one-eyed pinhead is required for development of the ventral midline of the zebrafish (Danio rerio) neural tube.

Axial midline structures play a central role as signalling centres during the development of the vertebrate embryo. We have isolated mutant alleles of a new locus, one-eyed pinhead (oep), in the zebrafish that are characterized by cyclopia and impaired development of the floor plate. oep mutants fail to establish expression of axial (zebrafish HNF3beta) and sonic hedgehog in the midline of the neural plate but form a notochord that expresses both genes. In the spinal cord of the 1-day-old embryo, mutation of oep impairs floor-plate but not motor-neuron development. Floor-plate development is absolutely dependent on oep only at early stages, since partial recovery of the floor plate can be detected at 48 h in the spinal cord, suggesting compensatory pathways. Ectopic expression of sonic hedgehog and a dominant-negative protein kinase A regulatory subunit induces expression of floor-plate marker genes in the oep mutant neural tube in a manner indistinguishable from wild-type embryos. Our data suggest that the oep mutation does not impair Sonic hedgehog signalling and thus implicate a second process that acts synergistically with Sonic hedgehog signalling in the specification of the midline of the neuroectoderm and that can partially be compensated for during later development.