Zebrafish organizer development and germ-layer formation require nodal-related signals

Abstract

The vertebrate body plan is established during gastrulation, when cells move inwards to form the mesodermal and endodermal germ layers. Signals from a region of dorsal mesoderm, which is termed the organizer, pattern the body axis by specifying the fates of neighbouring cells1,2. The organizer is itself induced by earlier signals1. Although members of the transforming growth factor-β (TGF-β) and Wnt families have been implicated in the formation of the organizer, no endogenous signalling molecule is known to be required for this process1. Here we report that the zebrafish squint (sqt)3 and cyclops (cyc)4 genes have essential, although partly redundant, functions in organizer development and also in the formation of mesoderm and endoderm. We show that the sqt gene encodes a member of the TGF-β superfamily that is related to mouse nodal. cyc encodes another nodal-related protein5,6, which is consistent with our genetic evidence that sqt and cyc have overlapping functions. The sqt gene is expressed in a dorsal region of the blastula that includes the extraembryonic yolk syncytial layer (YSL). The YSL has been implicated as a source of signals that induce organizer development and mesendoderm formation2,7. Misexpression of sqt RNA within the embryo or specifically in the YSL induces expanded or ectopic dorsal mesoderm. These results establish an essential role for nodal-related signals in organizer development and mesendoderm formation.

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Figure 1: Disruption of mesoderm and endoderm in sqt;cyc double mutants.
Figure 2: The sqt gene encodes a nodal-related signal.
Figure 3: Microinjection of sqt RNA induces gsc expression in wild type and sqt mutants.
Figure 4: Expression of sqt mRNA in embryos at blastula and gastrula stages.

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Acknowledgements

We thank C. Erter, C. Wright, M. Rebagliati and I. Dawid for sharing and allowing us to cite their unpublished data; members of the Talbot and Schier laboratories for discussions; T. Lepage, D.Kimelman and C.-P. Heisenberg for reagents and fish stocks; S. McManus for fish care; and A. Ruiz i Altaba and G. Fishell for comments on the manuscript. We acknowledge postdoctoral fellowship support from the NIH (B.F. and H.I.S.) and ACS (S.T.D.). This work was supported by grants from the NIH (W.S.T. and A.F.S.) and an NYU Whitehead Fellowship (W.S.T.).

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Correspondence to William S. Talbot.

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