Origin of Mediterranean insular endemics in the Boraginales: integrative evidence from molecular dating and ancestral area reconstruction

Aim The presence of numerous reliable fossils and the occurrence of many endemic island species make the Boraginales particularly suitable for integrative biogeographical studies. In this paper we aim to elucidate the time frame and events associated with the origin of selected borages endemic to the Mediterranean climate zone. More specifically, we describe and examine the alternative palaeo- and neoendemic hypotheses for their origin. Location Corsica and Sardinia (continental fragment islands) and the Canary Islands (an oceanic island archipelago). Methods Eighty-nine accessions, representing 30 genera from five families ascribed to the Boraginales, were examined for six chloroplast DNA regions. We used an integrative approach including phylogenetic analyses (MrBayes), Bayesian molecular dating (T3 package) with four fossil constraints on nodes, and biogeographical reconstructions (diva) to elucidate the temporal and spatial origins of the Corso-Sardinian and Canary Island endemics. Results Species of Echium endemic to the Canary Islands diverged from their continental sister clade during the Miocene (15.3 +/- 5.4 Ma), probably after the rise of the oldest islands (c. 20 Ma). Corso-Sardinian endemics of Borago diverged from their primarily North African sister clade during the late Miocene-Pliocene (c. 6.9 +/- 3.6 Ma), well after the initial fragmentation of the islands (c. 30 Ma). Similarly, Corso-Sardinian endemics of Anchusa diverged from the South African Anchusa capensis during the Pliocene-Pleistocene (c. 2.7 +/- 2.1 Ma). Main conclusions The present study reveals an Anatolian origin for Anchusa, Borago and Echium and underlines the importance of the Eastern Mediterranean region as a possible reservoir for plant evolution in the Mediterranean Basin. For Anchusa and Borago, the divergence from their respective sister clades on the two types of islands post-dated the formation of the islands, thus supporting the neo-endemic hypothesis, whereas the dating results for the origin of Echium endemics were less conclusive.