Evolution and ecology of phenotypic plasticity in parasite life history strategies

寄生虫在宿主和病媒体内经历的环境是高度多变的。例如，随着宿主生病，资源变得稀缺，针对寄生虫的免疫反应不断发展，寄生虫往往不得不与共享宿主的其他菌株和物种竞争。考虑到寄生虫生活在他人体内，与他人进行生死斗争，关于寄生虫如何应对其生活方式的挑战的知识非常肤浅。了解寄生虫进化以应对它们在宿主和媒介内部面临的挑战环境的策略是这项提议的目的。寄生虫策略很重要，因为它们是决定病态宿主(毒力)和寄生虫传染性(传播)的因素。在进化生态学中，生活史特征的可塑性是众所周知的对多变环境中生活的适应。适应性可塑性使有机体能够随着环境的变化而改变其表型，从而在不同环境中保持健康。一个多世纪以来，人们一直知道寄生虫的环境复杂，形状复杂，但由于寄生虫被视为策略僵化的生物，它们特征的可塑性--以及它是否有益--被忽视了。越来越多的证据显示，寄生虫的特征具有相当大的可塑性，这些特征支撑着在感染期间存活并传播给新宿主的能力。我们最近的研究表明，寄生虫会根据它们在宿主中经历的条件(资源可获得性、与其他菌株的竞争以及抗寄生虫药物)来调整它们的特征。我们知道，我们想通过测试预测寄生虫以最大化适应能力的方式调整自己特征的理论来研究为什么寄生虫会这样做。我们将确定寄生虫特征的变异在多大程度上是由寄生虫的策略和宿主与寄生虫之间的复杂相互作用来解释的，可塑性是否有利于寄生虫，以及可塑性是否进化到使寄生虫能够应对可变的环境条件。因此，我们的项目将揭示寄生虫的生存和传播策略是多么复杂，并揭示可用于疾病控制的弱点。疟疾(疟原虫)寄生虫提供了一个很好的系统来解释形成寄生虫毒力和传播表型的特征变异的生态和进化驱动因素。决定疟疾寄生虫适合性的一个关键特征是将资源分配到无性复制阶段(寄主内生存所必需的)和有性繁殖阶段(传播所必需的)之间的权衡，这在进化生物学中被称为“生殖努力”。我们的项目将确定宿主和寄生虫因素如何促进生殖努力的可塑性，量化跨环境梯度和寄生虫遗传背景产生的模式，测试寄生虫的策略是否以有利于生存和传播的方式根据环境进行调整，并通过测试在恒定环境条件下选择是否失去可塑性来调查可塑性是否代价高昂。解释寄生虫毒力和传染性的变异是进化生物学的一个主要目标，预测寄生虫如何适应环境变得越来越重要。例如，寄生虫生态的变化(如宿主-寄生虫-病媒群落和栖息地的组成)与野生动物、农作物、牲畜和人类的新传染病的出现有关。对于应用科学来说，更好地了解寄生虫是必要的，因为进化不断侵蚀控制疾病的努力，这些疾病导致相当大的死亡率和发病率。

The environments experienced by parasites inside hosts and vectors are highly changeable. For example, resources become scarce as hosts get sick, immune responses directed at parasites develop, and parasites often have to compete with other strains and species sharing their host. Given that parasites live in the bodies of others, with which they are engaged in a life-and-death struggle, knowledge about how parasites cope with the challenges of their lifestyle is remarkably shallow. Understanding the strategies that parasites have evolved to cope with the challenging environments they face inside hosts and vectors is the aim of this proposal. Parasite strategies are important because they are determinants of how sick hosts get (virulence) and how infectious parasites are (transmission). In evolutionary ecology, plasticity in life-history traits is a well-known adaptation to life in variable environments. Adaptive plasticity enables organisms to change their phenotype in response to changes in their circumstances in ways that maintain fitness across environments. That the environment is complex and shapes phenotypes has been known for over a century but because parasites have been viewed as creatures with inflexible strategies, plasticity in their traits - and whether it is beneficial - has been overlooked. Mounting evidence is revealing considerable plasticity in parasite traits that underpin the ability to survive during infections and to transmit to new hosts. Our recent research shows that parasites adjust their traits in response to the conditions (resource availability, competition with other strains, and anti-parasite drugs) they experience in the host. We know want to investigate why parasites do this by testing theory that predicts they adjust their traits in ways that maximise fitness. We will identify how much variation in parasite traits is explained by parasite strategies and complex interactions between hosts and parasites, whether plasticity benefits parasites, and whether plasticity has evolved to enable to parasites to cope with variable environmental conditions. Therefore, our project will reveal how sophisticated parasite strategies for survival and transmission really are, and reveal weaknesses that could be exploited for disease control.Malaria (Plasmodium) parasites offer an excellent system to explain the ecological and evolutionary drivers of variation in traits that shape the virulence and transmission phenotypes of parasites. A key fitness-determining trait for malaria parasites is the trade-off between the allocation of resources to asexually replicating stages (required for within-host survival) and sexually reproducing stages (required for transmission), which is called "reproductive effort" in evolutionary biology. Our project will determine how host and parasite factors contribute to plasticity in reproductive effort, quantify the patterns generated across environmental gradients and parasite genetic backgrounds, test whether parasite strategies are adjusted in response to the environment in ways that benefit survival and transmission, and investigate whether plasticity is costly by testing if it is lost under selection in constant environmental conditions. Explaining variation in the virulence and infectiousness of parasites is a major aim in evolutionary biology and predicting how parasites adapt to their environment is becoming increasingly important. For example, changes in parasite ecology (e.g. composition of host-parasite-vector communities, and habitats) are implicated in the emergence of new infectious diseases of wildlife, crops, livestock, and humans. For applied science, a better understanding of parasites is necessary because evolution continually erodes efforts to control diseases responsible for considerable mortality and morbidity.