Researchers focus on origin of full-color vision

Researchers at the University have found evidence that trichromatic vision, or full-color vision, originated in prosimians, a primate suborder, about 55 million years ago rather than in higher primates 35 to 40 million years ago.

Wen-Hsiung Li, Professor in Ecology & Evolution at the University, and his postdoctoral fellow Ying Tan published their findings in the Nov. 4 issue of Nature.

Three protein pigments in the retina, called opsins, produce full-color vision in primates. These pigments absorb light of various wavelengths, and the brain processes their combined input to produce full-color images.

An autosomal (nonsex-determining) gene encodes one of the pigments (the short-wavelength opsin), and similar opsin genes encode the other two pigments (the middle- and long-wavelength opsins) on the X chromosome.

Primates with dichromatic vision (the ability to see shades of only two colors) have two opsin genes, one on an autosome (nonsex-determining chromosome) and one on the X chromosome.

Scientists have long believed that the prosimians, such as lemurs and galagos, have dichromatic vision at best. In fact, some nocturnal prosimians have been shown to have a single class of color photopigment in their retinas and thus lack color vision entirely, seeing only in black and white.

Diurnal (active during the day) prosimians that previously have been investigated produce two classes of opsins and so have dichromatic color vision (similar to classical red/green human color blindness).

In several prosimians, Li and Tan discovered a polymorphism, a gene variation that codes for either a middle- or long-wavelength opsin.

This polymorphism, found on the X chromosome, together with the autosomal short-wavelength opsin gene should enable females with a middle-wavelength opsin on one X chromosome and a long-wavelength opsin on the other to produce three classes of opsin. These females should possess the ability to see in full color.

Since the variation is X-linked, trichromatic vision is possible only in females who carry the middle-wavelength variant on one X chromosome and the long-wavelength variant on the other X. Females with a middle- or long-wavelength variant on both X chromosomes and males, who have one X and one Y chromosome, possess dichromatic vision.

“Nobody had discovered this polymorphism before,” noted Li, “although a similar, more recent polymorphism allows the New World monkeys to see in full-color vision.”

Li and Tan analyzed genes from tissue samples taken from 20 different kinds of prosimians, mostly lemurs. They found the polymorphism in two diurnal species and in one nocturnal species.

Because color vision is useful only in daylight, Li was not surprised to find the polymorphism in the diurnal prosimians. The polymorphism in the one nocturnal species adds weight to the theory that nocturnal species originally evolved from prosimians that had been diurnal.

“The nocturnal condition, at least in the species in which we discovered the polymorphism, is recent. In nocturnal prosimians that lack the polymorphism, we can assume that they have been nocturnal for far longer, and the polymorphism conferring trichromatic vision has been lost,” said Li.