A Distinct Visual Pathway Mediates High-Intensity Light Adaptation of the Circadian Clock in Drosophila.

To provide organisms with a fitness advantage, circadian clocks have to react appropriately to changes in their environment. High-intensity (HI) light plays an essential role in the adaptation to hot summer days, which especially endanger insects of desiccation or prey visibility. Here, we show that solely increasing light intensity leads to an increased midday siesta in Drosophila behavior. Interestingly, this change is independent of the fly's circadian photoreceptor cryptochrome and is solely caused by a small visual organ, the Hofbauer-Buchner eyelets. Using receptor knock-downs, immunostaining, and recently developed calcium tools, we show that the eyelets activate key core clock neurons, namely the s-LN_vs, at HI. This activation delays the decrease of PERIOD (PER) in the middle of the day and propagates to downstream target clock neurons that prolong the siesta. We show a new pathway for integrating light-intensity information into the clock network, suggesting new network properties and surprising parallels between Drosophila and the mammalian system.SIGNIFICANCE The ability of animals to adapt to their ever-changing environment plays an important role in their fitness. A key player in this adaptation is the circadian clock. For animals to predict the changes of day and night, they must constantly monitor, detect and incorporate changes in the environment. The appropriate incorporation and reaction to high-intensity (HI) light is of special importance for insects because they might suffer from desiccation during hot summer days. We show here that different photoreceptors have specialized functions to integrate low-intensity, medium-intensity, or HI light into the circadian system in Drosophila These results show surprising parallels to mammalian mechanisms, which also use different photoreceptor subtypes to respond to different light intensities.