The persistence and widespread occurrence of cross-fertilization is widely recognized as one of the most interesting challenges for the theory of evolution by natural selection. One promising theory is that, by creating genetically variable progeny, cross-fertilization is favored as a way to reduce the risk of infection by parasites that evolve to attack common host genotypes. Here we test some important predictions and assumptions of this theory. The specific objectives are five fold: (1) to determine whether parasites can evolve under laboratory conditions to infect individual host genotypes, and whether such a response entails a trade-off in ability to infect other host genotypes; (2) to create and experimentally infect hybrid snails to determine the genetic basis of infection; (3) to extend a seven-year study of the evolutionary dynamics of host genotypes and their infection by parasites in a natural population; (4) to measure the rate of mutation accumulation as a test of an alternative theory that cross fertilization helps eliminate harmful mutations; and (5) to provide a direct experimental test of the parasite hypothesis. The results will be of interest to epidemiologists, crop plant pathologists, and conservation biologists interested in the interplay between host genetic diversity and the effects of coevolving parasites and pathogens. The results will also be of wide interest to evolutionary biologists interested in the effects of natural selection on the reproductive strategies of plants and animals.
