopsis, a type of fruit.

<span class="mw-page-title-main">Einkorn wheat</span> Primitive wheat

Einkorn wheat can refer either to a wild species of wheat (Triticum) or to its domesticated form. The wild form is T. boeoticum, and the domesticated form is T. monococcum. Einkorn is a diploid species of hulled wheat, with tough glumes ('husks') that tightly enclose the grains. The cultivated form is similar to the wild, except that the ear stays intact when ripe and the seeds are larger. The domestic form is known as "petit épeautre" in French, "Einkorn" in German, "einkorn" or "littlespelt" in English, "piccolo farro" in Italian and "escanda menor" in Spanish. The name refers to the fact that each spikelet contains only one grain.

<span class="mw-page-title-main">Emmer</span> Type of wheat

Emmer wheat or hulled wheat is a type of awned wheat. Emmer is a tetraploid. The domesticated types are Triticum turgidum subsp. dicoccum and T. t. conv. durum. The wild plant is called T. t. subsp. dicoccoides. The principal difference between the wild and the domestic forms is that the ripened seed head of the wild plant shatters and scatters the seed onto the ground, while in the domesticated emmer, the seed head remains intact, thus making it easier for humans to harvest the grain.

<span class="mw-page-title-main">Durum</span> Species of wheat used for food

Durum wheat, also called pasta wheat or macaroni wheat, is a tetraploid species of wheat. It is the second most cultivated species of wheat after common wheat, although it represents only 5% to 8% of global wheat production. It was developed by artificial selection of the domesticated emmer wheat strains formerly grown in Central Europe and the Near East around 7000 BC, which developed a naked, free-threshing form. Like emmer, durum wheat is awned. It is the predominant wheat that grows in the Middle East.

<span class="mw-page-title-main">Spelt</span> Species of grain

Spelt, also known as dinkel wheat or hulled wheat, is a species of wheat that has been cultivated since approximately 5000 BCE.

<span class="mw-page-title-main">Khorasan wheat</span> Species of grass

Khorasan wheat or Oriental wheat is a tetraploid wheat species. The grain is twice the size of modern-day wheat, and has a rich, nutty flavor.

<span class="mw-page-title-main">Founder crops</span> Original agricultural crops

The founder crops or primary domesticates are a group of flowering plants that were domesticated by early farming communities in Southwest Asia and went on to form the basis of agricultural economies across Eurasia. As originally defined by Daniel Zohary and Maria Hopf, they consisted of three cereals, four pulses, and flax. Subsequent research has indicated that many other species could be considered founder crops. These species were amongst the first domesticated plants in the world.

<i>Aegilops</i> Genus of grasses

Aegilops is a genus of Eurasian and North American plants in the grass family, Poaceae. They are known generally as goatgrasses. Some species are known as invasive weeds in parts of North America.

<span class="mw-page-title-main">Common wheat</span> Species of plant

Common wheat, also known as bread wheat, is a cultivated wheat species. About 95% of wheat produced worldwide is common wheat; it is the most widely grown of all crops and the cereal with the highest monetary yield.

<span class="mw-page-title-main">Farro</span> Food made from the grains of certain wheat species

Farro refers to the grains of three wheat species, which are sold dried and cooked in water until soft. It is eaten plain or is often used as an ingredient in salads, soups, and other dishes.

<span class="mw-page-title-main">Triticeae</span> Tribe of grasses

Triticeae is a botanical tribe within the subfamily Pooideae of grasses that includes genera with many domesticated species. Major crop genera found in this tribe include wheat, barley, and rye; crops in other genera include some for human consumption, and others used for animal feed or rangeland protection. Among the world's cultivated species, this tribe has some of the most complex genetic histories. An example is bread wheat, which contains the genomes of three species with only one being a wheat Triticum species. Seed storage proteins in the Triticeae are implicated in various food allergies and intolerances.

<i>Aegilops tauschii</i> Species of grass

Aegilops tauschii, the Tausch's goatgrass or rough-spike hard grass, is an annual grass species. It is native to Crimea, the Caucasus region, western and Central Asia, Afghanistan, Pakistan, the western Himalaya, and parts of China, and has been introduced to other locales, including California.

<span class="mw-page-title-main">Triticeae glutens</span> Seed storage protein in mature wheat seeds

Gluten is the seed storage protein in mature wheat seeds. It is the sticky substance in bread wheat which allows dough to rise and retain its shape during baking. The same, or very similar, proteins are also found in related grasses within the tribe Triticeae. Seed glutens of some non-Triticeae plants have similar properties, but none can perform on a par with those of the Triticeae taxa, particularly the Triticum species. What distinguishes bread wheat from these other grass seeds is the quantity of these proteins and the level of subcomponents, with bread wheat having the highest protein content and a complex mixture of proteins derived from three grass species.

<i>Triticum carthlicum</i> Species of grass

Triticum carthlicum Nevski, 1934, the Persian wheat, is a wheat with a tetraploid genome.

Triticum urartu, also known as red wild einkorn wheat, and a form of einkorn wheat, is a grass species related to wheat, and native to western Asia. It is a diploid species whose genome is the A genome of the allopolyploid hexaploid bread wheat Triticum aestivum, which has genomes AABBDD.

<i>Triticum zhukovskyi</i> Species of grass

Triticum zhukovskyi, or Zhukovsky's wheat, is a hexaploid wheat, very closely resembling the Triticum timopheevii, a tetraploid variety of wheat. T. zhukovskyi was first observed in Western Georgia in close proximity to Triticum timopheevii and Triticum monococcum and is believed to be an amphiploid arising from the cross of T. timopheevii and T. monococcum.

Triticum araraticum is a wild tetraploid species of wheat. T. araraticum is one of the least studied wheat species in the world.

<span class="mw-page-title-main">Ancient grains</span> Small, hard, dry seeds used as food

Ancient grains is a marketing term used to describe a category of grains and pseudocereals that are purported to have been minimally changed by selective breeding over recent millennia, as opposed to more widespread cereals such as corn, rice and modern varieties of wheat, which are the product of thousands of years of selective breeding. Ancient grains are often marketed as being more nutritious than modern grains, though their health benefits over modern varieties have been disputed by some nutritionists.

Ernest Robert Sears was an American geneticist, botanist, pioneer of plant genetics, and leading expert on wheat cytogenetics. Sears and Sir Ralph Riley (1924–1999) are perhaps the two most important founders of chromosome engineering in plant breeding.

References

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  19. ↑ Hulled wheats : proceedings of the First International Workshop on Hulled Wheats, 21-22 July 1995, Castelvecchio Pascoli, Tuscany, Italy. Padulosi, S. (Stefano), Hammer, K. (Karl), Heller, J. (Joachim), International Plant Genetic Resources Institute. Rome: IPGRI. 1996. ISBN   978-9290432883. OCLC   36382216.{{cite book}}: CS1 maint: others (link)
  20. ↑ Bowden, W. M. 1959. "The taxonomy and nomenclature of the wheats, barleys, and ryes and their wild relatives". Canadian Journal of Botany 37:657–684.
  21. ↑ "Wheat Taxonomy". Kansas State University Wheat Genetics Resource Center. Retrieved October 14, 2022.
  22. ↑ "Wheat Taxonomy". Kansas State University . Archived from the original on May 17, 2022.
  23. ↑ GRIN-Global Taxonomy
  24. ↑ "Triticum classification according to Dorofeev & Migushova 1979". Archived from the original on May 22, 2000.

Sources

Taxonomy

Genetics

Morphology

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