Natural Hybrids and Gene Flow between Upland and Lowland Switchgrass

Switchgrass (Panicum virgatum L.) is a perennial grass native to the North American tallgrass prairie and savanna habitats and is broadly adapted to the central and eastern United States. Upland and lowland ecotypes represent the two major taxa within switchgrass, which have distinct but overlapping distributions. The purpose of this study was to survey a broad array of putative upland and lowland accessions for the possible presence of natural hybrids or hybrid derivatives and evidence of historic gene flow between the two ecotypes. All plants were classified as upland, lowland, or intermediate based on visual assessment of phenotype, using large nurseries of known upland or lowland plants as controls. A total of 480 plants were surveyed for 19 simple sequence repeat (SSR) markers and sequenced using five chloroplast DNA (cpDNA) segments. Genetic structure analysis revealed 21 individuals with strong evidence for intertaxa hybrid origin and another 25 individuals with moderate evidence for intertaxa hybrid origin. All but two of these individuals originated from remnant populations of the central or eastern Gulf Coast or along the Atlantic Seaboard, a region that is populated with significant quantities of both upland and lowland ecotypes. We propose the central and eastern Gulf Coast glacial refuge as the primary center of origin and diversity for switchgrass, with the western Gulf Coast as the secondary center of origin and diversity. Much of this diversity appears to have been preserved along one of the major northward postglacial migration routes, the Atlantic Seaboard.