Symbionts shape host innate immunity in honeybees.

The gut microbiome plays a critical role in the health of many animals. Honeybees are no exception, as they host a core microbiome that affects their nutrition and immune function. However, the relationship between the honeybee immune system and its gut symbionts is poorly understood. Here, we explore how the beneficial symbiontSnodgrassella alviaffects honeybee immune gene expression. We show that both live and heat-killedS. alviprotect honeybees from the opportunistic pathogenSerratia marcescensand lead to the expression of host antimicrobial peptides.Honeybee immune genes respond differently to liveS. alvicompared to heat-killedS. alvi,the latter causing a more extensive immune expression response. We show a preference for Toll pathway upregulation over the Imd pathway in the presence of both live and heat-killedS. alvi. Finally, we find that liveS. alviaids in clearance ofS. marcescensfrom the honeybee gut, supporting a potential role for the symbiont in colonization resistance. Our results show that colonization by the beneficial symbiontS. alvitriggers a replicable honeybee immune response. These responses may benefit the host and the symbiont, by helping to regulate gut microbial members and preventing overgrowth or invasion by opportunists.