There is growing empirical evidence that animal hosts actively control the density of their mutualistic symbionts according to their requirements. Such active regulation can be facilitated by compartmentalization of symbionts within host tissues, which confers a high degree of control of the symbiosis to the host. Here, we build a general theoretical framework to predict the underlying ecological drivers and evolutionary consequences of host-controlled endosymbiont density regulation for a mutually obligate association between a host and a compartmentalized, vertically transmitted symbiont. Building on the assumption that the costs and benefits of hosting a symbiont population increase with symbiont density, we use state-dependent dynamic programming to determine an optimal strategy for the host, i.e., that which maximizes host fitness, when regulating the density of symbionts. Simulations of active host-controlled regulation governed by the optimal strategy predict that the density of the symbiont should converge to a constant level during host development, and following perturbation. However, a similar trend also emerges from alternative strategies of symbiont regulation. The strategy which maximizes host fitness also promotes symbiont fitness compared to alternative strategies, suggesting that active host-controlled regulation of symbiont density could be adaptive for the symbiont as well as the host. Adaptation of the framework allowed the dynamics of symbiont density to be predicted for other host-symbiont ecologies, such as for non-essential symbionts, demonstrating the versatility of this modelling approach.

中文翻译：

---

共生体密度的宿主控制调节的理论模型。

越来越多的经验证据表明，动物宿主根据自己的需求主动控制共生体的密度。这种主动调节可以通过宿主组织内共生体的区室化来促进，这赋予宿主对共生体的高度控制。在这里，我们建立了一个通用的理论框架来预测宿主控制的内共生体密度调节的潜在生态驱动因素和进化后果，以实现宿主与分隔的垂直传播共生体之间的相互约束关联。基于承载共生体种群的成本和收益随着共生体密度而增加的假设，我们使用状态相关的动态规划来确定宿主的最佳策略，即在调节共生体密度时最大化宿主适应性。由最佳策略控制的主动宿主控制调节的模拟预测，在宿主发育期间和扰动之后，共生体的密度应收敛到恒定水平。然而，共生体调控的替代策略也出现了类似的趋势。与其他策略相比，最大化宿主适应性的策略还可以促进共生体适应性，这表明宿主主动控制的共生体密度调节可能适用于共生体和宿主。该框架的适应性使得可以预测其他宿主-共生体生态的共生体密度动态，例如非必需共生体，证明了这种建模方法的多功能性。