Planarian Hh Signaling Regulates Regeneration Polarity and Links Hh Pathway Evolution to Cilia

Hedgehog, Flatworms, and Regeneration The Hedgehog (Hh) signaling pathway plays pivotal roles during embryonic development, post-embryonic tissue maintenance, and disease. In spite of having at its core a collection of highly conserved components, the Hh pathway displays unusual differences in signal transduction mechanisms among flies, fish, and mammals. Using planarians (nonparasitic flatworms), Rink et al. (p. 1406 , published online 22 October) report a key role for Hh signaling during adult regeneration and provide evidence that the perplexing diversity of transduction mechanisms may have arisen from animal-specific losses of an ancestral association between cilia and Hh signaling.