DESCRIPTION (provided by applicant): Effective and safe stem cell therapies must build upon knowledge of how stem cells generate precise numbers of differentiated cells to meet the body's needs. In adult organ renewal, each stem cell division triggers a pivotal decision between asymmetric, symmetric-stem, and symmetric-terminal fates. To sustain constant numbers of stem and differentiated cells, these three fate outcomes must be collectively balanced. Conversely, dysplasia or degeneration arises if fate balance is lost. Yet in contrast to the well-studied pathways that execute fate outcomes, the upstream events that decide between fate outcomes are virtually unknown. Our long-term goal is to understand the mechanisms that arbitrate the organ-wide balance of division fates. Toward this goal, here we probe the cellular basis of symmetric and asymmetric fate decisions-in vivo and in real time-by combining live imaging with the versatile genetic tools of Drosophila. Using the adult Drosophila midgut, we have made a path breaking innovation by developing long-term imaging of epithelial renewal at high cellular resolution in live animals. Our methodology enables individual stem cell divisions to be captured in their native context and fate decisions to be visualized in real time. We will investigate three fundamental questions about the cellular and molecular nature of fate decisions. In Aim 1, we ask whether fate decisions are made by the dividing mother stem cell, by equipotent daughter cells, or a combination. Using live imaging, we will directly test the mother-control mechanisms of oriented cell division and fate determinant partitioning, and the daughter-control mechanism of Notch-mediated lateral inhibition. We will evaluate whether different mechanisms bias toward different fates, and examine whether initial fate decisions can be overturned by later-acting mechanisms. Aim 2 builds upon exciting preliminary data those stem cells are motile, which provokes the question of whether motility influences fates by altering proximity to spatially localized signals. We will determine how motility impacts fate decisions, probe the interplay between motility and fate outcomes, and identify the cytoskeletal regulators that instigate motility. In Aim 3, we turn to the adhesion junctions that define epithelal architecture and ask how this multicellular adhesive network integrates into fate decisions. We will separately perturb basal, lateral, and apical adhesion receptors on stem, daughter, and differentiated neighbor cells and parse how distinct receptors on different cell surfaces influence fate decision mechanisms and outcomes. Because epithelial stem cell biology and architecture are broadly conserved, the fate decision mechanisms uncovered here will potentially extend to epithelial organs in vertebrates, including humans. Ultimately, understanding the basic mechanisms that decide between division fates will open new therapeutic avenues to combat s...