a. Heterozygote advantage
We found no evidence for an association between stickleback allelic diversity and parasite diversity, at either the scale of individual fish or among populations. At the individual level (Fig. 2A), the best mixed effect generalized linear regression model was the base model (AIC: 4226.02) that only included the fixed effect of log fish body length (coefficient = 1.24, z = 8.78, p < 2e-16) and the random intercept of sampling site (standard deviation: 0.47) conveying that populations differ in parasite diversity (as previously shown, (Bolnick et al., 2020)). Adding the fixed effect of MHC alleles (coefficient = 0.01, z = 1.22, p = 0.22) into the base model slightly decreased model fit (AIC: 4226.53). Adding the quadratic MHC fixed effect further decreased model fit (AIC: 4239.6). No additional explanatory power was given by site by MHC diversity interactions (linear or quadratic), so we have no support for the possibility that MHC diversity is under selection in some populations but not others (a summary of all the models described in this section is available in Table S3). This result suggested that MHC diversity did not have a significant impact on parasite richness.
In the model just described, the random effect of sampling site explained a large portion of the total variance of parasite richness, as noted previously (Bolnick et al., 2020). Populations ranged from as few as an average of 0.5 macroparasite taxa per individual, to nearly 5 taxa per individual (Fig. 2B). We also observed significant variation in MHC allelic diversity between populations, ranging from as low as an average of 5.31 alleles per fish, to as high as 9.71 alleles per fish (Fig. 2B). Among populations, average MHC diversity was unrelated to average parasite richness (coefficient = 0.03, F = 0.02, df = 24, p = 0.89, Fig. 2B).