MOLECULAR-GENETICS OF SPECTRAL TUNING IN NEW-WORLD MONKEY COLOR-VISION

Although most New World monkeys have only one X-linked photopigment lo cus, many species have three polymorphic alleles at the locus. The thr ee alleles in the squirrel monkey and capuchin have spectral peaks nea r 562, 550, and 535 nm, respectively, and the three alleles in the mar moset and tamarin have spectral peaks near 562, 556, and 543 nm, respe ctively. To determine the amino acids responsible for the spectral sen sitivity differences among these pigment variants, we sequenced all ex ons of the three alleles in each of these four species. From the deduc ed amino acid sequences and the spectral peak information and from pre vious studies of the spectral tuning of X-linked pigments in humans an d New World monkeys, we estimated that the Ala --> Ser, Ile --> Phe, G ly --> Ser, Phe --> Tyr, and Ala --> Tyr substitutions at residue posi tions 180, 229, 233, 277, and 285, respectively, cause spectral shifts of about 5, -2, -1, 8, and 15 nm. On the other hand, the substitution s His --> Tyr, Met --> Val or Leu, and Ala --> Tyr at positions 116, 2 75, and 276, respectively, have no discernible spectral tuning effect, though residues 275 and 276 are inside the transmembrane domains. Man y substitutions between Val and lie or between Val and Ala have occurr ed in the transmembrane domains among the New World monkey pigment var iants but apparently have no effect on spectral tuning. Our study sugg ests that, in addition to amino acid changes involving a hydroxyl grou p, large changes in residue size can also cause a spectral shift in a visual pigment.