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.

冰期后扩散过程中北部微型避难所（northern microrefugia）所可能发挥的关键作用，正对“南欧是欧洲生物群的避难所与起源地”这一传统认知提出挑战。在地下水生境中，那些被推测具备高效扩散能力的类群所拥有的广阔地理分布范围，愈发被认为实则由多个分布范围狭窄的隐存种（cryptic species）集合所组成。此外，即便通过分子证据证实了某类群拥有较大的分布范围，也难以揭示其扩散的时空动态。本研究借助系统发育推断（phylogenetic inferences）、物种界定方法以及贝叶斯系统地理学扩散模型，对原栉水虱属（Proasellus，等足目Isopoda，栉水虱科Asellidae）的5个形态种（morphospecies）是否源自远距避难所进行冰期后定殖的可能性展开检验。除1个类群外，其余所有形态种均为单系类群，但它们总计包含15至17个隐存种。其中3个隐存种的分布范围跨度超过650千米，与对应名义形态种的分布范围相当。基于线粒体标记（mitochondrial markers）的贝叶斯扩散模型结果显示，扩散速率存在显著的时空异质性，表明短暂的扩散窗口在塑造物种分布范围中起到了核心作用。仅发现1个物种经历了近期、推测为冰期后的分布区扩张。汝拉山脉（Jura）与阿尔卑斯山麓（Alpine foothills）可能通过同时充当物种多样化热点区域与更新世避难所（Pleistocene refugia），在维持北部地区原栉水虱属物种多样性方面发挥了重要作用。对扩散速率时空异质性的深入解析，揭示出不同物种间存在迥异的定殖动态，这与“欧洲的物种均源自远距避难所进行全球冰期后定殖”的观点并不相符。