Coevolution between mutualists and parasites in symbiotic communities may lead to the evolution of lower virulence

Most eukaryotes harbor a diverse community of parasitic, mutualistic and commensal microbial symbionts. Although the diversity of these microbial symbiotic communities has recently drawn considerable attention, theory regarding the evolution of interactions among symbionts and with the host is still in nascent stages. Here we evaluate the role of interactions among co-infecting symbionts in the evolution of symbiont virulence towards the host. To do so, we place the virulence-transmission trade-off into a community context and model the evolution of symbiont trophic modes along the continuum from parasitism (virulence) to mutualism (negative virulence). We establish a framework for studying multiple infections of a host by the same symbiont species, and co-infection by multiple species, using a concept of shared costs, wherein the negative consequences of virulence, or harm, toward the host are shared among symbionts. Our results show that mutualism can be maintained under infection by ...