Circadian rhythms in visual responsiveness in the behaviourally arrhythmic Drosophila clock mutant ClkJrk

An organism’s biological day is characterised by a pattern of anticipatory physiological and behavioural changes that are governed by circadian clocks to align with the 24-hour cycling environment. Here, we used flash electroretinograms (ERGs) and Steady State Visually Evoked Potentials (SSVEPs) to examine how visual responsiveness in wild-type Drosophila melanogaster and the circadian clock mutant ClkJrk varies over circadian time. We show that the ERG parameters of wild-type flies vary over the circadian day with a higher luminance response during the subjective night. The SSVEP response that assesses contrast sensitivity also showed a time of day dependence including two prominent peaks within a 24-hour period and a maximal response at the end of the subjective day, indicating a trade-off between luminance and contrast sensitivity. Moreover, the behaviourally arrhythmic ClkJrk mutants maintained a circadian profile in both luminance and contrast sensitivity but unlike the wild-types, which show bimodal profiles in their visual response, ClkJrk flies show a weakening of the bimodal character with visual responsiveness tending to peak once a day. We conclude that the ClkJrk mutation mainly affects one of two functionally coupled oscillators, and that the visual system is partially separated from the locomotor circadian circuits that drive bouts of morning and evening activity. As light exposure is a major mechanism for entrainment, our work suggests that a detailed temporal analysis of electrophysiological responses is warranted to better identify the time window at which circadian rhythms are most receptive to light-induced phase shifting.