Genetic architecture, demographic history, and genomic differentiation of Populus davidiana revealed by whole-genome resequencing.

Forest trees are an excellent resource from which to understand population differentiation and heterogeneous genome variation patterns due to the majority of forest trees being distributed widely and able to adapt to different climates and environments. Populus davidiana is among the most geographically widespread and ecologically important tree species in China. Whole-genome resequencing data of 75 individual examples of P. davidiana throughout China were conducted, finding that all examples from different regions were clearly divided into either Northeast (N), Central (C), and South (S) populations. The ancestors of P. davidiana diverged into Northern group, comprising both N and C and Southern populations approximately 792,548 years ago. This time point of differentiation suggests that divergence of P. davidiana populations might have been triggered by the mid-Pleistocene transition. The three populations experienced considerable periods of bottleneck following divergence, with population expansion beginning around 5,000 years ago after the end of the last glacial maximum. We found N to be the center of origin of P. davidiana in China. The migration route of P. davidiana in China was from N to S. Although the majority of the regions of genomic differentiation between N and S populations can be explained by neutral processes, a number of tested outlier regions were also found to have been significantly influenced by natural selection. Our results highlight that linked selection and rates of recombination were important factors in genomic differentiation between the N and S populations. Finally, we identified a substantial number of functional genes related to climate change during population differentiation and adaptive evolution.