Researchers have intentionally genetically modified a common beetle to develop a third functional eye, right in the middle of its forehead. Unravelling the biological mechanisms behind this occurrence could help researchers understand how evolution draws upon pre-existing developmental and genetic "building blocks" to create novel complex traits, or "old" traits in novel places. The study, published in the journal PNAS, also provides deeper insights into an earlier experiment that accidentally produced an extra eye as part of a study to understand how the insect head develops. "Developmental biology is beautifully complex in part because there is no single gene for an eye, a brain, a butterfly's wing or a turtle's shell," said Armin P Moczek, professor at Indiana University (IU) in the US. "Instead, thousands of individual genes and dozens of developmental processes come together to enable the formation of each of these traits," said Moczek. As a consequence, the evolution of novel features often requires many fewer genetic changes than biologists originally thought. Researchers grew an extra functional eye - technically, a "fusion" of two sets of extra eyes - following the knockdown of a single gene, a technique widely available to scientists in most organisms. The unexpected formation of a complex, functional eye in a novel location in the process is "a remarkable example of the ability of developmental systems to channel massive perturbations towards orderly and functional outcomes," Moczek said. To create a fully functional eye in the centre of a beetle's head, Moczek's team deactivated a single gene called orthodenticle, or odt, which research has previously shown to play a role in instructing the formation of the head during development. "This study experimentally disrupts the function of a single, major gene," Moczek said. "And, in response to this disruption, the remainder of head development reorganises itself to produce a highly complex trait in a new place: a compound eye in the middle of the head," he said. To confirm the eye was a true extra eye, the team conducted multiple tests to prove the structure had the same cell types, expressed the same genes, grew proper nerve connections and elicited the same behavioural response as a normal eye. What makes the results so exciting - beyond the eye's Frankenstein novelty - is the relatively simple genetic technique used to achieve the gene knockdown, said IU postdoctoral researcher Eduardo E Zattara, lead author on the study.
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