Evolution in the Arctic: a phylogeographic analysis of the circumarctic plant, Saxifraga oppositifolia (Purple saxifrage)

A survey of chloroplast DNA variation in the circumarctic-alpine plant, Saxifraga oppositifolia, has resolved two highly divergent cpDNA lineages with geographically widespread and mainly allopatric distributions that are largely concordant with those of two subspecies, that is, subspp. oppositifolia and glandulisepala. These subspecies differ for a single morphological trait and hence level of morphological divergence does not equate to molecular divergence within the species. The two cpDNA lineages were estimated to have diverged from their most recent common ancestor 5.37-3.76 Ma, that is, during the early to mid-Pliocene. A nested clade analysis was conducted in an attempt to determine how past episodes of range fragmentation, range expansion and long distance dispersal may have influenced the geographical distribution of cpDNA haplotypes. In Alaska-a known refugium for the species during the last ice-age-high levels of cpDNA diversity may be partly explained by divergence between populations that were isolated in different ice-free regions. It remains to be established whether the two subspecies of S. oppositifolia exhibit some form of reproductive isolation from each other under conditions of sympatry.