Historical biogeography of Aganope, Brachypterum and Derris (Fabaceae, tribe Millettieae): insights into the origins of Palaeotropical intercontinental disjunctions and general biogeographical patterns in Southeast Asia

Aim The historical biogeography of three Palaeotropical legume genera, Aganope, Brachypterum and Derris, was investigated with the aim of (1) evaluating competing hypotheses on the origins of Palaeotropical intercontinental disjunctions (PIDs), and (2) inferring spatio-temporal diversification patterns in tropical Southeast Asia. Location Palaeotropics. Methods Plastid (trnL–F IGS, psbA–trnH IGS and trnK–matK ORF) and nuclear ribosomal (ITS/5.8S) DNA sequence data, covering the geographical distribution of all three genera, were analysed using an uncorrelated-rates relaxed molecular clock model. Ancestral areas were reconstructed using a likelihood approach implementing the dispersal–extinction–cladogenesis model (Lagrange) and a Bayesian approach to dispersal–vicariance analysis (S-DIVA). Results A wide ancestral distribution in Africa and Asia was inferred for the Aganope stem and crown groups, with a vicariance event between Africa and Asia in the early Miocene. The Southeast Asian mainland was inferred as the ancestral area for both the Brachypterum and the Derris crown groups. The reconstructions indicated numerous dispersal events westwards to India, and eastwards across Wallace's Line to New Guinea from the middle Miocene onwards. Two dispersal events from Asia to Africa, in the Miocene–Pliocene in Brachypterum and in the Pliocene–Pleistocene in Derris, were deduced. Main conclusions The PID in Aganope is likely to be the result of vicariance, caused by climatic deterioration subsequent to the Middle Miocene Climatic Optimum. The inferred PIDs in Brachypterum and Derris in the middle Miocene to Pliocene–Pleistocene are consistent with long-distance dispersal. The biogeographical patterns of Brachypterum and Derris are similar to patterns identified in other Southeast Asian plant taxa, and highly congruent with geological events in Southeast Asia facilitating dispersal from the early Miocene onwards. Preadaptation to several environmental conditions and habitats including mangrove swamps, and high dispersal capabilities by hydrochory may explain the wide distributions of some species and frequent dispersal across oceanic water bodies separating western and eastern Malesia.