Aside from primates, most mammals are largely colorblind. Now researchers have found that transgenic mice can acquire the ability to detect new color differences if given a gene for making an additional light-sensing eye protein. The findings have implications for understanding how color vision evolved.
Primates can distinguish the colors of the rainbow better than other mammals because their eyes contain three photopigment proteins. Each photopigment is sensitive to light of a particular wavelength, and the primate visual system detects colors by comparing the relative activity of cells in the retina that bear each of the three photopigments. Most other mammals, however, only make two photopigments, limiting their color discrimination. Scientists have suggested that trichromatic color vision arose in primates when one of the two photopigment genes they already had mutated to produce a third photopigment.
A sudden mutation like this could have given primates an instant advantage when it came to finding food--but only if their visual system were able to make sense of the new information. Certain differences in retina anatomy between primates and other mammals led many researchers to suspect that only primates had the right kind of wiring to make use of a sudden addition of a third photopigment.
But perhaps not. In the new experiment, vision scientist Gerald Jacobs at the University of California, Santa Barbara, teamed up with geneticist Jeremy Nathans at Johns Hopkins Medical School in Baltimore, Maryland, and other colleagues to add a human photopigment gene to mice. Electrical recordings from the retinas of the engineered mice indicated that the added photopigment had enabled their color-sensing cone cells to respond to long wavelength red light, which normal mice can't see. Next the team gave the mice a battery of behavioral tests that required them to poke their nose at panels in their enclosure to indicate which of three panels was a different color than the other two. Right answers earned a tiny drop of soy milk ("It's kind of hippie-ish, but they really enjoy it," Jacobs says.) The engineered mice passed with flying colors, so to speak, making distinctions that regular mice cannot, the researchers report in tomorrow's Science.
The work supports the idea that a single gene mutation could have produced trichromatic color vision and immediate changes in behavior, says Daniel Osorio, a vision scientist at Sussex University, U.K. At the same time, Osorio says, it creates a mystery about why such color vision didn't evolve in other mammals, too.