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

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.