Signals and signal-transduction systems in the control of development in Hydra and Hydractinia.

Pattern control in Hydra has traditionally been assigned to the determining influence of morphogens and neuropeptides. However, at present, arachidonic acid and its derivative 12-S-HETE are the only identified, potential signal molecules known to promote head and bud formation. More potent factors might exist but are not yet identified. Nonetheless, it is possible to evoke the development of an almost unlimited number of supernumerary head structures and to induce ectopic foot formation by interference with the PI-PKC signal transducing system. Such an interference can also rescue the regeneration-deficient mutant reg-16. Regarding signals in the development of Hydractinia, metamorphosis is induced by an external key stimulus, i.e. a lipid derived from environmental bacteria. The reception of this stimulus involves PKC-mediated responses. Upon its reception, a neuropeptide is released as an internal, synchronising signal. Members of the novel LWamide family of peptides appear to represent this internal signal. In postmetamorphic development, a glycoprotein SIF serves as an inducer of stolon formation.