In certain species of dung beetles in the genus Onthophagus, researchers have found that knockdown of a gene called orthodenticle not only interferes with horn development but also results in the development of ectopic eye development on the midline of the beetle’s head—in other words, the beetle grows an extra eye. And further study shows that the eye works, to some degree, as researchers found beetles with ectopic eyes and without normal eyes still responded to light. (Photo credit: Eduardo Zattara, Ph.D.)

By Viviane Callier

One of evolution’s oldest tricks is to apply old genetic building blocks at new times or places in development to create a new structure. But that isn’t the only way to innovate: A new study on dung beetles published this week in Proceedings of the National Academy of Sciences shows that eliminating the activity of orthodenticle, a gene previously shown to play a role in positioning the horns on the head, not only messes up horn development. In some species in the genus Onthophagus, removing otd also causes a novel ectopic eye—in other words, an extra eye—to develop on top of the beetle’s head.

Viviane Callier

“This is not the first time that ectopic eyes are formed by altering the expression of a single gene,” said Eduardo Zattara, Ph.D., postdoctoral fellow who led the work in Armin Mozcek’s lab at Indiana University in Bloomington. “But these are the first ectopic eyes that are shown to be functional. These are also the first ectopic eyes to pop up by knocking down a gene [as opposed to overexpressing a gene],” Zattara explained.

In flies, a gene called pax6/eyeless produces ectopic eyes wherever it is overexpressed in the body. But those eyes are not functional because the photoreceptors never connect to the nervous system in the right way.

Zattara and Moczek were first interested in otd because it positions and patterns the beetle horns. Different species show a great variety of horn shapes and sizes, and the researchers hoped that otd might hold the key to unlocking that diversity. Along with a team of researchers, they knocked down the activity of otd in a handful of beetle species using RNA interference and expected to see defects in the horns, which is, in many cases, what they found. But an additional unexpected trait appeared in some of the specimens: structures looking like compound eyes on the midline, at the top of the head.

A histological section of a four-eyed beetle shows regular (bottom right inset) and ectopic (upper right inset) compound eyes. (Photo credit: Eduardo Zattara, Ph.D.)

To better understand what the eye-like structures really were, the team analyzed the gene expression of that tissue and showed that it resembled that of a regular compound eye. They used histological methods and electron microscopy to look at the internal structures of the ectopic eye, and found that it had lenses and was lined with photoreceptors just like a regular eye. Then, to find out if those ectopic eyes worked at all, they did a behavioral assay, removing the beetles’ regular eyes and watching to see if the beetles responded to a bright light. They found that the beetles with ectopic eyes and without normal ones did respond to the light by moving away from it. That suggested that the ectopic eyes are able to send signals to the brain.

“This result shows that developmental systems are very modular and self-contained, to the point where if everything else is the same but you change one key parameter, you can make something as complex as an eye develop outside of its normal context,” Zattara said. “That makes it much easier to suddenly evolve something quite complex and new in a different place, and shows how the way development works can facilitate evolutionary novelties.”

Viviane Callier is trained as an insect physiologist and is now a freelance science writer in Washington, DC. Twitter: @vcallier. Email: vcallier@gmail.com