Wheat is a member of the grass family (Gramineae), as are the other grains - rye, barley, and oats - that are suspected of harming people with celiac disease. Of course, triticale, a cross between wheat and rye, is toxic, as would be expected for any other similar crosses that included genetic material from one or more of the toxic grains.
The grasses are a relatively recent evolutionary development. They are angiosperms, flowering plants, that developed somewhere between 100 and 200 million years ago (Cronquist 1968). Fossil evidence for grasses goes back only about 65 million years. They became widespread during the Oligocene epoch about 25 to 40 million years ago (Thomasson 1980), which, when it is considered that life has been evolving on earth for about 4 billion years, is a relatively short time. Cereal grains presumably evolved more recently within the time frame of grass evolution.
The ancestors of wheat, barley, and rye were diploid species with 7 chromosomes in the haploid state, or 14 chromosomes in vegetative cells. Barley first diverged from the common ancestral line, followed by rye. The line eventually gave rise to a series of diploid species that may be classed as Triticum species (Morris and Sears 1967). A natural cross occurred at some unknown time between two slightly diverged Triticum species, presumably Triticum urartu and Triticum speltoides, that gave rise to a new species through a process of polyploid formation in which chromosomes become doubled.
Without chromosome doubling, a cross between related species, such as those of Triticum, can take place, but the offspring is infertile because of a failure by the chromosomes to pair during meiosis. Polyploid formation occurs naturally, albeit rarely, perhaps through a process involving unreduced gametes (Harlan and de Wet 1975). This process can also be achieved in the laboratory through the use of chemicals (such as colchicine) that interfere with spindle formation to block chromosomal separation at anaphase. Either way, the result is a doubled set of chromosomes, a condition that allows for each original chromosome set to pair with its identical replicated set, thus avoiding the chromosome pairing problems that occur in crosses between species. The result of allopolyploid formation between T. urartu and T. spel-toides was a new fertile species, called a tetraploid because it incorporated four sets of chromosomes in vegetative cells (designated. AABB). Durum wheats (Triticum turgidum var. durum), used for pasta making, belong to one of the varietal groups of the. AABB tetraploids.
Some time after wheat cultivation began, a tetra-ploid wheat crossed accidentally with a weed growing in the same field (Triticum tauschii var. strangulata), followed by chromosome doubling to give rise to a hexaploid species, which, because the genome of T. tauschii was designated D, had the composition AABBDD. Bread wheats (Triticum aestivum var. aes-tivum) produce a more elastic dough than durum wheats and have properties that lend themselves to the retention of gases produced during yeast fermentation (leavening) of doughs. It was in the context of the evolution of wheat and closely related species that proteins arose with amino acid sequences capable of initiating damage in persons with celiac disease.
World History
