High polymorphism of Mhc-E locus in non-human primates: Alleles with identical exon 2 and 3 are found in two different species

Thirteen Mhc-E new sequences were found in eight individuals belonging to the Cercopithecinae family, i.e.: Macaca mulatta, Macaca fascicularis and Cercopithecus aethiops when studying E locus polymorphism. No changes were found in the invariant residues which are required for the correct conformation of the peptide presenting region which are conserved in classical Mhc class I molecules from fish and reptiles to humans; however, polymorphism of Mhc-E alleles is not limited to the three typical hypervariable regions per domain as it is in classical class I alleles. The rate of synonymous and non-synonymous substitutions in the DNA sequence corresponding to the antigen binding site, compared to the remainder of exons 2 and 3 shows that the peptide-binding site is under high evolutionary pressure for stability since only synonymous substitutions have been found to be accepted in apes. Also, a clear example of trans-species evolution of allelism is found: two identical exon 2 and exon 3 sequences there exist belonging to individuals from different species (Mamu-Mhc-E*0101 and Mafa-Mhc-E*04). In addition, two Macaca mulatta individuals show an Mhc-E locus duplication. Finally, phylogenetic tree analysis shows that Mhc class I molecules found in Saguinus oedipus (described as Mhc-G homologues) are closer to Mhc-E sequences.