More wheat (Triticum) is produced annually than any other cereal crop. Overall, wheat is perhaps the single most important food crop for humans. It is grown throughout the temperate regions of the world, but only in the highlands of the tropics and subtropics. Major wheat producers include Russia, the United States, China, India, France, Canada, Australia, Turkey, Pakistan, and Argentina. There are several wild and cultivated wheat species and thousands of cultivars. The two economically important species today are Bread Wheat (T. aestivum), accounting for around 95% of world wheat production, and Durum or Macaroni Wheat (T. turgidum durum), accounting for around 5% (Peng et al. 2011).

Wheat is among the most ancient of domesticated crops. It was apparently derived from wild species in the Fertile Crescent of southwestern Asia around 8000 B.C., together with Barley (Hordeum vulgare) and pulses. Einkorn (T. monococcum) and Emmer Wheat (T. turgidum dicoccum) represent early lineages of domesticated wheats. The grains of these species retain their hulls after threshing. Today, Einkorn and Emmer are grown only to a very limited extent. Modern Bread Wheat (T. aestivum) and Durum Wheat (T. turgidum durum) are "free-threshing", producing naked grains. After Bread Wheat entered cultivation, it spread into Europe, North Africa, and Asia. Through most of the 16th century, wheat was confined to these continents, but in the subsequent two centuries it was taken to North and South America and to South Africa. Somewhat later, it reached Australia and New Zealand.

Although major advances in understanding the domestication history of wheat were made in the early decades of the 20th century, significant questions remain unresolved even with modern tools of genetic analysis. Dvorak et al. (2012) recognize six Triticum species (some authorities recognize more species than this):

1) Two diploid species (diploids have the familiar two sets of chromosomes in each cell), T. monococcum and T. urartu

2) Two tetraploid species (with four sets of chromosomes), T. turgidum and T. timopheevii

3) Two hexaploid species (with six sets of chromosomes), T. aestivum and T. zhukovskyi

Domestication of wild Emmer Wheat (T. turgidum dicoccoides) yielded domesticated Emmer Wheat (T. turgidum dicoccum), from which "free-threshing" (hull-free) tetraploid wheat such as Durum Wheat (T. turgidum durum) evolved. It has been known since the 1920s that four of the six sets of chromosomes of hexaploid Bread Wheat (T. aestivum) come from T. turgidum, but the source of the remaining two sets of chromosomes was not identified until the 1940s, when it was identified as the diploid wild goatgrass Aegilops tauschii.

The ability of domesticated wheat to exchange genes with certain other grasses is a potentially serious concern in light of expanding efforts to incorporate herbicide resistance and other traits--beneficial traits we would not want transferred to weeds--in new wheat varieties using genetic engineering. Hegde and Waines (2004) reviewed available literature on the reproductive ecology of Bread Wheat and on introgression (infiltration of genes) between Bread Wheat and its wild relatives in the genus Aegilops and with feral Rye (Secale cereale) in North America. Willenborg and Van Acker (2008) discussed aspects of the biology and ecology of Bread Wheat that make the transfer of traits between cultivated wheat and weeds more or less likely.

(Vaughan and Geissler 1997; Goncharov 2011; Peng et al. 2011; Dvorak et al. 2012)