Genotype frequency differences in Halipegus occidualis-infected and uninfected Helisoma anceps

Allozyme frequencies in Helisoma anceps infected with the hemiurid trematode, Halipegus occidualis, were compared with those of uninfected H. anceps from a small, North Carolina farm pond. Of 6 loci found to be polymorphic, the frequencies of esterase-1 and leucine aminopeptidase were different in infected and uninfected snails. Genetic heterozygosity, as determined by starch gel electrophoresis, was greater in uninfected H. anceps relative to infected individuals. These observations combined with the high prevalence (up to 60%), complete castration in patent infections, and the absence of an encapsulation response in infected snails, suggest that factors conferring incompatibility may have been selected for in the H. anceps population within the pond.