Introduction and Expression of Recombinant Genes in Ascidian Embryos

Vasopressin Promoter Transgenic and Vasopressin Gene-Edited Ascidian, Ciona intestinalis Type A (Ciona robusta): Innervation, Gene Expression Profiles, and Phenotypes

Oxytocin (OT) and vasopressin (VP) superfamily neuropeptides are distributed in not only vertebrates but also diverse invertebrates. However, no VPergic innervation of invertebrates has ever been documented. In the ascidian, Ciona intestinalis Type A (Ciona robusta), an OT/VP superfamily peptide was identified, and the Ciona vasopressin (CiVP) induces oocyte maturation and ovulation. In the present study, we characterize the innervation and phenotypes of genetically modified Ciona: CiVP promoter-Venus transgenic and CiVP mutants. CiVP promoter-Venus transgenic Ciona demonstrated that CiVP gene was highly expressed in the cerebral ganglion and several nerves. Fluorescence was also detected in the ovary of young CiVP promoter-Venus transgenic ascidians, suggesting that the CiVP gene is also expressed temporarily in the ovary of young ascidians. Furthermore, a marked decrease of post-vitellogenic (stage III) follicles was observed in the ovary of CiVP mutants, whereas pre-vitellogenic (stage I) and vitellogenic (stage II) follicles were increased in the mutant ovary, compared with that of wildtype Ciona. Gene expression profiles showed that the expression of various genes, including genes related to ovarian follicle growth, was altered in the ovary of CiVP mutants. Altogether, these results indicated that CiVP, mainly as a neuropeptide, plays pivotal roles in diverse biological functions, including growth of early-stage ovarian follicles via regulation of the expression of a wide variety of genes. This is the first report describing a VP gene promoter-transgenic and VP gene-edited invertebrate and also on its gene expression profiles and phenotypes.

Thyroid hormone receptor: A new player in epinephrine-induced larval metamorphosis of the hard-shelled mussel

Many marine invertebrate larvae undergo a dramatic morphological and physiological transition from a planktonic larva to a benthic juvenile. The mechanisms of this metamorphosis in bivalves are mainly unknown. The recent identification in bivalves of a thyroid hormone receptor (TR) gene raises the possibility that as occurs in vertebrate metamorphosis, TRs regulate this developmental process. An evolutionary study of TR receptors revealed they are ubiquitous in the molluscs. Knock-down of the TR gene in pediveliger larvae of the hard-shelled mussel, Mytilus coruscus (Mc), using electroporation of siRNA significantly (p < 0.01) reduced TR gene expression. TR gene knock-down decreased pediveliger larval metamorphosis by 54% and was associated with a significant (p < 0.01) reduction in viability compared to control larvae. The TR in the hard-shelled mussel appears to be an essential regulatory factor for the successful epinephrine-induced metamorphosis of the pediveliger larvae to post-larvae. It is hypothesised that the knock-down of TR by siRNA transfection affects the "competence" of pediveliger larvae for the metamorphic transition by reducing their ability to respond to the inducer. The involvement of TR in the epinephrine-induced metamorphosis of a mollusc, the hard-shelled mussel, suggests the role of TR in this process probably emerged early during evolution.

Microinjection of Exogenous Nucleic Acids into Eggs: Ciona Species

Microinjection is a common technique used to deliver nucleic acids into eggs and embryos in Ciona species. There are three Ciona species that are commonly used for research-Ciona intestinalis type A (C. robusta), C. intestinalis type B (C. intestinalis), and C. savignyi. Here, we present the microinjection methods using eggs and embryos of C. intestinalis type A and C. savignyi; however, our methods would also be applicable to eggs and embryos of C. intestinalis type B. Microinjection is a classical and widely used delivery method, which involves the use of a glass micropipette, a hollow glass needle with a microscopic tip, to inject nucleic acids into eggs and embryos under a stereo microscope. The required amount of nucleic acids is much smaller for microinjection than for electroporation, another delivery method. Proteins, and other chemicals, such as fluorescent dye, can be introduced with nucleic acids using a microinjection.

Introduction

The chordate ascidians, the major group of tunicate s, is the best animal group for studying molecular and cellular processes underlying formation of a chordate body plan. For these studies, transgenic technologies are powerful. Transgenesis of ascidians has a long history of more than 20 years, and many practical tips have been accumulated. This book is aimed at summarizing the accumulated techniques in ascidians in addition to concrete research in which transgenic techniques have played pivotal roles. This book is useful for fast assimilation of the techniques and for learning the unique devices developed by the enthusiasm of ascidian researchers.

piRNA-like small RNAs are responsible for the maternal-specific knockdown in the ascidian Ciona intestinalis Type A

The mRNAs stored in eggs are crucial for embryogenesis. To address functions of maternal mRNAs, we recently reported the novel method MASK (maternal mRNA-specific knockdown), which we used to specifically knockdown maternal transcripts in the ascidian Ciona intestinalis Type A. In MASK, the cis element of a maternal gene is fused with eGFP or Kaede reporter gene, and the cassette is introduced into Ciona genome by transposon-mediated transgenesis. In eggs of the transgenic lines, the maternal expression of the gene whose cis element is used for driving the reporter gene is suppressed. The zygotic expression of the gene is not suppressed, suggesting that the MASK method can distinguish between maternal and zygotic functions of a gene. Here we investigated the cis and trans factors responsible for MASK results. In the ovaries in which knockdown of a maternal gene occurs, a number of antisense small RNAs are expressed that are complementary to the sequence of the knocked-down genes. We suspect that these antisense small RNAs are the factor responsible for MASK results. The antisense small RNAs have several features that are seen in PIWI-interacting RNAs (piRNAs), suggesting that MASK is likely to use a piRNA-mediated mechanism to knock down maternal mRNAs.

Depletion of Maternal Cyclin B3 Contributes to Zygotic Genome Activation in the Ciona Embryo

Most animal embryos display a delay in the activation of zygotic transcription during early embryogenesis [1]. This process is thought to help coordinate rapid increases in cell number during early development [2]. The timing of zygotic genome activation (ZGA) during the maternal-to-zygotic transition (MZT) remains uncertain despite extensive efforts. We explore ZGA in the simple protovertebrate, Ciona intestinalis. Single-cell RNA sequencing (RNA-seq) assays identified Cyclin B3 (Ccnb3) as a putative mediator of ZGA. Maternal Ccnb3 transcripts rapidly diminish in abundance during the onset of zygotic transcription at the 8-cell and 16-cell stages. Disruption of Ccnb3 activity results in precocious activation of zygotic transcription, while overexpression abolishes normal activation. These observations suggest that the depletion of maternal Cyclin B3 products is a critical component of the MZT and ZGA. We discuss evidence that this mechanism might play a conserved role in the MZT of other metazoans, including mice and humans.

Morphology and Physiology of the Ascidian Nervous Systems and the Effectors

Neurobiology in ascidians has made many advances. Ascidians have offered natural advantages to researchers, including fecundity, structural simplicity, invariant morphology, and fast and stereotyped developmental processes. The researchers have also accumulated on this animal a great deal of knowledge, genomic resources, and modern genetic techniques. A recent connectomic analysis has shown an ultimately resolved image of the larval nervous system, whereas recent applications of live imaging and optogenetics have clarified the functional organization of the juvenile nervous system. Progress in resources and techniques have provided convincing ways to deepen what we have wanted to know about the nervous systems of ascidians. Here, the research history and the current views regarding ascidian nervous systems are summarized.

Electroporation in Ascidians: History, Theory and Protocols

Embryonic development depends on the orchestration of hundreds of regulatory and structural genes to initiate expression at the proper time, in the correct spatial domain(s), and in the amounts required for cells and tissues to become specified, determined, and ultimately to differentiate into a multicellular embryo. One of the key approaches to studying embryonic development is the generation of transgenic animals in which recombinant DNA molecules are transiently or stably introduced into embryos to alter gene expression, to manipulate gene function or to serve as reporters for specific cell types or subcellular compartments. In some model systems, such as the mouse, well-defined approaches for generating transgenic animals have been developed. In other systems, particularly non-model systems, a key challenge is to find a way of introducing molecules or other reagents into cells that produces large numbers of embryos with a minimal effect on normal development. A variety of methods have been developed, including the use of viral vectors, microinjection, and electroporation. Here, I describe how electroporation was adapted to generate transgenic embryos in the ascidian, a nontraditional invertebrate chordate model that is particularly well-suited for studying gene regulatory activity during development. I present a review of the electroporation process, describe how electroporation was first implemented in the ascidian, and provide a series of protocols describing the electroporation process, as implemented in our laboratory.

Transcriptional regulation of a horizontally transferred gene from bacterium to chordate

The horizontal transfer of genes between distantly related organisms is undoubtedly a major factor in the evolution of novel traits. Because genes are functionless without expression, horizontally transferred genes must acquire appropriate transcriptional regulations in their recipient organisms, although the evolutionary mechanism is not known well. The defining characteristic of tunicates is the presence of a cellulose containing tunic covering the adult and larval body surface. Cellulose synthase was acquired by horizontal gene transfer from Actinobacteria. We found that acquisition of the binding site of AP-2 transcription factor was essential for tunicate cellulose synthase to gain epidermal-specific expression. Actinobacteria have very GC-rich genomes, regions of which are capable of inducing specific expression in the tunicate epidermis as the AP-2 binds to a GC-rich region. Therefore, the actinobacterial cellulose synthase could have been potentiated to evolve its new function in the ancestor of tunicates with a higher probability than the evolution depending solely on a spontaneous event.

CRISPR/Cas9-mediated gene knockout in the ascidian Ciona intestinalis

Knockout of genes with CRISPR/Cas9 is a newly emerged approach to investigate functions of genes in various organisms. We demonstrate that CRISPR/Cas9 can mutate endogenous genes of the ascidian Ciona intestinalis, a splendid model for elucidating molecular mechanisms for constructing the chordate body plan. Short guide RNA (sgRNA) and Cas9 mRNA, when they are expressed in Ciona embryos by means of microinjection or electroporation of their expression vectors, introduced mutations in the target genes. The specificity of target choice by sgRNA is relatively high compared to the reports from some other organisms, and a single nucleotide mutation at the sgRNA dramatically reduced mutation efficiency at the on-target site. CRISPR/Cas9-mediated mutagenesis will be a powerful method to study gene functions in Ciona along with another genome editing approach using TALE nucleases.

Transposon-mediated targeted and specific knockdown of maternally expressed transcripts in the ascidian Ciona intestinalis

Maternal mRNAs play crucial roles during early embryogenesis of ascidians, but their functions are largely unknown. In this study, we developed a new method to specifically knockdown maternal mRNAs in Ciona intestinalis using transposon-mediated transgenesis. We found that GFP expression is epigenetically silenced in Ciona intestinalis oocytes and eggs, and this epigenetic silencing of GFP was used to develop the knockdown method. When the 5' upstream promoter and 5' untranslated region (UTR) of a maternal gene are used to drive GFP in eggs, the maternal gene is specifically knocked down together with GFP. The 5' UTR of the maternal gene is the major element that determines the target gene silencing. Zygotic transcription of the target gene is unaffected, suggesting that the observed phenotypes specifically reflect the maternal function of the gene. This new method can provide breakthroughs in studying the functions of maternal mRNAs.

A glycine receptor is involved in the organization of swimming movements in an invertebrate chordate

BACKGROUND

Rhythmic motor patterns for locomotion in vertebrates are generated in spinal cord neural networks known as spinal Central Pattern Generators (CPGs). A key element in pattern generation is the role of glycinergic synaptic transmission by interneurons that cross the cord midline and inhibit contralaterally-located excitatory neurons. The glycinergic inhibitory drive permits alternating and precisely timed motor output during locomotion such as walking or swimming. To understand better the evolution of this system we examined the physiology of the neural network controlling swimming in an invertebrate chordate relative of vertebrates, the ascidian larva Ciona intestinalis.

RESULTS

A reduced preparation of the larva consisting of nerve cord and motor ganglion generates alternating swimming movements. Pharmacological and genetic manipulation of glycine receptors shows that they are implicated in the control of these locomotory movements. Morphological molecular techniques and heterologous expression experiments revealed that glycine receptors are inhibitory and are present on both motoneurones and locomotory muscle while putative glycinergic interneurons were identified in the nerve cord by labeling with an anti-glycine antibody.

CONCLUSIONS

In Ciona intestinalis, glycine receptors, glycinergic transmission and putative glycinergic interneurons, have a key role in coordinating swimming movements through a simple CPG that is present in the motor ganglion and nerve cord. Thus, the strong association between glycine receptors and vertebrate locomotory networks may now be extended to include the phylum chordata. The results suggest that the basic network for 'spinal-like' locomotion is likely to have existed in the common ancestor of extant chordates some 650 M years ago.

Cell type and function of neurons in the ascidian nervous system

Ascidians, or sea squirts, are primitive chordates, and their tadpole larvae share a basic body plan with vertebrates, including a notochord and a dorsal tubular central nervous system (CNS). The CNS of the ascidian larva is formed through a process similar to vertebrate neurulation, while the ascidian CNS is remarkably simple, consisting of about 100 neurons. Recent identification of genes that are specifically expressed in a particular subtype of neurons has enabled us to reveal neuronal networks at single-cell resolution. Based on the information on neuron subtype-specific genes, different populations of neurons have been visualized by whole-mount in situ hybridization, immunohistochemical staining using specific antibodies, and fluorescence labeling of cell bodies and neurites by a fluorescence protein reporter driven by neuron-specific promoters. Neuronal populations that have been successfully visualized include glutamatergic, cholinergic, gamma-aminobutyric acid/glycinergic, and dopaminergic neurons, which have allowed us to propose functional regionalization of the CNS and a neural circuit for locomotion. Thus, the simple nervous system of the ascidian larva can serve as an attractive model system for studying the development, function, and evolution of the chordate nervous system.

Decoding cis-regulatory systems in ascidians

Ascidians, or sea squirts, are lower chordates, and share basic gene repertoires and many characteristics, both developmental and physiological, with vertebrates. Therefore, decoding cis-regulatory systems in ascidians will contribute toward elucidating the genetic regulatory systems underlying the developmental and physiological processes of vertebrates. cis-Regulatory DNAs can also be used for tissue-specific genetic manipulation, a powerful tool for studying ascidian development and physiology. Because the ascidian genome is compact compared with vertebrate genomes, both intergenic regions and introns are relatively small in ascidians. Short upstream intergenic regions contain a complete set of cis-regulatory elements for spatially regulated expression of a majority of ascidian genes. These features of the ascidian genome are a great advantage in identifying cis-regulatory sequences and in analyzing their functions. Function of cis-regulatory DNAs has been analyzed for a number of tissue-specific and developmentally regulated genes of ascidians by introducing promoter-reporter fusion constructs into ascidian embryos. The availability of the whole genome sequences of the two Ciona species, Ciona intestinalis and Ciona savignyi, facilitates comparative genomics approaches to identify cis-regulatory DNAs. Recent studies demonstrate that computational methods can help identify cis-regulatory elements in the ascidian genome. This review presents a comprehensive list of ascidian genes whose cis-regulatory regions have been subjected to functional analysis, and highlights the recent advances in bioinformatics and comparative genomics approaches to cis-regulatory systems in ascidians.

Computational discovery of DNA motifs associated with cell type-specific gene expression in Ciona

Temporally and spatially co-expressed genes are expected to be regulated by common transcription factors and therefore to share cis-regulatory elements. In the ascidian Ciona intestinalis, the whole-genome sequences and genome-scale gene expression profiles allow the use of computational techniques to investigate cis-elements that control transcription. We collected 5' flanking sequences of 50 tissue-specific genes from genome databases of C. intestinalis and a closely related species Ciona savignyi. We searched for DNA motifs over-represented in upstream regions of a group of co-expressed genes. Several motifs were distributed predominantly in upstream regions of photoreceptor, pan-neuronal, or muscle-specific gene groups. One muscle-specific motif, M2, was distributed preferentially in regions from -200 to -100 bp relative to the translational start sites. Promoters of muscle-specific genes of C. intestinalis were isolated, connected with a green fluorescent protein gene (GFP), and introduced into C. intestinalis embryos. In muscle cells, these promoters specifically drove GFP expression, which mutations of the M2 sites greatly reduced. When M2 sites were located upstream of a basal promoter, the reporter GFP was specifically expressed in muscle cells. These results suggest the validity of our computational prediction of cis-regulatory elements. Thus, bioinformatics can help identify cis-regulatory elements involved in chordate development.

Generation of Ci-Brachyury-GFP stable transgenic lines in the ascidian Ciona savignyi

We report generation of stable transgenic lines of the ascidian Ciona savignyi carrying a Ciona intestinalis-Brachyury-promoter/Green Fluorescent Protein-reporter (Ci-Bra-GFP) construct. The transgenic lines were made using a technique in which the endonuclease I-SceI was coinjected into fertilized eggs with a transgene construct containing flanking recognition sites for I-SceI. Two founder animals, out of 12 F(0) adults tested, were found to transmit the transgene to their offspring (F(1)s) at frequencies of 42% and 23%. The transgene was further inherited by the F(2) in a Mendelian fashion and displayed nonmosaic expression, indicating integration into the genome. The Mendelian inheritance and the absence of mosaicism persisted through the F(3) and F(4) generations. Southern blot analyses showed that the transgene was organized in tandem arrays of no more than 10 copies. Using these Ci-Bra-GFP transgenics, we describe cellular movements and shape changes involved in notochord morphogenesis in both wildtype and mutant embryos.

The development of three identified motor neurons in the larva of an ascidian, Halocynthia roretzi

The generation of distinct classes of motor neurons underlies the development of complex motile behavior in all animals and is well characterized in chordates. Recent molecular studies indicate that the ascidian larval central nervous system (CNS) exhibits anteroposterior regionalization similar to that seen in the vertebrate CNS. To extend the understanding about the diversity of motor neurons in the ascidian larva, we have identified the number, position, and projection of individual motor neurons in Halocynthia roretzi, using a green fluorescent protein under the control of a neuron-specific promoter. Three pairs of motor neurons, each with a distinct shape and innervation pattern, were identified along the anteroposterior axis of the neural tube: the anterior and posterior pairs extend their axons toward dorsal muscle cells, whereas the middle pair project their axons toward ventral muscle. Overexpression of a dominant-negative form of a potassium channel in these cells resulted in paralysis on the injected side, thus these cells must constitute the major population of motor neurons responsible for swimming behavior. Lim class homeobox genes have been known as candidate genes that determine subtypes of motor neurons. Therefore, the expression pattern of Hrlim, which is a Lim class homeobox gene, was examined in the motor neuron precursors. All three motor neurons expressed Hrlim at the tailbud stage, although each down-regulated Hrlim at a different time. Misexpression of Hrlim in the epidermal lineage led to ectopic expression of TuNa2, a putative voltage-gated channel gene normally expressed predominantly in the three pairs of motor neurons. Hrlim may control membrane excitability of motor neurons by regulating ion channel gene expression.

Regulation of synaptotagmin gene expression during ascidian embryogenesis

The ascidian embryo, a model for the primitive mode of chordate development, rapidly forms a dorsal nervous system which consists of a small number of neurons. Here, we have characterized the transcriptional regulation of an ascidian synaptotagmin (syt) gene to explore the molecular mechanisms underlying development of synaptic transmission. In situ hybridization showed that syt is expressed in all neurons described in previous studies and transiently in the embryonic epidermis. Neuronal expression of syt requires induction from the vegetal side of the embryo, whereas epidermal expression occurs autonomously in isolated ectodermal blastomeres. Introduction of green fluorescent protein reporter gene constructs into the ascidian embryos indicates that a genomic fragment of the 3.4-kb 5' upstream region contains promoter elements of syt gene. Deletion analysis of the promoter suggests that syt expression in neurons and in the embryonic epidermis depends on distinct cis-regulatory regions.

Ascidian Wnt-5 gene is involved in the morphogenetic movement of notochord cells

Wnt proteins play important roles in many developmental events. Wnts are divided into two groups according to biological function. The Wnt-5a class proteins function in morphogenetic movement during embryogenesis. Previously, a Wnt-5 homolog has been isolated from the ascidian, Halocynthia roretzi. HrWnt-5 is expressed in the notochord until the tail-bud stage, implying a role in the notochord. In this study, the function of HrWnt-5 was investigated. When HrWnt-5 mRNA was injected into fertilized eggs, the embryos showed morphologic defects at around the neurula stage. The anterior-posterior axis was shorter than in control embryos. These defects were caused by the abnormal movement of notochord cells. However, the overexpression of HrWnt-5 mRNA did not affect the differentiation of tissues, suggesting that HrWnt-5 solely regulates the morphogenetic movement. Although endogenous HrWnt-5 is expressed in the notochord, the overexpression of HrWnt-5 mRNA caused the defects, suggesting that the amount of HrWnt-5 mRNA in the notochord is strictly regulated. These results suggest that HrWnt-5 regulates the morphogenetic movement of notochord cells during ascidian embryogenesis.

Large-scale cDNA analysis of the maternal genetic information in the egg of Halocynthia roretzi for a gene expression catalog of ascidian development

The ascidian egg is a well-known mosaic egg. In order to investigate the molecular nature of the maternal genetic information stored in the egg, we have prepared cDNAs from the mRNAs in the fertilized eggs of the ascidian, Halocynthia roretzi. The cDNAs of the ascidian embryo were sequenced, and the localization of individual mRNA was examined in staged embryos by whole-mount in situ hybridization. The data obtained were stored in the database MAGEST (http://www.genome.ad.jp/magest) and further analyzed. A total of 4240 cDNA clones were found to represent 2221 gene transcripts, including at least 934 different protein-coding sequences. The mRNA population of the egg consisted of a low prevalence, high complexity sequence set. The majority of the clones were of the rare sequence class, and of these, 42% of the clones showed significant matches with known peptides, mainly consisting of proteins with housekeeping functions such as metabolism and cell division. In addition, we found cDNAs encoding components involved in different signal transduction pathways and cDNAs encoding nucleotide-binding proteins. Large-scale analyses of the distribution of the RNA corresponding to each cDNA in the eight-cell, 110-cell and early tailbud embryos were simultaneously carried out. These analyses revealed that a small fraction of the maternal RNAs were localized in the eight-cell embryo, and that 7.9% of the clones were exclusively maternal, while 40.6% of the maternal clones showed expression in the later stages. This study provides global insights about the genes expressed during early development.

Determinants of cell and positional fate in ascidian embryos

Ascidians have played a major role in studies to understand the function of cytoplasmic determinants in animal development. Special qualities, including eggs with colored cytoplasmic regions, an invariant cleavage pattern and cell lineage, embryos with low cell numbers, larvae with typical chordate features and only six different tissues, rapid development, and a small genome, combine to make these animals a unique system for studying cytoplasmic determinants. There is evidence for determinants that specify the cleavage pattern; the differentiation of epidermal, endodermal, and muscle cells; and cell movements associated with gastrulation. The muscle determinants appear to be modified in concert with tail and muscle regression in species that have evolved an anural, or tailless, larva. Several lines of evidence suggest that determinants may be localized maternal mRNAs, which encode transcription factors or signal transduction components responsible for initiating differential gene activity. Different approaches and strategies are being used to isolate and characterize the function of these localized maternal mRNAs.

(beta)-catenin mediates the specification of endoderm cells in ascidian embryos

In the present study, we addressed the role of (beta)-catenin in the specification of embryonic cells of the ascidians Ciona intestinalis and C. savignyi and obtained the following results: (1) During cleavages, (beta)-catenin accumulated in the nuclei of vegetal blastomeres, suggesting that it plays a role in the specification of endoderm. (2) Mis- and/or overexpression of (beta)-catenin induced the development of an endoderm-specific alkaline phosphatase (AP) in presumptive notochord cells and epidermis cells without affecting differentiation of primary lineage muscle cells. (3) Downregulation of (beta)-catenin induced by the overexpression of cadherin resulted in the suppression of endoderm cell differentiation. This suppression was compensated for by the differentiation of extra epidermis cells. (4) Specification of notochord cells did not take place in the absence of endoderm differentiation. Both the overexpression of (beta)-catenin in presumptive notochord cells and the downregulation of (beta)-catenin in presumptive endoderm cells led to the suppression of Brachyury gene expression, resulting in the failure of notochord specification. These results suggest that the accumulation of (beta)-catenin in the nuclei of endoderm progenitor cells is the first step in the process of ascidian endoderm specification.

Conservation of the developmental role of Brachyury in notochord formation in a urochordate, the ascidian Balocynthia roretzi

The notochord is one of the characteristic features of the phylum Chordata. The vertebrate Brachyury gene is known to be essential for the terminal differentiation of chordamesoderm into notochord. In the ascidian, which belongs to the subphylum Urochordata, differentiation of notochord cells is induced at the late phase of the 32-cell stage through cellular interaction with adjacent endoderm cells as well as neighboring notochord cells. The ascidian Brachyury gene (As-T) is expressed exclusively in the notochord-lineage blastomeres, and the timing of gene expression at the 64-cell stage precisely coincides with that of the developmental fate restriction of the blastomeres. In addition, experimental studies have demonstrated a close relationship between the inductive events and As-T expression. In the present study, we show that overexpression of As-T by microinjection of the synthesized As-T RNA results in the occurrence, without the induction, of notochord-specific features in the A-line presumptive notochord blastomeres. We also show that overexpression of As-T RNA leads to ectopic expression of notochord-specific features in non-notochord lineages, including those of spinal cord and endoderm. These results strongly suggest that the developmental role of the Brachyury is conserved throughout chordates in notochord formation.

Green Fluorescent Protein in the sea urchin: new experimental approaches to transcriptional regulatory analysis in embryos and larvae

The use of Green Fluorescent Protein (GFP) as a reporter for expression transgenes opens the way to several new experimental strategies for the study of gene regulation in sea urchin development. A GFP coding sequence was associated with three different previously studied cis-regulatory systems, viz those of the SM50 gene, expressed in skeletogenic mesenchyme, the CyIIa gene, expressed in archenteron, skeletogenic and secondary mesenchyme, and the Endo16 gene, expressed in vegetal plate, archenteron and midgut. We demonstrate that the sensitivity with which expression can be detected is equal to or greater than that of whole-mount in situ hybridization applied to detection of CAT mRNA synthesized under the control of the same cis-regulatory systems. However, in addition to the important feature that it can be visualized nondestructively in living embryos, GFP has other advantages. First, it freely diffuses even within fine cytoplasmic cables, and thus reveals connections between cells, which in sea urchin embryos is particularly useful for observations on regulatory systems that operate in the syncytial skeletogenic mesenchyme. Second, GFP expression can be dramatically visualized in postembryonic larval tissues. This brings postembryonic larval developmental processes for the first time within the easy range of gene transfer analyses. Third, GFP permits identification and segregation of embryos in which the clonal incorporation of injected DNA has occurred in any particular desired region of the embryo. Thus, we show explicitly that, as expected, GFP transgenes are incorporated in the same nuclei together with other transgenes with which they are co-injected.

Ascidian tyrosinase gene: its unique structure and expression in the developing brain

Tadpole larvae of ascidians have two sensory pigment cells in the brain. One is the otolith cell that functions as a gravity receptor, the other pigment cell is part of a primitive photosensory structure termed the ocellus. These sensory cells, like vertebrate pigment cells, contain membrane-bounded melanin granules and are considered to reflect a crucial position in the evolutionary process of this cell type. To investigate the molecular changes accompanying the evolution of pigment cells, we have isolated from Halocynthia roretzi a gene encoding tyrosinase, a key enzyme in melanin biosynthesis. The cDNA has an open reading frame (ORF) of 596 amino acids, which is 36-39% identical in amino acid sequence to vertebrate tyrosinases. In addition, the sequence analysis of both cDNA and genomic clones reveals an unusual organization of the tyrosinase gene, an extraordinary 3' untranslated region of the transcripts with significant homology to the coding sequence, and a single short intron in the sequence encoding a cytoplasmic domain. Expression of the gene is detected first in two pigment precursor cells positioned in the neural plate of early neurulae, and later in two melanin-containing pigment cells within the brain of late tailbud embryos. Its expression pattern correlates well with the appearance of tyrosinase enzyme activity in the developing brain. These results provide the first description of pigment cell differentiation at the molecular level in the ascidian embryo, and also will contribute to a better understanding of the evolution of chordate pigment cells.

Characterization of a notochord-specific enhancer from the Brachyury promoter region of the ascidian, Ciona intestinalis

We present evidence that the embryo of the ascidian, Ciona intestinalis, is an easily manipulated system for investigating the establishment of basic chordate tissues and organs. Ciona has a small genome, and simple, well-defined embyronic lineages. Here, we examine the regulatory mechanisms underlying the differentiation of the notochord. Particular efforts center on the regulation of a notochord-specific Ciona Brachyury gene (Ci-Bra). An electroporation method was devised for the efficient incorporation of transgenic DNA into Ciona embryos. This method permitted the identification of a minimal, 434 bp enhancer from the Ci-Bra promoter region that mediates the notochord-restricted expression of both GFP and lacZ reporter genes. This enhancer contains a negative control region that excludes Ci-Bra expression from inappropriate embryonic lineages, including the trunk mesenchyme and tail muscles. Evidence is presented that the enhancer is activated by a regulatory element which is closely related to the recognition sequence of the Suppressor of Hairless transcription factor, thereby raising the possibility that the Notch signaling pathway plays a role in notochord differentiation. We discuss the implications of this analysis with regard to the evolutionary conservation of integrative enhancers, and the subdivision of the axial and paraxial mesoderm in vertebrates.