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Cavefish's genes programmed to destroy its eyesight

The blind cavefish, a curiosity of nature, has a clever genetic trick that destroys its sight, thus giving itself an advantage in a pitch-dark watery world, scientists studying the creature believe.

Astyanax mexicanus lives in deep, lightless caves off the Mexican coast.

Soon after the cavefish starts developing in the egg, its eyes begin to degenerate and the fish is born blind. In contrast, fish of the same species which live in the open sea, where there is light, grow eyes and have normal vision.

University of Maryland biologist William Jeffery and colleagues say they have traced the genes which cause the ocular degeneration in cavefish.

The culprit lies with two so-called hedgehog genes which control the proteins that nurture an embryo's development, coaxing it along the right developmental path.

`sonic hedgehog'

The genes are the delightfully-named sonic hedgehog (shh) and tiggywinkle (twhh), named respectively after the video game logo and a character in the Beatrix Potter children's stories.

Jeffery's team found that shh and twhh control a wider group of proteins than is normal in these fish.

In this case, the genes abnormally order the destruction of cells in the lens while the cavefish's eyes are at a primitive stage of development.

The lens tissue breaks down and gradually sinks back into the eye orbit as the embryo continues to grow.

That raises questions about the assumption that blindness in the cavefish derives from an accumulation of flaws in the genes involved in eye development, the study says.

Instead, these particular variations of the shh and twhh genes may have evolved naturally because they confer an advantage on the fish, it hypothesises.

By being born blind, the cavefish does not waste energy or brainpower on eyesight, a faculty that is useless in total darkness.

The study is to be published today in Nature, the weekly British science magazine.
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