Fully funded PhD position available:

Circadian docks orchestrate daily rhythms of physiological and behavioral processes in almost ail living organisms. This régulation relies on a molecular mechanism featuring transcriptional feedback loops, resulting in oscillations of clock gene expression. In Drosophi/a melanogaster, about 150 clock neurons form a self-sustaining mechanism, synchronizing through inter-neuronal communication to create a unified neuronal network and robust behavioral rhythms. A key aspect is the clock's ability to synchronize with day-night cycles by using external eues, or zeitgebers, with light being the most influential. Under 12/12 light-dark cycle, flies are crepuscular with morning and evening activity peaks during dawn and dusk, separated by daytime sleep aka siesta and nighttime sleep. Drosophila circadian photoreception involves two pathways: one utilizing the blue-light-sensitive photoreceptor Cryptochrome (Cry) expressed in approximately half of clock neurons, and the other involving the visual System expressing Rhodopsins.This thesis explores the régulation of the sleep-wake cycle and clock resetting through light input pathways, specifically focusing on the visual System. It includes three parts. The first part aimed at unravelling how Rhodopsin-dependent light pathways reset the circadian oscillator at the molecular level in the different clock neurons. Although it is known that light-activated Cry binds to the Timeless (Tim) clock protein, leading to circadian molecular clock resetting, the mechanism employed by visual System pathways remains elusive. Using cry mutants and a prolonged light puise protocol, the study analysed the initial molecular changes in clock neurons after light exposure. Results demonstrated that the visual System pathway also targets Tim, with its levels decreasing in response to light. The second part focused on the rôle of individual Rhodopsin-expressing photoreceptors in circadian entrainment under different light intensifies. Multi-mutant flies expressing only one of the six rhodopsins were used to discern their contributions. In low light conditions, where the NorpA-dependent phototransduction pathway is active, Rh1 and Rh6 emerged as the primary contributors to entrainment. Contrastingly, in high light conditions involving both canonical and non-canonical phototransduction pathways, ail six rhodopsin- expressing photoreceptors were found to provide entrainment, with Rh1, Rh5, and Rh6 being the most efficient in this process. The third part of this study focuses on the régulation of sleep via light in Drosophila, where sleep is characterized by sustained periods of quiescence, or 5-minute inactivity intervals associated with an increased arousal threshold. This régulation is orchestrated by the interplay of two processes: the homeostatic System and the circadian System. The research delves into the rôle of light, particularly through the light receptors Cry and the visual System, under high light intensity conditions. Additionally, the study explores the involvement of different phototransduction pathways and histaminergic downstream pathways in this sleep régulation process. The findings suggest that both Cry and the visual System are sufficient and required for the proper functioning of sleep. In contrast to entrainment, the canonical phototransduction pathway and the two histamine receptors are required to regulate the siesta effectively.