Identification of astrocyte-like cells in an adult ascidian during regeneration of the central nervous system

The mechanisms underlying regeneration of the central nervous system (CNS) following lesions have been studied extensively in both vertebrate and invertebrate models. To shed light on regeneration, ascidians, a sister group of vertebrates and with remarkable ability to regenerate their brains, constitute an appropriate model system. Glial cells have been implicated in regeneration in vertebrates; however, their role in the adult ascidian CNS regeneration is unknown. A model of degeneration and regeneration using the neurotoxin 3-acetylpyridine (3AP) in the brain of the ascidian Styela plicata was used to identify astrocyte-like cells and investigate their role. We studied the CNS of control ascidians (injected with artificial sea water) and of ascidians whose CNS was regenerating (1 and 10 days after the injection with 3AP). Our results show that the mRNA of the ortholog of glutamine synthetase (GS), a glial-cell marker in vertebrates, is increased during the early stages of regeneration. Confirming the identity of GS, the protein was identified via immunostaining in a cell population during the same regeneration stage. Last, a single ortholog of GS (GSII) is present in ascidian and amphioxus genomes, while two types exist in fungi, some invertebrates, and vertebrates, suggesting that ascidians have lost the GSI type. Taken together, our findings revealed that a cell population expressing glial-cell markers may play a role in regeneration in adult ascidians. This is the first report of astrocyte-like cells in the adult ascidian CNS, and contributes to understanding of the evolution of glial cells among metazoans.

Sensory cells in tunicates: insights into mechanoreceptor evolution

Tunicates, the sister group of vertebrates, offer a unique perspective for evolutionary developmental studies (Evo-Devo) due to their simple anatomical organization. Moreover, the separation of tunicates from vertebrates predated the vertebrate-specific genome duplications. As adults, they include both sessile and pelagic species, with very limited mobility requirements related mainly to water filtration. In sessile species, larvae exhibit simple swimming behaviors that are required for the selection of a suitable substrate on which to metamorphose. Despite their apparent simplicity, tunicates display a variety of mechanoreceptor structures involving both primary and secondary sensory cells (i.e., coronal sensory cells). This review encapsulates two decades of research on tunicate mechanoreception focusing on the coronal organ's sensory cells as prime candidates for understanding the evolution of vertebrate hair cells of the inner ear and the lateral line organ. The review spans anatomical, cellular and molecular levels emphasizing both similarity and differences between tunicate and vertebrate mechanoreception strategies. The evolutionary significance of mechanoreception is discussed within the broader context of Evo-Devo studies, shedding light on the intricate pathways that have shaped the sensory system in chordates.

The oral sensory organs in Bathochordaeus stygius (Tunicata Appendicularia) are unique in structure and homologous to the coronal organ

BACKGROUND

Appendicularia consists of approximately 70 purely marine species that belong to Tunicata the probable sister taxon to Craniota. Therefore, Appendicularia plays a pivotal role for our understanding of chordate evolution. In addition, appendicularians are an important part of the epipelagic marine plankton. Nevertheless, little is known about appendicularian species, especially from deeper water.

RESULTS

Using µCT, scanning electron microscopy, and digital 3D-reconstruction techniques we describe three pairs of complex oral sensory organs in the mesopelagic appendicularian Bathochordaeus stygius. The oral sensory organs are situated at the anterior and lateral margin of the mouth and inside the mouth cavity. A single organ consists of 22-90 secondary receptor cells that project apical cilia through a narrow hole in the epidermis. The receptor cells are innervated by branches of the second brain nerve.

CONCLUSIONS

Based on position, morphology, and innervation we suggest that the oral sensory organs are homologues of the coronal organs in other tunicates. We discuss the hypothesized homology of coronal organs and the lateral line system of primary aquatic vertebrates. The complex oral sensory organs of B. stygius are unique in tunicates and could be adaptations to the more muffled environment of the mesopelagic.

VR for Cognition and Memory

This chapter will provide a review of research into human cognition through the lens of VR-based paradigms for studying memory. Emphasis is placed on why VR increases the ecological validity of memory research and the implications of such enhancements.

Central nervous system regeneration in ascidians: cell migration and differentiation

Adult ascidians have the capacity to regenerate the central nervous system (CNS) and are therefore excellent models for studies on neuroregeneration. The possibility that undifferentiated blood cells are involved in adult neuroregeneration merits investigation. We analyzed the migration, circulation, and role of hemocytes of the ascidian Styela plicata in neuroregeneration. Hemocytes were removed and incubated with superparamagnetic iron oxide nanoparticles (SPION), and these SPION-labeled hemocytes were injected back into the animals (autologous transplant), followed by neurodegeneration with the neurotoxin 3-acetylpyridine (3AP). Magnetic resonance imaging showed that 1, 5, and 10 days after injury, hemocytes migrated to the intestinal region, siphons, and CNS. Immunohistochemistry revealed that the hemocytes that migrated to the CNS were putative stem cells (P-element-induced wimpy testis + or PIWI + cells). In the cortex of the neural ganglion, migrated hemocytes started to lose their PIWI labeling 5 days after injury, and 10 days later started to show β-III tubulin labeling. In the neural gland, however, the hemocytes remained undifferentiated during the entire experimental period. Transmission electron microscopy revealed regions in the neural gland with characteristics of neurogenic niches, not previously reported in ascidians. These results showed that migration of hemocytes to the hematopoietic tissue and to the 3AP-neurodegenerated region is central to the complex mechanism of neuroregeneration.

Sensing the world and its dangers: An evolutionary perspective in neuroimmunology

Detecting danger is key to the survival and success of all species. Animal nervous and immune systems cooperate to optimize danger detection. Preceding studies have highlighted the benefits of bringing neurons into the defense game, including regulation of immune responses, wound healing, pathogen control, and survival. Here, we summarize the body of knowledge in neuroimmune communication and assert that neuronal participation in the immune response is deeply beneficial in each step of combating infection, from inception to resolution. Despite the documented tight association between the immune and nervous systems in mammals or invertebrate model organisms, interdependence of these two systems is largely unexplored across metazoans. This review brings a phylogenetic perspective of the nervous and immune systems in the context of danger detection and advocates for the use of non-model organisms to diversify the field of neuroimmunology. We identify key taxa that are ripe for investigation due to the emergence of key evolutionary innovations in their immune and nervous systems. This novel perspective will help define the primordial principles that govern neuroimmune communication across taxa.

Amphioxus neuroglia: Molecular characterization and evidence for early compartmentalization of the developing nerve cord

Glial cells play important roles in the development and homeostasis of metazoan nervous systems. However, while their involvement in the development and function in the central nervous system (CNS) of vertebrates is increasingly well understood, much less is known about invertebrate glia and the evolutionary history of glial cells more generally. An investigation into amphioxus glia is therefore timely, as this organism is the best living proxy for the last common ancestor of all chordates, and hence provides a window into the role of glial cell development and function at the transition of invertebrates and vertebrates. We report here our findings on amphioxus glia as characterized by molecular probes correlated with anatomical data at the transmission electron microscopy (TEM) level. The results show that amphioxus glial lineages express genes typical of vertebrate astroglia and radial glia, and that they segregate early in development, forming what appears to be a spatially separate cell proliferation zone positioned laterally, between the dorsal and ventral zones of neural cell proliferation. Our study provides strong evidence for the presence of vertebrate-type glial cells in amphioxus, while highlighting the role played by segregated progenitor cell pools in CNS development. There are implications also for our understanding of glial cells in a broader evolutionary context, and insights into patterns of precursor cell deployment in the chordate nerve cord.

Behavioural Responses of the Colonial Sea Squirt Botrylloides violaceus Oka to Suspended Food Micro-Particles in Laboratory Cultures

Violet sea squirts are noteworthy model organisms, because they provide insights into various physiologic processes, including cell senescence, ageing, apoptosis and allorecognition. Consequently, their culture is critical to permit experimental studies. Most papers refer to short periods of rearing using various feeds, both living and conserved, missing a formal justification for their use or indications of their actual nutritional value. Here, we use two behavioural responses—the percentage of open siphons and the frequency of zooid contractions—as compared to the abundance of suspended microparticles during feeding tests, to identify feeds able to promote filter-feeding. The results will enable to formulate compound diets that maximise positive physiological responses. Our tests demonstrated that plant items, such as dry microalgae and cyanobacteria (Arthrospira platensis, commercially known as Spirulina), along with living planktonic Haptophyta (Isochrysis galbana), trigger clear positive reactions, represented by a higher frequency of zooid contractions and larger proportions of open siphons. These responses correspond to decreases in the concentrations of suspended microparticles during the experiment and indicate higher filter-feeding activity. In contrast, feeds commonly administered to colonies, such as milk powder, dried eggs and artificial plankton, triggered negative behavioural responses, and their intake was lower during the feeding trials.

Spawning induction, development and culturing of the solitary ascidian Polycarpa mytiligera, an emerging model for regeneration studies

BACKGROUND

Ascidians (phylum Chordata, class Ascidiacea) represent the closest living invertebrate relatives of the vertebrates and constitute an important model for studying the evolution of chordate development. The solitary ascidian Polycarpa mytiligera exhibits a robust regeneration ability, unique among solitary chordates, thus offering a promising new model for regeneration studies. Understanding its reproductive development and establishing land-based culturing methods is pivotal for utilizing this species for experimental studies. Its reproduction cycle, spawning behavior, and developmental processes were therefore studied in both the field and the lab, and methods were developed for its culture in both open and closed water systems.

RESULTS

Field surveys revealed that P. mytiligera's natural recruitment period starts in summer (June) and ends in winter (December) when seawater temperature decreases. Laboratory experiments revealed that low temperature (21 °C) has a negative effect on its fertilization and development. Although spontaneous spawning events occur only between June and December, we were able to induce spawning under controlled conditions year-round by means of gradual changes in the environmental conditions. Spawning events, followed by larval development and metamorphosis, took place in ascidians maintained in either artificial or natural seawater facilities. P. mytiligera's fast developmental process indicated its resemblance to other oviparous species, with the larvae initiating settlement and metamorphosis at about 12 h post-hatching, and reaching the juvenile stage 3 days later.

CONCLUSIONS

Polycarpa mytiligera can be induced to spawn in captivity year-round, independent of the natural reproduction season. The significant advantages of P. mytiligera as a model system for regenerative studies, combined with the detailed developmental data and culturing methods presented here, will contribute to future research addressing developmental and evolutionary questions, and promote the use of this species as an applicable model system for experimental studies.

An α7-related nicotinic acetylcholine receptor mediates the ciliary arrest response in pharyngeal gill slits of Ciona

Ciliary movement is a fundamental process to support animal life, and the movement pattern may be altered in response to external stimuli under the control of nervous systems. Juvenile and adult ascidians have ciliary arrays around their pharyngeal gill slits (stigmata), and continuous beating is interrupted for seconds by mechanical stimuli on other parts of the body. Although it has been suggested that neural transmission to evoke ciliary arrest is cholinergic, its molecular basis has not yet been elucidated in detail. Here, we attempted to clarify the molecular mechanisms underlying this neurociliary transmission in the model ascidian Ciona Acetylcholinesterase histochemical staining showed strong signals on the laterodistal ciliated cells of stigmata, hereafter referred to as trapezial cells. The direct administration of acetylcholine (ACh) and other agonists of nicotinic ACh receptors (nAChRs) onto ciliated cells reliably evoked ciliary arrest that persisted for seconds in a dose-dependent manner. While the Ciona genome encodes ten nAChRs, only one of these called nAChR-A7/8-1, a relative of vertebrate α7 nAChRs, was found to be expressed by trapezial cells. Exogenously expressed nAChR-A7/8-1 on Xenopus oocytes responded to ACh and other agonists with consistent pharmacological traits to those observed in vivo Further efforts to examine signaling downstream of this receptor revealed that an inhibitor of phospholipase C (PLC) hampered ACh-induced ciliary arrest. We propose that homomeric α7-related nAChR-A7/8-1 mediates neurociliary transmission in Ciona stigmata to elicit persistent ciliary arrest by recruiting intracellular Ca2+ signaling.

De novo neurogenesis in a budding chordate: Co-option of larval anteroposterior patterning genes in a transitory neurogenic organ

During metamorphosis of solitary ascidians, part of the larval tubular nervous system is recruited to form the adult central nervous system (CNS) through neural stem-like cells called ependymal cells. The anteroposterior (AP) gene expression patterning of the larval CNS regionalize the distribution of the ependymal cells, which contains the positional information of the neurons of the adult nervous system. In colonial ascidians, the CNS of asexually developed zooids has the same morphology of the one of the post-metamorphic zooids. However, its development follows a completely different organogenesis that lacks embryogenesis, a larval phase and metamorphosis. In order to describe neurogenesis during asexual development (blastogenesis), we followed the expression of six CNS AP patterning genes conserved in chordates and five neural-related genes to determine neural cell identity in Botryllus schlosseri. We observed that a neurogenesis occurs de novo on each blastogenic cycle starting from a neurogenic transitory structure, the dorsal tube. The dorsal tube partially co-opts the AP patterning of the larval CNS markers, and potentially combine the neurogenesis role and provider of positional clues for neuron patterning. This study shows how a larval developmental module is reused in a direct asexual development in order to generate the same structures.

Biomechanics in Soft Mechanical Sensing: From Natural Case Studies to the Artificial World

Living beings use mechanical interaction with the environment to gather essential cues for implementing necessary movements and actions. This process is mediated by biomechanics, primarily of the sensory structures, meaning that, at first, mechanical stimuli are morphologically computed. In the present paper, we select and review cases of specialized sensory organs for mechanical sensing-from both the animal and plant kingdoms-that distribute their intelligence in both structure and materials. A focus is set on biomechanical aspects, such as morphology and material characteristics of the selected sensory organs, and on how their sensing function is affected by them in natural environments. In this route, examples of artificial sensors that implement these principles are provided, and/or ways in which they can be translated artificially are suggested. Following a biomimetic approach, our aim is to make a step towards creating a toolbox with general tailoring principles, based on mechanical aspects tuned repeatedly in nature, such as orientation, shape, distribution, materials, and micromechanics. These should be used for a future methodical design of novel soft sensing systems for soft robotics.

Developmental signature, synaptic connectivity and neurotransmission are conserved between vertebrate hair cells and tunicate coronal cells

In tunicates, the coronal organ represents a sentinel checking particle entrance into the pharynx. The organ differentiates from an anterior embryonic area considered a proto-placode. For their embryonic origin, morphological features and function, coronal sensory cells have been hypothesized to be homologues to vertebrate hair cells. However, vertebrate hair cells derive from a posterior placode. This contradicts one of the principle historical criteria for homology, similarity of position, which could be taken as evidence against coronal cells/hair cells homology. In the tunicates Ciona intestinalis and C. robusta, we found that the coronal organ expresses genes (Atoh, Notch, Delta-like, Hairy-b, and Musashi) characterizing vertebrate neural and hair cell development. Moreover, coronal cells exhibit a complex synaptic connectivity pattern, and express neurotransmitters (Glu, ACh, GABA, 5-HT, and catecholamines), or enzymes for their synthetic machinery, involved in hair cell activity. Lastly, coronal cells express the Trpa gene, which encodes an ion channel expressed in hair cells. These data lead us to hypothesize a model in which competence to make secondary mechanoreceptors was initially broadly distributed through placode territories, but has become confined to different placodes during the evolution of the vertebrate and tunicate lineages.

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.

Solitary Ascidians as Model Organisms in Regenerative Biology Studies

Regeneration, the process of replacing lost or damaged body parts, has long captured human imagination and is a key feature among all animal phyla. Due to their close phylogenetic relationship to vertebrates and their high regenerative abilities, ascidians (Chordata, Ascidiacea) are often used as models to shed light on the cellular and genetic process involved in tissue regeneration. Surprisingly, ascidian regeneration studies are based on only a few model species. In this chapter, we point out the important potential of solitary ascidians in regenerative and stem cell studies. We review recent studies of regeneration among solitary ascidians and discuss the cellular mechanism of tissue regeneration and the possible involvement of circulating cells in these processes. New data regarding the relationship between age and regeneration abilities of the solitary ascidian Polycarpa mytiligera (Stolidobranchia, Styelidae) are presented. The unique regeneration abilities found in P. mytiligera following evisceration of its digestive system and following amputation of its neural complex and siphon-associated structures and nerves imply on its potential to serve as a novel model system for understanding tissue regeneration.

Structure and ultrastructure of eyes and brains of Thalia democratica (Thaliacea, Tunicata, Chordata)

Salps are marine planktonic chordates that possess an obligatory alternation of reproductive modes in subsequent generations. Within tunicates, salps represent a derived life cycle and are of interest in considerations of the evolutionary origin of complex anatomical structures and life history strategies. In the present study, the eyes and brains of both the sexual, aggregate blastozooid and the asexual, solitary oozooid stage of Thalia democratica (Forskål, ) were digitally reconstructed in detail based on serial sectioning for light and transmission electron microscopy. The blastozooid stage of T. democratica possesses three pigment cup eyes, situated in the anterior ventral part of the brain. The eyes are arranged in a way that the optical axes of each eye point toward different directions. Each eye is an inverse eye that consists of two different cell types: pigment cells (pigc) and rhabdomeric photoreceptor cells (prcs). The oozooid stage of T. democratica is equipped with a single horseshoe-shaped eye, positioned in the anterior dorsal part of the brain. The opening of the horseshoe-shaped eye points anteriorly. Similar to the eyes of the blastozooid, the eye of the oozooid consists of pigment cells and rhabdomeric photoreceptor cells. The rhabdomeric photoreceptor cells possess apical microvilli that form a densely packed presumably photosensitive receptor part adjacent to the concave side of the pigc. We suggest correspondences of the individual eyes in the blastozooid stage to respective parts of the single horseshoe-shaped eye in the oozooid stage and hypothesize that the differences in visual structures and brain anatomies evolved as a result of the aggregate life style of the blastozooid as opposed to the solitary life style of the oozooid.

The nervous system of the adult ascidian Ciona intestinalis Type A (Ciona robusta): Insights from transgenic animal models

The nervous system of ascidians is an excellent model system to provide insights into the evolutionary process of the chordate nervous system due to their phylogenetic positions as the sister group of vertebrates. However, the entire nervous system of adult ascidians has yet to be functionally and anatomically investigated. In this study, we have revealed the whole dorsal and siphon nervous system of the transgenic adult ascidian of Ciona intestinalis Type A (Ciona robusta) in which a Kaede reporter gene is expressed in a pan-neuronal fashion. The fluorescent signal of Kaede revealed the innervation patterns and distribution of neurons in the nervous system of Ciona. Precise microscopic observation demonstrated the clear innervation of the anterior and posterior main nerves to eight and six lobes of the oral and atrial siphons, respectively. Moreover, visceral nerves, previously identified as unpaired nerves, were found to be paired; one nerve was derived from the posterior end of the cerebral ganglion and the other from the right posterior nerve. This study further revealed the full trajectory of the dorsal strand plexus and paired visceral nerves on either side from the cerebral ganglion to the ovary, and precise innervation between the cerebral ganglion and the peripheral organs including the gonoduct, cupular organ, rectum and ovary. The differential innervation patterns of visceral nerves and the dorsal strand plexus indicate that the peripheral organs including the ovary undergo various neural regulations. Collectively, the present anatomical analysis revealed the major innervation of the dorsal and siphon nervous systems of adult Ciona.

Early animal evolution and the origins of nervous systems

Understanding the evolution of early nervous systems is hazardous because we lack good criteria for determining homology between the systems of distant taxa; the timing of the evolutionary events is contested, and thus the relevant ecological and geological settings for them are also unclear. Here I argue that no simple approach will resolve the first issue, but that it remains likely that animals evolved relatively late, and that their nervous systems thus arose during the late Ediacaran, in a context provided by the changing planktonic and benthic environments of the time. The early trace fossil provides the most concrete evidence for early behavioural diversification, but it cannot simply be translated into increasing nervous system complexity: behavioural complexity does not map on a one-to-one basis onto nervous system complexity, both because of possible limitations to behaviour caused by the environment and because we know that even organisms without nervous systems are capable of relatively complex behaviour.

Larval nervous systems: true larval and precocious adult

The apical organ of ciliated larvae of cnidarians and bilaterians is a true larval organ that disappears before or at metamorphosis. It appears to be sensory, probably involved in metamorphosis, but knowledge is scant. The ciliated protostome larvae show ganglia/nerve cords that are retained as the adult central nervous system (CNS). Two structures can be recognized, viz. a pair of cerebral ganglia, which form the major part of the adult brain, and a blastoporal (circumblastoporal) nerve cord, which becomes differentiated into a perioral loop, paired or secondarily fused ventral nerve cords and a small perianal loop. The anterior loop becomes part of the brain. This has been well documented through cell-lineage studies in a number of spiralians, and homologies with similar structures in the ecdysozoans are strongly indicated. The deuterostomes are generally difficult to interpret, and the nervous systems of echinoderms and enteropneusts appear completely enigmatic. The ontogeny of the chordate CNS can perhaps be interpreted as a variation of the ontogeny of the blastoporal nerve cord of the protostomes, and this is strongly supported by patterns of gene expression. The presence of 'deuterostomian' blastopore fates both in an annelid and in a mollusk, which are both placed in families with the 'normal' spiralian gastrulation type, and in the chaetognaths demonstrates that the chordate type of gastrulation could easily have evolved from the spiralian type. This indicates that the latest common ancestor of the deuterostomes was very similar to the latest common pelago-benthic ancestor of the protostomes as described by the trochaea theory, and that the neural tube of the chordates is morphologically ventral.

Convergence of ion channel genome content in early animal evolution

Multicellularity has evolved multiple times, but animals are the only multicellular lineage with nervous systems. This fact implies that the origin of nervous systems was an unlikely event, yet recent comparisons among extant taxa suggest that animal nervous systems may have evolved multiple times independently. Here, we use ancestral gene content reconstruction to track the timing of gene family expansions for the major families of ion-channel proteins that drive nervous system function. We find that animals with nervous systems have broadly similar complements of ion-channel types but that these complements likely evolved independently. We also find that ion-channel gene family evolution has included large loss events, two of which were immediately followed by rounds of duplication. Ctenophores, cnidarians, and bilaterians underwent independent bouts of gene expansion in channel families involved in synaptic transmission and action potential shaping. We suggest that expansions of these family types may represent a genomic signature of expanding nervous system complexity. Ancestral nodes in which nervous systems are currently hypothesized to have originated did not experience large expansions, making it difficult to distinguish among competing hypotheses of nervous system origins and suggesting that the origin of nerves was not attended by an immediate burst of complexity. Rather, the evolution of nervous system complexity appears to resemble a slow fuse in stem animals followed by many independent bouts of gene gain and loss.

3-acetylpyridine-induced degeneration in the adult ascidian neural complex: Reactive and regenerative changes in glia and blood cells

Ascidians are interesting neurobiological models because of their evolutionary position as a sister-group of vertebrates and the high regenerative capacity of their central nervous system (CNS). We investigated the degeneration and regeneration of the cerebral ganglion complex of the ascidian Styela plicata following injection of the niacinamide antagonist 3-acetylpyridine (3AP), described as targeting the CNS of several vertebrates. For the analysis and establishment of a new model in ascidians, the ganglion complex was dissected and prepared for transmission electron microscopy (TEM), routine light microscopy (LM), immunohistochemistry and Western blotting, 1 or 10 days after injection of 3AP. The siphon stimulation test (SST) was used to quantify the functional response. One day after the injection of 3AP, CNS degeneration and recruitment of a non-neural cell type to the site of injury was observed by both TEM and LM. Furthermore, weaker immunohistochemical reactions for astrocytic glial fibrillary acidic protein (GFAP) and neuronal βIII-tubulin were observed. In contrast, the expression of caspase-3, a protein involved in the apoptotic pathway, and the glycoprotein CD34, a marker for hematopoietic stem cells, increased. Ten days after the injection of 3AP, the expression of markers tended toward the original condition. The SST revealed attenuation and subsequent recovery of the reflexes from 1 to 10 days after 3AP. Therefore, we have developed a new method to study ascidian neural degeneration and regeneration, and identified the decreased expression of GFAP and recruitment of blood stem cells to the damaged ganglion as reasons for the success of neuroregeneration in ascidians.

Evolutionary diversification of secondary mechanoreceptor cells in tunicata

BACKGROUND

Hair cells are vertebrate secondary sensory cells located in the ear and in the lateral line organ. Until recently, these cells were considered to be mechanoreceptors exclusively found in vertebrates that evolved within this group. Evidence of secondary mechanoreceptors in some tunicates, the proposed sister group of vertebrates, has recently led to the hypothesis that vertebrate and tunicate secondary sensory cells share a common origin. Secondary sensory cells were described in detail in two tunicate groups, ascidians and thaliaceans, in which they constitute an oral sensory structure called the coronal organ. Among thaliaceans, the organ is absent in salps and it has been hypothesised that this condition is due to a different feeding system adopted by this group of animals. No information is available as to whether a comparable structure exists in the third group of tunicates, the appendicularians, although different sensory structures are known to be present in these animals.

RESULTS

We studied the detailed morphology of appendicularian oral mechanoreceptors. Using light and electron microscopy we could demonstrate that the mechanosensory organ called the circumoral ring is composed of secondary sensory cells. We described the ultrastructure of the circumoral organ in two appendicularian species, Oikopleura dioica and Oikopleura albicans, and thus taxonomically completed the data collection of tunicate secondary sensory cells. To understand the evolution of secondary sensory cells in tunicates, we performed a cladistic analysis using morphological data. We constructed a matrix consisting of 19 characters derived from detailed ultrastructural studies in 16 tunicate species and used a cephalochordate and three vertebrate species as outgroups.

CONCLUSIONS

Our study clearly shows that the circumoral ring is the appendicularian homologue of the coronal organ of other tunicate taxa. The cladistic analysis enabled us to reconstruct the features of the putative ancestral hair cell in tunicates, represented by a simple monociliated cell. This cell successively differentiated into the current variety of oral mechanoreceptors in the various tunicate lineages. Finally, we demonstrated that the inferred evolutionary changes coincide with major transitions in the feeding strategies in each respective lineage.

Pattern of cell proliferation during budding in the colonial ascidian Diplosoma listerianum

Many invertebrates reproduce asexually by budding, but morphogenesis and the role of cell proliferation in this diverse and nonconserved regeneration-like process are generally poorly understood and particularly little investigated in didemnid ascidians. We here analyzed cell proliferation patterns and telomerase activity during budding in the colonial didemnid ascidian Diplosoma listerianum, with special focus on the thoracic bud where a new brain develops de novo. To help define developmental stages of the thoracic bud, the distribution of acetylated tubulin was also examined. We found extensive cell proliferation in both the thoracic and abdominal buds of D. listerianum as well as higher telomerase activity in bud tissue compared to adult tissues. In the parent adult, proliferation was found in various tissues, but was especially intense in the adult esophagus and epicardial structures that protrude into the proliferating and developing buds, confirming these tissues as the primary source of the cells that form the buds. The neural complex in the thoracic bud forms from a hollow tube that appears to separate into the neural gland and the cerebral ganglion. Whereas most of the bud undergoes proliferation, including the hollow tube and the neural gland, the cerebral ganglion shows little or no proliferation. Pulse-chase labeling experiments indicate that the ganglion, as well as the myocardium, in adult zooids are instead composed of postmitotic cells.

Generation of prolactin-like neurons in the dorsal strand of ascidians

The adult ascidian neural complex forms from a thin tube called the neurohypophyseal duct and from the primordium of the cerebral ganglion from the sensory vesicle in metamorphosing larvae. Neurohypophyseal duct cells, located in the anterior left side of the sensory vesicle of swimming larvae, are derived from the anterior embryonic neural plate, which expresses common transcription factors in vertebrates and urochordates. The cerebral ganglion primordium is probably derived from the posterior sensory vesicle during metamorphosis. After metamorphosis begins, the duct elongates anteriorly and fuses with the stomodeal ectoderm, where the dorsal tubercle, a large ciliated structure that opens into the upper part of the pharynx, later develops. The rudiment of the cerebral ganglion and the duct elongate posteriorly. The duct also differentiates into the neural gland. The dorsal wall of the neural gland in adult ascidians has a thick epithelium (placode), the central part of which forms the dorsal strand by repeated invaginations along the visceral nerve. Both gonadotropin-releasing hormone (GnRH) neurons and prolactin-like (non-GnRH) neurons are generated in the dorsal strand and migrate to the cerebral ganglion along the visceral nerve throughout adulthood. Thus, the epithelium derived from the neurohypophyseal duct possesses neurogenic potential to generate neural stem cells of the central (cerebral ganglion) and peripheral (dorsal strand) nervous systems. The generation of prolactin-like neurons and their migration into the brain with GnRH neurons suggest that the ascidian dorsal strand is homologous to the craniate olfactory placode, and provide unequivocal support for the existence of the clade Olfactores.

Variability of hair cells in the coronal organ of ascidians (Chordata, Tunicata)

The tunicate ascidians are nonvertebrate chordates that possess mechanoreceptor cells in the coronal organ in the oral siphon, which monitor the incoming water flow. Like vertebrate hair cells, the mechanoreceptor–coronal cells are secondary sensory (axonless) cells accompanied by supporting cells and they exhibit morphological diversities of apical specialisations: they are multiciliate in ascidians of the order Enterogona, whereas they are more complex and possess one or two cilia accompanied by stereovilli, also graded in length, in ascidians of the order Pleurogona. In morphology, embryonic origin, and arrangement, coronal sensory cells closely resemble vertebrate hair cells. We describe here the coronal organs of five ascidians ( Pyura haustor (Stimpson, 1864), Pyura stolonifera (Heller, 1878), Styela gibbsii (Stimpson, 1864), Styela montereyensis (Dall, 1872), and Polyandrocarpa zorritensis (Van Name, 1931)), belonging to Pleurogona, also comprising species of one family (Pyuridae), not yet considered, and thus completing our overview of the order. Each species possesses at least two kinds of secondary sensory cells, some of them characterized by stereovilli graded in length. In some species, the coronal sensory cells exhibit secretory activity; in P. haustor, a mitotic sensory cell has also been found. We compare the coronal organ in both ascidians and with other chordate sensory organs formed of secondary sensory cells, and discuss their possible homologies.

Natural apoptosis during the blastogenetic cycle of the colonial ascidian Botryllus schlosseri: a morphological analysis

Colonies of the compound ascidian Botryllus schiosseri undergo regular generation changes, during which adult zooids are progressively resorbed and replaced by growing buds. The generation change, or take-over, is characterized by massive cell death by apoptosis, as indicated by nuclear condensation, activation of caspases, overexpression of molecules recognized by antibodies against mammalian Bax, Fas, and FasL, changes in the expression of surface molecules by senescent cells of zooid tissues, and recruitment of circulating phagocytes in zooid tissues which ensure the complete clearing of dying cells. The entire process lasts 24-36 h at 20 degrees C and has been subdivided, on the basis of the degree of contraction of old zooids, into four substages. In the present work, we carried out a detailed morphological analysis of the events occurring in zooid tissues during the take-over substages. Results Indicate that traces of apoptosis can be found in the epidermis, peribranchial epithelium, and heart in the late substage but are easily found in the branchial basket 2-4 h after the beginning of the generation change, thus confirming the antero-posterior progression of cell death, at least in the alimentary system.

Placode formation and generation of gonadotropin-releasing hormone (GnRH) neurons in ascidians

Neurogenic placodes, a chordate innovation, generate several neuronal populations, including gonadotropin-releasing hormone (GnRH) neurons which are crucial for vertebrate and solitary ascidian urochordate reproduction. The dorsal strand placode of ascidians Is derived from the anterior ridge of the embryonic neural plate and thus shares a common developmental origin and expression of various transcription factors with vertebrate placodes. Despite their importance for understanding vertebrate origins, the evolutionary and developmental origins of the neurogenic placode remain obscure. Here I demonstrate the formation of an elaborate neurogenic placode, which forms the dorsal strand, on part of the neural gland epithelium in a solitary ascidian urochordate, Halocynthia roretzi. Two modes of GnRH neurogenesis in the dorsal strand (a peripheral organ) and the migration of GnRH neurons into the brain along the visceral nerve are also described. Ontogenetically, GnRH neurons are first detected in the dorsal strand and cerebral ganglion of very young Juveniles at almost the same time, demonstrating that ascidians possess morphological and developmental features in common with vertebrates. These results further indicate that the onset of peripheral GnRH neurogenesis and the ability of neurons to migrate into the brain predate the divergence of ascidians and vertebrates. Thus, based on the generation of GnRH neurons, the dorsal strand in ascidians may be homologous to the vertebrate olfactory placode. These organs are derived from the anterior region of the embryonic neural plate, which expresses several transcription factors that invertebrate chordates and vertebrates have in common. These results provide unequivocal support for the clade Olfactories (tunicates + vertebrates).

Sensory cells associated with the tentacular tunic of the ascidian Polyandrocarpa misakiensis (Tunicata: Ascidiacea)

New probable mechanosensory cell groups were found in Polyandrocarpa misakiensis. In this species, the tunic with epithelium penetrates into the oral and atrial tentacles (oral and atrial tentacular tunic), which is continuous with a tunic layer intervening between the descending and ascending epithelium of the siphons. In the oral tentacles, the tunic only extends into the basal part, but in the atrial siphon the tunic extends the full length of the tentacle. Intraepithelial sensory cells were found in the basal part of the oral and atrial tentacular tunic. These sensory cells are usually solitary or paired in the atrial tentacles, and a few cells are grouped together in the oral tentacles. In the oral tentacles, the apicolateral parts of the sensory cells are joined together by two types of junctions, i.e., adherent junctions and modified tight junctions. The supporting cells and sensory cells are connected by adherent junctions. Certain sensory cells are coupled to what seem to be neurosecretory cells. The sensory cells have one apical cilium surrounded by microvilli and a basal axonal process. The apical cytoplasm of the sensory cells has a few organelles. The basal cytoplasm of the sensory cells is dense with rough endoplasmic reticulum, mitochondria, lipid droplets, and dense bodies. The morphology of these sensory cells is comparable with that of the ciliated intraepithelial sensory neurons found in many tunicates, such as those of cupular organ and capusular organ.

Does hair cell differentiation predate the vertebrate appearance?

It is generally accepted that the three main chordate groups (tunicates, cephalochordates and vertebrates) originated from a common ancestor having the basic features of the chordate body plan, i.e. a neural tube and a notochord flanked by striated musculature. There is now increasing evidence that tunicates, rather than cephalochordates, are the vertebrate sister-group. Correlated with this, tunicates have sensory structures similar to those derived from placodes or neural crest in vertebrates. In this context, we discuss here whether the precursors of vertebrate hair cells, which are placodal in origin, were present in ancestral chordates. The ascidian tunicates possess a coronal organ, consisting of a row of mechanosensory cells that runs around the base of the oral siphon. Its function is to monitor the incoming water flow. The cells are secondary sensory cells, i.e. they lack axons and synapse with neurons whose somata lie in the cerebral ganglion. They are accompanied by supporting cells and, as in vertebrates, have varying morphologies in the species so far examined: in one order (Enterogona), they are multiciliate; in the other (Pleurogona), they may possess an apical apparatus, consisting of one or two cilia accompanied by stereovilli, that are graded in length. Coronal cells thus resemble vertebrate hair cells closely in their morphology, embryonic origin and arrangement, which suggests they originated early in ancestral chordates. We are continuing our study of the coronal organ in other ascidian species, and report new data here on Botrylloides leachi, which conforms with the pattern of Pleurogona and, in particular, with previously published results on other botryllid ascidians.

Localization of CiCBR in the invertebrate chordate Ciona intestinalis: evidence of an ancient role for cannabinoid receptors as axonal regulators of neuronal signalling

CiCBR is a G-protein-coupled receptor in the sea-squirt Ciona intestinalis and the first ortholog of vertebrate CB(1) and CB(2) cannabinoid receptors to be identified in an invertebrate (Elphick et al. [2003] Gene 302:95-101). Here we have used Western blotting and immunocytochemistry to examine expression of CiCBR in adult Ciona, employing novel antibodies to the C-terminal tail of CiCBR. Consistent with the expected mass for CiCBR, a approximately 47-kDa band was detected in Ciona membranes, and immunocytochemical analysis of serial sections of Ciona revealed intense immunoreactivity in the cerebral ganglion localised in a dense meshwork of fibers in the neuropile. Accordingly, Western blot analysis of neural complex homogenates revealed the presence of a approximately 47-kDa band. CiCBR immunoreactivity was also observed in axons exiting the ganglion in the anterior and posterior nerves, and analysis of whole-mount preparations revealed that these axons project over the interior surface of the oral and atrial siphons. Isolated CiCBR-immunoreactive axons not associated with the anterior and posterior nerves were observed projecting through the cortical layer of the cerebral ganglion. Central and peripheral CiCBR-immunoreactive fibers were studded with intensely stained varicosities, indicative of a role for CiCBR in regulation of axonal release of neurotransmitters, neuromodulators, or neurohormones. Collectively, our data suggest that the well-established role that the CB(1) receptor has as an axonal regulator of neurotransmitter release in mammals may have originated with ancestral-type cannabinoid receptors in invertebrate chordates before the emergence of CB(1)- and CB(2)-type receptors in vertebrates.

Botryllus schlosseri: a model ascidian for the study of asexual reproduction

Botryllus schlosseri, a cosmopolitan colonial ascidian reared in the laboratory for more than 50 years, reproduces both sexually and asexually and is used as a model organism for studying a variety of biological problems. Colonies are formed of numerous, genetically identical individuals (zooids) and undergo cyclical generation changes in which the adult zooids die and are replaced by their maturing buds. Because the progression of the colonial life cycle is intimately correlated with blastogenesis, a shared staging method of bud development is required to compare data coming from different laboratories. With the present review, we aim (1) to introduce B. schlosseri as a valuable chordate model to study various biological problems and, especially, sexual and asexual development; (2) to offer a detailed description of bud development up to adulthood and the attainment of sexual maturity; (3) to re-examine Sabbadin's (1955) staging method and re-propose it as a simple tool for in vivo recognition of the main morphogenetic events and recurrent changes in the blastogenetic cycle, as it refers to the developmental stages of buds and adults.

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.

Innervation of ascidian siphons and their responses to stimulation

The distribution of sensory cells and nerves was studied in the siphons of Corella inflata Huntsman, 1912 and Corella willmeriana Herdman, 1898 by immunohistology and electron microscopy. Each siphon has about 8000 primary sensory neurons. A coronal organ of the compound type is present on the oral tentacles. Convergence in the afferent pathway is estimated at >10:1. A new category of cells associated with the velar sphincter muscle is described at the tentacle bases. Responses to stimulation were recorded using flow meters. Both siphons are sensitive to touch and near-field vibrations. Removal of the oral tentacles did not diminish vibration sensitivity. Gentle stimulation of the oral siphon evokes crossed responses in which the atrial siphon closes and the velar sphincter contracts. Stronger stimulation produces squirts with closure of both siphons and branchial ciliary arrest. Experiments with polystyrene beads show that the oral tentacles are sensitive to contact with inflowing particles. Beads of 500–600 μm diameter evoked rejection responses 88% of the time, 355–425 μm beads 61%, and beads <125 μm less than 8%. These responses, attributed to the coronal organ, were lost after amputation of the tentacles. Electrophysiology confirmed that crossed responses and squirting are centrally mediated reflexes, but local conduction pathways also exist and survive deganglionation.

ADAPTIVE ROLES OF PROGRAMMED CELL DEATH DURING NERVOUS SYSTEM DEVELOPMENT

The programmed cell death (PCD) of developing cells is considered an essential adaptive process that evolved to serve diverse roles. We review the putative adaptive functions of PCD in the animal kingdom with a major focus on PCD in the developing nervous system. Considerable evidence is consistent with the role of PCD in events ranging from neurulation and synaptogenesis to the elimination of adult-generated CNS cells. The remarkable recent progress in our understanding of the genetic regulation of PCD has made it possible to perturb (inhibit) PCD and determine the possible repercussions for nervous system development and function. Although still in their infancy, these studies have so far revealed few striking behavioral or functional phenotypes.

Coronal organ of ascidians and the evolutionary significance of secondary sensory cells in chordates

A new mechanoreceptor organ, the coronal organ, in the oral siphon of some ascidians belonging to the order Pleurogona has recently been described. In contrast to the known mechanoreceptor organs of ascidian atrium that consist of sensory neurons sending their own axons to the cerebral ganglion, coronal sensory cells are secondary mechanoreceptors, i.e., axonless cells forming afferent and efferent synapses with neurites of neurons located in the ganglion. Moreover, coronal cells exhibit an apical apparatus composed of a cilium accompanied or flanked by rod-like microvilli (stereovilli). Because of the resemblance of these cells to vertebrate hair cells, their ectodermal origin and location in a linear array bordering the bases of the oral tentacles and velum, the coronal organ has been proposed as a homologue to the vertebrate acousticolateralis system. Here we describe the morphology of the coronal organs of six ascidians belonging to the suborders Phlebobranchia and Aplousobranchia (order Enterogona). The sensory cells are ciliated, lack typical stereovilli, and at their bases form synapses with neurites. In two species, the sensory cells are accompanied by large cells involved in synthesis and secretion of protein. We hypothesize that the coronal organ with its secondary sensory cells represents a plesiomorphic feature of ascidians. We compare the coronal organ with other chordate sensory organs formed of secondary sensory cells, i.e., the ventral lip receptors of appendicularians, the oral secondary sensory cells of cephalochordates, and the acousticolateralis system of vertebrates, and we discuss their homologies at different levels of organization.

Common and divergent pathways in alternative developmental processes of ascidians

Colonial ascidians offer opportunities to investigate how developmental events are integrated to generate the animal form, since they can develop similar individuals (oozooids from eggs, blastozooids from pluripotent somatic cells) through very different reproductive processes, i.e. embryogenesis and blastogenesis. Moreover, thanks to their key phylogenetic position, they can help in the understanding of the molecular mechanisms of morphogenesis and their evolution in chordates. We review organogenesis of the ascidian neural complex comparing embryos and buds in terms of topology, developmental mechanisms and terminology. We propose a new interpretation of bud territories, and reconsider nervous system development based on recent results suggesting that ascidians have vertebrate placodal and neural-crest-like cells. Comparing embryonic and blastogenic development in Botryllus schlosseri, we propose that the bud has territories with a placodal potentiality, suggesting that chordate ancestors possessed neurogenic placodes, and that the genetic pathways regulating neurogenic placode formation were co-opted for new developmental processes, such as blastogenesis.

Stomodeal and neurohypophysial placodes in Ciona intestinalis: insights into the origin of the pituitary gland

The ascidian larva has a central nervous system which shares basic characteristics with craniates, such as tripartite organisation and many developmental genes. One difference, at metamorphosis, is that this chordate-like nervous system regresses and the adult's neural complex, composed of the cerebral ganglion and associated neural gland, forms. It is known that neural complex differentiation involves two ectodermal structures, the neurohypophysial duct, derived from the embryonic neural tube, and the stomodeum, i.e. the rudiment of the oral siphon; nevertheless, their precise role remains to be clarified. We have shown that in Ciona intestinalis, the neural complex primordium is the neurohypophysial duct, which in the early larva is a short tube, blind anteriorly, with its lumen in continuity with that of the central nervous system, i.e. the sensory vesicle. The tube grows forwards and fuses with the posterior wall of the stomodeum, a dorsal ectodermal invagination of the larva. The duct then loses posterior communication with the sensory vesicle and begins to grow on the roof of the vesicle itself. The neurohypophysial duct differentiates into the neural gland rudiment; its dorsal wall begins to proliferate neuroblasts, which migrate and converge to build up the cerebral ganglion. The most anterior part of the neural gland organizes into the ciliated duct and funnel, whereas the most posterior part elongates and gives rise to the dorsal strand. The hypothesis that the neurohypophysial duct/stomodeum complex possesses cell populations homologous to the craniate olfactory and adenohypophysial placodes and hypothalamus is discussed.

Evolutionary origins of vertebrate placodes: insights from developmental studies and from comparisons with other deuterostomes

Ectodermal placodes comprise the adenohypophyseal, olfactory, lens, profundal, trigeminal, otic, lateral line, and epibranchial placodes. The first part of this review presents a brief overview of placode development. Placodes give rise to a variety of cell types and contribute to many sensory organs and ganglia of the vertebrate head. While different placodes differ with respect to location and derivative cell types, all appear to originate from a common panplacodal primordium, induced at the anterior neural plate border by a combination of mesodermal and neural signals and defined by the expression of Six1, Six4, and Eya genes. Evidence from mouse and zebrafish mutants suggests that these genes promote generic placodal properties such as cell proliferation, cell shape changes, and specification of neurons. The common developmental origin of placodes suggests that all placodes may have evolved in several steps from a common precursor. The second part of this review summarizes our current knowledge of placode evolution. Although placodes (like neural crest cells) have been proposed to be evolutionary novelties of vertebrates, recent studies in ascidians and amphioxus have proposed that some placodes originated earlier in the chordate lineage. However, while the origin of several cellular and molecular components of placodes (e.g., regionalized expression domains of transcription factors and some neuronal or neurosecretory cell types) clearly predates the origin of vertebrates, there is presently little evidence that these components are integrated into placodes in protochordates. A scenario is presented according to which all placodes evolved from an adenohypophyseal-olfactory protoplacode, which may have originated in the vertebrate ancestor from the anlage of a rostral neurosecretory organ (surviving as Hatschek's pit in present-day amphioxus).

Historical introduction, overview, and reproductive biology of the protochordates

This issue of the Canadian Journal of Zoology exhaustively reviews most major aspects of protochordate biology by specialists in their fields. Protochordates are members of two deuterostome phyla that are exclusively marine. The Hemichordata, with solitary enteropneusts and colonial pterobranchs, share a ciliated larva with echinoderms and appear to be closely related, but they also have many chordate-like features. The invertebrate chordates are composed of the exclusively solitary cephalochordates and the tunicates with both solitary and colonial forms. The cephalochordates are all free-swimming, but the tunicates include both sessile and free-swimming forms. Here I explore the history of research on protochordates, show how views on their relationships have changed with time, and review some of their reproductive and structural traits not included in other contributions to this special issue.