Brain circadian clock neurons drive fitness advantages in Drosophila.

The adaptive significance of circadian clocks is widely assumed due to their ubiquity; yet, direct empirical evidence remains scarce. Evaluating these benefits is often confounded by pleiotropic effects in conventional circadian null mutants. To address this, we selectively altered the circadian period exclusively within brain clock neurons in Drosophila melanogaster. Multi-generational competition assays revealed that flies with aberrant rhythms exhibit a significant fitness disadvantage under standard light-dark (LD 12:12) cycles. This disadvantage was abolished under constant light, confirming that the selection pressure is specifically mediated by the circadian clock. Furthermore, paternity assays conducted under LD 12:12 indicated that the timing of brain clock neurons influences male reproductive success, providing a potential mechanistic link between clock-controlled behavior and fitness. Intriguingly, we found that these fitness costs are highly photoperiod-dependent. Under short-day conditions (LD 8:16), the short-period strain (dbt S ) maintained a significantly higher overall frequency than the long-period strain (dbt L ). Our behavioral observations suggest that this difference may be associated with the quality of activity rhythms; specifically, dbt S lacked defined morning peaks and showed suppressed nocturnal activity, potentially narrowing its window for reproductive interactions compared to dbt L . These findings illustrate that the circadian system does not merely track a 24-h cycle but functions as a flexible feature that enables flies to cope with changing day lengths by aligning their mating behavior with the most favorable time of day.