Geographic patterns of nucleotide diversity and population differentiation in three closely related European pine species from the Pinus mugo complex

Nucleotide polymorphism at 12 nuclear loci and two mitochondrial gene fragments was studied in three closely related pine species from the Pinus mugo complex in populations across the species distributional range in Europe. Despite large differences in the census sizes of the populations, high and similar levels of nucleotide diversity (θsil = ∼0.013–0.017) were found at nuclear loci in the three pine species. More rapid decay of overall linkage disequilibrium (LD) and recombination to diversity ratio (ρ/θ) was observed across the species distributional range in P. mugo (ρ = 0.0369 ± 0.0028; ρ/θ = ∼2.2) than in P. uncinata (ρ = 0.0054 ± 0.0011; ρ/θ = ∼0.4) and P. uliginosa (ρ = 0.0051 ± 0.0010, ρ/θ = ∼0.4). However, regional groups of P. mugo showed similar levels of LD and ρ/θ ratio to the other species. An excess of rare nucleotide variants was found in P. mugo at four loci, but, overall, the allelic frequency spectrum in the three species did not deviate significantly from neutrality (multilocus Tajima's D = −0.681, D = −0.118 and D = −0.266, P > 0.05, respectively). Bayesian clustering methods showed no clear correspondence of clusters to species or geographical regions. Some differences between populations and species were found in a hierarchical analysis of molecular variance (AMOVA) and in the distribution of the mitochondrial DNA haplotypes, suggesting rather limited gene flow between the taxa and ongoing divergence. As all three pine taxa have similar genetic backgrounds, they form an excellent system for searching for loci involved in adaptive variation and speciation.