Molecular Phylogeography Reveals Two Geographically And Temporally Separated Floristic Exchange Tracks Between Southeast Asia And Northern Australia

Aim Exchange of plant lineages between Australia and Southeast Asia has had a substantial impact on the evolution of Australia's northern, tropical flora, with important ramifications for its conservation and biosecurity. Despite this, floristic exchange tracks between northern Australia and Southeast Asia remain poorly understood. To address this, we conducted a molecular phylogeographic case study to identify exchange tracks between Australia and Southeast Asia.Location India, Southeast Asia, Australia and Pacific islands.Taxon The widespread tropical monsoonal tree species Aglaia elaeagnoidea (Meliaceae).Methods We conducted a DArTseq phylogeographic study of 141 herbarium and silica-dried samples sourced from across the range of A. elaeagnoidea. We analysed 176,331 single nucleotide polymorphisms (SNPs) across 90,456 loci using multivariate, admixture, genetic differentiation and coalescent methods to characterise phylogeographic and phylogenetic patterns. These analyses were considered in the context of an environmental niche model for the last glacial maximum.Results Two exchange tracks were identified: one from New Guinea to Cape York Peninsula in north-east Australia, and a second from Timor-Leste to the Kimberley Plateau of north-west Australia. The Cape York Peninsula track is contemporary, characterised by ongoing genetic exchange, whereas the Kimberley Plateau track is historic, facilitated by multiple past exposures of the Arafura Shelf during the Pleistocene. Overall, we suggest that phylogeographic patterns of A. elaeagnoidea have resulted from a combination of repeated range expansion and contraction cycles concurrent with Quaternary climate fluctuations and stochastic dispersal events.Main conclusions This study provides the first molecular phylogeographic evidence for two floristic exchange tracks between northern Australia and Southeast Asia. It also highlights the influence of Quaternary climate fluctuations on the complex biogeography of the region, and supports the idea that the Kimberley Plateau and Cape York Peninsula in northern Australia have separate biogeographic histories.