WEBSTER, Joanne P.; Imperial College Faculty of Medicine: Snails and Schistosomes – Sex and Success.

Biomphalaria glabrata are simultaneous hermaphroditic freshwater snails that act as intermediate hosts for the macroparasitic trematode Schistosoma mansoni, the causative agent of schistosomiasis. Heritability and strain-specificity of both snail resistance and susceptibility to schistosome infection have been documented in this system. Genetic variability for each trait may be maintained, in part, through trade-offs between high fitness costs associated with infection and those resulting from resistance: susceptible genotypes suffer from increased mortality when infected whereas resistant genotypes suffer reduced egg fertility, regardless of infection status. 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 additional benefits of resistance genes. One such possibility could be differential mating success between genotypes. Here, we focus on a series of recent laboratory experiments examining the differential roles and interplay of host and parasite phenotype and genotype on snail mate choice and reproductive strategy. In particular, the partner and gender choice of snails, together with their propensity to outcross or self-fertilize, is examined across a multi-factorial range of potential partner combinations. These include host geographic strain, resistance genotype, rareness and/or infection status. Moreover, infection status is differentiated in relation to the geographic strain, infection intensity and/or virulence genotype of the parasite. The results are discussed in terms of the potential for frequency-dependent mate choice in this system and their theoretical and applied implications.