Salvador-Warts-Hippo Signaling Promotes Drosophila Posterior Follicle Cell Maturation Downstream of Notch

The Salvador Warts Hippo (SWH) network limits tissue size in Drosophila and vertebrates [1]. Decreased SWH pathway activity gives rise to excess proliferation and reduced apoptosis. The core of the SWH network is composed of two serine/threonine kinases Hippo (Hpo) and Warts (Wts), the scaffold proteins Salvador (Sav) and Mats, and the transcriptional coactivator Yorkie (Yki) [1]. Two band 4.1 related proteins, Merlin (Mer) and Expanded (Ex), have been proposed to act upstream of Hpo, which in turn activates Wts ([1] for review). Wts phosphorylates and inhibits Yki, repressing the expression of Yki target genes [2-4]. Recently, several planar cell polarity (PCP) genes have been implicated in the SWH network in growth control [5-8]. Here, we show that, during oogenesis, the core components of the SWH network are required in posterior follicle cells (PFCs) competent to receive the Gurken (Grk)/TGFalpha signal emitted by the oocyte to control body axis formation. Our results suggest that the SWH network controls the expression of Hindsight, the downstream effector of Notch, required for follicle cell mitotic cycle-endocycle switch. The PCP members of the SWH network are not involved in this process, indicating that signaling upstream of Hpo varies according to developmental context.