Structure and early embryonic expression of the zebrafish engrailed-2 gene

The Drosophila homeobox gene engrailed (en) is needed for correct embryonic development, and related sequences are active during vertebrate embryogenesis. Here we report the protein coding sequence and embryonic expression pattern of the zebrafish engrailed-2 gene (eng-2) which is directly homologous to En-2 in mice and Xenopus. The predicted zebrafish Eng-2 protein shares 65% overall identity to its Xenopus counterpart. In addition to the highly conserved homeodomain region, sequence conservation is present within three short stretches in the N-terminal region. The embryonic expression of the eng-2 gene was analysed by in situ hybridization to whole-mount embryos and tissue sections. Transcripts are first detected in two lateral bands at the 10-h stage, when epiboly is completed. Within the next 2 h of development, these two bands migrate and fuse at the midline. By the time the neural keel becomes visible (11-12 h), a transverse stripe of eng-2 expressing cells is seen at the presumptive midbrain-hindbrain boundary. Later this stripe becomes significantly compressed along the AP axis, and in 24-h embryos eng-2 transcripts are detected mainly in the posterior midbrain. In the hindbrain, eng-2 expression seems restricted to the primordium of the cerebellum. A second site of activity was observed in each somite where specific myotomal cells, the muscle pioneers, express eng-2. Our observations are discussed in relation to early regionalization of the central nervous system (CNS) and the generation of morphological borders.