Evolution of <i>Caenorhabditis elegans</i> host defense under selection by the bacterial parasite <i>Serratia marcescens</i>

Parasites can impose strong selection on hosts. In response, some host populations have adapted via the evolution of defenses that prevent or impede infection by parasites. However, host populations have also evolved life history shifts that maximize host fitness despite infection. Outcrossing and self-fertilization can have contrasting effects on evolutionary trajectories of host populations. While selfing and outcrossing are known to affect the rate at which host populations adapt in response to parasites, these mating systems may also influence the specific traits that underlie adaptation to parasites. Here, we determined the role of evolved host defense versus altered life history,in mixed mating (selfing and outcrossing) and obligately outcrossing C. elegans host populations after experimental evolution with the bacterial parasite, S. marcescens. Similar to previous studies, we found that both mixed mating and obligately outcrossing host populations adapted to S. marcescens exposure, and that the obligately outcrossing populations exhibited the greatest rates of adaptation. Regardless of the host population mating system, exposure to parasites did not significantly alter reproductive timing or total fecundity over the course of experimental evolution. However, both mixed mating and obligately outcrossing host populations exhibited significantly reduced mortality rates in the presence of the parasite after experimental evolution. Therefore, adaptation in both the mixed mating and obligately outcrossing populations was driven, at least in part, by the evolution of increased host defense and not changes in host life history. Thus, the host mating system altered the rate of adaptation, but not the nature of adaptive change in the host populations.寄生虫可对宿主施加强烈的选择压力。作为响应，部分宿主种群通过演化出能够阻止或阻碍寄生虫感染的防御机制实现了适应性进化。不过，宿主种群还演化出了生活史策略的转变，以便在遭受感染的前提下最大化自身适合度。异交（outcrossing）与自交（self-fertilization）对宿主种群的演化轨迹可产生截然不同的影响。尽管已知自交与异交会影响宿主种群应对寄生虫时的适应性演化速率，但这类交配系统或许还会调控宿主针对寄生虫产生适应性演化所依赖的具体性状。本研究针对与细菌性寄生虫粘质沙雷氏菌（S. marcescens）开展实验演化后的混交（自交与异交）及专性异交秀丽隐杆线虫（C. elegans）宿主种群，探究了宿主演化出的防御机制与改变后的生活史各自所发挥的作用。与既往研究一致，我们发现混交与专性异交的宿主种群均能适应粘质沙雷氏菌的侵染，且专性异交种群的适应性演化速率最快。无论宿主种群的交配系统如何，在实验演化过程中，寄生虫暴露并未显著改变宿主的繁殖时机或总繁殖力。但经实验演化后，混交与专性异交的宿主种群在暴露于该寄生虫时的死亡率均显著降低。因此，混交与专性异交种群的适应性演化至少部分是由宿主防御能力增强的演化所驱动，而非宿主生活史的改变。由此可见，宿主的交配系统仅会改变适应性演化的速率，而非宿主种群适应性演化的本质。