University of California, Irvine (UCI) researchers have explored how UV, blue, and red light affect the central brain neurons that allow circadian clock timing to be reset. The researchers found that blue and UV light create a sustained light response that is key to a kind of nonimage-forming vision that averages environmental light levels to determine the time of day and inform internal circadian processes. Red light evokes a less sustained response. The researchers generated transgenic fruit flies (Drosophila melanogaster) expressing cryptochrome (dCRY), a light-sensitive protein, in the conserved electron transport chain and then measured the flies’ neuronal electrophysiological phototransduction and behavioral responses to light. Light-evoked responses in the flies indicated that nonimage-forming vision in invertebrates relies on the redox chemistry of dCRYs. Biological redox chemistry is typically associated with metabolism. “The protein ancestors of cryptochromes were ultraviolet light-activated DNA repair enzymes that appeared in evolution well over 3 billion years ago before the appearance of our present-day oxygen-rich atmosphere that protects us from harmful ultraviolet radiation,” professor Todd C. Holmes said. “These first light-sensing mechanisms evolved when single-cell organisms developed the ability to repair their DNA damaged from UV light after coming too close to the surface of water. At that time, there was no life on land. It is remarkable that this ancient form of nonimage-forming vision persists to the present day.” UCI professor Todd C. Holmes and his team use Drosophila melanogaster to discover how dCRY-mediated clock-resetting light responses affect the circadian clock. Courtesy of UCI School of Medicine. Light is the primary regulator of circadian rhythms and evokes a wide range of time-of-day specific behaviors. By gaining an understanding of how insects respond to short-wavelength light, researchers hope to develop new, environmentally friendly alternatives to controlling harmful insects, such as mosquitoes and flies, and reduce the need for toxic pesticides. The research was published in the Proceedings of the National Academy of Sciences (www.doi.org/10.1073/pnas.1905023116).