Polarized F-actin establishes cell interactions required for the formation of a stem cell niche

<jats:p>Lifelong stem cells are maintained by a cellular microenvironment called the niche, which enables tissue homeostasis. Proper niche construction is essential for persistent function, but studying niche formation is challenged by the inaccessibility of most niches to in vivo visualization during development. Innovations imaging the Drosophila testis are now allowing investigation of niche inception. F-actin polarizes to precise cell interfaces during testis niche assembly. Yet it is unknown whether polarization directs niche cell motility, or reflects adhesive sorting in response to formation of niche cell contacts. By adapting a method to optogenetically manipulate cortical F-actin via disruption of Rho1, we interrogate the role for cytoskeletal polarization during niche formation with tissue and temporal specificity. Rho1-mediated disruption of F-actin polarization caused defects in niche anterior assembly and architecture. Also, fewer cells adopted bona fide niche identity, given diminished Fas3, N-Cadherin, and Islet. These disrupted niches fail in signaling to germ cells to establish stem cell identity. We reveal that polarized F-actin is crucial for establishing cell contacts to form a functional niche, and to maintain cell identity in the developing tissue.</jats:p>