Insect secretions stabilising obligate microbial symbioses

Insects deploy numerous secretions to sustain beneficial microbes throughout development. These feature egg-associated jelly, capsules, caplets, and smears that are shown to protect metabolically constrained symbionts during vulnerable extracellular phases, such as transmission and colonisation of host tissues. Recent molecular dissections of these secretions have begun to answer two key mechanistic questions: what are they composed of, and how do they compensate for the metabolic constraints imposed by reductive evolution? Advances in functional genetics, proteomics, glycoproteomics, and structural biochemistry have revealed the co-option and repurposing of canonical insect protein families and metabolites for symbiont protection. They have further shown that host secretions can be load-bearing for symbiont viability, compensating not only for physical exposure but for the transcriptional and metabolic limitations imposed by reductive genome evolution. These advances are reviewed here, emphasising the regulatory and mechanistic features of insect investment in obligate partnerships.