Abstract
Circadian rhythms are entrained by light to follow the daily solar cycle. We show that
Drosophila uses at least three light input pathways for this entrainment: (1) cryptochrome, acting in the pacemaker cells themselves, (2) the compound eyes, and (3) extraocular photoreception, possibly involving an internal structure known as the Hofbauer-Buchner eyelet, which is located underneath the compound eye and projects to the pacemaker center in the brain. Although influencing the circadian system in different ways, each input pathway appears capable of entraining circadian rhythms at the molecular and behavioral level. This entrainment is completely abolished in
glass
60j cry
b
double mutants, which lack all known external and internal eye structures in addition to being devoid of cryptochrome.