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).