Phylogeography of the endemic Gymnocypris chilianensis (Cyprinidae): sequential westward colonization followed by allopatric evolution in response to cyclical Pleistocene glaciations on the Tibetan Plateau.

The schizothoracine Gymnocypris chilianensis is restricted to the Shiyang, Ruoshui and Shule Rivers, listed from east to west, along the northeast edge of the Tibetan Plateau. This distribution provides a valuable system to test hypotheses about postglacial colonization. We used mitochondrial DNA sequence data (a control region and the cytochrome b gene; 1894 bp) to assess the phylogeographic structure of this species based on 278 specimens sampled from throughout the species' entire geographical range. We found three lineages corresponding geographically to the three rivers, suggesting three independent glacial differentiation centers within the northeast edge of the Tibetan Plateau. The phylogenetic analysis suggested that the Shiyang River population forms a lineage that separated from the other populations of G. chilianensis at the basal phylogenetic split within this species. The molecular data further demonstrated a clear pattern of decreasing genetic diversity from the eastern Shiyang River towards the central Ruoshui River and western Shule River lineages, a pattern consistent with sequential western colonization. We therefore propose a phylogeographic scenario for G. chilianensis of a gradual westerly expansion from the Shiyang River population along the northeast edge of the Tibetan Plateau, with subsequent allopatric evolution at approximately 0.37 and 0.05 million years ago (Ma), through at least two glacial maxima. Together with the genetic evidence reported in other species, our findings suggest that this common biogeographic pattern emphasizes the importance of the northeastern edge region of the Tibetan Plateau as a hotspot of genetic diversity for some taxa.