Figure 3. Testing predictions of frequency-dependent selection: rare allele advantage. A. The Z values of all MHC-parasite regressions in one sample site (Lawson Lake), as an example. Each model used data from one population and regressed the infection intensity of a focal parasite against the presence/absence of a focal MHC allele. Each row in the grid represents a MHC allele, and each column represents a parasite taxa. The color filling represents the effect size. The darker the blue color indicates a stronger positive association (having the allele confers higher infection load). The darker the orange color indicates a stronger negative association (e.g., resistance). The MHC-parasite regressions heavily biased by extreme values are filled as grey and excluded from further analysis. Two examples (highlighted in black boxes in A) are plotted in B. B. The infection intensity of parasite Unionidae is negatively and positively influenced by the presence/absence of MHC allele prot_577 and prot_673, respectively. Note the y axis of B is on log scale. C. The relationship between the allele prevalence of an MHC allele and its effect size (Z value) on the infection intensity of certain parasite. Each dot represents a MHC-parasite combination in a particular population. The solid line depicts the result of linear regression using all the data. The grey area surrounding the regression line indicates 95% confidence interval.