Evolutionary rescue in a host-pathogen system results in coexistence not clearance

The evolutionary rescue of host populations may prevent extinction from novel pathogens. However, the conditions that facilitate rapid evolution of hosts, in particular the population variation in host susceptibility, and the effects of host evolution in response to pathogens on population outcomes remain largely unknown. We constructed an individual-based model to determine the relationships between genetic variation in host susceptibility and population persistence in an amphibian-fungal pathogen (Batrachochytrium dendrobatidis) system. We found that host populations can rapidly evolve reduced susceptibility to a novel pathogen and that this rapid evolution led to a 71 fold increase in the likelihood of host-pathogen coexistence. However, the increased rates of coexistence came at a cost to host populations; fewer populations cleared infection, population sizes were depressed, and neutral genetic diversity was lost. Larger adult host population sizes and greater adaptive genetic variation prior to the onset of pathogen introduction led to substantially reduced rates of extinction suggesting that populations with these characteristics should be prioritized for conservation when species are threatened by novel infectious diseases.

宿主种群的进化拯救（evolutionary rescue）可避免其在遭遇新型病原体时发生灭绝。然而，当前学界对两大关键科学问题仍缺乏清晰认知：一是促进宿主快速演化的条件，尤其是宿主易感性的种群间变异；二是宿主针对病原体的演化对种群结局的具体影响。本研究构建了基于个体的模型（individual-based model），以探究两栖动物-蛙壶菌（Batrachochytrium dendrobatidis）宿主-病原体系统中，宿主易感性的遗传变异与种群存续之间的关联。研究发现，宿主种群可快速演化出对新型病原体的更低易感性，且该快速演化可使宿主-病原体共存的概率提升71倍。但共存概率的提升需宿主种群付出相应演化代价：能够清除感染的种群数量减少、种群规模受到抑制，且中性遗传多样性发生流失。在病原体入侵前期，更大的成体宿主种群规模与更高的适应性遗传变异水平，可显著降低种群灭绝速率。这提示，当物种受到新型传染性疾病威胁时，应优先对具备上述特征的种群开展保护工作。