Experimental evidence for the absence of a functional gut microbiome in the solitary bee Megachile tosticauda

The eusocial honey bee is a model for insect microbiome research, with socially transmitted gut communities that play key roles in health and development. In contrast, solitary bees lack social transmission pathways and often have an environmentally acquired microbiome, which may or may not be functionally important. Using 16S rRNA gene metabarcoding, previous work has described the bacterial communities in the solitary resin bee Megachile tosticauda pollen provisions, brood, and adult bees, but their functional significance has yet to be studied. Here, we investigate the importance of live bacteria for larval development and survival with an antibiotic-feeding experiment, and test whether bacteria are present in adult guts using scanning electron microscopy. Removing live bacteria in the food and gut of feeding larvae had no significant effect on survival or growth. Microscopy revealed no bacterial colonization of the adult gut, and the dominant taxon detected in larval pollen could not be cultured under targeted conditions. These results suggest an absence of a beneficial gut microbiome in M. tosticauda. Based on our findings, we propose that some bacteria detected by DNA-based methods in M. tosticauda may represent relic DNA, surface-associated or transient bacteria. Our findings highlight fundamental differences in the bacterial associations between social and solitary bees.