The wheat tribe, Triticeae, includes many examples of hybridization and allopolyploidy. The St genome of Pseudoroegneria seems especially inclined to join other genomes in a variety of allopolyploid combinations. 

Allotetraploid Elymus

Elymus is an entirely allopolyploid genus in the wheat tribe, Triticeae. Elymus comprises several different combinations of parental genome donors, but they all share at least one set of Pseudoroegneria (bluebunch wheatgrass) genomes, designated St. 

Native North American Elymus species are allotetraploid, combining the Pseudoroegneria genome with that of Hordeum (H; wild barley) in an StStHH configuration. Phylogenetic analyses of North American Elymus based on DNA sequence data from the chloroplast genome, and from nuclear genes encoding starch synthase, phosphoenolpyruvate carboxylase, and beta-amylase, confirm the StStHH genome complement. They further reveal introgression from and Asian Pseudoroegneria species into North American Pseudoroegneria and Elymus.

We have added a selection of Eurasion StStHH Elymus species to determine whether the St and H genomes of the Eurasion species are distinct from those of the North American species, which would indicate separate origins of StStHH tetraploids. The phylogenetic data to date, based on phosphoenolpyruvate carboxylase, beta-amylase, and starch synthase gene trees suggest a single, North American origin for the the Eurasian and North American StStHH tetraploids.

The Y genome of the Asian StStYY tetraploid species is derived from an unknown donor (which may or may not be extant). Phylogenetic analyses of the phosphoenolpyruvate carboxylase, beta-amylase, and starch synthase gene sequences confirm that the St and Y genomes represent distinct evolutionary lineages, and further show that St genes have been acquired from multiple Pseudoroegneria species.
 

Allohexaploid Elymus repens

This native of Europe and Asia is now widespread throughout the United States. Its phylogenetic affinities within the Triticeae are poorly understood, but earlier cytogenetic studies suggest that it is derived from Pseudoroegneria (St genome) and Hordeum (H genome), resulting in an StStStStHH hexaploid. I have been using data from the chloroplast genome and three nuclear genes (granule-bound starch synthase I, beta-amylase and phosphoenolpyruvate carboxylase) to clarify its relationships to the diploid members of the tribe. Together, the results indicate a complex origin of this enigmatic allohexaploid weed.

Chloroplast DNA - a Pseudoroegneria donor. Results from this maternally-inherited genome suggests that Pseudoroegneria was one of the genome donors to E. repens. (Most St-containing polyploids examined to date have a Pseudoroegneria-like chloroplast genome.) cpDNA tree.

Nuclear genes - three genome donors. The placement of E. repens sequences on the nuclear genes trees is partially consistent with the cytogenetic studies; they show that Pseudoroegneria and Hordeum were genome donors. In addition, however, they all show that a third distinct donor, currently unidentified, was involved in the origin of the hexaploid. Nuclear trees.

Introgression involving the starch synthase gene. In addition to the Pseudoroegneria, Hordeum, and unknown-donor gene copies seen on all three trees, Elymus repens has two additional starch synthase copies. One is nearly identical to another introduced weedy species, Taeniatherum caput-medusae, and the other is derived from the Poeae-Aveneae clade outside of the Triticeae. Because these are unique to one gene tree, they are interpreted as products of introgression. GBSSI trees.

Reticulation and speciation in Coreopsis

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