As you might expect, crayfish species that live in complete darkness for millions of years lose traits tied to light. Their eyes disappear. Their bodies no longer have pigment and are a translucent white. Their antennae and pinchers grow elongated, the better to feel their way in the darkness.
But what about the genes that control color sensitivity in vision?Contrary to what you'd expect, a Brigham Young University biologist has found that blind cave crayfish apparently still have fully functional genes to control color reception. Because they don't have eyes, he surmises that the gene must be tied to some other feature that the crayfish needs, such as its daily activity cycle.
The discovery by Keith A. Crandall, assistant professor of zoology BYU, may be a breakthrough. "We conclude that rhodopsin (a visual pigment in the eye) has an additional, previously recognized function . . . unrelated to light absorption," wrote Crandall and David M. Hills of the University of Texas, Austin, in a report published as "scientific correspondence" in the prestigious British journal Nature.
The scientists examined genes that control rhodopsin in three species of blind cave crayfish and three species of ordinary freshwater crayfish. They judged whether genes were still useful by counting changes in them.
If a gene were valuable to the survival of the animal, then it would tend to remain intact over the millennia. Animals with defects in the gene would be more likely to die out, if the gene served some purpose. But if the gene no longer served a function, random mutations would creep in and accumulate over the centuries, making hash of the genetic code. That wouldn't matter if the gene wasn't useful anyhow.
The blind cave crayfish were from species that had been cut off from the light for an estimated 4 million to 20 million years. Yet to the scientists' surprise, the rhodopsin gene seemed to be intact, without the accumulation of random changes that would occur if it were as useless as eyes to the cave-crawlers.
"The body uses rhythms for all sorts of things, including developmental biology," Crandall told the Deseret News. Developmental biology involves the timing of when genes are turned on and off - for example, when a young man begins to develop a beard.
Besides color reception, Crandall and Hills suspect the gene is also connected with circadian cycles, the rhythms that the body goes through every day.
Asked if other blind cave animals, such as salamanders, also have intact color vision genes, Crandall said he would expect so. "In fact, I have made some primers (chemical preparations used to study DNA) to look into that," he said. "There's a host of cave organisms" that could be used, including shrimp, crickets and bugs.
The researchers have not tried to transplant the rhodopsin gene from cave animals to their sighted counterparts, in order to double-check the function. But the gene's makeup seems to be "highly consistent" between the sighted and blind crayfish, Crandall said.
If the gene has a dual function in cave animals, "that implies that it also has that function in everything else, and we just haven't thought about it," he said. The same gene for color reception is present in many species, including man.
"It is an extremely well-studied gene, and it has been associated with color vision and absorption of light for at least 25 years now," he said. Color-blindness in people is tied to the same gene. "We know exactly what mutation in this gene is tied to color-blindness."