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）均携带着由寄生型、互利共生型与共栖型微生物共生体（microbial symbionts）组成的多样化群落。尽管此类微生物共生群落的多样性近来已受到广泛关注，但有关共生体之间及其与宿主间相互作用演化的理论研究仍处于起步阶段。本研究旨在评估共感染共生体之间的相互作用，对共生体针对宿主的毒力演化所产生的影响。为此，我们将毒力-传播权衡（virulence-transmission trade-off）置于群落研究框架之下，并针对共生体营养模式（trophic modes）的演化构建模型，该模型覆盖了从寄生（对应毒力）到互利共生（对应负毒力）的连续谱系。我们借助共享成本（shared costs）这一概念，搭建了可用于研究同一共生体物种对宿主的多次感染，以及多物种共感染的研究框架；该概念指出，共生体之间会共享其对宿主造成的毒力（即损害）所带来的负面后果。研究结果表明，当共享成本足够低时，多共生体感染宿主的情况下仍可维持互利共生关系；而当共享成本较高时，共生体更易演化出更强的毒力并呈现寄生状态。最后，针对多物种共感染的场景，我们发现：若互利共生体的存在可缓解病原体毒力所带来的部分成本，则共生群落往往更易演化至共栖状态，并维持互利共生关系。