Neuronal form in the central nervous system of the tadpole larva of the ascidian Ciona intestinalis

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.

Neuronal subtype specification in the spinal cord of a protovertebrate

The visceral ganglion (VG) comprises the basic motor pool of the swimming ascidian tadpole and has been proposed to be homologous to the spinal cord of vertebrates. Here, we use cis-regulatory modules, or enhancers, from transcription factor genes expressed in single VG neuronal precursors to label and identify morphologically distinct moto- and interneuron subtypes in the Ciona intestinalis tadpole larva. We also show that the transcription factor complement present in each differentiating neuron correlates with its unique morphology. Forced expression of putative interneuron markers Dmbx and Vsx results in ectopic interneuron-like cells at the expense of motoneurons. Furthermore, by perturbing upstream signaling events, we can change the transcription factor expression profile and subsequent identity of the different precursors. Perturbation of FGF signaling transforms the entire VG into Vsx+/Pitx+ putative cholinergic interneurons, while perturbation of Notch signaling results in duplication of Dmbx+ decussating interneurons. These experiments demonstrate the connection between transcriptional regulation and the neuronal subtype diversity underlying swimming behavior in a simple chordate.

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.

Functional analysis of synaptotagmin gene regulatory regions in two distantly related ascidian species

We have studied the structure and function of a promoter region of the Halocynthia synaptotagmin (Hr-Syt) gene, which is abundantly expressed in neuronal cells. Our previous analysis suggested that the expression of Hr-Syt is regulated by at least one epidermal and two neuronal regulatory regions. In this study, the regulatory regions of Hr-Syt promoter were further characterized by using two species of ascidians, Halocynthia roretzi and Ciona intestinalis embryos. A putative GATA transcription factor binding site in the epidermal regulatory region has ectodermal enhancer activity in the Halocynthia embryo. Neuronal expression of Hr-Syt was regulated by multiple redundant enhancer regions. Among these enhancer regions, a 200-bp (-2900/-2700) region drove the reporter expression in neurons in both species of ascidian. Although the synaptotagmin promoter sequences did not show overall similarity between Hr-Syt and Ciona synaptotagmin (Ci-Syt), 5'-upsteream two short sequences of Ci-Syt have similarity to the -2766/-2732 region of the Hr-Syt promoter. The homeodomain binding sites in this region are required for the neuronal enhancer activity. These results suggest that GATA and homeodomain transcription factors regulate the expression of synaptotagmin.

Glutamatergic networks in the Ciona intestinalis larva

Glutamate is a major neurotransmitter in the excitatory synapses of both vertebrate and invertebrate nervous systems and is involved in many neural processes including photo-, mechano-, and chemosensations, neural development, motor control, learning, and memory. We identified and characterized the gene (Ci-VGLUT) encoding a member of the vesicular glutamate transporter subfamily, a specific marker of glutamatergic neurons, in the ascidian Ciona intestinalis. The Ci-VGLUT gene is expressed in the adhesive organ, the epidermal neurons, and the brain vesicle, but not in the visceral ganglion. The Ci-VGLUT promoter and an anti-Ci-VGLUT antibody were used to analyze the distribution and axonal connections of prospective glutamatergic neurons in the C. intestinalis larva. The green fluorescent protein (GFP) reporter driven by the 4.6-kb upstream region of Ci-VGLUT recapitulated the endogenous gene expression patterns and visualized both the cell bodies and neurites of glutamatergic neurons. Papillar neurons of the adhesive organs, almost all epidermal neurons, the otolith cell, and ocellus photoreceptor cells were shown to be glutamatergic. Each papillar neuron connects with a rostral epidermal neuron. Axons from rostral epidermal neurons, ocellus photoreceptor cells, and neurons underlying the otolith terminate in the posterior brain vesicle. Some caudal epidermal neurons also send long axons toward the brain vesicle. The posterior brain vesicle contains a group of Ci-VGLUT-positive neurons that send axons posteriorly to the visceral ganglion. Our results suggest that glutamatergic neurotransmission plays a major role in sensory systems and in the integration of the sensory inputs of the ascidian larva.

Comparative genomics identifies a cis-regulatory module that activates transcription in specific subsets of neurons in Ciona intestinalis larvae

The larval nervous system of the ascidian Ciona intestinalis exhibits an abstract form of the vertebrate nervous system. The Ci-Galphai1 gene, which encodes a G-protein alpha subunit, is specifically expressed in distinct sets of neurons in C. intestinalis larvae, including papillar neurons of the adhesive organ, ocellus photoreceptor cells, and cholinergic and GABAergic neurons in the central nervous system (CNS). A GFP reporter gene driven by the 4.2-kb 5' flanking region of Ci-Galphai1 recapitulated the endogenous gene expression patterns. Comparative genomic analysis of the Galphai1 gene between C. intestinalis and Ciona savignyi identified an 87-bp highly conserved non-coding sequence located between -3176 and -3090 bp upstream of the gene. Deletion of this conserved upstream sequence resulted in the complete loss of reporter expression in the central nervous system, while reporter expression in the adhesive organ and mesenchyme cells remained unaffected. The conserved upstream sequence can activate gene expression from basal promoters in the brain vesicle, although it requires additional cis-regulatory sequences to fully activate the CNS-specific gene expression. These results suggest that different types of central neurons share a common transcriptional activation mechanism that is different from that of papillar neurons.

Neurons of the ascidian larval nervous system in Ciona intestinalis: I. Central nervous system

The tadpole larva of ascidians, basal living relatives of vertebrates, has a chordate body plan. The CNS has many homologies with that of vertebrates yet only about 100 neurons. These few, possibly fixed in number and composition, nevertheless govern a diverse repertoire of behaviors. To elucidate the circuits of the CNS first requires that we recognize each neuron type, for which we used electroporation to transfect precleavage embryos with a plasmid containing green fluorescent protein (GFP) driven by the promoter of the synaptotagmin gene. Hatched larvae were fixed and GFP 3-D reconstructions of confocal image stacks compiled into images of 31 whole or partial larvae, either with many GFP-labelled neurons or with few, each clearly visible. Neuron counts in the sensory vesicle (SV) and visceral ganglion (VG) indicated that between 75% (SV) and 69% (VG) of previously reported numbers of neurons were transfected. Based on their position, shape, and projections, the following neurons were identified in the SV: a prominent eminens neuron, possibly with direct input from papillar neurons, a large ventroposterior interneuron, photoreceptors of the ocellus, and putative antenna cells of the otolith. In the VG, we identified at least four subtypes of motor neuron, including an ovoid cell that may innervate distal tail muscle cells and contrapelo cells with ascending projections, unique among VG neurons. The caudal nerve cord contained the first reported neurons, the somata of planate neurons. These neurons are the first identified types, and will be used to construct a map of the nervous system for this model basal chordate.

The ascidian homolog of the vertebrate homeobox gene Rx is essential for ocellus development and function

The tadpole larvae prosencephalon of the ascidian Ciona intestinalis contains a single large ventricle, along the inner walls of which lie two sensory organs: the otolith (a gravity-sensing organ) and the ocellus (a photo-sensing organ composed of a single cup-shaped pigment cell, about 20 photoreceptor cells, and three lens cells). Comparison has been drawn between the morphology and physiology of photoreceptor cells in the ascidian ocellus and the vertebrate eye. The development of vertebrate and invertebrate eyes requires the activity of several conserved genes and it is regulated by precise expression patterns and cell fate decisions common to several species. We have isolated a Ciona homeobox gene (Ci-Rx) that belongs to the paired-like class of homeobox genes. Rx genes have been identified from a variety of organisms and have been demonstrated to have a role in vertebrate eye formation. Ci-Rx is expressed in the anterior neural plate in the middle tailbud stage and subsequently in the larval stage in the sensory vesicle around the ocellus. Loss of Ci-Rx function leads to an ocellus-less phenotype that shows a loss of photosensitive swimming behavior, suggesting the important role played by Ci-Rx in basal chordate photoreceptor cell differentiation and ocellus formation. Furthermore, studies on Ci-Rx regulatory elements electroporated into Ciona embryos using LacZ or GFP as reporter genes indicate the presence of Ci-Rx in pigment cells, photoreceptors, and neurons surrounding the sensory vesicle. In Ci-Rx knocked-down larvae, neither basal swimming activity nor shadow responses develop. Thus, Rx has a role not only in pigment cells and photoreceptor formation but also in the correct development of the neuronal circuit that controls larval photosensitivity and swimming behavior. The results suggest that a Ci-Rx "retinal" territory exists, which consists of pigment cells, photoreceptors, and neurons involved in transducing the photoreceptor signals.

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.

Neurogenic and non-neurogenic placodes in ascidians

The late differentiation of the ectodermal layer is analysed in the ascidians Ciona intestinalis and Botryllus schlosseri, by means of light and electron microscopy, in order to verify the possible presence of placodal structures. Cranial placodes, ectodermal regions giving rise to nonepidermal cell types, are classically found exclusively in vertebrates; however, data are accumulating to demonstrate that the nonvertebrate chordates possess both the genetic machinery involved in placode differentiation, and ectodermal structures that are possible homologues of vertebrate placodes. Here, the term "placode" is used in a broad sense and defines thickenings of the ectodermal layer that can exhibit an interruption of the basal lamina where cells delaminate, and so are able to acquire a nonepidermal fate. A number of neurogenic placodes, ones capable of producing neurons, have been recognised; their derivatives have been analysed and their possible homologies with vertebrate placodes are discussed. In particular, the stomodeal placode may be considered a multiple placode, being composed of different sorts of placodes: part of it, which differentiates hair cells, is discussed as homologous to the octavo-lateralis placodes, while the remaining portion, giving rise to the ciliated duct of the neural gland, is considered homologous to the adenohypophyseal placode. The neurohypophyseal placode may include the homologues of the hypothalamus and vertebrate olfactory placode; the rostral placode, producing the sensorial papillae, may possibly be homologous to the placodes of the adhesive gland of vertebrates.

Roles of Hroth, the ascidian otx gene, in the differentiation of the brain (sensory vesicle) and anterior trunk epidermis in the larval development of Halocynthia roretzi

Otx genes are expressed in the anterior neural tube and endoderm in all of the chordates so far examined. In mouse embryos, important roles of otx genes in the brain development have been well documented. However, roles of otx genes in other chordate species have been less characterized. To advance our understanding about roles of otx genes in chordates, we have studied Hroth, otx of the ascidian, Halocynthia roretzi. Hroth is expressed in the anterior part of the neural tube (the sensory vesicle), the endoderm and anterior epidermis in the development. In this study, we investigated roles of Hroth in the larval development through an antisense morpholino oligonucleotides (MOs) approach. Embryos injected with Hroth-targeting MO (Hroth knockdown embryos) developed into larvae without the adhesive organ, sensory pigment cells and cavity of the sensory vesicle. The tissues, in which defects were observed, are derived from anterior-animal cells of the embryo in early cleavage stages. During cleavage stages, Hroth is also expressed in the endoderm precursors of the vegetal hemisphere. However, Hroth expression in the anterior endoderm precursors do not seem to be essential for the above defects, since MO injection into the anterior-animal but not anterior-vegetal pair cells at the 8-cell stage gave the defects. Analysis of marker gene expression demonstrated that the fate choice of the sensory vesicle precursors and the specification of the sensory vesicle territory occurred normally, but the subsequent differentiation of the sensory vesicle was severely affected in Hroth knockdown embryos. The anterior trunk epidermis including the adhesive organ-forming region was also affected, indicating that anterior epidermal patterning requires Hroth function. Based on these findings, similarities and differences in the roles of otx genes between ascidians and mice are discussed.

Time course of programmed cell death in Ciona intestinalis in relation to mitotic activity and MAPK signaling

Programmed cell death (PCD) in the ascidian species Ciona intestinalis (Tunicata; Chordata) is investigated from early larvae to juvenile stages, by means of digoxigenin-based terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling (TUNEL) technique. At first, PCD in the swimming larva affects trunk mesenchyme and central nervous system (CNS), then it participates extensively to metamorphosis, until it is restricted to developing organs of juveniles. Analysis of patterns of cell death and division in the larval CNS question old models on the genesis of the adult C. intestinalis brain. Upon performing immunochemical and functional assays for mitogen-activated protein kinase (MAPK) kinase kinase-1 (MEKK1), MAPK kinase 1/2 (MEK1/2), c-Jun NH2-terminal kinase (JNK), and dual phosphorylated extracellular regulated kinase 1/2 (dpERK1/2), the neurogenic competence of the larval brain appears to rely on a combinatorial regulation of PCD by the mitogen-activated protein kinase signaling cascade. These results show that, in tunicates, PCD consists of a multistep program implicated in growth and patterning with various roles.

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.

Eutely, cell lineage, and fate within the ascidian larval nervous system: determinacy or to be determined?

The larval central nervous system (CNS) of the ascidian Ciona intestinalis (L., 1767) arises from an embryonic neural plate and contains sufficiently few cells, about 330, to enable definitive counts. On the basis of such counts, there is evidence both for cell constancy (eutely) in the larval CNS and for small variations in the overall numbers of cells and among defined cell types within this total. However, evidence for the range of such deviations and the existence of a true phenotypic wild type are lacking. The record of cell lineage, i.e., the mitotic ancestry of each cell, and the fates of some of these cells have recently received increased documentation in both the genus Ciona and Halocynthia roretzi (von Drasche, 1884). Relatively few generations of cells, between 10 and 14, form the entire CNS in C. intestinalis, and cell death does not occur prior to larval hatching. The tiny complement of larval CNS cells can therefore be seen as the product of a small fixed number of determinate cleavages, and variations in cell number as the product of minor deviations in this mitotic ancestry. Within these lineage records, some cell fates have already been identified, but knowledge of most is lacking because the cells lack markers or other identifying features. Nevertheless, this tiny nervous system offers the prospect that all its cells can one day be identified, and their developmental histories and larval functions analyzed, cell by cell.

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.

Synaptic connectivity and neuronal morphology: two sides of the same coin

Neurons often possess elaborate axonal and dendritic arbors. Why do these arbors exist and what determines their form and dimensions? To answer these questions, I consider the wiring up of a large highly interconnected neuronal network, such as the cortical column. Implementation of such a network in the allotted volume requires all the salient features of neuronal morphology: the existence of branching dendrites and axons and the presence of dendritic spines. Therefore, the requirement of high interconnectivity is, in itself, sufficient to account for the existence of these features. Moreover, the actual lengths of axons and dendrites are close to the smallest possible length for a given interconnectivity, arguing that high interconnectivity is essential for cortical function.

The neurobiology of the ascidian tadpole larva: recent developments in an ancient chordate

With little more than 330 cells, two thirds within the sensory vesicle, the CNS of the tadpole larva of the ascidian Ciona intestinalis provides us with a chordate nervous system in miniature. Neurulation, neurogenesis and its genetic bases, as well as the gene expression territories of this tiny constituency of cells all follow a chordate plan, giving rise in some cases to frank structural homologies with the vertebrate brain. Recent advances are fueled by the release of the genome and EST expression databases and by the development of methods to transfect embryos by electroporation. Immediate prospects to test the function of neural genes are based on the isolation of mutants by classical genetics and insertional mutagenesis, as well as by the disruption of gene function by morpholino antisense oligo-nucleotides. Coupled with high-speed video analysis of larval swimming, optophysiological methods offer the prospect to analyze at single-cell level the function of a CNS built on a vertebrate plan.

Interaction between noradrenaline or adrenaline and the beta 1-adrenergic receptor in the nervous system triggers early metamorphosis of larvae in the ascidian, Ciona savignyi

Molecular mechanisms underlying the metamorphosis of larvae, e.g., ligand and receptor interaction, have to be determined and roles for the nervous system in marine invertebrates are not well understood. We report here that treatment of swimming larvae of the ascidian Ciona savignyi with noradrenaline or adrenaline promoted morphological changes in early metamorphosis, e.g., tail resorption. Antagonists of the beta-adrenergic receptor, propranolol, and the beta(1)-adrenergic receptor, metoprolol, inhibited the noradrenaline-induced tail resorption, while an antagonist of the alpha-adrenergic receptor, phentolamine, and of the beta(2)- adrenergic receptor, butoxamine, had no inhibitory effects. In addition, a selective agonist of the beta-adrenergic receptor, isoproterenol, the concentration of which was lower than the effective concentration of the neurotransmitters, facilitated tail resorption. Immunohistochemical studies, using an anti-dopamine-hydroxylase antibody, showed that neurotransmitters such as noradrenaline and adrenaline localized around the brain vesicle of the larvae during metamorphosis. The beta(1)-adrenergic receptor stained with antibodies was localized on the nervous system. Temporal expression of the beta(1)-adrenergic receptor was intense in the nervous system in the larvae competent for metamorphosis. We propose that interactions between noradrenaline or adrenaline and the beta(1)-adrenergic receptor in the nervous system mediate the process of metamorphosis of Ciona larvae.

Large-scale characterization of genes specific to the larval nervous system in the ascidian Ciona intestinalis

The central and peripheral nervous systems (CNS and PNS) of the ascidian tadpole larva are comparatively simple, consisting of only about 350 cells. However, studies of the expression of neural patterning genes have demonstrated overall similarity between the ascidian CNS and the vertebrate CNS, suggesting that the ascidian CNS is sufficiently complex to be relevant to those of vertebrates. Recent progress in the Ciona intestinalis genome project and cDNA project together with considerable EST information has made Ciona an ideal model for investigating molecular mechanisms underlying the formation and function of the chordate nervous system. Here, we characterized 56 genes specific to the nervous system by determining their full-length cDNA sequences and confirming their spatial expression patterns. These genes included those that function in the nervous systems of other animals, especially those involved in photoreceptor-mediated signaling and neurotransmitter release. Thus, the nervous system-specific genes in Ciona larvae will provide not only probes for determining their function but also clues for exploring the complex network of nervous system-specific genes.

Analyzing gene regulation in ascidian embryos: new tools for new perspectives

Ascidians are marine protochordates at the evolutionary boundary between invertebrates and vertebrates. Ascidian larvae provide a simple system for unraveling gene regulation networks underlying the formation of the basic chordate body plan. After being used for over a century as a model for embryological studies, ascidians have become, in the past decade, an increasingly popular organism for studying gene regulation. Part of the renewed appeal of this system is the use of electroporation to introduce transgenic DNAs into developing embryos. This method is considerably more efficient than conventional microinjection assays and permits the simultaneous transformation of hundreds of embryos. Electroporation has allowed the identification and characterization of cis-regulatory DNAs that mediate gene expression in a variety of tissues, including the notochord, tail muscles, CNS, and endoderm. Electroporation has also provided a simple method for misexpressing patterning genes and producing dominant mutant phenotypes. Recent studies have used electroporation to create "knock-out" phenotypes by overexpressing dominant negative forms of particular proteins. Here we review the past and present uses of electroporation in ascidian development, and speculate on potential future uses.

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.

The larval ascidian nervous system: the chordate brain from its small beginnings

The body plan of the tadpole larva of ascidians, or sea-squirts, is widely presumed to be close to that of the hypothetical ancestor of all chordate animal groups, including vertebrates. This is nowhere more obvious than in the organization and development of the dorsal tubular nervous system. Several recent developments advocate this model neural system for studies on neurobiology and neurogenesis. These include advances in our understanding of development in ascidian embryos and of differentiation among the cellular progeny of its neural plate; the application of transgenic and mutant approaches to studies on ascidian larval neurones; and the prospect of advances in genomic analyses. In addition to providing ways to study a working chordate brain in miniature, all these offer insights into the ancestral condition of the developing vertebrate brain.