Data from: Natural selection on a measure of parasite resistance varies across ages and environmental conditions in a wild mammal

Parasites detrimentally affect host fitness, leading to expectations of positive selection on host parasite resistance. However, since immunity is costly, host fitness may be maximized at low, but non-zero, parasite infection intensities. These hypotheses are rarely tested on natural variation in free-living populations. We investigated selection on a measure of host parasite resistance in a naturally-regulated Soay sheep population using a longitudinal data set, and found negative correlations between parasite infection intensity and annual fitness in lambs, male yearlings, and adult females. However, having accounted for confounding effects of body weight, the effect was only significant in lambs. Associations between fitness and parasite resistance were environment-dependent, being strong during low-mortality winters, but negligible during harsher high-mortality winters. There was no evidence for stabilising selection. Our findings reveal processes that may shape variation in parasite resistance in natural populations, and illustrate the importance of accounting for correlated traits in selection analysis.寄生虫会对宿主适合度造成不利影响，据此可预期宿主的寄生虫抗性将受到正向选择。然而由于免疫应答存在成本，宿主适合度可能在寄生虫感染强度较低但非零的状态下达到最优。此类假说鲜有在自由生活种群的自然变异中得到验证。本研究借助纵向数据集，在受自然调控的索艾羊（Soay sheep）种群中针对宿主寄生虫抗性的相关指标开展选择分析，结果发现羔羊、雄性一岁羊与成年雌性个体的寄生虫感染强度与年度适合度呈负相关。然而在控制体重的混杂效应后，该关联仅在羔羊群体中达到显著水平。宿主适合度与寄生虫抗性之间的关联具有环境依赖性：在低死亡率冬季该关联较强，而在严苛的高死亡率冬季则可忽略不计。未发现存在稳定化选择的相关证据。本研究结果揭示了可能塑造自然种群中寄生虫抗性变异的相关过程，并阐明了在选择分析中考量相关性状的重要性。