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First published online 29 August 2007
doi: 10.1242/dev.002352

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Sequential and combinatorial inputs from Nodal, Delta2/Notch and FGF/MEK/ERK signalling pathways establish a grid-like organisation of distinct cell identities in the ascidian neural plate

Developmental Biology Unit, Université Pierre et Marie Curie (Paris 6) and CNRS, Observatoire Océanologique, 06230 Villefranche-sur-Mer, France.

^* Author for correspondence (e-mail: clare.hudson{at}obs-vlfr.fr)

Accepted 26 July 2007

The ascidian neural plate has a grid-like organisation, with six rows and eight columns of aligned cells, generated by a series of stereotypical cell divisions. We have defined unique molecular signatures for each of the eight cells in the posterior-most two rows of the neural plate - rows I and II. Using a combination of morpholino gene knockdown, dominant-negative forms and pharmacological inhibitors, we tested the role of three signalling pathways in defining these distinct cell identities. Nodal signalling at the 64-cell stage was found to be required to define two different neural plate domains - medial and lateral - with Nodal inducing lateral and repressing medial identities. Delta2, an early Nodal target, was found to then subdivide each of the lateral and medial domains to generate four columns. Finally, a separate signalling system along the anteroposterior axis, involving restricted ERK1/2 activation, was found to promote row I fates and repress row II fates. Our results reveal how the sequential integration of three signalling pathways - Nodal, Delta2/Notch and FGF/MEK/ERK - defines eight different sub-domains that characterise the ascidian caudal neural plate. Most remarkably, the distinct fates of the eight neural precursors are each determined by a unique combination of inputs from these three signalling pathways.

Key words: Ciona, Ascidian, Neural patterning, Nodal, Delta, Notch, FGF, MEK, ERK

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