Data from: Rapid change in parasite infection traits over the course of an epidemic in a wild host–parasite population

By combining a field study with controlled laboratory experimentation, we examined how infection traits of the sterilizing bacterium, Pasteuria ramosa, changed over the course of a growing season in a natural population of its crustacean host Daphnia magna. The number of parasite transmission spores per infected host increased ten-fold over the course of the season, concomitant with a decline in the density of infected hosts. Plausible explanations for this variation include changes in environmental conditions, changes in host quality, or that parasite migration or natural selection caused a genetic change in the parasite population. We sought to distinguish some of these possibilities in a laboratory experiment. Thus, we preserved field-collected parasite spores throughout the season, and later exposed a set of hosts to a fixed dose of these spores under controlled laboratory conditions. Parasites collected late in the season were more infectious and grew more rapidly than parasites collected early in the season. This result is compatible with the hypothesis that the observed increase in infectivity in the field was due to genetic change, i.e. evolution in the P. ramosa population.

本研究结合野外实地研究与受控实验室实验，探究了致宿主绝育的细菌帕氏芽孢杆菌（Pasteuria ramosa）在其甲壳类宿主大型溞（Daphnia magna）的自然种群中，一个生长季内的感染性状变化规律。单个感染宿主携带的寄生虫传播孢子（transmission spores）数量在该生长季内提升了十倍，与此同时感染宿主的种群密度出现下降。针对这一动态变化，现有合理推测包括环境条件改变、宿主质量变化，或是寄生虫迁移、自然选择引发寄生虫种群发生遗传改变。本研究旨在通过实验室实验区分其中部分可能性。为此，我们在整个生长季中留存了野外采集的寄生虫孢子，后续在受控实验室条件下，将一批宿主暴露于固定剂量的上述孢子中开展暴露实验。相较于早期采集的寄生虫，生长季后期采集的寄生虫感染性更强，增殖速度也更快。该结果与野外观察到的感染性提升源于遗传改变（即帕氏芽孢杆菌种群发生演化）的假说相符。