Genetic Structure And Demographic History Of Populus Davidiana

Populus davidiana Dode(Salicaceae),a temperate deciduous tree distributed continuously in mainland China,which is a keystone species in boreal forest communities.Although a series of research achievements have been made on the genetic structure of P.davidiana,the number of molecular marker used in different studies was inconsistent or limited.At present,there is no consistent or convincing conclusion about P.davidiana ’s genetic pattern and demographic history.Based on the above research background.A total of 502 individuals of P.davidiana were sampled from 32 natural populations throughout its distribution range in mainland China.Firstly,the genetic structure and demographic history of P.davidiana were preliminarily explored through 6 single-copy nuclear loci and 3 chloroplast loci.Futhermore,we applied a whole-genome resequencing approach here to delineate the problem based on a large number of unbiased SNPs distributed across the genome.In order to study on the genetic structure,population divergences and demographic for the management of P.davidiana thoroughly,the main research results are as follows:(1)The 32 P.davidiana populations can be divided into three groups: northeast(D B),central(HB)and southern(NF).The pairwise Fst values between different regions in northeast,central and southwest are 0.192-0.362 and all values were significant,indi cating that the genetic differentiation among the three geographical groups was obvious.(2)We revealed that the northeast China had endemic haplotypes,the haplotypes and nucleotide diversity in northeast regions were not lower than that in southern China.Ecological Niche Model simulation showed that the current distribution of P.davidiana is 25 °N and55 °N,and in the last glaciation(LGM)and interglaciation(LIG),there was also a suitable area for the growth of P.davidiana in the northeastern region.Bayesian skyline plots analysis showed that the effective population size of northeast populations was relatively constant in history,and has not experienced obvious population expansion or contraction.Based on this,this study preliminarily inferred that P.davidiana had quaternary ice age refuges in theNortheast.The estimated divergence time between Nouth and south populations(TDIV)was～0.8 million years ago(Mya),We speculated that the differentiation was probably influenced by the "Kun-Huang diastrophisms" and Naynayxungla Glaciation.The divergence time between Noutheast and central populations was ～0.08 million years ago(Mya),which coincided with the time node of the Last Glacial Maximum(LGM).(3)Three groups in northeast,central and southwest China were constructed according to the simulation results from NGSadmix,MEGA7.0 and PCAngsd results.The pairwise Fst values showed strong genetic differentiation among groups.To investigate patterns of interspecific genetic differentiation across the genome,we calculated the standard measure of genetic divergence,Fst,between Noutheast and South populations.The results showed that although a contribution of background selection to the observed patterns cannot be completely discounted,the independent action of positive selection in both Noutheast and South populations was expected to be the dominant driver for the evolution of reduced diversity and increased differentiation in most islands of differentiation.GO analysis in highly differentiated regions of different populations revealed significant enrichment of genes associated with photosynthesis pathways and plant disease resistance,and the metabolic pathways related to photosynthesis and plant disease resistance played an important role in the differentiation of different populations..