Molecular phylogenetics of the exoneurine allodapine bees reveal an ancient and puzzling dispersal from Africa to Australia

Previous phylogenetic studies of the bee tribe Allodapini suggested a puzzling biogeographic problem: one of the key basal divergences involved separation of the southern African and southern Australian clades at a very early stage in allodapine evolution, but no taxa occur in the Palaearctic or Asian regions that might suggest a Laurasian dispersal route. However, these studies lacked sufficient sequence data and appropriate maximum likelihood partition models to provide reliable phylogenetic estimates and enable alternative biogeographic hypotheses to be distinguished. Using Bayesian and penalized likelihood approaches and an expanded sequence and taxon set we examine phylogenetic relationships between the Australian, African, and Malagasy groups and estimate divergence times for key nodes. We show that divergence of the three basal Australian clades (known as the exoneurines) occurred at least 25 Mya following a single colonization event, and that this group diverged from the African + Madagascan clade at least 30 Mya, but actual divergence dates are likely to be much older than these very conservative limits. The bifurcation order of the exoneurine clades was not resolved and analyses could not rule out the existence of a hard polytomy, suggesting rapid radiation after colonization of Australia. Their divergence involved major transitions in life history traits and these placed constraints on the kinds of social organization that subsequently evolved in each lineage. Early divergence between the African, Malagasy, and Australian clades presents a major puzzle for historical biogeography: node ages are too recent for Gondwanan vicariance hypotheses, but too early for Laurasian dispersal scenarios. We suggest a scenario involving island hopping across the Indian Ocean via a series of now largely submerged elements of the Kergulen Plateau and Broken Ridge provinces, both of which are known to have had subaerial formations during the Cenozoic.