Gene delivery available in molluscan cells by strong promoter discovered from bivalve-infectious virus.

Understanding gene functions in marine invertebrates has been limited, largely due to the lack of suitable assay systems. Such a system requires investigative methods that are reproducible and can be quantitatively evaluated, such as a cell line, and a strong promoter that can drive high expression of a transgene. In this study, we established primary cell culture from a marine bivalve mollusc, Mizuhopecten yessoensis. Using scallop primary cells, we optimized electroporation conditions for transfection and carried out a luciferase-based promoter activity assay to identify strong promoter sequences that can drive expression of a gene of interest. We evaluated potential promoter sequences from genes of endogenous and exogenous origin and discovered a strong viral promoter derived from a bivalve-infectious virus, ostreid herpesvirus-1 (OsHV-1). This promoter, we termed OsHV-1 promoter, showed 24.7-fold and 16.1-fold higher activity than the cytomegalovirus immediate early (CMV IE) promoter and the endogenous EF1α promoter, the two most commonly used promoters in bivalves so far. Our GFP assays showed that the OsHV-1 promoter is active not only in scallop cells but also in HEK293 cells and zebrafish embryos. The OsHV-1 promoter practically enables functional analysis of marine molluscan genes, which can contribute to unveiling gene-regulatory networks underlying astonishing regeneration, adaptation, reproduction, and aging in marine invertebrates.

Vitellogenin uptake activity in the intestinal ducts of intraovarian embryos in a viviparous teleost Xenotoca eiseni.

In the viviparous teleost species belonging to the family Goodeidae, intraovarian embryos absorb maternal supplements while they grow during the gestation period. They take up the components via trophotaeniae, a hindgut-derived placental structure. Our previous study using a goodeid species Xenotoca eiseni revealed that intraovarian embryos absorb the yolk protein vitellogenin (Vtg) via the trophotaenia. However, another group indicated yolk components accumulate in the intestinal lumen of X. eiseni embryos. Here, we investigated whether the intestinal duct is capable of protein uptake, as is the trophotaenia. Immunohistochemical studies indicated that endogenous vitellogenin is detected in the intestinal epithelial cells of the intraovarian embryo. Tracer analysis using FITC-Vtg also indicated that intestinal tissues can take up protein. The endocytosis-related genes expressed in trophotaenia were also detected in the intestinal tissues of the embryo. Lipid transporter genes which are not expressed in the trophotaenia were detected in the embryonic intestine. This evidence suggests that the intraovarian embryo of X. eiseni possesses two distinct sites for uptake of the maternal proteins. However, the presumed functions of the embryonic intestine and trophotaenia might be not identical. The study provides a new perspective on how mother-to-embryo matrotrophic interactions have changed in the evolution of viviparous teleosts.

Mother-to-embryo vitellogenin transport in a viviparous teleost Xenotoca eiseni.

Vitellogenin (Vtg), a yolk nutrient protein that is synthesized in the livers of female animals, and subsequently carried into the ovary, contributes to vitellogenesis in oviparous animals. Thus, Vtg levels are elevated during oogenesis. In contrast, Vtg proteins have been genetically lost in viviparous mammals, thus the yolk protein is not involved in their oogenesis and embryonic development. In this study, we identified Vtg protein in the livers of females during the gestation of the viviparous teleost, Xenotoca eiseni Although vitellogenesis is arrested during gestation, biochemical assays revealed that Vtg protein was present in ovarian tissues and lumen fluid. The Vtg protein was also detected in the trophotaeniae of the intraovarian embryo. Immunoelectron microscopy revealed that Vtg protein is absorbed into intracellular vesicles in the epithelial cells of the trophotaeniae. Furthermore, extraneous Vtg protein injected into the abdominal cavity of a pregnant female was subsequently detected in the trophotaeniae of the intraovarian embryo. Our data suggest that the yolk protein is one of the matrotrophic factors supplied from the mother to the intraovarian embryo during gestation in X. eiseni.

Visualization of Sox10-positive chromatoblasts by GFP fluorescence in flounder larvae and juveniles using electroporation.

Japanese flounder are left-right asymmetrical, with features, such as dark, ocular-side specific pigmentation. This pigmentation arises during metamorphic stages, along with the asymmetric differentiation of adult-type chromatophores. Additionally, among juveniles, tank-reared specimens commonly show ectopic pigmentation on their blind sides. In both cases, neural crest-derived Sox10-positive progenitor cells at the dorsal fin base are hypothesized to contribute to chromatophore development. Here, we developed a method to visualize Sox10-positive cells via green fluorescent protein (GFP) fluorescence to directly monitor their migration and differentiation into chromatophores in vivo. Electroporation was applied to introduce GFP reporter vectors into the dorsal fin base of larvae and juveniles. Cre-loxP system vectors were also tested to enable cell labeling even after a decrease in sox10 expression levels. In larvae, undifferentiated Sox10-positive progenitor cells were labeled in the dorsal fin base, whereas newly differentiated adult-type chromatophores were seen dispersed on the ocular side. In juveniles, Sox10-positive cells were identified in the connective tissue of the dorsal fin base and observed prominently in areas of ectopic pigmentation, including several labeled melanophores. Thus, it was suggested that during metamorphic stages, Sox10-positive cells at the dorsal fin base contribute to adult-type chromatophore development, whereas in juveniles, they persist as precursors in the connective tissue, which in response to stimuli migrate to generate ectopic pigmentation. These findings contribute to elucidating pigmentation mechanisms, as well as abnormalities seen in hatchery-reared flounders. The electroporation method may be adapted to diverse animals as an accessible gene transfer method in various research fields, including developmental and biomedical studies.

Development of the anterior lateral line system through local tissue-tissue interactions in the zebrafish head.

BACKGROUND: The distribution of sensory organs is important for detecting environmental signals efficiently. The mechanosensory receptors of the lateral line system, neuromasts, are stereotypically distributed over the head and body surface of fish, although how neuromasts arise in these predetermined positions during development remains unclear. RESULTS: We investigated the development of the anterior lateral line (ALL) system in zebrafish head. The ALL neuromasts formed in the predetermined positions through proliferation and differentiation of (a) nonmigratory lateral line primordia, (b) migratory primordia, (c) interneuromast cells connecting preexisting neuromasts, and (d) budding primordia. We demonstrated that R-spondin2 (Rspo2), an activator of Wnt/ß-catenin signaling, is required for the development of a particular set of neuromasts associated with hyomandibular cartilage. Further genetic analyses suggested that Rspo2, which emanates from the hyoid mesenchyme, acts on the adjacent neuromast progenitor cells to stimulate their proliferation through activating Wnt/ß-catenin signaling. CONCLUSION: This study has revealed novel mechanisms for neuromast positioning through local tissue-tissue interactions, providing insights into the development and evolution of the vertebrate head.

Delivery of exogenous proteins into eggs by injection into the mother's ovary (IMO) in zebrafish.

Genome editing has had profound effects on biological experimentation and can now be applied to many organisms, including non-conventional models. However, the introduction of genome editing components is time- and labor-consuming and sometimes requires special skills for microinjection. In this study, we developed a technique to deliver exogenous proteins into eggs by injection into the mother's ovary (IMO), which leads to the delivery of CRISPR/Cas9 into the eggs of oviparous animals, including fish. To test this technique, we examined whether exogenous proteins tagged with GFP or luciferase (Luc), and fluorescent-labeled RNP (Cas9 and sgRNA complex), can be delivered into eggs by IMO. When GFP-Luc or Cas9-Luc was delivered by IMO, their incorporation into fertilized eggs was confirmed by GFP fluorescence or luciferase activity; proteins were accumulated in the yolk. Cas9-RNP (targeting tyrosinase) was also incorporated into the eggs. However, genome editing of the target gene, tyrosinase, was not observed yet. This is presumably because the RNP delivered by IMO was packed in the yolk granules and did not reach into the embryonic nuclei. Thus, this report shows that exogenous molecules including Cas9-RNP were successfully delivered into fertilized eggs by IMO. Transferring the delivered RNP into nuclei will be critical for successful genome editing via the IMO delivery system.

Energy landscape study of water splitting and H2 evolution at a ruthenium(II) pincer complex.

A bird's-eye view of the water splitting and H2 generation at a ruthenium(II) pincer complex is presented. Using a combination of density functional theory and efficient algorithms for exploration of potential energy hypersurface (PES), a total of 197 local minima and 186 transition states are identified, and a new mechanism for water splitting and H2 evolution via hydroxycarbonyl intermediates is presented. Furthermore, a global feature of the reaction PES, so-called potential energy landscape, is discussed on analyzing the obtained structures. As a result, the landscape is characterized by hierarchical structure, namely, PES consists of many "superbasins (SBs)" that are separated by relatively high energy barriers corresponding to bond breaking around Ru(II) center. Each SB involves a set of conformational isomers that can be interchanged with each other through relatively small barriers. To the best of our best knowledge, this is the first report on the quantum chemical computation of the hierarchical structure of PES for a realistic, catalytic reaction system.

Developmental regulatory system of ocular-side-specific asymmetric pigmentation in flounder: Critical role of retinoic acid signaling.

The body color of the Pleuronectiformes is bilaterally asymmetric between right and left halves, with a dark ocular-side and a white blind-side. This body color asymmetry develops by restricted differentiation of melanophores and xanthophores on the ocular-side during metamorphosis, accompanied by migration of one eye to the future ocular-side. In this study, we elucidated the developmental regulatory system of this lateralized pigmentation. We found that in flounder, Sox10-positive chromatophore progenitors appear bilaterally both in the ocular- and blind-side skin of metamorphosing larvae, and that those arriving at the ocular-side skin differentiate into gch2-positive chromatoblasts and then chromatophores. Transient exposure of metamorphosing larvae to retinoic acid (RA)-induced progenitors on the blind-side to differentiate into gch2-positive chromatoblasts. On the contrary, exposure to an RA receptor antagonist, BMS493, suppressed the differentiation of gch2-positive chromatoblasts on the ocular-side. Thus, we demonstrated that RA is essential for flounder chromatophore progenitors to differentiate into chromatoblasts. At the time of chromatoblast differentiation on the ocular-side, cyp26b1, which inactivates RA, was upregulated on the blind-side skin compared with the ocular-side. Therefore, we surmise that ocular-side-specific pigmentation is regulated by the inhibition of RA-signaling by cyp26b1 on the blind-side.

Plasmid delivery by electroporation into fish skeletal muscle for recombinant protein secretion and uptake by oocytes.

We examined the efficiency of electroporation for the delivery of plasmid into the skeletal muscle and also examined the subsequent secretion of recombinant protein into the circulation system, using zebrafish, Japanese flounder, and bubble-eye goldfish. The expression area of GFP fluorescence was markedly expanded by electroporation. Introduced plasmid was retained in the muscle cells and continued to express GFP for at least 180 days in zebrafish, indicating the long lifespan of plasmid DNA in the muscle cell. Luciferase and a fusion of growth hormone (GH) and luciferase were secreted into the blood from muscle electroporated with CMV:secNluc and CMV:GH-Luc plasmids, respectively, indicating that recombinant proteins such as peptide hormones can be supplied to the blood by plasmid electroporation into muscle. Interestingly, luciferase activity was detected from fertilized eggs laid by zebrafish females that had been electroporated with CMV:secNluc, indicating that maturing oocytes incorporated recombinant protein from the blood stream that had been secreted from the muscle. The plasmid electroporation system reported here also may work for the delivery of recombinant proteins, such as Cas9, into the oocytes. Since the GH-Luc fusion protein was detected in the lymph of the eye-sac of bubble-eye goldfish, this fish may be useful for the production of recombinant protein.

Identification of Sox10-positive cells at the dorsal fin base of juvenile flounder that are correlated with blind-side skin ectopic pigmentation.

Flounder develop left-right asymmetric body color, with a dark ocular side and white blind side. However, ectopic pigmentation often occurs on the blind side when juveniles are reared in tanks. To examine the developmental mechanism underlying ectopic pigmentation, we first examined the pigmentation process on the blind side and the localization of chromatoblasts during spontaneous and regeneration-stimulated ectopic pigmentation. Wild-caught juveniles that had completed metamorphosis in a natural environment were reared in tanks, where they exhibited ectopic pigmentation on the blind side that was initiated at the base of the dorsal and anal fins, with chromatoblasts appearing at the edges of scales and melanophores spreading on the scale papilla beneath the epidermis. During tissue regeneration at the base of the dorsal fin in juvenile before ectopic pigmentation, melanophores and chromatoblasts newly appeared on regenerated blind-side skin, resulting in rapid pigmentation at the wounded site. During regeneration-stimulated pigmentation, gch2-positive chromatoblasts were detected only under the regenerated epidermis. Next, we found that Sox10-positive cells were localized in connective tissue at the base of the dorsal fin and that when connective tissue was labeled with DiO, DiO-labeled melanophores appeared in regenerated skin of the blind side after wounding. Therefore, we conclude that in flounder juveniles, Sox10-positive progenitors of pigment cell lineage reside at the base of the dorsal fin and start migrating to the blind-side skin in response to specific stimuli, resulting in ectopic pigmentation. Ectopic pigmentation in flounder could be a good model for examining the flexibility of pigment cell differentiation.

Expression profiles of RA synthases and catabolic enzymes in newly hatched and metamorphosing larvae of Japanese flounder, Paralichthys olivaceus.

Retinoic acid (RA) plays various embryogenesis and post-embryogenesis roles in vertebrates. As exposure of metamorphosing flounder larvae to RA has teratogenic effects on skin color and vertebral column development, harmonized RA synthesis and catabolism are likely essential in metamorphic development. To approach understanding of the roles of RA in flounder metamorphic development, we here examined the tissue mRNA expression of RA synthases (aldh1a1, aldh1a2, aldh1a3) and catabolic enzymes (cyp26a1, cyp26b1, cyp26c1) in newly hatched and metamorphosing larvae, and three-month-old juveniles by in situ hybridization (ISH). No ISH signal was detected for any genes from the skin and vertebral column susceptible to the teratogenic effects by RA. Since the intestine expressed aldh1a2 at high level in larvae but not in juvenile, it is a possibility that the larval intestine serves as a source of RA, and RA catabolic enzymes function at the level below sensitivity of ISH at vertebral column and skin development. We found that aldh1a2 and aldh1a3 were expressed along the margin of the tectum and the neurohypophysis of pituitary, respectively, both in contact with the cerebrospinal fluid (CSF), and cyp26b1 at the posterior tectum and cerebellum. We hypothesize that RA is supplied from the tectum and pituitary via the CSF for brain growth and maintenance, and cyp26b1 locally regulates RA contents in the brain.

External Asymmetry and Pectoral Fin Loss in the Bamboo Sole (Heteromycteris japonica): Small-Sized Sole with Potential as a Pleuronectiformes Experimental Model.

Pleuronectiform fish develop marked external asymmetry in eye location and skin color at metamorphosis. The bamboo sole, Heteromycteris japonica, also exhibits loss of the pectoral fins at metamorphosis. Because of its small body size, short generation time, and long spawning season, we focused on the bamboo sole as an experimental model to investigate metamorphic asymmetry and pectoral fin loss during development. In the present study, we utilized a small-scale culture system to evaluate bamboo sole larvae and larval development, and a microinjection system for fertilized eggs. The culture system described here uses an 18 L culture tank for rotifers (the first diet for larvae) and 5 L plastic beakers for larval culture. Under this system, most larvae completed metamorphosis, including one-eye migration and pigmentation of the ocular side, by 23 days postfertilization (dpf) at 25°C. Larvae at density of 120-150 per liter were grown from hatching to 23 dpf with a survival ratio of 60-75% per beaker. Pectoral fins, including coracoid and disk cartilage, formed but were completely lost in late metamorphosis without formation of proximal radials and fin rays. The microinjection system designed here is adequate for the bamboo sole and allows injection of 100 one-cell-stage embryos per day. We expect that the culture and microinjection systems described here will facilitate the use of the bamboo sole as an experimental model organism in developmental biology.

Analyses of the cellular clock gene expression in peripheral tissue, caudal fin, in the Japanese flounder, Paralichthys olivaceus.

Understanding the systems for maintaining the circadian rhythms that give organisms the flexibility to adapt to environmental changes is important in both aquaculture and fish chronobiology, because nursery lighting conditions can affect the survival and growth rates of larvae. We previously demonstrated that in flounder, the suprachiasmatic nucleus (SCN) exhibits daily rhythm in per2 expression, in sharp contrast to zebrafish, in which the SCN does not exhibit clear per2 expression rhythm. To examine whether a hierarchy exists in systems that maintain the expression rhythm of peripheral clock genes in flounder, in the present study we analyzed the in vivo and in vitro expression of three clock genes, per2, per1, and cry1, in the caudal fin and the effects of cortisol and melatonin administration on the expression of each clock gene. In vivo, the fin maintained a daily expression rhythm of all three genes, even in 24-h darkness (DD) when shifted from 12-h light:12-h dark (LD) conditions, but fin explants lost the expression rhythm after a short time of tissue culture, even under LD conditions. Cortisol, but not melatonin, significantly upregulated the expression of the three clock genes in fin both in vitro and in vivo. Therefore, we hypothesize that the SCN-pituitary-adrenal cortex pathway plays a role in the oscillation of the peripheral clock in flounder. However, in vivo, peak expression of per2 and cry1 was shifted 2-4h earlier under DD conditions, and their expression was upregulated in response to short exposures to light when larvae were kept under DD conditions. Therefore, we also hypothesize that in addition to the SCN, a light-responsive coordinating factor also functions in photo-entrainment of the peripheral clock in flounder.

Diethylnitrosamine-induced expression of germline-specific genes and pluripotency factors, including vasa and oct4, in medaka somatic cells.

Various methods have been developed to reprogram mammalian somatic cells into pluripotent cells as well as to directly reprogram somatic cells into other cell lineages. We are interested in applying these methods to fish, and here, we examined whether mRNA expression of germline-specific genes (vasa, nanos2, -3) and pluripotency factors (oct4, sox2, c-myc, nanog) is inducible in somatic cells of Japanese medaka (Oryzias latipes). We found that the expression of vasa is induced in the gut and regenerating fin by exposure to a carcinogen, diethylnitrosamine (DEN). Induction of vasa in the gut started on the 5th day of treatment with >50 ppm DEN. In addition, nanos2, -3, oct4, sox2, klf4, c-myc, and nanog were also expressed simultaneously in some vasa-positive gut and regenerating fin samples. Vasa-positive cells were detected by immunohistochemistry (IHC) in the muscle surrounding the gut and in the wound epidermis, blastema, and fibroblast-like cells in regenerating fin. In vasa:GFP transgenic medaka, green fluorescent protein (GFP) fluorescence appeared in the wound epidermis and fibroblast-like cells in the regenerating fin following DEN exposure, in agreement with the IHC data. Our data show that mRNA expression of genes relevant to germ cell specification and pluripotency can be induced in fish somatic cells by exposure to DEN, suggesting the possibility of efficient and rapid cell reprogramming of fish somatic cells.

Modular development of the teleost trunk along the dorsoventral axis and zic1/zic4 as selector genes in the dorsal module.

Teleost fish exhibit remarkable diversity in morphology, such as fins and coloration, particularly on the dorsal side. These structures are evolutionary adaptive because their back is highly visible to other individuals. However, owing to the late phenotypic appearance (from larva to adult) and lack of appropriate mutants, the genetic mechanisms that regulate these dorsoventrally asymmetric external patterns are largely unknown. To address this, we have analyzed the spontaneous medaka mutant Double anal fin (Da), which exhibits a mirror-image duplication of the ventral half across the lateral midline from larva to adult. Da is an enhancer mutant for zic1 and zic4 in which their expression in dorsal somites is lost. We show that the dorsoventral polarity in Da somites is lost and then demonstrate using transplantation techniques that somites and their derived tissues globally determine the multiple dorsal-specific characteristics of the body (fin morphology and pigmentation) from embryo to adult. Intriguingly, the zic1/zic4 expression in the wild type persists throughout life in the dorsal parts of somite derivatives, i.e. the myotome, dermis and vertebrae, forming a broad dorsal domain in the trunk. Comparative analysis further implies a central role for zic1/zic4 in morphological diversification of the teleost body. Taken together, we propose that the teleost trunk consists of dorsal/ventral developmental modules and that zic1/zic4 in somites function as selector genes in the dorsal module to regulate multiple dorsal morphologies.

Excess Retinoic Acid Induces Fusion of Centra by Degenerating Intervertebral Ligament Cells in Japanese flounder, Paralichthys olivaceus.

In marine aquaculture fish, excessive supplement of vitamin A (VA) to zooplanktons for larval culture and experimental exposure of larvae to retinoic acid (RA: active form of VA) have been known to cause vertebral deformity. However, the tissues in the developing vertebral column that are affected by RA and the progression of vertebral deformity remain undetermined. To examine these questions, we histologically traced the progress of vertebral deformity induced by RA in Japanese flounder (Paralichthys olivaceus). Larvae were exposed to RA for 3 days at mid-metamorphosis (G-stage), a critical stage for vertebral deformity. Intervertebral ligament, which is known to form intervertebral joints in cooperation with the notochord, was severely degenerated by RA, leading to fusion of centra. During further development to adult, growth of centra was severely suppressed in an anterior-posterior direction in RA-treated fish and the notochord tissue was lost from fused centra, resulting in complete loss of intervertebral joints and fusion of centra. We conclude that RA initially damages the intervertebral ligaments, and these defects lead to fusion, narrowing of centra, and loss of intervertebral joints in the vertebral column. The cumulative effect of these modifications is a truncated body form.

Role of notochord cells and sclerotome-derived cells in vertebral column development in fugu, Takifugu rubripes: histological and gene expression analyses.

Despite the common structure of vertebrates, the development of the vertebral column differs widely between teleosts and tetrapods in several respects, including the ossification of the centrum and the function of the notochord. In contrast to tetrapods, vertebral development in teleosts is not fully understood, particularly for large fish with highly ossified bones. We therefore examined the histology and gene expression profile of vertebral development in fugu, Takifugu rubripes, a model organism for genomic research. Ossification of the fugu centrum is carried out by outer osteoblasts expressing col1a1, col2a1, and sparc, and the growing centra completely divide the notochord into double cone-shaped segments that function as intercentral joints. In this process, the notochord basal cells produce a thick notochord sheath exhibiting Alcian-blue-reactive cartilaginous properties and composing the intercentral ligament in cooperation with the external ligament connective tissue. Synthesis of the matrix by the basal cells was ascertained by an in vitro test. Expression of twist2 indicates that this connective tissue is descended from the embryonic sclerotome. Notochord basal cells express sox9, ihhb, shh, and col2a1a, suggesting that the signaling system involved in chondrocyte proliferation and matrix production also functions in notochord cells for notochord sheath formation. We further found that the notochord expression of both ntla and shh is maintained in the fugu vertebral column, whereas it is turned off after embryogenesis in zebrafish. Thus, our results demonstrate that, in contrast to zebrafish, a dynamic morphogenesis and molecular network continues to function in fugu until the establishment of the adult vertebral column.

Early development of circadian rhythmicity in the suprachiamatic nuclei and pineal gland of teleost, flounder (Paralichthys olivaeus), embryos.

Circadian rhythms enable organisms to coordinate multiple physiological processes and behaviors with the earth's rotation. In mammals, the suprachiasmatic nuclei (SCN), the sole master circadian pacemaker, has entrainment mechanisms that set the circadian rhythm to a 24-h cycle with photic signals from retina. In contrast, the zebrafish SCN is not a circadian pacemaker, instead the pineal gland (PG) houses the major circadian oscillator. The SCN of flounder larvae, unlike that of zebrafish, however, expresses per2 with a rhythmicity of daytime/ON and nighttime/OFF. Here, we examined whether the rhythm of per2 expression in the flounder SCN represents the molecular clock. We also examined early development of the circadian rhythmicity in the SCN and PG. Our three major findings were as follows. First, rhythmic per2 expression in the SCN was maintained under 24 h dark (DD) conditions, indicating that a molecular clock exists in the flounder SCN. Second, onset of circadian rhythmicity in the SCN preceded that in the PG. Third, both 24 h light (LL) and DD conditions deeply affected the development of circadian rhythmicity in the SCN and PG. This is the first report dealing with the early development of circadian rhythmicity in the SCN in fish.

Ocular-side lateralization of adult-type chromatophore precursors: development of pigment asymmetry in metamorphosing flounder larvae.

The adult-type chromatophores of flounder differentiate at metamorphosis in the skin of ocular side to establish asymmetric pigmentation. In young larva and before metamorphosis, adult-type melanophores that migrate to the ocular side during metamorphosis reside at the base of the dorsal fin as latent precursors. However, the migration route taken by these precursor cells and the mechanisms by which lateralization and asymmetric pigmentation develop on the ocular side are unknown. To further investigate this migration and lateralization, we used in situ hybridization with gch2 probe, a marker for melanoblasts and xanthoblasts (precursors of adult type chromatophores), to examine the distribution of chromatophore precursors in metamorphosing larvae. The gch2-positive precursors were present in the myoseptum as well as in the skin. This finding indicated that these precursors migrated from the dorsal part of the fin to the skin via the myoseptum. Additionally, there were much fewer gch2-positive cells in the myoseptum of the blind side than in the skin and myoseptum of the ocular side, and this finding indicated either that migration of the precursor cells into the myoseptum of blind side was inhibited or that the precursors were eliminated from the myoseptum of the blind side. Therefore, we propose that the signals responsible for development of asymmetric pigmentation in flounder reside not only in the skin but on a larger scale and in multiple tissues throughout the lateral half of the trunk.

Evolutionary developmental biology and genomics.

Reciprocal questions often frame studies of the evolution of developmental mechanisms. How can species share similar developmental genetic toolkits but still generate diverse life forms? Conversely, how can similar forms develop from different toolkits? Genomics bridges the gap between evolutionary and developmental biology, and can help answer these evo-devo questions in several ways. First, it informs us about historical relationships, thus orienting the direction of evolutionary diversification. Second, genomics lists all toolkit components, thereby revealing contraction and expansion of the genome and suggesting mechanisms for evolution of both developmental functions and genome architecture. Finally, comparative genomics helps us to identify conserved non-coding elements and their relationship to genome architecture and development.