Single cell analysis reveals the molecular signaling and cellular composition of the regenerating Hydra head

The extent to which animals can regenerate cells, tissues, or body parts varies largely. Hydra has a remarkable ability to undergo full body regeneration. Bisected polyps can regenerate the head and foot, and whole polyps can form from aggregates of cells. This capability is made possible by a cluster of cells known as the head organizer. This organizer has the capability to self-regulate and to induce a second axis. Previous studies have found Wnt3 and other developmental genes associated with head organizer function. Yet, the cellular composition and molecular program of regenerating tissue remains largely unknown. In this study, we used single cell RNA-seq from a regeneration time course to identify the molecular and cellular features of Hydra head regeneration. We identified nine distinct cell types in the regenerating head tissue, including candidate head organizer cells. We found Wnt-signaling and early wound response genes co-expressed with Wnt3 , and all were more highly expressed in the head organizer cell cluster. In addition, we found that Wnt3 expression is likely being regulated by conserved developmental transcription factors. Our study reveals coordination of early wound response, developmental transcription factors, and transposable elements during Hydra tissue regeneration and provides insight into the evolution of development and regeneration programs. ### Competing Interest Statement The authors have declared no competing interest.