Associations between parasitism and host genotype in natural populations of Daphnia (Crustacea: Cladocera)

1. Numerous models of parasite - host coevolution have shown that parasitism could play a key role in a wide range of biological phenomena. One critical assumption underlying models of parasite-host coevolution is that natural selection acts on genetic variation for host susceptibility, but this assumption has rarely been tested in the wild. 2. Clonal hosts are particularly suitable for detecting associations between genotypes and parasitism, because multi-locus gene complexes are preserved during reproduction, and following clonal selection, hosts which differ at resistance loci may also show differences at neutral marker loci. In this study, allozymes were used to search for associations between multi-locus host genotypes (clones) and parasitism within 25 cyclically parthenogenetic populations of Daphnia. 3. Rates of parasitism were observed ranging from 0% to 48% of adult females. and infection by microparasites was found to significantly reduce host fecundity. 4. Significant differences were detected between the clonal composition of parasitized and healthy Daphnia in 12 populations, showing that the proportion of individuals infected varies among clones. Six populations were sampled repeatedly to determine whether this variation was associated with temporal changes in both host clonal frequencies and population genetic characteristics (Hardy-Weinberg equilibrium. genotypic linkage disequilibrium). Three of the six temporally sampled populations showed changes in genetic structure that were consistent with the expectations of parasite-mediated selection, while the other three did not. These latter three populations all had low parasite prevalence (indicating low selection intensities) and or comprised primarily rare clones, thus limiting the statistical power of the study to detect parasite-mediated effects. 5. The results show the presence of genetic variation for the likelihood of contracting disease within Daphnia populations. These associations between infection and host genotype could result from genetic variation for parasite resistance or within-pond spatial arrangement of both host genotypes and parasite prevalences. Either factor would lead to parasite-mediated selection and the modulation of gene frequecies within populations.