b1. Frequency-dependent selection: is there a rare allele
advantage
Although we found many MHC-parasite associations, we found no tendency
for alleles to be protective when rare, or vulnerable to infection when
common. That is, the Z value of a given allele’s effect on a given
parasite in a given site was independent of that alleles’ prevalence in
that focal site. Across all sites, there were a total of 5623
MHC-parasite combinations that were moderately frequent and so included
in the analysis. After excluding the models heavily influenced by
outlier values with high leverage, we obtained 4130 linear regression
models (see Fig. 3A for an example of this result from one population).
45 models had significant negative MHC-parasite associations and 45
models had significant positive MHC-parasite associations (see Fig. 3B
for an example). Some MHC alleles, and parasites, are repeated across
multiple regression models from different populations. None of the
models were significant after we corrected for multiple-comparison with
BH method. A mixed effect linear model showed that the impact of allele
prevalence on Z values was not significant (coefficient = 0.09, t =
1.14, p = 0.27) (Fig. 3C). Therefore, this result did not provide
evidence for rare allele advantage of MHC alleles on parasite infection
intensities.