Bayesian phylogeographic inferences reveal contrasting colonization dynamics among European groundwater isopods.

The potentially important role of northern microrefugia during postglacial dispersal is challenging the view of southern Europe as a refuge and source area of European biota. In groundwaters, large geographic ranges of presumably good dispersers are increasingly suspected to consist of assemblages of cryptic species with narrow ranges. Moreover, a large species range, even when confirmed by molecular evidence, tells us little about the spatiotemporal dynamics of dispersal. Here, we used phylogenetic inferences, species delineation methods and Bayesian phylogeographic diffusion models to test for the likelihood of postglacial colonization from distant refugia among five morphospecies of Proasellus (Isopoda, Asellidae). All morphospecies except one were monophyletic, but they comprised a total of 15-17 cryptic species. Three cryptic species retained ranges that spanned a distance >650 km, similar to that of the nominal morphospecies. Bayesian diffusion models based on mitochondrial markers revealed considerable spatiotemporal heterogeneity in dispersal rates, suggesting that short-time dispersal windows were instrumental in shaping species ranges. Only one species was found to experience a recent, presumably postglacial, range expansion. The Jura and Alpine foothills probably played a major role in maintaining diversity within Proasellus in northern regions by acting both as diversification hotspots and Pleistocene refugia. Gaining insight into the spatiotemporal heterogeneity of dispersal rates revealed contrasting colonization dynamics among species that were not consistent with a global postglacial colonization of Europe from distant refugia.