Ecdysone Regulation of Stem Cell Maintenance in the Drosophila Testis Niche. Yijie Li, Qing Ma, Erika Matunis*. Dept of Cell Biology, 725 N. Wolfe Street, The Johns Hopkins University School of Medicine, Baltimore, MD, 21205.

   The Drosophila testis is a good model to study stem cell biology within an intact local microenvironment, or niche. In this tissue, quiescent somatic cells called hub cells create a niche for two types of stem cells: germline stem cells (GSCs) and cyst stem cells (CySCs). Although several local signals are known to regulate stem cells in the testis, the roles of systemic signals in this tissue are less well understood. Ecdysone is critical for development and for oogenesis, but was not thought to function in the testis niche. Here, we find that the Ecdysone receptor (EcR) and Ultraspiracle (USP), the two components of the ecdysone receptor complex, are expressed and activated in the Drosophila testis niche. We also find that EcR and its downstream targets ftz-f1 and E75 are required in the CySC lineage and hub cells for GSC and CySC maintenance. Furthermore, the EcR co-activator Taiman (Tai) is also expressed in the hub and required in the hub for stem cell maintenance. However, Tai is not autonomously required in the CySC or GSC lineage, indicating that different co-activators likely act in hub cells and stem cells. Finally, overactivation of Tai in the CySC or GSC lineage leads to CySC and GSC loss, likely through apoptosis, while overactivation in the hub has no effect. Together, our work indicates that ecdysone signaling promotes stem cell maintenance in Drosophila testis niche by acting both autonomously within CySCs, and non-autonomously in hub cells with distinct co-activators function in the hub and CySCs.