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.  Different Elymus species have different combinations of parental genome donors, but they all share at least one set of Pseudoroegneria (bluebunch wheatgrass) genomes, designated St.  Based on cytogenetic data, the native North American species are allotetraploid, combining the Pseudoroegneria genome with that of Hordeum (H; wild barley) in an StStHH configuration.  I have examined species of North American Elymus, along with representatives of nearly all of the diploid genera in the tribe, using DNA sequence data from a nuclear gene for starch synthase and several chloroplast DNA markers. Megan has studied them further using a PEP carboxylase gene.  The data confirm the identity of the suspected parental genome donors, Hordeum and Pseudoroegneria. 

Ongoing: We are examining Eurasion tetraploid Elymus species, to find whether the St and H genomes of the Eurasion species are distinct from those of the North American species, which would indicate a distinct lineage of StStHH tetraploids.  The Eurasion sample also includes StStYY tetraploids, in which the Y genome is derived from an unknown donor (which may or may not be extant).  This might help to clarify the phylogenetic affinities of the Y genome, even if we can not identify a specific diploid genome donor.  

Future possibilities include the addition of Elymus representatives from South America (in collaboration with Mary Barkworth and Mirta Arriaga) and Australia (in collaboration with Surrey Jacobs). 

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.

Ongoing: The representatives of E. repens examined so far are all introduced, North American accessions.  Sometimes, displaced weedy species form hybrids and/or polyploids with native or other introduced species, forming new combinations that are not seen in their home ranges.  To address whether the unexpected genomic composition of my introduced E. repens accessions is truly characteristic of the species as a whole, I am working on accessions of native populations from India, Turkey, Iran, Afghanistan, and Russia for examination within a similar phylogenetic framework. 

For more information on ongoing projects, visit the other People in the Lab.