RHODOPSIN 7: An ancient non-retinal photoreceptor for contrast vision, darkness detection, and circadian regulation

1 Abstract Animals rely on light not only for vision but also to adapt behavior to their environment. In Drosophila, most rhodopsins mediate retinal image formation, but RHODOPSIN 7 (RH7) is widely expressed in the brain and optic lobe, acting as a non-retinal light sensor reminiscent of mammalian melanopsin. We combined expression mapping, behavioral assays, and phylogenetic comparisons to investigate its function. RH7 detects light-on and light-off transitions, modulates contrast, motion, and brightness perception, and contributes to circadian clock synchronization. Flies lacking RH7 are less active during the dark phase, while strains with constitutively active protein show increased activity. Phylogenetic analyses suggest that RH7 is an ancient rhodopsin present across panarthropods, representing an intermediate between G protein-coupled receptors and specialized rhodopsins, highlighting a key step in the evolution of non-visual light perception. These findings show that RH7 functions as a melanopsin-like sensor, shaping behavior and illuminating the origins of light detection. Competing Interest Statement The authors have declared no competing interest. Footnotes In this revised version, we have made only minor edits to the manuscript. Specifically, we have: Revised the abstract for clarity and conciseness. Corrected minor citation inconsistencies. Adjusted figure alignments and formatting for improved readability. No changes were made to the data, analyses, or conclusions.