On the embryonic origin of adult melanophores: the role of ErbB and Kit signalling in establishing melanophore stem cells in zebrafish

Rapid and robust generation of cardiomyocyte-specific crispants in zebrafish using the cardiodeleter system

CRISPR-Cas9 has accelerated loss-of-function studies in zebrafish, but creating tissue-specific mutant lines is still labor intensive. While some tissue-specific Cas9 zebrafish lines exist, standardized methods for gene targeting, including guide RNA (gRNA) delivery, are lacking, limiting broader use in the community. To tackle these limitations, we develop a cardiomyocyte-specific Cas9 line, the cardiodeleter, that efficiently generates biallelic mutations in combination with gene-specific gRNAs. We create transposon-based guide shuttles that deliver gRNAs targeting a gene of interest while permanently labeling cells susceptible to becoming mutant. We validate this modular approach by deleting five genes (ect2, tnnt2a, cmlc2, amhc, and erbb2), resulting in the loss of the corresponding protein or phenocopy of established mutants. We provide detailed protocols for generating guide shuttles, facilitating the adoption of these techniques in the zebrafish community. Our approach enables rapid generation of tissue-specific crispants and analysis of mosaic phenotypes, making it a valuable tool for cell-autonomous studies and genetic screening.

Optimization of methods for rapid and robust generation of cardiomyocyte-specific crispants in zebrafish using the cardiodeleter system

CRISPR/Cas9 has massively accelerated the generation of gene loss-of-function models in zebrafish. However, establishing tissue-specific mutant lines remains a laborious and time-consuming process. Although a few dozen tissue-specific Cas9 zebrafish lines have been developed, the lack of standardization of some key methods, including gRNA delivery, has limited the implementation of these approaches in the zebrafish community. To tackle these limitations, we have established a cardiomyocyte-specific Cas9 line, the cardiodeleter , which efficiently generates biallelic mutations in combination with gene-specific gRNAs. We have also optimized the development of transposon-based guide shuttles that carry gRNAs targeting a gene of interest and permanently label the cells susceptible to becoming mutant. We validated this modular approach by deleting five genes ( ect2 , tnnt2a , cmlc2 , amhc , and erbb2 ), all resulting in the loss of the corresponding protein or phenocopying established mutants. Additionally, we provide detailed protocols describing how to generate guide shuttles , which will facilitate the dissemination of these techniques in the zebrafish community. Our approach enables the rapid generation of tissue-specific crispants and analysis of mosaic phenotypes, bypassing limitations such as embryonic lethality, making it a valuable tool for cell-autonomous studies and genetic screenings.

Melanocyte lineage dynamics in development, growth and disease

Melanocytes evolved to produce the melanin that gives colour to our hair, eyes and skin. The melanocyte lineage also gives rise to melanoma, the most lethal form of skin cancer. The melanocyte lineage differentiates from neural crest cells during development, and most melanocytes reside in the skin and hair, where they are replenished by melanocyte stem cells. Because the molecular mechanisms necessary for melanocyte specification, migration, proliferation and differentiation are co-opted during melanoma initiation and progression, studying melanocyte development is directly relevant to human disease. Here, through the lens of advances in cellular omic and genomic technologies, we review the latest findings in melanocyte development and differentiation, and how these developmental pathways become dysregulated in disease.

Toxic effects and comparison of common amino antioxidants (AAOs) in the environment on zebrafish: A comprehensive analysis based on cells, embryos, and adult fish

The ubiquity of amino antioxidants (AAOs) in the environment has attracted increasing attention, given their potential toxicity. This investigation represents a pioneering effort, systematically scrutinizing the toxicological effects of four distinct AAOs across the developmental spectrum of zebrafish, encompassing embryonic, larvae, and adult stages. The results indicate that four types of AAO exhibit varying degrees of cell proliferation toxicity. Although environmentally relevant concentrations of AAOs exhibit a comparatively circumscribed impact on zebrafish embryo development, heightened concentrations (300 μg/L) of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and N-isopropyl-N'-phenyl-p-phenylenediamine (IPPD) distinctly evoke developmental toxicity. Behavioral analysis results indicate that at concentrations of 20 and 300 μg/L, the majority of AAOs significantly reduced the swimming speed and activity of larvae. Moreover, each AAO triggers the generation of reactive oxygen species (ROS) in larvae, instigating diverse levels of oxidative stress. The study delineates parallel toxicological patterns in zebrafish exposed to 300 μg/L of 6PPD and IPPD, thereby establishing a comparable toxicity profile. The comprehensive toxicity effects among the four AAOs is as follows: IPPD >6PPD > N-Phenyl-1-naphthylamine (PANA) > diphenylamine (DPA). These findings not only enrich our comprehension of the potential hazards associated with AAOs but also provide data support for structure-based toxicity prediction models.

The melano-macrophage: The black leukocyte of fish immunity

Melanin and the process of melanin synthesis or melanogenesis have central roles in the immune system of insects, and production of melanin-synthesizing enzymes from their haemocytes may be induced following activation through danger signals. Melanin-containing macrophage-like cells have been extensively studied in amphibians and they are also present in reptiles. In fish, melano-macrophages are especially recognized with respect to melano-macrophage centres (MMCs), hypothesized to be analogues of germinal centres in secondary lymphoid organs of mammals and some birds. Melano-macrophages are in addition present in several inflammatory conditions, in particular melanised focal changes, or black spots, in the musculature of farmed Atlantic salmon, Salmo salar. Melanins are complex compounds that may be divided into different forms which all have the ability to absorb and scatter light. Other functions include the quenching of free radicals and a direct effect on the immune system. According to the common view held in the pigment cell community, vertebrate melanin synthesis with melanosome formation may only occur in cells of ectodermal origin. However, abundant information suggests that also myeloid cells of ectothermic vertebrates may be classified as melanocytes. Here, we discuss these opposing views and review relevant literature. Finally, we review the current status on the research concerning melanised focal muscle changes that represent the most severe quality problem in Norwegian salmon production, but also other diseases where melano-macrophages play important roles.

col1a2+ fibroblasts/muscle progenitors finetune xanthophore countershading by differentially expressing csf1a/1b in embryonic zebrafish

Animals evolve diverse pigment patterns to adapt to the natural environment. Countershading, characterized by a dark-colored dorsum and a light-colored ventrum, is one of the most prevalent pigment patterns observed in vertebrates. In this study, we reveal a mechanism regulating xanthophore countershading in zebrafish embryos. We found that csf1a and csf1b mutants altered xanthophore countershading differently: csf1a mutants lack ventral xanthophores, while csf1b mutants have reduced dorsal xanthophores. Further study revealed that csf1a is expressed throughout the trunk, whereas csf1b is expressed dorsally. Ectopic expression of csf1a or csf1b in neurons attracted xanthophores into the spinal cord. Blocking csf1 signaling by csf1ra mutants disrupts spinal cord distribution and normal xanthophores countershading. Single-cell RNA sequencing identified two col1a2+ populations: csf1ahighcsf1bhigh muscle progenitors and csf1ahighcsf1blow fibroblast progenitors. Ablation of col1a2+ fibroblast and muscle progenitors abolished xanthophore patterns. Our study suggests that fibroblast and muscle progenitors differentially express csf1a and csf1b to modulate xanthophore patterning, providing insights into the mechanism of countershading.

2,2',4,4'-tetrabromodiphenyl ether causes depigmentation in zebrafish larvae via a light-mediated pathway

Polybrominated diphenyl ethers (PBDEs) are organic pollutants widely detected in various environmental media due to their high persistence and bioaccumulation. PBDE-induced visual impairment and neurotoxicity were previously demonstrated using zebrafish (Danio rerio) models, and recent research reported the phenotypic depigmentation effect of PBDEs at high concentrations on zebrafish, but whether those effects are still present at environment-relevant levels is still unclear. Herein, we performed both phenotypic examination and mechanism investigation in zebrafish embryos (48 hpf) and larvae (5 dpf) about their pigmentation status when exposing to PBDE congener BDE-47 (2,2',4,4'-tetrabrominated diphenyl ether) at levels from 0.25 to 25 μg/L. Results showed that low-level BDE-47 can restrain the relative melanin abundance of zebrafish larvae to 70.47% (p < 0.05) and 61.54% (p < 0.01) respectively under 2.5 and 25 μg/L BDE-47 compared with control, and the thickness of retinal pigment epithelium (RPE) remarkably reduced from 571.4 nm to 350.3 nm (p < 0.001) under 25 μg/L BDE-47 exposure. We also observed disrupted expressions of melanin synthesis genes and disorganized mitfa differentiation patterns based on Tg(mifta:EGFP), as well as visual impairment resulting from thinner RPE. Considering both processes of visual development and melanin synthesis are highly sensitive to ambient light conditions, we prolonged the light regime of maintaining zebrafish larvae from 14 hours light versus 10 hours dark (14L:10D) to 18 hours light versus 6 hours dark (18L:6D). Lengthening photoperiod successfully rescued the fluorescent level of mitfa in zebrafish epidermis and most gene expressions associated with melanin synthesis under 25 μg/L BDE-47 exposure to the normal level. In conclusion, our work reported the effects of low-level PBDEs on melanin production using zebrafish embryos and larvae, and identified the potential role of a light-mediated pathway in the neurotoxic mechanism of PBDEs.

The Unreasonable Effectiveness of Reaction Diffusion in Vertebrate Skin Color Patterning

In 1952, Alan Turing published the reaction-diffusion (RD) mathematical framework, laying the foundations of morphogenesis as a self-organized process emerging from physicochemical first principles. Regrettably, this approach has been widely doubted in the field of developmental biology. First, we summarize Turing's line of thoughts to alleviate the misconception that RD is an artificial mathematical construct. Second, we discuss why phenomenological RD models are particularly effective for understanding skin color patterning at the meso/macroscopic scales, without the need to parameterize the profusion of variables at lower scales. More specifically, we discuss how RD models (a) recapitulate the diversity of actual skin patterns, (b) capture the underlying dynamics of cellular interactions, (c) interact with tissue size and shape, (d) can lead to ordered sequential patterning, (e) generate cellular automaton dynamics in lizards and snakes, (f) predict actual patterns beyond their statistical features, and (g) are robust to model variations. Third, we discuss the utility of linear stability analysis and perform numerical simulations to demonstrate how deterministic RD emerges from the underlying chaotic microscopic agents.

Transcriptomic profiling of tissue environments critical for post-embryonic patterning and morphogenesis of zebrafish skin

Pigment patterns and skin appendages are prominent features of vertebrate skin. In zebrafish, regularly patterned pigment stripes and an array of calcified scales form simultaneously in the skin during post-embryonic development. Understanding the mechanisms that regulate stripe patterning and scale morphogenesis may lead to the discovery of fundamental mechanisms that govern the development of animal form. To learn about cell types and signaling interactions that govern skin patterning and morphogenesis, we generated and analyzed single-cell transcriptomes of skin from wild-type fish as well as fish having genetic or transgenically induced defects in squamation or pigmentation. These data reveal a previously undescribed population of epidermal cells that express transcripts encoding enamel matrix proteins, suggest hormonal control of epithelial-mesenchymal signaling, clarify the signaling network that governs scale papillae development, and identify a critical role for the hypodermis in supporting pigment cell development. Additionally, these comprehensive single-cell transcriptomic data representing skin phenotypes of biomedical relevance should provide a useful resource for accelerating the discovery of mechanisms that govern skin development and homeostasis.

A Novel Perspective on Neuronal Control of Anatomical Patterning, Remodeling, and Maintenance

While the nervous system may be best known as the sensory communication center of an organism, recent research has revealed a myriad of multifaceted roles for both the CNS and PNS from early development to adult regeneration and remodeling. These systems work to orchestrate tissue pattern formation during embryonic development and continue shaping pattering through transitional periods such as metamorphosis and growth. During periods of injury or wounding, the nervous system has also been shown to influence remodeling and wound healing. The neuronal mechanisms responsible for these events are largely conserved across species, suggesting this evidence may be important in understanding and resolving many human defects and diseases. By unraveling these diverse roles, this paper highlights the necessity of broadening our perspective on the nervous system beyond its conventional functions. A comprehensive understanding of the complex interactions and contributions of the nervous system throughout development and adulthood has the potential to revolutionize therapeutic strategies and open new avenues for regenerative medicine and tissue engineering. This review highlights an important role for the nervous system during the patterning and maintenance of complex tissues and provides a potential avenue for advancing biomedical applications.

kcnj13 regulates pigment cell shapes in zebrafish and has diverged by cis-regulatory evolution between Danio species

Teleost fish of the genus Danio are excellent models to study the genetic and cellular bases of pigment pattern variation in vertebrates. The two sister species Danio rerio and Danio aesculapii show divergent patterns of horizontal stripes and vertical bars that are partly caused by the divergence of the potassium channel gene kcnj13. Here, we show that kcnj13 is required only in melanophores for interactions with xanthophores and iridophores, which cause location-specific pigment cell shapes and thereby influence colour pattern and contrast in D. rerio. Cis-regulatory rather than protein coding changes underlie kcnj13 divergence between the two Danio species. Our results suggest that homotypic and heterotypic interactions between the pigment cells and their shapes diverged between species by quantitative changes in kcnj13 expression during pigment pattern diversification.

Single-Cell Transcriptome and Pigment Biochemistry Analysis Reveals the Potential for the High Nutritional and Medicinal Value of Purple Sea Cucumbers

The sea cucumber Apostichopus japonicus has important nutritional and medicinal value. Unfortunately, we know little of the source of active chemicals in this animal, but the plentiful pigments of these animals are thought to function in intriguing ways for translation into clinical and food chemistry usage. Here, we found key cell groups with the gene activity predicted for the color morphology of sea cucumber body using single-cell RNA-seq. We refer to these cell populations as melanocytes and quinocytes, which are responsible for the synthesis of melanin and quinone pigments, respectively. We integrated analysis of pigment biochemistry with the transcript profiles to illuminate the molecular mechanisms regulating distinct pigment formation in echinoderms. In concert with the correlated pigment analysis from each color morph, this study expands our understanding of medically important pigment production, as well as the genetic mechanisms for color morphs, and provides deep datasets for exploring advancements in the fields of bioactives and nutraceuticals.

Characterising the Effect of Wnt/β-Catenin Signalling on Melanocyte Development and Patterning: Insights from Zebrafish (Danio rerio)

Zebrafish (Danio rerio) is a well-established model organism for studying melanocyte biology due to its remarkable similarity to humans. The Wnt signalling pathway is a conserved signal transduction pathway that plays a crucial role in embryonic development and regulates many aspects of the melanocyte lineage. Our study was designed to investigate the effect of Wnt signalling activity on zebrafish melanocyte development and patterning. Stereo-microscopic examinations were used to screen for changes in melanocyte count, specific phenotypic differences, and distribution in zebrafish, while microscopic software tools were used to analyse the differences in pigment dispersion of melanocytes exposed to LiCl (Wnt enhancer) and W-C59 (Wnt inhibitor). Samples exposed to W-C59 showed low melanocyte densities and defects in melanocyte phenotype and patterning, whereas LiCl exposure demonstrated a stimulatory effect on most aspects of melanocyte development. Our study demonstrates the crucial role of Wnt signalling in melanocyte lineage and emphasises the importance of a balanced Wnt signalling level for proper melanocyte development and patterning.

The journey from melanocytes to melanoma

Over the past decade, melanoma has led the field in new cancer treatments, with impressive gains in on-treatment survival but more modest improvements in overall survival. Melanoma presents heterogeneity and transcriptional plasticity that recapitulates distinct melanocyte developmental states and phenotypes, allowing it to adapt to and eventually escape even the most advanced treatments. Despite remarkable advances in our understanding of melanoma biology and genetics, the melanoma cell of origin is still fiercely debated because both melanocyte stem cells and mature melanocytes can be transformed. Animal models and high-throughput single-cell sequencing approaches have opened new opportunities to address this question. Here, we discuss the melanocytic journey from the neural crest, where they emerge as melanoblasts, to the fully mature pigmented melanocytes resident in several tissues. We describe a new understanding of melanocyte biology and the different melanocyte subpopulations and microenvironments they inhabit, and how this provides unique insights into melanoma initiation and progression. We highlight recent findings on melanoma heterogeneity and transcriptional plasticity and their implications for exciting new research areas and treatment opportunities. The lessons from melanocyte biology reveal how cells that are present to protect us from the damaging effects of ultraviolet radiation reach back to their origins to become a potentially deadly cancer.

Zebrafish pigment cells develop directly from persistent highly multipotent progenitors

Neural crest cells are highly multipotent stem cells, but it remains unclear how their fate restriction to specific fates occurs. The direct fate restriction model hypothesises that migrating cells maintain full multipotency, whilst progressive fate restriction envisages fully multipotent cells transitioning to partially-restricted intermediates before committing to individual fates. Using zebrafish pigment cell development as a model, we show applying NanoString hybridization single cell transcriptional profiling and RNAscope in situ hybridization that neural crest cells retain broad multipotency throughout migration and even in post-migratory cells in vivo, with no evidence for partially-restricted intermediates. We find that leukocyte tyrosine kinase early expression marks a multipotent stage, with signalling driving iridophore differentiation through repression of fate-specific transcription factors for other fates. We reconcile the direct and progressive fate restriction models by proposing that pigment cell development occurs directly, but dynamically, from a highly multipotent state, consistent with our recently-proposed Cyclical Fate Restriction model.

Requirement of Zebrafish Adcy3a and Adcy5 in Melanosome Dispersion and Melanocyte Stripe Formation

cAMP-PKA signaling plays a pivotal role in melanin synthesis and melanosome transport by responding to the binding of the α-melanocyte-stimulating hormone (α-MSH) to melanocortin-1 receptor (MC1R). Adenylate cyclases (ADCYs) are the enzymes responsible for the synthesis of cAMP from ATP, which comprises nine transmembrane isoforms (ADCYs 1-9) and one soluble adenylate cyclase (ADCY 10) in mammals. However, little is known about which and how ADCY isoforms regulate melanocyte generation, melanin biosynthesis, and melanosome transport in vivo. In this study, we have generated a series of single and double mutants of Adcy isoforms in zebrafish. Among them, adcy3a^-/- and adcy5^-/- double mutants cause defects in melanosome dispersion but do not impair melanoblast differentiation and melanocyte regeneration during the embryonic or larval stages. Activation of PKA, the main effector of cAMP signaling, significantly ameliorates the defects in melanosome dispersion in adcy3a^-/- and adcy5^-/- double mutants. Mechanistically, Adcy3a and Adcy5 regulate melanosome dispersion by activating kinesin-1 while inhibiting cytoplasmic dynein-1. In adult zebrafish, Adcy3a and Adcy5 participate in the regulation of the expression of microphthalmia transcription factor (Mitfa) and melanin synthesis enzymes Tyr, Dct, and Trp1b. The deletion of Adcy3a and Adcy5 inhibits melanin production and reduces pigmented melanocyte numbers, causing a defect in establishing adult melanocyte stripes. Hence, our studies demonstrate that Adcy3a and Adcy5 play essential but redundant functions in mediating α-MSH-MC1R/cAMP-PKA signaling for regulating melanin synthesis and melanosome dispersion.

Modeling Spitz melanoma in zebrafish using sequential mutagenesis

Spitz neoplasms are a diverse group of molecularly and histologically defined melanocytic tumors with varying biologic potentials. The precise classification of Spitz neoplasms can be challenging. Recent studies have revealed recurrent fusions involving multiple kinases in a large proportion of Spitz tumors. In this study, we generated a transgenic zebrafish model of Spitz melanoma using a previously identified ZCCHC8-ROS1 fusion gene. Animals developed grossly apparent melanocytic proliferations as early as 3 weeks of age and overt melanoma as early as 5 weeks. By 7 weeks, ZCCHC8-ROS1 induced a histologic spectrum of neoplasms ranging from hyperpigmented patches to melanoma. Given the swift onset of these tumors during development, we extended this approach into adult fish using a recently described electroporation technique. Tissue-specific expression of ZCCHC8-ROS1 in adults led to melanocyte expansion without overt progression to melanoma. Subsequent electroporation with tissue-specific CRISPR, targeting only tp53 was sufficient to induce transformation to melanoma. Our model exhibits the use of sequential mutagenesis in the adult zebrafish, and demonstrates that ZCCHC8-ROS1 induces a spectrum of melanocytic lesions that closely mimics human Spitz neoplasms.

Summary: We describe the first animal model of Spitz neoplasms and demonstrate its use for modeling sequential mutagenesis and its potential for studying melanocyte development in vivo.

Aldh2 is a lineage-specific metabolic gatekeeper in melanocyte stem cells

Melanocyte stem cells (McSCs) in zebrafish serve as an on-demand source of melanocytes during growth and regeneration, but metabolic programs associated with their activation and regenerative processes are not well known. Here, using live imaging coupled with scRNA-sequencing, we discovered that, during regeneration, quiescent McSCs activate a dormant embryonic neural crest transcriptional program followed by an aldehyde dehydrogenase (Aldh) 2 metabolic switch to generate progeny. Unexpectedly, although ALDH2 is well known for its aldehyde-clearing mechanisms, we find that, in regenerating McSCs, Aldh2 activity is required to generate formate – the one-carbon (1C) building block for nucleotide biosynthesis – through formaldehyde metabolism. Consequently, we find that disrupting the 1C cycle with low doses of methotrexate causes melanocyte regeneration defects. In the absence of Aldh2, we find that purines are the metabolic end product sufficient for activated McSCs to generate progeny. Together, our work reveals McSCs undergo a two-step cell state transition during regeneration, and that the reaction products of Aldh2 enzymes have tissue-specific stem cell functions that meet metabolic demands in regeneration.

Summary: In zebrafish melanocyte regeneration, quiescent McSCs respond by re-expressing a neural crest identity, followed by an Aldh2-dependent metabolic switch to generate progeny.

Tfap2b specifies an embryonic melanocyte stem cell that retains adult multifate potential

Melanocytes, the pigment-producing cells, are replenished from multiple stem cell niches in adult tissue. Although pigmentation traits are known risk factors for melanoma, we know little about melanocyte stem cell (McSC) populations other than hair follicle McSCs and lack key lineage markers with which to identify McSCs and study their function. Here we find that Tfap2b and a select set of target genes specify an McSC population at the dorsal root ganglia in zebrafish. Functionally, Tfap2b is required for only a few late-stage embryonic melanocytes, and is essential for McSC-dependent melanocyte regeneration. Fate mapping data reveal that tfap2b⁺ McSCs have multifate potential, and are the cells of origin for large patches of adult melanocytes, two other pigment cell types (iridophores and xanthophores), and nerve-associated cells. Hence, Tfap2b confers McSC identity in early development, distinguishing McSCs from other neural crest and pigment cell lineages, and retains multifate potential in the adult zebrafish.

Cell Fate Decisions in the Neural Crest, from Pigment Cell to Neural Development

The neural crest shows an astonishing multipotency, generating multiple neural derivatives, but also pigment cells, skeletogenic and other cell types. The question of how this process is controlled has been the subject of an ongoing debate for more than 35 years. Based upon new observations of zebrafish pigment cell development, we have recently proposed a novel, dynamic model that we believe goes some way to resolving the controversy. Here, we will firstly summarize the traditional models and the conflicts between them, before outlining our novel model. We will also examine our recent dynamic modelling studies, looking at how these reveal behaviors compatible with the biology proposed. We will then outline some of the implications of our model, looking at how it might modify our views of the processes of fate specification, differentiation, and commitment.

Cyclical fate restriction: a new view of neural crest cell fate specification

Neural crest cells are crucial in development, not least because of their remarkable multipotency. Early findings stimulated two hypotheses for how fate specification and commitment from fully multipotent neural crest cells might occur, progressive fate restriction (PFR) and direct fate restriction, differing in whether partially restricted intermediates were involved. Initially hotly debated, they remain unreconciled, although PFR has become favoured. However, testing of a PFR hypothesis of zebrafish pigment cell development refutes this view. We propose a novel 'cyclical fate restriction' hypothesis, based upon a more dynamic view of transcriptional states, reconciling the experimental evidence underpinning the traditional hypotheses.

Pigment pattern morphospace of Danio fishes: evolutionary diversification and mutational effects

Molecular and cellular mechanisms underlying variation in adult form remain largely unknown. Adult pigment patterns of fishes in the genus Danio, which includes zebrafish, Danio rerio, consist of horizontal stripes, vertical bars, spots and uniform patterns, and provide an outstanding opportunity to identify causes of species level variation in a neural crest derived trait. Understanding pigment pattern variation requires quantitative approaches to assess phenotypes, yet such methods have been mostly lacking for pigment patterns. We introduce metrics derived from information theory that describe patterns and pattern variation in Danio fishes. We find that these metrics used singly and in multivariate combinations are suitable for distinguishing general pattern types, and can reveal even subtle phenotypic differences attributable to mutations. Our study provides new tools for analyzing pigment pattern in Danio and potentially other groups, and sets the stage for future analyses of pattern morphospace and its mechanistic underpinnings.

Summary: A multidimensional morphospace for pigment patterns yields quantitative insights into the evolution and genetics of diverse pigment patterns across zebrafish and related species.

Development and genetics of red coloration in the zebrafish relative Danio albolineatus

Animal pigment patterns play important roles in behavior and, in many species, red coloration serves as an honest signal of individual quality in mate choice. Among Danio fishes, some species develop erythrophores, pigment cells that contain red ketocarotenoids, whereas other species, like zebrafish (D. rerio) only have yellow xanthophores. Here, we use pearl danio (D. albolineatus) to assess the developmental origin of erythrophores and their mechanisms of differentiation. We show that erythrophores in the fin of D. albolineatus share a common progenitor with xanthophores and maintain plasticity in cell fate even after differentiation. We further identify the predominant ketocarotenoids that confer red coloration to erythrophores and use reverse genetics to pinpoint genes required for the differentiation and maintenance of these cells. Our analyses are a first step toward defining the mechanisms underlying the development of erythrophore-mediated red coloration in Danio and reveal striking parallels with the mechanism of red coloration in birds.

Immunoglobulin superfamily receptor Junctional adhesion molecule 3 (Jam3) requirement for melanophore survival and patterning during formation of zebrafish stripes

Adhesive interactions are essential for tissue patterning and morphogenesis yet difficult to study owing to functional redundancies across genes and gene families. A useful system in which to dissect roles for cell adhesion and adhesion-dependent signaling is the pattern formed by pigment cells in skin of adult zebrafish, in which stripes represent the arrangement of neural crest derived melanophores, cells homologous to melanocytes. In a forward genetic screen for adult pattern defects, we isolated the pissarro (psr) mutant, having a variegated phenotype of spots, as well as defects in adult fin and lens. We show that psr corresponds to junctional adhesion protein 3b (jam3b) encoding a zebrafish orthologue of the two immunoglobulin-like domain receptor JAM3 (JAM-C), known for roles in adhesion and signaling in other developing tissues, and for promoting metastatic behavior of human and murine melanoma cells. We found that zebrafish jam3b is expressed post-embryonically in a variety of cells including melanophores, and that jam3b mutants have defects in melanophore survival. Jam3b supported aggregation of cells in vitro and was required autonomously by melanophores for an adherent phenotype in vivo. Genetic analyses further indicated both overlapping and non-overlapping functions with the related receptor, Immunoglobulin superfamily 11 (Igsf11) and Kit receptor tyrosine kinase. These findings suggest a model for Jam3b function in zebrafish melanophores and hint at the complexity of adhesive interactions underlying pattern formation.

Nerve-associated Schwann cell precursors contribute extracutaneous melanocytes to the heart, inner ear, supraorbital locations and brain meninges

Melanocytes are pigmented cells residing mostly in the skin and hair follicles of vertebrates, where they contribute to colouration and protection against UV-B radiation. However, the spectrum of their functions reaches far beyond that. For instance, these pigment-producing cells are found inside the inner ear, where they contribute to the hearing function, and in the heart, where they are involved in the electrical conductivity and support the stiffness of cardiac valves. The embryonic origin of such extracutaneous melanocytes is not clear. We took advantage of lineage-tracing experiments combined with 3D visualizations and gene knockout strategies to address this long-standing question. We revealed that Schwann cell precursors are recruited from the local innervation during embryonic development and give rise to extracutaneous melanocytes in the heart, brain meninges, inner ear, and other locations. In embryos with a knockout of the EdnrB receptor, a condition imitating Waardenburg syndrome, we observed only nerve-associated melanoblasts, which failed to detach from the nerves and to enter the inner ear. Finally, we looked into the evolutionary aspects of extracutaneous melanocytes and found that pigment cells are associated mainly with nerves and blood vessels in amphibians and fish. This new knowledge of the nerve-dependent origin of extracutaneous pigment cells might be directly relevant to the formation of extracutaneous melanoma in humans.

Transcriptomic Analysis of Skin Color in Anole Lizards

Color and color pattern are critical for animal camouflage, reproduction, and defense. Few studies, however, have attempted to identify candidate genes for color and color pattern in squamate reptiles, a colorful group with over 10,000 species. We used comparative transcriptomic analyses between white, orange, and yellow skin in a color-polymorphic species of anole lizard to 1) identify candidate color and color-pattern genes in squamates and 2) assess if squamates share an underlying genetic basis for color and color pattern variation with other vertebrates. Squamates have three types of chromatophores that determine color pattern: guanine-filled iridophores, carotenoid- or pteridine-filled xanthophores/erythrophores, and melanin-filled melanophores. We identified 13 best candidate squamate color and color-pattern genes shared with other vertebrates: six genes linked to pigment synthesis pathways, and seven genes linked to chromatophore development and maintenance. In comparisons of expression profiles between pigment-rich and white skin, pigment-rich skin upregulated the pteridine pathway as well as xanthophore/erythrophore development and maintenance genes; in comparisons between orange and yellow skin, orange skin upregulated the pteridine and carotenoid pathways as well as melanophore maintenance genes. Our results corroborate the predictions that squamates can produce similar colors using distinct color-reflecting molecules, and that both color and color-pattern genes are likely conserved across vertebrates. Furthermore, this study provides a concise list of candidate genes for future functional verification, representing a first step in determining the genetic basis of color and color pattern in anoles.

Schwann cell precursors generate sympathoadrenal system during zebrafish development

The autonomic portion of the peripheral nervous system orchestrates tissue homeostasis through direct innervation of internal organs, and via release of adrenalin and noradrenalin into the blood flow. The developmental mechanisms behind the formation of autonomic neurons and chromaffin cells are not fully understood. Using genetic tracing, we discovered that a significant proportion of sympathetic neurons in zebrafish originates from Schwann cell precursors (SCPs) during a defined period of embryonic development. Moreover, SCPs give rise to the main portion of the chromaffin cells, as well as to a significant proportion of enteric and other autonomic neurons associated with internal organs. The conversion of SCPs into neuronal and chromaffin cells is ErbB receptor dependent, as the pharmacological inhibition of the ErbB pathway effectively perturbed this transition. Finally, using genetic ablations, we revealed that SCPs producing neurons and chromaffin cells migrate along spinal motor axons to reach appropriate target locations. This study reveals the evolutionary conservation of SCP-to-neuron and SCP-to-chromaffin cell transitions over significant growth periods in fish and highlights relevant cellular-genetic mechanisms. Based on this, we anticipate that multipotent SCPs might be present in postnatal vertebrate tissues, retaining the capacity to regenerate autonomic neurons and chromaffin cells.

Nile Tilapia: A Model for Studying Teleost Color Patterns

The diverse color patterns of cichlid fishes play an important role in mate choice and speciation. Here we develop the Nile tilapia (Oreochromis niloticus) as a model system for studying the developmental genetics of cichlid color patterns. We identified 4 types of pigment cells: melanophores, xanthophores, iridophores and erythrophores, and characterized their first appearance in wild-type fish. We mutated 25 genes involved in melanogenesis, pteridine metabolism, and the carotenoid absorption and cleavage pathways. Among the 25 mutated genes, 13 genes had a phenotype in both the F0 and F2 generations. None of F1 heterozygotes had phenotype. By comparing the color pattern of our mutants with that of red tilapia (Oreochromis spp), a natural mutant produced during hybridization of tilapia species, we found that the pigmentation of the body and eye is controlled by different genes. Previously studied genes like mitf, kita/kitlga, pmel, tyrb, hps4, gch2, csf1ra, pax7b, and bco2b were proved to be of great significance for color patterning in tilapia. Our results suggested that tilapia, a fish with 4 types of pigment cells and a vertically barred wild-type color pattern, together with various natural and artificially induced color gene mutants, can serve as an excellent model system for study color patterning in vertebrates.

A complex genetic architecture in zebrafish relatives Danio quagga and D. kyathit underlies development of stripes and spots

Vertebrate pigmentation is a fundamentally important, multifaceted phenotype. Zebrafish, Danio rerio, has been a valuable model for understanding genetics and development of pigment pattern formation due to its genetic and experimental tractability, advantages that are shared across several Danio species having a striking array of pigment patterns. Here, we use the sister species D. quagga and D. kyathit, with stripes and spots, respectively, to understand how natural genetic variation impacts phenotypes at cellular and organismal levels. We first show that D. quagga and D. kyathit phenotypes resemble those of wild-type D. rerio and several single locus mutants of D. rerio, respectively, in a morphospace defined by pattern variation along dorsoventral and anteroposterior axes. We then identify differences in patterning at the cellular level between D. quagga and D. kyathit by repeated daily imaging during pattern development and quantitative comparisons of adult phenotypes, revealing that patterns are similar initially but diverge ontogenetically. To assess the genetic architecture of these differences, we employ reduced-representation sequencing of second-generation hybrids. Despite the similarity of D. quagga to D. rerio, and D. kyathit to some D. rerio mutants, our analyses reveal a complex genetic basis for differences between D. quagga and D. kyathit, with several quantitative trait loci contributing to variation in overall pattern and cellular phenotypes, epistatic interactions between loci, and abundant segregating variation within species. Our findings provide a window into the evolutionary genetics of pattern-forming mechanisms in Danio and highlight the complexity of differences that can arise even between sister species. Further studies of natural genetic diversity underlying pattern variation in D. quagga and D. kyathit should provide insights complementary to those from zebrafish mutant phenotypes and more distant species comparisons.

Pigment patterns of fishes are diverse and function in a wide range of behaviors. Common pattern themes include stripes and spots, exemplified by the closely related minnows Danio quagga and D. kyathit, respectively. We show that these patterns arise late in development owing to alterations in the development and arrangements of pigment cells. In the closely related model organism zebrafish (D. rerio) single genes can switch the pattern from stripes to spots. Yet, we show that pattern differences between D. quagga and D. kyathit have a more complex genetic basis, depending on multiple genes and interactions between these genes. Our findings illustrate the importance of characterizing naturally occurring genetic variants, in addition to laboratory induced mutations, for a more complete understanding of pigment pattern development and evolution.

Trio cooperates with Myh9 to regulate neural crest-derived craniofacial development

Trio is a unique member of the Rho-GEF family that has three catalytic domains and is vital for various cellular processes in both physiological and developmental settings. TRIO mutations in humans are involved in craniofacial abnormalities, in which patients present with mandibular retrusion. However, little is known about the molecular mechanisms of Trio in neural crest cell (NCC)-derived craniofacial development, and there is still a lack of direct evidence to assign a functional role to Trio in NCC-induced craniofacial abnormalities. Methods: In vivo, we used zebrafish and NCC-specific knockout mouse models to investigate the phenotype and dynamics of NCC development in Trio morphants. In vitro, iTRAQ, GST pull-down assays, and proximity ligation assay (PLA) were used to explore the role of Trio and its potential downstream mediators in NCC migration and differentiation. Results: In zebrafish and mouse models, disruption of Trio elicited a migration deficit and impaired the differentiation of NCC derivatives, leading to craniofacial growth deficiency and mandibular retrusion. Moreover, Trio positively regulated Myh9 expression and directly interacted with Myh9 to coregulate downstream cellular signaling in NCCs. We further demonstrated that disruption of Trio or Myh9 inhibited Rac1 and Cdc42 activity, specifically affecting the nuclear export of β-catenin and NCC polarization. Remarkably, craniofacial abnormalities caused by trio deficiency in zebrafish could be partially rescued by the injection of mRNA encoding myh9, ca-Rac1, or ca-Cdc42. Conclusions: Here, we identified that Trio, interacting mostly with Myh9, acts as a key regulator of NCC migration and differentiation during craniofacial development. Our results indicate that trio morphant zebrafish and Wnt1-cre;Trio^fl/fl mice offer potential model systems to facilitate the study of the pathogenic mechanisms of Trio mutations causing craniofacial abnormalities.

A globin-family protein, Cytoglobin 1, is involved in the development of neural crest-derived tissues and organs in zebrafish

The zebrafish is an excellent model animal that is amenable to forward genetics approaches. To uncover unknown developmental regulatory mechanisms in vertebrates, we conducted chemical mutagenesis screening and identified a novel mutation, kanazutsi (kzt). This mutation is recessive, and its homozygotes are embryonic lethal. Mutant embryos suffered from a variety of morphological defects, such as head flattening, pericardial edema, circulation defects, disrupted patterns of melanophore distribution, dwarf eyes, a defective jaw, and extensive apoptosis in the head, which indicates that the main affected tissues are derived from neural crest cells (NCCs). The expression of tissue-specific markers in kzt mutants showed that the early specification of NCCs was normal, but their later differentiation was severely affected. The mutation was mapped to chromosome 3 by linkage analyses, near cytoglobin 1 (cygb1), the product of which is a globin-family respiratory protein. cygb1 expression was activated during somitogenesis in somites and cranial NCCs in wild-type embryos but was significantly downregulated in mutant embryos, despite the normal primary structure of the gene product. The kzt mutation was phenocopied by cygb1 knockdown with low-dose morpholino oligos and was partially rescued by cygb1 overexpression. Both severe knockdown and null mutation of cygb1, established by the CRISPR/Cas9 technique, resulted in far more severe defects at early stages. Thus, it is highly likely that the downregulation of cygb1 is responsible for many, if not all, of the phenotypes of the kzt mutation. These results reveal a requirement for globin family proteins in vertebrate embryos, particularly in the differentiation and subsequent development of NCCs.

Evolution of the potassium channel gene Kcnj13 underlies colour pattern diversification in Danio fish

The genetic basis of morphological variation provides a major topic in evolutionary developmental biology. Fish of the genus Danio display colour patterns ranging from horizontal stripes, to vertical bars or spots. Stripe formation in zebrafish, Danio rerio, is a self-organizing process based on cell-contact mediated interactions between three types of chromatophores with a leading role of iridophores. Here we investigate genes known to regulate chromatophore interactions in zebrafish that might have evolved to produce a pattern of vertical bars in its sibling species, Danio aesculapii. Mutant D. aesculapii indicate a lower complexity in chromatophore interactions and a minor role of iridophores in patterning. Reciprocal hemizygosity tests identify the potassium channel gene obelix/Kcnj13 as evolved between the two species. Complementation tests suggest evolutionary change through divergence in Kcnj13 function in two additional Danio species. Thus, our results point towards repeated and independent evolution of this gene during colour pattern diversification.

Novel Glial Cell Functions: Extensive Potency, Stem Cell-Like Properties, and Participation in Regeneration and Transdifferentiation

Glial cells are the most abundant cells in both the peripheral and central nervous systems. During the past decade, a subpopulation of immature peripheral glial cells, namely, embryonic Schwann cell-precursors, have been found to perform important functions related to development. These cells have properties resembling those of the neural crest and, depending on their location in the body, can transform into several different cell types in peripheral tissues, including autonomic neurons. This review describes the multipotent properties of Schwann cell-precursors and their importance, together with innervation, during early development. The heterogeneity of Schwann cells, as revealed using single-cell transcriptomics, raises a question on whether some glial cells in the adult peripheral nervous system retain their stem cell-like properties. We also discuss how a deeper insight into the biology of both embryonic and adult Schwann cells might lead to an effective treatment of the damage of both neural and non-neural tissues, including the damage caused by neurodegenerative diseases. Furthermore, understanding the potential involvement of Schwann cells in the regulation of tumor development may reveal novel targets for cancer treatment.

Insights Into the Albinism Mechanism for Two Distinct Color Morphs of Northern Snakehead, Channa argus Through Histological and Transcriptome Analyses

The great northern snakehead (Channa argus) is one of the most important economic and conservational fish in China. In this study, the melanocytes in the skin of two distinct color morphs C. argus were investigated and compared through employment of the microscopic analysis, hematoxylin and eosin (H&E) and Masson Fontana staining. Our results demonstrated the uneven distribution of melanocytes with extremely low density and most of them were in the state of aging or death. Meanwhile, there was no obvious pigment layer and melanocytes distribution pattern found in the albino-type (AT), while the melanocytes were evenly distributed with abundance in the bicolor-type (BT). The transcriptome analysis through Illumina HiSeq sequencing showed that a total of 34.93 Gb Clean Data was obtained, and Q30 base percentage reached 92.66%. The BT and AT northern snakeheads transcriptome data included a total of 56,039,701 and 60,410,063 clean reads (n = 3), respectively. In gene expression analyses, the sample correlation coefficients (r) were ranged between 0.92 and 1.00; the contribution of PC1 and PC2 were 50.25 and 13.73% by using PCA cluster analysis, the total number of DEGs were 1024 (559 up-regulated and 465 down-regulated), and the number of annotated DEGs was 767 (COG 172, KEGG 262, GO 288, SwissProt 548, Pfam 579 and NR 765). Additionally, 46,363 ± 873 and 44,947 ± 392 single nucleotide polymorphisms (SNPs) were compiled via genetic structure analysis, respectively. Ten key pigment-related genes were screened using qRT-PCR. And all of them revealed extremely higher expression levels in the skin of BT than those of AT. This is the first study to analyze the mechanism of albino characteristics of Channa via histology and transcriptomics, and also provide the oretical and practical support for the protection and development of germplasm resources for C. argus.

PRL3-DDX21 Transcriptional Control of Endolysosomal Genes Restricts Melanocyte Stem Cell Differentiation

Melanocytes, replenished throughout life by melanocyte stem cells (MSCs), play a critical role in pigmentation and melanoma. Here, we reveal a function for the metastasis-associated phosphatase of regenerating liver 3 (PRL3) in MSC regeneration. We show that PRL3 binds to the RNA helicase DDX21, thereby restricting productive transcription by RNAPII at master transcription factor (MITF)-regulated endolysosomal vesicle genes. In zebrafish, this mechanism controls premature melanoblast expansion and differentiation from MSCs. In melanoma patients, restricted transcription of this endolysosomal vesicle pathway is a hallmark of PRL3-high melanomas. Our work presents the conceptual advance that PRL3-mediated control of transcriptional elongation is a differentiation checkpoint mechanism for activated MSCs and has clinical relevance for the activity of PRL3 in regenerating tissue and cancer.

A quantitative modelling approach to zebrafish pigment pattern formation

Pattern formation is a key aspect of development. Adult zebrafish exhibit a striking striped pattern generated through the self-organisation of three different chromatophores. Numerous investigations have revealed a multitude of individual cell-cell interactions important for this self-organisation, but it has remained unclear whether these known biological rules were sufficient to explain pattern formation. To test this, we present an individual-based mathematical model incorporating all the important cell-types and known interactions. The model qualitatively and quantitatively reproduces wild type and mutant pigment pattern development. We use it to resolve a number of outstanding biological uncertainties, including the roles of domain growth and the initial iridophore stripe, and to generate hypotheses about the functions of leopard. We conclude that our rule-set is sufficient to recapitulate wild-type and mutant patterns. Our work now leads the way for further in silico exploration of the developmental and evolutionary implications of this pigment patterning system.

Neural innervation as a potential trigger of morphological color change and sexual dimorphism in cichlid fish

Many species change their coloration during ontogeny or even as adults. Color change hereby often serves as sexual or status signal. The cellular and subcellular changes that drive color change and how they are orchestrated have been barely understood, but a deeper knowledge of the underlying processes is important to our understanding of how such plastic changes develop and evolve. Here we studied the color change of the Malawi golden cichlid (Melanchromis auratus). Females and subordinate males of this species are yellow and white with two prominent black stripes (yellow morph; female and non-breeding male coloration), while dominant males change their color and completely invert this pattern with the yellow and white regions becoming black, and the black stripes becoming white to iridescent blue (dark morph; male breeding coloration). A comparison of the two morphs reveals that substantial changes across multiple levels of biological organization underlie this polyphenism. These include changes in pigment cell (chromatophore) number, intracellular dispersal of pigments, and tilting of reflective platelets (iridosomes) within iridophores. At the transcriptional level, we find differences in pigmentation gene expression between these two color morphs but, surprisingly, 80% of the genes overexpressed in the dark morph relate to neuronal processes including synapse formation. Nerve fiber staining confirms that scales of the dark morph are indeed innervated by 1.3 to 2 times more axonal fibers. Our results might suggest an instructive role of nervous innervation orchestrating the complex cellular and ultrastructural changes that drive the morphological color change of this cichlid species.

GABA-A receptor and mitochondrial TSPO signaling act in parallel to regulate melanocyte stem cell quiescence in larval zebrafish

Tissue regeneration and homeostasis often require recruitment of undifferentiated precursors (adult stem cells; ASCs). While many ASCs continuously proliferate throughout the lifetime of an organism, others are recruited from a quiescent state to replenish their target tissue. A long-standing question in stem cell biology concerns how long-lived, non-dividing ASCs regulate the transition between quiescence and proliferation. We study the melanocyte stem cell (MSC) to investigate the molecular pathways that regulate ASC quiescence. Our prior work indicated that GABA-A receptor activation promotes MSC quiescence in larval zebrafish. Here, through pharmacological and genetic approaches we show that GABA-A acts through calcium signaling to maintain MSC quiescence. Unexpectedly, we identified translocator protein (TSPO), a mitochondrial membrane-associated protein that regulates mitochondrial function and metabolic homeostasis, as a parallel regulator of MSC quiescence. We found that both TSPO-specific ligands and induction of gluconeogenesis likely act in the same pathway to promote MSC activation and melanocyte production in larval zebrafish. In contrast, TSPO and gluconeogenesis appear to act in parallel to GABA-A receptor signaling to regulate MSC quiescence and vertebrate pigment patterning.

Histone variant dictates fate biasing of neural crest cells to melanocyte lineage

In the neural crest lineage, progressive fate restriction and stem cell assignment are crucial for both development and regeneration. Whereas fate commitment events have distinct transcriptional footprints, fate biasing is often transitory and metastable, and is thought to be moulded by epigenetic programmes. Therefore, the molecular basis of specification is difficult to define. In this study, we established a role for a histone variant, H2a.z.2, in specification of the melanocyte lineage from multipotent neural crest cells. H2a.z.2 silencing reduces the number of melanocyte precursors in developing zebrafish embryos and from mouse embryonic stem cells in vitro We demonstrate that this histone variant occupies nucleosomes in the promoter of the key melanocyte determinant mitf, and enhances its induction. CRISPR/Cas9-based targeted mutagenesis of this gene in zebrafish drastically reduces adult melanocytes, as well as their regeneration. Thereby, our study establishes the role of a histone variant upstream of the core gene regulatory network in the neural crest lineage. This epigenetic mark is a key determinant of cell fate and facilitates gene activation by external instructive signals, thereby establishing melanocyte fate identity.

Topological data analysis of zebrafish patterns

Self-organized pattern behavior is ubiquitous throughout nature, from fish schooling to collective cell dynamics during organism development. Qualitatively these patterns display impressive consistency, yet variability inevitably exists within pattern-forming systems on both microscopic and macroscopic scales. Quantifying variability and measuring pattern features can inform the underlying agent interactions and allow for predictive analyses. Nevertheless, current methods for analyzing patterns that arise from collective behavior capture only macroscopic features or rely on either manual inspection or smoothing algorithms that lose the underlying agent-based nature of the data. Here we introduce methods based on topological data analysis and interpretable machine learning for quantifying both agent-level features and global pattern attributes on a large scale. Because the zebrafish is a model organism for skin pattern formation, we focus specifically on analyzing its skin patterns as a means of illustrating our approach. Using a recent agent-based model, we simulate thousands of wild-type and mutant zebrafish patterns and apply our methodology to better understand pattern variability in zebrafish. Our methodology is able to quantify the differential impact of stochasticity in cell interactions on wild-type and mutant patterns, and we use our methods to predict stripe and spot statistics as a function of varying cellular communication. Our work provides an approach to automatically quantifying biological patterns and analyzing agent-based dynamics so that we can now answer critical questions in pattern formation at a much larger scale.

Developmental and Cellular Basis of Vertical Bar Color Patterns in the East African Cichlid Fish Haplochromis latifasciatus

The East African adaptive radiations of cichlid fishes are renowned for their diversity in coloration. Yet, the developmental basis of pigment pattern formation remains largely unknown. One of the most common melanic patterns in cichlid fishes are vertical bar patterns. Here we describe the ontogeny of this conspicuous pattern in the Lake Kyoga species Haplochromis latifasciatus. Beginning with the larval stages we tracked the formation of this stereotypic color pattern and discovered that its macroscopic appearance is largely explained by an increase in melanophore density and accumulation of melanin during the first 3 weeks post-fertilization. The embryonal analysis is complemented with cytological quantifications of pigment cells in adult scales and the dermis beneath the scales. In adults, melanic bars are characterized by a two to threefold higher density of melanophores than in the intervening yellow interbars. We found no strong support for differences in other pigment cell types such as xanthophores. Quantitative PCRs for twelve known pigmentation genes showed that expression of melanin synthesis genes tyr and tyrp1a is increased five to sixfold in melanic bars, while xanthophore and iridophore marker genes are not differentially expressed. In summary, we provide novel insights on how vertical bars, one of the most widespread vertebrate color patterns, are formed through dynamic control of melanophore density, melanin synthesis and melanosome dispersal.

Galanin Signaling in the Brain Regulates Color Pattern Formation in Zebrafish

Color patterns are prominent features of many animals and are of high evolutionary relevance. In basal vertebrates, color patterns are composed of specialized pigment cells that arrange in multilayered mosaics in the skin. Zebrafish (Danio rerio), the preeminent model system for vertebrate color pattern formation, allows genetic screens as powerful approaches to identify novel functions in a complex biological system. Adult zebrafish display a series of blue and golden horizontal stripes, composed of black melanophores, silvery or blue iridophores, and yellow xanthophores. This stereotyped pattern is generated by self-organization involving direct cell contacts between all three types of pigment cells mediated by integral membrane proteins [1, 2, 3, 4, 5]. Here, we show that neuropeptide signaling impairs the striped pattern in a global manner. Mutations in the genes coding either for galanin receptor 1A (npm/galr1A) or for its ligand galanin (galn) result in fewer stripes, a pale appearance, and the mixing of cell types, thus resembling mutants with thyroid hypertrophy [6]. Zebrafish chimeras obtained by transplantations of npm/galr1A mutant blastula cells indicate that mutant pigment cells of all three types can contribute to a normal striped pattern in the appropriate host. However, loss of galr1A expression in a specific region of the brain is sufficient to cause the mutant phenotype in an otherwise wild-type fish. Increased thyroid hormone levels in mutant fish suggest that galanin signaling through Galr1A in the pituitary is an upstream regulator of the thyroid hormone pathway, which in turn promotes precise interactions of pigment cells during color pattern formation.

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Zebrafish stripes are generated by three types of self-organizing pigment cells

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Galanin signaling through Galr1A impairs zebrafish stripe formation globally

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Galr1A function in a specific brain region is required for pigment cell interactions

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Galanin signaling functions to downregulate thyroid hormone levels

Regulation of zebrafish melanocyte development by ligand-dependent BMP signaling

Preventing terminal differentiation is important in the development and progression of many cancers including melanoma. Recent identification of the BMP ligand GDF6 as a novel melanoma oncogene showed GDF6-activated BMP signaling suppresses differentiation of melanoma cells. Previous studies have identified roles for GDF6 orthologs during early embryonic and neural crest development, but have not identified direct regulation of melanocyte development by GDF6. Here, we investigate the BMP ligand gdf6a, a zebrafish ortholog of human GDF6, during the development of melanocytes from the neural crest. We establish that the loss of gdf6a or inhibition of BMP signaling during neural crest development disrupts normal pigment cell development, leading to an increase in the number of melanocytes and a corresponding decrease in iridophores, another neural crest-derived pigment cell type in zebrafish. This shift occurs as pigment cells arise from the neural crest and depends on mitfa, an ortholog of MITF, a key regulator of melanocyte development that is also targeted by oncogenic BMP signaling. Together, these results indicate that the oncogenic role ligand-dependent BMP signaling plays in suppressing differentiation in melanoma is a reiteration of its physiological roles during melanocyte development.

Zebrafish Pigment Pattern Formation: Insights into the Development and Evolution of Adult Form

Vertebrate pigment patterns are diverse and fascinating adult traits that allow animals to recognize conspecifics, attract mates, and avoid predators. Pigment patterns in fish are among the most amenable traits for studying the cellular basis of adult form, as the cells that produce diverse patterns are readily visible in the skin during development. The genetic basis of pigment pattern development has been most studied in the zebrafish, Danio rerio. Zebrafish adults have alternating dark and light horizontal stripes, resulting from the precise arrangement of three main classes of pigment cells: black melanophores, yellow xanthophores, and iridescent iridophores. The coordination of adult pigment cell lineage specification and differentiation with specific cellular interactions and morphogenetic behaviors is necessary for stripe development. Besides providing a nice example of pattern formation responsible for an adult trait of zebrafish, stripe-forming mechanisms also provide a conceptual framework for posing testable hypotheses about pattern diversification more broadly. Here, we summarize what is known about lineages and molecular interactions required for pattern formation in zebrafish, we review some of what is known about pattern diversification in Danio, and we speculate on how patterns in more distant teleosts may have evolved to produce a stunningly diverse array of patterns in nature.

Neural crest development: insights from the zebrafish

Our understanding of the neural crest, a key vertebrate innovation, is built upon studies of multiple model organisms. Early research on neural crest cells (NCCs) was dominated by analyses of accessible amphibian and avian embryos, with mouse genetics providing complementary insights in more recent years. The zebrafish model is a relative newcomer to the field, yet it offers unparalleled advantages for the study of NCCs. Specifically, zebrafish provide powerful genetic and transgenic tools, coupled with rapidly developing transparent embryos that are ideal for high-resolution real-time imaging of the dynamic process of neural crest development. While the broad principles of neural crest development are largely conserved across vertebrate species, there are critical differences in anatomy, morphogenesis, and genetics that must be considered before information from one model is extrapolated to another. Here, our goal is to provide the reader with a helpful primer specific to neural crest development in the zebrafish model. We focus largely on the earliest events-specification, delamination, and migration-discussing what is known about zebrafish NCC development and how it differs from NCC development in non-teleost species, as well as highlighting current gaps in knowledge.

Maintenance of Melanocyte Stem Cell Quiescence by GABA-A Signaling in Larval Zebrafish

In larval zebrafish, melanocyte stem cells (MSCs) are quiescent, but can be recruited to regenerate the larval pigment pattern following melanocyte ablation. Through pharmacological experiments, we found that inhibition of γ-aminobutyric acid (GABA)-A receptor function, specifically the GABA-A ρ subtype, induces excessive melanocyte production in larval zebrafish. Conversely, pharmacological activation of GABA-A inhibited melanocyte regeneration. We used clustered regularly interspaced short palindromic repeats/Cas9 to generate two mutant alleles of gabrr1, a subtype of GABA-A receptors. Both alleles exhibited robust melanocyte overproduction, while conditional overexpression of gabrr1 inhibited larval melanocyte regeneration. Our data suggest that gabrr1 signaling is necessary to maintain MSC quiescence and sufficient to reduce, but not eliminate, melanocyte regeneration in larval zebrafish.

Bar, stripe and spot development in sand-dwelling cichlids from Lake Malawi

Background

Melanic patterns such as horizontal stripes, vertical bars and spots are common among teleost fishes and often serve roles in camouflage or mimicry. Extensive research in the zebrafish model has shown that the development of horizontal stripes depends on complex cellular interactions between melanophores, xanthophores and iridophores. Little is known about the development of horizontal stripes in other teleosts, and even less is known about bar or spot development. Here, we compare chromatophore composition and development of stripes, bars and spots in two cichlid species of sand-dwellers from Lake Malawi—Copadichromis azureus and Dimidiochromis compressiceps.

Results

(1) In D. compressiceps, stripes are made of dense melanophores underlaid by xanthophores and overlaid by iridophores. Melanophores and xanthophores are either loose or absent in interstripes, and iridophores are dense. In C. azureus, spots and bars are composed of a chromatophore arrangement similar to that of stripes but are separated by interbars where density of melanophores and xanthophores is only slightly lower than in stripes and iridophore density appears slightly greater. (2) Stripe, bar and spot chromatophores appear in the skin at metamorphosis. Stripe melanophores directly differentiate along horizontal myosepta into the adult pattern. In contrast, bar number and position are dynamic throughout development. As body length increases, new bars appear between old ones or by splitting of old ones through new melanophore appearance, not migration. Xanthophore and iridophore distributions follow melanophore patterns. (3) Metamorphic pigmentation arises in cichlids in a fashion similar to that described in zebrafish: melanophore progenitors derived from the medial route of neural crest migration migrate from the vicinity of the neural tube to the skin during metamorphosis.

Conclusion

The three pigment cell types forming stripes, bars and spots arise in the skin at metamorphosis. Stripes develop by differentiation of melanophores along horizontal myosepta, while bars do not develop along patent anatomical boundaries and increase in number in relation with body size. We propose that metamorphic melanophore differentiation and migratory arrest upon arrival to the skin lead to stripe formation, while bar formation must be supported by extensive migration of undifferentiated melanophores in the skin.

Morphological Characteristics and Comparative Transcriptome Analysis of Three Different Phenotypes of Pristella maxillaris

Pristella maxillaris is known as the X-ray fish based on its translucent body. However, the morphological characteristics and the molecular regulatory mechanisms of these translucent bodies are still unknown. In this study, the following three phenotypes, a black-and-gray body color or wild-type (WT), a silvery-white body color defined as mutant I (MU1), and a fully transparent body with a visible visceral mass named as mutant II (MU2), were investigated to analyze their chromatophores and molecular mechanisms. The variety and distribution of pigment cells in the three phenotypes of P. maxillaris significantly differed by histological assessment. Three types of chromatophores (melanophores, iridophores, and xanthophores) were observed in the WT, whereas MU1 fish were deficient in melanophores, and MU2 fish lacked melanophores and iridophores. Transcriptome sequencing of the skin and peritoneal tissues of P. maxillaris identified a total of 166,089 unigenes. After comparing intergroup gene expression levels, more than 3,000 unigenes with significantly differential expression levels were identified among three strains. Functional annotation and Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses of the differentially expressed genes (DEGs) identified a number of candidates melanophores and iridophores genes that influence body color. Some DEGs that were identified using transcriptome analysis were confirmed by quantitative real-time PCR. This study serves as a global survey of the morphological characteristics and molecular mechanism of different body colors observed in P. maxillaris and thus provides a valuable theoretical foundation for the molecular regulation of the transparent phenotype.