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a b s t r a c t This phylogenetic study focuses on a subset of the species in Elymus—specifically, the endemic Asian tet-raploids presumed to combine the St genome from Pseudoroegneria with the Y genome from an unknown donor. The primary goals were to (1) determine whether the St and Y genomes are derived from phylo-genetically distinct donors; (2) identify the closest relative, and potentially the likely donor, of the Y gen-ome; and (3) interpret variation among StStYY species in terms of multiple origins and/or introgression. The goals were addressed using phylogenetic analyses of sequences from three low-copy nuclear genes: phosphoenolpyruvate carboxylase, b-amylase, and granule-bound starch synthase I. Data sets include 16 StStYY individuals representing nine species, along with a broad sample of representatives from most of the monogenomic (i.e., non-allopolyploid) genera in the tribe. To briefly summarize the results: (1) the data clearly support an allopolyploid origin for the Asian tetraploids, involving two distinct donors; (2) the Y genome was contributed by a single donor, or multiple closely-related donors; (3) the phylogenetic position of the Elymus Y genome varies among the three trees and its position is not strongly supported, so the identity of the donor remains a mystery; and (4) conflicts among the gene trees with regard to the St-genome sequences suggest introgression involving both Elymus and Pseudoroegneria.! 2009 Published by Elsevier Inc. 1