NODAL INDUCES ECTOPIC GOOSECOID AND LIM1 EXPRESSION AND AXIS DUPLICATION IN ZEBRAFISH

One of the first intercellular signalling events in the vertebrate emb ryo leads to mesoderm formation and axis determination. In the mouse, a gene encoding a new member of the TGF-beta superfamily, nodal, is di srupted in a mutant deficient in mesoderm formation (Zhou et al., 1993 , Nature 361, 543). nodal mRNA is found in prestreak mouse embryos, co nsistent with a role in the development of the dorsal axis. To examine the biological activities of nodal, we have studied the action of thi s factor in eliciting axis determination in the zebrafish, Danio rerio . Injection of nodal mRNA into zebrafish embryos caused the formation of ectopic axes that included notochord and somites. Axis duplication was preceded by the generation of an apparent ectopic shield (organize r equivalent) in nodal-injected embryos, as indicated by the appearanc e of a region over-expressing gsc and lim1; isolation and expression i n the shield of the lim1 gene is reported here. These results suggest a role for a nodal-like factor in pattern formation in zebrafish.