Tales from the outside: The exosymbionts of solitary bees

The symbiosis between metazoans and microbes is exemplified in the case of bees. The presence of exosymbiotic microbes appears to be particularly important for solitary bees, which eat, digest, and assimilate bacteria and fungi within the pollen-provision. When denied microbe-colonized pollen, larvae suffer reduced fitness suggesting that these exosymbionts are essential for solitary bee survival. To address whether this phenomenon is broadly generalizable across solitary bee fauna, we examined the effects of exosymbiont presence/absence across both oligolectic and polylectic solitary bees. Findings from the study suggest that regardless of whether the pollen was provisioned by conspecifics or heterospecifics, the presence of microbes in pollen-provisions was critical to the survival of both oligoleges and polyleges. Next, we examined whether the source of the microbes, i.e, microbes provisioned by a conspecific forager, as opposed to microbes provisioned by a heterospecific forager, effected the extent of microbially-derived benefits. Here, naturally occurring microbes were always present within the pollen-provision, although the sources of the microbes and pollen were manipulated. Our results indicated that microbial- and pollen-sourcing each had significant impacts on bee development, and the effect sizes of each were quite similar. This suggested that the microbial community in a pollen-provision was as important for bee development as pollen composition. Altogether, these findings imply that pollen-borne exosymbionts are essential for solitary bee development, regardless of bee foraging strategy. Furthermore, the particular microbial community composition is as critical as the pollen source itself, both factors being significant and co-equal drivers of bee fitness.