Interspecific gene flow and ecological selection in a pine (Pinus sp.) contact zone

Witold Wachowiak,Weronika B Żukowska,Stephen Cavers

doi:10.1007/s00606-014-1182-x

Abstract

Nucleotide polymorphisms in a set of nuclear genes were studied in a sympatric population of pines Pinus mugo and Pinus sylvestris that includes trees classified as pure species and polycormic (multi-stemmed) individuals of potentially hybrid origin. Patterns of genetic diversity were compared between those groups of samples and to the reference allopatric populations of the species in Europe. Polymorphisms at the gene loci clearly distinguished pure parental species as measured by conventional frequency-based statistics and Bayesian assignment of samples into separate genetic clusters. Most individuals classified based on phenotypic assessments as putative hybrids were genetically very similar to P. mugo showing no existing average net divergence and genetic assignment to the same genetic cluster. On the other hand, individuals of P. sylvestris showed homogenous genetic background to the reference populations of the species from Central and Northern Europe. Ten individuals of admixed genetic composition were found in all three groups of samples; however, the majority of hybrids except one individual were identified across the samples classified as P. mugo and polycormic pines. Those trees that contained a mixture of nuclear gene haplotypes observed in the reference populations of pure species and cpDNA from P. mugo, most likely represent the first generation of hybrids. Analysis of the allelic frequency spectra and compound neutrality tests identified deviations from neutrality at several genes. This contact zone seems suitable for selection of a mapping population both in hybrid and parental species for admixture mapping to effectively search for polymorphisms that may play role in species adaptive variation and speciation.

Highlights

Natural hybridisation is an important process that creates recombinants from interspecific mating between divergent parental taxa where they come into geographic contact (Arnold and Martin 2010)
As the polymorphism at the genomic regions used in our study clearly distinguishes both putative parental species, our genetic data support the suggestion that exceptional morphology of some oligo- and polycormic individuals from peatbog populations may be due to environmental variation but they most likely represent P. mugo (Wachowiak et al 2006)
Our study shows that the examined contact zone includes the majority of pure parental species individuals and some proportion of hybrids (*17 %)

Summary

Introduction

Natural hybridisation is an important process that creates recombinants from interspecific mating between divergent parental taxa where they come into geographic contact (Arnold and Martin 2010). Hybridization occurs in roughly 10 % of animal species and 25 % of plant species and it may have various evolutionary consequences for the taxa involved (Baack and Rieseberg 2007). It may cause the swamping of the species with the smaller effective population size by gene flow from the more abundant species, integration of genetic material from one species into another through repeated back-crossing (introgression), homoploid hybrid speciation in which the new hybrid lineages become reproductively isolated from parental populations, and the transfer of adaptive traits across species boundaries (Baack and Rieseberg 2007). As the ranges of P. sylvestris and the taxa from the P. mugo complex overlapped in some part of their distribution, hybridisation between the species has likely contributed to high diversification observed especially within the P. mugo complex

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Conclusion