The novel peptide HEADY specifies apical fate in a simple radially symmetric metazoan.

One of the first steps in animal development is axis formation, during which an uneven distribution of signals and/or transcription factors results in the establishment of polarity in the embryo. Hydra, one of the simplest metazoan animals, shows characteristics of a permanent embryo. Even adult polyps have a striking capacity to regenerate, suggesting that molecular mechanisms underlying de novo pattern formation are permanently active and self regulatory. Here we show that HEADY, a short, amidated peptide, plays a central role in the specification of apical fate in this simple metazoan. The HEADY gene, whose transcripts accumulate at the apical organizing center, is required for specification of apical fate, as disruption of HEADY function by dsRNA mediated interference (RNAi) results in severe defects in head formation. Conversely, an instructive role of HEADY in head specification is demonstrated by the application of synthetic HEADY peptide, which induces formation of secondary axes with head morphology. Thus, the HEADY peptide acts as developmental switch to pattern the apical-basal axis of Hydra, providing a first insight into how initial asymmetry is specified in lower metazoan animals.