Blind cavefish whose eyes have withered while living in complete darkness over the course of evolutionary time can be made to see again. In some cases, the offspring of mated pairs originating from distinct cave populations regain vision, researchers found. The result shows that mutations in different genes are responsible for eye loss in separate cavefish lineages that may not have been exposed to light for the last one million years.
"Evolution has many ways to accomplish the same end result, which in the case of cave fish is blindness," said NYU Biology Professor Richard Borowsky, the study's lead author. "For this reason, the genes that are mutated in one population that lead to blindness are different in other, independently evolved populations. Thus, when you cross them, the genetic deficiencies in one lineage are compensated for by strengths in the other, and vice-versa."
The study examined four populations of blind cave fish, Astyanax mexicanus, which inhabit different caves in northeast Mexico. Blind for millennia, these fish evolved from eyed, surface fish. The researchers' genetic analysis showed that the evolutionary impairment of eye development, as well as the loss of pigmentation and other cave-related changes, resulted from mutations at multiple gene sites.
In order to gauge how genetic make-up could bring about the restoration of vision, the researchers created hybrids of the different cave fish populations. Among these various hybrids, they found that nearly 40 percent in some hybrid crosses could see.
"These fish are descended from ancestors that have been isolated in the dark for nearly one million years and most likely haven't had the capacity for vision for at least half that time," said Borowsky. "But by recombining the right genes through hybridization, you can partially restore vision. Not only are the structures of the eye restored to the point where they regain function, but all the connections to the brain for proper processing of information not used for that enormous length of time are restored."
Borowsky added that the findings could pave the way for greater understanding of human eyes.
"These genes that have had their function altered by mutation are the same genes that normally play important roles in the development and maintenance of the eye in humans as well as in fishes," he explained. "The cave fish system gives us an experimental model for learning about human eye development and diseases."
Earlier studies found that the evolutionary impairment of eye development -- as well as the loss of pigmentation and other cave-related changes -- results from mutations at multiple gene sites, or loci. Reports also showed that eye loss has evolved independently at least three times and that at least some of the genes involved differ between the different cave populations.
The research, supported by grants from the National Science Foundation and the National Institutes of Health, appears in a recent issue of the journal Current Biology.
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