Microbial Evolution in Allodapine Bees: Perspectives From Trophallactic, Socially Plastic Pollinators

This review seeks a deeper functional understanding of wild bee microbiomes by focusing on a tribe of bees where natural history and behavioral ecology are well known but investigations of microbiology are just beginning. Opportunities to improve our future knowledge of pathogens to insect pollinators are explored-which have broad ramifications for crop pollination services, considering the current overdependence on a few managed species that face a multitude of health threats. The bee tribe Allodapini (Apidae: Xylocopinae) has the potential to offer comparative insights on the evolution of bee microbiomes, owing to a unique combination of life history traits relevant to pollination service delivery across sub-Saharan Africa, Southern Asia, and Australia. Allodapines exhibit facultatively social colony organization that offer evolutionary perspectives on the formation of group living not afforded by obligately eusocial insects, which have already transgressed the solitary-social threshold. Progressive provisioning of brood (in the absence of brood cells) facilitates a network exchange of nutrients (via trophallaxis) that we speculate may culminate in an intra-colony "network microbiome". A literature review of pathogenic (bacterial, fungal, viral, and protozoan) associates of allodapine bees reveals considerably less research than for carpenter (Ceratina, Xylocopa), bumble (Bombus), and honey (Apis) bees. Interrogation of published genomes (Exoneura, Exoneurella) discovered novel microsporidian and protozoan parasites and relatives of known bee bacteria (Commensalibacter, Sodalis). Some Xylocopa exhibit microbial profiles typical of corbiculate bee core gut microbiomes, but no comparative evidence among allodapines was found. Allodapines visit flowers of 13 horticultural crops (fruits, vegetables, oilseeds, tree-nuts) and 50 native genera (predominantly Myrtaceae, Proteacae, Myoporaceae, Goodeniaceae). The ability to parse intrinsic and extrinsic factors influencing microbiome patterns within and between species means that allodapine bees provide the opportunity for an integrated approach to bee socio-eco-evo-immunology.