YAP/Yorkie in the germline modulates the age-related decline of germline stem cells and niche cells.

The properties and behaviour of stem cells rely heavily on signaling from the local microenvironment. At the apical end of Drosophila testis, self-renewal and differentiation of germline stem cells (GSCs) are tightly controlled by distinct somatic cells that comprise a specialised stem cell niche known as the hub. The hub maintains GSC homeostasis through adhesion and cell signaling. The Salvador/Warts/Hippo (SWH) pathway, which suppresses the transcriptional co-activator YAP/Yki via a kinase cascade, is a known regulator of stem cell proliferation and differentiation. Here, we show that increasing YAP/Yki expression in the germline, as well as reducing Warts levels, blocks the decrease of GSC numbers observed in aging flies, with only a small increase on their proliferation. An increased expression of YAP/Yki in the germline or a reduction in Warts levels also stymies an age-related reduction in hub cell number, suggesting a bilateral relationship between GSCs and the hub. Conversely, RNAi-based knockdown of YAP/Yki in the germline leads to a significant drop in hub cell number, further suggesting the existence of such a SC-to-niche relationship. All together, our data implicate the SWH pathway in Drosophila GSC maintenance and raise questions about its role in stem cell homeostasis in aging organisms.