The eye is often put on a pedestal as an example of the complexity of nature, but evolution has its shortcuts. A new experiment from Indiana University has shown how natural selection can create complex traits out of existing genetic "building blocks", and to illustrate this the team used a relatively simple genetic tool to grow a functional third eye on the forehead of beetles.

Bugs have been given extra eyes in past genetic studies, but it takes a lot of tinkering and the end results never function like normal eyes anyway. In those experiments, major regulatory genes need to be activated in that new part of the body, like a leg or wing. The IU researchers liken the process to building with Lego bricks.

"Evolving a novel physical trait is much like building a novel structure out of Legos, by re-using and recombining 'old' genes and developmental processes within new contexts," says Armin Moczek, senior author of the new study. "You can make new things over and over or in new places using the same old set of 'bricks.' But in Legos, we know the rules of assembly: which pieces go together and which things don't. In biology, we still struggle to understand the respective counterparts."

The experiment builds on previous work that grew an extra eye accidentally. In that study, the researchers shut off a gene known as orthodenticle (otd), which affects development of an organism's head. They expected that the altered beetles wouldn't grow horns, but they didn't expect that an extra set of eyes would grow in their place.

This time around, the team deliberately disrupted that gene, to fuse that extra pair into one compound eye. Then, they examined the new organ's function, using several tests to confirm that it had the same cell types, expressed the same genes, grew the required nerve connections and behaved like a normal eye. Best of all, the researchers say, is that it was relatively simple to do, requiring a minor tweak of a gene that's common to most organisms.

"This study experimentally disrupts the function of a single, major gene," says Moczek. "And, in response to this disruption, the remainder of head development reorganizes itself to produce a highly complex trait in a new place: a compound eye in the middle of the head. Moreover, the darn thing actually works!"

While the three-eyed beetles sound more like a sideshow attraction than a useful scientific creation, the researchers say that the experiment advances our understanding of how complex traits can evolve, and might open doors for the development of artificial organs for transplantation.

The research was published in the journal Proceedings of the National Academy of Sciences.

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