Mice get full-colour vision
23 March 2007
Genetically engineered mice that see the world in full colour, or at least in the same colours as humans see, have been created in a US lab. Most mammals lost the ability to distinguish colours during the age of the dinosaurs.
Birds and reptiles have three or more light sensitive protein pigments called opsins in the cells of their retina that absorb different wavelengths. But most mammals have only two, so their ability to discriminate between different light wavelengths is much reduced.
It is thought that this dichromatic vision developed during the Jurassic, when mammals survived as nocturnal creatures. They had no use for colour vision when they hid during the day, trying to avoid the dominant creatures of the time.
The only mammals that have since re-developed full colour vision are primates, in which new genes have emerged coding for a third opsin. These mutations have occurred independently several times in different primate groups - the ability to distinguish between ripe and unripe fruit is thought to have given a big advantage to ancestral monkeys.
Gerald Jacobs' team at the University of California, Santa Barbara, describe a study using a 'knock-in' mice strain, into which was inserted the gene encoding the third human opsin. Mice were shown three illuminated panels - two lit by light of identical wavelength and the third with a different wavelength at the same light intensity. The mice were trained to touch the odd-one-out; a successful choice earned them a food reward.
Mice with the human gene could detect light at a longer wavelength than normal for the species, and could discriminate between different wavelengths.
'This is a good study and it's interesting on two different levels', said Daniel Osorio, professor of neuroscience at the University of Sussex, UK.
First, it shows that the mouse brain is flexible enough to develop the circuitry needed to process colour vision data. 'It is not obvious that just by putting these sensory receptors into the eyes means that you then have colour vision; the brain wiring has to be in place as well.'
It also raises questions about why primates alone among the mammals have developed colour vision. It used to be thought that this was due to their unique neural architecture in which a single cone cell in the retina sends information to its own dedicated brain neuron. In mice and other mammals, multiple cone receptors feed into the same neuron. 'So they pool information at an early stage of processing and you would have thought that the wavelength information would no longer be useful.'
G H Jacobs et al, Science, 2007, 317, 1723
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