Genetic variability is often predicted to enhance host fitness in the face of parasitism, yet this idea is rarely tested in an experimental setting, particularly with animal hosts. To assess this question, we used a relatively resistant line of snail hosts (Biomphalaria glabrata) to generate inbred and outcrossed progeny that were then either exposed or sham-exposed to the trematode parasite, Schistosoma mansoni. Results showed no difference in prevalence between the groups; however, large differences appeared in other host life history traits, particularly reproduction. Outcrossed progeny produced large numbers of eggs relative to inbred progeny especially in the face of infection. Furthermore, eggs produced by outcrossed snails took less time to hatch and exhibited greater hatching success compared to their inbred counterparts. Parasite reproduction demonstrated the opposite trend, with fewer parasites emerging from outcrossed snails compared to inbred individuals. This work shows that the introduction of genetic variation into inbred snail populations can have important implications for the viability of host populations and disease transmission.

In this study, we investigated the interaction between host outcrossing and infection in the Biomphalaria glabrata-Schistosoma mansoni system. Snails collected from three susceptible isofemale lines were mated with either siblings or snails recently derived from a field site in Brazil. Resulting inbred and outcrossed progeny were then exposed to S. mansoni larvae and monitored for a 10-week period. Interestingly, all snails exhibited equal susceptibility whether they were the result of inbreeding or outcrossing. However, further examination of both host and parasite life-history traits uncovered significant differences between the groups. In uninfected snails, outcrossed progeny tended to exhibit greater fitness relative to inbred progeny. When snails were parasitized, these differences were magnified in certain life-history traits, particularly host reproduction and survival. As an extension of the work, we also investigated virulence within this host-parasite system. Estimates of parasite reproduction and host size were combined to generate a novel "exploitation index," and these indices were regressed with host survivorship. As predicted, there was a significant and negative correlation between the variables, but this was restricted to a single snail line. Results from this study demonstrate that infection outcomes (as measured by prevalence) may not differ between inbred and outcrossed hosts. However, outcrossing may enhance snail fitness through life-history trait expression.