Data from: Resist globally, infect locally: a trans-continental test of adaptation by stickleback and their tapeworm parasite

Parasite infections are a product of both ecological processes affecting host-parasite encounter rates, and evolutionary dynamics affecting host susceptibility. However, few studies examine natural infection variation from both ecological and evolutionary perspectives. Here, we describe the ecological and evolutionary factors generating variation in infection rates by a tapeworm (Schistocephalus solidus) in a vertebrate host, the threespine stickleback (Gasterosteus aculeatus). To explore ecological aspects of infection, we measured tapeworm prevalence in Canadian stickleback inhabiting two distinct environments: marine and freshwater. Consistent with ecological control of infection, the tapeworm is very rare in marine environments even though marine fish are highly susceptible. Conversely, commonly-infected freshwater stickleback exhibit substantial resistance in controlled laboratory trials, suggesting that high exposure risk overwhelms their recently-evolved resistance. We also tested for parasite adaptation to its host by performing trans-continental reciprocal infections, using stickleback and tapeworm populations from Europe and western Canada. More infections occurred in same-continent host-parasite combinations, indicating parasite 'local' adaptation at least on the scale of continents. However, the recently-evolved immunity of freshwater hosts applies to both local and foreign parasites. The pattern of adaptation described here is not wholly compatible with either of the common models of host-parasite coevolution (e.g., matching or targeted recognition). Instead, we propose a hybrid, eco-evolutionary model to explain the remarkable pattern of global host resistance and local parasite infectivity.寄生虫感染是影响宿主-寄生虫相遇率的生态过程，与影响宿主易感性的进化动力学共同作用的产物。然而，鲜有研究同时从生态与进化视角探究自然感染的变异规律。本研究以脊椎动物宿主三棘刺鱼（Gasterosteus aculeatus）为研究对象，阐明了导致其感染叠宫绦虫（Schistocephalus solidus）的感染速率存在变异的生态与进化因子。为探究感染的生态学特征，我们对栖息于海洋与淡水两种截然不同生境中的加拿大刺鱼种群的叠宫绦虫感染率进行了测定。结果与感染的生态调控假说一致：尽管海洋刺鱼本身具有较高的易感性，但该绦虫在海洋环境中极为罕见。与之相反，实验室受控试验显示，普遍感染绦虫的淡水刺鱼种群展现出显著的抗性，这表明高暴露风险掩盖了它们新近进化出的抗性。我们还通过跨大陆互易感染实验，测试了寄生虫对宿主的适应性：实验所用刺鱼与绦虫种群分别来自欧洲与加拿大西部。同大陆范围内的宿主-寄生虫组合出现了更多的感染事件，这表明寄生虫至少在大陆尺度上存在‘本地’适应性。然而，淡水宿主新近进化出的免疫能力，对本地与外来寄生虫均有效。本研究揭示的适应性模式，并不完全契合目前两种主流的宿主-寄生虫协同进化模型（如匹配识别模型或靶向识别模型）。据此，我们提出了一个混合的生态-进化模型，以解释全球范围内宿主抗性与寄生虫本地感染性并存的显著格局。