Patterns of polymorphism resulting from long-range colonization in the Mediterranean conifer Aleppo pine.

The evolutionary outcomes of range expansion/contraction depend on the biological system considered and the interactions among the evolutionary forces in place. In this study, we examined the demographic history and the local polymorphism patterns of candidate genes linked to drought tolerance of a widespread Mediterranean conifer (Pinus halepensis). To that end, we used cpSSRs and coalescence modelling of nuclear genes to infer the demographic history of natural populations covering the species range. Ten drought-response candidate genes were then examined for their patterns of polymorphism and tested for selection considering plausible demographic scenarios. Our results revealed a marked loss of genetic diversity from the relictual Greek population towards the western range of the species, as well as molecular signatures of intense bottlenecks. Moreover, we found an excess of derived polymorphisms in several genes sampled in the western part of the range a potential result of the action of natural selection on populations confronted with new environments following long-range colonization. Wide-range expansions contractions of forest trees are accompanied by strong selective pressures, resulting in distinct evolutionary units. This knowledge is of crucial importance for the conservation and management of forests in the face of climate change.