Human eyes, fly eyes, and horseshoe crab eyes, to name a few, differ so greatly that it would seem nature invented eyes dozens of times across the animal kingdom. A blow to this argument came 2 years ago, when a mouse-eye gene spliced into fruit flies prompted them to form extra fly eyes on their legs and elsewhere. Now this eye-catching feat has been duplicated in fruit flies using a squid gene. The finding, reported in today's issue of the Proceedings of the National Academy of Sciences, is the strongest evidence yet that all animals with eyes inherited them from a common ancestor.
A group led by molecular biologist Stanislav Tomarev of the National Eye Institute in Bethesda, Maryland, and developmental geneticist Walter Gehring of the University of Basel in Switzerland found that the squid Loligo opalescens, a mollusk, carries a gene very similar in its nucleotide sequence to a mouse gene called Pax-6. In squid embryos, they found, the gene is active in the brain, olfactory organs, and the iris and lens of the eye--all places where the protein Pax-6 is needed in mice. What's more, when the researchers engineered Drosophila fruit flies to express squid Pax-6 in locations where the flies' wings, legs, and antennae normally grow, extraneous fly eyes sprouted there, just as they do when the fly version of Pax-6, called eyeless, is artificially activated in these areas.
The results strengthen the idea that eyes evolved only once, with Pax-6 and eyeless acting as key regulators of eye development at least since the mollusks' evolutionary path diverged from other animals some 500 million to 600 million years ago, the researchers say. "If [the gene] were only active in flies and mammals, then you could suggest there was independent recruitment of the same gene to make eyes" in different species, says Tomarev. "But when it happens many times in many organisms, that's harder to imagine." Graeme Mardon, a developmental geneticist at Baylor University in Houston, agrees that the study suggests the eye is a singular invention: "These are very nice data to have," he says.