Extra ocular photic entrainment in Drosophila melanogaster.

The ability to anticipate the daily changes occurring in the photic environment, by adjusting their physiology and behavior accordingly, should provide living organisms with a selective advantage. Organisms could thus sample the relevant entraining stimuli at early dawn and/or late dusk. At these times of the day the principal spectral components of sunlight reaching the Earth's surface consist of relatively low levels of irradiance in the blue and red parts of the visible spectrum. Biological circadian systems could entrain to twilight conditions by sampling either one or both of these components. Previous studies, based on the characterization of phase response curves, following brief exposures to relatively high intensity light of varying wavelength, demonstrated the particular sensitivity of the Drosophila circadian system to blue light. In this study, we addressed the capacity of the circadian system of Drosophila to respond to long periods of exposure to red light. Thus flies were initially exposed to 12 h : 12 h LD cycles with full spectrum white light. Following a 1-4 h phase shift, the flies were exposed to LD cycles with red light. Our results suggest that, under these experimental conditions, red light of wavelength between 650-700 nm, can function as an entraining stimulus. Furthermore, analysis of the circadian locomotor activity profiles in visually impaired flies, suggests that in wild type flies locomotor activity is triggered by the circadian clock at key times during the day. Once triggered, the whole cycle (i.e. onset, peak and offset) of locomotor activity occurring both at dawn and dusk can proceed autonomously. However, the occurrence of a lights-off signal (typically at dusk) before the autonomous cessation of locomotor activity, leads to a light-driven termination of such activity. In addition, so1 (eyeless) mutant flies show the presence of a single evening locomotor activity peak during the whole circadian day, suggesting that in wild type flies the morning and evening activity peaks may be under separate control.