The south-western Carpathians as a centre of ancient diversity of freshwater gammarid amphipods: insights from the Gammarus fossarum species complex

Gammarus fossarum is a diverse species complex of epigean freshwater amphipods throughout Europe. Due to their poor dispersal capabilities and ubiquity, these crustaceans may serve as a model for investigating the influence of historical factors on the contemporary distribution and diversity patterns of freshwater macrozoobenthos. Here, we investigate the fine-scale phylogeographic structure of this complex across its range in the southwestern Carpathian Mountains, which comprises two areas that are geographically isolated from its main European distribution area as well as from each other. Given the Tertiary age of many freshwater Gammarus species, we hypothesize that the southwestern Carpathian populations reflect a relict distribution pattern. We used two mitochondrial and three nuclear markers from 32 localities to reconstruct phylogenetic relationships and estimate the timings of divergence among southwestern Carpathian and non-Carpathian lineages. Cryptic diversity was evaluated from mitochondrial markers by employing phylogenetic and distance-based methods. We distinguished at least 16 cryptic microendemic taxa, some of them coexisting, distributed in the southwestern Carpathians in a mosaic-like pattern. These lineages form a monophyletic group together with several lineages from southeastern Europe. Estimated divergence times indicate a Middle Miocene origin of this clade, with many deep splits dating back to more than 10 Ma. This time frame corresponds with a period of intense geological subsidence in the region that gave birth to the Pannonian Basin. We conclude that subsidence could have been an important driver of diversification in freshwater Gammarus and that the southwestern Carpathians represent an ancient centre of diversity for these crustaceans.

凹尾钩虾（Gammarus fossarum）是广泛分布于欧洲的地表淡水端足类复合物种类群。由于其扩散能力薄弱且分布极为普遍，这类甲壳动物可作为探究历史因素对淡水大型底栖动物当代分布与多样性格局影响的理想模式类群。本研究针对该复合类群在喀尔巴阡山脉西南部的分布范围，解析其精细尺度的系统地理结构——该区域包含两块地理上既与欧洲主要分布区相互隔离、又彼此互不连通的分布地块。鉴于多数淡水钩虾类群具有第三纪的演化历史，我们提出假说：喀尔巴阡西南部的种群呈现孑遗分布格局。我们依托采自32个采样点的2个线粒体标记与3个核标记，重建了喀尔巴阡西南部与非喀尔巴阡支系间的系统发育关系，并估算了类群分化时间。同时结合系统发育法与基于遗传距离的方法，基于线粒体标记评估了该复合类群的隐存多样性。研究共鉴定出至少16个隐存微特有类群，其中部分类群存在同域分布现象，它们以镶嵌状格局分布于喀尔巴阡西南部区域。这些支系与来自欧洲东南部的若干支系共同构成一个单系群。估算得到的分化时间显示，该单系群起源于中中新世，其中多数深层分化事件的时间可追溯至10 Ma之前。这一时间框架与该区域发生剧烈地质沉降、进而形成潘诺尼亚盆地的时期高度吻合。综上，我们认为地质沉降可能是淡水钩虾类群演化分化的重要驱动因素，而喀尔巴阡西南部乃是这类甲壳动物的古老多样性中心。