Abstract
BackgroundThe 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.ResultsWe 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 F1 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.ConclusionsOur 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.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-016-0776-6) contains supplementary material, which is available to authorized users.
Highlights
The factors that contribute to and maintain hybrid zones between distinct species are highly variable, depending on hybrid origins, frequencies and fitness
Private single-nucleotide polymorphisms (SNPs) for each parent species were recovered at each locus, with shared SNPs being more common between P. × jrtyschensis and P. nigra than between P. × jrtyschensis and P. laurifolia (Table 2)
Structure revealed that when K was set to 2 in Structure with USEPOPINFO = 1, P. nigra and P. laurifolia individuals clustered into two separate groups, while individuals of P. × jrtyschensis were admixed, containing a mixture of the genomes of the two groups representing the putative parent species (Fig. 3c)
Summary
The factors that contribute to and maintain hybrid zones between distinct species are highly variable, depending on hybrid origins, frequencies and fitness. 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. Hybrids comprise a mosaic of diverse genotypes that are highly variable according to their respective distributions In such a model, both environment-independent and -dependent selections against hybrids co-exist, combining the hypotheses of both the tension zone model and the bounded hybrid superiority model. It is very important to know the genetic composition of such hybrid zones, with regard to genotype frequencies, before we can identify the factors that may contribute to maintaining these hybrid populations as a result of either intrinsic or extrinsic fitness
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