Shared polymorphism between gorilla and human major histocompatibility complex DRB loci.

Abstract

A high degree of polymorphism and high nucleotide diversity mark the functional genes of the major histocompatibility complex (Mbc). Alleles at the different Mbc loci can be classified into distinct lineages that are shared between species and, therefore, are presumed to have been founded before speciation. We have sequenced the most polymorphic part of 25 gorilla Mbc-DRB genes from six individuals. (The DRB genes code for the beta-polypeptide chain of the alpha beta heterodimer that constitutes one family of the class II MHC molecules.) Fifteen of the sequences identify new alleles at four DRB loci; each of the six gorillas was heterozygous at one of the loci at least. Thirteen of the alleles could be assigned to lineages identified previously; the remaining two alleles represent new lineages. All the major human DRB allelic lineages are now known to be shared with apes, and all must have originated before the human-gorilla-chimpanzee divergence more than six million years (my) ago. The presence of some of the gorilla and human lineages in Old World monkeys suggests that these lineages emerged before the divergence of apes and cercopithecids. We argue that the major allelic lineages at the DRB1 locus began to diverge shortly after the rounds of duplication that generated the different DRB loci now found in the hominoids and that this event occurred more than 30 my ago. Comparison of closely related gorilla DRB sequences indicates that polymorphism may be generated by several mechanisms: point mutations, slippage during DNA replication, and recombination. Deduced gene linkages provide evidence for transspecies evolution of haplotype polymorphism.