The Hippo pathway acts downstream of the Hedgehog signaling to regulate follicle stem cell maintenance in the Drosophila ovary.

The Hippo pathway is conserved and plays important roles in organ size control. The core components of the Hippo pathway are two kinases Hippo (Hpo), Warts (Wts), and a transcription-co-activator Yorkie (Yki). Yki activity is regulated by phosphorylation, which affects its nuclear localization and stability. To determine the role of the Hippo pathway in stem cells, we examine follicle stem cells (FSCs) in the Drosophila ovary. Yki is detected in the nucleus of FSCs. Knockdown of yki in the follicle cell lineage leads to a disruption of the follicular epithelium. Mitotic clones of FSCs mutant for hpo or wts are maintained in the niche and tend to replace the other FSCs, and FSCs mutant for yki are rapidly lost, demonstrating that the Hippo pathway is both required and sufficient for FSC maintenance. Using genetic interaction analyses, we demonstrate that the Hedgehog pathway acts upstream of the Hippo pathway in regulating FSC maintenance. The nuclear localization of Yki is enhanced when the Hedgehog signaling is activated. Furthermore, a constitutively active but not a wild-type Yki promotes FSC maintenance as activation of the Hedgehog signaling does, suggesting that the Hedgehog pathway regulates Yki through a post-translational mechanism in maintaining FSCs.

The Hippo pathway controls polar cell fate through Notch signaling during Drosophila oogenesis.

During Drosophila oogenesis, the somatic follicle cells form an epithelial layer surrounding the germline cells to form egg chambers. In this process, follicle cell precursors are specified into polar cells, stalk cells, and main-body follicle cells. Proper specification of these three cell types ensures correct egg chamber formation and polarization of the anterior-posterior axis of the germline cells. Multiple signaling cascades coordinate to control the follicle cell fate determination, including Notch, JAK/STAT, and Hedgehog signaling pathways. Here, we show that the Hippo pathway also participates in polar cell specification. Over-activation of yorkie (yki) leads to egg chamber fusion, possibly through attenuation of polar cell specification. Loss-of-function experiments using RNAi knockdown or generation of mutant clones by mitotic recombination demonstrates that reduction of yki expression promotes polar cell formation in a cell-autonomous manner. Consistently, polar cells mutant for hippo (hpo) or warts (wts) are not properly specified, leading to egg chamber fusion. Furthermore, Notch activity is increased in yki mutant cells and reduction of Notch activity suppresses polar cell formation in yki mutant clones. These results demonstrate that yki represses polar cell fate through Notch signaling. Collectively, our data reveal that the Hippo pathway controls polar cell specification. Through repressing Notch activity, Yki serves as a key repressor in specifying polar cells during Drosophila oogenesis.