Figure 3: Adaptive significance of MHC frequency shifts.

Mean (+1 s.e.m.) parasite load as a function of the presence ('with') or absence ('without') of the respective resistance haplotypes (a) No13.No18 and (b) No01.No12 in G1 and G2 generations. Haplotype No13.No18 confers resistance against C. lacustris (G1, T=−7.4, d.f.=196, P<0.001; G2, d.f.=385, T=−2.61, P=0.009), whereas the haplotype No01.No12 confers resistance against A. crassus (G1, T=−3.875, d.f.=196, P=0.001, G2 T=−2.353, d.f.=392, P=0.021). Parasite data were log+1 transformed to meet normality assumptions. Parasite load is the difference between the average individual infection with and without the resistance haplotype to the average infection within its family. If values are negative, infection is lower than the average family infection whereas, if values are positive, infection is higher than the average family infection. Blue bars represent 13 (out of 15) segregating fish families exposed to C. lacustris and purple bars represent 12 (out of 15) segregating fish families exposed to A. crassus. Parasites were counted during fish dissection. The other families did not contain individuals both with and without the respective haplotypes and were therefore excluded to guarantee comparability. No13.No18 G1 (N=197) and G2 (N=386). No01.No12 G1 (N=197) and G2 (N=393).