Ferris Charlotte, Best Alex
School of Mathematics and Statistics, University of Sheffield, Sheffield, S3 7RH, UK.
School of Mathematics and Statistics, University of Sheffield, Sheffield, S3 7RH, UK.
Theor Popul Biol. 2019 Dec;130:182-190. doi: 10.1016/j.tpb.2019.07.015. Epub 2019 Aug 12.
There are many mechanisms that hosts can evolve to defend against parasites, two of which are resistance and tolerance. These defences often have different evolutionary behaviours, and it is important to consider how each individual mechanism may respond to changes in environment. In particular, host defence through tolerance is predicted to be unlikely to lead to variation, despite many observations of diversity in both animal and plant systems. Hence understanding the drivers of diversity in host defence and parasite virulence is vital for predicting future evolutionary changes in infectious disease dynamics. It has been suggested that heterogeneous environments might generally promote diversity, but the effect of temporal fluctuations has received little attention theoretically or empirically, and there has been no examination of how temporal fluctuations affects the evolution of host tolerance. In this study, we use a mathematical model to investigate the evolution of host tolerance in a temporally fluctuating environment. We show that investment in tolerance increases in more variable environments, giving qualitatively different evolutionary behaviours when compared to resistance. Once seasonality is introduced evolutionary branching though tolerance can occur and create diversity within the population, although potentially only temporarily. This branching behaviour arises due to the emergence of a negative feedback with the maximum infected density on a cycle, which is strongest when the infected population is large. This work reinforces the qualitative differences between tolerance and resistance evolution, but also provides theoretical evidence for the theory that heterogeneous environments promote host-parasite diversity, hence constant environment assumptions may omit important evolutionary outcomes.
宿主可以进化出多种抵御寄生虫的机制，其中两种是抗性和耐受性。这些防御机制通常具有不同的进化行为，因此考虑每种机制如何应对环境变化非常重要。特别是，尽管在动物和植物系统中都观察到了多样性，但通过耐受性进行的宿主防御预计不太可能导致变异。因此，了解宿主防御和寄生虫毒力多样性的驱动因素对于预测传染病动态的未来进化变化至关重要。有人认为，异质环境通常可能促进多样性，但时间波动的影响在理论上或实证研究中很少受到关注，而且尚未研究时间波动如何影响宿主耐受性的进化。在本研究中，我们使用数学模型来研究在时间波动环境中宿主耐受性的进化。我们表明，在变化更大的环境中，对耐受性的投入会增加，与抗性相比，会产生质的不同的进化行为。一旦引入季节性，通过耐受性的进化分支就可能发生，并在种群中产生多样性，尽管可能只是暂时的。这种分支行为的出现是由于在一个周期内与最大感染密度产生了负反馈，当感染种群较大时这种反馈最强。这项工作强化了耐受性和抗性进化之间的质的差异，但也为异质环境促进宿主 - 寄生虫多样性这一理论提供了理论证据，因此恒定环境假设可能会忽略重要的进化结果。
