Phylogeography of Pterocarya hupehensis reveals the evolutionary patterns of a Cenozoic relict tree around the Sichuan Basin.

Environmental factors such as mountain tectonic movements and monsoons can enhance genetic differentiation by hindering inter- and intra-specific gene flow. However, the phylogeographic breaks detected within species may differ depending on the different molecular markers used, and biological traits may be a major confounding factor. Pterocarya hupehensis is a vulnerable relict species distributed throughout the Sichuan Basin. Here, we investigated the phylogeographic patterns and evolutionary history of P. hupehensis using chloroplast DNA and restriction site-associated DNA sequencing data from 18 populations around the Sichuan Basin. The 24 chloroplast haplotypes separated into western and eastern lineages at approximately 16.7 Mya, largely coincident with a strengthening of the East Asian monsoon system during the early to middle Miocene. Both cpDNA and nuclear DNA datasets consistently identified distinct western and eastern lineages whose phylogeographic break conformed to the boundary of the Sino-Himalayan and Sino-Japanese forest sub-kingdoms. However, in contrast to the nuclear gene data, the cpDNA data revealed further divergence of the eastern lineage into northern and southern groups along the Yangtze River, a result that likely reflects differences in the extent of pollen vs seed dispersal. During the temperature decline in the penultimate (Riss) glacial period of the Pleistocene epoch, P. hupehensis experienced a genetic bottleneck event, and ecological niche modeling suggests that a subsequent population expansion occurred during the last interglacial period. Our findings not only establish a basis for conservation of this species, but also serve as a case study for the effects of geography and climate change on the evolutionary history of wind-pollinated relict plants.