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First published online July 11, 2006
doi: 10.1242/10.1242/dev.02466

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A signalling relay involving Nodal and Delta ligands acts during secondary notochord induction in Ciona embryos

Biologie du Développement, UMR 7009 CNRS/Universite Pierre et Marie Curie (Paris VI), Observatoire Océanologique, F-06230 Villefranche-sur-Mer, France.

e-mail: clare.hudson{at}obs-vlfr.fr; yasuo{at}obs-vlfr.fr

Accepted 30 May 2006

The notochord is one of the defining features of chordates. The ascidian notochord is a rod like structure consisting of a single row of 40 cells. The anterior 32 `primary' notochord cells arise from the A-line (anterior vegetal) blastomeres of the eight-cell stage embryo, whereas the posterior 8 `secondary' notochord cells arise from the B-line (posterior vegetal) blastomeres of the eight-cell stage embryo. Specification of notochord precursors within these two lineages occurs in a spatially and temporally distinct manner. We show that specification of the secondary but not the primary notochord in Ciona intestinalis requires a relay mechanism involving two signalling pathways. First, we show evidence that acquisition of secondary notochord fate is dependent upon lateral Nodal signalling sources, situated in the adjacent b-line animal cells. Expression of the notochord specific gene Ci-Brachyury in the secondary notochord precursor was downregulated following selective inhibition of Nodal signal reception in B-line derivatives and also, strikingly, following selective inhibition of Nodal signal reception in A-line cell derivatives. Within the A-line, Nodal signals are required for localised expression of Delta2, which encodes a divergent form of Delta ligand. Using four distinct reagents to inhibit Delta2/Notch signals, we showed that Delta2 signalling from A-line cells, which activates the Notch/Su(H) pathway in adjacent B-line cells, is required for specification of the secondary notochord precursor. We propose a model whereby laterally produced Nodal acts to specify the secondary notochord precursor both directly in the B-line cells and via Delta2 induction in adjacent A-line cells.

Key words: Nodal, Delta, Notch, Notochord, Ascidian, Ciona, Tunicate, Signalling relay

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