Data from: Variation in costs of parasite resistance among natural host populations

Organisms that can resist parasitic infection often have lower fitness in the absence of parasites. These costs of resistance can mediate host evolution during parasite epidemics. For example, large epidemics will select for increased host resistance. In contrast, small epidemics (or no disease) can select for increased host susceptibility when costly resistance allows more susceptible hosts to outcompete their resistant counterparts. Despite their importance for evolution in host populations, costs of resistance (which are also known as resistance trade-offs) have mainly been examined in laboratory-based host-parasite systems. Very few examples come from field-collected hosts. Furthermore, little is known about how resistance trade-offs vary across natural populations. We addressed these gaps using the freshwater crustacean Daphnia dentifera and its natural yeast parasite, Metschnikowia bicuspidata. We found a cost of resistance in two of the five populations we studied – those with the most genetic variation in resistance and the smallest epidemics in the previous year. However, yeast epidemics in the current year did not alter slopes of these trade-offs before and after epidemics. In contrast, the no-cost populations showed little variation in resistance, possibly because large yeast epidemics eroded that variation in the previous year. Consequently, our results demonstrate variation in costs of resistance in wild host populations. This variation has important implications for host evolution during epidemics in nature.

能够抵抗寄生虫感染的生物，在无寄生虫存在时通常具有更低的适合度（fitness）。这类抗性代价可在寄生虫流行期间调控宿主的演化进程。例如，大规模寄生虫流行会正向选择抗性更强的宿主，推动宿主抗性提升。与之相对，当抗性代价使得易感宿主相较于抗性宿主更具竞争优势时，小规模流行（或无病害发生）会正向选择更易感染的宿主，提升宿主整体易感性。
尽管抗性代价（亦称抗性权衡，resistance trade-offs）对宿主种群演化至关重要，但相关研究主要集中于实验室宿主-寄生虫系统（host-parasite system），基于野外采集宿主的案例极少。此外，学界对自然种群间抗性权衡的差异仍知之甚少。
本研究以淡水甲壳类动物齿突溞（Daphnia dentifera）及其天然酵母寄生虫二孢梅奇酵母（Metschnikowia bicuspidata）为研究对象，填补了上述研究空白。我们在所研究的5个种群中发现，其中2个种群存在抗性代价——这两个种群的抗性遗传变异最为丰富，且上一年度的寄生虫流行规模最小。
不过，本年度的酵母流行并未改变这些权衡关系在流行前后的斜率。与之相对，无抗性代价的种群其抗性遗传变异程度极低，这可能是因为上一年度大规模的酵母流行已消耗了这类遗传变异。
综上，本研究结果证实了野生宿主种群中存在抗性代价的变异，这类变异对自然环境中寄生虫流行期间的宿主演化具有重要意义。