Data from: No measurable fitness cost to experimentally evolved host defense in the Caenorhabditis elegans-Serratia marcescens host-parasite system

Host susceptibility to parasites can vary over space and time. Costs associated with the maintenance of host defense are thought to account for a portion of this variation. Specifically, tradeoffs wherein elevated defense is maintained at the cost of fitness in the absence of the parasite may cause levels of host defense to change over time and differ between populations. In previous studies we found that populations of the host nematode, Caenorhabditis elegans evolved greater levels of parasite avoidance and resistance against the bacterial parasite, Serratia marcescens. Here, we passaged these host populations either in the presence or absence of the parasite to test for a cost of elevated host defenses. After 16 generations, we found that elevated levels of host defense were maintained during evolution in both the presence and absence of the parasite. Further, this maintenance of defense was not the result of limited standing genetic variation, but rather the absence of a measurable cost associated with defense. Therefore, costs associated with host defense may not broadly account for differences in host susceptibility across space and time.

宿主对寄生虫的易感性可随空间与时间发生动态变化。学界普遍认为，宿主防御维持过程中产生的适应性代价可解释该变异的一部分成因。具体而言，在无寄生虫存在的情况下，以宿主适合度为代价维持增强型防御的进化权衡，可能导致宿主防御水平随时间改变，并在不同种群间产生差异。在过往研究中，我们发现宿主线虫秀丽隐杆线虫（Caenorhabditis elegans）的多个种群，进化出了更强的寄生虫规避能力与对细菌性寄生虫粘质沙雷氏菌（Serratia marcescens）的抗性。本研究中，我们将这些宿主种群分别在有、无寄生虫的条件下进行传代培养，以验证增强型宿主防御是否存在适应性代价。经过16代传代后，我们发现无论进化过程中是否存在寄生虫，宿主的增强型防御水平均得以维持。进一步分析表明，防御水平的维持并非源于现存遗传变异的局限性，而是因为未检测到与防御相关的适应性代价。因此，宿主防御相关的适应性代价，或许并不能广泛解释宿主易感性在空间与时间维度上的差异。