A MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-I ALLELE SHARED BY 2 SPECIESOF CHIMPANZEE

Little is known regarding the rates at which natural selection can mod ify or retain antigen presenting alleles at the major histocompatibili ty complex (MHC). Discovery of identical [1101 base pairs (bp)] coding regions at the MWC class I C locus in Pan troglodytes and Pan paniscu s, chimpanzee species that diverged similar to 2.3 million years ago, now indicates that a class I allotype can survive for at least this pe riod. Remarkable conservation was also reflected in the (1799 bp) intr ons where a maximum of only six substitutions distinguished live allel es (three from P. troglodytes and two from P., paniscus) that encoded the identical heavy chain allotype. Analysis of a more distantly relat ed human allele, HLA-Cw0702, corroborated that intron variation was n on-uniform along the gene, Thus we provide a clear reference frame for the lifetime of an MHC class I allotype, a direct estimate of allelic substitution rates, and evidence for an unusual evolution of MH(I cla ss I introns.