Evolution of the high molecular weight glutenin loci of the A, B, D, and G genomes of wheat.

We used PCR to obtain phylogenetically informative sequences from the high molecular weight glutenin genes of wheat. The validity of partial sequence comparisons as a means of studying glutenin phylogenetics was established by constructing neighbour-joining trees from partial alignments of 12 published glutenin allele sequences. PCR was then used to obtain 20 novel glutenin allele sequences from various Triticum and Aegilops species, including a 3000 year old preserved wheat. A neighbour-joining tree derived from all known glutenin allele sequences had eight clades, representing the eight loci from which the allele sequences were derived, and was split into two halves, one comprising alleles from the Glu-1-1 loci and the other comprising Glu-1-2 alleles. The topology was compatible with the postulated relationships between the A, B, D, and G genomes. The Glu gene duplication event was tentatively dated at 7.2-10.0 million years ago (MYA), the origin of the four genomes at 5.0-6.9 MYA, and the split between the B and G genomes at 2.5-3.5 MYA. The Glu-B1-1 alleles in cultivated wheats fell into two subgroups that diverged 1.4-2.0 MYA, suggesting that emmer was domesticated twice. The D allele sequences were relatively diverse, indicating that the hybridization event that resulted in the hexaploid bread wheats might have occurred more than once.