Interplay between hosts genetic diversity and disease transmission in a host-parasite association

Enhanced genetic variability through outcrossing is believed to be evolutionary favorable because it potentially provides a greater suite of responses in the face of environmental change and can suppress the negative fitness effects of deleterious recessive allele accumulation.  Parasites can act as a strong selective force in host populations and the magnitude of this force is predicted to increase for this idea is both limiting and conflicting.  In this study, we experimentally investigated the life history responses of inbred and outcrossed hosts (Biomphalaria glabrata) in the presence or absence of the human parasite, Schistosoma mansoni.  Surprisingly, exposure to S. mansoni resulted in high infection levels regardless of host genetic background suggesting no outcrossing advantage.  However, further examination of both host and parasite life-history traits uncovered significant differences based on crossing status.  In general, outcrossed progeny survived longer and exhibited greater reproduction success compared to inbred progeny.  In addition, S. mansoni larvae tended to be released in lower numbers from outcrossed snails relative to their inbred counterparts.  These experiments demonstrate that progeny resulting from parental outcrossing have a fitness advantage in the face of parasitism, which may have consequences for disease transmission dynamics in the field.