Figure 2. Testing predictions of heterozygote advantage. A. The relationship between the number of MHC alleles per individual and its residual parasite richness. For purposes of plotting, the residual parasite richness for each fish was calculated by using a mixed-effect generalized linear model to control for the random effect of sampling site and the fixed effect of the log body length (covariates in the statistical analysis we report in the text). Each dot represents a fish. The quadratic fit using either the full dataset or each population was represented by thick black line or thin grey lines, respectively. None of the quadratic fit using data from individual site was significant after correcting for multiple comparisons. B. The relationship between the average number of MHC alleles per-fish in a given population and the average per-fish parasite richness in that population. Each dot represents a population. The grey area represents 95% confidence interval. Note that x and y axes are reversed from panel A to B. In A we are specifically testing whether individual genotype affects their parasite burden, as the reverse direction of causation is not plausible. In B we are considering the hypothesis that existing parasite diversity is the driver of evolution of MHC diversity.