Blind cavefish whose eyes have withered while living in complete darkness over the course of evolutionary time can be made to see again, according to a report in the January 8th Current Biology, a publication of Cell Press. In some cases, the offspring of mated pairs originating from distinct cave populations regain vision, they 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.
"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 non-functional in the different populations," said Richard Borowsky of New York University. "[In the hybrids], the deficiencies in one lineage are compensated for by the good gene copies in the other lineage, and vice versa."
Scientists know of twenty-nine populations of the blind cavefish (Astyanax mexicanus) living in different caves in North-Eastern Mexico, he said. They evolved from eyed, surface-dwelling forms which only reached the area in the mid-Pleistocene, about a million years ago.
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.
"Given the large number of mutations at different loci that have accumulated in these populations, we reasoned that hybridization among independently evolved populations might restore visual function," Borowsky said.
And indeed, it did. While purebred cavefish are all blind, the hybrid progeny of different cave populations all had some individuals that exhibited a clear visual response. The farther apart the caves of the cavefish parents were, the more likely it was that their offspring could see, he added. That pattern is consistent with the idea that populations separated by greater distances are more distantly related and therefore have less overlap in the genes responsible for their blindness, he said.
In addition to the insights about the evolution of cavefish in particular, the findings also speak to a more general principle. "Evolution's palette is varied," Borowsky said. "There are numerous genetic ways to accomplish the same change."
The researcher is Richard Borowsky, Department of Biology, New York University, New York, New York, USA.