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

Rhodopsins are G protein-coupled receptors (GPCRs) specialized for light detection. In Drosophila, most rhodopsins are expressed in the retina and mediate image formation. However, RHODOPSIN 7 (RH7) exhibits a wider distribution, particularly in the optic lobe and in certain brain neurons, including a subset of dorsal clock neurons (DNs). In contrast to the classical rhodopsins, RH7 acts outside the retina and perceives ambient light for behavioral adaptation. We investigated its role using phylogenetic and sequence analyses, expression studies, and a series of behavioral assays. Our results show that RH7 detects simple light-on and light-off transitions and modulates circuits involved in contrast sensitivity, motion detection, and circadian synchronization, among others. The special role of RH7, independent of image formation, is supported by behavioral analyses of Rh70 mutants, Rh7-rescue experiments, and studies using the Darkfly strain. From an evolutionary perspective, RH7 potentially represents an intermediate stage between ancient GPCRs and specialized rhodopsins and provides insight into the early steps of photoreception evolution.