The mechanisms by which circadian pacemaker systems transmit timing information to control behavior are largely unknown. Here, we define two critical features of that mechanism in Drosophila. We first describe animals mutant for the pdf neuropeptide gene, which is expressed by most of the candidate pacemakers (LNv neurons). Next, we describe animals in which pdf neurons were selectively ablated. Both sets of animals produced similar behavioral phenotypes. Both sets entrained to light, but both were largely arrhythmic under constant conditions. A minority of each pdf variant exhibited weak to moderate free-running rhythmicity. These results confirm the assignment of LNv neurons as the principal circadian pacemakers controlling daily locomotion in Drosophila. They also implicate PDF as the principal circadian transmitter.

A new rhythm mutation was isolated based on its elimination of per-controlled luciferase cycling. Levels of period or timeless clock gene products in the mutant are flat in daily light-dark cycles or constant darkness (although PER and TIM oscillate normally in temperature cycles). Consistent with the fact that light normally suppresses TIM, cryb is an apparent null mutation in a gene encoding Drosophila's version of the blue light receptor cryptochrome. Behaviorally, cryb exhibits poor synchronization to light-dark cycles in genetic backgrounds that cause external blindness or demand several hours of daily rhythm resets, and it shows no response to brief light pulses. cryb flies are rhythmic in constant darkness, correlating with robust PER and TIM cycling in certain pacemaker neurons.