The major histocompatibility complex: a key to a better understanding of evolution

The study of the MHC has provided valuable information with regard to evolution. There are now clear indications that the modern MHC of vertebrates evolved from a common ancestral gene, although the evolution of the 'true' MHC (acute graft rejection) was probably convergent, i.e., it developed only when conferring a selective adavantage. For the species thoroughly investigated, it was also found that MHC genes of classes I, II, and III occur in duplicate. This duplication as well as the high degree of polymorphism of MHC genes must have had a selective advantage. In spite of gene duplication and the high degree of variation (polymorphism), the evolution of the MHC genes has been rather conservative. The fact that the genes of the MHC have remained together during evolution suggests a functional relationship among these genes. Many similarities were shown to exist among the MHC products, even when distantly related species such as the mouse and man were compared. Since the MHC undoubtedly plays a major role in the defence against viral and bacterial invasion, it is to be expected that certain MHC variants will be favored and others eliminated under the selective pressure of infectious diseases. Studies of human populations exposed to major epidemics in the past indicate that this is indeed the case. Finally, the biochemical and serologic comparison of MHC products of different species provided information on evolutionary distances between species: closely related species showed striking similarities, while the similarities were less pronounced in more distantly related species. This knowledge of how the MHC has developed to its present forms in different species has given additional information on the divergence of groups of species and provides a key to a better understanding of evolution.Chemicals/CAS: Complement, 9007-36-7