Published online 22 March 2007 | Nature | doi:10.1038/news070319-12


Mice made to see a rainbow of colours

All you need to see more is more pigments in the eye.

Mice can usually only see a dull mix of yellow, blues and greys.Mice can usually only see a dull mix of yellow, blues and greys.Getty

Simply by inserting a piece of DNA that codes for a human eye pigment into the genome of a mouse, scientists have introduced a rainbow array of colour to the dull mix of yellows, blues and greys that normally make up a mouse's visual world.

This suggests that the mammalian brain is very flexible and can interpret signals not normally encountered. It also hints that just a single genetic mutation could have added reds and greens to the visual palette of our ancestors tens of millions of years ago.

Gerald Jacobs from the University of California in Santa Barbara and his colleagues have genetically engineered mice with a human pigment in their eye as well as the normal mouse pigments and shown that this does appear to give the mice the ability to see colours they could not see before.

"The implications are astounding," says David Williams, an expert in vision at the University of Rochester in New York state. "It's stunning to think the rest of the nervous system in the mouse has developed to be able to process the new information."

Most mammals have just two kinds of photopigment in their retinas: one is encoded in the X chromosome and the other in an autosomal (non-sex) chromosome. But many primates, including humans, have a third photopigment, encoded by a second gene on the X chromosome. This allows for a much broader appreciation of colour.

In looking at the evolution of full colour, or trichromatic, vision in humans, most scientists turn to New World monkeys, which have an arrangement mid-way between the two- and three-photopigment systems. They have only one photopigment gene on their X chromosome, but there are different versions of the gene, producing different pigments. As a result, female monkeys (which carry two X chromosomes, and so can potentially have two different pigment genes) can end up with three different photopigments in their eyes.

It's plausible that millions of years ago a single mutation resulted in two different versions of the photopigment gene becoming located on the same X chromosome. That could have paved the way for trichromatic vision in both males and females in descendant primates, says Jeremy Nathans, a co-author of the mouse study.

But was an extra photopigment all that was needed to evolve trichromatic vision? Or does seeing the world in all its colourful splendour require extra brain power too?

Seeing the light

To tackle that question, Jacobs and his colleagues genetically engineered a mouse that effectively mimics the New World monkey's eye setup.

The team made female mice in which one X chromosome carries a normal mouse photopigment gene, while the other X chromosome has a human pigment gene. Like female New World monkeys, these mice were able to produce three types of photopigment, the team reports in Science1.

These photopigments had previously been shown to be able to detect light2. But whether the mice could actually use them to see more colours was unclear. To find out, the team gave the mice a kind of colour-blindness test.

They presented the mice with three circular panels each of which was lit up by light with a wavelength within the 500-600 nm range - that's the bit of the visual spectrum that looks green, yellow or red to us. In each trial, one panel was lit up differently from the other two; pressing on the odd-one-out would result in the mouse getting a soy milk reward.


Most of the transgenic mice could clearly see a difference when the lights were more than 10 nm different: three out of five of them pushed the correct panel with their nose or paw 80% of the time during 10,000 trials. As for the two transgenic mice who didn't do so well, the researchers speculate that they didn't have a good mix of the different photopigments in their eyes.

That means that the mouse brain is perfectly capable of deciphering a stream of new information from the eye. "This is a beautiful demonstration that fairly sophisticated processing can arise from a single change at the front end of the visual system," says Williams.

The result also hints that perhaps the right kind of photopigment is all that's needed for humans to get owl-like night vision, or to see ultraviolet colours.

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  • References

    1. Jacobs G. H., Williams G. A., Cahill H. & Nathans J.. Science, 315 . 1723 - 1725 (2007). | Article | PubMed | ChemPort |
    2. Smallwood P. M., et al. Proc. Natl Acad. Sci. USA, 100 . 11706 - 11711 (2003). | Article | PubMed | ChemPort |