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.

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We focus on a photorefractive response under the influence of temperature in conjunction with reduction. A series of reduced BaTiO 3 :Rh crystals is made. Their optical absorptions, dark decays, and light-induced decays are investigated. The temperature-dependent dark decay and light-induced decay are performed by an argon ion laser operated at 514.5 nm. The experimental results show that the photorefractive response can be improved by reduction in cooperation with the elevation of temperature. For different types of Rh-doped barium titanate crystals, the elevation of temperature has different influences in their photorefractive responses. The sample reduced at 10 Ϫ14 atm has the best response of 15 ms at 100°C. © 2004 Society of Photo-Optical Instrumentation Engineers.