Phylogeography of a widespread Asian subtropical tree: genetic east–west differentiation and climate envelope modelling suggest multiple glacial refugia
Fossil-based biome reconstructions predict that during the Last Glacial Maximum (LGM), the subtropical zone of East Asia was reduced to a narrow southern belt. In contrast, previous phylogeographical studies of subtropical plant species, many of which are rare, indicated different glacial refugia north of this predicted area. Here, we aim to elucidate the phylogeographical structure and putative refugia of Castanopsis eyrei, a widely distributed tree of subtropical evergreen broad-leaved forests of China.
We compiled distribution data and employed climate envelope model projections to predict potential areas at the LGM. Microsatellite data and chloroplast DNA (cpDNA) sequence data were obtained for 31 populations sampled throughout the species' range. Microsatellites were analysed with Bayesian clustering. Relationships among cpDNA haplotypes were depicted in a statistical parsimony network. We analysed patterns of variation within and among populations and clusters and along latitudinal clines.
Modelling revealed a potential LGM distribution of C. eyrei in a broad but interrupted belt overlapping the southern part of the present range. Nuclear microsatellites revealed two main clusters, suggesting a split between the western and eastern range, and a south-to-north decline in genetic variation. The eastern cluster harboured significantly higher nuclear genetic diversity. Sixteen closely related cpDNA haplotypes were identified. Populations were strongly differentiated at cpDNA markers, but lacked phylogeographical structure. Both data sets revealed higher genetic differentiation in the western cluster than in the eastern cluster.
Our results suggest at least two putative refugia during the LGM, further refugia-within-refugia substructure and a post-glacial northwards recolonization. Topographical differences between the mountainous western and the lowland eastern refugia may have affected the patterns of genetic differentiation between the extant populations. Incongruence between nuclear and chloroplast data might be attributed to ancestral polymorphism of cpDNA and chloroplast capture, but does not contradict the hypothesis of multiple refugia. Our results are likely to represent a template for evolutionary history and phylogeography in this region.