Systematic analysis of embryonic expression profiles of zinc finger genes in Ciona intestinalis

The recent decoding of a number of animal genomes has provided unprecedented information regarding evolution and gene structures, but this information must be supplemented with precise gene annotations and the temporal and spatial expression patterns of individual genes. In the present study, we systematically identified and characterized 566 zinc finger genes in the genome of Ciona intestinalis, an emerging model system for genome-wide studies of development and evolution. Of these genes, 356 genes encoded a potential transcription factor based on putative nucleic acid binding activity or domains of unknown function. We further examined the expression patterns of 225 genes during embryogenesis, and, when considered with a previous study [Imai, K.S., Hino, K., Yagi, K., Satoh, N., Satou, Y., 2004. Gene expression profiles of transcription factors and signaling molecules in the ascidian embryo: towards a comprehensive understanding of gene networks. Development 131, 4047-4058], we have characterized the developmental expression patterns of nearly 85% of the potential zinc finger-containing transcription factors. Overall, zinc finger genes are preferentially maternally expressed with little larval expression during development. The present study provides a valuable reference for genome-wide studies in this species and for future studies wishing to examine zinc finger gene expression patterns in other animals.

Live imaging of fluorescent proteins in chordate embryos: from ascidians to mice

Although we have advanced in our understanding of the molecular mechanisms intrinsic to the morphogenesis of chordate embryos, the question of how individual developmental events are integrated to generate the final morphological form is still unresolved. Microscopic observation is a pivotal tool in developmental biology, both for determining the normal course of events and for contrasting this with the results of experimental and pathological perturbations. Since embryonic development takes place in three dimensions over time, to fully understand the events required to build an embryo we must investigate embryo morphogenesis in multiple dimensions in situ. Recent advances in the isolation of naturally fluorescent proteins, and the refinement of techniques for in vivo microscopy offer unprecedented opportunities to study the cellular and molecular events within living, intact embryos using optical imaging. These technologies allow direct visual access to complex events as they happen in their native environment, and thus provide greater insights into cell behaviors operating during embryonic development. Since most fluorescent protein probes and modes of data acquisition are common across species, we have chosen the mouse and the ascidian, two model organisms at opposite ends of the chordate clade, to review the use of some of the current genetically-encoded fluorescent proteins and their visualization in vivo in living embryos for the generation of high-resolution imaging data.

Ci-Rga, a gene encoding an MtN3/saliva family transmembrane protein, is essential for tissue differentiation during embryogenesis of the ascidian Ciona intestinalis

A novel gene (Ci-Rga) essential for tissue differentiation during embryogenesis of the ascidian Ciona intestinalis is reported here. This gene was identified through functional screening of Ciona genes required for development by translational inhibition experiments with morpholino antisense oligonucleotides. The deduced protein of Ci-Rga contains two copies of a domain with unknown function called the MtN3/saliva domain. Phylogenetic analysis showed that Ci-Rga belongs to the MtN3/saliva family of genes conserved among metazoans and plants, and is an ortholog of mouse Rga (Recombination-activating gene 1 gene activation). During Ciona embryogenesis, both maternal and zygotic transcripts of Ci-Rga were expressed. Translational inhibition of Ci-Rga with specific morpholino resulted in abnormal embryos in which the cleavage pattern became atypical and expression of marker genes for each of the six major tissues, namely the endoderm, muscle, mesenchyme, notochord, neural tissue, and epidermis, was lost or suppressed at the tailbud stage. Although differentiation of all the six major tissues was affected by Ci-Rga knock-down, the degree of abnormalities and the timing of appearance of abnormalities were different among tissues. Expression analysis of developmentally important genes involved in the fate specification, such as Ci-Bra, Ci-Twist-like1a, Ci-Otx, Ci-Fgf9/16/20, Ci-Lhx3, Ci-FoxD, and Ci-Tbx6b, showed that an initial step of the fate specification of notochord, mesenchyme, and neural tissue, but not of endoderm or muscle, is impaired in the knock-down embryo. These results showed that Ci-Rga is a multifunctional gene essential for tissue differentiation during embryogenesis, and is primarily required for the fate specification of notochord, mesenchyme, and neural tissue, and provide some insights into the function of this little-known group of genes.

Establishment of animal–vegetal polarity during maturation in ascidian oocytes

Mature ascidian oocytes are arrested in metaphase of meiosis I (Met I) and display a pronounced animal-vegetal polarity: a small meiotic spindle lies beneath the animal pole, and two adjacent cortical and subcortical domains respectively rich in cortical endoplasmic reticulum and postplasmic/PEM RNAs (cER/mRNA domain) and mitochondria (myoplasm domain) line the equatorial and vegetal regions. Symmetry-breaking events triggered by the fertilizing sperm remodel this primary animal-vegetal (a-v) axis to establish the embryonic (D-V, A-P) axes. To understand how this radial a-v polarity of eggs is established, we have analyzed the distribution of mitochondria, mRNAs, microtubules and chromosomes in pre-vitellogenic, vitellogenic and post-vitellogenic Germinal Vesicle (GV) stage oocytes and in spontaneously maturing oocytes of the ascidian Ciona intestinalis. We show that myoplasm and postplasmic/PEM RNAs move into the oocyte periphery at the end of oogenesis and that polarization along the a-v axis occurs after maturation in several steps which take 3-4 h to be completed. First, the Germinal Vesicle breaks down, and a meiotic spindle forms in the center of the oocyte. Second, the meiotic spindle moves in an apparently random direction towards the cortex. Third, when the microtubular spindle and chromosomes arrive and rotate in the cortex (defining the animal pole), the subcortical myoplasm domain and cortical postplasmic/PEM RNAs are excluded from the animal pole region, thus concentrating in the vegetal hemisphere. The actin cytoskeleton is required for migration of the spindle and subsequent polarization, whereas these events occur normally in the absence of microtubules. Our observations set the stage for understanding the mechanisms governing primary axis establishment and meiotic maturation in ascidians.

Fertilization regulates apoptosis of Ciona intestinalis extra-embryonic cells through thyroxine (T4)-dependent NF-κB pathway activation during early embryonic development

In Ciona intestinalis, the elimination of extra-embryonic test cells during early stage of development is delayed by a fertilization signal. Test cells undergo a caspase-dependent apoptosis event repressed by thyroxine (T4)-activated NF-kappaB. When apoptosis was experimentally blocked, the hatching stage was delayed. The incubation of unfertilized eggs with a 1-h-fertilized egg extract or purified T4 restored apoptosis in test cells at a similar timing than found in fertilized eggs. Ciona expresses specific genes forming a functional IkappaB/NF-kappaB pathway. One, Ci-p65, was transiently induced upon fertilization via T4 and found to exert its anti-apoptotic role in test cells nuclei as well as in a reconstituted cell system. Blocking NF-kappaB activity by dexamethasone-induced overexpression of Ci-IkappaB abrogated the repression of apoptosis in test cells. Overall, the data are consistent for defining a central coupling role of both T4 and NF-kappaB during early embryo development.

Ca2+ and Na+ current patterns during oocyte maturation, fertilization, and early developmental stages ofCiona intestinalis

Using the whole-cell voltage clamp technique, the electrical changes in oocyte and embryo plasma membrane were followed during different meiotic and developmental stages in Ciona intestinalis. We show, for the first time, an electrophysiological characterization of the plasma membrane in oocytes at the germinal vesicle (GV) stage with high L-type calcium (Ca2+) current activity that decreased through meiosis. Moreover, the absence of Ca2+ reduced germinal vesicle breakdown (GVBD), which is consistent with a role of Ca2+ currents in the prophase/metaphase transition. In mature oocytes at the metaphase I (MI) stage, Ca2+ currents decreased and then disappeared and sodium (Na+) currents first appeared remaining high up to the zygote stage. Intracellular Ca2+ release was higher in MI than in GV, indicating that Ca2+ currents in GV may contribute to fill the stores which are essential for oocyte contraction at fertilization. The fertilization current generated in Na+ free sea water was significantly lower than the control; furthermore, oocytes fertilized in the absence of Na+ showed high development of anomalous "rosette" embryos. Current amplitudes became negligible in embryos at the 2- and 4-cell stage, suggesting that signaling pathways that mediate first cleavage do not rely on ion current activities. At the 8-cell stage embryo, a resumption of Na+ current activity and conductance occurred, without a correlation with specific blastomeres. Taken together, these results imply: (i) an involvement of L-type Ca2+ currents in meiotic progression from the GV to MI stage; (ii) a role of Na+ currents during electrical events at fertilization and subsequent development; (iii) a major role of plasma membrane permeability and a minor function of specific currents during initial cell line segregation events.

Oligonucleotide-based microarray analysis of retinoic acid target genes in the protochordate, Ciona intestinalis

Oligonucleotide-based microarray analyses were carried out to identify retinoic acid target genes in embryos of the ascidian Ciona intestinalis. Of 21,938 spots, 50 (corresponding to 43 genes) showed over twofold up-regulation in retinoic acid-treated tail bud embryos. In situ hybridization verified retinoic acid-induced up-regulation of 23 genes. Many of them were expressed in the anterior tail region, where a retinaldehyde dehydrogenase homolog is expressed. Homologs of vertebrate genes involved in neurogenesis and/or neuronal functions (e.g., COUP-TF, Ci-Hox1, and SCO-spondin) were expressed in the central nervous system of Ciona embryos, and activated by retinoic acid. Genes encoding transcription factors (e.g., Ci-lmx1.2, vitamin D receptor, and Hox proteins) and apoptosis-related proteins (e.g., transglutaminase and an apoptosis-inducing factor homolog) were also activated by retinoic acid. Simultaneous treatment of embryos with retinoic acid and puromycin revealed a few direct targets, including genes encoding Ci-Hox1, Ci-Cyp26, and an Rnf126-like ring finger protein.

A bHLH transcription factor gene, Twist-like1, is essential for the formation of mesodermal tissues of Ciona juveniles

Ascidian larval mesenchyme cells, comprising about 900 cells, are derived from the A7.6, B8.5 and B7.7 blastomere pairs in the 110-cell embryo. Previous studies showed that the properties of mesenchyme cells are not uniform among the three lines in embryos of Ciona savignyi and Ciona intestinalis. After metamorphosis, the larval mesenchyme cells form the mesodermal tissues or organs of the adult body. In the present study, the developmental fates of A7.6-, B8.5- and B7.7-line mesenchyme cells were traced using DiI to determine the origins of juvenile mesodermal tissues of C. savignyi. It was demonstrated that each of the A7.6-, B8.5- and B7.7-line mesenchyme cells is distributed in different positions of the larval trunk, and then give rise to the different mesodermal tissues of juveniles. Twist-like 1 is a transcription factor gene essential for the specification of larval mesenchyme cells. Knockdown of this gene with specific morpholino antisense oligonucleotides affected not only the specification of larval mesenchyme cells, but also the formation of most of the mesodermal tissues of juveniles. The juvenile mesodermal tissues in the Twist-like 1-knockdown specimen were never compensated by the surrounding tissues. The present results therefore indicate that Twist-like 1 is required for the differentiation of most mesodermal precursors of adults.

Unraveling genomic regulatory networks in the simple chordate, Ciona intestinalis

The draft genome of the primitive chordate, Ciona intestinalis, was published three years ago. Since then, significant progress has been made in utilizing Ciona's genomic and morphological simplicity to better understand conserved chordate developmental processes. Extensive annotation and sequencing of staged EST libraries make the Ciona genome one of the best annotated among those that are publicly available. The formation of the Ciona tadpole depends on simple, well-defined cellular lineages, and it is possible to trace the lineages of key chordate tissues such as the notochord and neural tube to the fertilized egg. Electroporation methods permit the targeted expression of regulatory genes and signaling molecules in defined cell lineages, as well as the rapid identification of regulatory DNAs underlying cell-specific gene expression. The recent sequencing of a second Ciona genome (C. savignyi) permits the use of simple alignment algorithms for the identification of conserved noncoding sequences, including microRNA genes and enhancers. Detailed expression profiles are now available for almost every gene that encodes a regulatory protein or cell-signaling molecule. The combination of gene-expression profiles, comparative genome analysis, and gene-disruption assays should permit the determination of high-resolution genomic regulatory networks underlying the specification of basic chordate tissues such as the heart, blood, notochord, and neural tube.

Surfing with the tunicates into the post-genome era

This year is the centenary of Edward G. Conklin's signal findings in embryology: the elucidation of complete cell lineages and the discovery of localized maternal determinants. Conklin used ascidian embryos to elucidate universal principles in embryology. A century later, ascidians, or sea squirts, have not only entered the post-genome era, but in many ways are leading the way to the promise of a "systems-level" understanding of complex processes such as notochord formation, neurogenesis, and even behavior.

Ciona intestinalis: chordate development made simple

Thanks to their transparent and rapidly developing mosaic embryos, ascidians (or sea squirts) have been a model system for embryological studies for over a century. Recently, ascidians have entered the postgenomic era, with the sequencing of the Ciona intestinalis genome and the accumulation of molecular resources that rival those available for fruit flies and mice. One strength of ascidians as a model system is their close similarity to vertebrates. Literature reporting molecular homologies between vertebrate and ascidian tissues has flourished over the past 15 years, since the first ascidian genes were cloned. However, it should not be forgotten that ascidians diverged from the lineage leading to vertebrates over 500 million years ago. Here, we review the main similarities and differences so far identified, at the molecular level, between ascidian and vertebrate tissues and discuss the evolution of the compact ascidian genome.

Ci-Tbx6b and Ci-Tbx6c are key mediators of the maternal effect gene Ci-macho1 in muscle cell differentiation in Ciona intestinalis embryos

Maternally deposited mRNA encoding the Zic family zinc-finger protein Ci-macho1 is a determinant responsible for muscle cell differentiation in Ciona intestinalis embryos. In a previous study, we identified possible Ci-macho1 downstream genes, which include seven transcription factor genes and seven signaling molecule genes (Yagi, K., Satoh, N., Satou, Y., 2004. Identification of downstream genes of the ascidian muscle determinant gene Ci-macho1. Dev. Biol. 274, 478-489), suggesting complex Ci-macho1 downstream cascades. Here, we show that of the Ci-macho1 downstream genes, only Ci-Tbx6b and Ci-Tbx6c promote ectopic differentiation of muscle cells when misexpressed in non-muscle blastomeres. Overexpression of Ci-Tbx6b or Ci-Tbx6c in Ci-macho1 knockdown embryos is able to compensate for the functional loss of Ci-macho1 and promote differentiation of muscle cells. In addition, we show that knockdown of each of Ci-Tbx6b or Ci-Tbx6c suppresses the initiation of muscle protein gene expression, and both gene products appear to recognize a similar binding sequence. However, later expression of muscle protein genes at the tailbud stage is only reduced in Ci-Tbx6b knockdown embryos and undisturbed in Ci-Tbx6c knockdown embryos. Although ectopic expression or knockdown of Ci-ZicL alone does not affect muscle cell differentiation, simultaneous knockdown of Ci-Tbx6b, Ci-Tbx6c, and Ci-ZicL completely abolishes muscle cell differentiation, as in the case of knockdown of Ci-macho1 and Ci-ZicL. These results strongly suggest that muscle cell differentiation in Ciona embryos is controlled by four key factors: maternal macho1 and zygotic Tbx6b, Tbx6c, and ZicL. The two T-box genes are primary mediators of macho1 function, and cooperation between the zygotically expressed transcription factors is indispensable for muscle cell differentiation in Ciona embryos.

Microarray analysis of localization of maternal transcripts in eggs and early embryos of the ascidian, Ciona intestinalis

The establishment of body axes and specification of early embryonic cells depend on maternally supplied transcripts and/or proteins, several of which are localized at specific regions of fertilized eggs and early embryos. The ascidian is known to exhibit a mosaic mode of development, and this mode is largely dependent on localized maternal factors. Using blastomere isolation, microarray and whole-mount in situ hybridization, the present study of Ciona intestinalis demonstrates that maternal transcripts of a total of 17 genes are localized at the posterior-most region of fertilized eggs and early embryos. Ten of them are newly identified in the present study, while the remaining seven genes have already been characterized in previous studies. In addition, maternal transcripts of two genes, in addition to 14 genes encoded by the mitochondrial genome, showed a mitochondria-like distribution. Despite the present comprehensive approach, we could not identify maternal transcripts that are clearly localized to the animal-pole side, the vegetal-pole side, the anterior-side or other specific regions of the early embryo. Therefore, we concluded that the posterior-most localization and mitochondria-like distribution appear to be major specialized patterns of maternal transcripts in early Ciona embryos.

Molecular evidence from Ciona intestinalis for the evolutionary origin of vertebrate sensory placodes

Cranial sensory placodes are focused areas of the head ectoderm of vertebrates that contribute to the development of the cranial sense organs and their associated ganglia. Placodes have long been considered a key character of vertebrates, and their evolution is proposed to have been essential for the evolution of an active predatory lifestyle by early vertebrates. Despite their importance for understanding vertebrate origins, the evolutionary origin of placodes has remained obscure. Here, we use a panel of molecular markers from the Six, Eya, Pax, Dach, FoxI, COE and POUIV gene families to examine the tunicate Ciona intestinalis for evidence of structures homologous to vertebrate placodes. Our results identify two domains of Ciona ectoderm that are marked by the genetic cascade that regulates vertebrate placode formation. The first is just anterior to the brain, and we suggest this territory is equivalent to the olfactory/adenohypophyseal placodes of vertebrates. The second is a bilateral domain adjacent to the posterior brain and includes cells fated to form the atrium and atrial siphon of adult Ciona. We show this bares most similarity to placodes fated to form the vertebrate acoustico-lateralis system. We interpret these data as support for the hypothesis that sensory placodes did not arise de novo in vertebrates, but evolved from pre-existing specialised areas of ectoderm that contributed to sensory organs in the common ancestor of vertebrates and tunicates.

Dynamic changes in developmental gene expression in the basal chordate Ciona intestinalis

Large-scale expressed sequence tags (EST) analysis was used to demonstrate a number of dynamic changes in the global gene expression profile of the basal chordate Ciona intestinalis over the course of its development. The fertilized egg was found to store a great variety of maternal transcripts and, as development proceeds, the organism expresses a progressively smaller repertoire of genes. In addition, a significant portion of genes involved in embryogenesis were observed to be downregulated during metamorphosis, at which point the adult appears to utilize a different set of genes to form its body. At least 25% of the genes involved in development were found to be used multiple times. This kind of information is essential to form a comprehensive understanding of the overarching expression-control plan by which the basic chordate body is formed.

Regulatory gene expressions in the ascidian ventral sensory vesicle: evolutionary relationships with the vertebrate hypothalamus

In extant chordates, the overall patterning along the anteroposterior and dorsoventral axes of the neural tube is remarkably conserved. It has thus been proposed that four domains corresponding to the vertebrate presumptive forebrain, midbrain-hindbrain transition, hindbrain, and spinal cord were already present in the common chordate ancestor. To obtain insights on the evolution of the patterning of the anterior neural tube, we performed a study aimed at characterizing the expression of regulatory genes in the sensory vesicle of Ciona intestinalis, the anteriormost part of the central nervous system (CNS) related to the vertebrate forebrain, at tailbud stages. Selected genes encoded primarily for homologues of transcription factors involved in vertebrate forebrain patterning. Seven of these genes were expressed in the ventral sensory vesicle. A prominent feature of these ascidian genes is their restricted and complementary domains of expression at tailbud stages. These patterning markers thus refine the map of the developing sensory vesicle. Furthermore, they allow us to propose that a large part of the ventral and lateral sensory vesicle consists in a patterning domain corresponding to the vertebrate presumptive hypothalamus.

Making very similar embryos with divergent genomes: conservation of regulatory mechanisms of Otx between the ascidians Halocynthia roretzi and Ciona intestinalis

Ascidian embryos develop with a fixed cell lineage into simple tadpoles. Their lineage is almost perfectly conserved, even between the evolutionarily distant species Halocynthia roretzi and Ciona intestinalis, which show no detectable sequence conservation in the non-coding regions of studied orthologous genes. To address how a common developmental program can be maintained without detectable cis-regulatory sequence conservation, we compared in both species the regulation of Otx, a gene with a shared complex expression pattern. We found that in Halocynthia, the regulatory logic is based on the use of very simple cell line-specific regulatory modules, the activities of which are conserved, in most cases, in the Ciona embryo. The activity of each of these enhancer modules relies on the conservation of a few repeated crucial binding sites for transcriptional activators, without obvious constraints on their precise number, order or orientation, or on the surrounding sequences. We propose that a combination of simplicity and degeneracy allows the conservation of the regulatory logic, despite drastic sequence divergence. The regulation of Otx in the anterior endoderm by Lhx and Fox factors may even be conserved with vertebrates.

A modular cis-regulatory system controls isoform-specific pitx expression in ascidian stomodaeum

Pituitary homeobox (pitx) genes have been identified in vertebrates as critical molecular determinants of various craniofacial ontogenetic processes including pituitary organogenesis. Accordingly, a prominent conserved feature of pitx genes in chordates is their early expression in the anterior neural boundary (ANB) and oral ectoderm, also known as the stomodaeum. Here we used the ascidian model species Ciona intestinalis to investigate pitx gene organization and cis-regulatory logic during early stages of oral development. Two distinct Ci-pitx mRNA variants were found to be expressed in mutually exclusive embryonic domains. Ci-pitx and vertebrate pitx2 genes display remarkably similar exon usage and organization, suggesting ancestry of the pitx transcriptional unit and regulation in chordates. We next combined phylogenetic footprinting, transient transgenesis, and confocal imaging methods to study the Ci-pitx cis-regulatory system, with special emphasis on the regulation of isoform-specific ANB/stomodaeal expression. Among 10 conserved noncoding sequences (CNSs) interspersed in C. intestinalis and Ciona savignyi pitx loci, we identified two separate cis-regulatory modules (CRMs) that drive ANB/stomodaeal expression in complementary spatiotemporal patterns. We discuss the developmental relevance of these data that provide an entry point to investigate the gene regulatory networks (GRNs) that position and shape oral structures in chordates.

Identification of neuron-specific promoters in Ciona intestinalis

We isolated 5' flanking regions of four genes, Ci-Galphai1, Ci-arr, Ci-vAChTP, and Ci-vGAT, each of which is expressed in distinct sets of neurons in the central nervous system of the ascidian Ciona intestinalis, and we examined their function by introducing green fluorescent protein (GFP)-fusion constructs into Ciona embryos. The reporter gene driven by the 5' flanking region of Ci-Galphai1, Ci-arr, and Ci-vAChTP recapitulated the endogenous gene expression patterns, while that of Ci-vGAT can drive GFP expression in particular subsets of neurons expressing the endogenous gene. Deletion analysis revealed that the Ci-Galphai1 promoter consists of multiple regulatory modules controlling the expression in different types of cells. The GFP fluorescence enabled visualization of cell bodies and axons of different sets of neurons in ascidian larvae. These promoters can be a powerful tool for studying molecular mechanisms of neuronal development as well as neuron networks and functions in ascidians.

Toxicity assessment of the antifouling compound zinc pyrithione using early developmental stages of the ascidian Ciona intestinalis

This study investigated the toxicity of zinc pyrithione (Zpt) on the early stages of development of the ascidian Ciona intestinalis. Larval morphological abnormalities were studied after the exposure of C. intestinalis embryos at different stages of development. The median effective concentrations (EC50) ranged from 226-590 nM. The larval settlement stage was the most sensitive to Zpt. Toxic effects of Zpt on larval settlement were detected at 9 nM (EC10). The inhibition of C. intestinalis embryonic development was also used to study the loss of toxicity in Zpt solutions exposed to direct sunlight and laboratory UV light. The results showed that the toxicity of Zpt solutions decreased but did not disappear after 4 h exposure to direct sunlight (EC50 = 484 nM) or UV light (EC50 = 453 nM), compared to control Zpt solutions prepared in dark conditions. On the basis of the present data, predicted no effect concentrations of Zpt to C. intestinalis larvae are lower than predicted environmental concentrations of Zpt in certain polluted areas and therefore, may pose a risk to C. intestinalis populations.

Promoter activity of the retinoic acid receptor gene in theCiona intestinalis embryo

Retinoic acid regulates development of many chordate-specific characteristics. The genes encoding retinoic acid receptors (RARs) were isolated only from chordates. In the present study, a genomic DNA fragment containing the promoter region of the RAR gene (Ci-RAR) in the protochordate Ciona intestinalis was isolated. The transgene containing 1,326 bp of the Ci-RAR 5' flanking region was expressed in the epidermis, central nervous system, and muscle. The sequence element necessary for expression in the epidermis seemed to be subdivided into two elements. One was required for activation in the dorsal midline epidermis and the other for the rest of the epidermis. The element responsible for activation in the central nervous system was characterized by putative binding sites for Fox, Ets, and GATA transcription factors. An E-box sequence was required for the transgene expression in the tail muscle cells.

Live imaging and morphometric analysis of embryonic development in the ascidianCiona intestinalis

The ascidian Ciona intestinalis is one of the model organisms of choice for comparative investigations of chordate development and for unraveling the molecular mechanisms underlying morphogenesis and cell fate specification. Taking advantage of the availability of various genetically encoded fluorescent proteins and of defined cis-regulatory elements, we combined transient transgenesis with laser scanning confocal imaging to acquire and quantitate 3D time-lapse data from living Ciona embryos. We used Ciona tissue-specific enhancers to drive expression of spectrally distinct fluorescent protein reporters to label and simultaneously visualize axially and paraxially positioned mesodermal derivatives, as well as neural precursors in individual embryos. We observed morphogenetic movements, without perturbing development, from the early gastrula throughout the larval stage, including gastrulation, neurulation, convergent extension of the presumptive notochord, and tail elongation. These multidimensional data allowed us to establish a reference system of metrics to quantify key developmental events including blastopore closure and muscle extension. The approach we describe can be used to document morphogenetic cell and tissue rearrangements in living embryos and paves the way for a live digitized anatomical atlas of Ciona.