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doi: 10.1242/10.1242/dev.00200
1 Laboratoire de Génétique et Physiologie du Développement.
IBDM. CNRS/INSERM/Université de la Méditerranée/AP de
Marseille. Parc Scientifique de Luminy, Case 907. 13288, Marseille Cedex 9,
France.
2 Unité de Biologie du Développement (UMR 7009), CNRS/UPMC,
Station Zoologique, Observatoire Océanologique, 06230
Villefranche-sur-Mer, France
* Present address: Unité de Biologie du Développement (UMR 7009),
CNRS/UPMC, Station Zoologique, Observatoire Océanologique, 06230
Villefranche-sur-Mer, France
Authors for correspondence (e-mail:
clare.hudson{at}obs-vlfr.fr
and
lemaire{at}ibdm.univ-mrs.fr)
Accepted 15 October 2002
Ascidians are invertebrate chordates with a larval body plan similar to that of vertebrates. The ascidian larval CNS is divided along the anteroposterior axis into sensory vesicle, neck, visceral ganglion and tail nerve cord. The anterior part of the sensory vesicle comes from the a-line animal blastomeres, whereas the remaining CNS is largely derived from the A-line vegetal blastomeres. We have analysed the role of the Ras/MEK/ERK signalling pathway in the formation of the larval CNS in the ascidian, Ciona intestinalis. We show evidence that this pathway is required, during the cleavage stages, for the acquisition of: (1) neural fates in otherwise epidermal cells (in a-line cells); and (2) the posterior identity of tail nerve cord precursors that otherwise adopt a more anterior neural character (in A-line cells). Altogether, the MEK signalling pathway appears to play evolutionary conserved roles in these processes in ascidians and vertebrates, suggesting that this may represent an ancestral chordate strategy.
Key words: Ciona, MEK, FGF, ERK, Ascidian, Tunicate, Neural patterning, Neural induction
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