Experimental evolution of sexual host populations in response to sterilizing parasites

Hypothesis: Sexual host populations rapidly evolve specific resistance against parasites, and consequently diverge from populations under relaxed parasite-mediated selection or selection imposed by different parasite populations. Organisms: The freshwater snail, Potamopyrgus antipodarum, and two different populations of a naturally prevalent sterilizing trematode, Microphallus sp. Methods: Experimental populations of the host snail were exposed to one of the two parasite populations, or allowed to evolve in the absence of parasites. After three generations of experimental evolution, replicate hosts were exposed to parasites from one of the parasite source populations, and resistance was compared across parasite treatments. Results: Host populations rapidly diverged as each evolved to their experimental parasite populations. This resistance was associated with an increased susceptibility, relative to the control and other parasite treatments, to the other experimental parasite population. Hence, resistance to infection by Microphallus is specific, rather than generalized.