Genetic discontinuity, breeding-system change and population history of Arabis alpina in the Italian Peninsula and adjacent Alps

Arabis alpina is a widespread plant of European arctic and alpine environments and belongs to the same family as Arabidopsis thaliana. It grows in all major mountain ranges within the Italian glacial refugia and populations were sampled over a 1300 km transect from Sicily to the Alps. Diversity was studied in nuclear and chloroplast genome markers, combining phylogeographical and population genetic approaches. Alpine populations had significantly lower levels of nuclear genetic variation compared to those in the Italian Peninsula, and this is associated with a pronounced change in within-population inbreeding. Alpine populations were significantly inbred (F-IS = 0.553), possibly reflecting a change to the self-incompatibility system during leading edge colonization. The Italian Peninsula populations were approaching Hardy-Weinberg equilibrium (outbreeding, F-IS = 0.076) and genetic variation was highly structured, consistent with independent local 'refugia within refugia' and the fragmentation of an established population by Quaternary climate oscillations. There is very little evidence of genetic exchange between the Alps and the Italian Peninsula main distribution ranges. The Alps functioned as a glacial sink for A. alpina, while the Italian Peninsula remains a distinct and separate long-term refugium. Comparative analysis indicated that inbreeding populations probably recolonized the Alps twice: (i) during a recent postglacial colonization of the western Alps from a Maritime Alps refugium; and (ii) separately into the central Alps from a source outside the sampling range. The pronounced geographical structure and inbreeding discontinuities are significant for the future development of A. alpina as a model species.