Dung beetle’s horns not so novel after all

A revelation that the dung beetle’s thoracic horns grew from wing genes challenges biology textbook definitions of novelty, in an evolutionary sense.

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Many scarab beetles possess rigid projections of the exoskeleton, collectively referred to as horns. Sizes, shapes and locations of horns vary extensively between species. Credit: Yonggang Hu

Novel traits or features are generally thought to have no relationship to any structure in an animal’s ancestry, nor be related to any other body part of that organism; they are thought to pop up out of the blue in the evolution of a species.

In that respect, the horns of the enigmatic Scarabaeidae dung beetle are popularly showcased as an example of evolutionary novelty, because their ancestors are hornless and the features don’t appear to be related to any other structure in the insect.

A new study, published in the journal Science, has found otherwise. 

Yonggang Hu and co-authors from Indiana University, US, discovered that the prothoracic horn developed from the same core network of genes as insect wings on neighbouring thoracic segments.

This ancient network of genes existed before wings, horns and even insects, in every segment along the body.

To conduct their experiment, the researchers used RNA interference to knock out genes associated with wing development in the larvae and pupae of three different beetle species: Onthophagus sagittarius, O. Taurus and O. binodis

In all three species, they were surprised to find that deactivating the genes thwarted the development of prothoracic horns as well as wings.

They also showed that manipulating other genes forced the horns to grow into “ectopic” wings – extra wings on the wrong part of the body – demonstrating that the horns and wings both derived from the same gene network.

“This work forces us to rethink what we mean by ‘novelty’,” says senior author Armin Moczek. “Each insect segment possesses this gene network, and as such, it is an ancient feature of their makeup.

“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.”

In a related commentary, H. Frederik Nijhout from Duke University, US, writes that scientists have long suspected novel features don’t arise from new genetic mutations as hypothesised, but until now, a better explanation was lacking.

The new discovery builds on insights by modern developmental genetics, which according to Nijhout “reveal[s] that a surprisingly small toolbox of regulatory genes controls the development of exceptionally diverse and seemingly unrelated structures”.

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