Stem cell specification and maintenance is necessary for proper organ development and tissue homeostasis. The testes of the fruit fly Drosophila melanogaster is one of the most thoroughly characterized systems for studying this process. The two populations of stem cells in the Drosophila testes, the germline stem cells (GSCs) and a somatic population of cyst stem cells (CySCs), are both localized to the anterior of the testes in a stem cell niche. The niche proper (also called the hub), GSCs, and CySCs communicate with each other to balance stem cell maintenance and differentiation. In order to determine how such a system forms, recent experiments have investigated GSC development and found that functional, asymmetrically dividing GSCs are first established from primordial germ cells (PGCs) around 23 hours after egg laying at the embryo to larval transition (Sheng et al, 2009). This occurs at the same time that the cells that comprise the stem cell niche coalesce to form the hub. Here, I examine CySC establishment and what mechanism regulates CySC maintenance and specification during embryonic and early larval testes development. I found that functional, asymmetrically dividing CySCs are present at the time of GSC establishment and that the Jak-STAT signaling pathway is both necessary and sufficient for CySC maintenance shortly thereafter. Jak-STAT signaling, however, does not appear to be necessary for early stages of CySC specification during embryogenesis. In a separate set of experiments, when Jak-STAT signaling is inhibited in the soma after stem cell establishment, cells are lost in the posterior testes, possibly due to cell death or loss of adhesion. I found that this is not the result of defects in CySC establishment or the initial differentiation of cyst cells. Together, these findings show that the CySCs are established soon after GSCs during morphogenesis and require Jak-Stat signaling to be maintained soon thereafter. Additionally, Jak-Stat signaling has a previously uncharacterized function later in differentiation that is suspected to involve adhesion or viability. Continued research should further our understanding of what factors are involved in CySC establishment and when precisely it occurs as well as the precise function of the Jak-STAT pathway in the posterior testes
