Beetles come in a staggering array of shapes and sizes. And so do the horns that grow out of the heads and bodies of the males of many species, which they use to battle over mates. Now, a study reveals a surprising link between fight and flight in beetles: The same set of genes that codes for their wings also helps build their horns.
“It’s a radical idea,” says Yoshinori Tomoyasu, an expert in insect development at Miami University in Oxford, Ohio, who was not involved in the study. The horns on beetles’ bodies, he says, have long been considered a textbook example of an “evolutionary novelty”—a trait that “appears to have evolved suddenly, without any connection to an old [body part or structure].”
Beetles have horns on their heads and the midregion of their body called the thorax, which is broken into three segments. Two of those segments bear wings whereas the one closest to the head, which is wingless, has proved to be a hot spot of evolutionary innovation. “Insects have just gone crazy modifying this segment,” says Frederik Nijhout, an evolutionary biologist at Duke University in Durham, North Carolina. Treehoppers form helmetlike structures on it, grasshoppers grow massive crests out of it, and beetles top it with a diverse collection of horns. The new study “may have discovered the mechanism by which you can get this tremendous variation,” adds Nijhout, who was not involved in the work.
Yonggang Hu, a developmental biologist at Indiana University in Bloomington, suspected a link between beetle horns and wings after discovering last year that gin traps—defensive jawlike organs on beetles’ abdomens—rely on wing genes to grow. To find out whether these same genes guide horn growth, Hu and his team genetically tinkered with dung beetles. They focused on three species, each with a uniquely shaped thorax horn ranging from a nub that disappears in adulthood to a narrow cone to a sturdy wedge.
The scientists designed fragments of RNA that would find and destroy specific genes critical for wing development. But when they injected the RNA into dung beetle larvae, it wasn’t just the wings that were reduced in size or completely absent. The expected horns also either came in small or failed to grow on the bodies of all three beetle species.
Their experiments, published today in Science, reveal that the wing genes get turned on in the earliest stage of horn growth. Then, once a bump begins to protrude from the thorax, the wing genes turn off and a new group of genes takes over to enlarge and sculpt the horn.
Hu believes the notion that certain traits, such as beetle horns, evolved out of the blue needs revising. “We have to rethink the definition of evolutionary novelty,” he says.
With more and more studies exploring the genetic underpinnings of traits, Tomoyasu says it’s only a matter of time before more work reveals that other presumed evolutionary novelties actually emerged from ancient groups of genes. The new finding is a “game changer,” he says—one that adds to a growing understanding among biologists that many seemingly novel recent traits have far deeper roots than previously thought.