b2. Frequency-dependent selection: are allele effects inconsistent
across lakes?
We found some cases where the allele effects are inconsistent across
lakes (significant MHC allele × site interaction effects), suggesting
there were potentially Red Queen style co-evolutionary arms race between
stickleback population and parasites. We obtained 788 generalized linear
models for each moderately prevalent MHC-parasite combinations which
were present in more than one site. 53 models had a significant
interaction term but an insignificant MHC term (11 of them involved
parasites with birds as final hosts), suggesting inconsistent allelic
effects across sites. For example, the allele prot_110 confers
susceptibility (higher infection) in McCreight Stream, but has no effect
in McCreight Lake (Fig. 4C). Although none of the models were
significant after correction for multiple-comparison with BH method, the
number of significant models was greater than null expectation with a
significance level of 0.05 (792 * 0.05 = 39.6), so some of the model
results were truly significant despite the issue of multiple comparison.
66 models had a significant MHC term but an insignificant interaction
term (13 of them involved parasites with birds as final hosts) (Fig.
4B), suggesting that those alleles had consistent effects on the focal
parasite across sites, and thus did not fit localized Red Queen race
dynamics. Parasites with birds as final hosts were not more likely to be
found in the models with significant MHC term than in the models with
significant interaction term (Chi-squared test, X2=0.004, p=0.95).