Since GW182 downregulation result in a phenotype reminiscent of those of flies with no PDF signaling, and since GW182 is expressed in both PDF-positive and -negative circadian neurons, it could affect either PDF expression/release or PDFR signaling. To distinguish between these two hypotheses, we determined the circadian neurons in which GW182 is required. We first crossed gw182 RNAi transgenic

flies to Pdf-GAL4/UAS-dcr2 (PD2) flies to downregulate GW182 only in PDF-positive circadian neurons. This tissue-specific downregulation had no effect on circadian behavior in LD and DD ( Figures 3A and 3B; Table 1), which strongly suggests that GW182 is primarily required Cell Cycle inhibitor in PDF-negative circadian neurons. Although we have previously observed that Pdf-GAL4 is as efficient as tim-GAL4 to downregulate genes in PDF-positive LNvs ( Dubruille et al., 2009), we cannot entirely exclude the possibility that there is higher residual GW182 expression in these neurons when using Pdf-GAL4. We therefore also combined TD2 with Pdf-GAL80 (PG80), to block expression of the dsRNAs in PDF-positive LNvs. The phenotypes were comparable to those with TD2 alone, although slightly weaker ( Figures 3A and 3B; Table

1). Seventy-five percent of TD2/GWRNAi-1; PG80/+ flies were arrhythmic (98% without PG80), morning peak was blunted, and the evening peak phase advanced. We therefore conclude that GW182′s primary role is in PDF-negative see more circadian neurons, which strongly suggest that Tolmetin it functions in the PDFR pathway ( Lear et al., 2009). The results presented so far strongly suggest that GW182

plays a positive role in the PDFR signaling pathway. If indeed this is the case, flies in which expression of gw182 dsRNAs is combined with a severely hypomorphic Pdfr mutation should behave similarly as single-mutant flies. If, on the contrary, GW182 and PDFR affect two separate pathways, we would expect an additive effect. Since the morning peak of activity is almost entirely eliminated in both gw182 RNAi flies and Pdfr mutant flies, and since both are almost completely arrhythmic in DD, the only phenotype that can show additive effects is the evening peak. We observed no additive effects when combining a Pdfr mutation with GW182 downregulation on the phase of evening activity ( Figures 3C and 3D). This absence of additive effect is not caused by a limitation in how early the evening peak can be advanced. Indeed, the evening peak in perS mutant flies ( Konopka and Benzer, 1971) is more advanced than in gw182 or Pdfr mutants and could even be further advanced when perS was combined with gw182 downregulation ( Figures 3C and 3D). The absence of additive effect is thus specific to the gw182-RNAi/Pdfr mutant combination and, therefore, strongly suggests that GW182 and PDFR are in the same signaling pathway.