REDUCTION OF 2 FUNCTIONAL GAMMA-GLOBIN GENES TO ONE - AN EVOLUTIONARYTREND IN NEW-WORLD MONKEYS (INFRAORDER PLATYRRHINI)

Nucleotide sequences were determined for the gamma(1)- and gamma(2)-gl obin 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 gamm a 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 til e basal ancestor of this clade; (ii-iv) in pitheciid titi, saki, and c ebid capuchin monkeys by potentially debilitating nucleotide substitut ions in the proximal CCAAT box of the gamma(1) promoters; and (v and v i) in cebid owl and squirrel monkeys by crossovers that fused 5' seque nce from gamma(1) with 3' sequence from gamma(2). In the five clades w ith gamma(1) and gamma(2) loci separated by intel genic sequences (the fifth clade being the cebid marmosets), the gamma(2) genes retained a n unaltered proximal CCAAT motif and their gamma(2) promoters accumula ted fewer nucleotide substitutions than did the gamma(1) promoters, Th us, phylogenetic considerations indicate that the stem platyrrhines, a ncestral to all New World monkeys, had gamma(2) as the primary fetally expressed gamma gene. A further inference is that when the earlier st em anthropoid gamma gene duplicated, gamma(2) (at its greater downstre am distance from epsilon) could evade embryonic activation by the locu s control region but could he fetally activated once released by regul atory mutations from fetal repressors.