Genetic Diversity And Population Structure Of Three Poplar Species In Section Tacamahaca

Section Tacamahaca comprises a variety of poplar trees,which mainly distribute in North America and Asia.Populus cathayana,Populus ussuriensis and Populus yunnanensis are endemic species in China..P.cathayana is widely distributed in north China,northwest China,and Qinghai-Tibet Plateau.Because of its fast growth,wood property and strong adaptability,P.cathayana is not only an important tree species used for fast-growing timber,shelterbelt and afforestation,but also is a vital poplar genetic and breeding resource.P.ussuriensis has strong cold tolerance and it is distributed centrally northeast China as a major tree species for local forest regeneration.P.yunnanensis is a typical southern poplar species and is mainly distributed in Yunnan,Guizhou and Sichuan.In this study,the natural populations of these three species,541 individuals of P.cathayana were collected from 34 populations,64 individuals of P.yunnanensis were collected from 6 populations,56 individuals of Populus ussuriensis were collected from 6 populations,was used to analysis of genetic diversity and population structure using nuclear SSR and chloroplast DNA(cpDNA)markers.The results will provide theoretical basis for the evaluation,conservation,inheritance and improvement of germplasm resources.In this study,the genetic diversity of P.cathayana was investigated using 38 SSR markers with high polymorphism and uniform distribution on 19 chromosomes.We found that 38,36 and 34 pari of SSR markers showed polymorphism in P.cathayana,P.ussuriensis,and P.yunnanensis,respectively.A total of 165 alleles were detected by SSR primers in P.cathayana.The average number of alleles(Ne)was 4.323.The number of effective alleles was 2.776.And the average genetic differentiation coefficient(Fst)was 0.332.The coefficient of inbreding(Fis)was 0.180.The average polymorphism information content(PIC)was 0.750.A total of 92 alleles were detected by SSR primers in P.ussuriensis.The average number of alleles was 2.551.The number of effective alleles(Ne)was 1.895.And the average genetic differentiation coefficient(Fst)was 0.409.The coefficient of inbreding(Fis)was 0.308.The average polymorphism information content(PIC)was 0.545.A total of 154 alleles were detected by SSR primers in P.yunnanensis.The average number of alleles was 4.588.The number of effective alleles(Ne)was 2.262.And the average genetic differentiation coefficient(Fst)was 0.238.The coefficient of inbreding(Fis)was-0.139.The average polymorphism information content(PIC)was 0.421.The results showed that the SSR markers used in this studyare effective to reveal their genetic diversity.The genetic differentiation of the three poplars was in the middle,among which the genetic differentiation of P.ussuriensis was the highest.P.cathayana showed that the observed heterozygosity(Ho)and expected heterozygosity(He)were 0.543 and 0.407,the coefficient of inbreding(Fis)was 0.242,and the Shanning information index is 1.066;P.ussuriensis showed that the observed heterozygosity(Ho)and expected heterozygosity(He)were 0.020 and 0.024,the coefficient of inbreding(Fis)was 0.047,and the Shanning information index is 0.040;P.yunnanensis showed that the observed heterozygosity(Ho)and expected heterozygosity(He)were 0.360 and 0.528,the coefficient of inbreding(Fis)was 0.341,and the Shanning information index is 0.561.AMOVA showed that 74%,82% and 59% of the genetic variation were respectivelyfrom within the population for P.cathayana,P.ussuriensis and P.yunnanensis indicating that the genetic variation within the population was greater than that of between populations.The results of clustering and principal coordinate analysis showed that P.cathayana and P.yunnanensis both exhibited geographical distribution patterns,while the genetic distances between populations of P.ussuriensis did not show similar status.P.cathayana and P.yunnanensis are geographically segregated.It is considered that the geographical isolation of P.cathayana and P.yunnanensis results in the obvious geographical distribution of genetic structure.However,P.ussuriensis has a high degree of artificial nurturing,resulting in greater genetic differentiation among populations and greater artificial disturbance in gene flow among populations,which leads to the genetic differentiation among populations.Additionally,under different selection pressures,different populations underwent independent evolutionary processes,which may lead to unrelated genetic distance and geographical distance between populations.Four cpDNA markers were used to analyze three poplar,which showed high polymorphism.These fragments constitute 72,15 and 13 chloroplast haplotypes in P.cathayana,P.ussuriensis and P.yunnanensis,respectively.The distribution and frequency of haplotypes in different populations were quite different.P.cathayana population has the largest number of haplotypes,while P.ussuriensis and P.yunnanensis have fewer haplotypes.This is mainly due to the difference in sample size.Among them,27 populations of P.cathayana contain specific haplotypes,59 haplotypes are specific haplotypes;5 populations of P.ussuriensis contain specific haplotypes,14 haplotypes;4 populations of P.ussuriensis contain specific haplotypes and 12.In terms of nucleotide diversity and genetic diversity,P.cathayana showed that haplotype diversity was 3.043 and nucleotide diversity was 2.539.P.ussuriensis showed that haplotype diversity was 1.768 and nucleotide diversity was 4.632 and P.yunnanensis haplotype diversity was 0.402 and nucleotide diversity was 0.200 which indicates that P.cathayana has the highest genetic diversity,while P.yunnanensis genetic diversity is the lowest.The genetic diversity of P.ussuriensis is at the medium level.According to the distribution of haplotypes in different populations,the clustering analysis showed that there was a grouping phenomenon in P.cathayana and P.yunnanensis,but there was no obvious grouping phenomenon between P.ussuriensis.SSR markers and cpDNA markers revealed that the genetic diversity of three poplar species was moderate level evaluate by nuclear DNA and cpDNA.The population structure of P.cathayana and P.ussuriensis showed geographical distribution patterns.The result of gene flow indicated that the gene flow between populations of the same genetic subtype was larger than that among populations of different genetic subtypes.