Phylogeography at the crossroad: Pleistocene range expansion throughout the Mediterranean and back-colonization from the Canary Islands in the legume Bituminaria bituminosa

Abstract

Aim: Species with widespread distributions offer excellent opportunities for investi-gating recent biogeographical patterns across broad scales. Here, we tested the hy-pothesis that, due to its intermediate geographical location, NW Africa is pivotal inexplaining the phylogeographical patterns of taxa with Mediterranean–Macaronesiandistributions using a legume species with short generation times.Location: Mediterranean, with a focus on NW Africa and the Canary Islands.Taxon: Pitch trefoil (Bituminaria bituminosa).Methods: We generated genetic data and performed phylogeographical and demo-graphic analyses at two geographical scales: Mediterranean Basin (MB), using plastidsequences (115 individuals), and Macaronesia, using plastid sequences (182 individu -als) and 10 nuclear microsatellite loci (220 individuals). We also performed a literaturesurvey focusing on phylogeographical studies of other circum-Mediterranean taxa.Results: Northwest Africa was identified as a centre of genetic diversity (19 out of38 haplotypes) and demographic expansion of B. bituminosa in the MB during thePleistocene. Our literature review revealed two main phylogeographical patterns inwidespread species: pre- Mediterranean evergreen sclerophylls versus Pleistocenefacultative-deciduous (including Bituminaria) taxa, but on average, both functionalgroups show a similar, large genetic diversity (~40% of haplotypes) in NW Africa. Atthe Macaronesian scale, we found that Canarian Bituminaria is composed of two ge-netic sublineages that coexist and hybridize on the central islands and in the mainlandMacaronesian enclave (Anti-Atlas region). Demographic analyses rejected the pro-gression rule as the model of island colonization but strongly suggested that Anti-Atlas populations are the result of back-colonization from the easternmost islandsbefore the Last Glacial Maximum (LGM).Conclusions: Bituminaria displays a pattern of Quaternary eastward expansion in theMB that appears to be paralleled by several members of its functional plant group.Thus, our study reveals a previously undescribed dual role of NW Africa in plant bio -geography, acting both as a source of species expansion to the rest of the MB and aLGM refugium of plant populations with a Macaronesian island origin.