Spatial genetic structure of the endemic alpine plant Salix serpillifolia: genetic swamping on nunataks due to secondary colonization?

Pleistocene climatic changes affected the current distribution and genetic structure of alpine plants. Some refugial areas for the high elevation species have been proposed in the Alps, but whether they could survive on nunataks, is still controversial. Here, the spatial genetic structure in Salix serpillifolia revealed by chloroplast (cpSSR) and nuclear (nSSR) microsatellites was compared with the MaxEnt-modelled geographic distributions under current and past (Last Glacial Maximum) climate conditions. Our results suggest that the genetic pattern of differentiation detected in S. serpillifolia may be explained by the existence of Pleistocene refugia, including nunataks. The geographical patterns of variation obtained from the chloroplast and nuclear markers were not fully congruent. The spatial genetic structure that was based on nSSRs was more homogenous, while the cpSSR-based pattern pointed at strong genetic structure along the Alps. Five populations from the Central Alps had a combination of local and unique cpSSR clusters and admixture of those occurring in the Western and Eastern Alps. These findings may indicate the local survival of small populations of S. serpillifolia that were subsequently populated by new colonists in the postglacial period.