The switch to parasitism on an ecological and evolutionary scale

在某些条件下无害的病原体在不同的情况下会突然变得极其有害。事实上，寄生的水平在自然界中不断变化，随着生态和进化时间的推移，一些物种沿着从良性到致病的连续体转变。但这个问题更根本的是寄生本身的进化过程。了解促进寄生获得的生态和进化因素非常重要，因为它使流行病学家能够更准确地预测传染病何时何地出现，并制定有效的控制措施。我将研究寄生虫的生活史进化，表达宿主利用策略的变化，并确定导致过渡到寄生生活方式的选择压力。兼性寄生虫为解决这些问题提供了一个独特而有趣的机会，因为它们经常从自由生活转变为寄生生活方式。兼性外寄生螨，Macrocheles subbadius饲料和繁殖的高度短暂的栖息地-腐烂的仙人掌。然而，螨在某些情况下通过附着并以嗜恶果蝇宿主为食而成为寄生虫。因此，螨似乎能够根据环境表达感染性（寄生宿主的能力）。苍蝇和螨在野外自然共存，并且非常适合实验室微观实验。我计划操纵环境条件来测试环境异质性对螨行为和感染的生态后果（目的1）。为了确定感染性性状的遗传性，我将通过人工选择具有不同感染性水平的螨来进行实验进化;所选品系将用于测量寄生进化与其他关键寄生虫生活史性状之间的进化权衡（目的2）。此外，我将确定在很长一段时间内选择增加对宿主依赖的生态因素，即，向寄生的进化过渡（目标3）。

Pathogenic organisms that are harmless under certain conditions can suddenly become extremely harmful under different circumstances. Indeed levels of parasitism vary continuously in nature, with some species shifting along a continuum from benign to pathogenic over ecological and evolutionary time. But even more fundamental to this issue is the process by which parasitism per se evolved. Understanding the ecological and evolutionary factors that facilitate the acquisition of parasitism is important because it allows epidemiologists to more accurately predict when and where infectious diseases will emerge, and develop effective control measures. I will investigate the life-history evolution of parasites that express variation in host exploitation strategies, and identify the selective pressures that lead to the transition to a parasitic lifestyle. Facultative parasites present a unique and interesting opportunity for addressing these questions because they regularly shift from free-living to parasitic lifestyles. The facultative ectoparasitic mite, Macrocheles subbadius feeds and reproduces on highly ephemeral habitats - rotting cacti. However, mites become parasitic under certain circumstances by attaching to and feeding on cactophilic drosophilid hosts. Therefore, it appears the mites are capable of context-dependent expression of infectivity (ability to parasitize a host). The flies and mites co-occur naturally in the wild and are highly amendable to laboratory microcosm experiments. I plan to manipulate environmental conditions to test the ecological consequences of environmental heterogeneity on mite behaviour and infection (aim 1). To determine the heritability of the infectivity trait, I will perform experimental evolution by artificially selecting for mites with divergent levels of infectivity; the selected lines will be used to measure the evolutionary trade-offs between the evolution of parasitism and other key parasite life history traits (aim 2). Additionally, I will identify the ecological factors that select for increased reliance on hosts over a long period of time, i.e., the evolutionary transition to parasitism (aim 3).