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

Aim: Species with widespread distributions offer excellent
opportunities for investigating recent biogeographical patterns across
broad scales. Here, we tested the hypothesis that, due to its intermediate
geographical location, NW Africa is pivotal in explaining the
phylogeographical patterns of taxa with Mediterranean-Macaronesian
distributions using a legume species with short generation times.
Region: 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 demographic analyses at two geographical scales:
Mediterranean Basin (MB), using plastid sequences (115 individuals), and
Macaronesia, using plastid sequences (182 individuals) and 10 nuclear
microsatellite loci (220 individuals). We also performed a literature
survey focusing on phylogeographical studies of other circum-Mediterranean
taxa. Results: North-west Africa was identified as a center of
genetic diversity (19 out of 38 haplotypes) and demographic expansion of
B. bituminosa in the MB during the Pleistocene. Our literature
review revealed two main phylogeographical patterns in widespread species:
pre-Mediterranean evergreen sclerophylls vs. Pleistocene
facultative-deciduous (including Bituminaria) taxa, but on average both
functional groups show a similar, large genetic diversity (c. 40% of
haplotypes) in NW Africa. At the Macaronesian scale, we found that
Canarian Bituminaria is composed of two genetic sublineages that
coexist and hybridize on the central islands and in the mainland
Macaronesian enclave (Anti-Atlas region). Demographic analyses rejected
the progression rule as the model of island colonization, but strongly
suggested that Anti-Atlas populations are the result of back-colonization
from the easternmost islands before the Last Glacial Maximum (LGM).
Conclusions: Bituminaria displays a pattern of
Quaternary eastward expansion in the MB 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 biogeography,
acting both as a source of species expansion to the rest of the MB and a
LGM refugium of plant populations with a Macaronesian island origin.