Mechanisms of symbiont-conferred protection against natural enemies: an ecological and evolutionary framework

• Many vertically-transmitted symbionts protect insect hosts from pathogenic and parasitic natural enemies.
• Vertically-transmitted symbionts can also alter the ability of insects to vector pathogens and parasites.
• Protection involves a variety of mechanisms (toxin production, resource competition, activation of host immunity).
• Mechanisms of protection are influenced by host–symbiont evolutionary history.
• Mechanisms of protection can influence evolution of host physiological processes.

Many vertically-transmitted microbial symbionts protect their insect hosts from natural enemies, including host-targeted pathogens and parasites, and those vectored by insects to other hosts. Protection is often achieved through production of inhibiting toxins, which is not surprising given that toxin production mediates competition in many environments. Classical models of macroecological interactions, however, demonstrate that interspecific competition can be less direct, and recent research indicates that symbiont-protection can be mediated through exploitation of limiting resources, and through activation of host immune mechanisms that then suppress natural enemies. Available data, though limited, suggest that effects of symbionts on vectored pathogens and parasites, as compared to those that are host-targeted, are more likely to result from symbiont activation of the host immune system. We discuss these different mechanisms in light of their potential impact on the evolution of host physiological processes.

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