Date of Award


Document Type

Open Access Thesis

Degree Name

Master of Science (MS)



First Advisor

Chris Maher

Second Advisor

Ken Weber

Third Advisor

Cate Miller


The major histocompatibility complex (MHC) is the most polymorphic gene

region in jawed vertebrates. Its gene products play a critical role in determining

individual and population fitness by presenting foreign antigens to immune system

cells, thereby initiating immune response to pathogen and parasite infections and

cancer. I surveyed the literature to investigate how selection maintains such

important adaptive diversity in mammalian populations and how MHC diversity

affects individual and population fitness. Balancing selection in the form of either

heterozygote advantage or rare allele advantage has long been proposed as the key

mechanism for maintaining the extreme diversity in MHC gene alleles. Deviations f

rom Hardy-Weinberg frequencies, excess heterozygosity and high rates of

nonsynonymous base substitutions, as well as

trans-species polymorphisms, point toward past and ongoing balancing selection on

MHC genes. Additionally, mate choice based on MHC genotype is proposed as a

mechanism that contributes to maintenance of the polymorphism. Although most

studies provided some degree of evidence in support of heterozygote advantage,

rare allele advantage, or mate choice in the establishment or maintenance of MHC

polymorphism, the relative importance of the three selective forces reviewed in this

paper remains controversial. My survey suggests that, based on different

demographic and ecological factors, these three nonexclusive mechanisms affect

MHC diversity to varying degrees in different mammalian species.


Literature review thesis

Included in

Biology Commons



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