Testing the origins of the Balkan serpentine endemics in Onosma (Boraginaceae): a molecular phylogenetic approach

Plant life on serpentine soils has been a topic of research for decades, but the evolutionary dynamics behind the origin of specialized lineages endemic to such a stressful habitat are still incompletely understood. This issue was addressed using the genus Onosma (Boraginaceae) as model system and a molecular phylogenetic approach. Nuclear DNA ITS sequences were generated for a broad sample of taxa including seven obligate endemics allopatrically distributed on the ophiolitic "islands" in the Balkans, along with accessions of serpentine-tolerant and non-tolerant species. Bayesian inference of phylogeny showed that Europaean obligate endemics belong to at least five clades without direct relationships to each other. Lack of a common ancestor and of correlation between geographic and genetic distances suggested polyphyletic evolution of endemic species group. Preference for non-serpentine habitats is suggested as the ancestral condition but key preadaptive traits such as drought tolerance and ability to cope with high Mg soil concentrations may have fostered multiple events of colonization of ultramafics. Based on a dated tree calibrated with the known age of the Macaronesian clade of the sister genus Echium, such events can be placed towards the limit between the Pliocene and Pleistocene (ca. 2.5 mya).