Chloroplast phylogeography of a temperate tree Pteroceltis tatarinowii (Ulmaceae) in China

Pteroceltis tatarinowii Maxim. (Ulmaceae), one of the most widespread temperate canopy trees in mainland China, is the sole representative of the genus Pteroceltis. To illuminate the biogeographic and demographic history of this temperate tree species, we carried out a survey of chloroplast DNA sequence variation (trnS-trnG and psbA-trnH) within and among 28 populations (284 individuals in total) representing most of the distributional range of the species. Based on a total of 13 haplotypes identified, P. tatarinowii was found to harbor surprisingly high levels of haplotype and nucleotide diversity (hT= 0.71; pT= 2.83 X 10-3), possibly associated with its long evolutionary history and wide-scale geographical distribution. Significant chloroplast DNA population subdivision was detected (GST= 0.898; NST= 0.938), suggesting low levels of recurrent gene flow through seeds among populations and significant phylogeographical structure (NST > GST, P < 0.05). The ancestral haplotypes show clear-cut geographical distribution, and most regions possess a unique set of haplotypes, suggesting multiple potential refugia of the species occurring in montane areas of South China. The haplotype mismatch distributions analysis indicates that populations from North China underwent a spatial northward expansion, which might reflect one of the repeated Pleistocene south-to-north shifts of temperate deciduous forest in North China following cold periods.