The stickleback-Schistocephalus host-parasite system as a model for understanding the effect of a macroparasite on host reproduction

Reproductive life history traits are closely linked to fitness. Thus, changes in host reproductive traits associated with parasitic infection are of keen interest to the study of the evolutionary ecology of host-parasite systems. Alterations of host reproductive function may reflect defensive host adaptations to mitigate the effect of parasitism, adaptive parasitic manipulations to promote pathogen reproduction and transmission, or non-adaptive side effects of infection. Parasitic infection often results in reduced host fitness through decreased reproductive performance representing parasite virulence. The pathology of Schistocephalus on reproduction in threespine stickleback (Gasterosteus aculeatus) classically has been viewed as largely resulting in the inability of infected host fish within a population to produce gametes and spawn. We show, however, that this model does not apply to stickleback populations world-wide. Several Alaskan populations of stickleback do not fit the classical model because a large fraction of individuals in each population may produce gametes, engage in reproductive behaviour, and spawn in the face of substantial infection. The significance of inter-population variation in reproductive performance of host stickleback is considered. Using the trophically transmitted Schistocephalus as a model, the suite of traits expressed in hosts of trophically transmitted parasites should be systematically investigated. Beyond the genetic constitutions of host and parasite, environmental conditions appear to play a major role in the manifestations of infection and must be considered more broadly in developing a complete understanding of the system.