Sunda to Sahul dispersals in Trichosanthes (Cucurbitaceae): a dated phylogeny reveals five independent dispersal events to Australasia

AimThe Cucurbitaceae genus Trichosanthes is widespread in Asia and Australia, and previous studies have shown that the genus originated in Asia, and that three independent lineages dispersed through the Sunda archipelago to Australasia. The timing and routes of these three dispersals, as well as the dispersal of two widespread species found in Australia and New Guinea, were investigated. LocationSunda-Sahul dispersals with a focus on New Guinea and Australia. MethodsA combined dataset of nuclear ribosomal (ITS1-5.8S-ITS2) and plastid DNA (matK, ndhF, rpl20-rps12, rps16) was used for maximum likelihood and Bayesian phylogenetic analysis to infer the relationships of the studied taxa. A fossil-calibrated molecular dating was used to time the dispersal events, and a biogeographical analysis was used to study the origin and dispersal of the genus. ResultsThe two widespread species, T. pilosa and T. cucumerina, form monophyletic groups in Australia, suggesting single dispersals. Molecular dating analysis dates four of the dispersal events to the Miocene, and two to the Oligo-Miocene boundary and the initial Sahul shelf collision with the Philippine plate. Most known Sahul-Philippine dispersals concern species that migrated from the Sahul shelf, whereas dispersals south are fewer. Southward Miocene dispersals include species that were present on the Sunda shelf before the tectonic formation of the Makassar Straits. All lineages that dispersed to Australasia have undergone extensive diversification following dispersal. Lineages adapted to wet tropical climates have speciated mostly in New Guinea, and lineages adapted to monsoon tropical climates have speciated mostly in northern Australia. Main conclusionsDispersals in Trichosanthes pre-date human colonization of Australasia, suggesting natural long-distance dispersal and establishment of all lineages. Diversification within lineages found in both Australia and New Guinea is limited, corroborating frequent land connections between these areas during the Pleistocene. Sunda-Sahul dispersals are likely to have been more common through time than previously suggested, especially in genera with floating fruit such as Trichosanthes.