Microbiome diversity and composition across development stages of the Blue Orchard Bee, Osmia lignaria (Megachilidae)

Host-associated microbes are increasingly recognized as important drivers of bee health. Surveys of bee microbiomes have primarily sampled social bees, yet non-social bees constitute the majority of species. We employed 16S and ITS amplicon sequencing to describe the diversity and composition of bacterial and fungal communities across multiple developmental stages of Osmia lignaria, an important native and managed solitary mason bee. Bacterial and fungal diversity were not significantly different across bee development. However, the composition of bacteria and fungi significantly changed between larval and fully pigmented adult stages, in agreement with dramatic changes in host morphology during metamorphosis. Many of the microbial taxa found in provisions were also present in larvae, indicating that immature bees likely acquire their microbiome from food. Notably, the most prevalent bacterial genus was Arsenophonus, a symbiont with many recorded phenotypes, ranging from reproductive parasitism to beneficial endosymbiont. Arsenophonus was found in samples from provisions and eggs, yet reached higher read counts in larvae and fully pigmented adults. The Arsenophonus amplicon sequencing variants (ASVs) detected in this study had high sequence similarity with a symbiont that displays the son-killing phenotype, suggesting that the ASVs in O. lignaria are also reproductive parasites. The causative agent of chalkbrood disease in bees, Ascosphaera, was also detected in provisions and larvae. Most other taxa present were plant pathogens or commonly found in soil. This study highlights that O. lignaria may harbor horizontally and vertically transmitted microbial taxa with diverse consequences for bee fitness.