EVOLUTION OF PRIMATE ABO BLOOD-GROUP GENES AND THEIR HOMOLOGOUS GENES

There are three common alleles (A, B, and O) at the human ABO blood gr oup locus. We compared nucleotide sequences of these alleles, and rela tively large numbers of nucleotide differences were found among them. These differences correspond to the divergence time of at least a few million years, which is unusually large for a human allelic divergence under neutral evolution. We constructed phylogenetic networks of huma n and nonhuman primate ABO alleles, and at least three independent app earances of B alleles from the ancestral A form were observed. These r esults suggest that some kind of balancing selection may have been ope rating at the ABO locus. We also constructed phylogenetic trees of ABO and their evolutionarily related alpha-1,3-galactosyltransferase gene s, and the divergence time between these two gene families was estimat ed to be roughly 400 MYA.