Cytogenetic studies of North American Elymus suggest that the genus is an allopolyploid derivative of Pseudoroegneria (St) and Hordeum (H). To test this, we conducted a phylogenetic analysis of North American Elymus species within a broad sample of diploid Triticeae taxa using cloned sequences from one member of the nuclear gene family encoding phosphoenolpyruvate carboxylase. The phylogeny supports the hypothesis that Pseudoroegneria and Hordeum are the diploid progenitors of the North American Elymus tetraploids. Each tetraploid Elymus individual has two distinct forms of the gene, and each form is in a strongly supported clade with sequences from either Pseudoroegneria or Hordeum, suggesting that these Elymus species have an St H genomic content. This pattern is consistent with a single (or multiple very similar) polyploid ancestor(s) of the North American tetraploids, confirming earlier results based on granule-bound starch synthase I gene sequence data. We also examined the utility of the phosphoenolpyruvate carboxylase gene to reconstruct the evolutionary history of the Triticeae by comparing it to starch synthase gene sequence data. Both nuclear data sets are phylogenetically informative, but suggest somewhat different evolutionary histories among genera within the tribe.
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