Evolution of eusociality in the Australian arid-zone bee genus Exoneurella

Eusociality is a complex form of social behaviour with colonies comprised of distinct queen and worker castes. Most of the evolutionary origins of this behaviour have been obscured by time, with all extant taxa in those groups displaying eusociality without the original transitional forms. In the Australian Allodapini, however, there is only one known eusocial bee, Exoneurella tridentata. The three other described species in this genus (E. lawsoni, E. eremophila and E. setosa) display almost-solitary to weakly social behaviour, with populations of E. eremophila and E. setosa in NW Victoria displaying casteless group living. Despite the large differences in behaviour all of the Exoneurella live in arid to semi-arid climates, so social behaviour is unlikely to be purely driven by climatic conditions. In contrast the sister groups to the Exoneurella, Exoneura and Brevineura live in temperate zones of Australia. These species range in behaviour from almost-solitary to facultatively eusocial, but all colonies have strong hierarchical reproductive structure. Here we use the software package BAMM to compare patterns of speciation between these groups and discuss how this links to theories on the evolution of eusociality in the Australian allodapines, with habitat and past climate change.