Organization and metamorphic remodeling of the nervous system in juveniles of Phoronopsis harmeri (Phoronida): insights into evolution of the bilaterian nervous system

Peripheral and central employment of acid-sensing ion channels during early bilaterian evolution

Nervous systems are endowed with rapid chemosensation and intercellular signaling by ligand-gated ion channels (LGICs). While a complex, bilaterally symmetrical nervous system is a major innovation of bilaterian animals, the employment of specific LGICs during early bilaterian evolution is poorly understood. We therefore questioned bilaterian animals' employment of acid-sensing ion channels (ASICs), LGICs that mediate fast excitatory responses to decreases in extracellular pH in vertebrate neurons. Our phylogenetic analysis identified an earlier emergence of ASICs from the overarching DEG/ENaC (degenerin/epithelial sodium channel) superfamily than previously thought and suggests that ASICs were a bilaterian innovation. Our broad examination of ASIC gene expression and biophysical function in each major bilaterian lineage of Xenacoelomorpha, Protostomia, and Deuterostomia suggests that the earliest bilaterian ASICs were probably expressed in the periphery, before being incorporated into the brain as it emerged independently in certain deuterostomes and xenacoelomorphs. The loss of certain peripheral cells from Ecdysozoa after they separated from other protostomes likely explains their loss of ASICs, and thus the absence of ASICs from model organisms Drosophila and Caenorhabditis elegans. Thus, our use of diverse bilaterians in the investigation of LGIC expression and function offers a unique hypothesis on the employment of LGICs in early bilaterian evolution.

Developmental atlas of the indirect-developing sea urchin Paracentrotus lividus: From fertilization to juvenile stages

The sea urchin Paracentrotus lividus has been used as a model system in biology for more than a century. Over the past decades, it has been at the center of a number of studies in cell, developmental, ecological, toxicological, evolutionary, and aquaculture research. Due to this previous work, a significant amount of information is already available on the development of this species. However, this information is fragmented and rather incomplete. Here, we propose a comprehensive developmental atlas for this sea urchin species, describing its ontogeny from fertilization to juvenile stages. Our staging scheme includes three periods divided into 33 stages, plus 15 independent stages focused on the development of the coeloms and the adult rudiment. For each stage, we provide a thorough description based on observations made on live specimens using light microscopy, and when needed on fixed specimens using confocal microscopy. Our descriptions include, for each stage, the main anatomical characteristics related, for instance, to cell division, tissue morphogenesis, and/or organogenesis. Altogether, this work is the first of its kind providing, in a single study, a comprehensive description of the development of P. lividus embryos, larvae, and juveniles, including details on skeletogenesis, ciliogenesis, myogenesis, coelomogenesis, and formation of the adult rudiment as well as on the process of metamorphosis in live specimens. Given the renewed interest for the use of sea urchins in ecotoxicological, developmental, and evolutionary studies as well as in using marine invertebrates as alternative model systems for biomedical investigations, this study will greatly benefit the scientific community and will serve as a reference for specialists and non-specialists interested in studying sea urchins.

Novel data on the innervation of the lophophore in adult phoronids (Lophophorata, Phoronida)

The structure of the lophophore nervous system may help clarify the status of the clade Lophophorata, whose monophyly is debated. In the current study, antibody labeling and confocal laser scanning microscopy revealed previously undescribed main nerve elements in the lophophore in adult phoronids: Phoronis australis and Phoronopsis harmeri. In both species, the nervous system includes a dorsal ganglion, a tentacle nerve ring, an inner nerve ring, intertentacular groups of perikarya, and tentacle nerves. The dorsal ganglion and tentacle nerve ring contain many serotonin-like immunoreactive perikarya of different sizes. The inner nerve ring is described for the first time in adult phoronids with complex lophophore. It contains a thin bundle of serotonin-like immunoreactive neurites. The tentacles possess abfrontal, frontal, and laterofrontal nerves. The abfrontal nerves originate from the tentacle nerve ring; the frontal tentacle nerves extend from the inner nerve ring in P. harmeri and from the intertentacular frontal nerves in P. australis. The intertentacular groups of perikarya are found in phoronids for the first time. These small nerve centers connect with neither the tentacle nerve ring nor the inner nerve ring, giving rise to the laterofrontal tentacle nerves. The discovery of the inner nerve ring in adult phoronids makes the architecture of the lophophore nervous system similar in all lophophorates and thereby supports the monophyly of this group. The presence of intertentacular nerves, perikarya, and groups of perikarya is a typical feature of the nervous system in lophophorate presumably coordinating movements of the tentacles and thereby increasing the efficiency of lophophore functioning.

Neurons and Glia Cells in Marine Invertebrates: An Update

The nervous system (NS) of invertebrates and vertebrates is composed of two main types of cells: neurons and glia. In both types of organisms, nerve cells have similarities in biochemistry and functionality. The neurons are in charge of the synapse, and the glial cells are in charge of important functions of neuronal and homeostatic modulation. Knowing the mechanisms by which NS cells work is important in the biomedical area for the diagnosis and treatment of neurological disorders. For this reason, cellular and animal models to study the properties and characteristics of the NS are always sought. Marine invertebrates are strategic study models for the biological sciences. The sea slug Aplysia californica and the squid Loligo pealei are two examples of marine key organisms in the neurosciences field. The principal characteristic of marine invertebrates is that they have a simpler NS that consists of few and larger cells, which are well organized and have accessible structures. As well, the close phylogenetic relationship between Chordata and Echinodermata constitutes an additional advantage to use these organisms as a model for the functionality of neuronal cells and their cellular plasticity. Currently, there is great interest in analyzing the signaling processes between neurons and glial cells, both in vertebrates and in invertebrates. However, only few types of glial cells of invertebrates, mostly insects, have been studied, and it is important to consider marine organisms' research. For this reason, the objective of the review is to present an update of the most relevant information that exists around the physiology of marine invertebrate neuronal and glial cells.

Evolution of the bilaterian mouth and anus

It is widely held that the bilaterian tubular gut with mouth and anus evolved from a simple gut with one major gastric opening. However, there is no consensus on how this happened. Did the single gastric opening evolve into a mouth, with the anus forming elsewhere in the body (protostomy), or did it evolve into an anus, with the mouth forming elsewhere (deuterostomy), or did it evolve into both mouth and anus (amphistomy)? These questions are addressed by the comparison of developmental fates of the blastopore, the opening of the embryonic gut, in diverse animals that live today. Here we review comparative data on the identity and fate of blastoporal tissue, investigate how the formation of the through-gut relates to the major body axes, and discuss to what extent evolutionary scenarios are consistent with these data. Available evidence indicates that stem bilaterians had a slit-like gastric opening that was partially closed in subsequent evolution, leaving open the anus and most likely also the mouth, which would favour amphistomy. We discuss remaining difficulties, and outline directions for future research.

Innervation of the lophophore suggests that the phoronid Phoronis ovalis is a link between phoronids and bryozoans

The validity of the Lophophorata as a monophyletic group remains controversial. New data on the innervation of the lophophore, which is a unique feature of the lophophorates, may help clarify the status of the Lophophorata and provide new information on the early evolution of the group. In this paper, the organization of the nervous system of the lophophore is described in adults of the minute phoronid Phoronis ovalis. The lophophore nervous system includes a dorsal ganglion, a tentacular nerve ring, an inner ganglion, an inner nerve ring, and six nerves in each tentacle. The inner ganglion and inner nerve ring, which is associated with sensory cells, are described for the first time in adult phoronids. The general plan of the nervous system of the lophophore and tentacles is similar in P. ovalis and bryozoans. These new results suggest the presence of two nerve centers and two nerve rings in the last common ancestor of phoronids and bryozoans. During evolution, bryozoans may have lost the outer nerve center and outer nerve ring, whereas phoronids may have lost the inner nerve center and inner nerve ring. These morphological results evidence the lophophorates are monophyletic.

The first data on the innervation of the lophophore in the rhynchonelliform brachiopod Hemithiris psittacea: what is the ground pattern of the lophophore in lophophorates?

Background

The nervous system in brachiopods has seldom been studied with modern methods. An understanding of lophophore innervation in adult brachiopods is useful for comparing the innervation of the same lophophore type among different brachiopods and can also help answer questions about the monophyly of the lophophorates. Although some brachiopods are studied with modern methods, rhynchonelliform brachiopods still require investigation. The current study used transmission electron microscopy, immunocytochemistry, and confocal laser scanning microscopy to investigate the nerve system of the lophophore and tentacles in the rhynchonelliform Hemithiris psittacea.

Results

Four longitudinal nerves pass along each brachium of the lophophore: the main, accessory, second accessory, and lower. The main brachial nerve extends at the base of the dorsal side of the brachial fold and gives rise to the cross nerves, passing through the extracellular matrix to the tentacles. Cross nerves skirt the accessory brachial nerve, branch, and penetrate into adjacent outer and inner tentacles, where they are referred to as the frontal tentacular nerves. The second accessory nerve passes along the base of the inner tentacles. This nerve consists of Ʊ-like parts, which repetitively skirt the frontal and lateral sides of the inner tentacle and the frontal sides of the outer tentacles. The second accessory nerve gives rise to the latero-frontal nerves of the inner and outer tentacles. The abfrontal nerves of the inner tentacles also originate from the second accessory nerve, whereas the abfrontal nerves of the outer tentacles originate from the lower brachial nerve. The lower brachial nerve extends along the outer side of the lophophore brachia and gives rise to the intertentacular nerves, which form a T-like branch and penetrate the adjacent outer tentacles where they are referred to as abfrontal nerves. The paired outer radial nerves start from the lower brachial nerve, extend into the second accessory nerve, and give rise to the lateroabfrontal tentacular nerves of the outer tentacles.

Conclusions

The innervation of the lophophore in the rhynchonelliform Hemithiris psittacea differs from that in the inarticulate Lingula anatina in several ways. The accessory brachial nerve does not participate in the innervation of the tentacles in H. psittacea as it does in L. anatina. The second accessory nerve is present in H. psittacea but not in L. anatina. There are six tentacular nerves in the outer tentacles of H. psittacea but only four in all other brachiopods studied to date. The reduced contribution of the accessory brachial nerve to tentacle innervation may reflect the general pattern of reduction of the inner lophophoral nerve in both phoronids and brachiopods. Bryozoan lophophores, in contrast, have a weakened outer nerve and a strengthened inner nerve. Our results suggest that the ancestral lophophore of all lophophorates had a simple shape but many nerve elements.

Development of the nervous system in Platynereis dumerilii (Nereididae, Annelida)

Background

The structure and development of the nervous system in Lophotrochozoa has long been recognized as one of the most important subjects for phylogenetic and evolutionary discussion. Many recent papers have presented comprehensive data on the structure and development of catecholaminergic, serotonergic and FMRFamidergic parts of the nervous system. However, relatively few papers contain detailed descriptions of the nervous system in Annelida, one of the largest taxa of Lophotrochozoa. The polychaete species Platynereis dumerilii has recently become one of the more popular model animals in evolutionary and developmental biology. The goal of the present study was to provide a detailed description of its neuronal development. The data obtained will contribute to a better understanding of the basic features of neuronal development in polychaetes.

Results

We have studied the development of the nervous system in P. dumerilii utilizing histo- and immunochemical labelling of catecholamines, serotonin, FMRFamide related peptides, and acetylated tubulin. The first neuron differentiates at the posterior extremity of the protrochophore, reacts to the antibodies against both serotonin and FMRFamide. Then its fibres run forwards along the ventral side. Soon, more neurons appear at the apical extreme, and their basal neurites form the basel structure of the developing brain (cerebral neuropil and circumesophageal connectives). Initial development of the nervous system starts in two rudiments: anterior and posterior. At the nectochaete stage, segmental ganglia start to differentiate in the anterior-to-posterior direction, and the first structures of the stomatogastric and peripheral nervous system appear. All connectives including the unpaired ventral cord develop from initially paired nerves.

Conclusions

We present a detailed description of Platynereis dumerilii neuronal development based on anti-acetylated tubulin, serotonin, and FMRFamide-like immunostaining as well as catecholamine histofluorescence. The development of the nervous system starts from peripheral pioneer neurons at both the posterior and anterior poles of the larva, and their neurites form a scaffold upon which the adult central nervous system develops. The anterior-to-posterior mode of the ventral ganglia development challenges the primary heteronomy concept. Comparison with the development of Mollusca reveals substantial similarities with early neuronal development in larval Solenogastres.

Electronic supplementary material

The online version of this article (doi:10.1186/s12983-017-0211-3) contains supplementary material, which is available to authorized users.

Distribution of serotonin and FMRF-amide in the nervous system of different zooidal types of cheilostome bryozoa: A case study of Arctonula arctica

In the White Sea bryozoans Arctonula arctica, the structure of the nervous system and distribution of 5-hydroxytryptamine (5-HT) and FMRF-amide were studied for the first time using immunohistochemical methods and confocal scanning microscopy. The neurotransmitters studied have been actively involved into the integrative processes, gut functioning, and regulation of motion activity. In avicularia, 5-HT and FMRF-amide receptors are capable of performing the same functions, except for participation in the gut functioning, because they have no digestive system.

The nervous system of the lophophore in the ctenostome Amathia gracilis provides insight into the morphology of ancestral ectoprocts and the monophyly of the lophophorates

BACKGROUND

The Bryozoa (=Ectoprocta) is a large group of bilaterians that exhibit great variability in the innervation of tentacles and in the organization of the cerebral ganglion. Investigations of bryozoans from different groups may contribute to the reconstruction of the bryozoan nervous system bauplan. A detailed investigation of the polypide nervous system of the ctenostome bryozoan Amathia gracilis is reported here.

RESULTS

The cerebral ganglion displays prominent zonality and has at least three zones: proximal, central, and distal. The proximal zone is the most developed and contains two large perikarya giving rise to the tentacle sheath nerves. The neuroepithelial organization of the cerebral ganglion is revealed. The tiny lumen of the cerebral ganglion is represented by narrow spaces between the apical projections of the perikarya of the central zone. The cerebral ganglion gives rise to five groups of main neurite bundles of the lophophore and the tentacle sheath: the circum-oral nerve ring, the lophophoral dorso-lateral nerves, the pharyngeal and visceral neurite bundles, the outer nerve ring, and the tentacle sheath nerves. Serotonin-like immunoreactive nerve system of polypide includes eight large perikarya located between tentacles bases. There are two analmost and six oralmost perikarya with prominent serotonergic "gap" between them. Based on the characteristics of their innervations, the tentacles can be subdivided into two groups: four that are near the anus and six that are near the mouth. Two longitudinal neurite bundles - medio-frontal and abfrontal - extend along each tentacle.

CONCLUSION

The zonality of the cerebral ganglion, the presence of three commissures, and location of the main nerves emanating from each zone might have caused by directive innervation of the various parts of the body: the tentacles sheath, the lophohpore, and the digestive tract. Two alternative scenarios of bryozoan lophophore evolution are discussed. The arrangement of large serotonin-like immunoreactive perikarya differs from the pattern previously described in ctenostome bryozoans. In accordance with its position relative to the same organs (tentacles, anus, and mouth), the lophophore outer nerve ring corresponds to the brachiopod lower brachial nerve and to the phoronid tentacular nerve ring. The presence of the outer nerve ring makes the lophophore innervation within the group (clade) of lophophorates similar and provides additional morphological evidence of the lophophore homology and monophyly of the lophophorates.

Report on the 13th symposium on invertebrate neurobiology held 26-30 August 2015 at the Balaton Limnological Institute, MTA Centre for ecological research of the Hungarian Academy of Sciences, Tihany, Hungary

This report summarizes the lectures and posters presented at the International Society for Invertebrate Neurobiology's 13th symposium held 26-30 August 2015, at the Balaton Limnological Institute, MTA Centre for Ecological Research, Tihany, Hungary. The symposium provided an opportunity for scientists working on a range of topics in invertebrate neurobiology to meet and present their research and discuss ways to advance the discipline.

Neuromuscular development in Patellogastropoda (Mollusca: Gastropoda) and its importance for reconstructing ancestral gastropod bodyplan features

Within Gastropoda, limpets (Patellogastropoda) are considered the most basal branching taxon and its representatives are thus crucial for research into evolutionary questions. Here, we describe the development of the neuromuscular system in Lottia cf. kogamogai. In trochophore larvae, first serotonin‐like immunoreactivity (lir) appears in the apical organ and in the prototroch nerve ring. The arrangement and number of serotonin‐lir cells in the apical organ (three flask‐shaped, two round cells) are strikingly similar to those in putatively derived gastropods. First, FMRFamide‐lir appears in veliger larvae in the Anlagen of the future adult nervous system including the cerebral and pedal ganglia. As in other gastropods, the larvae of this limpet show one main and one accessory retractor as well as a pedal retractor and a prototroch muscle ring. Of these, only the pedal retractor persists until after metamorphosis and is part of the adult shell musculature. We found a hitherto undescribed, paired muscle that inserts at the base of the foot and runs towards the base of the tentacles. An apical organ with flask‐shaped cells, one main and one accessory retractor muscle is commonly found among gastropod larvae and thus might have been part of the last common ancestor.

The lophophore innervation pattern of the inarticulate brachiopod Lingula anatina (Brachiopoda) supports monophyly of Lophophorata

Lophophore innervation in the brachiopod Lingula anatina has been investigated using immunocytochemistry and laser confocal microscopy. Three prominent nerves, namely, the main brachial nerve, the accessory brachial nerve, and the lower brachial nerve, have been found to extend along each brachium of the lophophore. Tentacle innervation is also described in detail. Comparative analysis revealed homologous nerves in lophophores of brachiopods, phoronids, and bryozoans. Similarities in tentacle innervation in these phyla of invertebrates have been detected. The results obtained confirm lophophore homology in different groups of lophophorates and provide evidence for monophyly of Lophophorata.

Metamorphic remodeling of morphology and the body cavity in Phoronopsis harmeri (Lophotrochozoa, Phoronida): the evolution of the phoronid body plan and life cycle

BACKGROUND

Phoronids undergo a remarkable metamorphosis, in which some parts of the larval body are consumed by the juvenile and the body plan completely changes. According to the only previous hypothesis concerning the evolution of the phoronid body plan, a hypothetical ancestor of phoronids inhabited a U-shaped burrow in soft sediment, where it drew the anterior and posterior parts of the body together and eventually fused them. In the current study, we investigated the metamorphosis of Phoronopsis harmeri with light, electron, and laser confocal microscopy.

RESULTS

During metamorphosis, the larval hood is engulfed by the juvenile; the epidermis of the postroral ciliated band is squeezed from the tentacular epidermis and then engulfed; the larval telotroch undergoes cell death and disappears; and the juvenile body forms from the metasomal sack of the larva. The dorsal side of the larva becomes very short, whereas the ventral side becomes very long. The terminal portion of the juvenile body is the ampulla, which can repeatedly increase and decrease in diameter. This flexibility of the ampulla enables the juvenile to dig into the sediment. The large blastocoel of the larval collar gives rise to the lophophoral blood vessels of the juvenile. The dorsal blood vessel of the larva becomes the definitive median blood vessel. The juvenile inherits the larval protocoel, mesocoel, and metacoel. Late in metamorphosis, however, the protocoel loses its epithelial structure: the desmosomes between cells and the basal lamina under the cells disappear. This loss may reflect a reduction of the protocoel, which is a characteristic of some recent phoronids.

CONCLUSIONS

Based on our investigation of P. harmeri metamorphosis, we hypothesize that the phoronid ancestor was worm-like animal that possessed preoral, tentacular, and trunk coeloms. It lived on the soft sediment and collected food with its tentacles. When threatened, this worm-like ancestor buried itself in the soft sediment by means of the ventral protrusion into which the loop of the intestine and the blood vessels were drawn. We propose that this behavior gave rise to the body plan of all recent phoronids. The evolution of phoronid life cycle seems having more in common with"intercalation" than "terminal addition" theories.

Modern Data on the Innervation of the Lophophore in Lingula anatina (Brachiopoda) Support the Monophyly of the Lophophorates

Evolutionary relationships among members of the Lophophorata remain unclear. Traditionally, the Lophophorata included three phyla: Brachiopoda, Bryozoa or Ectoprocta, and Phoronida. All species in these phyla have a lophophore, which is regarded as a homologous structure of the lophophorates. Because the organization of the nervous system has been traditionally used to establish relationships among groups of animals, information on the organization of the nervous system in the lophophore of phoronids, brachiopods, and bryozoans may help clarify relationships among the lophophorates. In the current study, the innervation of the lophophore of the inarticulate brachiopod Lingula anatina is investigated by modern methods. The lophophore of L. anatina contains three brachial nerves: the main, accessory, and lower brachial nerves. The main brachial nerve is located at the base of the dorsal side of the brachial fold and gives rise to the cross neurite bundles, which pass through the connective tissue and connect the main and accessory brachial nerves. Nerves emanating from the accessory brachial nerve account for most of the tentacle innervation and comprise the frontal, latero-frontal, and latero-abfrontal neurite bundles. The lower brachial nerve gives rise to the abfrontal neurite bundles of the outer tentacles. Comparative analysis revealed the presence of many similar features in the organization of the lophophore nervous system in phoronids, brachiopods, and bryozoans. The main brachial nerve of L. anatina is similar to the dorsal ganglion of phoronids and the cerebral ganglion of bryozoans. The accessory brachial nerve of L. anatina is similar to the minor nerve ring of phoronids and the circumoral nerve ring of bryozoans. All lophophorates have intertentacular neurite bundles, which innervate adjacent tentacles. The presence of similar nerve elements in the lophophore of phoronids, brachiopods, and bryozoans supports the homology of the lophophore and the monophyly of the lophophorates.