Genetic structure and phylogeography of alpine relict populations of Ranunculus pygmaeus and Saxifraga cernua

Bauert M. R., Kälin M., Edwards P. J. and Baltisberger M. 2007. Genetic structure and phylogeography of alpine relict populations of Ranunculus pygmaeus and Saxifraga cernua. Bot. Helv. 117: 181-196. Ranunculus pygmaeus and Saxifraga cernua are arctic-alpine species with similar disjunct distributions: both occur as small, isolated relict populations in the Alps, while they are more widespread in the Arctic. To improve our understanding of their glacial and postglacial history, we investigated their genetic diversity within and among populations collected in the Alps and in the Arctic using 80 RAPD primers. We found only two genotypes of R. pygmaeus, one in the Alps and one in the Arctic. The absence of genetic diversity within each region is probably the consequence of postglacial colonization from a single source, followed by inbreeding in very small populations. In S. cernua, we found six genotypes among 11 populations in the Alps but no genetic variation within alpine populations. This limited genetic variation probably results from an extreme reduction and fragmentation of populations during successive glacial periods. In contrast, there was a high level of genetic variation both among and within all arctic populations of S. cernua. We suggest that this arose at least partly through the immigration of plants from multiple sources after the last glaciation. The higher genetic diversity of S. cernua compared to R. pygmaeus might also be related to their contrasting breeding systems: R. pygmaeus is an inbreeding diploid which propagates only by seeds, while S. cernua is a clonal polyploid which propagates mainly by vegetative means. Clonal growth, by prolonging the life span of a genotype, might contribute to the maintenance of genetic diversity under conditions which are difficult for sexual reproduction and seedling establishmen