Interactions between the foot and the head patterning systems in Hydra vulgaris

The Cnidarian, hydra, is an appealing model system for studying the basic processes underlying pattern formation. Classical studies have elucidated much basic information regarding the role of development gradients, and theoretical models have been quite successful at describing experimental results. However, most experiments and computer simulations have dealt with isolated patterning events such as the dynamics of head regeneration. More global events such as interactions among the head, bud, and foot patterning systems have not been extensively addressed. The characterization of monoclonal antibodies with position-specific labeling patterns and the recent cloning and characterization of genes expressed in position-specific manners now provide the tools for investigating global interactions between patterning systems. In particular, changes in the axial positional value gradient may be monitored in response to experimental perturbation. Rather than studying isolated patterning events, this approach allows us to study patterning over the entire animal. The studies reported here focus on interactions between the foot and the head patterning systems in Hydra vulgaris following induction of a foot in close proximity to a head, axial grafting of a foot closer to the head, or doubling the amount of basal tissue by lateral grafting of an additional peduncle-foot onto host animals. Resulting positional value changes as monitored by antigen (TS19) and gene (ks1 and CnNK-2) expression were assessed in the foot, head, and intervening tissue. The results of the experiments indicate that positional values changed rapidly, in a matter of hours, and that there were reciprocal interactions between the foot and the head patterning systems. Theoretical interpretations of the results in the form of computer simulations based on the reaction-diffusion model are presented and predict many, but not all, of the experimental observations. Since the lateral grafting experiment cannot, at present, be simulated, it is discussed in light of what has been learned from the axial grafting experiments and their simulations.