Phylogeographic analyses of poplar revealed potential glacial refugia and allopatric divergence in southwest China.

The distribution pattern and genetic structure of plant species have been profoundly influenced by climate oscillations. Phylogeographic analyses have been numerously conducted in biodiversity hotspot regions and some general conclusions have been documented. However, other regions have received less attentions and these places may serve as potential glacial refugia for plant species to survive the Quaternary glaciation. Here, we used six nuclear and three cpDNA markers to estimate the phylogeographic pattern of Populus davidiana, a widespread species distributed in southwest China. As a widely distributed species in southwest China, the nucleotide diversity of P. davidiana was relatively high (N_a = 6.28, H_O = 0.534, and H_E = 0.658). Genetic differentiation (F_ST) between the two main distribution regions, Yunnan and Guizhou provinces, was 0.21221. According to the composition of chloroplast haplotypes and the result of structure in these populations, we clearly distinguished two distantly sublineages corresponding to two distribution regions. Results of the Mantel test showed that there was a significant correlation between genetic distance and geographical distance (R² = 0.8252, p<.05). The topographically heterogeneous regions and the low dispersal ability of seed and pollen may lead to high genetic differentiation between these two regions. A potential glacial refugia for P. davidiana located in adjacent regions to the Hengduan range was revealed and allopatric divergence in separated glacial refugia may directly lead to the present phylogeographic pattern of this species.