HEAD FORMATION IN HYDRA IS DIFFERENT AT APICAL AND BASAL LEVELS

Hydra's head is a compound structure with a hypostome at the apical ex treme and a circle of tentacles more basally. During head regeneration , it is thought (P. M. Bode, T. A. Awad, O. Koizumi, Y. Nakashima, C. J. P. Grimmelikhuijzen and H. R. Bode (1988) Developinent 102, 223-235 ; R. Weinziger, L. M. Salgado, C. N. David and T. C. G. Bosch (1994) D evelopment 120, 2511-2517) that the conditions for tentacle formation are fulfilled before those for hypostome formation, Using a new hypost ome-specific marker, we have found that the order of hypostome and ten tacle formation is variable. In regenerating basal tissue, the hyposto me marker is expressed before tentacles appear but in apical tissue, t he tentacles appear first. This observation appears inconsistent with current views but can be explained by a hierarchical model (H. Meinhar dt (1993) Developmental Biology 157, 321-333) in which tentacles requi re an inductive influence of the hypostome. In basal regenerates, the hypostome forms first and then induces tentacles. In apical regenerate s, inductive factor remains from the amputated hypostome, and tentacle may form before the new hypostome. We have also observed that the mod e of expression of the tentacle marker differs in basal and apical tis sue. In basal tissue, the marker first appears in the definitive tenta cle zone; in apical tissue, the marker first appears in the position o f the presumptive hypostome and is then displaced to its final positio n, as described by previous workers. This observation is also expected according to the above-cited model.