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First published online October 23, 2009
doi: 10.1242/10.1242/dev.039693

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The entire zebrafish blastula-gastrula margin acts as an organizer dependent on the ratio of Nodal to BMP activity

¹ IGBMC, CNRS/INSERM/Université de Strasbourg, 1 rue Laurent Fries, BP10142, 67404 Illkirch Cedex, France.
² Department of Cell Biology, University of Virginia Health Sciences Center, PO Box 800732, 1300 Jefferson Park Avenue, Charlottesville, VA 22908, USA.

^* Author for correspondence (christhisse{at}virginia.edu)

Accepted 21 September 2009

Formation of the vertebrate embryo is known to depend on the activity of organizing centers. The dorsal Spemann organizer is the source of growth factor antagonists that participate in the creation of signaling gradients. In various species, the existence of head, trunk and trunk-tail inducers has been proposed to explain the formation of different parts of the embryo along the anteroposterior (A/P) axis. In zebrafish, two organizing centers have been described, the dorsal and tail organizers, located at the dorsal and ventral gastrula margins, respectively. Here, we report that organizer functions are executed not only by the dorsal and ventral margins, but also by all parts of the blastula-gastrula margin. The position of different marginal territories along the dorsoventral axis defines the A/P nature of the structures they are able to organize. At the molecular level, we show that this organizing activity results from the simultaneous activation of BMP and Nodal signaling pathways. Furthermore, the A/P character of the organized structures is not defined by absolute levels but instead by the ratio of BMP and Nodal activities. Rather than resulting from the activity of discrete centers, organization of the zebrafish embryo depends on the activity of the entire margin acting as a continuous and global organizer that is established by a gradual ventral-to-dorsal modulation of the ratio of marginal BMP to Nodal activity.

Key words: Zebrafish, Organizer, Margin, BMP, Nodal, Blastula, Gastrula

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