Tales of the unexpected: phylogeography of the arctic-alpine model plant Saxifraga oppositifolia (Saxifragaceae) revisited.

Arctic-alpine biota occupy enormous areas in the Arctic and the northern hemisphere mountain ranges and have undergone major range shifts during their comparatively short history. The origins of individual arctic-alpine species remain largely unknown. In the case of the Purple saxifrage, Saxifraga oppositifolia, an important model for arctic-alpine plants, phylogeographic studies have remained inconclusive about early stages of the species' spatiotemporal diversification but have provided evidence for long-range colonization out of a presumed Beringian origin to cover today's circumpolar range. We re-evaluated the species' large-scale range dynamics based on a geographically extended sampling including crucial areas such as Central Asia and the (south-)eastern European mountain ranges and employing up-to-date phylogeographic analyses of a plastid sequence data set and a more restricted AFLP data set. In accordance with previous studies, we detected two major plastid DNA lineages also reflected in AFLP divergence, suggesting a long and independent vicariant history. Although we were unable to determine the species' area of origin, our results point to Europe (probably the Alps) and Central Asia, respectively, as the likely ancestral areas of the two main lineages. AFLP data suggested that contact areas between the two clades in the Carpathians, Northern Siberia and western Greenland were secondary. In marked contrast to high levels of diversity revealed in previous studies, populations from the major arctic refugium Beringia did not exhibit any plastid sequence polymorphism. Our study shows that adequate sampling of the southern, refugial populations is crucial for understanding the range dynamics of arctic-alpine species.