Colour Vision

 

We all know that almost one in ten men and less than one in a hundred women are colour blind. But in comparison with many species of birds, insects and fish all humans are slightly colour blind. And chickens, for example, put our meagre colour vision completely to in the shade.

The latest research draws on these differences within and between species to explain not just how and why colour vision varies, but to reveal the physical and chemical details of the mechanisms that encode colours and those that determine how it develops, and even how it evolves.

One yardstick for comparison between colour vision systems is the range of wavelengths that can be detected. Our visible spectrum is limited by red at one end and violet at the other. But many insects, birds and fish can see ultraviolet light, and some snakes can see infra red.

The study of animals in their natural environment shows how these differences in sensitivity are driven by the competition to survive. An extended range of colour vision might help locate a specific food source. We have known for some time that bees use ultra-violet light to read the signposts that point the way to nectar in some plants. But the value of ultra-violet vision to the kestrel has only just been explained.

It turns out that the kestrel also uses ultra-violet light to find his lunch. In this case it is the humble vole, which has the misfortune to be incontinent, leaving a trail of urine behind it as it goes about its daily business. The poor vole’s urine contains chemicals that reflect ultra violet light, enabling the kestrel not only to track down individual voles, but also to tell at a glance whether there are many voles in the neighbourhood.

Colour vision involves more than just detecting light. To discriminate colours we need different types of colour sensor, known in vertebrates as cones, each of which responds differently to the various bands of wavelengths. Colours are then coded by the pattern of activity in the different cone types.

With two cone types a rudimentary form of colour vision, called dichromacy, is possible. Our distant evolutionary ancestors were probably dichromats, as are some species of monkey, and about two per cent of men.

Three cone types, as found in humans and many primates species, allows much better colour vision (trichromacy). But although we think of trichromatic colour vision as a sort of gold standard, many insects and fish have four colour receptors, and chickens have about six.

Increasing the number of colour receptors permits finer colour discriminations. As with sensitivity, the drive to improve colour discrimination can be linked to the search for lunch. A chicken that eats worms, and grain, and searches for them on the ground needs to have a highly developed ability to discriminate the colours that we can lump together as browns.

The ability to discriminate reds and greens (and oranges and yellows which lie in between red and green in the spectrum) was acquired fairly recently in our evolutionary history. Not surprisingly, it is also a help at lunchtime. It enables us to distinguish ripe from unripe fruit, and, if we have to go and find the fruit, to detect it against a background of green foliage.

With the aim of understanding not just how colour vision works, but why it works the way it does, research is branching out from the traditional study of behaviour and physiology, into ecology, chemistry and molecular biology.

Ecology is needed to show how the cones may be matched to the important colour differences in the environment. We cannot do this in humans because we now control the colour of our environment, and buy our food in the supermarket. But some of our primate relatives can be studied in the habitats in which they evolved.

Chemistry and molecular biology reveal the structure of the pigments that make the cones sensitive to light, and the location of the genes that code them. This makes it possible to explain both how trichromacy evolved in primates, and how colour blindness continues to crop up.

The cone pigments that permit red-green colour discrimination have the same general structure, and over 95 per cent of the chemical content is identical. This similarity leaves little doubt that the pigments evolved from a common ancestor. It also means that the genes that code the pigments can pair illegitimately, producing either intermediate pigments, or double copies of the same pigment, giving rise to the different forms of red-green colour blindness.

This new understanding of the chemistry and molecular biology means that colour blindness can be tested from a tissue sample. Last month, in a triumph of genetic detective work, John Dalton, who initiated research on colour blindness 200 years ago, had his own deficiency correctly diagnosed for the first time, following an analysis of tissue from his eyes which are preserved in a collection in Manchester.

 Posted by at 20:21