Phylogeographic patterns driven by river isolations in an island-endemic montane plant

Hainan Island in south China is a key part of the globally important Indo-Burma biodiversity hotspot, while the origin and maintenance of its species richness remains largely unexplored. In this study, we combined nuclear ITS and cpDNA trnL-trnF and ycf1b sequences to evaluate the genetic structure and phylogeographic patterns of the Hainan-endemic Primulina heterotricha (Gesneriaceae). The results showed significant phylogeographic patterns with low within-population genetic diversity and significant genetic differentiation among populations (Fst = 0.708 ± 0.319 and 0.826 ± 0.209 for nrDNA and cpDNA). Three clades were identified with little gene flow (Nm << 1 for nrDNA and cpDNA), which is supported by our STRUCTURE v.2.3.4 analyses. Our analyses detected two vicariance events at c. 0.83 and c. 0.48 Myr, and suggest that these three genetically-separated groups were isolated by two big rivers (Changhua River and Wanglou River), that likely acted as barriers to gene flow. The magnitude of isolation was positively correlated to the size of the rivers, with the greater barrier effect associated with the larger Changhua River. Our results highlight for the first time the critical role of riverine isolation in the patterns of intraspecific evolution of plant populations on Hainan Island.