Although major histocompatibility complex (MHC) class I molecules are, as a rule, highly polymorphic in mammalian species, those of the New World primate Saguinus oedipus (cotton-top tamarin) exhibit limited polymorphism. We have cloned and sequenced twelve MHC class I cDNAs from this species. Since cloned cotton-top tamarin cell lines express three to six MHC class I molecules, this species must have at least three functional MHC class I loci. There was, however, no evidence of locus-specific substitutions in the tamarin cDNAs. Unlike all other species studied, tamarin MHC class I cDNAs displayed limited nucleotide sequence variation. The sequence similarity between the two most divergent tamarin cDNAs was 95%. To ensure that the polymerase chain reaction (PCR) primers employed in these studies had amplified all of the tamarins' expressed MHC class I genes, we used another set of primers to amplify only exons 2 and 3 from RNA and DNA. PCR of genomic DNA resulted in the amplification of six distinct clones, of which only three were well expressed. Two of these nonexpressed genes were pseudogenes and the other was a nonclassical gene. Southern blot analysis demonstrated that the tamarin has 8-11 MHC class I genes, suggesting we had indeed cloned the majority of these genes. Cotton-top tamarins are, therefore, unique among mammalian species studied to date in that they express MHC class I molecules with limited nucleotide sequence variation.
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