Parasitism of asymmetrical pelvic phenotypes in stickleback

Subtle departures from bilateral symmetry in morphological traits result from environmental and genetic stresses and may signal an inferior genetic background, Because one correlate of an inferior genome is reduced resistance to infection. such asymmetry may provide a phenotypic signal of susceptibility to parasitism. I tested this hypothesis in a population of threespine stickleback (Gasterosteus aculeatus) with cestode and nematode infections and bilateral asymmetry of the pelvis. Seventeen percent of the fish had an asymmetrical pelvis and, of these, 78% had greater expression on the left side; this directionality suggests a genetic influence. Females had consistently greater left-side asymmetry than did males, The incidence of total infection (all parasite species) in the largest adult fish (>60 mm body length) was greater in asymmetrical phenotypes, and this occurred in both sexes and for each parasite species (Schistocephalus solidus, Cyathocephalus truncatus, Eustrongylides spp.), even when multiple-species infections were excluded. Contrary to prediction, however, in juvenile fish (<20 mm) and yearlings (20-40 mm) but nor subadults and adults (40-60 mm), asymmetrical phenotypes had significantly lower infection rates than symmetrical fish. This pattern occurred in both sexes. but the extent of the association varied over the 14 years of sampling. Consequently. if the directional asymmetry of the pelvis is under genetic control, asymmetry would be favoured during early ontogeny but selected against during the adult stages. The data support the hypothesis that asymmetry is a phenotypic signal of parasitism, but the unexpected bidirectionality of the association within a single population suggests increased complexity of the processes coupling asymmetry and genetic background.