RT Journal Article SR Electronic T1 Light-mediated circuit switching in the Drosophila neuronal clock network JF bioRxiv FD Cold Spring Harbor Laboratory SP 515478 DO 10.1101/515478 A1 M Schlichting A1 P Weidner A1 M Diaz A1 P Menegazzi A1 E Dalla-Benetta A1 C Helfrich-Förster A1 M Rosbash YR 2019 UL http://biorxiv.org/content/early/2019/01/08/515478.abstract AB The circadian clock is a timekeeper but also helps adapt physiology to the outside world. This is because an essential feature of clocks is their ability to adjust (entrain) to the environment, with light being the most important signal. Whereas Cryptochrome-mediated entrainment is well understood in Drosophila, integration of light information via the visual system lacks a neuronal or molecular mechanism. Here we show that a single photoreceptor sub-type is essential for long day adaptation. These cells activate key circadian neurons, namely the lLNvs, which release the neuropeptide PDF. Using a cell-specific CRISPR/Cas9 assay, we show that PDF directly interacts with neurons important for evening (E) activity timing. Interestingly, this pathway is specific for light entrainment and appears to be dispensable in constant darkness (DD). The results therefore indicate that external cues cause a rearrangement of neuronal hierarchy, which is a novel form of plasticity.