The mutant strain reg-16 of Hydra magnipapillata has less capacity to regenerate a head than the wild-type strain 105 when examined in a standard test. The present comparative study with various strains of Hydra shows: (1) The head-forming potential of reg-16 and also of a widely used strain of Hydra vulgaris is severely impaired by the presence of a just-emerging or latent bud. (2) Periodic treatment with dioctanoylglycerol (DAG), a known activator of protein kinase C (PKC), plus arachidonic acid (AA), the mother compound of the eicosanoid signal substances, enabled reg-16 and H. vulgaris to insert additional tentacles into their original whorl while the export of cells by budding was reduced. (3) On the other hand, during the period of daily treatment with DAG+AA, potentials for head and bud formation were stored. Gastric segments excised from pretreated animals formed more tentacles than untreated wild-type 105 and even formed supernumerary head structures; in addition, segments excised from all body regions quickly resumed budding in spite of starvation and while they regenerated a head. (4) With continued treatment supernumerary tentacles were elicited in all strains although with high frequency only in Hydra magnipapillata, wt105. However, while wt105 formed supernumerary head structures preferentially at ectopic sites, reg-16, H. vulgaris, and Hydra oligactis instead increased the number of tentacles in the apical head, which occasionally split into two heads. (5) The treatment with DAG+AA caused the budding zone to shift closer to the foot end. (6) In H. vulgaris a pulse treatment with a high dose of DAG frequently caused a mirror image duplication of the budding zone; in wt105 mere excessive feeding over 2-3 weeks may have the same effect. The hypothesis is proposed that a regenerating head and a beginning bud compete for hormone-like factors, which enable the cells to increase positional value, and for precursor cells. Periodic treatment with activators of PKC plus AA leads to an augmentation of these resources, and head structures and buds can be produced simultaneously. Traditional terms are reinterpreted correspondingly: the high level of "head inhibition" in reg-16 is interpreted as a low level of resources, in particular, of head-promoting factors, and the low head activation level as a low ability to make use of resources.
