Mate choice, frequency dependence, and the maintenance of resistance to parasitism in a simultaneous hermaphrodite

Biomphalaria glabrata are simultaneous hermaphroditic freshwater snails that act as intermediate hosts for the macroparasitic trematode Schistosoma mansoni, a causative agent of schistosomiasis. Heritability and strain-specificity of both snail resistance and susceptibility to schistosome infection have been demonstrated, genetic variability for which is maintained, in part, through trade-offs between high fitness costs associated with infection and those associated with resistance. However, despite such a high cost of resistance and a low prevalence of infection in natural snail populations, genes for resistance are maintained within snail populations over successive generations, including in the complete absence of parasite pressure in laboratory populations. This may be indicative of alternative benefits of resistance genes, in addition to parasite defense, such as differential mating success between genotypes. Here we examined the mate and gender choice of snails across a multi-factorial range of potential partner combinations. These included host-resistance or susceptibility genotype, host genotype frequency within the population, current parasite infection status, and parasite genotype. We demonstrate recognition and discrimination by host snails depending on host and/or parasite genotype for each of these factors. In particular, our results suggest that a rare mating advantage to resistant genotypes may be a potential explanation for the maintenance of highly costly resistance genes within intermediate host populations under conditions of low or zero parasite pressure.