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 (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 glumes.

Hybridisation and polyploidy

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. 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 A genome is from T. urartu (AA). The B genome is a descendant of the S genome 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 D genome 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 ('spelt') 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 (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. 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 (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 Triticeae § Genetics.

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