Natural selection at major histocompatibility complex loci of vertebrates

Annu Rev Genet. 1998;32:415-35. doi: 10.1146/annurev.genet.32.1.415.


The loci of the vertebrate major histocompatibility complex encode cell-surface glycoproteins that present peptides to T cells. Certain of these loci are highly polymorphic, and the mechanisms responsible for this polymorphism have been intensely debated. Four independent lines of evidence support the hypothesis that MHC polymorphisms are selectively maintained: (a) The distribution of allelic frequencies does not fit the neutral expectation. (b) The rate of nonsynonymous nucleotide substitution significantly exceeds the rate of synonymous substitution in the codons encoding the peptide-binding region of the molecule. (c) Polymorphisms have been maintained for long periods of time ("trans-species polymorphism"). (d) Introns have been homogenized relative to exons over evolutionary time, suggesting that balancing selection acts to maintain diversity in the latter, in contrast to the former.

Publication types

  • Research Support, U.S. Gov't, P.H.S.
  • Review

MeSH terms

  • Animals
  • Genes, MHC Class I
  • Genetic Variation
  • HLA Antigens / genetics
  • Histocompatibility Antigens Class I
  • Histocompatibility Antigens Class II
  • Humans
  • Major Histocompatibility Complex*
  • Models, Genetic
  • Phylogeny
  • Polymorphism, Genetic
  • Selection, Genetic*
  • Vertebrates


  • HLA Antigens
  • Histocompatibility Antigens Class I
  • Histocompatibility Antigens Class II