Origin of the Y genome in Elymus and its relationship to other genomes in Triticeae based on evidence from elongation factor G (EF-G) gene sequences

It is well known that Elymus arose through hybridization between representatives of different genera. Cytogenetic analyses show that all its members include the St genome in combination with one or more of four other genomes, the H, Y, P, and W genomes. The origins of the H, P, and W genomes are known, but not for the Y genome. We analyzed the single copy nuclear gene coding for elongation factor G (EF-G) from 28 accessions of polyploid Elymus species and 45 accessions of diploid Triticeae species in order to investigate origin of the Y genome and its relationship to other genomes in the tribe Triticeae. Sequence comparisons among the St, H, Y, P, W, and E genomes detected genome-specific polymorphisms at 66 nucleotide positions. The St and Y genomes are relatively dissimilar. The phylogeny of the Y genome sequences was investigated for the first time. They were most similar to the W genome sequences. The Y genome sequences were placed in two different groups. These two groups were included in an unresolved clade that included the W and E sequences as well as sequences from many annual species. The H genomes sequences were in a clade with the F, P, and Ns genome sequences as sister groups. These two clades were more closely related to each other and to the L and Xp genomes than they were to the St genome sequences. These data support the hypothesis that the Y genome evolved in a diploid species and has a different origin from the St genome.