Keep them in the dark for a million years. Then cross two strains of cave-dwelling fish, now totally blind. It turns out some of their kids will be able to see.
Fish and other creatures lose their sight after generations living in caves. Yet working vision genes from one parent can partly make up for defunct versions from the other parent, at least in young fish, reports Richard Borowsky of New York University.
The vision rescue experiment works only if mom and dad come from different caves, says Borowsky, illuminating how the blind cave strains evolved. Lineages of the same species, long separated in different caves may all end up blind, but different genes have mutated to converge on the same result, he concludes in the Jan. 8 Current Biology.
Other studies of the genetics of these fish have suggested this scenario of convergence, comments William Jeffery of the University of Maryland in College Park. “This really nails it down,” he says.
Blindness in cave fish has intrigued evolutionary biologists as well as biomedical researchers studying the genes underlying eye development and sight. Dozens of species of blind fish live in caves around the world, but the Mexican cave tetra (Astyanax mexicanus) is “just a beautiful experimental animal,” says Jeffery. Different forms will breed in the lab, including sighted forms that live in sunlit waters.
Borowsky mixed and matched strains from Mexico, where 29 caves shelter sightless varieties. He tested vision in the pure strains and hybrid crosses with a fish eye exam that works even for tiny fry.
To perform the test, researchers temporarily immobilize the fish in thick goo, which can be washed off. Then researchers move a circlet of alternating dark and light stripes around the fish and watch its eye motions. If the fish sees the stripes, its eyes twitch to track the stripe parade. When the researchers reverse the spin of the stripes, the fish’s eyes twitch the opposite way.
Borowsky crossed blind fish from one cave, Molino, with those from Tinaja cave, about 100 kilometers away. When the offspring were just over a week old, 39 percent of them passed his eye test.
When he crossed the Tinaja strain with fish from nearby Curva cave, though, none of the offspring could see. The fish from the neighboring caves probably shared too much history to have genes that could rescue each other, he says.
The cave hybrids typically lost their sight as they grew up. But when Borowsky crossed cave dwellers with surface fish, some of the offspring did retain vision into adulthood.
In a set of multigenerational crosses, Borowsky saw vision-inheritance patterns that confirm there’s little overlap between the genes that mutated in the far-flung cave systems. Evolution uses “a large and varied palette,” he says.
“This can be generalized,” says Horst Wilkens of the University of Hamburg in Germany. He predicts that plenty of cave species will turn out to have evolved their typical pale, blind form through a variety of genetic losses.