Demographic stability of the Australian temperate exoneurine bees (Hymenoptera: Apidae) through the Last Glacial Maximum

The period of the Last Glacial Maximum (LGM) is characterised by severe cooling and extreme aridity in Australia. Australian temperate-adapted bees experienced entirely new conditions over the LGM; however, few studies focused on the response of these key pollinators and their dependent plants to the LGM in Australia. In this study, we investigated the impact of the LGM climatic conditions on the population histories of the Australian temperate exoneurine bees regarded as critical native pollinators in the continent. To address this issue, we first identified the bee species using a combination of morphological and molecular approaches including phylogenetic and species delimitation methods. The analyses revealed three exoneurine genera, including Brevineura, Exoneura and Inquilina. Our estimated divergence time between Exoneura and Inquilina was close to that of previous phylogenetic studies of the exoneurine bees, which used different dating methods. The haplotype network analyses indicated the geographical structuring of the haplotypes within Exonuera and Inquilina species. We then utilised Bayesian skyline analyses to reconstruct demographic histories of the genus Exoneura for which sufficient haplotype diversity was found. The results showed that the population size of Exoneura species remained unchanged over the LGM or even increased shortly before it. The wide geographical distribution of the exoneurine species in Australia and earlier development of genetic and physiological adaptations are discussed as possibilities that enabled the species to maintain their population sizes over the LGM.