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The BMP signaling pathway is required together with the FGF pathway for notochord induction in the ascidian embryo

¹ Department of Biological Sciences, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8501, Japan
² Laboratoire de Génétique et Physiologie du Développement, Institut de Biologie du Développement de Marseille, CNRS-INSERM-Université de la Méditerranée-AP de Marseille, Campus de Luminy, Case 907, F-13288 Marseille Cedex 9, France

*Author for correspondence (e-mail: darras{at}ibdm.univ-mrs.fr)

Accepted April 25, 2001

The 40 notochord cells of the ascidian tadpole invariably arise from two different lineages: the primary (A-line) and the secondary (B-line) lineages. It has been shown that the primary notochord cells are induced by presumptive endoderm blastomeres between the 24-cell and the 64-cell stage. Signaling through the fibroblast growth factor (FGF) pathway is required for this induction. We have investigated the role of the bone morphogenetic protein (BMP) pathway in ascidian notochord formation. HrBMPb (the ascidian BMP2/4 homologue) is expressed in the anterior endoderm at the 44-cell stage before the completion of notochord induction. The BMP antagonist Hrchordin is expressed in a complementary manner in all surrounding blastomeres and appears to be a positive target of the BMP pathway. Unexpectedly, chordin overexpression reduced formation of both primary and secondary notochord. Conversely, primary notochord precursors isolated prior to induction formed notochord in presence of BMP-4 protein. While bFGF protein had a similar activity, notochord precursors showed a different time window of competence to respond to BMP-4 and bFGF. Our data are consistent with bFGF acting from the 24-cell stage, while BMP-4 acts during the 44-cell stage. However, active FGF signaling was also required for induction by BMP-4. In the secondary lineage, notochord specification also required two inducing signals: an FGF signal from anterior and posterior endoderm from the 24-cell stage and a BMP signal from anterior endoderm during the 44-cell stage.

Key words: Notochord induction, BMP, Chordin, bFGF, Ascidian embryo, Asymmetric division, Halocynthia roretzi

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