From hydra to vertebrates: Models for the transition from radial- to bilateral-symmetric body plans

Abstract

The development of a higher organism needs many coupled patternforming reactions. Crucial are interactions in which a local self-enhancing reaction is coupled to an antagonistic reaction of longer range. Using the pattern of head, tentacle and foot formation in the small freshwater polyp hydra as a model system it is shown (1) how polar pattern can emerge; (2) how a polar pattern can be maintained during substantial growth; (3) how structures next to each other can be generated and (4) how two organizing regions can be forced to appear at a maximum distance from each other at the two terminal poles. The understanding of the organization along the single axis of the radial-symmetric hydra was a key to understand the evolution of bilateral-symmetric body plans. Many observations can be explained under the assumption that the body of hydra-like ancestors evolved into the brain of higher organisms, that generation of a midline was a subtle patterning process for which evolution has found different solutions, that the ancestral hydra-type organizer became the organizer for the AP axis in higher organisms, and that the trunk is a later evolutionary addition. © Springer-Verlag Berlin Heidelberg 2013.