Phylogeography of Bellamya (Mollusca: Gastropoda: Viviparidae) snails on different continents: contrasting patterns of diversification in China and East Africa.

BACKGROUND: Species diversity is determined by both local environmental conditions that control differentiation and extinction and the outcome of large-scale processes that affect migration. The latter primarily comprises climatic change and dynamic landscape alteration. In the past few million years, both Southeast Asia and Eastern Africa experienced drastic climatic and geological oscillations: in Southeast Asia, especially in China, the Tibetan Plateau significantly rose up, and the flow of the Yangtze River was reversed. In East Africa, lakes and rivers experienced frequent range expansions and regressions due to the African mega-droughts. To test how such climatic and geological histories of both regions relate to their respective regional species and genetic diversity, a large scale comparative phylogeographic study is essential. Bellamya, a species rich freshwater snail genus that is widely distributed across China and East Africa, represents a suitable model system to address this question. We sequenced mitochondrial and nuclear DNA for members of the genus from China and used published sequences from Africa and some other locations in Asia to investigate their phylogeny and distribution of genetic diversity.

RESULTS: Our phylogenetic analysis revealed two monophyletic groups, one in China and one in East Africa. Within the Chinese group, Bellamya species show little genetic differentiation. In contrast, we observe fairly deep divergence among the East African lakes with almost every lake possessing its unique clade. Our results show that strong divergence does not necessarily depend on intrinsic characteristics of a species, but rather is related to the landscape dynamics of a region.

CONCLUSION: Our phylogenetic results suggest that the Bellamya in China and East Africa are independent phylogenetic clades with different evolutionary trajectories. The different climate and geological histories likely contributed to the diverging evolutionary patterns. Repeated range expansions and regressions of lakes likely contributed to the great divergence of Bellamya in East Africa, while reversal of the river courses and intermingling of different lineages had an opposite effect on Bellamya diversification in China.