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 co...