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Front Biosci. 1998 May 26;3:d509-16.

Natural selection and the evolutionary history of major histocompatibility complex loci.

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Department of Biology and Institute of Molecular Evolutionary Genetics, The Pennsylvania State University, University Park, PA 16802, USA. austin@


The major histocompatibility complex (MHC) is a multi-gene family unique to the vertebrates, whose products function to present peptides to T cells. Certain MHC loci are highly polymorphic, and this polymorphism is maintained by a form of balancing selection, probably overdominant selection. This selection has several consequences for MHC biology that make these genes different from neutrally evolving genes: an enhanced rate of nonsynonymous nucleotide substitution in codons encoding the peptide-binding region; long-lasting ("trans-species") polymorphism; and homogenization of introns relative to exons as a result of recombination and subsequent genetic drift. The MHC also reveals evidence of processes shared with other multi-gene families, including gene duplication and deletion and a low level of inter-locus recombination.

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