Rapid plant speciation associated with the last glacial period: reproductive isolation and genetic drift in sedges

Recent studies have demonstrated that reproductive isolation in combination with relatively strong natural selection or genetic drift may cause rapid speciation. Dramatic isolation scenarios such as allopolyploidy or differentiation among islands are often invoked to explain such cases of rapid speciation, but rapid speciation may also be driven by less obvious chromosomal or ecological differentiation. Carex waponahkikensis is a narrowly endemic sedge of two counties in Maine, USA, and nearby New Brunswick, Canada, sister to the common and widespread eastern North American C. scoparia. These species are diploid, largely sympatric and recently diverged from one another. In this study we examined genetic data to infer the phylogeographic and evolutionary history of C. waponahkikensis. We performed phylogeographic analyses of ten SSR loci in populations representing a wide distribution of both species and a high percentage of known localities of C. waponahkikensis. We found a clear genetic separation between C. scoparia and C. waponahkikensis. Although we also found clear genetic structure in C. scoparia, we found essentially no genetic variation in C. waponahkikensis. The estimated time of divergence between the species is remarkably recent, suggesting divergence following the last glacial maximum. Carex waponahkikensis appears to have originated in one of the most rapid non-polyploid speciation processes documented in plants as result of a strong bottleneck: reproductive isolation of a few populations, probably as a result of geographical isolation during the last glacial maximum, leading to accelerated genetic drift, fixation of chromosomal variants and resulting differentiation.