RESEARCH ARTICLE Patterns ofMHC-G-Like andMHC-B Diversification in NewWorld Monkeys

The MHC class I (MHC-I) region in NewWorld monkeys (Platyrrhini) has remained relatively understudied. To evaluate the diversification patterns and transcription behavior of MHC-I in Platyrrhini, we first analyzed public genomic sequences from theMHC-G-like subregion in Saimiri boliviensis, Ateles geoffroyi and Callicebus moloch, and from theMHC-B subre-gion in Saimiri boliviensis. While S. boliviensis showed multiple copies of bothMHC-G-like (10) and –B (15) loci, A. geoffroyi and C.moloch had only three and fourMHC-G-like genes, respectively, indicating that not all Platyrrhini species have expanded their MHC-I loci. We then sequencedMHC-G-like and-B cDNAs from nine Platyrrhini species, recover-ing two to five unique cDNAs per individual for both loci classes. In two Saguinus species, however, noMHC-B cDNAs were found. In phylogenetic trees,MHC-G-like cDNAs formed genus-specific clusters whereas theMHC-B cDNAs grouped by Platyrrhini families, sug-gesting a more rapid diversification of the former. Furthermore, cDNA sequencing in 12 capuchin monkeys showed that they transcribe at least fourMHC-G-like and fiveMHC-B polymorphic genes, showing haplotypic diversity for gene copy number and signatures of positive natural selection at the peptide binding region. Finally, a quantitative index for MHC:KIR affinity was proposed and tested to predict putative interacting pairs. Altogether, our data indicate that i)MHC-I genes has expanded differentially among Platyrrhini species, ii) Callitrichinae (tamarins and marmosets)MHC-B loci have limited or tissue-specific expression, iii) MHC-G-like genes have diversified more rapidly thanMHC-B genes, and iv) the MHC-I diversity is generated mainly by genetic polymorphism and gene copy number variation, likely promoted by natural selection for ligand binding