Expression of the zebrafish gene hlx-1 in the prechordal plate and during CNS development.

The zebrafish hlx-1 gene belongs to the H2.0 subfamily of homeobox genes and is closely related to the mouse Dbx gene with respect to both homeodomain homology (96.7%) and neural expression during embryogenesis. Analysis of hlx-1 expression by in situ hybridization reveals several particularly interesting features. In late gastrula embryos, hlx-1 transcripts are detected within a circular area in the region of the presumptive rostral brain. Subsequently, the expression domain becomes restricted to the hypoblast and undergoes dynamic changes involving conversion into a longitudinal stripe which elongates and retracts following a temporal sequence. The site of transient hlx-1 expression along the ventral midline of the rostral neurectoderm, which in part corresponds to the prechordal plate, suggests a role in the determination of head mesoderm as well as in patterning of the rostral brain. As the midline stripe gradually disappears, the hlx-1 gene becomes regionally expressed within the diencephalon and at a specific dorsoventral level along the hindbrain and spinal cord. In the hindbrain, expression is initiated in dorsoventrally restricted transversal stripes which correlate with the segmental pattern of rhombomeres. The stripes fuse into bilateral columns that are later converted to two series of paired transversal stripes at the rhombomere borders. This pattern is consistent with the proposed subdivision of hindbrain segments into rhombomere centers separated by border regions.