The Study On Clonal Structure, Genetic Diversity And Interspecific Relationship Of The Genus Populus Along The Erqis River

Large areas of the populus natural forests were distributed along the Erqis River in Xinjiang, which including the four Populus sections of Leuce,Aigeiros,Tacamahaca,Turanga, and the Populus tremula,Populus alba,Populus canescens,Populus nigra,Populus laurifolia,Populus×jrtyschensis just distribute naturally in this region of China. The Erqis River has abundant gene resource and idiographic value of the ecological and genetic research. The distributing pattern,breeding methods,genetic diversity and phylogenetic relationship of Populus natural forests were studied based on field surveys and experiments. Major results are described as follows:1. We analyzed the clonal structure and diversity of Populus sections of Leuce, then the results following:(1) Through the field survey, we found that the Leuce natural forests present a independently distributing pattern of male or female patch, many independent patches along the valley formed a large patch, and a number of major patches then formed a population. The study results at a microgeographical scale of single patch showed that all swatches in a single distributing patch of male or female were a identical clone, the single patch had only one genet, that was to say the structure of patch is monoclonal, and the Leuce natural forests propagated offspring by asexual clonal reproduction means (rooting) to maintain the population continuation.(2) The populations of Populus tremula and Populus alba had an abundant clonal diversity with the mean Simpson's index was 0.987 and 0.983 based on a macrogeographical scale of the livelong valley. The high clonal diversity might be maintained if the populations were initially founded by multiple genets that differed genetically and the existent frond was the progeny of clonal genets by root clone.(3) Because the independently distributing patch of male or female was monoclonal structure and there was genetic variation among patches along the valley, we described the distributing pattern of the Leuce natural forests along the Erqis River based on the studying at microgeographical scale (single patch) and macrogeographical scale (the livelong valley). Because of human activities and environmental degradation, then habitat fragmentation made a large patch form many adjacent patches which would have been belonged to the same clone. In addition, the disjunct patches within the same populations was a same clone system, showed that the distance of root tillering of Populus tremula and Populus alba was long, and the clonal spatial distribution was large.(4) The Shannon index(I) and Nei index(h) of Populus tremula was 0.7659 and 0.3657 and the 3 populations were ranked as S1>S3>S2 according to genetic diversity based on He; The Shannon index(I) and Nei index(h) of Populus alba was 0.3199 and 0.2122 and the 4 populations were ranked asY4>Y3>Y1>Y2 according to genetic diversity based on He. The genetic diversity of Populus alba was significantly lower than the Populus tremula.2. We analyzed the population genetic diversity of Populus nigra and Populus laurifolia by SSR molecular marker, then the results following:(1) For Populus nigra, 89 alleles were detected based on 12 SSR primers and the average number of alleles per locus Na= 7.4167, the effective number of alleles Ne= 3.8872, the mean observed heterozygosity Ho= 0.4249, the mean expected heterozygosity He= 0.3940; Populus laurifolia, 24alleles were detected based on 12 SSR primers and the average number of alleles per locus Na=2.0000, the effective number of alleles Ne=1.3967, the mean observed heterozygosity Ho=0.3452, the mean expected heterozygosity He=0.2200. the mean expected heterozygosity (He) of Populus laurifolia and Populus nigra was less than the mean observed heterozygosity(Ho), indicating that their heterozygote was excess .(2) The Shannon index(I) and Nei index(h) of Populus laurifolia was 0.7659 and 0.3657 and the 6 populations were ranked as K3>K1>K5>K6>K2>K4 according to genetic diversity based on He; The Shannon index(I) and Nei index(h) of Populus nigra was 0.3199 and 0.2122 and the 4 populations were ranked as H2>H3>H4>H1 according to genetic diversity based on He. The genetic diversity of Populus nigra was significantly lower than the Populus laurifolia.(3) A low level of genetic differentiation among Populus laurifolia populations was detected(Fst=0.0714) ,which indicated that the variation within groups is a major source of genetic variation, and a higher estimate of gene flow(Nm=3.2533) coincided with a high level of genetic identity(I) among the populations(from 0.9156 to 0.9817); analogously, a low level of genetic differentiation among Populus nigra populations was detected(Fst=0.0243), which indicated that the variation within groups was a major source of genetic variation, and a higher estimate of gene flow(Nm=10.0585) coincided with a high level of genetic identity(I) among the populations(from 0.9825 to 0.9972).(4) The index of Fit and Fis of Populus laurifolia was -0.0860 and -0.1695 and the index of Fit and Fis of Populus nigra was -0.5754 and -0.6146, which indicated that their heterozygote was excess.(5) The clustering analysis suggested that 228 swatches were classified two filiations: the first filiation was Populus laurifolia and the second filiation was Populus nigra and Populus×jrtyschensis, which proved that Populus×jrtyschensis should be ranged to the Populus nigra group and it presented the characteristics of Populus nigra at the genomic DNA level. There were considerable genetic variation among the species of Populus laurifolia and the genetic similarity coefficient of many Populus nigra individuals was 1.00.(6) Populus×jrtyschensis, a natural hybrid of Populus laurifolia and Populus nigra, had considerable phenotypic variation. Some individuals looked like Populus nigra, some looked like Populus laurifolia and some were intermediate type, so it was difficult to identify from the modality. The sample of K3-16, originally identified be Populus laurifolia through the phenotype, was clustered to Populus×jrtyschensis, and the samples of Ke2-2 and Ke2-3 and Ke3-18, originally identified be Populus×jrtyschensis through the phenotype, were clustered to Populus laurifolia. This showed that there had backcross between Populus×jrtyschensis and Populus laurifolia, then resulting in complicated morphological variation and the stand was the compound types of many hybrid generations. Identification of species by using the DNA molecular markers was more accurate than phenotypic characteristics.3. The Populus plant chloroplast DNA trnL-trnF intergenic space sequence was amplified and sequenced by the primer walking technique based on PCR product fragments. The results showed that: there were 883 invariable loci and 108 variable loci (11.96%). The 20 polymorphic loci (2.21%) had insertion or deletion (A / T) and substitution (AG / AC / AT / CT / GT) and the content of sequence G + C was 32.6%. Relatively high level of haplotype diversity (Hd = 0.764) and nucleotide diversity (Pi = 0.00396) were detected in Populus cpDNA trnL-trnF sequence,which indicated that this DNA sequence had rich genetic variation. The Tajima's D index of neutral test was 0.39108 and accepted the null hypothesis, its difference was not significant at the level of P> 0.10, which showed that the evolution of cpDNA trnL-trnF sequence in Populus was neutral.The cpDNA trnL-trnF intergenic space sequence of 5 Populus species (Populus tremula,Populus alba,Populus laurifolia,Populus×jrtyschensis,Populus nigra) was highly homologous and monophyletic origin . The phylogenetic tree was clustered into three filiations of Populus tremula, Populus alba and Populus laurifolia, and the Populus nigra was situated at the base of the tree. Populus jrtyschensis, this hybrid was clustered together with Populus laurifolia, which showed that the cpDNA of Populus×jrtyschensis had an origin from its female parent Populus laurifolia. 17 haplotypes were identified based on nucleotide variation. Populus tremula had 2 haplotypes (Hap₁3, Hap₁4), and Hap₁3 was dominant; Populus alba had 4 haplotypes (Hap₁4, Hap₁5, Hap₁6, Hap₁7), and Hap₁5 was dominant, and the Hap₁4 was shared by the two species but its frequency was low; Populus nigra had 2 haplotypes (Hap₁, Hap₃), and Hap₁ was dominant; Populus laurifolia had the largest number of identified haplotype (Hap₂, Hap₅, Hap₆, Hap₇, Hap₈, Hap₉, Hap₁0, Hap₁1, Hap₁2), and Hap₂ was dominant; Populus×jrtyschensis had 2 haplotypes (Hap₂, Hap₄). The 2 species of Populus laurifolia and Populus jrtyschensis all had the Hap₂ haplotype, and Populus tremula and Populus alba all had the Hap₁4 haplotype.We analyzed and constructed phylogenetic tree of 17 haplotypes based on the neighbor-joining method (NJ) and the maximum likelihood method (MP). The phylogenetic tree obtained by different methods was basically coincident but had differences for Bootstrap value (BS). The results showed that all haplotypes formed a multiphyly cluster and had different presumptive ancestors; Populus laurifolia had the largest number of haplotype variation; the haplotypes in the same Populus Sect. were tend to cluster together and the haplotypes of Aigeiros and Tacamahaca had more closer phylogenetic relationship.