Novel masking effects of light are revealed in Drosophila by skeleton photoperiods

Here, we show that in a skeleton photoperiod where all midday light is removed from a standard laboratory 12:12 LD photoperiod, a large diurnal peak of activity is revealed that is continuous with the E peak seen in constant dark (DD). We further show that the circadian clock gene tim regulates light-dependent masking of daytime activity, but the clock gene per does not. Finally, relative to wild-type flies, mutants for both of these clock genes showed increased nighttime activity in the skeleton photoperiod but not in the standard photoperiod. This result suggests that nighttime activity is suppressed by the intact circadian clock, and in its absence, by exposure to a standard photoperiod. These results support and extend the literature addressing the complex interactions between masking and clock-controlled components of overt circadian rhythms.