Modeling pattern formation in hydra: a route to understanding essential steps in development.

Modeling of pattern formation in hydra has revealed basic mechanisms that underlie the reproducible generation of complex and self-regulating patterns. Organizing regions can be generated by a local self-enhancing reaction that is coupled with an inhibitory effect of longer range. Such reactions enable pattern formation even in an initially almost homogeneous assembly of cells. A long-ranging feedback of the organizer onto the competence to perform the pattern-forming reaction stabilizes the polar axial pattern during growth and allows for regeneration with preserved polarity. Hypostome formation is assumed to be under the control of two positive feedback loops in which Wnt3 is a common element. In addition to the well-established loop employing beta-catenin, a second cell-local loop is involved, possibly with Brachyury as an additional component. This model accounts for the different expression patterns of beta-catenin and Wnt3. Wnt molecules are proposed to play a dual role, functioning as activators and, after processing, as inhibitors. Since Wnt genes code for complete pattern-forming systems, gene duplication and diversification lead to a family of genes whose expression regions have a precise relation to each other. Tentacle formation is an example of positioning a second pattern-forming system by medium-ranging activation and local exclusion exerted by the primary system. A model for bud formation suggests that a transient pre-bud signal is involved that initiates the formation of the foot of the bud, close to the normal foot, as well as close to the bud tip. Many dynamic regulations, as observed in classical and molecular observations, are reproduced in computer simulations. A case is made that hydra can be regarded as a living fossil, documenting an evolutionary early axis formation before trunk formation and bilaterality were invented. Animated simulations are available in the supplementary information accompanying this paper.