Topology changes of the regenerating Hydra define actin nematic defects as mechanical organizers of morphogenesis.

Abstract Hydra is named after the mythological animal for its regenerative capabilities, but contrary to its mythological counterpart, it only regenerates one head when cut. Here we show that soft compression of head regenerating tissues induces the regeneration of viable, two headed animals. Topological defects in the supracellular nematic organization of actin were previously correlated with the new head regeneration site 1 . Soft compression creates new topological defects associated with additional heads. To test the necessity of topological defects in head regeneration, we changed the topology of the tissue. By compressing the head regenerating tissues along their body axis, topological defects of the foot and of the regenerating head fused together, forming a toroid with no defects. Perfectly ordered toroids did not regenerate over eight days and eventually disintegrated. Spheroids made from excised body column tissue partially lose their actin order during regeneration. Compression of spheroids generated toroids with actin defects. These tissues regenerated into toroidal animals with functional head and foot, and a bifurcated body. Our results show that topological defects in the actin order are necessary to shape the head of the regenerating Hydra, supporting the notion that actin topological defects are mechanical organizers of morphogenesis.