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

The Colorful Sex Chromosomes of Teleost Fish

Teleost fish provide some of the most intriguing examples of sexually dimorphic coloration, which is often advantageous for only one of the sexes. Mapping studies demonstrated that the genetic loci underlying such color patterns are frequently in tight linkage to the sex-determining locus of a species, ensuring sex-specific expression of the corresponding trait. Several genes affecting color synthesis and pigment cell development have been previously described, but the color loci on the sex chromosomes have mostly remained elusive as yet. Here, we summarize the current knowledge about the genetics of such color loci in teleosts, mainly from studies on poeciliids and cichlids. Further studies on these color loci will certainly provide important insights into the evolution of sex chromosomes.

Neural crest and the patterning of vertebrate craniofacial muscles

Patterning of craniofacial muscles overtly begins with the activation of lineage-specific markers at precise, evolutionarily conserved locations within prechordal, lateral, and both unsegmented and somitic paraxial mesoderm populations. Although these initial programming events occur without influence of neural crest cells, the subsequent movements and differentiation stages of most head muscles are neural crest-dependent. Incorporating both descriptive and experimental studies, this review examines each stage of myogenesis up through the formation of attachments to their skeletal partners. We present the similarities among developing muscle groups, including comparisons with trunk myogenesis, but emphasize the morphogenetic processes that are unique to each group and sometimes subsets of muscles within a group. These groups include branchial (pharyngeal) arches, which encompass both those with clear homologues in all vertebrate classes and those unique to one, for example, mammalian facial muscles, and also extraocular, laryngeal, tongue, and neck muscles. The presence of several distinct processes underlying neural crest:myoblast/myocyte interactions and behaviors is not surprising, given the wide range of both quantitative and qualitative variations in craniofacial muscle organization achieved during vertebrate evolution.

Generation of a double binary transgenic zebrafish model to study myeloid gene regulation in response to oncogene activation in melanocytes

A complex network of inflammatory genes is closely linked to somatic cell transformation and malignant disease. Immune cells and their associated molecules are responsible for detecting and eliminating cancer cells as they establish themselves as the precursors of a tumour. By the time a patient has a detectable solid tumour, cancer cells have escaped the initial immune response mechanisms. Here, we describe the development of a double binary zebrafish model that enables regulatory programming of the myeloid cells as they respond to oncogene-activated melanocytes to be explored, focussing on the initial phase when cells become the precursors of cancer. A hormone-inducible binary system allows for temporal control of expression of different Ras oncogenes (NRasQ61K, HRasG12V and KRasG12V) in melanocytes, leading to proliferation and changes in morphology of the melanocytes. This model was coupled to binary cell-specific biotagging models allowing in vivo biotinylation and subsequent isolation of macrophage or neutrophil nuclei for regulatory profiling of their active transcriptomes. Nuclear transcriptional profiling of neutrophils, performed as they respond to the earliest precursors of melanoma in vivo, revealed an intricate landscape of regulatory factors that may promote progression to melanoma, including Serpinb1l4, Fgf1, Fgf6, Cathepsin H, Galectin 1 and Galectin 3. The model presented here provides a powerful platform to study the myeloid response to the earliest precursors of melanoma.

Uveal melanoma driver mutations in GNAQ/11 yield numerous changes in melanocyte biology

Uveal melanoma (UM) is the most common primary intraocular cancer and has a high incidence of metastasis, which lacks any effective treatment. Here, we present zebrafish models of UM, which are driven by melanocyte-specific expression of activating GNAQ or GNA11 alleles, GNAQ/11^Q209L , the predominant initiating mutations for human UM. When combined with mutant tp53, GNAQ/11^Q209L transgenics develop various melanocytic tumors, including UM, with near complete penetrance. These tumors display nuclear YAP localization and thus phenocopy human UM. We show that GNAQ/11^Q209L expression induces profound melanocyte defects independent of tp53 mutation, which are apparent within 3 days of development. First, increases in melanocyte number, melanin content, and subcellular melanin distribution result in hyperpigmentation. Additionally, altered melanocyte migration, survival properties, and evasion of normal boundary cues lead to aberrant melanocyte localization and stripe patterning. Collectively, these data show that GNAQ/11^Q209L is sufficient to induce numerous protumorigenic changes within melanocytes.

Anterior-posterior gene expression differences in three Lake Malawi cichlid fishes with variation in body stripe orientation

Morphological differentiation among closely related species provides opportunities to study mechanisms shaping natural phenotypic variation. Here, we address variation in the orientation of melanin-colored body stripes in three cichlid species of the tribe Haplochromini. Melanochromis auratus displays a common pattern of dark, straight horizontal body stripes, whereas in Aristochromis christyi and Buccochromis rhoadesii, oblique stripes extend from the anterior dorsal to the posterior mid-lateral trunk. We first validated a stably reference gene, and then, investigated the chromatophore distribution in the skin by assessing the expression levels of the iridophore and melanophore marker genes, ltk and slc24a5, respectively, as well as pmel, a melanophore pigmentation marker gene. We found anterior-posterior differences in the expression levels of the three genes in the oblique-striped species. The higher anterior expression of ltk, indicates increased iridophore density in the anterior region, i.e., uneven horizontal distribution of iridophores, which coincides with the anterior dorsalization of melanophore stripe in these species. The obliqueness of the horizontal body stripes might be a result of distinct migratory or patterning abilities of melanophores in anterior and posterior stripe regions which could be reflected by variation in the expression of genes involved in melanophore patterning. To address this, we investigated anterior-posterior expression levels of a primary set of candidate target genes with known functions in melanophore migration and stripe patterning in the adult zebrafish, and their related gene regulatory network. Among these genes, those with differences in anterior-posterior expression showed only species-specific differential expression, e.g., sdf1a, col14a1a, ifitm5, and agpat3, with the exception of fbxw4/hagoromo (differentially expressed in an oblique-and the straight-striped species). In summary, distinct anterior-posterior gradients in iridophore density found to be more similar characteristic between the two oblique-striped species. Furthermore, the species-specific differential expression of genes involved in stripe patterning might also implicate distinct molecular processes underlying the obliqueness of body stripe in two closely related cichlid species.

Transparent Tiger barb Puntius tetrazona, a fish model for in vivo analysis of nocardial infection

Nocardiosis afflicts multiple species of cultured fish, resulting in substantial economic losses to the aquaculture industry, however, lack of detailed knowledge on disease pathogenesis has hampered the development of effective prevention and control strategies. In this study, we injected a green fluorescent protein (GFP)-labeled Nocardia seriolae strain into a transparent mutant strain of Tiger barb (Puntius tetrazona) to monitor tissue pathogen accumulation and tissue damage in vivo, and to clarify the relationship between pathogenic processes and overt symptoms. GFP-labeled bacteria were phagocytized by leukocytes and could proliferate within these cells, which in turn led to leukocyte aggregation, leukocyte death, and granuloma formation. In addition, intracellular bacteria could permanently colonize various tissues via leukocyte circulation, causing multi-organ infection as revealed by changes of tissue transparency. Histology revealed granulomatous lesions in organs such as muscle, kidney, and spleen that was corresponded to the tissue opacities in vivo. Confocal microscopy confirmed massive accumulations of GFP-labeled bacteria within these granulomas, which often contained a necrotic core. Tiger barb transparency allows for real-time observation of in vivo pathological changes within the same animal, and the pathogenic process can be evaluated based on the shape and size of body opacities. Thus, transparent Tiger barb is a promising model to study the pathogenesis of nocardiosis.

BAC Recombineering of the Agouti Loci from Spotted Gar and Zebrafish Reveals the Evolutionary Ancestry of Dorsal-Ventral Pigment Asymmetry in Fish

Dorsoventral pigment patterning, characterized by a light ventrum and a dark dorsum, is one of the most widespread chromatic adaptations in vertebrate body coloration. In mammals, this countershading depends on differential expression of agouti-signaling protein (ASIP), which drives a switch of synthesis of one type of melanin to another within melanocytes. Teleost fish share countershading, but the pattern results from a differential distribution of multiple types of chromatophores, with black-brown melanophores most abundant in the dorsal body and reflective iridophores most abundant in the ventral body. We previously showed that Asip1 (a fish ortholog of mammalian ASIP) plays a role in patterning melanophores. This observation leads to the surprising hypothesis that agouti may control an evolutionarily conserved pigment pattern by regulating different mechanisms in mammals and fish. To test this hypothesis, we compared two ray-finned fishes: the teleost zebrafish and the nonteleost spotted gar (Lepisosteus oculatus). By examining the endogenous pattern of asip1 expression in gar, we demonstrate a dorsoventral-graded distribution of asip1 expression that is highest ventrally, similar to teleosts. Additionally, in the first reported experiments to generate zebrafish transgenic lines carrying a bacterial artificial chromosome (BAC) from spotted gar, we show that both transgenic zebrafish lines embryos replicate the endogenous asip1 expression pattern in adult zebrafish, showing that BAC transgenes from both species contain all of the regulatory elements required for regular asip1 expression within adult ray-finned fishes. These experiments provide evidence that the mechanism leading to an environmentally important pigment pattern was likely in place before the origin of teleosts.

Developmental mechanisms of longitudinal stripes in the Japanese four-lined snake

The developmental mechanisms of color patterns formation and its evolution remain unclear in reptilian sauropsids. We, therefore, studied the pigment cell mechanisms of stripe pattern formation during embryonic development of the snake Elaphe quadrivirgata. We identified 10 post-ovipositional embryonic developmental stages based on external morphological characteristics. Examination for the temporal changes in differentiation, distribution, and density of pigment cells during embryonic development revealed that melanophores first appeared in myotome and body cavity but not in skin surface at Stage 5. Epidermal melanophores were first recognized at Stage 7, and dermal melanophores and iridophores appeared in Stage 9. Stripe pattern first appeared to establish at Stage 8 as a spatial density gradient of epidermal melanophores between the regions of future dark brown longitudinal stripes and light colored background. Our study, thus, provides a comprehensive pigment-cell-based understanding of stripe pattern formation during embryonic development. We briefly discuss the importance of the gene expression studies by considering the biologically relevant theoretical models with standard developmental staging for understanding reptilian color pattern evolution.

Sequence-Based Mapping and Genome Editing Reveal Mutations in Stickleback Hps5 Cause Oculocutaneous Albinism and the casper Phenotype

Here, we present and characterize the spontaneous X-linked recessive mutation casper, which causes oculocutaneous albinism in threespine sticklebacks (Gasterosteus aculeatus). In humans, Hermansky-Pudlak syndrome results in pigmentation defects due to disrupted formation of the melanin-containing lysosomal-related organelle (LRO), the melanosome. casper mutants display not only reduced pigmentation of melanosomes in melanophores, but also reductions in the iridescent silver color from iridophores, while the yellow pigmentation from xanthophores appears unaffected. We mapped casper using high-throughput sequencing of genomic DNA from bulked casper mutants to a region of the stickleback X chromosome (chromosome 19) near the stickleback ortholog of Hermansky-Pudlak syndrome 5 (Hps5). casper mutants have an insertion of a single nucleotide in the sixth exon of Hps5, predicted to generate an early frameshift. Genome editing using CRISPR/Cas9 induced lesions in Hps5 and phenocopied the casper mutation. Injecting single or paired Hps5 guide RNAs revealed higher incidences of genomic deletions from paired guide RNAs compared to single gRNAs. Stickleback Hps5 provides a genetic system where a hemizygous locus in XY males and a diploid locus in XX females can be used to generate an easily scored visible phenotype, facilitating quantitative studies of different genome editing approaches. Lastly, we show the ability to better visualize patterns of fluorescent transgenic reporters in Hps5 mutant fish. Thus, Hps5 mutations present an opportunity to study pigmented LROs in the emerging stickleback model system, as well as a tool to aid in assaying genome editing and visualizing enhancer activity in transgenic fish.

Genome-wide association analysis identifies potential regulatory genes for eumelanin pigmentation in chicken plumage

Plumage color in chicken is determined by the proportion of eumelanin and pheomelanin pigmentation. As the main ingredient in plumage melanin, eumelanin plays a key role in the dark black, brown and grey coloration. However, very few studies have been performed to identify the related genes and mutations on a genome-wide scale. Herein, a resource family consisting of one backcross population and two F2 cross populations between a black roster and Yukou Brown I parent stockbreed was constructed for identification of genes related to eumelanin pigmentation. Chickens with eumelanin in their plumage were classified as the case group, and the rest were considered the control group. A genome-wide association study of this phenotype and genotypes using Affymetrix 600K HD SNP arrays in this F2 family revealed 13 significantly associated SNPs and in 10 separate genes on chromosomes 1, 2, 3 and 5. Based on previous studies in model species, we inferred that genes, including NUAK family kinase 1 (NUAK1) and sonic hedgehog (SHH), may play roles in the development of neural crest cells or melanoblasts during the embryonic period, which may also affect the eumelanin pigmentation. Our results facilitate the understanding of the genetic basis of eumelanin pigmentation in chicken plumage.

Mechanisms of silver_nanoparticles induced hypopigmentation in embryonic zebrafish

Silver_nanoparticles (AgNPs) have been reported to inhibit specification of erythroid cells and to induce spinal cord deformities and cardiac arrhythmia in vertebrates, but have not been implicated in development of neural crest (NC) and pigment cells in an in vivo model yet. In current study, down-regulated expressions of NC genes pax7 and foxd3, melanophore genes mitfa and dct, and xanthophore gene gch2 in AgNPs-exposed embryos were revealed by microarray, qRT-PCR and whole-mount in situ hybridization (WISH). Then, the down-regulated expressions of melanophore genes mitfa and dct but not xanthophore gene gch2 in AgNPs-exposed embryos were found to be recovered by melanogenesis agonists palmitic acid and dibutyryl cyclic AMP (dbcAMP). Finally, Ag⁺ chelating and AgNPs coating compound l-cysteine was found to neutralize AgNPs-induced hypopigmentation in AgNPs-exposed embryos, and to recover the down-regulated expressions of both dct and gch2 to nearly normal level in embryos, suggesting that AgNPs-releasing Ag⁺ might mediate their biological effects on zebrafish pigmentation mostly. This study was firstly to unveil that AgNPs might specifically act up-stream of mitfa and pax7 genes to suppress specification and differentiation of melanophore and xanthophore lineages respectively by their releasing Ag⁺ during vertebrate embryogenesis.

A gene expression study of dorso-ventrally restricted pigment pattern in adult fins of Neolamprologus meeli, an African cichlid species

Fish color patterns are among the most diverse phenotypic traits found in the animal kingdom. Understanding the molecular and cellular mechanisms that control in chromatophore distribution and pigmentation underlying this diversity is a major goal in developmental and evolutionary biology, which has predominantly been pursued in the zebrafish model system. Here, we apply results from zebrafish work to study a naturally occurring color pattern phenotype in the fins of an African cichlid species from Lake Tanganyika. The cichlid fish Neolamprologus meeli displays a distinct dorsal color pattern, with black and white stripes along the edges of the dorsal fin and of the dorsal half of the caudal fin, corresponding with differences in melanophore density. To elucidate the molecular mechanisms controlling the differences in dorsal and ventral color patterning in the fins, we quantitatively assessed the expression of 15 candidate target genes involved in adult zebrafish pigmentation and stripe formation. For reference gene validation, we screened the expression stability of seven widely expressed genes across the investigated tissue samples and identified tbp as appropriate reference. Relative expression levels of the candidate target genes were compared between the dorsal, striped fin regions and the corresponding uniform, grey-colored regions in the anal and ventral caudal fin. Dorso-ventral expression differences, with elevated levels in both white and black stripes, were observed in two genes, the melanosome protein coding gene pmel and in igsf11, which affects melanophore adhesion, migration and survival. Next, we predicted potential shared upstream regulators of pmel and igsf11. Testing the expression patterns of six predicted transcriptions factors revealed dorso-ventral expression difference of irf1 and significant, negative expression correlation of irf1 with both pmel and igsf11. Based on these results, we propose pmel, igsf11 and irf1 as likely components of the genetic mechanism controlling distinct dorso-ventral color patterns in N. meeli fins.

Zebrafish adult pigment stem cells are multipotent and form pigment cells by a progressive fate restriction process: Clonal analysis identifies shared origin of all pigment cell types

Skin pigment pattern formation is a paradigmatic example of pattern formation. In zebrafish, the adult body stripes are generated by coordinated rearrangement of three distinct pigment cell-types, black melanocytes, shiny iridophores and yellow xanthophores. A stem cell origin of melanocytes and iridophores has been proposed although the potency of those stem cells has remained unclear. Xanthophores, however, seemed to originate predominantly from proliferation of embryonic xanthophores. Now, data from Singh et al. shows that all three cell-types derive from shared stem cells, and that these cells generate peripheral neural cell-types too. Furthermore, clonal compositions are best explained by a progressive fate restriction model generating the individual cell-types. The numbers of adult pigment stem cells associated with the dorsal root ganglia remain low, but progenitor numbers increase significantly during larval development up to metamorphosis, likely via production of partially restricted progenitors on the spinal nerves.

Pigeonetics takes flight: Evolution, development, and genetics of intraspecific variation

Intensive artificial selection over thousands of years has produced hundreds of varieties of domestic pigeon. As Charles Darwin observed, the morphological differences among breeds can rise to the magnitude of variation typically observed among different species. Nevertheless, different pigeon varieties are interfertile, thereby enabling forward genetic and genomic approaches to identify genes that underlie derived traits. Building on classical genetic studies of pigeon variation, recent molecular investigations find a spectrum of coding and regulatory alleles controlling derived traits, including plumage color, feather growth polarity, and limb identity. Developmental and genetic analyses of pigeons are revealing the molecular basis of variation in a classic example of extreme intraspecific diversity, and have the potential to nominate genes that control variation among other birds and vertebrates in general.

The zebrafish as a model for studying neuroblastoma

Neuroblastoma is a tumor arising in the peripheral sympathetic nervous system and is the most common cancer in childhood. Since most of the cellular and molecular mechanisms underlying neuroblastoma onset and progression remain unknown, the generation of new in vivo models might be appropriate to better dissect the peripheral sympathetic nervous system development in both physiological and disease states. This review is focused on the use of zebrafish as a suitable and innovative model to study neuroblastoma development. Here, we briefly summarize the current knowledge about zebrafish peripheral sympathetic nervous system formation, focusing on key genes and cellular pathways that play a crucial role in the differentiation of sympathetic neurons during embryonic development. In addition, we include examples of how genetic changes known to be associated with aggressive neuroblastoma can mimic this malignancy in zebrafish. Thus, we note the value of the zebrafish model in the field of neuroblastoma research, showing how it can improve our current knowledge about genes and biological pathways that contribute to malignant transformation and progression during embryonic life.

Comparative transcriptomics of anal fin pigmentation patterns in cichlid fishes

Background

Understanding the genetic basis of novel traits is a central topic in evolutionary biology. Two novel pigmentation phenotypes, egg-spots and blotches, emerged during the rapid diversification of East African cichlid fishes. Egg-spots are circular pigmentation markings on the anal fins of hundreds of derived haplochromine cichlids species, whereas blotches are patches of conspicuous anal fin pigmentation with ill-defined boundaries that occur in few species that belong to basal cichlid lineages. Both traits play an important role in the breeding behavior of this group of fishes. Knowledge about the origin, homology and underlying genetics of these pigmentation traits is sparse.

Results

Here, we present a comparative transcriptomic and differential gene expression analysis of egg-spots and blotches. We first conducted an RNA sequencing experiment where we compared egg-spot tissue with the remaining portion of egg-spot-free fin tissue using six individuals of Astatotilapia burtoni. We identified 1229 differentially expressed genes between the two tissue types. We then showed that rates of evolution of these genes are higher than average estimated on whole transcriptome data. Using quantitative real-time PCR, we found that 29 out of a subset of 46 differentially expressed genes showed an analogous expression pattern in another haplochromine species’ egg-spots, Cynotilapia pulpican, strongly suggesting that these genes are involved in the egg-spot phenotype. Among these are the previously identified egg-spot gene fhl2a, two known patterning genes (hoxC12a and bmp3) as well as other pigmentation related genes such as asip. Finally, we analyzed the expression patterns of the same gene subset in two species that feature blotches instead of egg-spots, one haplochromine species (Pseudocrenilabrus philander) and one ectodine species (Callochromis macrops), revealing that the expression patterns in blotches and egg-spots are rather distinct.

Conclusions

We identified several candidate genes that will serve as an important and useful resource for future research on the emergence and diversification of cichlid fishes’ egg-spots. Only a limited degree of conservation of gene expression patterns was detected between the egg-spots of the derived haplochromines and blotches from ancestral haplochromines, as well as between the two types of blotches, suggesting an independent origin of these traits.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3046-y) contains supplementary material, which is available to authorized users.

Isolation of 4,5-O-Dicaffeoylquinic Acid as a Pigmentation Inhibitor Occurring in Artemisia capillaris Thunberg and Its Validation In Vivo

There is a continual need to develop novel and effective melanogenesis inhibitors for the prevention of hyperpigmentation disorders. The plant Artemisia capillaris Thunberg (Oriental Wormwood) was screened for antipigmentation activity using murine cultured cells (B16-F10 malignant melanocytes). Activity-based fractionation using HPLC and NMR analyses identified the compound 4,5-O-dicaffeoylquinic acid as an active component in this plant. 4,5-O-Dicaffeoylquinic acid significantly reduced melanin synthesis and tyrosinase activity in a dose-dependent manner in the melanocytes. In addition, 4,5-O-dicaffeoylquinic acid treatment reduced the expression of tyrosinase-related protein-1. Significantly, we could validate the antipigmentation activity of this compound in vivo, using a zebrafish model. Moreover, 4,5-O-dicaffeoylquinic acid did not show toxicity in this animal model. Our discovery of 4,5-O-dicaffeoylquinic acid as an inhibitor of pigmentation that is active in vivo shows that this compound can be developed as an active component for formulations to treat pigmentation disorders.

The Physiological Characterization of Connexin41.8 and Connexin39.4, Which Are Involved in the Striped Pattern Formation of Zebrafish

The zebrafish has a striped skin pattern on its body, and Connexin41.8 (Cx41.8) and Cx39.4 are involved in striped pattern formation. Mutations in these connexins change the striped pattern to a spot or labyrinth pattern. In this study, we characterized Cx41.8 and Cx39.4 after expression in Xenopus oocytes. In addition, we analyzed Cx41.8 mutants Cx41.8I203F and Cx41.8M7, which caused spot or labyrinth skin patterns, respectively, in transgenic zebrafish. In the electrophysiological analysis, the gap junctions formed by Cx41.8 and Cx39.4 showed distinct sensitivity to transjunctional voltage. Analysis of non-junctional (hemichannel) currents revealed a large voltage-dependent current in Cx39.4-expressing oocytes that was absent in cells expressing Cx41.8. Junctional currents induced by both Cx41.8 and Cx39.4 were reduced by co-expression of Cx41.8I203F and abolished by co-expression of Cx41.8M7. In the transgenic experiment, Cx41.8I203F partially rescued the Cx41.8 null mutant phenotype, whereas Cx41.8M7 failed to rescue the null mutant, and it elicited a more severe phenotype than the Cx41.8 null mutant, as evidenced by a smaller spot pattern. Our results provide evidence that gap junctions formed by Cx41.8 play an important role in stripe/spot patterning and suggest that mutations in Cx41.8 can effect patterning by way of reduced function (I203F) and dominant negative effects (M7). Our results suggest that functional differences in Cx41.8 and Cx39.4 relate to spot or labyrinth mutant phenotypes and also provide evidence that these two connexins interact in vivo and in vitro.

Pigment cell mechanism of postembryonic stripe pattern formation in the Japanese four-lined snake

Postembryonic changes in the dermal and epidermal pigment cell architecture of the striped and nonstriped morph of the Japanese four-lined snake Elaphe quadrivirgata were examined to reveal stripe pattern formation after hatching. The striped and nonstriped morphs were distinguishable at the hatching, suggesting that the basis of stripe pattern was formed during embryonic development. In the striped morph, the color of stripes changed from red-brown in juveniles to vivid dark-brown in adults, and density of dermal melanophore increased much more in the stripe than background dorsal scales with growth. This increase in density of dermal melanophore was accompanied not only by the increased epidermal melanophore density but also by the change in vertical structures of dermal melanophore. By contrast, the density of epidermal and dermal melanophore evenly increased over the dorsal scales in the nonstriped morph. Thus, the increased vividness of the stripe pattern after hatching is achieved through localized increase of melanophore density particularly in the stripe region but not over the whole dorsal scales.

PAX transcription factors in neural crest development

The nine vertebrate PAX transcription factors (PAX1-PAX9) play essential roles during early development and organogenesis. Pax genes were identified in vertebrates using their homology with the Drosophila melanogaster paired gene DNA-binding domain. PAX1-9 functions are largely conserved throughout vertebrate evolution, in particular during central nervous system and neural crest development. The neural crest is a vertebrate invention, which gives rise to numerous derivatives during organogenesis, including neurons and glia of the peripheral nervous system, craniofacial skeleton and mesenchyme, the heart outflow tract, endocrine and pigment cells. Human and mouse spontaneous mutations as well as experimental analyses have evidenced the critical and diverse functions of PAX factors during neural crest development. Recent studies have highlighted the role of PAX3 and PAX7 in neural crest induction. Additionally, several PAX proteins - PAX1, 3, 7, 9 - regulate cell proliferation, migration and determination in multiple neural crest-derived lineages, such as cardiac, sensory, and enteric neural crest, pigment cells, glia, craniofacial skeleton and teeth, or in organs developing in close relationship with the neural crest such as the thymus and parathyroids. The diverse PAX molecular functions during neural crest formation rely on fine-tuned modulations of their transcriptional transactivation properties. These modulations are generated by multiple means, such as different roles for the various isoforms (formed by alternative splicing), or posttranslational modifications which alter protein-DNA binding, or carefully orchestrated protein-protein interactions with various co-factors which control PAX proteins activity. Understanding these regulations is the key to decipher the versatile roles of PAX transcription factors in neural crest development, differentiation and disease.

Zebrafish stripes as a model for vertebrate colour pattern formation

Colour patterns are prominent features of many animals and have important functions in communication, such as camouflage, kin recognition and mate choice. As targets for natural as well as sexual selection, they are of high evolutionary significance. The molecular mechanisms underlying colour pattern formation in vertebrates are not well understood. Progress in transgenic tools, in vivo imaging and the availability of a large collection of mutants make the zebrafish (Danio rerio) an attractive model to study vertebrate colouration. Zebrafish display golden and blue horizontal stripes that form during metamorphosis as mosaics of yellow xanthophores, silvery or blue iridophores and black melanophores in the hypodermis. Lineage tracing revealed the origin of the adult pigment cells and their individual cellular behaviours during the formation of the striped pattern. Mutant analysis indicated that interactions between all three pigment cell types are required for the formation of the pattern, and a number of cell surface molecules and signalling systems have been identified as mediators of these interactions. The understanding of the mechanisms that underlie colour pattern formation is an important step towards deciphering the genetic basis of variation in evolution.

Facing the environment: onset and development of UV markings in young fish

Most colour patterns in animals represent an elegant compromise between conspicuousness to ensure effective communication with preferred receivers and camouflage to avoid attracting the attention of unwanted predators. Many species, including several coral reef fishes, overcome this conflict by using ultraviolet (UV) colouration and signalling, as these colours are visible only over short distances and are often invisible to their predators. Despite a great interest in their behavioural significance and ecological influence on survival, little is known about when these colours first develop on the bodies of free-living animals. Here we show for the first time that the UV facial patterns of a coral reef fish do not develop in captivity but only when juveniles experience the socio-behavioural conditions of their natural environment. Using field and laboratory experiments, we determined that the onset and early development of these UV facial markings did not occur at metamorphosis. Instead, juveniles developed the UV markings during their first two weeks on the reef. Exposure to different reef environments revealed significant plasticity in the development of these markings. The direct or indirect (through intraspecific interactions) exposure to predators is a likely candidate trigger for the plastic development of these UV markings in the wild.

Pigment cell movement is not required for generation of Turing patterns in zebrafish skin

The zebrafish is a model organism for pattern formation in vertebrates. Understanding what drives the formation of its coloured skin motifs could reveal pivotal to comprehend the mechanisms behind morphogenesis. The motifs look and behave like reaction-diffusion Turing patterns, but the nature of the underlying physico-chemical processes is very different, and the origin of the patterns is still unclear. Here we propose a minimal model for such pattern formation based on a regulatory mechanism deduced from experimental observations. This model is able to produce patterns with intrinsic wavelength, closely resembling the experimental ones. We mathematically prove that their origin is a Turing bifurcation occurring despite the absence of cell motion, through an effect that we call differential growth. This mechanism is qualitatively different from the reaction-diffusion originally proposed by Turing, although they both generate the short-range activation and the long-range inhibition required to form Turing patterns.

Ferritin H subunit gene is specifically expressed in melanophore precursor-derived white pigment cells in which reflecting platelets are formed from stage II melanosomes in the periodic albino mutant of Xenopus laevis

“White pigment cells” are derived from melanophore precursors and contain both melanophore-specific and iridophore-specific pigment organelles. Whereas melanophores differentiate in the wild type regenerating tail, white pigment cells appear in the regenerating tail in the periodic albino mutant (a^p/a^p) of Xenopus laevis. The localization and density of white pigment cells in the mutant regenerating tail are similar to those of melanophores in the wild type regenerating tail. Here, white pigment cells in the mutant regenerating tail have been compared with melanophores in the wild type regenerating tail in the presence of phenylthiourea (PTU), which inhibits melanosome maturation in melanophores but does not affect reflecting platelet formation in white pigment cells. Ultrastructural analysis shows that reflecting platelet formation in white pigment cells is different from that in iridophores. Reflecting platelets in iridophores are formed from spherical vesicles with electron-dense material, whereas they are formed from stage II melanosomes characteristic of melanophore precursors in white pigment cells. Ultrastructural features of pigment organelles, except reflecting platelets, are similar between mutant melanophores and white pigment cells. In an attempt to identify specific genes in white pigment cells, a subtracted cDNA library enriched for mutant cDNAs has been prepared. Subtracted cDNA fragments have been cloned and selected by whole mount in situ hybridization. Among cDNA fragments examined so far, the ferritin H subunit gene is specifically expressed in white pigment cells, but not in melanophores. Pigment organellogenesis and specific gene expression in white pigment cells are also discussed.

Molecular control of the neural crest and peripheral nervous system development

A transient and unique population of multipotent stem cells, known as neural crest cells (NCCs), generate a bewildering array of cell types during vertebrate development. An attractive model among developmental biologists, the study of NCC biology has provided a wealth of knowledge regarding the cellular and molecular mechanisms important for embryogenesis. Studies in numerous species have defined how distinct phases of NCC specification, proliferation, migration, and survival contribute to the formation of multiple functionally distinct organ systems. NCC contributions to the peripheral nervous system (PNS) are well known. Critical developmental processes have been defined that provide outstanding models for understanding how extracellular stimuli, cell-cell interactions, and transcriptional networks cooperate to direct cellular diversification and PNS morphogenesis. Dissecting the complex extracellular and intracellular mechanisms that mediate the formation of the PNS from NCCs may have important therapeutic implications for neurocristopathies, neuropathies, and certain forms of cancer.

Pigment patterns in adult fish result from superimposition of two largely independent pigmentation mechanisms

Dorso-ventral pigment pattern differences are the most widespread pigmentary adaptations in vertebrates. In mammals, this pattern is controlled by regulating melanin chemistry in melanocytes using a protein, agouti-signalling peptide (ASIP). In fish, studies of pigment patterning have focused on stripe formation, identifying a core striping mechanism dependent upon interactions between different pigment cell types. In contrast, mechanisms driving the dorso-ventral countershading pattern have been overlooked. Here, we demonstrate that, in fact, zebrafish utilize two distinct adult pigment patterning mechanisms - an ancient dorso-ventral patterning mechanism, and a more recent striping mechanism based on cell-cell interactions; remarkably, the dorso-ventral patterning mechanism also utilizes ASIP. These two mechanisms function largely independently, with resultant patterns superimposed to give the full pattern.

Origins of adult pigmentation: diversity in pigment stem cell lineages and implications for pattern evolution

Teleosts comprise about half of all vertebrate species and exhibit an extraordinary diversity of adult pigment patterns that function in shoaling, camouflage, and mate choice and have played important roles in speciation. Here, we review studies that have identified several distinct neural crest lineages, with distinct genetic requirements, that give rise to adult pigment cells in fishes. These lineages include post-embryonic, peripheral nerve-associated stem cells that generate black melanophores and iridescent iridophores, cells derived directly from embryonic neural crest cells that generate yellow-orange xanthophores, and bipotent stem cells that generate both melanophores and xanthophores. This complexity in adult chromatophore lineages has implications for our understanding of adult traits, melanoma, and the evolutionary diversification of pigment cell lineages and patterns.

Thyroid hormone-dependent adult pigment cell lineage and pattern in zebrafish

Pigment patterns are useful for elucidating fundamental mechanisms of pattern formation and how these mechanisms evolve. In zebrafish, several pigment cell classes interact to generate stripes, yet the developmental requirements and origins of these cells remain poorly understood. Using zebrafish and a related species, we identified roles for thyroid hormone (TH) in pigment cell development and patterning, and in postembryonic development more generally. We show that adult pigment cells arise from distinct lineages having distinct requirements for TH and that differential TH dependence can evolve within lineages. Our findings demonstrate critical functions for TH in determining pigment pattern phenotype and highlight the potential for evolutionary diversification at the intersection of developmental and endocrine mechanisms.

Carotenoid-based coloration in cichlid fishes

Animal colors play important roles in communication, ecological interactions and speciation. Carotenoid pigments are responsible for many yellow, orange and red hues in animals. Whereas extensive knowledge on the proximate mechanisms underlying carotenoid coloration in birds has led to testable hypotheses on avian color evolution and signaling, much less is known about the expression of carotenoid coloration in fishes. Here, we promote cichlid fishes (Perciformes: Cichlidae) as a system in which to study the physiological and evolutionary significance of carotenoids. Cichlids include some of the best examples of adaptive radiation and color pattern diversification in vertebrates. In this paper, we examine fitness correlates of carotenoid pigmentation in cichlids and review hypotheses regarding the signal content of carotenoid-based ornaments. Carotenoid-based coloration is influenced by diet and body condition and is positively related to mating success and social dominance. Gaps in our knowledge are discussed in the last part of this review, particularly in the understanding of carotenoid metabolism pathways and the genetics of carotenoid coloration. We suggest that carotenoid metabolism and transport are important proximate mechanisms responsible for individual and population-differences in cichlid coloration that may ultimately contribute to diversification and speciation.

Growth and maturation in the zebrafish, Danio rerio: a staging tool for teaching and research

Zebrafish have been increasingly used as a teaching tool to enhance the learning of many biological concepts from genetics, development, and behavior to the understanding of the local watershed. Traditionally, in both research and teaching, zebrafish work has focused on embryonic stages; however, later stages, from larval through adulthood, are increasingly being examined. Defining developmental stages based on age is a problematic way to assess maturity, because many environmental factors, such as temperature, population density, and water quality, impact growth and maturation. Fish length and characterization of key external morphological traits are considered better markers for maturation state. While a number of staging series exist for zebrafish, here we present a simplified normalization table of post-embryonic maturation well suited to both educational and research use. Specifically, we utilize fish size and four easily identified external morphological traits (pigment pattern, tail fin, anal fin, and dorsal fin morphology) to describe three larval stages, a juvenile stage, and an adult stage. These simplified maturation standards will be a useful tool for both educational and research protocols.

The biology of melanocyte and melanocyte stem cell

The melanocyte stem cells of the hair follicle provide an attractive system for the study of the stem cells. Successful regeneration of a functional organ relies on the organized and timely orchestration of molecular events among distinct stem/progenitor cell populations. The stem cells are regulated by communication with their specialized microenvironment known as the niche. Despite remarkable progress in understanding stem cell-intrinsic behavior, the molecular nature of the extrinsic factors provided to the stem cells by the niche microenvironment remains poorly understood. In this regard, the bulge niche of the mammalian hair follicle offers an excellent model for study. It holds two resident populations of SCs: epidermal stem cells and melanocyte stem cells. While their behavior is tightly coordinated, very little of the crosstalk involved is known. This review summarized the recent development in trying to understand the regulation of melanocyte and melanocyte stem cells. A better understanding of the normal regulation and behaviors of the melanocytes and the melanocyte stem cells will help to improve the clinical applications in regenerative medicine, cancer therapy, and aging.

Multiple pigment cell types contribute to the black, blue, and orange ornaments of male guppies (Poecilia reticulata)

The fitness of male guppies (Poecilia reticulata) highly depends on the size and number of their black, blue, and orange ornaments. Recently, progress has been made regarding the genetic mechanisms underlying male guppy pigment pattern formation, but we still know little about the pigment cell organization within these ornaments. Here, we investigate the pigment cell distribution within the black, blue, and orange trunk spots and selected fin color patterns of guppy males from three genetically divergent strains using transmission electron microscopy. We identified three types of pigment cells and found that at least two of these contribute to each color trait. Further, two pigment cell layers, one in the dermis and the other in the hypodermis, contribute to each trunk spot. The pigment cell organization within the black and orange trunk spots was similar between strains. The presence of iridophores in each of the investigated color traits is consistent with a key role for this pigment cell type in guppy color pattern formation.

Collective cell migration of epithelial and mesenchymal cells

Directional cell migration is required for proper embryogenesis, immunity, and healing, and its underpinning regulatory mechanisms are often hijacked during diseases such as chronic inflammations and cancer metastasis. Studies on migratory epithelial tissues have revealed that cells can move as a collective group with shared responsibilities. First thought to be restricted to proper epithelial cell types able to maintain stable cell-cell junctions, the field of collective cell migration is now widening to include cooperative behavior of mesenchymal cells. In this review, we give an overview of the mechanisms driving collective cell migration in epithelial tissues and discuss how mesenchymal cells can cooperate to behave as a collective in the absence of bona fide cell-cell adhesions.

Possible involvement of cone opsins in distinct photoresponses of intrinsically photosensitive dermal chromatophores in tilapia Oreochromis niloticus

Dermal specialized pigment cells (chromatophores) are thought to be one type of extraretinal photoreceptors responsible for a wide variety of sensory tasks, including adjusting body coloration. Unlike the well-studied image-forming function in retinal photoreceptors, direct evidence characterizing the mechanism of chromatophore photoresponses is less understood, particularly at the molecular and cellular levels. In the present study, cone opsin expression was detected in tilapia caudal fin where photosensitive chromatophores exist. Single-cell RT-PCR revealed co-existence of different cone opsins within melanophores and erythrophores. By stimulating cells with six wavelengths ranging from 380 to 580 nm, we found melanophores and erythrophores showed distinct photoresponses. After exposed to light, regardless of wavelength presentation, melanophores dispersed and maintained cell shape in an expansion stage by shuttling pigment granules. Conversely, erythrophores aggregated or dispersed pigment granules when exposed to short- or middle/long-wavelength light, respectively. These results suggest that diverse molecular mechanisms and light-detecting strategies may be employed by different types of tilapia chromatophores, which are instrumental in pigment pattern formation.

The neural crest

The neural crest (NC) is a highly migratory multipotent cell population that forms at the interface between the neuroepithelium and the prospective epidermis of a developing embryo. Following extensive migration throughout the embryo, NC cells eventually settle to differentiate into multiple cell types, ranging from neurons and glial cells of the peripheral nervous system to pigment cells, fibroblasts to smooth muscle cells, and odontoblasts to adipocytes. NC cells migrate in large numbers and their migration is regulated by multiple mechanisms, including chemotaxis, contact-inhibition of locomotion and cell sorting. Here, we provide an overview of NC formation, differentiation and migration, highlighting the molecular mechanisms governing NC migration.

Pigment pattern formation in the guppy, Poecilia reticulata, involves the Kita and Csf1ra receptor tyrosine kinases

Males of the guppy (Poecilia reticulata) vary tremendously in their ornamental patterns, which are thought to have evolved in response to a complex interplay between natural and sexual selection. Although the selection pressures acting on the color patterns of the guppy have been extensively studied, little is known about the genes that control their ontogeny. Over 50 years ago, two autosomal color loci, blue and golden, were described, both of which play a decisive role in the formation of the guppy color pattern. Orange pigmentation is absent in the skin of guppies with a lesion in blue, suggesting a defect in xanthophore development. In golden mutants, the development of the melanophore pattern during embryogenesis and after birth is affected. Here, we show that blue and golden correspond to guppy orthologs of colony-stimulating factor 1 receptor a (csf1ra; previously called fms) and kita. Most excitingly, we found that both genes are required for the development of the black ornaments of guppy males, which in the case of csf1ra might be mediated by xanthophore–melanophore interactions. Furthermore, we provide evidence that two temporally and genetically distinct melanophore populations contribute to the adult camouflage pattern expressed in both sexes: one early appearing and kita-dependent and the other late-developing and kita-independent. The identification of csf1ra and kita mutants provides the first molecular insights into pigment pattern formation in this important model species for ecological and evolutionary genetics.

Interactions with iridophores and the tissue environment required for patterning melanophores and xanthophores during zebrafish adult pigment stripe formation

Skin pigment patterns of vertebrates are a classic system for understanding fundamental mechanisms of morphogenesis, differentiation, and pattern formation, and recent studies of zebrafish have started to elucidate the cellular interactions and molecular mechanisms underlying these processes. In this species, horizontal dark stripes of melanophores alternate with light interstripes of yellow or orange xanthophores and iridescent iridophores. We showed previously that the highly conserved zinc finger protein Basonuclin-2 (Bnc2) is required in the environment in which pigment cells reside to promote the development and maintenance of all three classes of pigment cells; bnc2 mutants lack body stripes and interstripes. Previous studies also revealed that interactions between melanophores and xanthophores are necessary for organizing stripes and interstripes. Here we show that bnc2 promotes melanophore and xanthophore development by regulating expression of the growth factors Kit ligand a (Kitlga) and Colony stimulating factor-1 (Csf1), respectively. Yet, we found that rescue of melanophores and xanthophores was insufficient for the recovery of stripes in the bnc2 mutant. We therefore asked whether bnc2-dependent iridophores might contribute to stripe and interstripe patterning as well. We found that iridophores themselves express Csf1, and by ablating iridophores in wild-type and mutant backgrounds, we showed that iridophores contribute to organizing both melanophores and xanthophores during the development of stripes and interstripes. Our results reveal an important role for the cellular environment in promoting adult pigment pattern formation and identify new components of a pigment-cell autonomous pattern-generating system likely to have broad implications for understanding how pigment patterns develop and evolve.

[Migration of melanocytic lineage-derived cells]

During development, neural crest cells-derived melanoblasts migrate along the dorso-lateral axis into the dermis, then cross the basal layer to reach the epidermis and differentiate into melanocytes. They finally colonize the hair follicles to become resident pigmented cells. Neoplastic transformation converts melanocytes into highly invasive melanoma cells, which can adopt two modes of interconvertible migration (mesenchymal and amoeboid). Through analysis of the coat color phenotype of natural mouse mutants and genetically modified animals, many of the genes regulating migration were identified. Deciphering of cell membrane protrusions and signaling molecules involved in melanoma cell motility was further achieved through 2D and 3D culture systems. Here, we summarize how these data allow a better understanding of the complex mechanisms controlling migration of normal and pathological cells of the melanocytic lineage.

Cellular and molecular mechanisms controlling the migration of melanocytes and melanoma cells

During embryonic development in vertebrates, the neural crest-derived melanoblasts migrate along the dorsolateral axis and cross the basal membrane separating the dermis from the epidermis to reach their final location in the interfollicular epidermis and epidermal hair follicles. Neoplastic transformation converts melanocytes into highly invasive and metastatic melanoma cells. In vitro, these cells extend various types of protrusions and adopt two interconvertible modes of migration, mesenchymal and amoeboid, driven by different signalling molecules. In this review, we describe the major contributions of natural mouse mutants, mouse models generated by genetic engineering and in vitro culture systems, to identification of the genes, signalling pathways and mechanisms regulating the migration of normal and pathological cells of the melanocyte lineage, at both the cellular and molecular levels.

Genomic diversity and evolution of the head crest in the rock pigeon

The geographic origins of breeds and the genetic basis of variation within the widely distributed and phenotypically diverse domestic rock pigeon (Columba livia) remain largely unknown. We generated a rock pigeon reference genome and additional genome sequences representing domestic and feral populations. We found evidence for the origins of major breed groups in the Middle East and contributions from a racing breed to North American feral populations. We identified the gene EphB2 as a strong candidate for the derived head crest phenotype shared by numerous breeds, an important trait in mate selection in many avian species. We also found evidence that this trait evolved just once and spread throughout the species, and that the crest originates early in development by the localized molecular reversal of feather bud polarity.