Phylogeographical and population genetics of Polyspora sweet in China provides insights into its phylogenetic evolution and subtropical dispersal.

BACKGROUND: Geological movements and climatic fluctuations stand as pivotal catalysts driving speciation and phylogenetic evolution. The genus Polyspora Sweet (Theaceae), prominently found across the Malay Archipelagos and Indochina Peninsula in tropical Asia, exhibits its northernmost distribution in China. In this study, we investigated the evolutionary and biogeographical history of the genus Polyspora in China, shedding light on the mechanisms by which these species respond to ancient geological and climatic fluctuations.

METHODS: Phylogenetic relationships of 32 representative species of Theaceae were reconstructed based on the chloroplast genome and ribosome 18-26 S rRNA datasets. Species divergence time was estimated using molecular clock and five fossil calibration. The phylogeography and population genetics in 379 individuals from 32 populations of eight species were analyzed using chloroplast gene sequences (trnH-psbA, rpoB-trnC and petN-psbM), revealing the glacial refugia of each species, and exploring the causes of the phylogeographic patterns.

RESULTS: We found that Chinese Polyspora species diverged in the middle Miocene, showing a tropical-subtropical divergence order. A total of 52 haplotypes were identified by the combined chloroplast sequences. Chinese Polyspora exhibited a distinct phylogeographical structure, which could be divided into two clades and eight genealogical subdivisions. The divergence between the two clades occurred approximately 20.67 Ma. Analysis of molecular variance revealed that the genetic variation mainly occurred between species (77.91%). At the species level, Polyspora axillaris consists of three lineages, while P. speciosa had two lineages. The major lineages of Chinese Polyspora diverged between 12 and 15 Ma during the middle to late Miocene. The peak period of haplotype differentiation in each species occurred around the transition from the last interglacial to the last glacial period, approximately 6 Ma ago.

CONCLUSION: The primary geographical distribution pattern of Chinese Polyspora was established prior to the last glacial maximum, and the population historical dynamics were relatively stable. The geological and climatic turbulence during the Quaternary glacial period had minimal impact on the distribution pattern of the genus. The genus coped with Quaternary climate turbulence by glacial in situ survival in multiple refuges. The Sino-Vietnam border and Nanling corridor might be the genetic mixing center of Polyspora.