Just one extra gene is all mice need to see vibrant colors.

Colored lights help show that the brains of genetically-altered mice can process information from new photoreceptors in their eyes. Here, a mouse deciding that the third colored panel looks different from the other two is rewarded with a drop of soy milk.

Some lab mice can see the world in a whole new light, thanks to HHMI investigator Jeremy Nathans and his colleague Gerald H. Jacobs. Their findings provide insight into the remarkable plasticity of mammalian brains, and shed light on a plausible means by which humans may have acquired the ability to see many colors.

Nathans, at Johns Hopkins University School of Medicine; Jacobs, of the University of California at Santa Barbara; and their colleagues introduced the gene for a human red-light-sensing pigment into mice, and showed that the new photopigment functioned correctly, allowing the mice to distinguish colors they previously could not detect.

Most mammals have only two types of photopigments in the color-sensitive cone cells of their retinas. In mice, one pigment detects ultraviolet light, while the other sees yellowish-green wavelengths. Many primates, including humans, have three color-sensing pigments, which gives us our rainbow palette of color vision.

The researchers wondered whether the gene alone would alter sensory perception, or if additional changes in the nervous system would be necessary. Nathans and Jacobs showed that just the addition of the new photopigment endowed the mice with broader color vision. Using electrophysiological tests, the researchers determined that the rodents' retinas responded to red light. Then they subjected the mice to a series of behavioral tests, which confirmed that the mice could indeed see red.

In essence, says Nathans, the brains of the mice completed all the necessary rewiring to make their new color receptors function. He views the findings, published in the March 23, 2007, issue of Science, as a lesson on how color vision, and possibly other sensory traits, might have evolved in humans. "Maybe the principal way in which sensory systems evolve is by genetic change at the front end—at the receptor cells," he says, "and the brain is flexible enough to immediately take advantage of those changes."

Scientific Image: Gerald Jacobs