Data from: 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 multiple symbionts when shared costs are sufficiently low, while greater virulence and parasitism toward the host are more likely when shared costs are high. Lastly, for co-infection by more than one species, we show that if the presence of a mutualist ameliorates some of the costs of pathogen virulence, then the symbiotic community may more often evolve to a more commensal state and maintain mutualisms.

绝大多数真核生物（eukaryotes）都拥有多样的寄生、互利共生与共栖微生物共生体群落。尽管此类微生物共生群落的多样性近来广受关注，但有关共生体间相互作用及其与宿主相互作用演化的理论仍处于起步阶段。在此，我们评估了共感染共生体间的相互作用在共生体对宿主的毒力演化中所扮演的角色。为此，我们将毒力-传播权衡（virulence-transmission trade-off）置于群落语境之中，并构建模型，模拟共生体营养模式沿从寄生（具毒力）到互利共生（负毒力）的连续谱系的演化过程。我们建立了一套研究框架，用以探究同一共生体物种对宿主的多重感染，以及多物种共感染的情形，该框架基于共享成本（shared costs）这一概念：即共生体对宿主造成的毒力或伤害所带来的负面后果，会在共生体间共享。我们的研究结果表明，当共享成本足够低时，多重共生体感染下仍可维持互利共生；而当共享成本较高时，共生体更易演化出对宿主更强的毒力与寄生性。最后，针对多物种共感染的情形，我们发现若互利共生体的存在可缓解病原体毒力带来的部分成本，则共生群落往往更易演化至更偏向共栖的状态，并维持互利共生关系。