Evolutionary ecology of host defense in a snail-trematode interaction

I examine immune defense of a snail (Potamopyrgus antipodarum) against a trematode parasite (Microphallus sp.). The parasite is adapted to infect local populations of the snail. I use laboratory exposure experiments and natural field observations to compare genetic models of host-parasite interactions. In Chapter 2, I use a dose-response experiment to show that sympatric parasites are better able to infect hosts in comparison to allopatric parasites, and that growth and survival of the host do not decrease even with extremely high doses of parasite, implying that the cost of defense is small. In Chapter 3, I show that defense cell (hemocyte) concentration of the host increases with parasite exposure, but this induction of defense cells does not depend on parasite source, host source, or the host-parasite combination. In Chapter 4, I use natural observations to show that host hemocyte concentration is highest in habitats having higher rates of parasite exposure and higher frequencies of diploid hosts. I then use a laboratory exposure experiment to show that triploid hosts maintain lower concentrations of defense cells than diploids in both parasite-free and parasite-rich environments. Because triploids have been shown to be more resistant to non-coevolving parasites and are more heterozygous than diploids, these data show that triploids are innately more resistant to parasites for reasons other than defense allocation. One potential reason for the increased resistance of triploids is increased heterozygosity at host-parasite interaction loci. In the last chapter, I report on experiments designed to detect a fitness cost of an induced immune defense. I find no evidence for any cost of defense in this system. Taken together, these experiments are consistent with a model of host self/non-self recognition as the basis for the interaction.