LAUSR

Understanding how polyploids originate provides insight into evolutionary processes and facilitates the genetic improvement of crop species. The exclusively polyploid genus Elymus contains approximately 150 perennial species, which provide ideal material for studies of polyploid speciation. Five basic genomes (haplomes), St, H, P, W, and Y, occur in different combinations in the genus. The St genome found in all Elymus species, was supposedly donated by Pseudoroegneria. The H, P, and W genomes are derived from Hordeum, Agropyron, and Australopyrum, respectively. However, the diploid donor of the Y genome is unknown. To study speciation and phylogenetic relationships within Elymus, DNA sequences of nuclear ribosomal ITS, and Adh2, Adh3, and Waxy genes of more than 40 accessions containing various genomes (StY, StH, StHY, StPY, StWY) were analyzed, together with those of the ancestral genera, Pseudoroegneria, Hordeum, Agropyron, and Australopyrum. Based on analyses of 5,210 aligned nucleotides, we obtained a well-resolved and well-supported estimate of genomic relationships of Elymus species which strongly supports data from prior cytological studies. The molecular data revealed a distinct phylogenetic relationship of the Elymus polyploids and their diploid donors, but the St and Y genomes did not show any separation. The phylogenetic trees illustrated clearly that the St, H, P, and W genomes in polyploid Elymus have been donated by Pseudoroegneria, Hordeum, Agropyron, and Australopyrum, respectively. Further analysis suggested that the St and Y genomes might have the same origin. In other words, the allopolyploid StY-genome Elymus may have an origin from autopolyploids (StSt), and the same genomes differentiated gradually within the polyploid species resulting in today’s StY-genome allotetraploid Elymus. This conclusion is strongly supported by many results obtained from previous cytological studies of Triticeae and other plant species. This represents a new mechanism of allopolyploidy in plants.