Visceral mesoderm signaling regulates assembly position and function of the Drosophila testis niche

Tissue homeostasis often requires a properly placed niche to support stem cells. The morphogenetic processes that position a niche are just being described. We recently showed that Drosophila testis pro niche cells, specified at disparate positions during early gonadogenesis, must assemble in one collective at the gonad anterior. Here, we identify Slit and FGF signals emanating from adjacent visceral mesoderm (Vm) that regulate assembly. In response to signaling, niche cells express islet, which we find is also required for positioning the niche. Without signaling, niche cells specified furthest from the anterior are unable to migrate, remaining dispersed. Function of the dispersed niche is severely disrupted, with pro-niche cells evading cell cycle quiescence, compromised in their ability to signal the incipient stem cell pool, and failing to orient stem cell divisions properly. Our work identifies both extrinsic signaling and intrinsic responses required for proper assembly and placement of the testis niche.