Towards a ground pattern reconstruction of bivalve nervous systems: neurogenesis in the zebra mussel Dreissena polymorpha

Expanded expression of pro-neurogenic factor SoxB1 during larval development of gastropod Lymnaea stagnalis suggests preadaptation to prolonged neurogenesis in Mollusca

Introduction

The remarkable diversity observed in the structure and development of the molluscan nervous system raises intriguing questions regarding the molecular mechanisms underlying neurogenesis in Mollusca. The expression of SoxB family transcription factors plays a pivotal role in neuronal development, thereby offering valuable insights into the strategies of neurogenesis.

Methods

In this study, we conducted gene expression analysis focusing on SoxB-family transcription factors during early neurogenesis in the gastropod Lymnaea stagnalis. We employed a combination of hybridization chain reaction in situ hybridization (HCR-ISH), immunocytochemistry, confocal microscopy, and cell proliferation assays to investigate the spatial and temporal expression patterns of LsSoxB1 and LsSoxB2 from the gastrula stage to hatching, with particular attention to the formation of central ring ganglia.

Results

Our investigation reveals that LsSoxB1 demonstrates expanded ectodermal expression from the gastrula to the hatching stage, whereas expression of LsSoxB2 in the ectoderm ceases by the veliger stage. LsSoxB1 is expressed in the ectoderm of the head, foot, and visceral complex, as well as in forming ganglia and sensory cells. Conversely, LsSoxB2 is mostly restricted to the subepithelial layer and forming ganglia cells during metamorphosis. Proliferation assays indicate a uniform distribution of dividing cells in the ectoderm across all developmental stages, suggesting the absence of distinct neurogenic zones with increased proliferation in gastropods.

Discussion

Our findings reveal a spatially and temporally extended pattern of SoxB1 expression in a gastropod representative compared to other lophotrochozoan species. This prolonged and widespread expression of SoxB genes may be interpreted as a form of transcriptional neoteny, representing a preadaptation to prolonged neurogenesis. Consequently, it could contribute to the diversification of nervous systems in gastropods and lead to an increase in the complexity of the central nervous system in Mollusca.

Comparative development of the serotonin- and FMRFamide-immunoreactive components of the nervous system in two distantly related ribbon worm species (Nemertea, Spiralia)

Introduction

Neurodevelopment in larval stages of non-model organisms, with a focus on the serotonin- and FMRFamide-immunoreactive components, has been in the focus of research in the recent past. However, some taxonomic groups remain understudied. Nemertea (ribbon worms) represent such an understudied clade with only few reports on nervous system development mostly from phylogenetically or developmentally derived species. It would be insightful to explore neurodevelopment in additional species to be able to document the diversity and deduce common patterns to trace the evolution of nervous system development.

Methods

Fluorescent immunohistochemical labeling with polyclonal primary antibodies against serotonin and FMRF-amide and a monoclonal antibody against synapsin performed on series of fixed larval stages of two nemertean species Cephalothrix rufifrons (Archinemertea, Palaeonemertea) and Emplectonema gracile (Monostilifera, Hoplonemertea) were analyzed with confocal laser scanning microscopy.

Results

This contribution gives detailed accounts on the development of the serotonin- and FMRFamide-immunoreactive subsets of the nervous system in two nemertean species from the first appearance of the respective signals. Additionally, data on synapsin-like immunoreactivity illustrates the general structure of neuropil components. Events common to both investigated species are the appearance of serotonin-like immunoreactive signals before the appearance of FMRF-like immunoreactive signals and the strict progression of the development of the lateral nerve cords from the anteriorly located, ring-shaped brain toward the posterior pole of the larva. Notable differences are (1) the proboscis nervous system that is developing much earlier in investigated larval stages of E. gracile and (2) distinct early, but apparently transient, serotonergic neurons on the frontal and caudal pole of the larva in E. gracile that seem to be absent in C. rufifrons.

Discussion

According to the results from this investigation and in line with previously published accounts on nervous system development, the hypothetical last common ancestor of Nemertea had a ring-shaped brain arranged around the proboscis opening, from which a pair of ventro-lateral nerve cords develops in anterior to posterior progression. Early frontal and caudal serotonergic neurons that later degenerate or cease to express serotonin are an ancestral character of Nemertea that they share with several other spiralian clades.

Comparative Neuroanatomy of Pediveliger Larvae of Various Bivalves from the Sea of Japan

Here, we describe the nervous system structures from pediveligers of eight bivalve species (Callista brevisiphonata, Mactromeris polynyma, Crenomytilus grayanus, Kellia japonica, Mizuhopecten yessoensis, and Azumapecten farreri) with different modes of life in their adult stages, corresponding to the ecological niches that they occupy (burrowing, cemented, byssally attached, and mobile forms). We have identified neuromorphological features of the central and peripheral nervous systems in larval bivalves. We show that the unpaired sensory apical organ is still present in pediveligers along with the developing paired cerebral ganglia characteristic of an adult mollusk. Pediveligers have the pleural ganglia connected to the pedal ganglia via the pedal nerve cords and to the visceral ganglia via the lateral nerve cords. We have found a number of structures of the peripheral nervous system whose presence varies between pediveligers of different species. Mactromeris, Callista, and Pododesmus have 5-HT-immunopositive stomatogastric neurons, whereas the Yesso and Farrer's scallops have an FMRFamide-immunopositive enteric nervous system. The innervation of the anterior part of the velum is connected to a system of the apical organ and cerebral ganglia, and the innervation of the posterior part is connected to the visceral ganglia. Most differences in the structure of the peripheral elements of the nervous system are species-specific and weakly depend on the ecological niche that pediveligers occupy.

Identification and Expressional Analysis of Putative PRDI-BF1 and RIZ Homology Domain-Containing Transcription Factors in Mulinia lateralis

Mollusca represents one of the ancient bilaterian groups with high morphological diversity, while the formation mechanisms of the precursors of all germ cells, primordial germ cells (PGCs), have not yet been clarified in mollusks. PRDI-BF1 and RIZ homology domain-containing proteins (PRDMs) are a group of transcriptional repressors, and PRDM1 (also known as BLIMP1) and PRDM14 have been reported to be essential for the formation of PGCs. In the present study, we performed a genome-wide retrieval in Mulinia lateralis and identified 11 putative PRDMs, all of which possessed an N-terminal PR domain. Expressional profiles revealed that all these prdm genes showed specifically high expression levels in the given stages, implying that all PRDMs played important roles during early development stages. Specifically, Ml-prdm1 was highly expressed at the gastrula stage, the key period when PGCs arise, and was specifically localized in the cytoplasm of two or three cells of blastula, gastrula, or trochophore larvae, matching the typical characteristics of PGCs. These results suggested that Ml-prdm1-positive cells may be PGCs and that Ml-prdm1 could be a candidate marker for tracing the formation of PGCs in M. lateralis. In addition, the expression profiles of Ml-prdm14 hinted that it may not be associated with PGCs of M. lateralis. The present study provides insights into the evolution of the PRDM family in mollusks and offers a better understanding of the formation of PGCs in mollusks.

Histamine and gamma-aminobutyric acid in the nervous system of Pygospio elegans (Annelida: Spionidae): structure and recovery during reparative regeneration

Background

In recent two decades, studies of the annelid nervous systems were revolutionized by modern cell labeling techniques and state-of-the-art microscopy techniques. However, there are still huge gaps in our knowledge on the organization and functioning of their nervous system. Most of the recent studies have focused on the distribution of serotonin and FMRFamide, while the data about many other basic neurotransmitters such as histamine (HA) and gamma-aminobutyric acid (GABA) are scarce.

Results

Using immunohistochemistry and confocal microscopy we studied the distribution of histamine and gamma-aminobutyric acid in the nervous system of a spionid annelid Pygospio elegans and traced their redevelopment during reparative regeneration. Both neurotransmitters show specific patterns in central and peripheral nervous systems. HA-positive cells are concentrated mostly in the brain, while GABA-positive cell somata contribute equally to brain and segmental ganglia. Some immunoreactive elements were found in peripheral nerves. Both substances were revealed in high numbers in bipolar sensory cells in the palps. The first signs of regenerating HAergic and GABAergic systems were detected only by 3 days after the amputation. Further redevelopment of GABAergic system proceeds faster than that of HAergic one.

Conclusions

Comparisons with other annelids and mollusks examined in this respect revealed a number of general similarities in distribution patterns of HAergic and GABAergic cells in different species. Overall, the differences in the full redevelopment of various neurotransmitters correlate with neuronal development during embryogenesis. Our results highlight the importance of investigating the distribution of different neurotransmitters in comparative morphological and developmental studies.

Neurogenesis of the scallop Azumapecten farreri: from the first larval sensory neurons to the definitive nervous system of juveniles

Background

Scallops are among the best-studied bivalve mollusks. However, adult nervous system and neurogenesis studies of scallops are limited. Here, we studied the localization of neurotransmitters (serotonin/5-HT, FMRFamide, catecholamines) in adult ganglia and larvae of Azumapecten farreri using histochemical and immunohistochemical methods.

Results

We found peptide FMRFamide in all adult scallop ganglia, whereas 5-HT-like immunoreactive (lir) somata were exclusively detected in the cerebropleural, pedal, and accessory ganglia. Scallop larval neurogenesis starts with the emergence of the 5-HT-lir neurons, which are part of the apical organ (AO) at the early veliger stage. Near the AO, paired anlagen of cerebral ganglion (CG) developed. 5-HT-lir neurites of the CG innervate the velum, ventral, and dorsal parts of the larva at the late veliger stage. Scallop pediveligers possess 5-HT-lir CG, pleural ganglia, and immunopositive signals in the developing enteric nervous system. FMRFamide-lir is first detected in dorsal, ventral, and AO cells of early veligers. Later, FMRFamide-lir extends to the visceral nervous cord, all ganglia, as well as in the enteric nervous system in pediveligers. Catecholaminergic neurons are detected near the larval mouth, in the vellum, and in the stomach in veligers.

Conclusions

We described the distribution of neurotransmitters of the ganglia in adult scallops and the larval neurodevelopment in A. farreri. Immunostaining of neurotransmitters showed that the gross anatomy of adult scallop ganglia, in general, is similar to that in other bivalves, but complicated by the complexity of the structure of the ganglia and the appearance of additional ganglia not described in other molluscs. A comparison of larval neuromorphology suggests that 5-HT-lir structures are more conservative than FMRF-lir structures in Bivalvia. Notably, the latter are much more distributed in scallop A. farreri larvae than in other studied bivalves.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12983-022-00468-7.

First Immunodetection of Sensory and Nervous Systems of Parasitic Larvae (Glochidia) of Freshwater Bivalve Nodularia douglasiae

Most freshwater mussels have an unusual life cycle that requires host fish species for larval (glochidia) development and dispersal. Glochidia have a unique morphological structure that adapts to parasitic lifestyles and survival. The morphology of the glochidial shells of most Unionoidea, a group of freshwater bivalve mollusks, has been studied in detail using light and scanning electron microscopy. This study summarizes our data on the glochidia shell morphology of the Asian mussel Nodularia douglasiae from two localities in the Primorsky Territory, the Russian Far East. In contrast to the shell morphology of glochidia, little is known about the neurodevelopment of the Unionoidea. Herein, we first demonstrate that the structures of the sensory, muscle, and nervous systems of the glochidia larvae of N. douglasiae differ dramatically from those of the comparable larval systems of marine bivalve species, as revealed through alpha-acetylated tubulin, serotonin (5-HT), and FMRFamide antibodies as well as phalloidin for detection of F-actin and whole-mount confocal microscopy. We found that the glochidia sensory system included four pairs of tubulin-lir multicilia hair cells. Non-ciliar tubulin-lir cells synthesize the neuropeptide FMRFamide and are identified as afferent neurons collecting information from peripheral tubulin-lir hair sensory cells to nervous regulators. The glochidia’s muscular system was represented by a smooth adductor, retractors, and minor muscle bundles associated with the shell and visceral organs. The 5-HT-lir larval system is arranged most simply and consists of two immunopositive neurons innervating the adductor. The FMRFamide-lir system is more complicated and consists of several neuronal centers comprising neuronal bodies and their neurites in different areas of the larva. The FMRFamide-lir neurons are closely associated with sensory hair cells, and others, together with 5-HT-lir neurons, may be involved in the anlagen of adult ganglia. Thus, the nervous system of N. douglasiae glochidia is drastically different from other mollusks and lophotrochozoans because of the absence of an apical organ and the location and composition of FMRFamide and 5-HT cells. Morphological, molecular, and behavioral investigations of Unionoidea taxa need to be further conducted to investigate the parasite-host relationship, nerve-dependent regulation of parasite behavior, and evolution of mollusks.

Single-Cell RNA Sequencing Atlas From a Bivalve Larva Enhances Classical Cell Lineage Studies

Ciliated trochophore-type larvae are widespread among protostome animals with spiral cleavage. The respective phyla are often united into the superclade Spiralia or Lophotrochozoa that includes, for example, mollusks, annelids, and platyhelminths. Mollusks (bivalves, gastropods, cephalopods, polyplacophorans, and their kin) in particular are known for their morphological innovations and lineage-specific plasticity of homologous characters (e.g., radula, shell, foot, neuromuscular systems), raising questions concerning the cell types and the molecular toolkit that underlie this variation. Here, we report on the gene expression profile of individual cells of the trochophore larva of the invasive freshwater bivalve Dreissena rostriformis as inferred from single cell RNA sequencing. We generated transcriptomes of 632 individual cells and identified seven transcriptionally distinct cell populations. Developmental trajectory analyses identify cell populations that, for example, share an ectodermal origin such as the nervous system, the shell field, and the prototroch. To annotate these cell populations, we examined ontology terms from the gene sets that characterize each individual cluster. These were compared to gene expression data previously reported from other lophotrochozoans. Genes expected to be specific to certain tissues, such as Hox1 (in the shell field), Caveolin (in prototrochal cells), or FoxJ (in other cillia-bearing cells) provide evidence that the recovered cell populations contribute to various distinct tissues and organs known from morphological studies. This dataset provides the first molecular atlas of gene expression underlying bivalve organogenesis and generates an important framework for future comparative studies into cell and tissue type development in Mollusca and Metazoa as a whole.

Dinophiliformia early neurogenesis suggests the evolution of conservative neural structures across the Annelida phylogenetic tree

Despite the increasing data concerning the structure of the adult nervous system in various Lophotrochozoa groups, the early events during the neurogenesis of rare and unique groups need clarification. Annelida are a diverse clade of Lophotrochozoa, and their representatives demonstrate a variety of body plans, lifestyles, and life cycles. Comparative data about the early development are available for Errantia, Sedentaria, Sipuncula, and Palaeoannelida; however, our knowledge of Dinophiliformia is currently scarce. Representatives of Dinophiliformia are small interstitial worms combining unique morphological features of different Lophotrochozoan taxa and expressing paedomorphic traits. We describe in detail the early neurogenesis of two related species: Dimorphilus gyrociliatus and Dinophilus vorticoides, from the appearance of first nerve cells until the formation of an adult body plan. In both species, the first cells were detected at the anterior and posterior regions at the early trochophore stage and demonstrated positive reactions with pan-neuronal marker anti-acetylated tubulin only. Long fibers of early cells grow towards each other and form longitudinal bundles along which differentiating neurons later appear and send their processes. We propose that these early cells serve as pioneer neurons, forming a layout of the adult nervous system. The early anterior cell of D. vorticoides is transient and present during the short embryonic period, while early anterior and posterior cells in D. gyrociliatus are maintained throughout the whole lifespan of the species. During development, the growing processes of early cells form compact brain neuropile, paired ventral and lateral longitudinal bundles; unpaired medial longitudinal bundle; and commissures in the ventral hyposphere. Specific 5-HT- and FMRFa-immunopositive neurons differentiate adjacent to the ventral bundles and brain neuropile in the middle trochophore and late trochophore stages, i.e. after the main structures of the nervous system have already been established. Processes of 5-HT- and FMRFa-positive cells constitute a small proportion of the tubulin-immunopositive brain neuropile, ventral cords, and commissures in all developmental stages. No 5-HT- and FMRFa-positive cells similar to apical sensory cells of other Lophotrochozoa were detected. We conclude that: (i) like in Errantia and Sedentaria, Dinophiliformia neurogenesis starts from the peripheral cells, whose processes prefigure the forming adult nervous system, (ii) Dinophiliformia early cells are negative to 5-HT and FMRFa antibodies like Sedentaria pioneer cells.

Ecdysis-related neuropeptide expression and metamorphosis in a non-ecdysozoan bilaterian

Ecdysis-related neuropeptides (ERNs), including eclosion hormone, crustacean cardioactive peptide, myoinhibitory peptide, bursicon alpha, and bursicon beta regulate molting in insects and crustaceans. Recent evidence further revealed that ERNs likely play an ancestral role in invertebrate life cycle transitions, but their tempo-spatial expression patterns have not been investigated outside Arthropoda. Using RNA-seq and in situ hybridization, we show that ERNs are broadly expressed in the developing nervous system of a mollusk, the polyplacophoran Acanthochitona fascicularis. While some ERN-expressing neurons persist from larval to juvenile stages, others are only present during settlement and metamorphosis. These transient neurons belong to the "ampullary system," a polyplacophoran-specific larval sensory structure. Surprisingly, however, ERN expression is absent from the apical organ, another larval sensory structure that degenerates before settlement is completed in A. fascicularis. Our findings thus support a role of ERNs in A. fascicularis metamorphosis but contradict the common notion that the apical organ-like structures shared by various aquatic invertebrates (i.e., cnidarians, annelids, mollusks, echinoderms) are of general importance for this process.

Bisphenol A interferes with first shell formation and development of the serotoninergic system in early larval stages of Mytilus galloprovincialis

Bisphenol A-BPA, a widespread plastic additive, is an emerging contaminant of high concern and a potential endocrine disruptor in mammals. BPA also represents a potential threat for aquatic species, especially for larval stages. In the marine bivalve Mytilus galloprovincialis, BPA has been previously shown to affect early larval development and gene transcription. In this work, the effects of BPA (0.05-0.5-5 μM) were further investigated at different times post fertilization (24-28-32-48 hpf). BPA induced concentration-dependent alterations in deposition of the organic matrix and calcified shell at different larval stages, as shown by double calcofluor/calcein staining, resulting in altered phenotypes at 48hpf. Transcription of Tyrosinase-TYR, that plays a key role in remodelling of the shell organic matrix, and of HOX1, a member of homeobox genes involved in larval shell formation and neurogenesis, were evaluated by In Situ Hybrydization-ISH. BPA altered the spatial pattern of expression of both genes, with distinct effects depending on the concentration and developmental stage. Moreover, BPA affected the time course of mRNA levels for TYR from 24 to 48hpf. BPA impaired development of serotonin-5-HT-immunoreactive neurons at different times pf; at 48hpf, the reduction in the number of serotoninergic neurons was associated with developmental delay and downregulation of the 5-HT receptor-5-HTR. All the effects were observed from the lowest concentration tested, corresponding to detectable BPA levels in contaminated coastal waters. These data demonstrate that BPA interferes with key processes occurring during the first developmental stages of mussels, thus representing a potential threat for natural populations.

Cloning and characterisation of NMDA receptors in the Pacific oyster, Crassostrea gigas (Thunberg, 1793) in relation to metamorphosis and catecholamine synthesis

Bivalve metamorphosis is a developmental transition from a free-living larva to a benthic juvenile (spat), regulated by a complex interaction of neurotransmitters and neurohormones such as L-DOPA and epinephrine (catecholamine). We recently suggested an N-Methyl-D-aspartate (NMDA) receptor pathway as an additional and previously unknown regulator of bivalve metamorphosis. To explore this theory further, we successfully induced metamorphosis in the Pacific oyster, Crassostrea gigas, by exposing competent larvae to L-DOPA, epinephrine, MK-801 and ifenprodil. Subsequently, we cloned three NMDA receptor subunits CgNR1, CgNR2A and CgNR2B, with sequence analysis suggesting successful assembly of functional NMDA receptor complexes and binding to natural occurring agonists and the channel blocker MK-801. NMDA receptor subunits are expressed in competent larvae, during metamorphosis and in spat, but this expression is neither self-regulated nor regulated by catecholamines. In-situ hybridisation of CgNR1 in competent larvae identified NMDA receptor presence in the apical organ/cerebral ganglia area with a potential sensory function, and in the nervous network of the foot indicating an additional putative muscle regulatory function. Furthermore, phylogenetic analyses identified molluscan-specific gene expansions of key enzymes involved in catecholamine biosynthesis. However, exposure to MK-801 did not alter the expression of selected key enzymes, suggesting that NMDA receptors do not regulate the biosynthesis of catecholamines via gene expression.

Peripheral sensory neurons govern development of the nervous system in bivalve larvae

Recent findings regarding early lophotrochozoan development have altered the conventional model of neurogenesis and revealed that peripheral sensory elements play a key role in the initial organization of the larval nervous system. Here, we describe the main neurogenetic events in bivalve mollusks in comparison with other Lophotrochozoa, emphasizing a novel role for early neurons in establishing larval nervous systems and speculating about the morphogenetic function of the apical organ. We demonstrate that during bivalve development, peripheral sensory neurons utilizing various transmitters differentiate before the apical organ emerges. The first neurons and their neurites serve as a scaffold for the development of the nervous system. During veliger stage, cerebral, pleural, and visceral ganglia form along the lateral (visceral) nerve cords in anterior-to-posterior axis. The pedal ganglia and corresponding ventral (pedal) nerve cords develop much later, after larval settlement and metamorphosis. Pharmacological abolishment of the serotonin gradient within the larval body disrupts the navigation of "pioneer" axons resulting in malformation of the whole nervous system architecture. Comparative morphological data on neurogenetic events in bivalve mollusks shed new light on the origin of the nervous system, mechanisms of early axon navigation, and sequence of the tetraneurous nervous system formation. Furthermore, this information improves our understanding of the basic nervous system architecture in larval Bivalvia and Mollusca.

Developmental architecture of the nervous system in Themiste lageniformis (Sipuncula): New evidence from confocal laser scanning microscopy and gene expression

Sipuncula is a clade of unsegmented marine worms that are currently placed among the basal radiation of conspicuously segmented Annelida. Their new location provides a unique opportunity to reinvestigate the evolution and development of segmented body plans. Neural segmentation is clearly evident during ganglionic ventral nerve cord (VNC) formation across Sedentaria and Errantia, which includes the majority of annelids. However, recent studies show that some annelid taxa outside of Sedentaria and Errantia have a medullary cord, without ganglia, as adults. Importantly, neural development in these taxa is understudied and interpretation can vary widely. For example, reports in sipunculans range from no evidence of segmentation to vestigial segmentation as inferred from a few pairs of serially repeated neuronal cell bodies along the VNC. We investigated patterns of pan-neuronal, neuronal subtype, and axonal markers using immunohistochemistry and whole mount in situ hybridization (WMISH) during neural development in an indirect-developing sipunculan, Themiste lageniformis. Confocal imaging revealed two clusters of 5HT⁺ neurons, two pairs of FMRF⁺ neurons, and Tubulin⁺ peripheral neurites that appear to be serially positioned along the VNC, similar to other sipunculans, to other annelids, and to spiralian taxa outside of Annelida. WMISH of a synaptotagmin1 ortholog in T. lageniformis (Tl-syt1) showed expression throughout the centralized nervous system (CNS), including the VNC where it appears to correlate with mature 5HT⁺ and FMRF⁺ neurons. An ortholog of elav1 (Tl-elav1) showed expression in differentiated neurons of the CNS with continuous expression in the VNC, supporting evidence of a medullary cord, and refuting evidence of ontogenetic segmentation during formation of the nervous system. Thus, we conclude that sipunculans do not exhibit any signs of morphological segmentation during development.

The evolution of molluscs

Molluscs are extremely diverse invertebrate animals with a rich fossil record, highly divergent life cycles, and considerable economical and ecological importance. Key representatives include worm-like aplacophorans, armoured groups (e.g. polyplacophorans, gastropods, bivalves) and the highly complex cephalopods. Molluscan origins and evolution of their different phenotypes have largely remained unresolved, but significant progress has been made over recent years. Phylogenomic studies revealed a dichotomy of the phylum, resulting in Aculifera (shell-less aplacophorans and multi-shelled polyplacophorans) and Conchifera (all other, primarily uni-shelled groups). This challenged traditional hypotheses that proposed that molluscs gradually evolved complex phenotypes from simple, worm-like animals, a view that is corroborated by developmental studies that showed that aplacophorans are secondarily simplified. Gene expression data indicate that key regulators involved in anterior-posterior patterning (the homeobox-containing Hox genes) lost this function and were co-opted into the evolution of taxon-specific novelties in conchiferans. While the bone morphogenetic protein (BMP)/decapentaplegic (Dpp) signalling pathway, that mediates dorso-ventral axis formation, and molecular components that establish chirality appear to be more conserved between molluscs and other metazoans, variations from the common scheme occur within molluscan sublineages. The deviation of various molluscs from developmental pathways that otherwise appear widely conserved among metazoans provides novel hypotheses on molluscan evolution that can be tested with genome editing tools such as the CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats/clustered regularly interspaced short palindromic repeats-associated protein9) system.

Nervous system development in the Pacific oyster, Crassostrea gigas (Mollusca: Bivalvia)

Background

Bivalves comprise a large, highly diverse taxon of invertebrate species. Developmental studies of neurogenesis among species of Bivalvia are limited. Due to a lack of neurogenesis information, it is difficult to infer a ground pattern for Bivalvia. To provide more comprehensive morphogenetic data on bivalve molluscs and relationships among molluscan clades, we investigated neurogenesis in the Pacific oyster, Crassostrea gigas, from the appearance of the first sensory cells to the formation of the larval ganglionic nervous system by co-immunocytochemistry of the neuronal markers FMRFamide or 5-HT and vesicular acetylcholine transporter (VAChT).

Results

Neurogenesis begins with the emergence of the apical serotonin-immunoreactive (5-HT-ir) sensory cells and paired sensory posttrochal dorsal and ventral FMRFamide-immunoreactive (FMRFamide-ir) cells at the early trochophore stage. Later, at the early veliger stage, the apical organ (AO) includes 5-HT-ir, FMRFamide-ir, and VAChT-ir cells. At the same stage, VAChT-ir cells appear in the posterior region of larvae and send axons towards the AO. Thus, FMRFamide-ir neurites and VAChT-ir processes form scaffolds for longitudinal neurite bundles develop into the paired ventral nerve cords (VNC). Later-appearing axons from the AO/CG neurons join the neurite bundles comprising the VNC. All larval ganglia appear along the VNC as paired or fused (epiathroid) clusters in late veliger and pediveliger larvae. We observed the transformation of the AO into the cerebral ganglia, which abundantly innervated the velum, and the transformation of ventral neurons into the pedal ganglia, innervating the foot, gills, and anterior adductor muscle. The visceral ganglia appear last in the pediveliger oyster and innervate the visceral mass and posterior adductor of premetamorphic larvae. In addition, a local FMRFamide-ir network was detected in the digestive system of pediveliger larvae. We identified VAChT-ir nervous elements in oyster larvae, which have not been observed previously in molluscs. Finally, we performed a morphology-based comparative analysis of neuronal structures among bivalve, conchiferan, and aculiferan species.

Conclusions

We described the development of the nervous system during the larval development in Crassostrea gigas. These data greatly advance the currently limited understanding of neurodevelopment in bivalves and mollusks, which has hampered the generation of a ground pattern reconstruction of the last common ancestor of Mollusca. Our morphological data support phylogenomic data indicating a closer Bivalvia-Gastropoda sister group relationship than the Bivalvia-Scaphopoda (Diasoma) group relationship.

Electronic supplementary material

The online version of this article (10.1186/s12983-018-0259-8) contains supplementary material, which is available to authorized users.