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.

[9] Then ~0.4 MYA T. diccocoides naturally crossed with Aegilops tauschii (DD), adding the D genome and yielding the hexaploid.

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.

[8] Polyploidy is important to wheat classification for three reasons: 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.

[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.

All wild wheats are hulled: they have tough glumes (husks) that tightly enclose the grains.

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 wheats need substantial extra pounding or milling to remove the tough glumes.

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.

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.

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.

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.

If the traditional classification is favoured, Dorofeev's work is a comprehensive scheme that meshes well with other less complete treatments.