Department of Biology

Realization that thoracic horns are built by the same gene network as wings reframes traditional ideas about emergence of 'new' traits.

When studying how organisms evolve, biologists consider most traits as derived from some earlier version already present in their ancestors. Only a very few traits are regarded as truly "novel."

Insects were wingless, then winged. Animals were blind, then had eyes.

And in biology textbooks, novelty has a strict definition: It must have no relationship to any structure found in an ancestor and no relationship to any other body part elsewhere in the organism.

By this definition, a dolphin's pectoral fins are not a novelty because they are modified forelimbs that already existed. Some experts argue that this creates a problem since it means that novelty must seemingly arise from nothing.

Now evidence has emerged—in a study published Nov. 21 in the journal Science—that better illuminates how new traits can evolve. And this evidence has come from an unexpected source: the small yet charismatic dung beetle.

Regarded as a textbook example of an evolutionary novelty, the thoracic horn is unique to horned beetles and appears to have no relationship to any other structure in the animal.

"This work forces us to rethink what we mean by 'novelty,'" Moczek said. "Each insect segment possesses this gene network, and as such, it is an ancient feature of their makeup. Yet, what each segment does with this network is so strikingly variable that it can yield traits that on the surface seem to have nothing in common, like wings and horns."

To conduct the study, Moczek, along with lead authors Yonggang Hu and David M. Linz, knocked down various genes in this "wing gene network" in three species of beetle: Onthophagus sagittarius, O. taurus, O. binodis. In all three species, the researchers were surprised to find that deactivation of genes in the network significantly affected the formation not just of wings but of thoracic horns, causing them to be reduced or completely absent.

They also showed that they could manipulate other genes and force the horns to transform into "ectopic" wings—or extra wings on the wrong part of the body—which provided additional evidence that thoracic horns and wings were interchangeable, alternative outputs of the same gene network.

This work also extends beyond just dung beetles and their thoracic horns.

"This new evidence is profound since it suggests that all of this vast diversity, all these novelties, could in fact be enabled by a single gene network that was used millions of years ago to form the flight wings on other body segments."

Hu, Linz and Moczek added that these results contribute to a growing call to re-examine the usefulness of defining morphological novelty as the absence of relatedness to other structures. Rather, this work suggests that novelty can emerge through such relatedness—and accordingly, textbook definitions of what constitutes novelty may need to be redefined.

This research was supported by the National Science Foundation and John Templeton Foundation.