Identification and Characterization of The Photorecetor of Insect Circadian Clock

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Locomotor activity of the adult normal and mutant fruit flies Drosophila melanogaster was investigated under various lighting conditions.Normal flies were given a short light flash of various intensities during the subjective night when the circadian locomotor activity decreases to the minimum level.An intense light induced both (1) an increase of locomotor activity during the light stimulation and (2) a phase-shift of the circadian locomotor activities as a result of the entrainment by the light flash.Flashes of lower intensity, however, induced only the increase of locomotor activity during the stimulation without phase-shifting the circadian activity.Eye mutants, like sine oculis or eye-missing where the structures of the total compound eyes are missing, did not show the direct light response to the flash while they showed almost normal phase-shift of the circadian activity.The dissociation of the locomotor activities in the normal flies and the absence of the direct light response in the eye-less mutants suggest (1) that the photoreceptor (s) of the circadian clock is not the compound eyes and (2) that the direct light response of the locomotor activity is nevertheless controlled by the input from the compound eyes.The ocelli-less mutants also showed a normal phase-shift in response to the light flash suggesting that the ocelli are not involved in the photoresponse of the circadian clock.While the nature of the photoreceptor of the circadian clock still remained unclear, this study has revealed the presence an input from the visual system affecting the locomotor activity without resetting the activity.