PROJECT SUMMARY Adult stem cells are the agents of organ renewal, remodeling and repair. Their hallmark ability to concomitantly self-renew and produce terminal progeny enables lifelong maintenance of organ form and function. At any given point in time, stem cells receive an ever-changing panoply of local and systemic signals that reinforce stemness, activate division, or direct cellular fate as needed to respond to the tissue’s evolving needs. These signals are deployed across space and time to marshal diverse stem cell behaviors for coordinated, organ-scale outputs such as tissue homeostasis. Such emergent properties are fundamental to the biology of adult tissues and essential for human health—yet, our grasp of their workings is rudimentary. My lab seeks to uncover the cellular mechanisms that underlie the robustness and flexibility of adult organ maintenance. The goal of our MIRA program is to build a comprehensive framework for understanding how each individual cell is guided by local and systemic signals for a net result of cellular equilibrium at the organ scale. Our model system is the adult Drosophila midgut, a stem cell-based, tubular epithelial organ that is functionally equivalent to the vertebrate stomach and small intestine. Our approach leverages unique live imaging capabilities—pioneered in our lab—and precision genetic tools to illuminate real-time cell dynamics in vivo and to probe the mechanisms that tune these dynamics. Here, we focus on three questions with broad significance to stem cell-based epithelial organs: 1) How does the spatial distribution of stem cells––which we find is non-random, due to autonomous stem cell motility– –impact the efficiency and robustness of organ turnover? 2) What are the real-time spatial kinetics of the EGF feedback signals that equilibrate stem cell divisions and differentiated cell death, and does ectopic manipulation of these kinetics support or negate a point-source model for organ size control? 3) How do new, differentiating cells, which are born outside of the epithelium’s sealed network of occluding junctions, integrate seamlessly into the organ as they differentiate? These studies build upon and expand our R01-funded work on organ-scale stem cell dynamics. Since the cellular life cycle is a universal feature of self-renewing organs, the tunable, population-level mechanisms that we uncover in the Drosophila midgut will provide a template for thinking about more complex organs, including those in humans.