<i>miR-124</i> acts During <i>Drosophila</i> Development to Determine the Phase of Adult Circadian Behavior

The circadian clock enables organisms to optimize their metabolism, physiology, and behavior with the time-of-day. However, circadian rhythms benefit organisms only if they are properly synchronized with the day/night cycle; circadian misalignment can have detrimental effects on animals' wellbeing and survival. We previously showed that in <i>Drosophila</i>, loss of the microRNA <i>miR-124</i> advances the phase of circadian evening locomotor activity by several hours under constant darkness conditions. Interestingly, we now report that loss of <i>miR-124</i> also delays morning activity under a light/dark cycle with a short photoperiod. We recapitulated these opposite phase phenotypes by eliminating <i>miR-124</i> during larval development, but not when this microRNA is lost during pupation to adulthood. The loss of <i>miR-124</i> results in significant miswiring within the circadian neural network and severely alters neural activity rhythms in the ventral Lateral Neurons (s-LNvs) and the posterior Dorsal Neurons 1 (DN1ps), which control the timing of morning and evening activity. Silencing the s-LNvs in <i>miR-124</i> mutant flies restores the phase of evening activity, while activating the DN1ps rescues the phases of both morning and evening activities. Our findings thus reveal the pivotal role of <i>miR-124</i> in sculpting the <i>Drosophila</i> circadian neural network during development and its long-lasting impact on circuit activity and adult circadian behavior.