Rethinking refugia: tree topology, divergence dates, and demographic history trace the distribution of the endangered Plymouth gentian (Sabatia kennedyana) from the Pleistocene glaciation to present day

Premise of study: Molecular population genetics is a powerful tool to infer how species responded to past environmental change. In the northern hemisphere, interest is increasing in how species responded to changes in ice coverage and temperature during the last glaciation maximum (LGM, between 18000–21000 yr ago) with a common assumption that glacial refugia were located at the southern edge of a species range. Methods: We reconstructed the glacial and postglacial phylogeography of Sabatia kennedyana, a member of the Atlantic Coastal Plains Flora with a current distribution from Nova Scotia (NS) to South Carolina, using both cpDNA and nuclear markers. We also examined clinal variation in morphological traits, in particular relative investment in asexual vs sexual growth. Key results: We find strong evidence that the species did not reside in southern glacial refugia, but rather in primary glacial refugia off the exposed continental shelf extending from Cape Cod and that this area was r...