Microbiome‐mediated immunity: Limited support from the gazelle dung beetle

Insects commonly live in environments rich in opportunistically pathogenic microbes and thus have evolved traits that decrease the likelihood of infection and subsequent loss of fitness. In many cases, these adaptations involve other microbes, which aid in preventing pathogen colonization via the production of antimicrobial compounds, pathogen blocking or immune priming, thus modulating the reactivity of the host immune system once challenged. Here, we use the gazelle dung beetle Digitonthophagus gazella to assess the role of the host's complex microbiome in supporting host immune functions, including basal protein concentration, phenoloxidase activity and lysozyme‐like activity in the larval hemolymph and the host's ability to overcome an experimentally induced and standardized immune challenge. We find that the inherited microbiome mediates an increase in larval mass and basal hemolymph protein concentration throughout development yet does not affect basal hemolymph phenoloxidase or lysozyme‐like activity. Further, the presence/absence of a microbiome did not affect the host's ability to survive experimentally induced exposure to Serratia marcescens . These results are discussed in light of both dung beetle ecology and insect‐microbiome interactions broadly. Taken together, results suggest that the D. gazella microbiome may function primarily to enhance development rather than immunity.