Evolutionary biologists have puzzled over the maintenance of genetic diversity despite natural selection eliminating all but the most fit genotypes. One explanation is that coevolution between hosts and their parasites can preserve genetic polymorphism, both within and between populations. Parasites are predicted to keep any particular genotype from dominating a population by targeting common host genotypes. This idea rests on a number of assumptions; primarily that parasites are harmful to their hosts, and thus impose a strong selection pressure, and that there is strong genetic specificity for infection. This latter assumption implies that a given parasite genotype should only be able to infect a subset of the host population, as determined by their genotype at disease resistance loci. If tight specificity for infection exists, and rare hosts have an advantage, parasites are predicted to be adapted to their local host population.
My dissertation research examines the coevolutionary dynamics of the sterilizing trematode, Microphallus, and its intermediate snail host, Potamopyrgus antipodarum. Specifically, I test the major assumptions of parasite-mediated, rare host advantage in this system. The first chapter examines the underlying assumption of genotypic specificity for infection in this system, while chapters two and three examine the process of negative frequency-dependent selection directly. These latter data come from a three-year experimental coevolution study in which parasites were cycled through host populations in the lab for six generations. The results are strongly in support of parasites tracking common host genotypes over time and of the ability of parasites to maintain genotypic diversity over time. This type of host genotype-specific tracking is predicted to lead to parasite local adaptation, whereby parasites are more infective to local hosts than to foreign hosts, since populations are expected to be cycling independently. Therefore, the final chapter examines parasite adaptation to local host populations across the South Island of New Zealand in order to determine whether parasite local adaptation is present at both small and large geographic distances, as would be predicted under frequency-dependent dynamics.
|Advisor:||Lively, Curt M.|
|Commitee:||Bever, James, Hall, Spencer R., Lynch, Mike|
|Department:||Ecology and Evolutionary Biology|
|School Location:||United States -- Indiana|
|Source:||DAI-B 69/11, Dissertation Abstracts International|
|Keywords:||Coevolution, Host-parasite coevolution, Infection genetics, Local adaptation, Parasites, Rare advantage, Red queen, Snail-trematode system|
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