Compensatory Branching Morphogenesis in the Drosophila Tracheal System Deanne Francis Amin Ghabrial Most organs and glands are composed of interconnected networks of tubes. Tubes carryout many important functions throughout the body, such as homeostasis, nutrient and oxygen transport. Surprisingly, given the importance of interconnected tubular networks, how connections between different tubes are maintained remains undetermined. To address this question we used the Drosophila tracheal system as a model to study tube connectivity. The Drosophila trachea is composed of multi-cellular, auto-cellular and seamless tubes. Multi-cellular tubes are composed of multiple interconnected cells, auto-cellular tubes form by wrapping and membrane self-adhesion, while seamless tubes form entirely intracellularly. In all epithelial tube types, the cell apical domain faces the lumen. In this work, I focused on the connection between the auto-cellular tube in the stalk cell and the seamless tube in the highly branched terminal cell. Taking a forward genetic approach, I analyzed three mutants that perturb connectivity: oak gall (okg), conjoined (cnj) and disjointed (dsj). The connectivity defects resulted from branching and invasion of WT stalk cells into mutant terminal cells. To determine the mechanisms that induce branching of WT stalk cells we mapped the mutations. We found that okg and cnj encode two vATPase subunits and dsj encodes an Archease. Terminal cells deficient for vATPase activity mis-localize apical proteins and stall apical membrane growth, which stimulates compensatory stalk cell hypertrophy and branching. While dsj, okg and cnj terminal cells shared terminal cell pruning defects were genetically downstream of the Target of Rapamycin (Tor) pathway, the compensatory branching of WT stalk cells adjacent to dsj terminal cell was independent of apical polarity. Additionally, we found that terminal cell injury induced adjacent stalk cell compensatory branching and stalk cells next to injured and genetically compromised terminal cells activate Jun Kinase and endoreplicate during invasion. Finally, inactivation of the Hippo growth pathway in stalk cells is sufficient to induce hypertrophy and branching. We conclude that genetically compromised or injured terminal cells activate hypertrophy and branching in a neighboring stalk cell.