Immunogenetics 33: 79-89, 1991 Original articles ll]llllllllO- genetics © Springer-Verlag 1991 Unusually limited nucleotide sequence variation of the expre major histocompatibility complex class I genes of a New Wo primate species (Saguinus oedipus) David I. Watkins 1, Theodore L. Garber 2, Zheng W. Chen 1, Gary Toukatly t, Austin L. Hughes 3, and N L. Letvin x 1 Harvard Medical School, New England Regional Primate Research Center, Southborough, MA 01772, USA 2 Texas A&M University, College of Veterinary Medicine, College Station, TX 77843, USA 3 Center for Demographic and Population Genetics, The University of Texas Health Science Center at Houston, Houston, TX 77225, USA Received September 17, 1990 Abstract. Although major histocompatibility complex (MHC) class I molecules are, as a rule, highly polymor- phic in mammalian species, those of the New World primate Saguinus oedipus (cotton-top tamarin) exhibit limited polymorphism. We have cloned and sequenced twelve MHCclass I cDNAs from thisspecies. 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 displayedlimited 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. * The nucleotide sequence data reported in this paper have been submit- ted to the GenBank nucleotide sequence database and have been assign- ed the accession numbers M38403-15. Address correspondence and offprint requests to: D.I. Watkins. Introduction The products of the major histocompatibility complex (MHC) class I loci are among the most polymorphic molecules in the animal kingdom (Klein 1986). The crystal structure of a human MHC class I molecule suggests that pepfides bind to a groove in the top of the molecule (Bjorkman et al. 1987). Bound peptide frag- ments are then presented to cytotoxic T lymphocytes (Townsend et al. 1985). Overdominant selective pres- sures at the peptide-binding groove are largely responsible for the extreme diversity of these molecules (Hughes and Nei 1988). Only 11 different MHC class I molecules have been defined in the cotton-top tamarin (Watkins et al. 1988a and data not shown). One of these MHC class I molecules is expressed by the lymphocytes of every tamarin ex- amined to date. To understand the molecular basis of this limited MHC class I polymorphism, twelve cDNAs en- coding tamarin MHC class I molecules and two pseudogenes have been sequenced. We have previously reported that these cDNAs are related to the human nonclassical genes HLA-G and HLA-F (Watkinset al. 1990b). In the present studies we have analyzed the extent of nucleotide sequence variation of the tamarins' MHC class I genes. Materials and methods Animals. Heparinized blood was obtained by venipuncture from healthy cotton-top tamarins originally housed at the New England Regional Primate Research Center, Southborough, Massachusetts (#779-76), the University of Bristol, Bristol, England (#46-85), and at a colony in