Exosymbiotic microbes within fermented pollen provisions are as important for the development of solitary bees as the pollen itself

Authors: DHARAMPAL, PRARTHANA - University Of Wisconsin; DANFORTH, BRIAN - University Of Wisconsin; Steffan, Shawn

Submitted to: Journal of Experimental Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/14/2022
Publication Date: 4/6/2022
Citation: Dharampal, P., Danforth, B., Steffan, S.A. 2022. Exosymbiotic microbes within fermented pollen provisions are as important for the development of solitary bees as the pollen itself. Journal of Experimental Biology. 12:e8788. https://doi.org/10.1002/ece3.8788.
DOI: https://doi.org/10.1002/ece3.8788

Interpretive Summary: Managed and wild solitary bee fauna are extremely important for US fruit pollination and production. Two studies were conducted to assay the importance of external microbial symbionts for developing solitary bee larvae. The first study examined whether the presence/absence of microbial exosymbionts were as important for generalist bees as specialists. In this study, the expectation was that if microbial exosymbionts were truly critical for the development of solitary bee larvae, this effect should be consistent across taxa and across foraging strategies. We found that for both the specialist (O. ribifloris) and generalist (O. lignaria) bees, the lack of microbes in larval pollen-provisions was immensely problematic, if not deadly, for the bee larvae. In the second study, the importance of microbial-sourcing was examined concurrently with pollen-sourcing. Given that microbes were always present in the pollen-provisions of the larvae, and that the microbial and pollen compositions were sourced from either conspecific or heterospecific bees, we could examine the main and interactive effects of microbial- and pollen-sourcing. Ostensibly, conspecific foragers would be providing the ‘right kind’ of microbes for their progeny. Similarly, the pollen composition provided by conspecific foragers would be expected to be the ‘right kind' of pollen, while the pollen composition provided by heterospecific foragers could be considered the ‘wrong’ pollen. Thus, any given larva in the second study was fed the ‘right/wrong’ microbes and the ‘right/wrong’ pollen. Analyses of microbial- and pollen-sourcing indicated that the main effect of each factor was highly significant, while the interaction term was not. This indicates that microbial- and pollen-sourcing were both significant drivers of bee fitness. Further, the impact of each was almost identical, suggesting that the particular identity of the microbial community in a pollen-provision was just as important for bee development as the pollen, itself. Altogether, these findings suggest that to conserve the managed and unmanaged, wild bee species in US agriculture, attention will need to be devoted to the external microbial symbionts in their pollen-provisions.

Technical Abstract: Developing bees depend on microbial symbionts within the gut as well as within their fermenting pollen-provisions. The presence of such external microbial symbionts (exosymbionts) appears to be particularly important for solitary bees, which suffer increased larval mortality and reduced fitness when denied access to microbe-digested pollen. To address whether this phenomenon was broadly generalizable across solitary bee fauna, we examined the effects of exosymbiont presence/absence across both oligolectic (specialist) and polylectic (generalist) bees. In a follow-up study, we examined explicitly whether solitary bee larvae benefit from feeding on microbes provisioned by a conspecific forager (i.e., the mother bee), as opposed to microbes provisioned by a heterospecific forager (i.e., other bee taxa). In the first study, for both the specialist and generalist bees, the presence of microbes in pollen-provisions was critical to larval survival, regardless of whether the pollen was provisioned by conspecifics or heterospecifics. Here, the presence of microbes in the pollen-provision had a greater positive effect on bee health than the type of pollen. In the second study, we found that microbial- and pollen-sourcing each had highly significant impacts on bee development. The effect sizes of each were quite similar, suggesting that the microbial community in a pollen-provision is just as important for bee development as the pollen composition. Importantly, the effects of conspecific-sourced microbes and pollen were strongly positive, both of which appeared to compensate for the negative impacts of heterospecific-sourced microbes or pollen. Altogether, these findings indicate that the microbial exosymbionts in the pollen-provisions of solitary bees may be as critical to bee growth and survival as the pollen composition, itself.