Blind Cavefish Can Produce Sighted Offspring

Brian Handwerk
for National Geographic News
January 8, 2008
It's a miracle! Blind cavefish, despite having adapted to their lightless environment for more than a million years, can produce sighted offspring in just a single generation, a new study reveals.

The ability was discovered when researchers mated fish from distinct populations that had been isolated in separate caves.

In some cases the first-generation offspring of such unions could see.

The find shows that the genetic mutations causing blindness are different in different lineages of the fish.

"Evolution's palette is varied," said study author Richard Borowsky of New York University in a statement.

"Restoration of the ability to see comes in a single generation because the populations residing in different caves are blind for different reasons—i.e., different sets of genes are nonfunctional in the different populations."

Genetic Reversal

The research, which was recently published in the journal Current Biology, focused on several of the 29 known blind cavefish (Astyanax mexicanus) populations found in northeastern Mexico.

The fish evolved from surface-dwelling ancestors during the past million years. (Related: "Eyeless 'Ghost Fish' Haunts Ozark Caves" [October 29, 2003].)

They have sightless eyes as embryos, but the organs decay as the animal ages and are eventually scaled over by the fish's body.

Previous work had suggested that the evolution of blindness, a loss of pigmentation, and other underground adaptations occurred independently in several locations and via mutations of different genes.

The new work "nailed down" this concept, said biologist William Jeffery of the University of Maryland in College Park.

The results also show how quickly physical adaptations can be reversed when interbreeding occurs between distinct populations.

The genetic deficiencies from each parent's lineage were easily overcome by the strengths of the other.

This means that even though the fish are blind, they basically have functional visual systems that have been deactivated by a few key mutations, said Jeffery, who was unaffiliated with the study.

"Everything must be in place except for the function of these key genes," Jeffery added.

The study also found that cavefish genetics reflect geography.

The farther apart two parents came from, the more likely it was their offspring would be able to see.

This suggests that geographically distant populations are genetically more distant and thus have less overlap in blindness-causing genes.

Eye Test

During the course of their research, the scientists also developed a definitive test for sight in blind cavefish—a boon to future research on the unusual creatures.

Studies from as far back as the 1970s had suggested some hybrid fish could see because their eyes were larger than their parents'—but definitive vision tests have been lacking.

The new test involves immobilizing recently hatched fish and placing them in a cylinder that flashes an alternating pattern of black and white stripes. If the fish can see, they move their eyes according to the color divisions.

"For the first time it really gives us an assay for detecting whether cavefish can see," Jeffrey said.

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