A microbiome rife with enemies of the host should cause selection for defensive traits in symbionts, yet such complex environments are also predicted to select for greater symbiont virulence. Why then do we so often observe defensive mutualists that protect hosts while causing little to no damage? To address this question, we build a symbiont-centered model that incorporates the evolution of two independent symbiont traits: defense and virulence. Virulence is modeled as a continuous trait spanning parasitism (positive virulence) and mutualism (negative virulence), thus accounting for the entire range of direct effects that symbionts have on host mortality. Defense is modeled as a continuous trait that ameliorates the costs to the host associated with infection by a deleterious parasite. We show that the evolution of increased defense in one symbiont may lead to the evolution of lower virulence in both symbionts and even facilitate pathogens evolving to mutualism. However, results are context dependent, and when defensive traits are costly, the evolution of greater defense may also lead to the evolution of greater virulence, breaking the common expectation that defensive symbionts are necessarily mutualists toward the host.

中文翻译：

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防御性共生与毒力的演变。

带有宿主敌人的微生物群落应引起共生体防御性状的选择，然而，预计这种复杂的环境也会选择更大的共生体毒力。那么为什么我们如此经常观察到防御性的互助主义者，他们在保护主机的同时却几乎没有造成破坏？为了解决这个问题，我们建立了一个以共生菌为中心的模型，该模型结合了两个独立的共生菌性状的演变：防御和毒力。毒力被建模为跨越寄生性（阳性毒力）和共生性（阴性毒力）的连续性状，因此解释了共生体对宿主死亡率的直接影响的整个范围。防御被建模为连续性状，可减轻与有害寄生虫感染相关的宿主的成本。我们表明，在一种共生体中防御力的增强进化可能导致在两种共生体中低毒力的进化，甚至促进病原体向共生进化。但是，结果取决于上下文，并且当防御性状代价高昂时，防御能力的增强也可能导致毒力的增强，这打破了人们普遍认为防御性共生体一定是对宿主的共存主义者的普遍期望。