Animal morphogenesis arises from the complex interplay between multiple mechanical and biochemical processes with mutual feedback. Developing an effective, coarse-grained description of morphogenesis is essential for understanding how these processes are coordinated across scales to form robust, functional outcomes. Here we show that the nematic order of the supra-cellular actin fibers in regenerating Hydra defines a slowlyvarying field, whose dynamics provide an effective description of the morphogenesis process. We show that topological defects in this field, which are long-lived yet display rich dynamics, act as organization centers with morphological features developing at defect sites. These observations suggest that the nematic orientation field can be considered a "mechanical morphogen" whose dynamics, in conjugation with various biochemical and mechanical signaling processes, result in the robust emergence of functional patterns during morphogenesis.Animal morphogenesis involves multiple mechanical and biochemical processes, spanning several orders of magnitude in space and time, from local dynamics at the molecular level to global, organism-scale morphology. How these numerous processes are coordinated and integrated across scales to form robust, functional outcomes remains an outstanding question [1][2][3][4] .Developing an effective, coarse-grained description of morphogenesis can provide essential insights towards addressing this important challenge. Here, we focus on whole-body regeneration in Hydra, a small fresh-water predatory animal, and provide an effective description of the morphogenesis process that is based on the dynamic organization of the supra-cellular actin fibers in regenerating tissues 5,6 .

Understanding how mechanics complement bio-signaling in defining patterns during morphogenesis is an outstanding challenge. Here, we utilize the multicellular polyp Hydra to investigate the role of the actomyosin cytoskeleton in morphogenesis. We find that the supra-cellular actin fiber organization is inherited from the parent Hydra and determines the body axis in regenerating tissue segments. This form of structural inheritance is non-trivial because of the tissue folding and dynamic actin reorganization involved. We further show that the emergence of multiple body axes can be traced to discrepancies in actin fiber alignment at early stages of the regeneration process. Mechanical constraints induced by anchoring regenerating Hydra on stiff wires suppressed the emergence of multiple body axes, highlighting the importance of mechanical feedbacks in defining and stabilizing the body axis. Together, these results constitute an important step toward the development of an integrated view of morphogenesis that incorporates mechanics.

Animal morphogenesis arises from the interaction of multiple biochemical and mechanical processes, spanning several orders of magnitude in space and time, from local dynamics at the molecular level to global, organism-scale morphology. How these numerous processes are coordinated and integrated across scales to form robust, functional outcomes remains an outstanding question 1-4 . Developing an effective, coarse-grained description of morphogenesis can provide essential insight towards addressing this important challenge. Here we show that the nematic order of the supra-cellular actin fibers in regeneratingHydra 5, 6 defines a slowly-varying field, whose dynamics provide an effective description of the morphogenesis process. The nematic orientation field necessarily contains defects constrained by the topology of the regenerating tissue. These nematic topological defects are long-lived, yet display rich dynamics that can be related to the major morphological events during regeneration. In particular, we show that these defects act as organization centers, with the main functional morphological features developing at defect sites. To our knowledge, this provides the first demonstration of the significance of topological defects for establishing the body plan of a living animal. Importantly, the early identification of topological defect sites as precursors of morphological features, suggests that the nematic orientation field can be considered a "mechanical morphogen" whose dynamics, in conjugation with various biochemical signaling processes, result in the robust emergence of functional patterns during morphogenesis.Hydra is a classic model system for morphogenesis, thanks to its simple body plan and remarkable regeneration capabilities. Historically, research on Hydra regeneration inspired the development of many of the fundamental concepts on the biochemical basis of morphogenesis, including the role of an "organizer" 7 , the idea of pattern formation by reaction-diffusion

One of the major events in animal morphogenesis is the emergence of a polar body axis. Here, we combine classic grafting techniques with live imaging to study the emergence of body axis polarity during whole body regeneration in Hydra. Composite tissues are made by fusing two rings, excised from separate animals, in different configurations that vary in the relative polarity and original position of the rings along the body axis of the parent animals. We find that under frustrating initial configurations, body axis polarity that is otherwise stably inherited from the parent animal, can become labile and even be reversed. The site of head regeneration exhibits a bias for the edges of the fused doublets, even when this involves polarity reversal in the tissue, emphasizing the importance of structural factors in head formation. The doublets' edges invariably contain defects in the organization of the supra-cellular actin fibers, which form as the tissue ring seals on each side. We suggest that the presence of a defect can act as an 'attractor' for head formation at the edge, even though a defect is neither required nor sufficient for head formation. The observation of head formation at an originally distal edge of the tissue upon polarity reversal, is not compatible with models of Hydra regeneration based solely on preexisting morphogen gradients. Rather, our results suggest that body axis determination is a dynamic process that involves mechanical feedback and signaling processes that are sensitive to the original polarity and position of the excised tissues.

The emergence and stabilization of a body axis is a major step in animal morphogenesis, determining the symmetry of the body plan as well as its polarity. To advance our understanding of the emergence of body axis polarity, we study regenerating Hydra. Axis polarity is strongly memorized in Hydra regeneration even in small tissue segments. What type of processes confer this memory? To gain insight into the emerging polarity, we utilize frustrating initial conditions by studying regenerating tissue strips which fold into hollow spheroids by adhering their distal ends of opposite original polarities. Despite the convoluted folding process and the tissue rearrangements during regeneration, these tissue strips develop in a reproducible manner, preserving the original polarity and yielding an ordered body plan. These observations suggest that the integration of mechanical and biochemical processes supported by their mutual feedback attracts the tissue dynamics towards a well-defined developmental trajectory biased by weak inherited cues from the parent animal. Hydra thus provide an example of dynamic canalization in which the dynamic rules are instilled, but, in contrast to the classical picture, the detailed developmental trajectory does not unfold in a programmatic manner.