Drosophila photoreceptors contain an autonomous circadian oscillator that can function without period mRNA cycling

Circadian oscillations in period (per) mRNA and per protein (PER) constitute, in part, a feedback loop that is required for circadian pacemaker function in Drosophila melanogaster. Oscillations in PER are required for oscillations in per mRNA, but the converse has not been rigorously tested because of a lack of measurable quantities of per mRNA and protein in the same cells. This circadian feedback loop operates synchronously in many neuronal and non-neuronal tissues, including a set of lateral brain neurons (LNs) that mediate rhythms in locomotor activity, but whether a hierarchy among these tissues maintains this synchrony is not known. To determine whether per mRNA cycling is necessary for PER cycling and whether cyclic per gene expression is tissue autonomous, we have generated per01 flies carrying a transgene that constitutively expresses per mRNA specifically in photoreceptors, a cell type that supports feedback loop function. These transformants were tested for different aspects of feedback loop function including per mRNA cycling, PER cycling, and PER nuclear localization. Under both light/dark (LD) cycling and constant dark (DD) conditions, PER abundance cycles in the absence of circadian cycling of per mRNA. These results show that per mRNA cycling is not required for PER cycling and indicate that Drosophila photoreceptors R1-R6 contain a tissue autonomous circadian oscillator.