Reduction of two functional gamma-globin genes to one: An evolutionary trend in New World monkeys (infraorder Platyrrhini)

Nucleotide sequences were determined for the gamma(1)- and gamma(2)-globin loci from representatives of the seven anciently separated clades in the three extant platyrrhine families (Atelidae, Pitheciidae, and Cebidae). These sequences revealed an evolutionary trend in New World monkeys either to inactivate the gamma(1) gene or to fuse it with the gamma(2) gene, i.e. to have only one functional fetally expressed gamma gene. This trend is clearly Evident in six of the seven clades: (i) it occurred in atelids by deletion of most of the gamma(1) gene in the basal ancestor of this clade; (ii-iv) in pitheciid titi, saki, and cebid capuchin monkeys by potentially debilitating nucleotide substitutions in the proximal CCAAT box of the gamma(1) promoters; and (v and vi) in cebid owl and squirrel monkeys by crossovers that fused 5' sequence from gamma(1) with 3' sequence from gamma(2). In the five clades with gamma(1) and gamma(2) loci separated by intel genic sequences (the fifth clade being the cebid marmosets), the gamma(2) genes retained an unaltered proximal CCAAT motif and their gamma(2) promoters accumulated fewer nucleotide substitutions than did the gamma(1) promoters, Thus, phylogenetic considerations indicate that the stem platyrrhines, ancestral to all New World monkeys, had gamma(2) as the primary fetally expressed gamma gene. A further inference is that when the earlier stem anthropoid gamma gene duplicated, gamma(2) (at its greater downstream distance from epsilon) could evade embryonic activation by the locus control region but could he fetally activated once released by regulatory mutations from fetal repressors.