A tourist's guide to the microbiome of the bee brood cell mini-ecosystem

Pollen and nectar provisions within a bee brood cell are infused with diverse bacterial and fungal taxa, and mounting evidence suggests that these microbes are vital for bee health and development. Solitary bee microbiomes originate from the pollen and nectar provided by their mother, and for many species, microbial symbionts may aid in preventing provision spoilage, facilitate nutrient liberation from pollen, offer a direct source of nutrition, and moderate entomopathogens. Despite their critical roles, the relationships between diverse bees and the microbial associates found in the pollen/nectar provisions are largely unknown, especially for ground-nesting bees. To study the diversity and inferred functional roles of the brood cell microbiome, we collected and analyzed via amplicon sequencing brood cell pollen provisions (~N=10) from 6 families, 18 tribes, and 39 species of bees. Our sampling spans diverse life-history strategies (solitary, facultatively social and weakly social), diet breadths (generalist or specialist: pollen collecting and oil collecting), as well as nesting substrates (ground nesting, stem nesting and wood nesting). Our results indicate that the brood cell bacterial richness is influenced by resources they collect and the nesting strategy they employ. Many bee species are dominated by lactic acid forming or acetic acid producing bacteria which can profoundly restrict bacterial diversity. From comparing brood cells within genera, and across diverse groups of bees, we can begin to develop both broad and in-depth perspectives on the microbial ecology of brood cell mini ecosystem, as well as how these relationships may influence bee biology, ecology, and evolution.