Genetic origin and composition of a natural hybrid poplar Populus × jrtyschensis from two distantly related species

BACKGROUND: The factors that contribute to and maintain hybrid zones between distinct species are highly variable, depending on hybrid origins, frequencies and fitness. In this study, we aimed to examine genetic origins, compositions and possible maintenance of Populus × jrtyschensis, an assumed natural hybrid between two distantly related species. This hybrid poplar occurs mainly on the floodplains along the river valleys between the overlapping distributions of the two putative parents. RESULTS: 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 (cpDNA) fragment and genetic polymorphisms at 20 SSR loci. We further sequenced eight nuclear genes for 168 individuals from 31 populations. Two groups of cpDNA haplotypes 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 assumed parental species. All examined populations of P. × jrtyschensis comprise mainly F(1) hybrids from interspecific hybridizations between 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. CONCLUSIONS: Our extensive examination of the genetic composition of P. × jrtyschensis suggested that it is typical of F(1)-dominated hybrid zones. This finding plus the low concentration of soil nitrogen in the floodplain soils support the F(1)-dominated bounded hybrid superiority hypothesis of hybrid zone maintenance for this particular hybrid poplar. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1186/s12870-016-0776-6) contains supplementary material, which is available to authorized users.