Eye-opening gene: how many times did eyes arise? - evolution of eye - Cover Story

Science News, May 10, 1997 by John Travis

Several years ago, Walter J. Gehring of the University of Basel in Switzerland was working on a zoology textbook. When it came time to write a section that dealt with the evolution of eyes, Gehring unhesitatingly recited the traditional view that eyes had evolved independently dozens of times.

For the next edition, he'll pen a different scenario.

The discovery of a gene shared by fruit flies, mice, squid, and humans and the creation of unusual fruit flies that sprout eyelike structures in places such as wings, legs, and antennae have persuaded Gehring that all modern animals with eyes evolved from a common ancestor that possessed a primitive image-forming organ.

In essence, he contends that the eye probably evolved just once in life's evolutionary history--an assertion not everyone is willing to accept.

The eye has always been a thought-provoking organ in discussions of evolution. Creationists have regularly pointed to it as something so complex and specialized that it could not have developed on its own.

Charles Darwin also considered eyes a formidable challenge to his theory of natural selection. "To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree," he wrote in On the Origin of Species.

Yet Darwin quickly dismissed this concern, arguing that the complex eyes of modern animals could have evolved slowly from light-sensitive nerve cells and not much else.

In more recent years, evolutionary biologists have been asking a different question: How often can such an organ develop from scratch? While many creatures have an ability to sense light, a survey of the animal world shows that a minority of the major animal groups, or phyla, have true eyes.

"Only 6 out of 30 phyla have complex eyes, which are able to give you images, but because [such eyes] give them so many evolutionary advantages, these phyla dominate," says Stanislav I. Tomarev of the National Eye Institute (NEI) in Bethesda, Md. Researchers estimate that species with complex eyes comprise 95 percent of the animals on the planet, notes Tomarev.

While image-forming eyes are commonplace, no one design for eyes dominates. Scientists have described almost a dozen distinct blueprints, from the alien-seeming compound eyes of insects and many other species to the cameralike single eyes of vertebrates like us.

The exotic appearance of the compound insect eye, with its hundreds of miniature eyes called ommatidia, helps explain why scientists have assumed that it evolved independently of the vertebrate eye.

Even superficially similar eyes provide evidence of independent evolution. At first glance, the eyes of cephalopods such as squid and octopuses closely resemble those of vertebrates. A closer examination reveals that the organs emerge from different embryonic tissues and differ considerably in the fine details of their construction. Consequently, the two groups of eyes have been thought a classic example of convergent evolution.

"They appeared independently and somehow evolved to form the very similar structures we observe now," says Tomarev.

Indeed, the majority of scientists studying eye evolution ultimately decided that the wide variety of eyes spread across the animal kingdom is evidence that the organ could not have developed just once. In 1977, L. Von Salvini-Plawen and Ernst Mayr, both of Harvard University, placed this conventional wisdom solidly on the record when they published a landmark paper concluding that eyes had arisen independently at least several dozen times.

That's where the story of eye evolution stood until 1993. That year, Gehring and Rebecca Quiring, also of the University of Basel, were studying fruit flies and looking for transcription factors--proteins that regulate the activity of genes.

Quiring finally identified a protein that binds to DNA, a common feature of transcription factors. Although it wasn't the kind of transcription factor Gehring was interested in, the researchers sent information on the discovery of this protein and its gene to a worldwide computer database to see if any similar genes, or homologues, had already been reported.

The database search highlighted two genes, one from mice that is called Pax-6 (or small eye) and one from people that is called Aniridia. Both genes, which are nearly identical to the fruit fly gene, encode proteins crucial to eye development. If mutations exist in both copies of the mouse gene, embryos don't form eyes at all. In people, a mutation in one of Aniridia's two copies usually produces defects in the eyes.

Gehring was surprised that the fly gene was so similar to the two vertebrate genes, but the real astonishment came when he realized that the insect gene also plays a role in eye development.

That finding, reported in the Aug. 5, 1994 Science, emerged after Gehring and his colleagues had mapped the location of the new gene. They found it at a chromosomal site harboring mutations in flies with developmental eye defects ranging from too-small compound eyes to a complete absence of the organs.