Expression of a zebrafish caudal homeobox gene correlates with the establishment of posterior cell lineages at gastrulation.

This paper deals with the first identification of a caudal cDNA containing a homeobox of the Drosophila caudal family in the zebrafish. A cDNA library from late gastrula stage embryos was constructed and screened with a mouse Cdx 1 homeobox probe. A 1.6 kb cDNA clone containing a homeobox related to other caudal homeoboxes was isolated and called cdx[Zf-cad1]. Analysis of the predicted 301 amino acid translation product reveals additional regions of homology outside the homeodomain with other members of the caudal family. Particularly, the cdx[Zf-cad1] putative protein shares a conserved N-terminal region with its chicken homolog CHox-cad. Transcripts are first detected just before the onset of gastrulation. At the beginning of gastrulation, a single 1.8 kb cdx[Zf-cad1] transcript is located near the blastoderm margin with a high level of expression restricted to the epiblast. At this stage, the hypoblast is clearly negative. At the end of gastrulation, cdx[Zf-cad1] is widely expressed in vegetal (i.e. prospective posterior) epiblast and hypoblast, with a somewhat weaker expression in the dorsal hypoblast. During somitogenesis, cdx[Zf-cad1] exhibits a posterior regionalization in the neurectoderm. In contrast, no expression is detected in the mesoderm of 22 h embryos (late somitogenesis). Posterior endoderm is also positive at this stage. cdx[Zf-cad1] transcripts cease to be detected about 48 h after fertilization. They are undetectable in the adult, particularly in female gonads. The pattern of cdx[Zf-cad1] expression during and after gastrulation is consistent with its possible involvement in the regionalization of the embryo at these stages.(ABSTRACT TRUNCATED AT 250 WORDS)