Polymorphism of photopigments in the squirrel monkey: a sixth phenotype

We describe here a trichromatic type of squirrel monkey that resembles Old World monkeys in having two well-separated photopigments in the red-green part of the spectrum; the cones of this phenotype have peak sensitivities close to 430, 536 and 564 nm. The existence of such animals is predicted by a genetic model that postulates three alleles for a single locus on the X-chromosome of the squirrel monkey. The three alleles correspond to three different photopigments in the red-green spectral range. A male monkey, or a homozygous female, will be dichromatic, combining short-wave cones with just one of the cone types in the red-green range. But a female monkey, if heterozygous at the locus, draws any two of the three alleles from the set. X-chromosome inactivation ensures that the two alleles are expressed in different subpopulations of retinal cone, giving the monkey the basis for trichromatic colour vision. This model requires three trichromatic types of female squirrel monkey. The photopigment complements of two types have previously been reported and microspectrophotometric data are now given for the third type. Behaviourally, this third type of trichromat gives precise Rayleigh matches that are intermediate between those of the other two types of trichromat. The polymorphism of photopigments in the squirrel monkey may be maintained by the heterozygous advantage enjoyed by the trichromatic females. This would be an instructive instance of heterozygous advantage because it is a case where X-chromosome inactivation plays a crucial role in segregating the two different gene-products into different cells.