Researchers at UCSB and at The Johns Hopkins University School of Medicine have demonstrated that a particular genetic modification enables mice to acquire close to full color vision. Their findings, which could have implications for understanding the evolution of color vision and other sensory systems in mammals, was published last month in the journal Science.

Though mice, like most mammals, typically view the world with a limited color palette—similar to what people with red-green color blindness see—scientists have transformed their vision by introducing a single human gene into a mouse chromosome. The human gene codes for a light sensor that mice do not normally possess, and its insertion allowed the mice to distinguish colors as never before.

Gerald Jacobs, a psychology research professor with the Neuroscience Research Institute at UCSB, and Jeremy Nathans, M.D., professor of molecular biology and genetics at Johns Hopkins and a Howard Hughes Medical Institute researcher, are the article’s lead authors. The other researchers were Gary A. Williams, a postdoctoral researcher at UCSB, and Hugh Cahill, a graduate student at The Johns Hopkins University School of Medicine.

Jacobs’s group developed behavioral tests to determine whether the female mice could discriminate among colored lights. They conducted tens of thousands of tests in which two different wavelengths or intensities of light were displayed on three test panels.

Mice received a drop of soymilk as a reward when they correctly identified which panel differed from the other two.

The genetically altered mice demonstrated their new visual ability by choosing the correct panel in 80 percent of the trials. By contrast, normal mice only chose correctly one third of the time, the score that one would obtain by guessing randomly among the three panels.

This study builds on earlier research by Jacobs and his colleagues that has centered on issues related to the biology and evolution of color vision among the primates. The new abilities of the genetically engineered mice indicate that the mammalian brain possesses a flexibility that permits a nearly instantaneous upgrade in the complexity of color vision, say Jacobs and Nathans.

The new research is the most definitive yet in shedding light on the first steps that led to the emergence of trichromacy—the variety of color vision found today in most primates, including humans.

Trichromacy is dependent on three types of photoreceptor cells in the retina that preferentially absorb light at different wavelengths. These are known as cone cells and each type contains a particular kind of light-absorbing sensor protein. When light strikes the retina and activates the cone cells, the brain compares the responses of the three photoreceptors, and it assesses their relative levels to produce what humans perceive as color.

Nathans worked out the structure of the genetic basis of human color vision variation beginning in the 1980s. At the same time, Jacobs deciphered the distinctive genetic mechanism that gave rise to trichromatic color vision in New World (South American) primates.

A way of genetically expressing selected receptor cells was found that produced receptors in different cone cells the same as those in New World primates. For the current study, the team selected altered mice with the right selection of receptors and compared their vision to that of normal mice.

According to the scientists, their findings have implications not just for the evolution of color vision, but for the evolution of sensory systems in general.

Previous experiments with the visual, olfactory (smell), and gustatory (taste) systems, have suggested that introducing a new sensory receptor can alter an animal’s behavior and nerve activity.