Diversification of Biscutella ser. Biscutella (Brassicaceae) followed post-Miocene geologic and climatic changes in the Mediterranean basin

Abstract

Biscutella ser. Biscutella (= ser. Lyratae Malin.) is a group of mostly annual or short-lived perennial plants, with petals gradually tapering at the base and lateral intrastaminal nectaries, endemic to the Mediterranean basin and the Middle East. Recent taxonomic work has revealed that a relative morphological homogeneity occurs in Europe and Asia, but a high plasticity is found in N Africa for most of the characters traditionally used for taxonomic arrangements. This fact had generally led to overestimation of the number of taxa, which currently is reduced to ten (namely 7 species and 3 additional varieties), some of them being narrow endemics. In the present contribution, on the basis of a previous detailed morphological study carried out by the authors, the first comprehensive phylogeny based on 47 DNA sequence data including concatenation of two plastid (rpl32-trnL and trnV) and one nuclear (ITS) regions, together with the first time-calibrated phylogenetic tree, allows reappraisal of evolutionary and biogeographic relationships among the accepted taxa in the series. According to all evidence gathered in the present study, the current distribution of B. ser. Biscutella, mostly centred in the southern parts of the Mediterranean basin and the Middle East, suggests that it evolved in relation with the major geological and climatic events occurred in the Mediterranean basin and Eurasia within the last 20 million years. The origin of Biscutella is dated ca 18.75 Mya, and the radiation of the series triggered ca 5.87 Mya with the Messinian Salinity Crisis. Rapid diversification occurred coetaneously to the Intensification of Northern Hemisphere Glaciation (ca 2.86 Mya) onwards, with parallel large-amplitude aridity cycles in Africa and southwestern Asia. In recent times, the divergence of lineages became faster in the W Mediterranean (ca 1.54 to 0.43 Mya), mostly related to geographical and ecological patterns of specialisation. In many cases, the distribution of the current species is apparently linked to ancient glacial refuges in S Mediterranean basin.