Polymorphisms in two homeologous gamma-gliadin genes and the evolution of cultivated wheat

The polymorphisms in two gamma -gliadin genes GAG56D and GAG56B on the D- a nd B-genomes of polyploid wheat, respectively, were investigated by sequenc ing PCR products and by PCR-RFLP. Of GAG56D, two alleles fo and ok were pre viously known to occur in hexaploid wheat. Here, we found that 16 sequenced fragments of GAG56D from six recognized subspecies of Triticum aestivum, i ncluding 13 contributed by this study, were identical to either the fo or t he ok allele. Considering published evidence, it was concluded that the inv estigated alleles of GAG56D stemmed from two different Aegilops tauschii pl ants and thus two independent origins of hexaploid wheat. Compared to GAG56 D-sequences obtained from 10 accessions of Ae. tauschii, the fo and ok alle les clustered with fragments from three accessions collected in the Caspian region. By sequencing fragments of GAG56B, four distinct allelic groups we re found among cultivated wheats, typical of bread wheat (p-aes), durum whe at of gliadin 45-type (a), durum wheat of gliadin 42-type (p-dur) and Timop heev's wheat (p-tim), respectively. Interestingly, the a allele found in gl iadin 45-type durum wheat was shared by European spelt cultivars, which str ongly supported the hypothesis that European spelt originated from a hybrid ization event between a tetra- and hexaploid wheat. The data also suggested that emmer might have been domesticated more than once. Phylogenetic analy sis of GAG56-fragments obtained from putative B/G-genome donors excluded al l candidate species as immediate donors of the B/G-genome, but instead indi cated a monophyletic origin of all GAG56B alleles found in polyploid wheat, i.e. including T. timopheevii.