Mutant analysis in the zebrafish Danio rerio has demonstrated distinct developmental roles for the T-box transcription factor Spadetail (Spt) and the Nodal-receptor cofactor One-Eyed Pinhead (Oep) in the formation of mesoderm and endoderm. Here, we show that spt and oep genetically interact and are together essential for the formation of cardiac and somitic mesoderm. These two mesodermal defects are dependent on different effectors of Nodal signalling; cardiac mesoderm formation involves the mix-like transcription factor Bonnie and Clyde (Bon), whereas somitogenesis is dependent on a different pathway. Analysis of the somite defect in Zoep;spt embryos has provided insights into the control of somitic mesoderm formation by Spt, which was previously implicated in the regulation of cell adhesion and motility. We show that the failure to form somites in Zoep;spt embryos is independent of this and that Spt must have an additional function. We propose that the major role of Spt in somitogenesis is to promote the differentiation of presomitic mesoderm from tailbud progenitors by antagonizing progenitor-type gene expression and behaviour.

T-box transcription factors are important determinants of embryonic cell fate and behaviour. Two T-box genes are expressed in the developing telencephalon of several vertebrate species, including amphibia, birds and mammals. Here we report the cloning of zebrafish T-brain-1 (tbr1) and eomesodermin (eom). As a prelude to genetic studies of neuro-ectodermal fate determination we studied their expression pattern during embryogenesis and early larval development. Eom is expressed in the presumptive telencephalon from around the 4-5 somite stage in bilaterally symmetric groups of cells; the number of positive cells increases dramatically with time and encompasses the entire dorsal telencephalon by the 22 somite stage. Tbr1 is expressed from the 18 somite stage in a subset of eom-expressing cells. By 24 hpf eom and tbr1 are expressed in largely overlapping domains in the dorsal telencephalon, tbr1 is expressed in postmitotic cells whereas eomes is also expressed in proliferative ventricular zone cells. Both genes are also found in a small domain of the diencephalon bordering the telencephalon. A detailed analysis of the expression of tbr1 and eom in the brain of 4 day old larvae shows that the two T-box genes are differentially expressed in various cell populations of the developing brain.

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In zebrafish, endoderm induction occurs in marginal blastomeres and requires Casanova (Cas), the first endoderm-specific factor expressed in the embryo. Whereas the transcription factors Gata5 and Bon are necessary and sufficient for cas expression in marginal blastomeres, Bon and Gata5 are unable to induce cas in animal pole cells, suggesting that cas expression requires an additional, unidentified factor(s). Here, we show that cas expression depends upon the T box transcription factor Eomesodermin (Eomes), a maternal determinant that is localized to marginal blastomeres. Eomes synergizes potently with Bon and Gata5 to induce cas, even in animal pole blastomeres. We show that Eomes is required for endogenous endoderm induction, acting via an essential binding site in the cas promoter. Direct physical interactions between Eomes, Bon, and Gata5 suggest that Eomes promotes endoderm induction in marginal blastomeres by facilitating the assembly of a transcriptional activating complex on the cas promoter.