Tracing the evolutionary trajectory of allotetraploid StH-genomic Elymus species

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Polyploidization has long been recognized as important causes of plant diversification and speciation. Gene flow through hybridization plays an important role 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. The StH genome species are found from the arctic to the subtropics, from sea level to above 5000m in the Himalayas, and their morphology is very diverse and complex. Given the wide distribution and the diverse adaptation of StH genome species, is speciation mainly due to a single origin followed by differential selection leading to adaptation and differentiation into a number of species, or multiple origins of hybridizations that have happened repeatedly at different times and places during evolutionary history? We will use several individuals per species and multiple sets of DNA sequence data to resolve a long-lasting controversy concerning multiple vs. single origins for StH species of Elymus and to trace reticulate events and their impact on phylogenetic relationships and speciation. 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 investigate the extent of introgression among Elymus species and its role in speciation. Population genetic approaches will be used to analyze the pattern of markers in overlapping populations, comparing them to the non-overlapping 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.