Adult tissue homeostasis relies on stem cells dividing to provide cells that differentiate and replenish lost cells. To prevent depletion of the stem cell pool, some of the daughter cells resulting from stem cell divisions retain stem cell identity and continue to proliferate, or self-renew. How stem cell self-renewal and differentiation are balanced is poorly understood. The niche, in which stem cells reside, provides spatially restricted signals that promote self-renewal. Daughter cells displaced away from the niche are thought to differentiate upon losing access to such signals. The sequence of events that occurs leading up to and following stem cell division is not well-understood. The Drosophila testis is a well-characterised model to study these behaviours. Here, cyst stem cells (CySCs) give rise to cyst cells that support germline development. Previous work in fixed tissues has shown that dysregulating CySC signalling can bias CySC fate outcomes. It remains to be demonstrated that, under normal physiological conditions, endogenous differences in signalling pathway activity are responsible for stem cell fate.

In this thesis, I present evidence that differentiation in the Drosophila testis is not a default state upon losing access to niche-derived signals but is actively induced by signalling, and that the germline also plays a role in CySC differentiation. To visualise signalling dynamics in vivo, we have adapted a kinase activity biosensor from mammalian cell culture. I demonstrate that this biosensor faithfully reports kinase activity in both larval and adult tissues, and that it can be implemented with live imaging to study real-time signalling dynamics in individual cells. Finally, I characterise CySC behaviours under normal physiological circumstances using time-lapse live imaging of adult Drosophila testis explants. Based on these data, I discuss the role that signalling pathways play in maintaining the balance between self-renewal and differentiation of stem cells during normal homeostasis.