Genetic Differentiation and Population Genetic Structure of the Chinese Endemic Dipteronia Oliv. Revealed by cpDNA and AFLP Data

Dipteronia Oliv. is an endangered genus found in China with two species, D. sinensis and D. dyeriana. Previous morphological, cytogenetic, and molecular studies have suggested that D. dyeriana is a species related to D. sinensis. However, it is unclear how the two species diverged and whether gene flow exists between these two species. Here, we performed a molecular study at the population level to characterize genetic differentiation and decipher the phylogeographic history for Dipteronia species based on newly sequenced chloroplast DNA (cpDNA) and amplified fragment length polymorphisms (AFLP) date retrieved from our previous studies. No haplotype was shared between the two species in the cpDNA network. However, the phylogenetic analysis suggested that a haplotype found in D. sinensis (H4) showed a closer relationship with haplotypes of D. dyeriana. Based on our estimated time of divergence, these two cpDNA haplotype lineages of Dipteronia diverged at about 31.19 Ma. Furthermore, two genetic clusters with asymmetric gene flow were supported based on the structure analysis, which corresponded with the two Dipteronia species, and we also detected a low level of asymmetric gene flow between these two species according to the MIGRATE analysis using AFLP data. During the Last Glacial Maximum (LGM, c.21 kya BP), the genus' predicted distribution was more or less similar to that at present, which was also supported by the mismatch analyses that showed no population expansion of the two Dipteronia populations after the LGM. The combined cpDNA and AFLP data revealed significant genetic differentiation between the two Dipteronia species with asymmetric gene flow, which can be explained by the varying phylogeographical histories of these two species.