Untangling evolutionary dynamic and speciation of allotetraploid StH-genomic Elymus species by population phylogenetics

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Hybridization and polyploidy origin have long been recognized as important causes of plant diversification and speciation. Gene flow through hybridization plays important roles in speciation. A better understanding of polyploidy formation processes and the consequences of gene flow in each polyploid species is of widespread interest for studying evolution. Wheatgrass genus Elymus is within the same group as wheat, barley and rye. Grasses within this genus have adapted to various climates and habitats, making them valuable genetic resources for cereal and forage breeding as well as an excellent group for research in evolution, taxonomy, and speciation. All wheatgrasses are made up of at least two different sets of chromosomes with most species having the StY and StH chromosome combination. There are about 50 species with StH combination. The St genome comes from ancestor species in genus Pseudoroegneria and H from wild barley. Based on morphological traits, the StH species in the genus Elymus were classified into seven sections. Since both environment and the stage of development could affect morphological traits, species with similar morphology may not be close genetically. For example, DNA analysis of three Eurasian and eight North American Elymus taxa did not fully support the morphological classification. Given that both genetic factors and environment could affect morphological traits, we hypothesized that the species in Elymus with similar morphology are not necessarily genetically related to each other. The origin and phylogeny of most polyploids remains unclear, a phylogenetic study of StH polyploids based on several individuals per species and multiple sets of single- or low copy nuclear locus is still lacking. Thus, we will use single copy nuclear gene and chloroplast DNA sequences to analyze Elymus species, each with 4-5 accessions along with their ancestor species. Sequence data will be analyzed to determine relationships among these species; the genetic relationships will be compared with the circumscription of the sections proposed for StH species based on morphology to test whether species with similar morphology are genetically related to each other. Understanding the dynamics of speciation with incomplete reproductive isolation is one of the challenges in evolutionary biology. Most allotetraploid Elymus species can hybridize and form partially or fully fertile hybrids, which causes gene flow between different species. Gene flow can prevent genetic differentiation necessary for hybrid speciation, but will result in continuous gene flow from one species into the other that will break the previously established species boundaries. The extent of gene flow and its role in creating diversity and speciation in Elymus has not been examined. We will use a population genetic approach to investigate the extent of introgression among Elymus species and its role in speciation. To fulfill this, AFLP, microsatellites and DNA sequence will be used to analyze 20 pure populations from E. caninus, E. fibrosus, E. mutabilis and E. alaskanus, and 15 mixed populations of these species. Population genetic approach will be used to analyze the pattern of markers in “mixed” populations, and compare them to the “pure” populations to determine how likely gene flow between overlap zones and separate zones is. These studies will provide new insight into the genomic and evolutionary consequences of introgression and hybridization speciation and guide conservation decisions. They will also have agronomic relevance because of the many agriculturally useful species in Elymus. A better understanding of the dynamic nature of past hybridization events will also lend insight into accurate classification and biodiversity management.