Pathogen resistance and genetic variation at MHC loci

Area of Science:

• Evolutionary biology
• Population genetics
• Immunogenetics

Background:

• High genetic variation at major histocompatibility complex (MHC) genes is well-documented.
• Balancing selection, including heterozygote advantage and frequency-dependent selection, is a proposed explanation for this diversity.
• The role of temporally varying environmental pressures, such as pathogen presence, remains an area of investigation.

Purpose of the Study:

• To investigate how temporal variation in pathogen presence influences genetic variation at multiallelic loci.
• To determine if pathogen-driven selection can explain the high polymorphism observed at MHC genes.
• To examine the impact of varying selection levels, dominance, and genetic drift on these genetic patterns.

Main Methods:

• Development of a basic population genetics model examining the effect of pathogen presence over generations.
• Mathematical formulation of fitnesses for homozygotes and heterozygotes under dominant resistance alleles.
• Analysis of allele frequency distributions and heterozygosity as a function of pathogen autocorrelation.
• Comparison of model predictions with empirical data from MHC genes in South American Indian populations.

Main Results:

• A stable, multiallelic polymorphism can be maintained by temporally variable pathogens, even without intrinsic heterozygote advantage.
• Positive autocorrelation in pathogen presence across generations leads to reduced observed heterozygosity.
• Lower selection, dominance, and genetic drift were found to influence allele frequencies and heterozygosity patterns.

Conclusions:

• Temporally fluctuating pathogen resistance can be a significant factor contributing to the high polymorphism at MHC genes.
• This mechanism provides a potential explanation for observed genetic diversity in host defense loci.
• The findings highlight the importance of considering environmental dynamics in understanding immune gene evolution.