Phylogeography And Changes Of Geographic Distribution Of Periploca Sepium,a Dominant Shrub Species In Northern China

Climate and geological-historical events have profoundly influenced the geographic distributions and population dynamics history of plants.However,the phylogeographic history and change patterns of geographic distributions of widespread shrub species in northern China responsed to climatic and geological historical events are still controversial.Periploca sepium Bunge is a deciduous vine shrub belonging to the genus Periploca in the family Apocynaceae,which widely distributed in Jilin,Shaanxi,Shandong,Gansu and other provinces in northern China,and provide the excellent model for the study of phylogeographic history of plants.In this study,we used next-generation high-throughput sequencing technology to assemble and annotate the whole chloroplast genome sequence and nuclear ITS gene sequence of P.sepium.Meanwhile,combined with the species distribution simulation method,the change patterns of geographic distribution of species in different historical periods were examined.The main results are as follows:1.Sampling 380 individuals from natural 62 populations of P.sepium,performing the second-generation sequencing through Illumina Hiseq,assembling and splicing the complete chloroplast genome sequence.Variation detection showed that the chloroplast genome was a closed circular tetrad structure,and its genome length variation was between 153,381-153,549 bp.74 single nucleotide polymorphisms（SNPs）variant sites and nine insertion/deletion（indels）sites were identified,resulting in a total of 32 haplotypes.The phylogenetic analysis showed that there was a high genetic differentiation among populations(G_ST=0.561,N_ST=0.585),and a high species-level genetic diversity（H_T=0.933,H_S=0.410）,and there was no significant phylogenetics(N_ST>G_ST,P>0.05).Population structure analysis showed that the genetic variation mainly occurred among populations(F_ST=0.559).Mismatch analysis found that the different geographic regions and species levels of P.sepium have not experienced the population expansion events.Ancestral distribution area reconstruction simulation suggested that there were migration and isolation events of gene flow in lineage branches in different geographic regions.Combined with the geographical distribution and phylogenetic relationship of chloroplast DNA haplotypes of P.sepium,it was speculated that the Qinling Mountains,Taihang Mountains and Yinshan Mountains were the main refuges of P.sepium during the glacial periods;Mount Min and Qinling Mountains may be origin and differentiation centers of P.sepium.2.We have assembled and spliced the nuclear gene ITS sequences of 370 samples from62 natural populations of P.sepium,and carried out genetic variation analysis on them.Sequence alignment test found that the length of the parental nuclear gene ITS sequence was between 1,727-1,896 bp.Seven variant loci（SNPs）and identified eight haplotypes were determined.At the nuclear gene level,P.sepium has low genetic diversity（H_T=0.284,H_S=0.195）and high genetic differentiation between populations(G_ST=0.314,N_ST=0.375);The species had a significant phylogenetic structure(N_ST>G_ST,P<0.05),and the genetic variation mainly came from within the population(F_ST=0.327).3.The Maxent program was used to simulate the distribution pattern of the species distribution in different historical periods.The results showed that the suitable area of this species during the Last Glacial Maximum（LGM）was less than that during the Last Interglacial Period（LIG）,while the suitable area for the species in the Middle Holocene（MH）was less.area expanded.Under the current climate scenario,the area of the distribution area of P.sepium further expanded,but the area of its optimum growth area was slightly reduced compared with the mid-Holocene.Niche detection analysis found that the dominant factors affecting the physical and geographical distribution of P.sepium were the mean temperature of driest quarter（Bio＿9）,the annual mean temperature（Bio＿1）,and the the precipitation of warmest quarter（Bio＿18）.Under different climatic scenarios in the future,most of the highly suitable habitat areas of P.sepium showed an increasing trend.In conclusion,the chloroplast genome sequence and nuclear ITS gene were used to analyze the phylogenetic history of the widespread shrub P.sepium in northern China,and used the method of species distribution area simulation to study the suitable distribution area and main ecological factors of the species in different historical periods.These results provide the typical case and related basis for understanding the responses of the widely distributed cold-and drought-resistant shrubs to climate fluctuations and geological events in northern China,and are also conducive to the rational development and utilization of P.sepium resources.