The Drosophila Female Germline Stem Cell Lineage Acts to Spatially Restrict DPP Function Within the Niche

Drosophila ovarian stem cells communicate with somatic cells to define the size of the stem cell niche. Identifying with One’s Niche The stem cell niche in the Drosophila germarium (which produces egg chambers) supports two to three self-renewing germline stem cells (GSCs) surrounded by various types of somatic cells. The morphogen Decapentaplegic (DPP), which is produced by somatic cells in the niche, defines the spatial limits of the niche. Cells inside the niche are exposed to high concentrations of DPP and are maintained as stem cells, whereas cells located outside the niche receive lower concentrations and differentiate. However, it has been unclear how DPP activity is spatially restricted. Liu et al. found that when GSCs lacked functional Stet, an intramembrane protease that processes precursors of EGFR ligands into their mature forms, epidermal growth factor receptor (EGFR) signaling in somatic cells decreased, thereby increasing the expression of a glypican necessary for DPP stability and transport. Thus, mutant germaria for stet contained more cells that were activated by DPP and had more GSC-like cells than did wild-type germaria, indicating an expanded range for DPP function and, consequently, niche activity. These results suggest that GSCs play an active role in defining the size of the niche. Maintenance of stem cells requires spatially restricted, niche-associated signals. In the Drosophila female germline stem cell (GSC) niche, Decapentaplegic (DPP) is the primary niche-associated factor and functions over a short range to promote GSC self-renewal rather than differentiation. Here, we show that the GSC lineage and, more specifically, the stem cells themselves participate in the spatial restriction of DPP function by activating epidermal growth factor receptor (EGFR)–mitogen-activated protein kinase (MAPK) signaling in the surrounding somatic cells. EGFR-MAPK signaling in somatic cells repressed the expression of dally, which encodes a glypican required for DPP movement and stability. Consequently, only GSCs close to the DPP source (the somatic cells in the niche) showed high signal activation and were maintained as stem cells, whereas cystoblasts outside the niche showed low signal activation and initiated differentiation. Thus, our data reveal that the reciprocal crosstalk between the GSCs and the somatic cells defines the spatial limits of DPP action and therefore the extent of the GSC niche.