Hidden Dynamical Canalization at the Onset of Hydra Morphogenesis

Abstract

The primary morphological transition in Hydra regeneration, from a quasi-spherical fragment to an elongated body, is preceded by a prolonged period during which its average form changes only modestly, while its instantaneous shape continues to fluctuate. We asked whether this preparatory stage contains signatures of dynamical organization. Using principal component analysis, we calculated the effective dimension of shape fluctuations, which characterizes how broadly they are distributed across PCA directions. Across all five conditions, spanning differences in tissue fragment size, axial origin and GdCl exposure, the effective dimension decreased progressively before overt tissue elongation. Total fluctuation variance increased about 2.8-fold, whereas the leading eigenvalue increased about fourfold. Late intervals had lower effective dimension than early intervals at similar coarse shapes. Calcium analysis in GdCl-treated tissues distinguished mean level, fast events and spatial heterogeneity. Fast events were associated with short-timescale shape changes. After accounting for temporal trends, mean Ca2+ and coarse morphology, higher spatial heterogeneity of Ca2+ activity was associated with lower effective dimension. The progressive concentration of morphological fluctuations along fewer dominant directions characterizes dynamical canalization associated with spatial Ca2+ heterogeneity.