Evolution of MHC-G in primates: a different kind of molecule for each group of species

When MHC-G molecules in primates (New World and Old World monkeys, Anthropoids and humans) were compared phylogenetically, very different evolutionary patterns within each species were found; their molecules did not have a straight forward and linear development throughout the postulated evolutionary pathway of primates. The earlier New World monkeys (South America) had relatively more alleles and the polymorphism was placed in the T-cell receptor (TcR), NK receptors and antigen binding sites; MHC-G probably works as a classical class I presenting molecule in these monkeys. MHC-G intron 2 from New World monkeys does not show the typical 23 bp deletion found in all other more recent primate species. Thus, it is possible that MHC-G molecules in New World monkeys belong to a different lineage than the MHC from higher primates. Another early lineage, Eurasian Old World monkeys, shows stop codons at exon 3: MHC-G proteins lacking the alpha2 domain may functionally suffice or otherwise reading-through stop-codon translational mechanisms may exist, as shown for other genes. Orangutans show lower (but significant) polymorphism than New World monkeys at NK, TcR and antigen binding regions; gorilla and chimpanzee show very low polymorphism. Humans only show three different HLA-G proteins with changes not affecting NK, TcR or antigen binding sites. It is observed that the more exposed the mother to allogeneic fetuses (polygamy), the less polymorphic HLA-G is observed within a given species. The data are concordant with the postulated immune inhibitory function for MHC-G in Old World monkeys, anthropoids and humans both at placental and inflammatory level.