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.

BackgroundThe transcription factor Sox6 has been implicated in regulating muscle fiber type-specific gene expression in mammals. In zebrafish, loss of function of the transcription factor Prdm1a results in a slow to fast-twitch fiber type transformation presaged by ectopic expression of sox6 in slow-twitch progenitors. Morpholino-mediated Sox6 knockdown can suppress this transformation but causes ectopic expression of only one of three slow-twitch specific genes assayed. Here, we use gain and loss of function analysis to analyse further the role of Sox6 in zebrafish muscle fiber type specification.MethodsThe GAL4 binary misexpression system was used to express Sox6 ectopically in zebrafish embryos. Cis-regulatory elements were characterized using transgenic fish. Zinc finger nuclease mediated targeted mutagenesis was used to analyse the effects of loss of Sox6 function in embryonic, larval and adult zebrafish. Zebrafish transgenic for the GCaMP3 Calcium reporter were used to assay Ca2+ transients in wild-type and mutant muscle fibres.ResultsEctopic Sox6 expression is sufficient to downregulate slow-twitch specific gene expression in zebrafish embryos. Cis-regulatory elements upstream of the slow myosin heavy chain 1 (smyhc1) and slow troponin c (tnnc1b) genes contain putative Sox6 binding sites required for repression of the former but not the latter. Embryos homozygous for sox6 null alleles expressed tnnc1b throughout the fast-twitch muscle whereas other slow-specific muscle genes, including smyhc1, were expressed ectopically in only a subset of fast-twitch fibers. Ca2+ transients in sox6 mutant fast-twitch fibers were intermediate in their speed and amplitude between those of wild-type slow- and fast-twitch fibers. sox6 homozygotes survived to adulthood and exhibited continued misexpression of tnnc1b as well as smaller slow-twitch fibers. They also exhibited a striking curvature of the spine.ConclusionsThe Sox6 transcription factor is a key regulator of fast-twitch muscle fiber differentiation in the zebrafish, a role similar to that ascribed to its murine ortholog.Electronic supplementary materialThe online version of this article (doi:10.1186/s13395-014-0026-2) contains supplementary material, which is available to authorized users.