Study Of Interspecific Gene Flow And Differentiation Between Notopterygium Incisum And N. Franchetii

N. incisum and N. franchetii are perennial herbs which grow in the Qinghai-Tibet Plateau and the nearby mountain areas. They are Chinese endemic plant which under threaten of extinction. The Tibetan Plateau uplift and the climate fluctuations during Pleistocene ice age have great impact on the speciation process and the evolutionary history of N. incisum and N. franchetii species, we adopted sequences of chloroplast, mitochondria and nuclear gene in 122 individuals from 21 populations in Qinghai, Gansu, Sichuan and Shaanxi, and aim to reveal the genetic diversity, population structure, evolutionary history as well as the mechanism of speciation with the presence of gene flow of these two species. The main conclusions are:1) The study of chloroplast DNA and mitochondria DNA shows that there were 4 haplotypes are found in the cpDNA OG28079 (261 bp) and the mtDNA OG537037 (872 bp) respectively. Both the cpDNA and the mtDNA contained 3 variation sites, and no variation site was detact in the cpDNA of N. franchetii. Based on the phylogenetic tree constructed by maximum parsimony (MP) methods and Maximum likelihood (ML) method, a close genetic relationship was found between N. incisum and N. franchetii. At the same time, the two species share the same haplotypes and have the overlapped distribution range. These evidences indicate that the possibility of secondary contact and introgression.2)Accroding to the analysis of the 8 nuclear DNA, the genetic diversity of N. incisum and N. franchetii species is corresponding low. The nucleotide diversity rate of N. franchetii (?T= 0.00303; ?s= 0.00422) is slightly higher than the figure for N. incisum (?T = 0.00367; ?s= 0.00408). Results of AMOVA and Permut analysis showed that the genetic divergence is mainly caused by the variation among populations within this two species. The average intraspecific variation rate is 79.02%, and the percentage of N. incisum and N. franchetii species are 89.94% and 75.28%, respectively. The haplotype variation is pretty high in most of the 8 nuclear DNA. Several exclusive haplotype of these two species were detected, and with few sharing haplotypes, the gene network diagrams show a distict distribution of these two types of haplotypes, therefore supposing the divergent level among N. incisum and N. franchetii species is high.3) An Isolation with Migration model was applied to sequences of nuclear gene and asymmetry bidirectional gene flow have been detected. The divergence time is 4.635×105. However, we prefer the allopatric speciation because the mean divergence time of each loci was similar. The presence of gene flow may due to the secondary contact or introgression after speciation. The effective population size of N. incisum and N. franchetii species (7.824×106 and 6.792×106 respectively) were shrunk dramatic comparing to the ancestral species size (4.249×107). Mismatch curve infers that the population size of the two species may repeatedly expanded. The consequences infer that these two species may encounter repetitive expanded and shrunk events in the evolutionary history.4) The distribution areas with high haplotype diversity were suggested to be the glacial refugia of N. incisum and N. franchetii, such as the northeast of the Qinghai-Tibet Plateau; the southwest of the Loess Plateau and the southwest of the Qinling Mountains.