Historical biogeography of Trigonostemon and Dimorphocalyx (Euphorbiaceae)

Trigonostemon and Dimorphocalyx are two morphologically similar genera in tropical Asia. We estimated their divergence times through a Bayesian clock analysis and reconstructed the historical biogeography using a likelihood analysis under the dispersal-extinction-cladogenesis (DEC) model and a statistical dispersal-vicariance analysis (S-DIVA). We have found that the two genera differ in their historical biogeography: Trigonostemon originated on the South-East Asian mainland, but one section dispersed to the Malay Peninsula and Borneo, where rapid speciation events occurred during the Pleistocene, whereas Dimorphocalyx originated on and extended to its current distribution from Borneo. The dispersal routes of both genera are well supported by the tectonic history and are comparable to the conclusions in previous case studies. Long-distance dispersals across Wallace’s line are of particular interest in biogeography. We compared the patterns of historical distribution and dispersal of our taxa and other comparable taxa in this area. Our data support the hypothesis that the Philippines is the most common stepping stone for crossing Wallace’s line. Furthermore, we consider that the frequent change of sea levels during the Pleistocene propelled the diversification of Trigonostemon section Trigonostemon in Borneo and the Malay Peninsula.