Old genes can learn new tricks

Washington, May 12 (ANI): Indiana University Bloomington researchers have challenged the age-old belief among evolutionary biologists that fundamental genes do not acquire new functions, reporting that two ancient genes were "co-opted" to help build a new trait in beetles - the fancy antlers that give horned beetles their name.

Biologist Armin Moczek and research associate Debra Rose say that the genes, Distal-less and homothorax, touch most aspects of insect larval development, and have, therefore, been considered off-limits to the evolution of new traits.

They studied two horned beetle species Moczek and Rose, and found that the genetic sequences of Distal-less and homothorax were hardly different, suggesting that the two genes have retained their unique identities because of selective pressures not to change.

The researchers said that what changed was not the genes themselves, but when and where they are turned on.

"Evolutionary biologists have a good idea of what it takes to change the shape of a wing, the length of a leg, or the anatomy of an eye. What we have struggled with, though, is how these traits originate in the first place. How do you evolve that first wing, limb or photoreceptor from a flightless, limbless and blind ancestor?" Moczek said.

The research team investigated these questions by examining three development genes that are so old that all insects have them: Distal-less, homothorax and a third, dachshund.

Moczek said that in horned beetles, each of the three genes is likely to have hundreds to thousands of downstream targets.

A tenuous consensus among evolutionary biologists was that such genes could not be easily modified because any modification would affect countless aspects of the insect's development, and reduce its fitness relative to its peers.

Moczek and Rose's PNAS paper confirms one aspect of this idea. All three genes were sequenced and found to be unchanged, not only among the individuals of each beetle species they examined, but also between the two species, Onthophagus taurus (Italy) and Onthophagus binodis (South Africa), whose lineages diverged about 24 million years ago.

The researchers, however, said that that was not the whole story.

They said that to understand the effects of the three genes on horned beetle development, they employed a new and promising technique, RNA interference, which disables the action of specific genes without compromising other genetic processes.

The team divided beetle larvae of both species into three treatment groups: no injection, buffer injection with nonsense RNA and buffer injection with RNA interference transcripts designed to disrupt one of three crucial developmental genes.

Moczek and Rose found two of the three genes - Distal-less and homothorax - to be used by both O. taurus and O. binodis in the development of beetle horns. While Distal-less was found to affect both the development of thorax horns (which form just behind the head) and head horns, homothorax was only found to influence thorax horn development.

The gene dachshund appears to have no effect whatsoever on horn development in either species.

"The evolution of novel features does not require the evolution of novel genes. A lot of innovation can grow from within the organism's genetic toolbox," Moczek said.

The researchers also learnt that all developmental genes were candidates for such recruitment, not just the genes whose development functions are considered non-essential or limited in their effects.

Moczek says that this study may compel evolutionary biologists to revisit pleiotropy, the foundational concept of one gene influencing many traits.

"It may be that our understanding of pleiotropy is too simplistic. Now that we know fundamental development genes can acquire new and diverse functions with relative ease, pleiotropy may not be nearly as constraining as we have thought," Moczek said.

The study has been published in the Proceedings of the National Academy of Sciences. (ANI)