MHC class I-processed pseudogenes in New World primates provide evidence for rapid turnover of MHC class I genes

The MHC class I genes of the New World primate, the cotton-top tamarin (Saguinus oedipus), are an exception to the high polymorphism and variability displayed by this multigene family. We report the isolation of the first two processed pseudogenes from the MHC region in primates. These two MHC class I-processed pseudogenes (MHC-PS1 and -PS2) were found in several species of New World primates, suggesting a possible explanation for the cotton-top tamarin's limited MHC class I diversity. The pattern of synonymous and nonsynonymous substitutions in PS1 suggests that the gene that gave rise to this processed pseudogene was once subject to selection for variability in the peptide binding region and might, therefore, have been functional. Additionally, PSI is not closely related to the expressed cotton-top tamarin's MHC class I genes, but does show some similarity to So-N1, a tamarin pseudogene from which no transcript has been found. Thus, PS1 may represent a remnant of a once active MHC class I gene that is no longer functional in the cotton-top tamarin. The MHC class I loci in primates, therefore, appear to be evolving by a continual process of duplication and inactivation. This process seems to be exaggerated in New World primates and may in part be responsible for the cotton-top tamarin's limited MHC class I diversity.