Data from: Resist globally, infect locally: a transcontinental 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 transcontinental 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 (i.e., matching infection or targeted recognition). Instead, we propose a hybrid, eco-evolutionary model to explain the remarkable pattern of global host resistance and local parasite infectivity.寄生虫感染是同时影响宿主-寄生虫接触率的生态过程，与影响宿主易感性的进化动态共同作用的产物。然而，鲜有研究同时从生态学与进化学视角探究自然感染的变异规律。本文旨在阐明导致脊椎动物宿主三刺鱼（Gasterosteus aculeatus）感染绦虫（Schistocephalus solidus）感染率变异的生态与进化机制。 为探究感染的生态学特征，我们对栖息于两种截然不同生境——海洋与淡水——的加拿大三刺鱼开展了绦虫感染率统计。结果与感染的生态调控机制相符：尽管海洋三刺鱼对该绦虫具有高度易感性，但绦虫在海洋环境中极为罕见。与之相反，尽管淡水三刺鱼在自然环境中普遍感染该绦虫，但在受控实验室试验中却表现出显著抗性，这表明野外的高接触风险抵消了它们新近演化出的抗性。 此外，我们利用来自欧洲与加拿大西部的三刺鱼和绦虫种群开展跨大陆交互感染实验，以此检验寄生虫对宿主的适应性。研究发现，同大陆宿主-寄生虫组合的感染率更高，表明寄生虫至少在大陆尺度上存在‘本地’适应性。但淡水宿主新近演化出的免疫能力可同时抵御本地与外来寄生虫。 本文所阐述的适应性模式，无法完全契合宿主-寄生虫协同演化的两种主流模型（即匹配感染模型或靶向识别模型）。为此，我们提出一种混合生态-进化模型，用以解释全球宿主抗性与本地寄生虫感染性并存的显著格局。