Ring distribution patterns-diversification or speciation? Comparative phylogeography of two small mammals in the mountains surrounding the Sichuan Basin.

Studying the genetic differentiation in a unique geographic area contributes to understanding the process of speciation. Here, we explore the spatial genetic structure and underlying formation mechanism of two congeneric species (Apodemus draco and A. chevrieri), which are mainly distributed in the mountains surrounding the lowland Sichuan Basin, in Southwest China. We applied a set of comparative phylogeographic analyses to determine their genetic diversification patterns, combining mitochondrial (Cytb and COI) and nuclear (microsatellite loci) markers, with dense sampling throughout the range (411 A. draco from 21 sites and 191 A. chevrieri from 22 sites). Moreover, we performed three complementary statistical methods to investigate the correlation between genotype and geographic and environmental components, and predicted the potential suitable distributional range under the present and historical climate conditions. Our results suggest that both species have experienced allopatric differentiation and admixture in historical periods, resulting in a ring-shape diversification, under the barrier effect of the Sichuan Basin. We infer that the tectonic events of the Qinghai-Tibetan Plateau (QTP) and climatic oscillations during the Quaternary played an important role on the genetic divergence of the two species by providing environmental heterogeneity and geographic variation. Our study reveals a case of two sympatric small mammals following a ring-shaped diversification pattern and provides an insight into the process of differentiation.