The disconnected visual system mutations in Drosophila melanogaster drastically disrupt circadian rhythms.

Mutations at the disconnected (disco) locus in Drosophila melanogaster cause cultures of this insect to eclose in an essentially arrhythmic manner and also nearly eliminate free-running circadian rhythms of locomotor activity. Yet disco mutants are not totally light-insensitive: Whereas they performed very poorly in tests of certain behavioral responses to visual stimuli, they were able to exhibit "forced" periodic locomotor activity under conditions of light-dark cycling. We discuss these results in the context of (1) the dispensability of this insect's external photoreceptors for entrainment of its circadian pacemaker, and (2) possible disco-induced abnormalities in the connections of extraocular photoreceptors to their targets in the central nervous system and/or abnormalities in the targets themselves--which presumably include elements of the fly's circadian clock.