Patterns of diversity in the wheat tribe (Triticeae) have been shaped by reticulate evolutionary processes, include extensive polyploidy. The tribe includes many auto- and allopolyploid taxa representing numerous distinct genomic combinations. The largest group of allopolyploids in the tribe is the genus Elymus, which, under a genomic system of classification, is made up entirely of allopolyploid species. By definition, all members of Elymus include at least one genome set from Pseudoroegneria (bluebunch wheatgrass), while the other genome set(s) may be derived from one or more of several other genera. The present work focuses on an allohexaploid species, Elymus repens, which was introduced to the United States and is now widespread throughout. Its genomic constitution is not well understood, but earlier cytogenetic studies suggested the presence of genomes from Pseudoroegneria and Hordeum (wild barley). The genomic complement of E. repens is examined here within a phylogenetic context, using three data sets from the chloroplast genome and one from the nuclear gene encoding granule-bound starch synthase. The data from E. repens are analyzed within a broad sampling of diploid genera from throughout the tribe. The results are more complex than expected: in addition to Hordeum and Pseudoroegneria, the two previously hypothesized genome donors, the data indicate the involvement of at least two additional donors, including a highly unexpected contribution from Taeniatherum (medusahead rye), along with an additional, unknown donor.

Key words: allopolyploidy, Elymus repens, Triticeae