Parasite evolution has been fruitfully examined using both theory and laboratory-based empirical studies; however, we know little of real-time parasite evolution in the field, because of the difficulty in dissecting the influences of non-genetic and genetic factors on parasite phenotype. We overcame this hurdle by studying a wild host-parasite population while simultaneously preserving field-collected parasite spores so a common garden experiment could later be performed under controlled laboratory conditions. We studied how parasite infection traits, including parasite prevalence and infection intensity, changed over the course of a growing season in a natural population of the crustacean Daphnia magna and its sterilizing bacterial parasite Pasteuria ramosa. We later exposed standard host genotypes to a fixed dose of spores collected at different times over the season in the laboratory.
Results/Conclusions
In the field, we observed that the number of parasite transmission spores per infected host increased ten-fold over the course of the growing season, and this increase was accompanied by a decline in host density. In the laboratory, we found parasites collected late in the season were more infective and grew more rapidly than parasites collected early in the season. Thus, we were able to demonstrate that changes in wild parasite infection traits were due to adaptive evolution (possibly in response to selection resulting from the decline in host density), and not due to non-genetic effects. To our knowledge, this is the first study to demonstrate real-time adaptive evolution in a wild parasite population.