Taxonomy of wheat

During 10,000 years of cultivation, numerous forms of wheat, many of them hybrids, have developed under a combination of artificial and natural selection. 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 led botanists such as Bowden to amalgamate Aegilops and Triticum as one genus, Triticum. This approach is still followed by some, but has not been widely adopted by taxonomists. Aegilops is morphologically highly distinct from Triticum, with rounded rather than keeled glumes.

Hybridisation and polyploidy

Aegilops is important in wheat evolution because of its role in two important hybridisation events. Wild emmer resulted from the hybridisation of a wild wheat, T. urartu, and an as yet unidentified goatgrass, probably closely related to Ae. speltoides. Hexaploid wheats 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. The is a descendant of the of an unidentified species related to Aegilops section Sitopsis. 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, adding the and yielding the hexaploid.

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 wheats.
Carl Linnaeus 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.

Important characters in wheat

Ploidy level

As with many grasses, polyploidy is common in wheat. 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.
The polyploid wheats are tetraploid, or hexaploid. 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. 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, and the results of hybridisation experiments, have shown that wheat genomes 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, 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:

A^m, also called A^b – present in wild einkorn.
A – present in T. urartu.
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 takes the genome composition as defining each species. As there are five known combinations in Triticum this translates into five super species:

A^m T. monococcum
A^u T. urartu
BA^u T. turgidum
GA^m T. timopheevii
BA^uD, 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 of the Near East. 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 that tightly enclose the grains. Each package of glumes, lemma and palaea, and grains is known as a spikelet. At maturity the rachis disarticulates, allowing the spikelets to disperse.
The first domesticated wheats, einkorn and emmer, were hulled like their wild ancestors, but with rachises that 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 or at subspecific level. The first advocate of the genetic approach was Bowden, in a 1959 classification. He, and subsequent proponents, argued that forms that were interfertile should be treated as one species. Thus emmer and hard wheat should both be treated as subspecies of a single tetraploid species defined by the genome BA^u. 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" 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.