Experimental evidence for major histocompatibility complex-allele-specific resistance to a bacterial infection

Area of Science:

• Immunogenetics
• Evolutionary biology
• Aquaculture

Background:

• The extreme polymorphism in major histocompatibility complex (MHC) genes is hypothesized to be maintained by balancing selection due to infectious diseases.
• Experimental evidence linking specific MHC alleles to pathogen resistance and balancing selection remains inconclusive.

Purpose of the Study:

• To investigate the relationship between specific MHC genotypes and survival in Atlantic salmon experimentally infected with Aeromonas salmonicida.
• To assess the selection pressure exerted by a pathogen on MHC alleles and evaluate the disease resistance patterns.

Main Methods:

• Utilized full-sibling broods of Atlantic salmon with controlled homozygote and heterozygote MHC genotypes.
• Experimentally infected juvenile salmon with Aeromonas salmonicida, monitoring individual survival rates.
• Minimized the influence of other genes by comparing survival within full-sibling groups with differing MHC genotypes.

Main Results:

• Demonstrated that Aeromonas salmonicida can impose intense selection pressure on specific MHC alleles, with fitness differences up to 0.5.
• Observed a co-dominant pattern of disease resistance and susceptibility, suggesting distinct immune responses based on MHC alleles.
• Found no survival advantage for heterozygous individuals compared to homozygous ones, which was an unexpected outcome.

Conclusions:

• Specific MHC alleles are under strong selection pressure from pathogens, supporting the balancing selection hypothesis.
• The interaction between MHC genotype and pathogen resistance is complex, with co-dominance observed in disease response.
• Heterozygote advantage was not detected in this study, challenging a common assumption in MHC-pathogen interaction research.