The zebrafish colourless gene regulates development of non-ectomesenchymal neural crest derivatives

Specification of neural crest into sensory neuron and melanocyte lineages

Elucidating the mechanisms by which multipotent cells differentiate into distinct lineages is a common theme underlying developmental biology investigations. Progress has been made in understanding some of the essential factors and pathways involved in the specification of different lineages from the neural crest. These include gene regulatory networks involving transcription factor hierarchies and input from signaling pathways mediated from environmental cues. In this review, we examine the mechanisms for two lineages that are derived from the neural crest, peripheral sensory neurons and melanocytes. Insights into the specification of these cell types may reveal common themes in the specification processes that occur throughout development.

Zebrafish arl6ip1 is required for neural crest development during embryogenesis

Background

Although the embryonic expression pattern of ADP ribosylation factor-like 6 interacting protein 1 (Arl6ip1) has been reported, its function in neural crest development is unclear.

Methods/Principal Findings

We found that knockdown of Arl6ip1 caused defective embryonic neural crest derivatives that were particularly severe in craniofacial cartilages. Expressions of the ectodermal patterning factors msxb, dlx3b, and pax3 were normal, but the expressions of the neural crest specifier genes foxd3, snai1b, and sox10 were greatly reduced. These findings suggest that arl6ip1 is essential for specification of neural crest derivatives, but not neural crest induction. Furthermore, we revealed that the streams of crestin- and sox10-expressing neural crest cells, which migrate ventrally from neural tube into trunk, were disrupted in arl6ip1 morphants. This migration defect was not only in the trunk neural crest, but also in the enteric tract where the vagal-derived neural crest cells failed to populate the enteric nervous system. We found that this migration defect was induced by dampened Shh signaling, which may have resulted from defective cilia. These data further suggested that arl6ip1 is required for neural crest migration. Finally, by double-staining of TUNEL and crestin, we confirmed that the loss of neural crest cells could not be attributed to apoptosis.

Conclusions/Significance

Therefore, we concluded that arl6ip1 is required for neural crest migration and sublineage specification.

Smooth muscle caldesmon modulates peristalsis in the wild type and non-innervated zebrafish intestine

BACKGROUND

The high molecular weight isoform of the actin-binding protein Caldesmon (h-CaD) regulates smooth muscle contractile function by modulating cross-bridge cycling of myosin heads. The normal inhibitory activity of h-CaD is regulated by the enteric nervous system; however, the role of h-CaD during intestinal peristalsis has never been studied.

METHODS

We identified a zebrafish paralog of the human CALD1 gene that encodes an h-CaD isoform expressed in intestinal smooth muscle. We examined the role of h-CaD during intestinal peristalsis in zebrafish larvae by knocking down the h-CaD protein using an antisense morpholino oligonucleotide. We also developed transgenic zebrafish that express inhibitory peptides derived from the h-CaD myosin and actin-binding domains, and examined their effect on peristalsis in wild-type zebrafish larvae and sox10 (colourless) mutant larvae that lack enteric nerves.

KEY RESULTS

Genomic analyses identified two zebrafish Caldesmon paralogs. The cald1a ortholog encoded a high molecular weight isoform generated by alternative splicing whose intestinal expression was restricted to smooth muscle. Propulsive intestinal peristalsis was increased in wild-type zebrafish larvae by h-CaD knockdown and by expression of transgenes encoding inhibitory myosin and actin-binding domain peptides. Peristalsis in the non-innervated intestine of sox10 (colourless) larvae was partially restored by h-CaD knockdown and expression of the myosin-binding peptide.

CONCLUSIONS & INFERENCES

Disruption of the normal inhibitory function of h-CaD enhances intestinal peristalsis in both wild-type zebrafish larvae and mutant larvae that lack enteric nerves, thus confirming a physiologic role for regulation of smooth muscle contraction at the actin filament.

Deciphering the cellular and molecular roles of cellular nucleic acid binding protein during cranial neural crest development

Cellular nucleic acid binding protein (Cnbp) is a highly conserved single-stranded nucleic acid binding protein required for rostral head development. The use of a morpholino that inhibits Cnbp mRNA translation previously revealed a role of Cnbp in balancing neural crest cell apoptosis and proliferation in the developing zebrafish. Here, we report the use of another morpholino that specifically modifies the splicing of Cnbp pre-mRNA resulting in a reduction of full-length mRNA levels along with the generation of a novel transcript coding for an isoform that may act as dominant negative proteins. The use of this morpholino resulted in more severe phenotypes that enabled us to demonstrate that Cnbp loss-of-function adversely affects the formation and survival of craniofacial cartilaginous structures not only controlling the ratio of cell proliferation and apoptosis but also defining skeletogenic neural crest cell fate.

Development of the zebrafish enteric nervous system

The enteric nervous system (ENS) is composed of neurons and glia that modulate many aspects of intestinal function. The ability to use both forward and reverse genetic approaches and to visualize development in living embryos and larvae has made zebrafish an attractive model in which to study mechanisms underlying ENS development. In this chapter, we review the recent work describing the development and organization of the zebrafish ENS and how this relates to intestinal motility. We also discuss the cellular, molecular, and genetic mechanisms that have been revealed by these studies and how they are providing new insights into human ENS diseases.

Chondrogenic and gliogenic subpopulations of neural crest play distinct roles during the assembly of epibranchial ganglia

In vertebrates, the sensory neurons of the epibranchial (EB) ganglia transmit somatosensory signals from the periphery to the CNS. These ganglia are formed during embryogenesis by the convergence and condensation of two distinct populations of precursors: placode-derived neuroblasts and neural crest- (NC) derived glial precursors. In addition to the gliogenic crest, chondrogenic NC migrates into the pharyngeal arches, which lie in close proximity to the EB placodes and ganglia. Here, we examine the respective roles of these two distinct NC-derived populations during development of the EB ganglia using zebrafish morphant and mutants that lack one or both of these NC populations. Our analyses of mutant and morphant zebrafish that exhibit deficiencies in chondrogenic NC at early stages reveal a distinct requirement for this NC subpopulation during early EB ganglion assembly and segmentation. Furthermore, restoration of wildtype chondrogenic NC in one of these mutants, prdm1a, is sufficient to restore ganglion formation, indicating a specific requirement of the chondrogenic NC for EB ganglia assembly. By contrast, analysis of the sox10 mutant, which lacks gliogenic NC, reveals that the initial assembly of ganglia is not affected. However, during later stages of development, EB ganglia are dispersed in the sox10 mutant, suggesting that glia are required to maintain normal EB ganglion morphology. These results highlight novel roles for two subpopulations of NC cells in the formation and maintenance of EB ganglia: chondrogenic NC promotes the early-stage formation of the developing EB ganglia while glial NC is required for the late-stage maintenance of ganglion morphology.

Fgf20b is required for the ectomesenchymal fate establishment of cranial neural crest cells in zebrafish

In cranial skeletal development, the establishment of the ectomesenchymal lineage within the cranial neural crest is of great significance. Fgfs are polypeptide growth factors with diverse functions in development and metabolism. Fgf20b knockdown zebrafish embryos showed dysplastic neurocranial and pharyngeal cartilages. Ectomesenchymal cells from cranial neural crest cells were significantly decreased in Fgf20b knockdown embryos, but cranial neural crest cells with a non-ectomesnchymal fate were increased. However, the proliferation and apoptosis of cranial neural crest cells were essentially unchanged. Fgfr1 knockdown embryos also showed dysplastic neurocranial and pharyngeal cartilages. The present findings indicate that Fgf20b is required for ectomesenchymal fate establishment via the activation of Fgfr1 in zebrafish.

Anterior Hox genes interact with components of the neural crest specification network to induce neural crest fates

Hox genes play a central role in neural crest (NC) patterning particularly in the cranial region of the body. Despite evidence that simultaneous loss of Hoxa1 and Hoxb1 function resulted in NC specification defects, the role of Hox genes in NC specification has remained unclear due to extended genetic redundancy among Hox genes. To circumvent this problem, we expressed anterior Hox genes in the trunk neural tube of the developing chick embryo. This demonstrated that anterior Hox genes play a central role in NC cell specification by rapidly inducing the key transcription factors Snail2 and Msx1/2 and a neural progenitor to NC cell fate switch characterized by cell adhesion changes and an epithelial-to-mesenchymal transition (EMT). Cells delaminated from dorsal and medial neural tube levels and generated ectopic neurons, glia progenitors, and melanocytes. The mobilization of the NC genetic cascade was dependent upon bone morphogenetic protein signaling and optimal levels of Notch signaling. Therefore, anterior Hox patterning genes participate in NC specification and EMT by interacting with NC-inducing signaling pathways and regulating the expression of key genes involved in these processes.

Cold exposure down-regulates zebrafish pigmentation

Vertebrates use adaptive mechanisms when exposed to physiologic stresses. However, the mechanisms of pigmentation regulation in response to physiologic stresses largely remain unclear. To address this issue, we developed a novel pigmentation model in adult zebrafish using coldwater exposure (cold zebrafish). When zebrafish were maintained at 17 °C, the pigmentation of their pigment stripes was reduced compared with zebrafish at 26.5 °C (normal zebrafish). In cold zebrafish, gene expression levels of tyrosinase and dopachrome tautomerase, which encode enzymes involved in melanogenesis, were down-regulated, suggesting that either down-regulation of melanin synthesis occurred or the number of melanophores decreased. Both regular and electron microscopic observation of zebrafish skin showed that the number of melanophores decreased, whereas aggregation of melanosomes was not changed in cold zebrafish compared with normal zebrafish. Taken together, we here show that cold exposure down-regulated adult zebrafish pigmentation through decreasing the number of melanophores and propose that the cold zebrafish model is a powerful tool for pigmentation research.

Characterization of zebrafish intestinal smooth muscle development using a novel sm22α-b promoter

Smooth muscle cells provide structural support for many tissues and control essential physiological processes, such as blood pressure and gastrointestinal motility. Relatively little is known about the early stages of intestinal smooth muscle development and its relationship to the development of the enteric nervous system, which regulates intestinal motility. Here, we report an evolutionarily conserved 523 base pair regulatory element within the promoter of the zebrafish sm22α-b (transgelin1) gene that directs transgene expression in smooth muscle cells of the intestine and other tissues. Comparative genomic analysis identified a conserved motif within this element consisting of two Serum Response Factor binding sites that is also present in the promoters of many mammalian smooth muscle genes. We established a stable line expressing GFP in smooth muscle cell and used this line to describe lineage relationships among cells within different intestinal smooth muscle layers and their co-development with the enteric nervous system (ENS).

Zebrafish: an exciting model for investigating the spatio-temporal pattern of enteric nervous system development

AIM

Recently, the zebrafish (Danio rerio) has been shown to be an excellent model for human paediatric research. Advantages over other models include its small size, externally visually accessible development and ease of experimental manipulation. The enteric nervous system (ENS) consists of neurons and enteric glia. Glial cells permit cell bodies and processes of neurons to be arranged and maintained in a proper spatial arrangement, and are essential in the maintenance of basic physiological functions of neurons. Glial fibrillary acidic protein (GFAP) is expressed in astrocytes, but also expressed outside of the central nervous system. The aim of this study was to investigate the spatio-temporal pattern of GFAP expression in developing zebrafish ENS from 24 h post-fertilization (hpf), using transgenic fish that express green fluorescent protein (GFP).

METHODS

Zebrafish embryos were collected from transgenic GFP Tg(GFAP:GFP)(mi2001) adult zebrafish from 24 to 120 hpf, fixed and processed for whole mount immunohistochemistry. Antibodies to Phox2b were used to identify enteric neurons. Specimens were mounted on slides and imaging was performed using a fluorescent laser confocal microscope.

RESULTS

GFAP:GFP labelling outside the spinal cord was identified in embryos from 48 hpf. The patterning was intracellular and consisted of elongated profiles that appeared to migrate away from the spinal cord into the periphery. At 72 and 96 hpf, GFAP:GFP was expressed dorsally and ventrally to the intestinal tract. At 120 hpf, GFAP:GFP was expressed throughout the intestinal wall, and clusters of enteric neurons were identified using Phox2b immunofluorescence along the pathway of GFAP:GFP positive processes, indicative of a migratory pathway of ENS precursors from the spinal cord into the intestine.

CONCLUSION

The pattern of migration of GFAP:GFP expressing cells outside the spinal cord suggests an organized, early developing migratory pathway to the ENS. This shows for the first time that Tg(GFAP:GFP)(mi2001) zebrafish model is an ideal one to study spatio-temporal patterning of early ENS development.

Differential effects of thin and thick filament disruption on zebrafish smooth muscle regulatory proteins

BACKGROUND

 The smooth muscle actin binding proteins Caldesmon and Tropomyosin (Tm) promote thin filament assembly by stabilizing actin polymerization, however, whether filament assembly affects either the stability or activation of these and other smooth muscle regulatory proteins is not known.

METHODS

Measurement of smooth muscle regulatory protein levels in wild type zebrafish larvae following antisense knockdown of smooth muscle actin (Acta2) and myosin heavy chain (Myh11) proteins, and in colourless mutants that lack enteric nerves. Comparison of intestinal peristalsis in wild type and colourless larvae.

KEY RESULTS

Knockdown of Acta2 led to reduced levels of phospho-Caldesmon and Tm. Total Caldesmon and phospho-myosin light chain (p-Mlc) levels were unaffected. Knockdown of Myh11 had no effect on the levels of either of these proteins. Phospho-Caldesmon and p-Mlc levels were markedly reduced in colourless mutants that have intestinal motility comparable with wild type larvae.

CONCLUSIONS & INFERENCES

These in vivo findings provide new information regarding the activation and stability of smooth muscle regulatory proteins in zebrafish larvae and their role in intestinal peristalsis in this model organism.

Schwann cells reposition a peripheral nerve to isolate it from postembryonic remodeling of its targets

Although much is known about the initial construction of the peripheral nervous system (PNS), less well understood are the processes that maintain the position and connections of nerves during postembryonic growth. Here, we show that the posterior lateral line nerve in zebrafish initially grows in the epidermis and then rapidly transitions across the epidermal basement membrane into the subepidermal space. Our experiments indicate that Schwann cells, which myelinate axons in the PNS, are required to reposition the nerve. In mutants lacking Schwann cells, the nerve is mislocalized and the axons remain in the epidermis. Transplanting wild-type Schwann cells into these mutants rescues the position of the nerve. Analysis of chimeric embryos suggests that the process of nerve relocalization involves two discrete steps - the degradation and recreation of the epidermal basement membrane. Although the outgrowth of axons is normal in mutants lacking Schwann cells, the nerve becomes severely disorganized at later stages. In wild-type embryos, exclusion of the nerve from the epidermis isolates axons from migration of their targets (sensory neuromasts) within the epidermis. Without Schwann cells, axons remain within the epidermis and are dragged along with the migrating neuromasts. Our analysis of the posterior lateral line system defines a new process in which Schwann cells relocate a nerve beneath the epidermal basement membrane to insulate axons from the postembryonic remodeling of their targets.

Glial cells revealed by GFAP immunoreactivity in fish gut

Glial fibrillary acidic protein (GFAP) is a commonly used marker to identify enteric glia in the mammalian gut. Little is however known about enteric glia in other vertebrates. The aim of the present study was to examine the distribution of GFAP immunoreactivity in adult and developing fish. In adult shorthorn sculpin (Myoxocephalus scorpius) and zebrafish (Danio rerio), GFAP immunoreactivity was seen in the myenteric plexus in all regions of the gut. Co-staining for the neuronal markers Hu C/D and acetylated tubulin showed that GFAP immunoreactivity was not associated with nerves. GFAP immunoreactivity was predominantly seen in processes with few glial cell bodies being demonstrated in adult fish. GFAP immunoreactivity was also found in the gut in larval zebrafish from 3 days post-fertilisation, i.e. at approximately the same time that differentiated enteric nerve cells first occur. Immunoreactivity was most prominent in areas with no or a low density of Hu-immunoreactive nerve cell bodies, indicating that the developing glia follows a different pattern from that of enteric neurons. The results suggest that GFAP can be used as a marker for enteric glia in fish, as in birds and mammals. The distribution of GFAP immunoreactivity implies that enteric glia are widespread in the fish gastrointestinal tract. Glia and neurons diverge early during development of the gastrointestinal tract.

Sox proteins in melanocyte development and melanoma

Over 10 years have passed since the first Sox gene was implicated in melanocyte development. Since then, we have discovered that SOX5, SOX9, SOX10 and SOX18 all participate as transcription factors that affect key melanocytic genes in both regulatory and modulatory fashions. Both SOX9 and SOX10 play major roles in the establishment and normal function of the melanocyte; SOX10 has been shown to heavily influence melanocyte development and SOX9 has been implicated in melanogenesis in the adult. Despite these advances, the precise cellular and molecular details of how these SOX proteins are regulated and interact during all stages of the melanocyte life cycle remain unknown. Improper regulation of SOX9 or SOX10 is also associated with cancerous transformation, and thus understanding the normal function of SOX proteins in the melanocyte will be key to revealing how these proteins contribute to melanoma.

Development of the autonomic nervous system: a comparative view

In this review we summarize current understanding of the development of autonomic neurons in vertebrates. The mechanisms controlling the development of sympathetic and enteric neurons have been studied in considerable detail in laboratory mammals, chick and zebrafish, and there are also limited data about the development of sympathetic and enteric neurons in amphibians. Little is known about the development of parasympathetic neurons apart from the ciliary ganglion in chicks. Although there are considerable gaps in our knowledge, some of the mechanisms controlling sympathetic and enteric neuron development appear to be conserved between mammals, avians and zebrafish. For example, some of the transcriptional regulators involved in the development of sympathetic neurons are conserved between mammals, avians and zebrafish, and the requirement for Ret signalling in the development of enteric neurons is conserved between mammals (including humans), avians and zebrafish. However, there are also differences between species in the migratory pathways followed by sympathetic and enteric neuron precursors and in the requirements for some signalling pathways.

Studying peripheral sympathetic nervous system development and neuroblastoma in zebrafish

The combined experimental attributes of the zebrafish model system, which accommodates cellular, molecular, and genetic approaches, make it particularly well-suited for determining the mechanisms underlying normal vertebrate development as well as disease states, such as cancer. In this chapter, we describe the advantages of the zebrafish system for identifying genes and their functions that participate in the regulation of the development of the peripheral sympathetic nervous system (PSNS). The zebrafish model is a powerful system for identifying new genes and pathways that regulate PSNS development, which can then be used to genetically dissect PSNS developmental processes, such as tissue size and cell numbers, which in the past haves proved difficult to study by mutational analysis in vivo. We provide a brief review of our current understanding of genetic pathways important in PSNS development, the rationale for developing a zebrafish model, and the current knowledge of zebrafish PSNS development. Finally, we postulate that knowledge of the genes responsible for normal PSNS development in the zebrafish will help in the identification of molecular pathways that are dysfunctional in neuroblastoma, a highly malignant cancer of the PSNS.

Autonomic innervation of the fish gut

The enteric nervous system follows a similar overall arrangement in all vertebrate groups. In fish, the majority of nerve cell bodies are found in the myenteric plexus, innervating muscles, blood vessels and glands. In this review, I describe similarities and differences in size, shape and transmitter content in enteric neurons in different fish species and also in comparison with other vertebrates, foremost mammals. The use of different histological and immunochemical methods is reviewed in a historical perspective including advantages and disadvantages of different methods. Lately, zebrafish have become an important model species for developmental studies of the nervous system, including the enteric nervous system, and this is briefly discussed. Finally, examples of how the enteric nervous system controls gut activity in fish is presented, focussing on the effect on gastrointestinal motility.

A new high-content model system for studies of gastrointestinal transit: the zebrafish

The zebrafish gastrointestinal (GI) tract displays an anatomy and cellular architecture that is similar to the human GI tract, with concentric layers of inner epithelia, connective tissue, circular muscle and outer longitudinal muscle layers. Propulsion of luminal content results from the integrated activity of smooth muscle cells, enteric neurons and the interstitial cells of Cajal (ICC). Zebrafish larvae are transparent and propagating contractions in the entire GI tract are easily visualized. A new moderate-throughput zebrafish-based GI transit assay is described in this issue of Neurogastroenterology and Motility. This assay utilizes intact zebrafish larvae which contain essential regulatory elements (ICC and enteric neurons). Forward genetic analysis, which identifies genes underlying specific phenotypes, is possible using the zebrafish system. The zebrafish model system compliments existing models for studies of GI motility and will contribute to the understanding of the regulation of GI motility, and to identification of novel drug targets.

Genetic model system studies of the development of the enteric nervous system, gut motility and Hirschsprung's disease

The enteric nervous system (ENS) is the largest and most complicated subdivision of the peripheral nervous system. Its action is necessary to regulate many of the functions of the gastrointestinal tract including its motility. Whilst the ENS has been studied extensively by developmental biologists, neuroscientists and physiologists for several decades it has only been since the early 1990s that the molecular and genetic basis of ENS development has begun to emerge. Central to this understanding has been the use of genetic model organisms. In this article, we will discuss recent advances that have been achieved using both mouse and zebrafish model genetic systems that have led to new insights into ENS development and the genetic basis of Hirschsprung's disease.

A zebrafish model for Waardenburg syndrome type IV reveals diverse roles for Sox10 in the otic vesicle

In humans, mutations in the SOX10 gene are a cause of the auditory-pigmentary disorder Waardenburg syndrome type IV (WS4) and related variants. SOX10 encodes an Sry-related HMG box protein essential for the development of the neural crest; deafness in WS4 and other Waardenburg syndromes is usually attributed to loss of neural-crest-derived melanocytes in the stria vascularis of the cochlea. However, SOX10 is strongly expressed in the developing otic vesicle and so direct roles for SOX10 in the otic epithelium might also be important. Here, we examine the otic phenotype of zebrafish sox10 mutants, a model for WS4. As a cochlea is not present in the fish ear, the severe otic phenotype in these mutants cannot be attributed to effects on this tissue. In zebrafish sox10 mutants, we see abnormalities in all otic placodal derivatives. Gene expression studies indicate deregulated expression of several otic genes, including fgf8, in sox10 mutants. Using a combination of mutant and morphant data, we show that the three sox genes belonging to group E (sox9a, sox9b and sox10) provide a link between otic induction pathways and subsequent otic patterning: they act redundantly to maintain sox10 expression throughout otic tissue and to restrict fgf8 expression to anterior macula regions. Single-cell labelling experiments indicate a small and transient neural crest contribution to the zebrafish ear during normal development, but this is unlikely to account for the strong defects seen in the sox10 mutant. We discuss the implication that the deafness in WS4 patients with SOX10 mutations might reflect a haploinsufficiency for SOX10 in the otic epithelium, resulting in patterning and functional abnormalities in the inner ear.

Stripes and belly-spots -- a review of pigment cell morphogenesis in vertebrates

Pigment patterns in the integument have long-attracted attention from both scientists and non-scientists alike since their natural attractiveness combines with their excellence as models for the general problem of pattern formation. Pigment cells are formed from the neural crest and must migrate to reach their final locations. In this review, we focus on our current understanding of mechanisms underlying the control of pigment cell migration and patterning in diverse vertebrates. The model systems discussed here - chick, mouse, and zebrafish - each provide unique insights into the major morphogenetic events driving pigment pattern formation. In birds and mammals, melanoblasts must be specified before they can migrate on the dorsolateral pathway. Transmembrane receptors involved in guiding them onto this route include EphB2 and Ednrb2 in chick, and Kit in mouse. Terminal migration depends, in part, upon extracellular matrix reorganization by ADAMTS20. Invasion of the ectoderm, especially into the feather germ and hair follicles, requires specific signals that are beginning to be characterized. We summarize our current understanding of the mechanisms regulating melanoblast number and organization in the epidermis. We note the apparent differences in pigment pattern formation in poikilothermic vertebrates when compared with birds and mammals. With more pigment cell types, migration pathways are more complex and largely unexplored; nevertheless, a role for Kit signaling in melanophore migration is clear and indicates that at least some patterning mechanisms may be highly conserved. We summarize the multiple factors thought to contribute to zebrafish embryonic pigment pattern formation, highlighting a recent study identifying Sdf1a as one factor crucial for regulation of melanophore positioning. Finally, we discuss the mechanisms generating a second, metamorphic pigment pattern in adult fish, emphasizing recent studies strengthening the evidence that undifferentiated progenitor cells play a major role in generating adult pigment cells.

Identification of neural crest and glial enhancers at the mouse Sox10 locus through transgenesis in zebrafish

Sox10 is a dynamically regulated transcription factor gene that is essential for the development of neural crest-derived and oligodendroglial populations. Developmental genes often require multiple regulatory sequences that integrate discrete and overlapping functions to coordinate their expression. To identify Sox10 cis-regulatory elements, we integrated multiple model systems, including cell-based screens and transposon-mediated transgensis in zebrafish, to scrutinize mammalian conserved, noncoding genomic segments at the mouse Sox10 locus. We demonstrate that eight of 11 Sox10 genomic elements direct reporter gene expression in transgenic zebrafish similar to patterns observed in transgenic mice, despite an absence of observable sequence conservation between mice and zebrafish. Multiple segments direct expression in overlapping populations of neural crest derivatives and glial cells, ranging from pan-Sox10 and pan-neural crest regulatory control to the modulation of expression in subpopulations of Sox10-expressing cells, including developing melanocytes and Schwann cells. Several sequences demonstrate overlapping spatial control, yet direct expression in incompletely overlapping developmental intervals. We were able to partially explain neural crest expression patterns by the presence of head to head SoxE family binding sites within two of the elements. Moreover, we were able to use this transcription factor binding site signature to identify the corresponding zebrafish enhancers in the absence of overall sequence homology. We demonstrate the utility of zebrafish transgenesis as a high-fidelity surrogate in the dissection of mammalian gene regulation, especially those with dynamically controlled developmental expression.

Development of enteric and vagal innervation of the zebrafish (Danio rerio) gut

The autonomic nervous system develops following migration and differentiation of precursor cells originating in the neural crest. Using immunohistochemistry on intact zebrafish embryos and larvae we followed the development of the intrinsic enteric and extrinsic vagal innervation of the gut. At 3 days postfertilization (dpf), enteric nerve cell bodies and fibers were seen mainly in the middle and distal intestine, while the innervation of the proximal intestine was scarcer. The number of fibers and cell bodies gradually increased, although a large intraindividual variation was seen in the timing (but not the order) of development. At 11-13 dpf most of the proximal intestine received a similar degree of innervation as the rest of the gut. The main intestinal branches of the vagus were similarly often already well developed at 3 dpf, entering the gut at the transition between the proximal and middle intestine and projecting posteriorly along the length of the gut. Subsequently, fibers branching off the vagus innervated all regions of the gut. The presence of several putative enteric neurotransmitters was suggested by using markers for neurokinin A (NKA), pituitary adenylate cyclase-activating polypeptide (PACAP), vasoactive intestinal polypeptide (VIP), nitric oxide, serotonin (5-hydroxytryptamine, 5-HT), and calcitonin gene-related peptide (CGRP). The present results corroborate the belief that the enteric innervation is well developed before the onset of feeding (normally occurring around 5-6 dpf). Further, the more detailed picture of how development proceeds at stages previously not examined suggests a correlation between increasing innervation and more regular and elaborated motility patterns.

Zebrafish endzone regulates neural crest-derived chromatophore differentiation and morphology

The development of neural crest-derived pigment cells has been studied extensively as a model for cellular differentiation, disease and environmental adaptation. Neural crest-derived chromatophores in the zebrafish (Danio rerio) consist of three types: melanophores, xanthophores and iridiphores. We have identified the zebrafish mutant endzone (enz), that was isolated in a screen for mutants with neural crest development phenotypes, based on an abnormal melanophore pattern. We have found that although wild-type numbers of chromatophore precursors are generated in the first day of development and migrate normally in enz mutants, the numbers of all three chromatophore cell types that ultimately develop are reduced. Further, differentiated melanophores and xanthophores subsequently lose dendricity, and iridiphores are reduced in size. We demonstrate that enz function is required cell autonomously by melanophores and that the enz locus is located on chromosome 7. In addition, zebrafish enz appears to selectively regulate chromatophore development within the neural crest lineage since all other major derivatives develop normally. Our results suggest that enz is required relatively late in the development of all three embryonic chromatophore types and is normally necessary for terminal differentiation and the maintenance of cell size and morphology. Thus, although developmental regulation of different chromatophore sublineages in zebrafish is in part genetically distinct, enz provides an example of a common regulator of neural crest-derived chromatophore differentiation and morphology.

Sequential actions of Pax3 and Pax7 drive xanthophore development in zebrafish neural crest

The Pax3/7 gene family has a fundamental and conserved role during neural crest formation. In people, PAX3 mutation causes Waardenburg syndrome, and murine Pax3 is essential for pigment formation. However, it is unclear exactly how Pax3 functions within the neural crest. Here we show that pax3 is expressed before other pax3/7 members, including duplicated pax3b, pax7 and pax7b genes, early in zebrafish neural crest development. Knockdown of Pax3 protein by antisense morpholino oligonucleotides results in defective fate specification of xanthophores, with complete ablation in the trunk. Other pigment lineages are specified and differentiate. As a consequence of xanthophore loss, expression of pax7, a marker of the xanthophore lineage, is reduced in neural crest. Morpholino knockdown of Pax7 protein shows that Pax7 itself is dispensable for xanthophore fate specification, although yellow pigmentation is reduced. Loss of xanthophores after reduction of Pax3 correlates with a delay in melanoblast differentiation followed by significant increase in melanophores, suggestive of a Pax3-driven fate switch within a chromatophore precursor or stem cell. Analysis of other neural crest derivatives reveals that, in the absence of Pax3, the enteric nervous system is ablated from its inception. Therefore, Pax3 in zebrafish is required for specification of two specific lineages of neural crest, xanthophores and enteric neurons.

Leukocyte tyrosine kinase functions in pigment cell development

A fundamental problem in developmental biology concerns how multipotent precursors choose specific fates. Neural crest cells (NCCs) are multipotent, yet the mechanisms driving specific fate choices remain incompletely understood. Sox10 is required for specification of neural cells and melanocytes from NCCs. Like sox10 mutants, zebrafish shady mutants lack iridophores; we have proposed that sox10 and shady are required for iridophore specification from NCCs. We show using diverse approaches that shady encodes zebrafish leukocyte tyrosine kinase (Ltk). Cell transplantation studies show that Ltk acts cell-autonomously within the iridophore lineage. Consistent with this, ltk is expressed in a subset of NCCs, before becoming restricted to the iridophore lineage. Marker analysis reveals a primary defect in iridophore specification in ltk mutants. We saw no evidence for a fate-shift of neural crest cells into other pigment cell fates and some NCCs were subsequently lost by apoptosis. These features are also characteristic of the neural crest cell phenotype in sox10 mutants, leading us to examine iridophores in sox10 mutants. As expected, sox10 mutants largely lacked iridophore markers at late stages. In addition, sox10 mutants unexpectedly showed more ltk-expressing cells than wild-type siblings. These cells remained in a premigratory position and expressed sox10 but not the earliest neural crest markers and may represent multipotent, but partially-restricted, progenitors. In summary, we have discovered a novel signalling pathway in NCC development and demonstrate fate specification of iridophores as the first identified role for Ltk.

Stem and other multipotent cells generate diverse cell-types, but our understanding of how they make these decisions, which is important for their therapeutic use, is incomplete. Neural crest cells are an important class of multipotent cells and generate multiple stem cell types. We have looked at how pigment cells are made from the neural crest in the zebrafish. The silver shine familiar in so many fish is due to specialised mirror-like pigment cells, called iridophores. We show that these cells are missing in zebrafish shady mutants. We identify the shady gene as encoding a cell signalling receptor, leukocyte tyrosine kinase (Ltk), that has recently been associated with human auto-immune disease. We show that in zebrafish this gene is most likely required to make iridophores from neural crest cells. Thus, we identify a novel pathway required for diversification of these multipotent cells. Our work defines the first role for Ltk in a vertebrate. It provides a mutant resource that will allow us to discover the full breadth of roles for this important gene. Furthermore, the loss of iridophores forms a simple visual screen for inhibition of LTK function and might well have implications in drug discovery.

colgate/hdac1 Repression of foxd3 expression is required to permit mitfa-dependent melanogenesis

Neural crest-derived pigment cell development has been used extensively to study cell fate specification, migration, proliferation, survival and differentiation. Many of the genes and regulatory mechanisms required for pigment cell development are conserved across vertebrates. The zebrafish mutant colgate (col)/histone deacetylase1 (hdac1) has reduced numbers, delayed differentiation and decreased migration of neural crest-derived melanophores and their precursors. In hdac1(col) mutants normal numbers of premigratory neural crest cells are induced. Later, while there is only a slight reduction in the number of neural crest cells in hdac1(col) mutants, there is a severe reduction in the number of mitfa-positive melanoblasts suggesting that hdac1 is required for melanoblast specification. Concomitantly, there is a significant increase in and prolonged expression of foxd3 in neural crest cells in hdac1(col) mutants. We found that partially reducing Foxd3 expression in hdac1(col) mutants rescues mitfa expression and the melanophore defects in hdac1(col) mutants. Furthermore, we demonstrate the ability of Foxd3 to physically interact at the mitfa promoter. Because mitfa is required for melanoblast specification and development, our results suggest that hdac1 is normally required to suppress neural crest foxd3 expression thus de-repressing mitfa resulting in melanogenesis by a subset of neural crest-derived cells.

A Rap GTPase interactor, RADIL, mediates migration of neural crest precursors

The neural crest (NC) is a highly motile cell population that gives rise to multiple tissue lineages during vertebrate embryogenesis. Here, we identify a novel effector of the small GTPase Rap, called RADIL, and show that it is required for cell adhesion and migration. Knockdown of radil in the zebrafish model results in multiple defects in NC-derived lineages such as cartilage, pigment cells, and enteric neurons. We specifically show that these defects are primarily due to the diminished migratory capacity of NC cells. The identification of RADIL as a regulator of NC migration defines a role for the Rap pathway in this process.

Kit-like immunoreactivity in the zebrafish gastrointestinal tract reveals putative ICC

Gastrointestinal (GI) motility results from the coordinated actions of enteric neurons, interstitial cells of Cajal (ICC), and smooth muscle cells. The GI tract of the zebrafish has a cellular anatomy that is essentially similar to humans. Although enteric nerves and smooth muscle cells have been described, it is unknown if ICC are present in the zebrafish. Immunohistochemistry and PCR were used determine expression for the zebrafish Kit orthologue in the zebrafish gastrointestinal tract. Cells displaying Kit-like immunoreactivity were identified in the muscular layers of the adult zebrafish gastrointestinal tract. Two layers of Kit-positive cells were identified, one with multipolar cells located between the longitudinal and circular smooth muscle layers and one with simple bipolar cells located deep in the circular muscle layer. Primers specifically designed to amplify mRNA coding for two zebrafish kit genes, kita and kitb, and two kit ligands, kitla and kitlb, amplified the expected transcript from total RNA isolated from zebrafish GI tissues. The Sparse mutant, a kita null mutant, showed reduced contraction frequency and increased size of the GI tract indicating a functional role for kita. These data establish the presence of a cellular network with Kit-like immunoreactivity in the myenteric plexus region of the zebrafish GI tract, adjacent to enteric neurons. Expression of kita and kitb, and the ligands kitla and kitlb, were verified in the adult GI tract. The anatomical arrangement of the Kit-positive cells strongly suggests that they are ICC.

A screen for genetic defects of the zebrafish ear

To advance the understanding of genetic mechanisms involved in the patterning and the differentiation of the vertebrate auditory system, we screened for mutations affecting ear development in the zebrafish larva. Fifteen recessive mutant alleles have been isolated and analyzed. The phenotypes of these mutants involve abnormalities in ear morphology, otolith formation, or both processes in parallel. Among morphological defects, we found mutations affecting early patterning of the otic vesicle, the morphogenesis of semicircular canals, and the expansion of the ear lumen. The two most severe mutant phenotypes involve the absence of anterior and posterior cristae, as well as a severely misshapen morphology of the ear. In the category of otolith mutants, we found defects in otolith formation, growth, and shape. As it proved to be the case in past screening efforts of this type, these mutant lines represent an asset in the studies of molecular mechanisms that regulate vertebrate ear development.

Genetic screen for mutations affecting development and function of the enteric nervous system

An intact enteric nervous system is required for normal gastrointestinal tract function. Several human conditions result from decreased innervation by enteric neurons; however, the genetic basis of enteric nervous system development and function is incompletely understood. In an effort to increase our understanding of the mechanisms underlying enteric nervous system development, we screened mutagenized zebrafish for changes in the number or distribution of enteric neurons. We also established a motility assay and rescreened mutants to learn whether enteric neuron number is correlated with gastrointestinal motility in zebrafish. We describe mutations isolated in our screen that affect enteric neurons specifically, as well as mutations that affect other neural crest derivatives or have pleiotropic effects. We show a correlation between the severity of enteric neuron loss and gastrointestinal motility defects. This screen provides biological tools that serve as the basis for future mechanistic studies.

The mother superior mutation ablates foxd3 activity in neural crest progenitor cells and depletes neural crest derivatives in zebrafish

The zebrafish mutation mother superior (mosm188) leads to a depletion of neural crest (NC) derivatives including the craniofacial cartilage skeleton, the peripheral nervous system (sympathetic neurons, dorsal root ganglia, enteric neurons), and pigment cells. The loss of derivatives is preceded by a reduction in NC-expressed transcription factors, snail1b, sox9b, sox10, and a specific loss of foxd3 expression in NC progenitor cells. We employed genetic linkage analysis and physical mapping to place the mosm188 mutation on zebrafish chromosome 6 in the vicinity of the foxd3 gene. Furthermore, we found that mosm188 does not complement the sym1/foxd3 mutation, indicating that mosm188 resides within the foxd3 locus. Injection of PAC clones containing the foxd3 gene into mosm188 embryos restored foxd3 expression in NC progenitors and suppressed the mosm188 phenotype. However, sequencing the foxd3 transcribed area in mosm188 embryos did not reveal nucleotide changes segregating with the mosm188 phenotype, implying that the mutation most likely resides outside the foxd3-coding region. Based on these findings, we propose that the mosm188 mutation perturbs a NC-specific foxd3 regulatory element. Further analysis of mosm188 mutants and foxd3 morphants revealed that NC cells are initially formed, suggesting that foxd3 function is required to maintain the pool of NC progenitors.

Sorting out Sox10 functions in neural crest development

For both vertebrate developmental and evolutionary biologists, and also for clinicians, the neural crest (NC) is a fundamental cell population. An understanding of Sox10 function in NC development is of particular significance since Sox10 mutations underlie several neurocristopathies. Surprisingly, experiments in different model organisms aimed at identifying Sox10's role(s) have suggested at least four distinct functions. Sox10 may be critical for formation of neural crest cells (NCCs), maintaining multipotency of crest cells, specification of derivative cell fates from these cells and their differentiation. Here, I discuss this controversy and argue that these functions are, in part, molecularly interrelated.

Signals and myogenic regulatory factors restrict pax3 and pax7 expression to dermomyotome-like tissue in zebrafish

Pax3/7 paired homeodomain transcription factors are important markers of muscle stem cells. Pax3 is required upstream of myod for lateral dermomyotomal cells in the amniote somite to form particular muscle cells. Later Pax3/7-dependent cells generate satellite cells and most body muscle. Here we analyse early myogenesis from, and regulation of, a population of Pax3-expressing dermomyotome-like cells in the zebrafish. Zebrafish pax3 is widely expressed in the lateral somite and, along with pax7, becomes restricted anteriorly and then to the external cells on the lateral somite surface. Midline-derived Hedgehog signals appear to act directly on lateral somite cells to repress Pax3/7. Both Hedgehog and Fgf8, signals that induce muscle formation within the somite, suppress Pax3/7 and promote expression of myogenic regulatory factors (MRFs) myf5 and myod in specific muscle precursor cell populations. Loss of MRF function leads to loss of myogenesis by specific populations of muscle fibres, with parallel up-regulation of Pax3/7. Myod is required for lateral fast muscle differentiation from pax3-expressing cells. In contrast, either Myf5 or Myod is sufficient to promote slow muscle formation from adaxial cells. Thus, myogenic signals act to drive somite cells to a myogenic fate through up-regulation of distinct combinations of MRFs. Our data show that the relationship between Pax3/7 genes and myogenesis is evolutionarily ancient, but that changes in the MRF targets for particular signals contribute to myogenic differences between species.

GFAP transgenic zebrafish

We have generated transgenic zebrafish that express green fluorescent protein (GFP) in glial cells driven by the zebrafish glial fibrillary acidic protein (GFAP) regulatory elements. Transgenic lines Tg(gfap:GFP) were generated from three founders; the results presented here are from the mi2001 line. GFP expression was first visible in the living embryo at the tail bud-stage, then in the developing brain by the 5-somite-stage ( approximately 12 h post-fertilization, hpf) and then spreading posteriorly along the developing spinal cord by the 12-somite stage (approximately 15 hpf). At 24 hpf GFP-expressing cells were in the retina and lens. By 72 hpf GFP expression levels were strong and localized to the glia of the brain, neural retina, spinal cord, and ventral spinal nerves, with moderate expression in the enteric nervous system and weaker levels in the olfactory sensory placode and otic capsule. GFP expression in glia co-localized with anti-GFAP antibodies, but did not co-localize with the neuronal antibodies HuC/D or calretinin in mature neurons.

Duplicate sfrp1 genes in zebrafish: sfrp1a is dynamically expressed in the developing central nervous system, gut and lateral line

The secreted frizzled-related proteins (Sfrp) are a family of soluble proteins with diverse biological functions having the capacity to bind Wnt ligands, to modulate Wnt signalling, and to signal directly via the Wnt receptor, Frizzled. In an enhancer trap screen for embryonic expression in zebrafish we identified an sfrp1 gene. Previous studies suggest an important role for sfrp1 in eye development, however, no data have been reported using the zebrafish model. In this paper, we describe duplicate sfrp1 genes in zebrafish and present a detailed analysis of the expression profile of both genes. Whole mount in situ hybridisation analyses of sfrp1a during embryonic and larval development revealed a dynamic expression profile, including: the central nervous system, where sfrp1a was regionally expressed throughout the brain and developing eye; the posterior gut, from the time of endodermal cell condensation; the lateral line, where sfrp1a was expressed in the migrating primordia and interneuromast cells that give rise to the sensory organs. Other sites included the blastoderm, segmenting mesoderm, olfactory placode, developing ear, pronephros and fin-bud. We have also analysed sfrp1b expression during embryonic development. Surprisingly this gene exhibited a divergent expression profile being limited to the yolk syncytium under the elongating tail-bud, which later covered the distal yolk extension, and transiently in the tail-bud mesenchyme. Overall, our studies provide a basis for future analyses of these developmentally important factors using the zebrafish model.

Zebrafish foxd3 is selectively required for neural crest specification, migration and survival

The vertebrate neural crest is a pluripotent cell population that generates a large variety of cell types, including peripheral neurons, cartilage and pigment cells. Mechanisms that control the patterning of the neural crest toward specific cell fates remain only partially understood. Zebrafish homozygous for the sympathetic mutation 1 (sym1) have defects in a subset of neural crest derivatives, such as peripheral neurons, glia and cartilage, but retain normal numbers of melanocytes. The sym1 mutation is a nucleotide deletion that disrupts the forkhead DNA-binding domain of the foxd3 gene, which encodes a conserved winged-helix transcription factor. We show that sym1 mutants have normal numbers of premigratory neural crest cells, but these cells express reduced levels of snai1b and sox10, implicating foxd3 as an essential regulator of these transcription factors in the premigratory neural crest. The onset of neural crest migration is also delayed in sym1 mutants, and there is a reduction in the number of migratory trunk neural crest cells, particularly along the medial migration pathway. TUNEL analysis revealed aberrant apoptosis localized to the hindbrain neural crest at the 15-somite stage, indicating a critical role for foxd3 in the survival of a subpopulation of neural crest cells. These results show that foxd3 selectively specifies premigratory neural crest cells for a neuronal, glial or cartilage fate, by inducing the expression of lineage-associated transcription factors in these cells and regulating their subsequent migration.

Deletion of long-range sequences at Sox10 compromises developmental expression in a mouse model of Waardenburg-Shah (WS4) syndrome

The transcription factor SOX10 is mutated in the human neurocristopathy Waardenburg-Shah syndrome (WS4), which is characterized by enteric aganglionosis and pigmentation defects. SOX10 directly regulates genes expressed in neural crest lineages, including the enteric ganglia and melanocytes. Although some SOX10 target genes have been reported, the mechanisms by which SOX10 expression is regulated remain elusive. Here, we describe a transgene-insertion mutant mouse line (Hry) that displays partial enteric aganglionosis, a loss of melanocytes, and decreased Sox10 expression in homozygous embryos. Mutation analysis of Sox10 coding sequences was negative, suggesting that non-coding regulatory sequences are disrupted. To isolate the Hry molecular defect, Sox10 genomic sequences were collected from multiple species, comparative sequence analysis was performed and software was designed (ExactPlus) to identify identical sequences shared among species. Mutation analysis of conserved sequences revealed a 15.9 kb deletion located 47.3 kb upstream of Sox10 in Hry mice. ExactPlus revealed three clusters of highly conserved sequences within the deletion, one of which shows strong enhancer potential in cultured melanocytes. These studies: (i) present a novel hypomorphic Sox10 mutation that results in a WS4-like phenotype in mice; (ii) demonstrate that a 15.9 kb deletion underlies the observed phenotype and likely removes sequences essential for Sox10 expression; (iii) combine a novel in silico method for comparative sequence analysis with in vitro functional assays to identify candidate regulatory sequences deleted in this strain. These studies will direct further analyses of Sox10 regulation and provide candidate sequences for mutation detection in WS4 patients lacking a SOX10-coding mutation.

Genetic network during neural crest induction: from cell specification to cell survival

The concerted action of extracellular signals such as BMP, Wnt, FGF, RA and Notch activate a genetic program required to transform a naïve ectodermal cell into a neural crest cell. In this review we will analyze the extracellular signals and the network of transcription factors that are required for this transformation. We will propose the division of this complex network of factors in two main steps: an initial cell specification step followed by a maintenance or cell survival step.

Intestinal growth and differentiation in zebrafish

Intestinal development in amniotes is driven by interactions between progenitor cells derived from the three primary germ layers. Genetic analyses and gene targeting experiments in zebrafish offer a novel approach to dissect such interactions at a molecular level. Here we show that intestinal anatomy and architecture in zebrafish closely resembles the anatomy and architecture of the mammalian small intestine. The zebrafish intestine is regionalized and the various segments can be identified by epithelial markers whose expression is already segregated at the onset of intestinal differentiation. Differentiation of cells derived from the three primary germ layers begins more or less contemporaneously, and is preceded by a stage in which there is rapid cell proliferation and maturation of epithelial cell polarization. Analysis of zebrafish mutants with altered epithelial survival reveals that seemingly related single gene defects have different effects on epithelial differentiation and smooth muscle and enteric nervous system development.

The three-sided romance of the lateral line: glia love axons love precursors love glia

The lateral line system of fish and amphibians is closely related to the inner ear in terms of evolution, morphology and physiology. Several recent papers have shed new light on the postembryonic development of this system, and have revealed an unexpected triangular relationship where migrating sensory precursors guide axons, axons guide glia and glia, in turn, control the formation of sensory organs. They have also revealed the crucial importance of controlled cell migration not only for patterning the system, but also for determining polarity (and therefore directional sensitivity) of the mechanosensory hair cells. The remarkable accessibility of the lateral line system may allow a detailed analysis of cell migration and polarization, and may help us better understand the complex interactions between sensory precursor cells, neurons and glia during development.

Melanophore sublineage-specific requirement for zebrafish touchtone during neural crest development

The specification, differentiation and maintenance of diverse cell types are of central importance to the development of multicellular organisms. The neural crest of vertebrate animals gives rise to many derivatives, including pigment cells, peripheral neurons, glia and elements of the craniofacial skeleton. The development of neural crest-derived pigment cells has been studied extensively to elucidate mechanisms involved in cell fate specification, differentiation, migration and survival. This analysis has been advanced considerably by the availability of large numbers of mouse and, more recently, zebrafish mutants with defects in pigment cell development. We have identified the zebrafish mutant touchtone (tct), which is characterized by the selective absence of most neural crest-derived melanophores. We find that although wild-type numbers of melanophore precursors are generated in the first day of development and migrate normally in tct mutants, most differentiated melanophores subsequently fail to appear. We demonstrate that the failure in melanophore differentiation in tct mutant embryos is due at least in part to the death of melanoblasts and that tct function is required cell autonomously by melanoblasts. The tct locus is located on chromosome 18 in a genomic region apparently devoid of genes known to be involved in melanophore development. Thus, zebrafish tct may represent a novel as well as selective regulator of melanoblast development within the neural crest lineage. Further, our results suggest that, like other neural crest-derived sublineages, melanogenic precursors constitute a heterogeneous population with respect to genetic requirements for development.

The Tomita collection of medaka pigmentation mutants as a resource for understanding neural crest cell development

All body pigment cells in vertebrates are derived from the neural crest. In fish the neural crest can generate up to six different types of pigment cells, as well as various non-pigmented derivatives. In mouse and zebrafish, extensive collections of pigmentation mutants have enabled dissection of many aspects of pigment cell development, including fate specification, survival, proliferation and differentiation. A collection of spontaneous mutations collected from wild medaka (Oryzias latipes) populations and maintained at Nagoya University includes more than 40 pigmentation mutations. The descriptions of their adult phenotypes have been previously published by Tomita and colleagues (summarised in Medaka (Killifish) Biology and Strains, 1975), but the embryonic phenotypes have not been systematically described. Here we examine these embryonic phenotypes, paying particular attention to the likely defect in pigment cell development in each, and comparing the spectrum of defects to those in the zebrafish and mouse collections. Many phenotypes parallel those of identified zebrafish mutants, although pigment cell death phenotypes are largely absent, presumably due to the different selective pressures under which the mutants were isolated. We have identified mutant phenotypes that may represent the Mitf/Kit pathway of melanophore specification and survival. We use in situ hybridisation with available markers to confirm a key prediction of this hypothesis. We also highlight a set of novel phenotypes not seen in the zebrafish collection. These mutants will be a valuable resource for pigment cell and neural crest studies and will strongly complement the mutant collections in other vertebrates.