Project Summary/Abstract: A key step in tissue repair and regeneration is to replace cells that have been lost or damaged by injury. One strategy occurs by restoring cell number through cell proliferation and another occurs by increasing cell size through polyploidization. Polyploid cells (two or more copies of the genome) have been observed in many mammalian organs following injury, but their role in tissue repair has only recently been recognized. Using the genetic model organism Drosophila melanogaster, we discovered that polyploid cells arise in the adult epithelium in response to injury and are required for wound repair. The importance of polyploidization as a means to assist with tissue repair we believe is more widespread than previously appreciated. Cells are often observed to re-enter the cell cycle in response to injury, but a major challenge in the field has been to faithfully distinguish between polyploidization and proliferative cell cycle events. Through single-cell analysis a detailed picture of the distribution, size, and ploidy of the nuclei throughout the repaired fly epithelium was revealed using my system. Remarkably, we found that the extent of polyploidy was tuned to precisely restore tissue mass and maintain organ size. My research has revealed an unexpected, beneficial function for polyploidy, despite its association with many degenerative diseases, including cancer. Therefore, the degree of polyploidy and the mechanisms that produce polyploid cells must be tightly regulated. One of the key regulators of polyploidization, we identified, is Yorkie (Yki). Yki is the conserved co-transcriptional activator of Hippo signaling pathway and its transcriptional targets appear to dictate whether cells polyploidizes or proliferate during tissue repair. The overall objective of this scientific program is to identify the mechanisms that regulate polyploidization in adult tissues in order to promote a compensatory and prevent a pathological, disease response. The critical next step towards achieving this goal are to elucidate the signals that activate Yki to produce polyploid cells and the downstream Yki-dependent gene expression program that regulates the degree of polyploidy in adult tissues. In summary, this research program will provide fundamental insight into mechanisms regulating polyploidization providing a foundation for how to control polyploidization in health and disease.