Geographic variation of chloroplast DNA in Platycarya strobilacea (Juglandaceae)

The monotypic genus Platycarya (Juglandaceae) is one of the most widespread temperate tree species in East Asia. In this research, we implemented a phylogeographical study using chloroplast DNA (cpDNA) (psbA-trnH and atpB-rbcL intergenic spacer) sequences on Platycarya strobilacea, in order to identify the locations of the species main refugia and migration routes. A total of 180 individuals of P. stobilacea from 27 populations from China and Jeju Island (Korea) were collected. The results revealed that P. strobilacea had 35 haplotypes for the two intergenic spacers and high genetic diversity (hT= 0.926). This surprisingly high diversity of haplotypes indicates its long evolutionary history, which is in agreement with previous phylogenetic analyses and fossil records. Significant cpDNA population subdivision was detected (GST= 0.720; NST= 0.862), suggesting low levels of recurrent gene flow through seeds among populations and significant phylogeographical structure (NST > GST, P < 0.05). The construction of phylogenetic relationships of the 35 chlorotypes detected four major cpDNA clades. Divergence dating analyses using BEAST suggest that the divergence of the major cpDNA clades occurred before the Miocene. Demographic analysis indicated that the Eastern clade underwent localized demographic expansions. The molecular phylogenetic data, together with the geographic distribution of the haplotypes, suggest the existence of multiple glacial refugia in most of its current range in China through Quaternary climatic oscillations.