Microbial secondary metabolites and their impacts on insect symbioses

• Insect-associated microbes use secondary metabolites to interact with each other and the host.
• Secondary metabolite function can be correlatively and causally established in the spirit of Koch's postulates.
• Interaction networks using secondary metabolites are structured by both ecological and evolutionary processes.
• Specific compounds can have multiple functions and evolve due to multiple selective pressures.
• The fitness of insect-microbe symbioses is an emergent property of interactions between their constituent organisms.

All insects host communities of microbes that interact both with the insect and each other. Secondary metabolites are understood to mediate many of these interactions, although examples having robust genetic, chemical and/or ecological evidence are relatively rare. Here, I review secondary metabolites mediating community interactions in the beewolf, entomopathogenic nematode and fungus-growing ant symbioses, using the logic of Koch's postulates to emphasize well-validated symbiotic functions mediated by these metabolites. I especially highlight how these interaction networks are structured by both ecological and evolutionary processes, and how selection acting on secondary metabolite production can be multidimensional.

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