Determination and regulation in the pigment cell lineage of the ascidian embryo

On the evolutionary origins and regionalization of the neural crest

The neural crest is a vertebrate-specific embryonic stem cell population that gives rise to a vast array of cell types throughout the animal body plan. These cells are first born at the edges of the central nervous system, from which they migrate extensively and differentiate into multiple cellular derivatives. Given the unique set of structures these cells comprise, the origin of the neural crest is thought to have important implications for the evolution and diversification of the vertebrate clade. In jawed vertebrates, neural crest cells exist as distinct subpopulations along the anterior-posterior axis. These subpopulations differ in terms of their respective differentiation potential and cellular derivatives. Thus, the modern neural crest is characterized as multipotent, migratory, and regionally segregated throughout the embryo. Here, we retrace the evolutionary origins of the neural crest, from the appearance of conserved regulatory circuitry in basal chordates to the emergence of neural crest subpopulations in higher vertebrates. Finally, we discuss a stepwise trajectory by which these cells may have arisen and diversified throughout vertebrate evolution.

Brain Sensory Organs of the Ascidian Ciona robusta: Structure, Function and Developmental Mechanisms

During evolution, new characters are designed by modifying pre-existing structures already present in ancient organisms. In this perspective, the Central Nervous System (CNS) of ascidian larva offers a good opportunity to analyze a complex phenomenon with a simplified approach. As sister group of vertebrates, ascidian tadpole larva exhibits a dorsal CNS, made up of only about 330 cells distributed into the anterior sensory brain vesicle (BV), connected to the motor ganglion (MG) and a caudal nerve cord (CNC) in the tail. Low number of cells does not mean, however, low complexity. The larval brain contains 177 neurons, for which a documented synaptic connectome is now available, and two pigmented organs, the otolith and the ocellus, controlling larval swimming behavior. The otolith is involved in gravity perception and the ocellus in light perception. Here, we specifically review the studies focused on the development of the building blocks of ascidians pigmented sensory organs, namely pigment cells and photoreceptor cells. We focus on what it is known, up to now, on the molecular bases of specification and differentiation of both lineages, on the function of these organs after larval hatching during pre-settlement period, and on the most cutting-edge technologies, like single cell RNAseq and genome editing CRISPR/CAS9, that, adapted and applied to Ciona embryos, are increasingly enhancing the tractability of Ciona for developmental studies, including pigmented organs formation.

Transcription Factors of the bHLH Family Delineate Vertebrate Landmarks in the Nervous System of a Simple Chordate

Tunicates are marine invertebrates whose tadpole-like larvae feature a highly simplified version of the chordate body plan. Similar to their distant vertebrate relatives, tunicate larvae develop a regionalized central nervous system and form distinct neural structures, which include a rostral sensory vesicle, a motor ganglion, and a caudal nerve cord. The sensory vesicle contains a photoreceptive complex and a statocyst, and based on the comparable expression patterns of evolutionarily conserved marker genes, it is believed to include proto-hypothalamic and proto-retinal territories. The evolutionarily conserved molecular fingerprints of these landmarks of the vertebrate brain consist of genes encoding for different transcription factors, and of the gene batteries that they control, and include several members of the bHLH family. Here we review the complement of bHLH genes present in the streamlined genome of the tunicate Ciona robusta and their current classification, and summarize recent studies on proneural bHLH transcription factors and their expression territories. We discuss the possible roles of bHLH genes in establishing the molecular compartmentalization of the enticing nervous system of this unassuming chordate.

Bisphenols disrupt differentiation of the pigmented cells during larval brain formation in the ascidian

The endocrine disruptor Bisphenol A (BPA), a widely employed molecule in plastics, has been shown to affect several biological processes in vertebrates, mostly via binding to nuclear receptors. Neurodevelopmental effects of BPA have been documented in vertebrates and linked to neurodevelopmental disorders, probably because some nuclear receptors are present in the vertebrate brain. Similarly, endocrine disruptors have been shown to affect neurodevelopment in marine invertebrates such as ascidians, mollusks or echinoderms, but whether invertebrate nuclear receptors are involved in the mode-of-action is largely unknown. In this study, we assessed the effect of BPA on larval brain development of the ascidian Phallusia mammillata. We found that BPA is toxic to P. mammillata embryos in a dose-dependent manner (EC50: 11.8μM; LC50: 21μM). Furthermore, micromolar doses of BPA impaired differentiation of the ascidian pigmented cells, by inhibiting otolith movement within the sensory vesicle. We further show that this phenotype is specific to other two bisphenols (BPE and BPF) over a bisphenyl (2,2 DPP). Because in vertebrates the estrogen-related receptor gamma (ERRγ) can bind bisphenols with high affinity but not bisphenyls, we tested whether the ascidian ERR participates in the neurodevelopmental phenotype induced by BPA. Interestingly, P. mammillata ERR is expressed in the larval brain, adjacent to the differentiating otolith. Furthermore, antagonists of vertebrate ERRs also inhibited the otolith movement but not pigmentation. Together our observations suggest that BPA may affect ascidian otolith differentiation by altering Pm-ERR activity whereas otolith pigmentation defects might be due to the known inhibitory effect of bisphenols on tyrosinase enzymatic activity.

Acquisition of multipotent and migratory neural crest cells in vertebrate evolution

The emergence of multipotent and migratory neural crest (NC) cells defines a key evolutionary transition from invertebrates to vertebrates. Studies in vertebrates have identified a complex gene regulatory network that governs sequential stages of NC ontogeny. Comparative analysis has revealed extensive conservation of the overall architecture of the NC gene regulatory network between jawless and jawed vertebrates. Among invertebrates, urochordates express putative NC gene homologs in the neural plate border region, but these NC-like cells do not have migratory capacity, whereas cephalochordates contain no NC cells but its genome contains most homologs of vertebrate NC genes. Whether the absence of migratory NC cells in invertebrates is due to differences in enhancer elements or an intrinsic limitation in potency remains unclear. We provide a brief overview of mechanisms that might explain how ancestral NC-like cells acquired the multipotency and migratory capacity seen in vertebrates.

Bisphenol A affects neural development of the ascidian Ciona robusta

Bisphenol A (BPA) is an organic pollutant derived from plastic degradation that has numerous and variable adverse effects on human health and wildlife. In particular, it has been reported that BPA can alter reproductive processes and nervous system development in vertebrates. Considering BPA presence in marine environment and the scant data available on its interaction with nervous system development, we analyzed the effect of BPA exposure on sperm viability, fertilization, embryogenesis, and neural differentiation of the ascidian Ciona robusta. Ascidians are members of the Phylum Tunicata, the sister group of Vertebrata, sharing with them fundamental developmental processes. Our results showed that first cell division was altered starting from 5 µM concentration. Lethal concentration (LC ₅₀ ) was estimated to be 5.2 µM. Larvae developed from treated embryos showed specific malformations to the pigment cells even at 0.1 µM, corresponding to the highest environmental concentration reported so far. Moreover, GABAergic and dopaminergic neurons proved to be target organs of BPA teratogenic action, in accordance with similar results reported in vertebrate animal models. Overall, our results suggest that BPA can exert its effects on nervous system acting on different pathways and underline that C. robusta is a valuable invertebrate animal model for preliminary screenings of effects of pollutants on vertebrates.

Developmental Roles and Evolutionary Significance of AMPA-Type Glutamate Receptors

Organogenesis and metamorphosis require the intricate orchestration of multiple types of cellular interactions and signaling pathways. Glutamate (Glu) is an excitatory extracellular signaling molecule in the nervous system, while Ca²⁺ is a major intracellular signaling molecule. The first Glu receptors to be cloned are Ca²⁺ -permeable receptors in mammalian brains. Although recent studies have focused on Glu signaling in synaptic mechanisms of the mammalian central nervous system, it is unclear how this signaling functions in development. Our recent article demonstrated that Ca²⁺ -permeable AMPA-type Glu receptors (GluAs) are essential for formation of a photosensitive organ, development of some neurons, and metamorphosis, including tail absorption and body axis rotation, in ascidian embryos. Based on findings in these embryos and mammalian brains, we formed several hypotheses regarding the evolution of GluAs, the non-synaptic function of Glu, the origin of GluA-positive neurons, and the neuronal network that controls metamorphosis in ascidians.

Revised lineage of larval photoreceptor cells in Ciona reveals archetypal collaboration between neural tube and neural crest in sensory organ formation

The Ciona intestinalis larva has two distinct photoreceptor organs, a conventional pigmented ocellus and a nonpigmented ocellus, that are asymmetrically situated in the brain. The ciliary photoreceptor cells of these ocelli resemble visual cells of the vertebrate retina. Precise elucidation of the lineage of the photoreceptor cells will be key to understanding the developmental mechanisms of these cells as well as the evolutionary relationships between the photoreceptor organs of ascidians and vertebrates. Photoreceptor cells of the pigmented ocellus have been thought to develop from anterior animal (a-lineage) blastomeres, whereas the developmental origin of the nonpigmented ocellus has not been determined. Here, we show that the photoreceptor cells of both ocelli develop from the right anterior vegetal hemisphere: those of the pigmented ocellus from the right A9.14 cell and those of the nonpigmented ocellus from the right A9.16 cell. The pigmented ocellus is formed by a combination of two lineages of cells with distinct embryonic origins: the photoreceptor cells originate from a medial portion of the A-lineage neural plate, while the pigment cell originates from the lateral edge of the a-lineage neural plate. In light of the recently proposed close evolutionary relationship between the ocellus pigment cell of ascidians and the cephalic neural crest of vertebrates, the ascidian ocellus may represent a prototypic contribution of the neural crest to a cranial sensory organ.

A direct role for murine Cdx proteins in the trunk neural crest gene regulatory network

Numerous studies in chordates and arthropods currently indicate that Cdx proteins have a major ancestral role in the organization of post-head tissues. In urochordate embryos, Cdx loss-of-function has been shown to impair axial elongation, neural tube (NT) closure and pigment cell development. Intriguingly, in contrast to axial elongation and NT closure, a Cdx role in neural crest (NC)-derived melanocyte/pigment cell development has not been reported in any other chordate species. To address this, we generated a new conditional pan-Cdx functional knockdown mouse model that circumvents Cdx functional redundancy as well as the early embryonic lethality of Cdx mutants. Through directed inhibition in the neuroectoderm, we provide in vivo evidence that murine Cdx proteins impact melanocyte and enteric nervous system development by, at least in part, directly controlling the expression of the key early regulators of NC ontogenesis Pax3,Msx1 and Foxd3 Our work thus reveals a novel role for Cdx proteins at the top of the trunk NC gene regulatory network in the mouse, which appears to have been inherited from their ancestral ortholog.

Developmental Biology: We Are All Walking Mutants

What is Developmental Biology? Developmental Biology is a discipline that evolved from the collective fields of embryology, morphology, and anatomy, which firmly established that structure underpins function. In its simplest terms, Developmental Biology has come to describe how a single cell becomes a completely formed organism. However, this definition of Developmental Biology is too narrow. Developmental Biology describes the properties of individual cells; their organization into tissues, organs, and organisms; their homeostasis, regeneration, aging, and ultimately death. Developmental Biology provides a context for cellular reprogramming, stem cell biology, regeneration, tissue engineering, evolutionary development and ecology, and involves the reiterated use of the same cellular mechanisms and signaling pathways throughout the lifespan of an organism. Using neural crest cells as an example, this review explores the contribution of Developmental Biology to our understanding of development, evolution, and disease.

Neural crest cell evolution: how and when did a neural crest cell become a neural crest cell

As vertebrates evolved from protochordates, they shifted to a more predatory lifestyle, and radiated and adapted to most niches of the planet. This process was largely facilitated by the generation of novel vertebrate head structures, which were derived from neural crest cells (NCC). The neural crest is a unique vertebrate cell population that is frequently termed the "fourth germ layer" because it forms in conjunction with the other germ layers and contributes to a diverse array of cell types and tissues including the craniofacial skeleton, the peripheral nervous system, and pigment cells among many other tissues and cell types. NCC are defined by their origin at the neural plate border, via an epithelial-to-mesenchymal transition (EMT), together with multipotency and polarized patterns of migration. These defining characteristics, which evolved independently in the germ layers of invertebrates, were subsequently co-opted through their gene regulatory networks to form NCC in vertebrates. Moreover, recent data suggest that the ability to undergo an EMT was one of the latter features co-opted by NCC. In this review, we discuss the potential origins of NCC and how they evolved to contribute to nearly all tissues and organs throughout the body, based on paleontological evidence together with an evaluation of the evolution of molecules involved in NCC development and their migratory cell paths.

The ascidian pigmented sensory organs: structures and developmental programs

The recent advances on ascidian pigment sensory organ development and function represent a fascinating platform to get insight on the basic programs of chordate eye formation. This review aims to summarize current knowledge, at the structural and molecular levels, on the two main building blocks of ascidian light sensory organ, i.e. pigment cells and photoreceptor cells. The unique features of these structures (e.g., simplicity and well characterized cell lineage) are indeed making it possible to dissect the developmental programs at single cell resolution and will soon provide a panel of molecular tools to be exploited for a deep developmental and comparative-evolutionary analysis.

Ephrin-mediated restriction of ERK1/2 activity delimits the number of pigment cells in the Ciona CNS

Recent evidence suggests that ascidian pigment cells are related to neural crest-derived melanocytes of vertebrates. Using live-imaging, we determine a revised cell lineage of the pigment cells in Ciona intestinalis embryos. The neural precursors undergo successive rounds of anterior-posterior (A-P) oriented cell divisions, starting at the blastula 64-cell stage. A previously unrecognized fourth A-P oriented cell division in the pigment cell lineage leads to the generation of the post-mitotic pigment cell precursors. We provide evidence that MEK/ERK signals are required for pigment cell specification until approximately 30min after the final cell division has taken place. Following each of the four A-P oriented cell divisions, ERK1/2 is differentially activated in the posterior sister cells, into which the pigment cell lineage segregates. Eph/ephrin signals are critical during the third A-P oriented cell division to spatially restrict ERK1/2 activation to the posterior daughter cell. Targeted inhibition of Eph/ephrin signals results in, at neurula stages, anterior expansion of both ERK1/2 activation and a pigment cell lineage marker and subsequently, at larval stages, supernumerary pigment cells. We discuss the implications of these findings with respect to the evolution of the vertebrate neural crest.

Progenitors of the protochordate ocellus as an evolutionary origin of the neural crest

The neural crest represents a highly multipotent population of embryonic stem cells found only in vertebrate embryos. Acquisition of the neural crest during the evolution of vertebrates was a great advantage, providing Chordata animals with the first cellular cartilage, bone, dentition, advanced nervous system and other innovations. Today not much is known about the evolutionary origin of neural crest cells. Here we propose a novel scenario in which the neural crest originates from neuroectodermal progenitors of the pigmented ocelli in Amphioxus-like animals. We suggest that because of changes in photoreception needs, these multipotent progenitors of photoreceptors gained the ability to migrate outside of the central nervous system and subsequently started to give rise to neural, glial and pigmented progeny at the periphery.

Identification of a rudimentary neural crest in a non-vertebrate chordate

Neural crest arises at the neural plate border, expresses a core set of regulatory genes, and produces a diverse array of cell types including ectomesenchyme derivatives that elaborate the vertebrate head^1,2. The evolution of neural crest has been postulated as a key event leading to the appearance of new cell types that fostered the transition from filter feeding to active predation in ancestral vertebrates³. However, the origin of neural crest remains controversial, since homologous cell types have not been unambiguously identified in non-vertebrate chordates^1,4. Here we show that the tunicate Ciona intestinalis possesses a cephalic melanocyte lineage (a9.49) similar to neural crest that can be reprogrammed into migrating ectomesenchyme by the targeted misexpression of Twist. Our results suggest that the neural crest melanocyte regulatory network predated the divergence of tunicates and vertebrates. We propose that the co-option of mesenchyme determinants, such as Twist, into the neural plate ectoderm was crucial for the emergence of the vertebrate “new head”³.

Evolutionary crossroads in developmental biology: the tunicates

The tunicates, or urochordates, constitute a large group of marine animals whose recent common ancestry with vertebrates is reflected in the tadpole-like larvae of most tunicates. Their diversity and key phylogenetic position are enhanced, from a research viewpoint, by anatomically simple and transparent embryos, compact rapidly evolving genomes, and the availability of powerful experimental and computational tools with which to study these organisms. Tunicates are thus a powerful system for exploring chordate evolution and how extreme variation in genome sequence and gene regulatory network architecture is compatible with the preservation of an ancestral chordate body plan.

FGF/MAPK/Ets signaling renders pigment cell precursors competent to respond to Wnt signal by directly controlling Ci-Tcf transcription

FGF and Wnt pathways constitute two fundamental signaling cascades, which appear to crosstalk in cooperative or antagonistic fashions in several developmental processes. In vertebrates, both cascades are involved in pigment cell development, but the possible interplay between FGF and Wnt remains to be elucidated. In this study, we have investigated the role of FGF and Wnt signaling in development of the pigment cells in the sensory organs of C. intestinalis. This species possesses the basic features of an ancestral chordate, thus sharing conserved molecular developmental mechanisms with vertebrates. Chemical and targeted perturbation approaches revealed that a FGF signal, spreading in time from early gastrulation to neural tube closure, is responsible for pigment cell precursor induction. This signal is transmitted via the MAPK pathway, which activates the Ci-Ets1/2 transcription factor. Targeted perturbation of Ci-TCF, a downstream factor of the canonical Wnt pathway, indicated its contribution to pigment cell differentiation Furthermore, analyses of the Ci-Tcf regulatory region revealed the involvement of the FGF effector, Ci-Ets1/2, in Ci-Tcf transcriptional regulation in pigment cell precursors. Our results indicate that both FGF and the canonical Wnt pathways are involved in C. intestinalis pigment cell induction and differentiation. Moreover, we present a case of direct transcriptional regulation exerted by the FGF signaling cascade, via the MAPK-ERK-Ets1/2, on the Wnt downstream gene Ci-Tcf. Several examples of FGF/Wnt signaling crosstalk have been described in different developmental processes; however, to our knowledge, FGF-Wnt cross-interaction at the transcriptional level has never been previously reported. These findings further contribute to clarifying the multitude of FGF-Wnt pathway interactions.

doublesex/mab3 related-1 (dmrt1) is essential for development of anterior neural plate derivatives in Ciona

Ascidian larvae have a hollow, dorsal central nervous system that shares many morphological features with vertebrate nervous systems yet is composed of very few cells. We show here that a null mutation in the gene dmrt1 in the ascidian Ciona savignyi results in profound abnormalities in the development of the sensory vesicle (brain), as well as other anterior ectodermal derivatives, including the palps and oral siphon primordium (OSP). Although the phenotype of the mutant embryos is variable, the majority have a complete loss of the most anterior structures (palps and OSP) and extensive disruption of sensory structures, such as the light-sensitive ocellus, in the sensory vesicle. dmrt1 is expressed early in the blastula embryo in a small group of presumptive ectodermal cells as they become restricted to anterior neural, OSP and palp fates. Despite the early and restricted expression of dmrt1, we were unable, using several independent criteria, to observe a defect in the mutant embryos until the early tailbud stage. We speculate that the variability and late onset in the phenotype may be due to partially overlapping activities of other gene products.

Chordate ancestry of the neural crest: New insights from ascidians

This article reviews new insights from ascidians on the ancestry of vertebrate neural crest (NC) cells. Ascidians have neural crest-like cells (NCLC), which migrate from the dorsal midline, express some of the typical NC markers, and develop into body pigment cells. These characters suggest that primordial NC cells were already present in the common ancestor of the vertebrates and urochordates, which have been recently inferred as sister groups. The primitive role of NCLC may have been in pigment cell dispersal and development. Later, additional functions may have appeared in the vertebrate lineage, resulting in the evolution of definitive NC cells.

Ascidian embryonic development: An emerging model system for the study of cell fate specification in chordates

The ascidian tadpole larva represents the basic body plan of all chordates in a relatively small number of cells and tissue types. Although it had been considered that ascidians develop largely in a determinative way, whereas vertebrates develop in an inductive way, recent studies at the molecular and cellular levels have uncovered several similarities in the way developmental fates are specified. In this review, we describe ascidian embryogenesis and its cell lineages, introduce several characteristics of ascidian embryos, describe recent advances in understanding of the mechanisms of cell fate specification, and discuss them in the context of what is known in vertebrates and other organisms.

Ci-FoxA-a is the earliest zygotic determinant of the ascidian anterior ectoderm and directly activates Ci-sFRP1/5

This work focuses on the anteroposterior patterning of the ectoderm in the invertebrate chordate Ciona intestinalis. Previous work indicated that, by the eight-cell stage, the anterior and posterior animal blastomeres have acquired different properties, including a differential responsiveness to inducing signals from the underlying mesendoderm. Here, we investigated the molecular basis of this distinction. For this, we studied the regulation of the earliest marker specific for the anterior ectoderm, Ci-sFRP1/5, which is activated at the 64-cell stage. We first found that the activation of this marker in the anterior ectoderm does not involve communication with other lineages. We then identified, by phylogenetic footprinting and deletion analysis, a short conserved minimal enhancer driving the onset of expression of Ci-sFRP1/5. We showed that this enhancer was a direct target of the Ci-FoxA-a gene, a FoxA/HNF3 orthologue expressed in anterior ectodermal and mesendodermal lineages from the eight-cell stage. Gain- and loss-of-function experiments revealed that Ci-FoxA-a is necessary and sufficient within the ectoderm to impose an ectodermal anterior identity, and to repress the posterior programme. Thus, Ci-FoxA-a constitutes a major early zygotic anterior determinant for the ascidian ectoderm, acting autonomously in this territory, prior to the onset of vegetal inductions. Interestingly, while vertebrate FoxA2 are also involved in the regionalization of the ectoderm, they are thought to act during gastrulation to control, in the mesendoderm, the expression of organizer signals. We discuss the evolution of chordate ectodermal patterning in light of our findings.

Experimental analyses of the function of the proepicardium using a new microsurgical procedure to induce loss-of-proepicardial-function in chick embryos

The proepicardium (PE) is a primarily extracardiac progenitor cell population that colonizes the embryonic heart and delivers the epicardium, the subepicardial and intramyocardial fibroblasts, and the coronary vessels. Recent data show that PE-derived cells additionally play important regulatory roles in myocardial development and possibly in the normal morphogenesis of the heart. Developmental Dynamics 233, 2005. Research on the latter topics profits from the fact that loss-of-PE-function can be experimentally induced in chick embryos. So far, two microsurgical techniques were used to produce such embryos: (1) blocking of PE cell transfer with pieces of the eggshell membrane, and (2) mechanical excision of PE. Both of these techniques, however, have their shortcomings. We have searched, therefore, for new techniques to eliminate the PE. Here, we show that loss-of-PE-function can be induced by photoablation of the PE. Chick embryos were treated in ovo by means of a window in the eggshell at Hamburger and Hamilton (HH) stage 16 (iday 3). The pericardial coelom was opened, and the PE was externally stained with a 1% solution of Rose Bengal by means of a micropipette. Photoactivation of the dye was accomplished by illumination of the operation field with visible light. Examination on postoperative day 1 (iday 4, HH stages 19/20) disclosed complete removal of PE in every experimental embryo. On iday 9 (HH stages 33/34), the survival rate of experimental embryos was 35.7% (15 of 42). Development of the PE-derivatives was compromised in the heart of every survivor. The abnormalities encompassed hydro- or hemopericardium, epicardium-free areas with aneurysmatic outward bulging of the ventricular wall, thin myocardium, defects of the coronary vasculature, and abnormal tissue bridges between the ventricles and the pericardial wall. Our results show that photoablation of the PE is a powerful technique to induce long-lasting loss-of-PE-function in chick embryos. We have additionally obtained new data that suggest that the embryonic epicardium may make important contributions to the passive mechanics of the developing heart.

Pigmentation in the sensory organs of the ascidian larva is essential for normal behavior

Free-living animals and their larvae utilize light and gravity as cues to navigate in open space. Detection and response to these environmental stimuli are important for the dispersal and settlement of ascidian larvae. Two pigmented structures in the brain of the ascidian larva, the ocellus and the otolith, have been shown to function as the photoreceptive and gravity sensitive organs, respectively. Here, we show that pigmentation is essential for proper phototactic and geotactic behavior in larvae of the ascidian species Ciona savignyi. Two recessive and complementing mutant lines of C. savignyi, immaculate and spotless, that specifically disrupt the pigmentation of the sensory organs during larval development are described. Homozygous mutant larvae are unable to respond properly to gravity and illumination cues while settling. Genetic analysis shows that spotless is caused by a point mutation within the tyrosinase gene that creates a premature stop codon, while the molecular nature of immaculate is unknown. Although the role of pigmentation in the ocellus of C. savignyi is similar to that in vertebrate visual systems, our results demonstrate a novel use of melanin in geotactic behavior.

Tracing cell fate in brain formation during embryogenesis of the ascidian Halocynthia roretzi

Ascidian eggs develop into tadpole larvae. They have a simple central nervous system (CNS) at the dorsal midline. The CNS is formed through neural tube formation at the neurula stage, as in vertebrates. The total number of cells in the CNS is approximately 300. In Halocynthia roretzi, the anterior part of the CNS, which consists of the brain (sensory vesicle) and the visceral ganglion, is exclusively derived from 10 blastomeres at the 110-cell stage. The anterior CNS is relatively complex and shows remarkable left-right asymmetry, with the lumen of the sensory vesicle, the otolith, and the ocellus on the right side, and the presumed hydrostatic pressure organ on the left side. We labeled these 10 precursor blastomeres - six in the animal hemisphere (a-line) and four in the vegetal hemisphere (A-line) - with lineage tracer, and examined the fates in swimming larvae. The clonal organization of the anterior CNS is essentially invariant among individuals, although slight variation in the clonal boundary was observed. There was no extensive mixing between descendants of each precursor. We observed no evidence of cell migration except for two neuronal cells derived from a8.25 blastomeres. The eventual fates of the bilateral blastomeres produced extensive left-right asymmetry. The results suggest that the anterior neural tube rotates in a clockwise direction when viewed from the posterior pole. Staged observation indicated that this rotation takes place during the last 5 h of embryogenesis. We describe detailed positions of descendants of each precursor blastomere. In particular, specific cells of sensory structures were identified by their morphology and staining with specific antibodies and probes. The otolith and ocellus pigment cells were derived from left and right a8.25 blastomeres. Lens cells of the ocellus have a right A8.8 origin, and most of the photoreceptor cells originated from the right A8.7. The presumed pressure organ was formed by descendants of left and right a8.19 and left a8.17 blastomeres. The description of cell lineages of the CNS would facilitate future research to analyze the mechanisms of development of the simple CNS of ascidian tadpole larvae.

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.

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.

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.

The central nervous system of the ascidian larva: mitotic history of cells forming the neural tube in late embryonic Ciona intestinalis

Ascidian larvae develop after an invariant pattern of embryonic cleavage. Fewer than 400 cells constitute the larval central nervous system (CNS), which forms without either extensive migration or cell death. We catalogue the mitotic history of these cells in Ciona intestinalis, using confocal microscopy of whole-mount embryos at stages from neurulation until hatching. The positions of cells contributing to the CNS were reconstructed from confocal image stacks of embryonic nuclei, and maps of successive stages were used to chart the mitotic descent, thereby creating a cell lineage for each cell. The entire CNS is formed from 10th- to 14th-generation cells. Although minor differences exist in cell position, lineage is invariant in cells derived from A-line blastomeres, which form the caudal nerve cord and visceral ganglion. We document the lineage of five pairs of presumed motor neurons within the visceral ganglion: one pair arises from A/A 10.57, and four from progeny of A/A 9.30. The remaining cells of the visceral ganglion are in their 13th and 14th generations at hatching, with most mitotic activity ceasing around 85% of embryonic development. Of the approximately 330 larval cells previously reported in the CNS of Ciona, we document the lineage of 226 that derive predominantly from A-line blastomeres.

Cloning and functional analysis of ascidian Mitf in vivo: insights into the origin of vertebrate pigment cells

The microphthalmia-associated transcription factor (Mitf) is a basic-helix-loop-helix-leucine zipper (bHLH-ZIP) transcription factor essential for the development and function of all melanin-producing pigment cells in vertebrates. To elucidate the evolutionary history of Mitf and the antiquity of its association with pigment cells, we have isolated and characterized HrMitf, a sole member of the Mitf-TFE bHLH-ZIP subfamily in the ascidian Halocynthia roretzi. Maternal HrMitf mRNA is detected in the fertilized egg and in the animal hemisphere from 4-cell stage through the gastrula stage. From the neurula through the early tailbud stage, HrMitf is preferentially expressed in the pigment-lineage cells that express the lineage-specific melanogenesis genes tyrosinase (HrTyr) and Tyrp. Overexpression of HrMitf induced ectopic expression of HrTyr enzyme activity in mesenchymal cells where the same enzyme activity was induced by overexpression of HrPax3/7, suggesting that a part(s) of the Pax3-Mitf-tyrosinase gene regulatory cascade seen in vertebrate melanocytes is operative during ascidian embryogenesis. We also show HrMitf and mouse Mitf-A, a Mitf isoform abundantly expressed in pigmented epithelial cells, share similar functional characteristics. These results suggest antiquity of the association of the Mitf-TFE subfamily with pigment cells and may support the idea that acquisition of multiple promoters (isoforms) by an ancestral Mitf gene has allowed the evolution of multiple pigment cell types.

Evolution and development of brain sensory organs in molgulid ascidians

The ascidian tadpole larva has two brain sensory organs containing melanocytes: the otolith, a gravity receptor, and the ocellus, part of a photoreceptor. One or both of these sensory organs are absent in molgulid ascidians. We show here that developmental changes leading to the loss of sensory pigment cells occur by different mechanisms in closely related molgulid species. Sensory pigment cells are formed through a bilateral determination pathway in which two or more precursor cells are specified as an equivalence group on each side of the embryo. The precursor cells subsequently converge at the midline after neurulation and undergo cell interactions that decide the fates of the otolith and ocellus. Molgula occidentalis and M. oculata, which exhibit a tadpole larva with an otolith but lacking an ocellus, have conserved the bilateral pigment cell determination pathway. Programmed cell death (PCD) is superimposed on this pathway late in development to eliminate the ocellus precursor and supernumerary pigment cells, which do not differentiate into either an otolith or ocellus. In contrast to molgulids with tadpole larvae, no pigment cell precursors are specified on either side of the M. occulta embryo, which forms a tailless (anural) larva lacking both sensory organs, suggesting that the bilateral pigment cell determination pathway has been lost. The bilateral pigment cell determination pathway and superimposed PCD can be restored in hybrids obtained by fertilizing M. occulta eggs with M. oculata sperm, indicating control by a zygotic process. We conclude that PCD plays an important role in the evolution and development of brain sensory organs in molgulid ascidians.

Early steps in the formation of neural tissue in ascidian embryos

Ascidians are simple invertebrate chordates whose lineage diverged from that of vertebrates at the base of the chordate tree. Their larvae display a typical chordate body plan, but are composed of a remarkably small number of cells. Ascidians develop with an invariant cell lineage, and their embryos can be easily experimentally manipulated during the cleavage stages. Their larval nervous system is organised in a similar way as in vertebrates but is composed of less than 130 neurones and around 230 glial cells. This remarkable simplicity offers an opportunity to understand, at the cellular and molecular levels, the ontogeny and function of each neural cell. Here, we first review the organisation of the ascidian nervous system and its lineage. We then focus on the current understanding of the processes of neural specification and patterning before and during gastrulation. We discuss these advances in the context of what is currently known in vertebrates.

Specification of developmental fates in ascidian embryos: molecular approach to maternal determinants and signaling molecules

Tadpole larvae of ascidians represent the basic body plan of chordates with a relatively small number and few types of cells. Because of their simplicity, ascidians have been intensively studied. More than a century of research on ascidian embryogenesis has uncovered many cellular and molecular mechanisms responsible for cell fate specification in the early embryo. This review describes recent advances in our understanding of the molecular mechanisms of fate specification mainly uncovered in model ascidian species--Halocynthia roretzi, Ciona intestinalis, and Ciona savignyi. One category of developmentally important molecules represents maternal localized mRNAs that are involved in cell-autonomous processes. In the second category, signaling molecules and downstream transcription factors are involved in inductive cell interactions. Together with genome-wide information, there is a renewed interest in studying ascidian embryos as a fascinating model system for understanding how single-celled eggs develop a highly organized chordate body plan.

HrzicN, a new Zic family gene of ascidians, plays essential roles in the neural tube and notochord development

Two axial structures, a neural tube and a notochord, are key structures in the chordate body plan and in understanding the origin of chordates. To expand our knowledge on mechanisms of development of the neural tube in lower chordates, we have undertaken isolation and characterization of HrzicN, a new member of the Zic family gene of the ascidian, Halocynthia roretzi. HrzicN expression was detected by whole-mount in situ hybridization in all neural tube precursors, all notochord precursors, anterior mesenchyme precursors and a part of the primary muscle precursors. Expression of HrzicN in a- and b-line neural tube precursors was detected from early gastrula stage to the neural plate stage, while expression in other lineages was observed between the 32-cell and the 110-cell stages. HrzicN function was investigated by disturbing translation using a morpholino antisense oligonucleotide. Embryos injected with HrzicN morpholino ('HrzicN knockdown embryos') exhibited failure of neurulation and tail elongation, and developed into larvae without a neural tube and notochord. Analysis of neural marker gene expression in HrzicN knockdown embryos revealed that HrzicN plays critical roles in distinct steps of neural tube formation in the a-line- and A-line precursors. In particular HrzicN is required for early specification of the neural tube fate in A-line precursors. Involvement of HrzicN in the neural tube development was also suggested by an overexpression experiment. However, analysis of mesodermal marker gene expression in HrzicN knockdown embryos revealed unexpected roles of this gene in the development of mesodermal tissues. HrzicN knockdown led to loss of HrBra (Halocynthia roretzi Brachyury) expression in all of the notochord precursors, which may be the cause for notochord deficiency. Hrsna (Halocynthia roretzi snail) expression was also lost from all the notochord and anterior mesenchyme precurosrs. By contrast, expression of Hrsna and the actin gene was unchanged in the primary muscle precursors. These results suggest that HrzicN is responsible for specification of the notochord and anterior mesenchyme. Finally, regulation of HrzicN expression by FGF-like signaling was investigated, which has been shown to be involved in induction of the a- and b-line neural tube, the notochord and the mesenchyme cells in Halocynthia embryos. Using an inhibitor of FGF-like signaling, we showed that HrzicN expression in the a- and b-line neural tube, but not in the A-line lineage and mesodermal lineage, depends on FGF-like signaling. Based on these data, we discussed roles of HrzicN as a key gene in the development of the neural tube and the notochord.

Ciona intestinalis cDNA projects: expressed sequence tag analyses and gene expression profiles during embryogenesis

Ascidians are primitive chordates. Their fertilized egg develops quickly into a tadpole-type larva, which consists of a small number but distinct types of cells, including those of epidermis, central nervous system with two sensory organs, endoderm and mesenchyme in the trunk, and notochord and muscle in the tail. This configuration of the ascidian tadpole is thought to represent the most simplified and primitive chordate body plan. In addition, the free-swimming and non-feeding larvae metamorphose into sessile and filter-feeding adults. The genome size of Ciona intestinalis is estimated to be about 160 Mb, and the number of genes approximately 15,500. The present Ciona cDNA projects focused on gene expression profiles of fertilized eggs, 32-110-cell stage embryos, tailbud embryos, larvae, and young adults. Expressed sequence tags (ESTs) of the 5'-most end and 3'-most end of more than 3000 clones were determined at each developmental stage, and the clones were categorized into independent clusters using the 3'-end sequences. Nearly 1000 clusters of them were then analyzed in detail of their sequences against a BLASTX search. This analysis demonstrates that, on average, half of the clusters showed proteins with sequence similarities to known proteins and the other half did not show sequence similarities to known proteins. Genes with sequence similarities were further categorized into three major subclasses, depending on their functions. Furthermore, the expression profiles of all of the clusters were analyzed by whole-mount in situ hybridization. This analysis highlights gene expression patterns characteristic to each developmental stage. As a result, the present study provides many new molecular markers for each of the tissues and/or organs that constitutes the Ciona tailbud embryo. This sequence information will be used for further comparative genome studies to explore molecular mechanisms involved in the formation of one of the most primitive chordate body plans. All of the data fully characterized may be viewed at the web site http://ghost.zool.kyoto-u.ac.jp.

Gene expression profiles in Ciona intestinalis cleavage-stage embryos

A total of 1612 expression sequence tags derived from Ciona intestinalis cleavage-stage embryos were examined to explore detailed gene expression profiles. The 3' sequences indicate that the 1612 clones can be categorized into 1066 independent clusters. DDBJ database search suggested that 496 of them showed significant matches to reported proteins with distinct functions. Among them 69 are associated with cell-cell communications and 41 with transcription factors. In situ hybridization of all 1066 clusters showed that 84 clusters exhibited blastomere-specific pattern of expression, and many of these genes seem to encode for novel proteins. One of the interesting findings is that most of them were expressed in the precursor cells of multiple tissues. Among them 28 genes were expressed in the marginal zone of the 32-cell embryo. The blastomeres in this region are thought to receive an inductive signal from the vegetal blastomeres. Many of the blastomere-specific genes did not show similarity to known proteins. The present analysis therefore provides new information for further analyses on the cell fate specification in the Ciona embryo.

Morphogenetic pattern formation during ascidian notochord formation is regulative and highly robust

The ascidian notochord forms through simultaneous invagination and convergent extension of a monolayer epithelial plate. Here we combine micromanipulation with time lapse and confocal microscopy to examine how notochord-intrinsic morphogenetic behaviors and interactions with surrounding tissues, determine these global patterns of movement. We show that notochord rudiments isolated at the 64-cell stage divide and become motile with normal timing; but, in the absence of interactions with non-notochordal tissues, they neither invaginate nor converge and extend. We find that notochord formation is robust in the sense that no particular neighboring tissue is required for notochord formation. Basal contact with either neural plate or anterior endoderm/lateral mesenchyme or posterior mesoderm are each alone sufficient to ensure that the notochord plate forms and extends a cylindrical rod. Surprisingly, the axis of convergent extension depends on the specific tissues that contact the notochord, as do other patterns of cell shape change, movement and tissue deformation that accompany notochord formation. We characterize one case in detail, namely, embryos lacking neural plates, in which a normal notochord forms but by an entirely different trajectory. Our results show ascidian notochord formation to be regulative in a fashion and to a degree never before appreciated. They suggest this regulative behavior depends on a complex interplay between morphogenetic tendencies intrinsic to the notochord plate and instructive and permissive interactions with surrounding tissues. We discuss mechanisms that could account for these data and what they imply about notochord morphogenesis and its evolution within the chordate phylum.

Induction of ascidian peripheral neuron by vegetal blastomeres

Ascidian tadpole larvae have a similar dorsal tubular nervous system as vertebrates. The induction of brain formation from a4.2-derived (a-line) cells requires signals from the A4.1-derived (A-line) cells. However, little is known about the mechanism underlying the development of the larval peripheral nervous system due to the lack of a suitable molecular marker. Gelsolin, an actin-binding protein, is specifically expressed in epidermal sensory neurons (ESNs) that mainly constitute the entire peripheral nervous system of the ascidian young tadpoles. Here, we address the role of cell interactions in the specification of ESNs using immunostaining with an anti-gelsolin antibody. Animal half (a4.2- and b4.2-derived) embryos did not give rise to any gelsolin-positive neurons, indicating that differentiation of ESNs requires signals from vegetal cells. Cell isolation experiments showed that A4.1 blastomeres induce gelsolin-positive neurons from a-line cells but not from b4.2-derived (b-line) cells. On the other hand, B4.1 blastomeres induce gelsolin-positive neurons both from b-line cells and a-line cells. This is in sharp contrast to the specification of brain cells which is not affected by the ablation of B4.1-derived (B-line) cells. Furthermore, basic fibroblast growth factor (bFGF) induced ESNs from the a-line cells and b-line cells in the absence of vegetal cells. Their competence to form ESNs was lost between the 110-cell stage and the neurula stage. Our results suggested that the specification of the a-line cells and b-line cells into ESNs is controlled by distinct inducing signals from the anterior and posterior vegetal blastomeres. ESNs in the trunk appear to be derived from the a8.26 blastomeres aligning on the edge of presumptive neural region where ascidian homologue of Pax3 is expressed. These findings highlight the close similarity of ascidian ESNs development with that of vertebrate placode and neural crest.

Ci-opsin1, a vertebrate-type opsin gene, expressed in the larval ocellus of the ascidian Ciona intestinalis

A novel gene encoding visual pigment, Ci-opsin1, was identified in a primitive chordate, the ascidian, Ciona intestinalis. Molecular phylogenetic analysis and the exon-intron organization suggest that Ci-opsin1 is closely related to the retinal and pineal opsins of vertebrates. During embryogenesis, Ci-opsin1 transcripts were first detected in part of the brain of mid tailbud embryos; its expression was confined to photoreceptor cells of the ocellus (eye spot) in the larval brain as development proceeded. These results suggest a common descent of the ascidian ocellus and the vertebrate eyes. The ocellus of ascidian larvae may represent an ancestral state of the vertebrate eye.

The BMP/CHORDIN antagonism controls sensory pigment cell specification and differentiation in the ascidian embryo

We have investigated the role of the bone morphogenetic protein (BMP) pathway during neural tissue formation in the ascidian embryo. The orthologue of the BMP antagonist, chordin, was isolated from the ascidian Halocynthia roretzi. While both the expression pattern and the phenotype observed by overexpressing chordin or BMPb (the dpp-subclass BMP) do not suggest a role for these factors in neural induction, BMP/CHORDIN antagonism was found to affect neural patterning. Overexpression of BMPb induced ectopic sensory pigment cells in the brain lineages that do not normally form pigment cells and suppressed pressure organ formation within the brain. Reciprocally, overexpressing chordin suppressed pigment cell formation and induced ectopic pressure organ. We show that pigment cell formation occurs in three steps. (1) During cleavage stages ectodermal cells are neuralized by a vegetal signal that can be substituted by bFGF. (2) At the early gastrula stage, BMPb secreted from the lateral nerve cord blastomeres induces those neuralized blastomeres in close proximity to adopt a pigment cell fate. (3) At the tailbud stage, among these pigment cell precursors, BMPb induces the differentiation of specifically the anterior type of pigment cell, the otolith; while posteriorly, CHORDIN suppresses BMP activity and allows ocellus differentiation.

Delta-Notch signaling and lateral inhibition in zebrafish spinal cord development

BACKGROUND

Vertebrate neural development requires precise coordination of cell proliferation and cell specification to guide orderly transition of mitotically active precursor cells into different types of post-mitotic neurons and glia. Lateral inhibition, mediated by the Delta-Notch signaling pathway, may provide a mechanism to regulate proliferation and specification in the vertebrate nervous system. We examined delta and notch gene expression in zebrafish embryos and tested the role of lateral inhibition in spinal cord patterning by ablating cells and genetically disrupting Delta-Notch signaling.

RESULTS

Zebrafish embryos express multiple delta and notch genes throughout the developing nervous system. All or most proliferative precursors appeared to express notch genes whereas subsets of precursors and post-mitotic neurons expressed delta genes. When we ablated identified primary motor neurons soon after they were born, they were replaced, indicating that specified neurons laterally inhibit neighboring precursors. Mutation of a delta gene caused precursor cells of the trunk neural tube to cease dividing prematurely and develop as neurons. Additionally, mutant embryos had excess early specified neurons, with fates appropriate for their normal positions within the neural tube, and a concomitant deficit of late specified cells.

CONCLUSIONS

Our results are consistent with the idea that zebrafish Delta proteins, expressed by newly specified neurons, promote Notch activity in neighboring precursors. This signaling is required to maintain a proliferative precursor population and generate late-born neurons and glia. Thus, Delta-Notch signaling may diversify vertebrate neural cell fates by coordinating cell cycle control and cell specification.

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.