The evolutionary ecology of host-specificity: experimental studies with Strongyloides ratti

Factors constraining the evolution of host-specificity were investigated us ing a gastrointestinal parasitic nematode, Strongyloides ratti. S. ratti is a natural parasite of rats which can also reproduce, with decreased succes s, in laboratory mice. Observed host-specificity arose from lower establish ment, reduced per capita fecundity and more rapid expulsion of parasites fr om mice relative to rats. Variation in the efficacy of thymus-dependent imm unity between host species (rats and mice) was insufficient to explain the majority of the observed differences in parasite establishment and reproduc tive success. The role of natural selection in determining host-specificity was addressed using experimental selection followed by reciprocal fitness assays in both host species. Experimental selection failed to modify the ho st-specificity of S. ratti to any measurable degree, suggesting either a la ck of genetic variation for this trait or the involvement of as yet unident ified factors underlying the differences in S. ratti fitness in rats and mi ce respectively. These results are discussed in relation to competing theor etical models of ecological specialization, host immunology and previous at tempts to experimentally alter the host-specificity of parasitic nematodes.