Genetic Origin And Population Genetic Composition Of Populus × Jrtyschensis

Numerous wild poplars are distributed along the Erqis River in Xinjiang, including Populus tremula、Populus alba、Populus canescens、Populus nigra、Populus laurifolia、Populus × jrtyschensis. This area harbors abundant and important naturl genetic resources for poplars. In this study, we mainly aimed to clarify genetic origin and population genetic compositions of Populus × jrtyschensis. In addition, we further investigated the genetic differentions between marginal populations of Populus nigra, which is one of the parent species of Populus × jrtyschensis.The factors that contribute to and maintain hybrid zones between distinct species are highly variable, depending on hybrid origins, frequencies and fitness. We examined genetic origins, compositions and possible maintenance of Populus × jrtyschensis, an assumed natural hybrid between Populus nigra and Populus laurifolia, two highly divergent species classified as belonging to two different sections of the genus. This hybrid poplar occurs mainly on the floodplains along the river valleys between the overlapping distributions of two putative parents.We collected 566 individuals from 45 typical populations of P. × jrtyschensis, P. nigra and P. laurifolia. We genotyped them based on the sequence variations of one maternally inherited chloroplast DNA fragment(cpDNA) and genetic polymorphisms at 20 SSR loci. We further sequenced eight nuclear genes for 168 individuals from 31 populations. Two groups of cpDNA haplotypes that are characteristic of P. nigra and P. laurifolia respectively, were both recovered for P. × jrtyschensis. Genetic structures and coalescent tests of two sets of nuclear population genetic data suggested that P. × jrtyschensis originated from hybridizations between the two species examined. All populations of P. × jrtyschensis comprise mainly F1 hybrids: interspecific hybridizationsbetween P. nigra and P. laurifolia. In the habitats of P. × jrtyschensis there are lower concentrations of soil nitrogen than in the habitats occupied by the other two species.Our extensive examination of the genetic composition of P. × jrtyschensis suggested that it is typical of F1-dominated hybrid zones. This finding plus the low concentration of soil nitrogen in the floodplain soils support the F1-dominated bounded hybrid superiority hypothesis of hybrid zone maintenance for this particular hybrid poplar.In addition, we also examined the structure of genetic diversity in marginal populations of black poplar, Populus nigra L.(Salicaceae). This species occurs mainly in Europe but its range extends to central Asia. We collected 117 individuals from 10 populations at the edge of the distributional range of the species in central Asia to examine the structure of genetic diversity based on genetic polymorphisms at 20 microsatellite markers. As expected, the genetic diversity within these marginal populations is relatively low, with an average observed heterozygosity Ho of 0.337 and an average expected heterozygosity He of 0.466, compared to the genetic diversity of populations from central distributions. However, we recovered very low genetic differentiation between populations, with an average Fst of 0.0745, a value similar to those reported for central populations. AMOVA analyses confirmed this result, showing that only 9.2% of the total variation could be attributed to between-population variance(P < 0.001). Our findings do not fully support hypotheses about the structure of genetic diversity in marginal populations formed from observations on other species. We suggest that a high rate of outcrossing and possible postglacial colonization at the edge of the distributional range of this long-lived poplar may explain the observed structure of the genetic diversity.