Drosophila melanogaster clock gene mutants exhibit a circadian rhythm in visual contrast response

Most organisms use a molecular timekeeping mechanism centered on the so-called “clock genes”, known to interact with one another in a 24-hour Transcriptional-Translational Feedback Loop (TTFL) to control circadian rhythms intracellularly. However, the discovery of circadian rhythmicity in the oxidation state of peroxiredoxins has suggested that an alternative metabolic oscillator may govern circadian rhythms independently of gene transcription. Although circadian rhythms have been documented in the morphology of the Drosophila visual system, much of the underlying physiology remains unclear. It was previously found that a circadian rhythm in the visual transduction amplitude of Drosophila persists in some “clock” gene mutants, indicating that the rhythm may persist independently of the TTFL. In this study the highly sensitive Steady State Visually Evoked Potential (SSVEP) assay was used to assess the visual function of the TTFL mutants ClkJrkst1 and per0 in order to determine whether a TTFL oscillator is driving oscillations in the visual contrast response of fruit flies, as well as dissect the contribution of individual neuron orders in the retina to the response. We have found that despite a complete loss of circadian rhythmicity in locomotor activity levels the ClkJrkst1 mutant exhibits robust circadian rhythms in contrast sensitivity, with a recurring peak 4 hours after anticipated light onset in the photoreceptors, lamina, and medullary neurons. We conclude that Drosophila possess a circadian rhythm in contrast sensitivity that can operate independently of clock gene transcription, and thus is likely synchronized instead by a metabolic oscillator.