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变化。现有假说认为，宿主防御维持所需的代价可部分解释这种易感性的差异。具体而言，存在演化权衡效应：在无寄生虫的环境中，宿主维持高水平防御会以自身适合度为代价，这可能导致宿主防御水平随时间改变，且不同种群间的防御水平存在差异。在既往研究中，我们发现宿主线虫秀丽隐杆线虫（Caenorhabditis elegans）的种群已演化出对细菌性寄生虫粘质沙雷氏菌（Serratia marcescens）更强的规避能力与抗性。本研究中，我们将这些宿主种群分别置于存在、不存在该寄生虫的环境中继代培养，以检验宿主防御水平升高是否伴随适应性代价。经过16代培养后，我们发现，无论演化过程中是否存在寄生虫，宿主防御水平的升高均得以维持。进一步分析表明，这种防御能力的维持并非源于有限的现存遗传变异，而是未检测到与防御相关的可测量代价。因此，宿主防御相关的适应性代价或许无法广泛解释不同时空背景下宿主寄生虫易感性的差异。