Light has profound behavioral effects on almost all animals, and nocturnal animals show sensitivity to extremely low light levels 1, 2, 3, 4. Crepuscular, i.e., dawn/dusk-active animals such as Drosophila melanogaster are thought to show far less sensitivity to light 5, 6, 7, 8. Here we report that Drosophila respond to extremely low levels of monochromatic blue light. Light levels three to four orders of magnitude lower than previously believed impact circadian entrainment and the light-induced stimulation of locomotion known as positive behavioral masking. We use GAL4;UAS-mediated rescue of tyrosine hydroxylase (DTH) mutant (ple) flies to study the roles of dopamine in these processes. We present evidence for two roles of dopamine in circadian behaviors. First, rescue with either a wild-type DTH or a DTH mutant lacking neural expression leads to weak circadian rhythmicity, indicating a role for strictly regulated DTH and dopamine in robust circadian rhythmicity. Second, the DTH rescue strain deficient in neural dopamine selectively shows a defect in circadian entrainment to low light, whereas another response to light, positive masking, has normal light sensitivity. These findings imply separable pathways from light input to the behavioral outputs of masking versus circadian entrainment, with only the latter dependent on dopamine.
