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First published online 14 February 2007
doi: 10.1242/dev.02804

Development 134, 1191-1201 (2007)
Published by The Company of Biologists 2007
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Head regeneration in wild-type hydra requires de novo neurogenesis

Marijana Miljkovic-Licina*, Simona Chera, Luiza Ghila and Brigitte Galliot{dagger}

Department of Zoology and Animal Biology, University of Geneva, Sciences III, 30 Quai Ernest Ansermet, CH-1211 Geneva 4, Switzerland.

{dagger} Author for correspondence (e-mail: brigitte.galliot{at}zoo.unige.ch)

Accepted 10 January 2007

Because head regeneration occurs in nerve-free hydra mutants, neurogenesis was regarded as dispensable for this process. Here, in wild-type hydra, we tested the function of the ParaHox gsx homolog gene, cnox-2, which is a specific marker for bipotent neuronal progenitors, expressed in cycling interstitial cells that give rise to apical neurons and gastric nematoblasts (i.e. sensory mechanoreceptor precursors). cnox-2 RNAi silencing leads to a dramatic downregulation of hyZic, prdl-a, gsc and cnASH, whereas hyCOUP-TF is upregulated. cnox-2 indeed acts as an upstream regulator of the neuronal and nematocyte differentiation pathways, as cnox-2(-) hydra display a drastic reduction in apical neurons and gastric nematoblasts, a disorganized apical nervous system and a decreased body size. During head regeneration, the locally restricted de novo neurogenesis that precedes head formation is cnox-2 dependent: cnox-2 expression is induced in neuronal precursors and differentiating neurons that appear in the regenerating tip; cnox-2 RNAi silencing reduces this de novo neurogenesis and delays head formation. Similarly, the disappearance of cnox-2+ cells in sf-1 mutants also correlates with head regeneration blockade. Hence in wild-type hydra, head regeneration requires the cnox-2 neurogenic function. When neurogenesis is missing, an alternative, slower and less efficient, head developmental program is possibly activated.

Key words: Cnidarian, Evolution, Apical patterning, Regeneration, Neurogenesis, Neuronal progenitors, Interstitial stem cells, RNA interference, ParaHox gene, ß-Tubulin


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