Project Summary/Abstract Organisms with regenerative abilities have been informative models for uncovering natural mechanisms by which tissue damage activates stem or progenitor cells for injury repair. Such systems require not only sources for newly forming differentiated cells and initial responses to wounding, but also critically, spatial information systems that signal tissue presence/absence in order to control appropriate regeneration outcomes and restore tissue to its original scale and cell number. While regenerative tissues have been extensively probed for the roles of injury-induced signals and the involvement of stem or progenitor cells, much less is known about the molecular and developmental processes that enable the restoration of form after injury and its maintenance through adult growth. This grant seeks to understand the factors involved in the early symmetry- breaking events after injury, the process of establishing and using signaling centers for control of regenerative growth, the mechanism by which whole-body regeneration can robustly restore tissue proportionality and restore homeostasis, and identify the control mechanisms used by progenitor cells in whole body regeneration. The work will compare regeneration mechanisms in two distantly evolved model systems, the planarian Schmidtea mediterranea and the acoel Hofstenia miamia, each capable of whole-body regeneration using Piwi-expressing pluripotent adult stem cells termed neoblasts. Using expression profiling, RNAi screening and spatial transcriptomics, these studies will reveal what factors and strategies of whole-body regeneration are ancient and conserved. These approaches will reveal foundational mechanisms used by organisms to control adult tissue repair, growth, and homeostasis.