Population Genetics And Interspecific Divergence Of Picea Asperata And P. Crassifolia

Picea asperata and P. crassifolia, which belong to the genus Picea of family Pinaceae, are endemic species to China and geographically parapatric distributed. P. asperata is mainly distributed on the east of the Qinghai-Tibetan Plateau (QTP), with an altitude of 2400 to 3600 meters in Sichuan or Gansu province, while P. crassifolia in the northeast of QTP or adjacent regions with an altitude of 1600 to 3800 meters in Qinghai, Gansu, Ningxia or Inner Mongolia. Previous studies show that there was significant difference in the shape of needle for Picea asperata and P. crassifolia, but the interspecific relationship of them was not reported. Thus, we focus on these two spruce species to figure out the population genetic structure and interspecific relationship of them. In this study, populations of each species were sampled from their major natural distribution range, with 88 individuals in 11 populations of P. asperata and 128 individuals in 15 populations of P. crassifolia, a total of 216 individuals in 26 populations. We sequenced three chloroplast DNA fragments, two mitochondrial DNA fragments and thirteen nuclear loci, and did population genetic related analysis based on the sequences we received. The analysis mainly included estimates of nucleotide diversity, neutrality tests, tests for linkage disequilibrium and recombination, analysis of population genetic structure, phylogenetic analyses and simulation of population demographic histories. Our results show that both of P. asperata and P. crassifolia have a low level of nucleotide diversity compared with other species of Picea, with the average total nucleotide diversity πt= 0.00264 and%πt =0.00304, the average estimates of silent sites sequence diversity πsil= 0.00393 and πsil=0.00334, respectively. This indicated that the population sizes of these two species may declined in the evolutionary history. The values of Tajima’s D as well as Fu and Li’s F* were positive for some locus at a significant level of P＜001. The mean Tajima’s D and Fu and Li’s D*and F*values were positive while the mean Fay and Wu’s H was negative in P. asperata and P. crassifolia across the thirteen nuclear loci, which indicating that there were bottle necks in the evolutionary history of these two species recently. AMOVA analysis showed that there was significant genetic difference between P. asperata and P. crassifolia, but the genetic variations within species harbored a larger proportion, which indicated that the divergence of these two species occurred in the recent time. The structure analyses showed that when K= 2, individuals of P. asperata and P. crassifolia failed to form two obvious clusters, which illustrated the speciation of these two species occurred recently with incomplete lineages sorting. The results of ABC analysis reveal that the divergence between P. asperata and P. crassifolia occurred in the late of Pleistocene, about 136.6 thousands years ago. The effective population sizes of P. asperata and P. crassifolia was much smaller than their ancestral species and there was greater gene flow between them. This study shows that the recent divergence event and the high gene flow may play an important role in the low level of genetic differences between P. asperata and P. crassifolia.