EXPRESSION OF 2 ZEBRAFISH ORTHODENTICLE-RELATED GENES IN THE EMBRYONIC BRAIN

To analyze the molecular mechanism of pattern formation in the anterio rmost regions of the zebrafish embryo, we isolated two zebrafish seque nces, zOtx1 and zOtx2, related to the Drosophila orthodenticle (otd) a nd two murine Otx genes. zOtx1 and zOtx2 encode predicted gene product s which are 82% and 94% identical to the corresponding mouse proteins. Transcripts of both zebrafish genes appear abruptly at high levels in a triangular patch at the animal pole of the mid-gastrula, a region w hich contains cells fated to become midbrain and forebrain. Between 9 and 14 h of development, zOtx transcripts disappear from forebrain reg ions in a manner characteristic for each gene, and from 14 to 24 h, pa rticular regions of the forebrain and midbrain express one or both gen es. The posterior limit of expression of both genes in 10-30-h embryos forms a sharp boundary at the posterior border of the midbrain. As in the mouse, the early expression patterns of the zOtx genes are consis tent with a role in defining mid-brain and forebrain territories. Howe ver, there are a number of interesting differences between the forebra in and midbrain regions which express the genes in the two species.