Allopatric divergence and secondary contacts in Euphorbia spinosa L.: influence of climatic changes on the split of the species.

Euphorbia spinosa, a perennial xerophilous shrub naturally distributed across the Italian peninsula, was selected for examination of the role of the Ligurian Alps and Apennines in glacial survival. The Italian Peninsula is considered to be one of the principal glacial refugia in Europe, but few plant population genetic and phylogeography studies have been undertaken in this region. The combined analysis of chloroplast and nuclear loci (ITS, cpSSR and ISSR) enabled us to detect extensive DNA variation and proved to be a very powerful tool for the reconstruction of the phylogeography. Molecular data support the hypothesis of a long-term separation of the Northwestern (Maritime Alps, Sardinia, Corsica, Northern Apennines) and Southeastern (Southern Apennines and Balkan area) lineages in glacial refugia. The existence of allopatrically fragmented lineages is most probably the result of isolation in different glacial refugia, possibly due to the Last Glacial Maximum cooling and the topographic complexity of the Italian peninsula. The most plausible hypothesis assumes the formation of two migration paths during more recent periods: the first one starting with southward migration and the second one moving northwards. The Central Apennines should be considered the confluence of migration routes radiating from separate refugia according to this hypothesis.