— A mouse drinks a drop of soymilk as a reward for correctly identifying which one of three illuminated panels is different from the other two (Image: Gerald Jacobs)
Mice engineered to produce a human protein in their eyes develop dramatically enhanced colour vision, a new study reveals.
The finding supports the idea that full colour vision offered an immediate advantage to our primate ancestors when it evolved about 40 million years ago, the researchers say.
"We were curious about the evolution of colour vision and interested to see if we could recreate one of the events," says biologist Jeremy Nathans at Johns Hopkins School of Medicine in Baltimore, Maryland, US.
Nathans and colleagues genetically altered mice by inserting a human gene that codes for the "L receptor" protein into the animals' DNA. This protein allows cells in the retina of the eye to respond to long wavelengths of light, which correspond to the reddish and orange hues that people perceive.
Normal mice in common with most mammals have only S and M receptor proteins, which are sensitive to short wavelengths of bluish light and medium wavelengths of green hues, respectively. So mice have "bi-chromatic" vision, which means they cannot see reddish hues.
The team tested the engineered mice to see whether having the L receptor protein would enhance their colour perception. Each rodent viewed three coloured lights and were rewarded if they picked the one that differed from the other two. Picking out the proper light garnered them a tasty drop of soy milk.
The engineered mice picked the correct light in 80% of the trials. By comparison, their normal counterparts only chose correctly one-third of the time no better than predicted by random chance alone.
For example, the engineered mice could easily discriminate a bright red light from two dim green lights. But the because the bright red light triggers the M receptor of normal mice just as strongly as the dim green lights do, the control animals were stumped.
The study demonstrates that the mouse brain is "primed" for expanded colour vision, the researchers say. It suggests that a simple mutation giving rise to the L receptor protein in our primate ancestors immediately expanded their visual perception.
"It has been unclear whether the simple addition of a photopigment is sufficient to yield a new dimension of colour vision, or whether you might need, in addition, some changes in the nervous system," says Gerald Jacobs, a vision researcher at the University of California, Santa Barbara, US, who took part in the study.
Humans, monkeys and apes have tri-chromatic vision that involves receptor proteins for short, medium and long wavelengths of light.
Scientists have hypothesised that this enhanced colour perception which includes greater sensitivity to reddish hues offered an evolutionary advantage by enabling our ancestors to better distinguish ripe fruit and avoid poisonous berries, and even to become more attuned to emotions (see Colour vision evolved to spot our blushes).
Some animals, Nathans point out, go one better than humans. Chickens, for example, have four different colour receptors.
Some scientists have suggested that gene therapy could one day help restore full colour vision in people who are red-green colour-blind a condition that makes it hard to discriminate between these hues. But Jacobs says that attempts to use this approach to treat colour-blindness in primates have given disappointing results and that it is "moderately unlikely" that a treatment will soon be developed for humans.
Journal reference: Science (DOI: 10.1126/science.1138838)