The Tbx1 ortholog org-1 is required to establish testis stem cell niche identity in Drosophila.

Stem cells require signals from a cellular microenvironment known as the niche that regulates identity, location and division of stem cells. Niche cell identity must be properly specified during development to form a tissue capable of functioning in the adult. Here, we show that the Tbx1 ortholog org-1 is expressed in Drosophila testis niche cells in response to Slit and FGF signals. org-1 is expressed during niche development and is required to specify niche cell identity. org-1 mutants specified fewer niche cells, and those cells showed disruption of niche-specific markers, including loss of the niche adhesion protein Fas3 and reduced hedgehog expression. We found that org-1 expression in somatic gonadal precursors is capable of inducing formation of additional niche cells. Disrupted niche identity in org-1 mutants caused defects in niche assembly and functionality. We found that the conserved transcription factor islet is expressed in response to org-1 and show that islet functions downstream to mediate niche identity and assembly. This work identifies a previously unreported role for org-1 in niche establishment.