THE EFFECTS OF HOST GENOTYPE AND SPATIAL-DISTRIBUTION ON TREMATODE PARASITISM IN A BIVALVE POPULATION

A basic assumption underlying models of host-parasite coevolution is t he existence of additive genetic variation among hosts for resistance to parasites. However, estimates of additive genetic variation are lac king for natural populations of invertebrates. Testing this assumption is especially important in view of current models that suggest parasi tes may be responsible for the evolution of sex, such as the Red Queen hypothesis. This hypothesis suggests that the twofold reproductive di sadvantage of sex relative to parthenogenesis can be overcome by the m ore rapid production of rare genotypes resistant to parasites. Here I present evidence of significant levels of additive genetic variance in parasite resistance for an invertebrate host-parasite system in natur e. Using families of the bivalve mollusc, Transennella tantilla, cultu red in the laboratory, then exposed to parasites in the field, I quant ified heritable variation in parasite resistance under natural conditi ons. The spatial distribution of outplanted hosts was also varied to d etermine environmental contributions to levels of parasite infection a nd to estimate potential interactions of host genotype with environmen t. The results show moderate but significant levels of heritability fo r resistance to parasites (h(2) = 0.36). The spatial distribution of h osts also significantly influenced parasite prevalence such that incre ased host aggregation resulted in decreased levels of parasite infecti on. Family mean correlations across environments were positive, indica ting no genotype-environment interaction. Therefore, these results pro vide support for important assumptions underlying coevolutionary model s of host-parasite systems.