The evolution of parasite virulence in the presence of resistance-conferring defensive symbionts

Defensive symbionts-organisms that confer protection to their hosts against natural enemies such as parasites, predators, or herbivores-are found throughout the natural world. Theoretical and empirical studies have shown that defensive symbionts can both interfere with ecological interactions between hosts and exploiters, as well as drive exploiter evolution. Defensive symbionts are also potential candidates for biocontrol agents to help manage infectious diseases or agricultural pests. The impact of defensive symbionts on parasite ecology and evolution has therefore recently received increased empirical and theoretical attention. In this theoretical study, we investigate the impact that a defensive symbiont which protects hosts from infection (conferred resistance) has on the evolution of parasite virulence. We also explore how the extent of protection conferred by the defensive symbiont coevolves with parasite virulence, and how symbiont and parasite evolution affect the ecology of the host population in both the short- and long-term. We show that, while costly resistance-conferring defensive symbionts always select for increased parasite virulence, the overall long-term ecological effect on the host population may still be positive due to reductions in disease prevalence. This contrasts with tolerance-conferring symbionts (which protect against virulence), where the long-term ecological effects on the host population are always negative. We also show when the defensive symbiont can successfully eliminate the parasite. Resistance-conferring defensive symbionts therefore offer more promise as evolutionarily robust biocontrols than those that only confer tolerance.