Trade-offs in the evolution of virulence in an indirectly transmitted macroparasite

The adaptive trade-off theory for the evolution and maintenance of parasite virulence requires that virulence be genetically correlated with other fit ness characteristics of the parasite. Many theoretical models rely oil a po sitive correlation between virulence and transmissibility. They assume that high parasite replication rates are associated with a high probability of transmission land, hence, increased parasite fitness), but also with high l evels of damage to the host (high virulence). Schistosomes are macroparasit es with an indirect life cycle involving a mammalian and a molluscan host. Here we demonstrate, through the development of five substrains, a genetic basis for schistosome virulence. We used these substrains further. in order to investigate the presence of parasite fitness traits that were genetical ly correlated with virulence. High Virulence in the (mouse) definitive host was, as predicted, positively correlated with parasite replication. In con trast, in the (snail) intermediate host high virulence was associated with low parasite replication rates. Variation in infectivity to and parasite re plication in the definitive host was suggested as a compensating mechanism for the maintenance of virulence in the snail host. This is the first repor t of a trade-off in parasite reproductive success across hosts in an indire ctly transmitted macroparasite.