Scientists Have Eye On Gene

Combining elements of the sublime and the macabre, scientists have created flies that grow large, perfectly formed eyes on the most inappropriate parts of their bodies: on their wings, on their legs, on the quivering tips of their antennae.

The experiment offers graphic evidence that scientists may have discovered what they call “the master control gene” for the formation of the eye, one of the most complex structures in nature.

Researchers in developmental biology have been struggling, with scant success, to identify the genetic signals that initiate the growth of the body’s specialized components, whether limb, liver or brain.

But the latest work, reported today in the journal Science, suggests that the gene with which the scientists prompted laboratory flies to sprout as many as 14 eyes apiece is indeed the kingpin of vision, the gene that touches off an intricate biochemical event able to transform a non-descript speck of cells into a fully outfitted eye.

Whether these extracurricular fly eyes can see remains to be determined.

Though the work was done with fruit flies, which are genetically amenable to such manipulations, the eye gene in the fly turns out to be similar to a gene identified in mammals, including humans, indicating that the equivalent gene in human embryos may direct the creation of the paired windows to the soul.

The gene used in the fly experiments is called eyeless, because the absence of the gene results in flies with no eyes at all.

“It’s an amazing example of how a single gene can switch on an entire developmental program,” said Dr. Walter Gehring of the University of Basel in Switzerland, the senior author of the report. “It came as a total surprise to us.”

Gehring estimates that at least 2,500 different genes participate in the construction of the eye, and that all those genes answer directly or indirectly to eyeless.

Gehring did the experiments with his colleagues Dr. Georg Halder and Dr. Patrick Callaerts.

“It’s the paper of the year,” said Dr. Charles Zuker, a neuroscientist and fruit fly biologist at the University of California School of Medicine in San Diego. “This is Frankensteinian science at its best.”

Other scientists expressed enthusiasm for the work, though some scorned the term “master control gene,” which they said was a glib phrase that ignored the highly interactive nature of the body’s development, the chattering talk and crosstalk that occurs while the multitudes of growing cells figure out who does what.

“This is quite a spectacular result, but I have problems with the idea of ‘master regulators,”’ said Dr. S. Larry Zipursky, a fruit fly researcher at the University of California School of Medicine in Los Angeles. “I think it’s an attractive way to get attention.”

The new work also suggests that conventional ideas about the evolution of the eye may be wrong. In view of the vast differences between the visual systems of many different organisms, scientists had long assumed that the eye might have been invented as many as 40 different times. A human eye, with its single lens, looks nothing like the fly’s compound eye, which is made up of 800 tiny eyes linked together like soap bubbles in a bath.

But the paper suggests that, given the similarities between the gene for a fly eye and that for a mammalian eye, the primordial eye may in fact have evolved only once, taking on manifold shapes and designs depending on the needs of the organism. Even the squid appears to have its own version of the eyeless gene.

The fact that vertebrates like people and invertebrates like insects and squid seem to share the same master control gene for eyes “is contrary to all the textbooks,” Gehring said. “I’ll freely admit that that includes my own.”

The latest edition of the Encyclopedia Britannica, for example, discusses as a well-known fact the autonomous evolution of the squid eye and the vertebrate eye.

Dr. Hermann Steller, a geneticist at the Massachusetts Institute of Technology, said, “This convincingly draws similarities between eyes that were thought to have developed independently, and it suggests that the first simple visual system must be very old,” dating back to before insects and vertebrates went their separate ways half a billion years ago, and perhaps long before that.