While Wnt/β-catenin signaling is known to be involved in the development of neural crest cells in zebrafish, it is unclear which Wnts are involved, and when they are required. To address these issues we employed a zebrafish line that was transgenic for an inducible inhibitor of Wnt/β-catenin signaling, and inhibited endogenous Wnt/β-catenin signaling at discrete times in development. Using this approach, we defined a critical period for Wnt signaling in the initial induction of neural crest,which is distinct from the later period of development when pigment cells are specified from neural crest. Blocking Wnt signaling during this early period interfered with neural crest formation without blocking development of dorsal spinal neurons. Transplantation experiments suggest that neural crest precursors must directly transduce a Wnt signal. With regard to identifying which endogenous Wnt is responsible for this initial critical period, we established that wnt8 is expressed in the appropriate time and place to participate in this process. Supporting a role for Wnt8, blocking its function with antisense morpholino oligonucleotides eliminates initial expression of neural crest markers. Taken together, these results demonstrate that Wnt signals are critical for the initial induction of zebrafish neural crest and suggest that this signaling pathway plays reiterated roles in its development.

The anamniote lateral line system, comprising mechanosensory neuromasts and electrosensory ampullary organs, is a useful model for investigating the developmental and evolutionary diversification of different organs and cell types. Zebrafish neuromast development is increasingly well understood, but neither zebrafish nor Xenopus is electroreceptive and our molecular understanding of ampullary organ development is rudimentary. We have used RNA-seq to generate a lateral line-enriched gene-set from late-larval paddlefish (Polyodon spathula). Validation of a subset reveals expression in developing ampullary organs of transcription factor genes critical for hair cell development, and genes essential for glutamate release at hair cell ribbon synapses, suggesting close developmental, physiological and evolutionary links between non-teleost electroreceptors and hair cells. We identify an ampullary organ-specific proneural transcription factor, and candidates for the voltage-sensing L-type Cav channel and rectifying Kv channel predicted from skate (cartilaginous fish) ampullary organ electrophysiology. Overall, our results illuminate ampullary organ development, physiology and evolution.DOI: http://dx.doi.org/10.7554/eLife.24197.001

foxd3 encodes a winged helix/forkhead class transcription factor expressed in the premigratory neural crest cells of many vertebrates. We have investigated the function of this gene in zebrafish neural crest by a loss of function approach using antisense morpholino oligonucleotides and immunostaining for Foxd3 protein. Knockdown of Foxd3 expression produces deficits in several differentiated neural crest derivatives, including jaw cartilage, peripheral neurons, and glia, and iridophore pigment cells. Other derivatives, such as melanophore and xanthophore pigment cells are not affected. Reduction in the expression of several lineage-specific markers becomes evident soon after the onset of neural crest migration, suggesting that Foxd3 knockdown affects these lineages at early stages in their development. In contrast, analysis of the expression of early neural crest markers indicates little effect on neural crest induction or initial emigration. Finally, cell transplantation suggests that with respect to dorsal root ganglia neurons the Foxd3 requirement is cell autonomous, although Foxd3 itself is not detectable in differentiated DRG neurons. These results suggest that in zebrafish Foxd3 may not be required for induction of neural crest identity but is necessary for the differentiation of a subset of neural crest cell fates, perhaps in precursors of particular neural crest lineages.

Electroreception is an ancient subdivision of the lateral line sensory system, found in all major vertebrate groups (though lost in frogs, amniotes and most ray-finned fishes). Electroreception is mediated by 'hair cells' in ampullary organs, distributed in fields flanking lines of mechanosensory hair cell-containing neuromasts that detect local water movement. neuromasts, and afferent neurons for both neuromasts and ampullary organs, develop from lateral line placodes. Although ampullary organs in the axolotl (a representative of the lobefinned clade of bony fishes) are lateral line placode-derived, non-placodal origins have been proposed for electroreceptors in other taxa. Here we show morphological and molecular data describing lateral line system development in the basal ray-finned fish Polyodon spathula, and present fate-mapping data that conclusively demonstrate a lateral line placode origin for ampullary organs and neuromasts. Together with the axolotl data, this confirms that ampullary organs are ancestrally lateral line placode-derived in bony fishes.

Gegenbaur’s classical hypothesis of jaw-gill arch serial homology is widely cited, but remains unsupported by either paleontological evidence (e.g. a series of fossils reflecting the stepwise transformation of a gill arch into a jaw) or developmental genetic data (e.g. shared molecular mechanisms underlying segment identity in the mandibular, hyoid and gill arch endoskeletons). Here we show that nested expression of Dlx genes – the “Dlx code” that specifies upper and lower jaw identity in mammals and teleosts – is a primitive feature of the mandibular, hyoid and gill arches of jawed vertebrates. Using fate-mapping techniques, we demonstrate that the principal dorsal and ventral endoskeletal segments of the jaw, hyoid and gill arches of the skate Leucoraja erinacea derive from molecularly equivalent mesenchymal domains of combinatorial Dlx gene expression. Our data suggest that vertebrate jaw, hyoid and gill arch cartilages are serially homologous, and were primitively patterned dorsoventrally by a common Dlx blueprint.