Photoreceptors for immediate effects of light on circadian behaviour

Animals need to sharpen behavioural output in the adaptation to a variable environment. Hereby, light is one of the most pivotal environmental signals and thus behavioural plasticity in response to light can be observed in diurnal animals, including humans. Furthermore, light is the main entraining signal of the clock, yet immediate effects of light enhance or overwrite circadian output and thereby mask circadian behaviour.In Drosophila, such masking effects are most evident as a lights-on response in two behavioural rhythms – the emergence of the adult insect from the pupa, called eclosion, and the diurnal rhythm of locomotor activity. Here, we show that the immediate effect of light on eclosion depends solely on R8 photoreceptors of the eyes. In contrast, the increase in activity by light at night is triggered by different cells and organs, that seem to compensate for the loss of each other, potentially to ensure behavioural plasticity.