Seeing red

Mice have been engineered with cone cells that are sensitive to red light.

Researchers have engineered mice to express an additional photoreceptor, a transformation that may mimic the evolution of trichromatic vision in primates, reports this week's Science. In the study, mice that express a human cone pigment sensitive to long-wavelength light can see colors that normal mice cannot.

"What this shows is that animals can develop quite sophisticated discrimination capabilities just by inserting a new class of receptors at the very front end of the visual system," said David Williams of the University of Rochester in New York, who was not involved in the study. "That's really fundamentally important in understanding how sensory systems develop."

Mice normally have two types of cone photoreceptors -- blue- and green-sensitive -- which gives them dichromatic vision. Many primates have trichromatic vision arising from the addition of a third, red-sensitive photopigment. In Old World primates, this third pigment comes from a separate gene on the X chromosome. In New World primates, the third pigment arises from a polymorphism in a single X-linked gene, which means that only females heterozygous at this locus have trichromatic vision.

Some scientists have suggested that adding a new cone pigment might be sufficient to extract new color information, "but there's been no real proof of this," said lead author Gerald Jacobs of the University of California, Santa Barbara.

One interesting point: The "design" of the third pigment in primates is different for the two groups of primates. Either we're not looking at Intelligent Design, or we've got two different designers trying not to infringe on each other's patents.

Researchers previously created a knock-in mouse in which some of the coding sequences for the normal medium-wavelength (green) pigment gene were replaced with human long-wavelength (red) cDNA. Breeding produced males and homozygous females possessing either green or red cones plus heterozygous females with a mixture of the two cone types.

To see whether the addition of this photopigment actually changed color vision in the mice, Jacobs and his co-workers trained mice to identify which of three panels was illuminated with a different color than the other two. After thousands of training trials, the researchers found that most heterozygous females could discriminate between colors that were roughly red and green, while mice with only green cones could not.

"This is really a landmark paper in sensory neuroscience," Williams told The Scientist. The results suggest that "all you need is the right sensory input and the brain will take care of the rest by itself."