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.