  Active matter is characterized by its ability to induce motion by
self-generated stress. In the case of a solid, such motion can lead to shape
transformations. The stress-generating components can be anisotropic endowing
the material with mesoscopic orientational order. It is currently unknown how
the specific postions and orientations of these active constituents influence
morphological changes. We study theoretically the effects of imposing
topological point defects in the arrangements of the stress-generating
components on the morphology of elastic active nematic shells. We show that
topological defects of charge +1 are uniquely capable of increasing, reducing
or maintaining the intrinsic curvature of the shell. These changes depend on
the nature of the active stress and the phase angle of the defect. We apply our
theory to experiments conducted on contracting actomyosin sheets. By combining
defects of different charges, we can generate shells with arbitrary complexity.
We confirm this flexibility by reproducing the shape of the freshwater polyp
Hydra, in which topological defects have been associated with morphological
features of the animal. In addition to understanding morphogenetic processes,
these principles can be applied to the design of programmable active mechanical
metamaterials that form the basis of autonomous soft robots.
