Tuesday, November 2, 2010

Life Lines 14


An experiment by a NYU biologist, Richard Borowsky, confirmed a prediction made by evolutionary biologists on the origin of these cave dwellers in Mexico. There are 29 independent locations or unconnected caves where these fish reside, most of them assigned to the species Astynax mexicanus. They are usually albinos and they lack eyes (or at most show a small pinhead where the eye should be). A century ago Fernandus Payne studied fruit flies raised in the dark for 69 generations to see if disuse would lead to loss of eye color or loss of vision. He found that the flies had perfectly normal vision and the identical red eye color of those who were raised under ordinary day and night conditions. Geneticists speculated that random mutations leading to loss of pigment in the skin and loss of eyes in the cave environment were actually beneficial because these conserved energy and reduced infections. In the 1980s other geneticists identified some of the mutant genes involved with loss of the eyes and skin pigment.

Borowsky came up with an interesting idea. If the mutations are independent, those farther away from each other are less likely to have had a common mutant origin, which got dispersed by floods into nearby caves. By crossing these 29 varieties, he confirmed that those farthest away were the ones that produced the restored vision they had lost sometime over the past one million years when they became trapped in caves. Why should that be?

About the same time Payne was working in 1906-1908 on fruit flies, George Shull at Cold Spring Harbor was crossing different strains of inbred corn to one another. He found that the hybrid offspring were larger, had more grains, and did much better in the soil. He had discovered hybrid vigor which revolutionized American agriculture. The same principle is at work in the blind cavefish. If the blindness is due to inbred mutations in cave 1 a different inbred strain with a different mutation may lead to blindness in cave 25. When the two are crossed to one another their offspring show hybrid vigor and their normal functions are stored. An organ like an eye is put together by many genes, but anyone of them, if its father’s and mother’s genes are both mutant for that particular gene, may cause blindness or loss of the eye. What is remarkable is the difference in time. It took less than 10,000 years to make corn from teosinte by farmers selecting for the best plants and using their seeds for the next season. It took nature almost one million years for mutants arising hundreds of thousands of years ago to have a biologist breed them together and restore their sight. Borowsky devised a flashing light pattern that shows eye movement when the flash is on for a sighted fish but a blind fish will not respond to the flash. The results of Borowsky’s work show that unless gene functions are sifted by a natural environment (like the hazards encountered in day and night daily rhythms), the genes for blindness will accumulate in the cave. In the cave, no such advantage exists for eyes whether they function or not and they will disappear (especially if eyed flies require more food or are more likely to get scratched and infected). In some of the cave areas the original species from which the blind cave fish arose no longer exists. By crossing these varieties, the lost ancestor species can be approximated if not restored.

No comments: