Origin And Speciation Of Picea Schrenkiana And P. Smithiana

The uplifts of the Qinghai-Tibet Plateau (QTP) and the Quaternary climatic oscillations are suggested to play important roles in species divergence that account for high diversity of current species of this region. It is difficult to elucidate the evolutionary histories of current plant species, especially trees with long generation times which are confounded by genetic drift and gene flow. In the present study we aim to analyze the origin and speciation of Picea schrenkiana and P. smithiana based on population genetic data from three chloroplast (cp) loci, two mitochondrial (mt) loci and11nuclear (nr) loci. These two species mainly occur in the Central Asian Highlands and Himalayas, respectively, where they are isolated from other Asian congeneric species by the Qinghai-Tibet Plateau (QTP) or adjacent deserts. Previous studies based on both morphological and molecular evidence suggest that they have contrasting phylogenetic relationships with P. likiangensis or P. wilsonii. We sequenced3chloroplast DNAs (trnL-trnF, trnS-trnG and ndhK/C),2mitochondrial DNAs (nadl intron b/c and nad5intronl) and11nuclear loci (4CL, EBS, GI, MOO2, M007D1, Sb16, Sb29, Sb62, se1364, se1390and xy1420) in279individuals sampled from30populations of P. schrenkiana, P. smithiana, P. likiangensis and P. wilsonii. We examined genetic variation among16DNAs of three genomes from30populations of these four species. NETWORK and BEAST analyses of chloroplast and mitochondrial markers supported that Picea schrenkiana appeared to be closely related to P. likiangensis, although statistical support for this was weak. However, phylogenetic analyses and speciation tests based on the nuclear data from11loci provided evidence that P. schrenkiana and P. smithiana are sister species. These two species diverged around five million years ago (Myr) while the divergence between them and the P. likiangensis-P. wilsonii clade occurred about18.4Myr. We also detected gene flow during and after these speciation events. Our results highlight the complex speciation histories of these alpine conifers due to interspecific gene flow and/or incomplete lineage sorting and the importance of the early QTP uplifts in promoting the origin of these important conifer species in the Asian highlands.