Phylogeography of the Arctic-Alpine Saxifraga oppositifolia (Saxifragaceae) and some related taxa based on cpDNA and its sequence variation

Saxifraga oppositifolia (Saxifragaceae) is an important model system for the evolution of Arctic-Alpine plant species. Sequences of the psbA-trnH intergenic spacer of chloroplast DNA and of the internal transcribed spacer region, ITS1-5.8S-ITS2, of the nuclear ribosomal DNA were used to investigate the intraspecific evolution and phylogeography of this species. Samples from nearly the species' entire circumpolar distribution were included in the analysis as well as samples from the closely related taxa S. smalliana from Alaska and S. blepharophylla and S. biflora from the Alps (S. aizoides served as outgroup). These latter taxa showed a low number of parsimony informative characters, in both cpDNA and ITS sequences, which separated them from S. oppositifolia. Two main cpDNA haplotypes were detected within S. oppositifolia, one with a Eurasian distribution and one with an East Asian-North American distribution. This confirmed the existence of two cpDNA lineages with different geographical distributions in this species, which had previously been reported based on a RFLP analysis. The ITS phylogeny was not useful with respect to the intraspecific evolution and phylogeography of S. oppositifolia, because it showed a largely unresolved topology with low statistical support. The cpDNA sequence analysis, however, also suggested a putative long-distance dispersal event. All investigated taxa had cpDNA haplotypes that were congruent with their geographical origin rather than their phylogeny. This could point to putative lineage sorting in S. oppositifolia and related taxa.