Project Summary/Abstract: Stem cells are required for tissue homeostasis and regeneration. Accomplishing these tasks requires intimate association with a niche, a cellular microenvironment that forms in a specialized tissue location with precise morphology to enable communication with stem cells. Niches are formed by cells that must be specified for niche identity, receive signals directing migration to the appropriate tissue compartment, and respond to those cues with changes in gene expression and cytoskeletal behavior. Studying this has proven challenging, as most niches are established during embryogenesis when the tissue is inaccessible to live imaging. Under previous GM funding, my lab work established an in vivo live imaging method to enable exploration of each of these facets in the assembling Drosophila testis niche, a tractable and conserved model. Foundational studies in the adult testis have repeatedly unveiled concepts that apply to other systems, yet before my work, we did not know how this niche formed. Our method permits direct in vivo visualization, revealing discreet steps of morphogenesis. This application will perform lab work to investigate the underlying mechanisms for each step. We ask (1) How are niche cells specified? (2) How do regulators of the cytoskeleton enable niche morphogenesis? and (3) What signals direct the location of niche assembly? My previous work showed that signals from adjacent visceral muscle (Vm) are required to assemble the testis niche during embryogenesis. In response to signals, niche cells express the transcription factor islet (mammalian ortholog, Isl1), which I found polarizes F-actin and regulates anterior niche assembly. An open question is whether F-actin polarization directly enables niche morphogenesis, or if it is polarized in consequence of niche assembly. This application will harness our in vivo imaging protocol along with an incisive optogenetic approach to test direct contributions of cytoskeletal regula