In dark pools in Mexican caves, the blind cavefish survives just fine without eyesight, finding food in an environment of low productivity, swimming without bumping into the sides of the rock walls.
A team of University of Maryland researchers has recently found an important clue into how blindness might have evolved in these fish over the past 10,000 years. As reported in the Oct. 14 edition of the journal Nature, two genes with cartoon-like names--sonic hedgehog (shh) and tiggy winkle hedgehog (twhh) -- appear to play a major role in eye degeneration in the blind cavefish.
William Jeffery, University of Maryland biology professor, and Yoshiyuki Yamamoto, until recently a post-doctoral student at the University of Maryland, along with David W. Stock, of the University of Colorado, found that the hedgehog genes are key to the blind fish's failure to develop sight in its embryonic stages. The findings also may have implications for learning more about some forms of human eyesight loss.
The blind cavefish, Astyanax mexicanus, is the sightless form of another fish of the same species that lives in fresh water and has normal eyesight. The blind cavefish, which evolved from this sighted ancestor, has taken on a number of traits brought on by living in total darkness. It doesn't have eyes, its lack of pigmentation makes it ghostly white instead of silver, and it has a larger jaw and more teeth and tastebuds than the sighted form.
The blind cavefish is a good species for studying evolution, said William Jeffery. "The sighted and blind forms of Astyanax are one of the only animals in which the evolution of development can be studied directly by comparing a living animal with its ancestral type."
To study how genes affect eyesight development in the blind cavefish's embryonic stages, the Maryland researchers zeroed in on the shh and twhh genes.
"We focused on the hedgehog genes because we suspected from previous studies that they had roles in inhibiting eye development,"Jeffery said.
"We know that loss of shh function in humans expands eye development, resulting in a single large eye in the middle of the forehead. Actually, the Cyclops in The Odyssey was probably not a product of Homer's imagination. Therefore, we reasoned that increased shh could do the opposite in cavefish, that is, function as an inhibitor of eye development."
Sonic Hedgehog Isn't Just a Video Game
The researchers found that blind cavefish have a larger than normal activity of sonic and tiggy winkle hedgehog genes in the area of the embryo where the eye is formed. The hedgehog genes activate a protein-signaling pathway that eventually causes cell death in the developing lens of the eye. The lens is critical for overall eye growth and development. Without it, the cavefish eye stops growing and degenerates.
To test the effects of that extra dose of shh and twhh, the scientists injected messenger RNA from the shh gene into one side of a sighted fish embryo. The messenger RNA directed the production of extra shh protein, which increased the dose of the hedgehog gene in that embryo.
They saw immediate effects on eye development. In 78 per cent of the normally sighted fish, eye development was arrested in the embryonic stage. Adults that developed from these embryos were missing an eye on the injected side.
They also transplanted lenses from the shh-injected fish embryo into a sighted fish embryo, to test whether the lens had been altered. A little more than a third of those embryos had arrested development in the eye region, showing that the lens was the target of increased shh activity.
In a third experiment, they looked at the effects of reducing the hedgehog gene activity. Cavefish embryos were treated with cyclopamine, an inhibitor of the hedgehog-signaling pathway. These embryos showed a partial restoration of eye development.
Understanding the benefits of enhanced hedgehog genes could provide key insights into the mechanisms controlling the evolution of blind cavefish.
"Other studies have shown that only a few genes (4-6) control the loss of eye development in cavefish, although the identity of these genes remained a mystery," said Jeffery. "Our results suggest that two of the critical genes may be shhand twhh. However, we have not excluded the possibility that related genes, which are turned on earlier in the embryo and regulate the two hedgehog genes, may harbor the actual mutations.
"Eye regression in cave animals has been attributed to loss-of-function mutations in eye genes, which may accumulate without penalty under conditions of relaxed selection for eyesight. It also could be that eye regression could be driven by natural selection for adaptive traits," Jeffery said.
Jeffrey says the findings also could have implications for understanding some types of loss of eyesight in humans. "The results could explain a human anomaly called hypertelorism, in which eyes develop much further apart in the head than normal, and eyesight is affected. Perhaps this disease, like loss of cavefish eyesight, can be attributed to increased activity of hedgehog genes. If so, then cavefish have taken advantage of a genetic pathway that is normally destructive to eyesight in humans."
Seeing into the Future
The University of Maryland researchers are now looking at the beneficial functions of increased hedgehog signaling, which could lead to the first comprehensive understanding of how and why many cave animals have lost their eyesight.
"Excellent candidates are the very things that have evolved to help cavefish find food and navigate -- large jaws, more teeth and taste buds, and sensory organs that detect the presence of rock walls," said Jeffery.
The Jeffery Laboratory at the University of Maryland harbors the largest blind cavefish colony in the world. Research is supported by the National Institutes of Health and the National Science Foundation.
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