High diversity in neuropeptide immunoreactivity patterns among three closely related species of Dinophilidae (Annelida)

The development of the adult nervous system in the annelid Owenia fusiformis

BACKGROUND

The evolutionary origins of animal nervous systems remain contentious because we still have a limited understanding of neural development in most major animal clades. Annelids - a species-rich group with centralised nervous systems - have played central roles in hypotheses about the origins of animal nervous systems. However, most studies have focused on adults of deeply nested species in the annelid tree. Recently, Owenia fusiformis has emerged as an informative species to reconstruct ancestral traits in Annelida, given its phylogenetic position within the sister clade to all remaining annelids.

METHODS

Combining immunohistochemistry of the conserved neuropeptides FVamide-lir, RYamide-lir, RGWamide-lir and MIP-lir with gene expression, we comprehensively characterise neural development from larva to adulthood in Owenia fusiformis.

RESULTS

The early larval nervous system comprises a neuropeptide-rich apical organ connected through peripheral nerves to a prototroch ring and the chaetal sac. There are seven sensory neurons in the prototroch. A bilobed brain forms below the apical organ and connects to the ventral nerve cord of the developing juvenile. During metamorphosis, the brain compresses, becoming ring-shaped, and the trunk nervous system develops several longitudinal cords and segmented lateral nerves.

CONCLUSIONS

Our findings reveal the formation and reorganisation of the nervous system during the life cycle of O. fusiformis, an early-branching annelid. Despite its apparent neuroanatomical simplicity, this species has a diverse peptidergic nervous system, exhibiting morphological similarities with other annelids, particularly at the larval stages. Our work supports the importance of neuropeptides in animal nervous systems and highlights how neuropeptides are differentially used throughout development.

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.

Nervous System Development and Neuropeptides Characterization in Embryo and Larva: Insights from a Non-Chordate Deuterostome, the Sea Cucumber Apostichopus japonicus

Here, we described the complex nervous system at five early developmental stages (blastula, gastrula, auricularia, doliolaria and pentactula) of a holothurian species with highly economic value, Apostichopus japonicus. The results revealed that the nervous system of embryos and larvae is mainly distributed in the anterior apical region, ciliary bands or rings, and the feeding and attachment organs, and that serotonergic immunoreactivity was not observed until the embryo developed into the late gastrula; these are evolutionarily conserved features of echinoderm, hemichordate and protostome larvae. Furthermore, based on available transcriptome data, we reported the neuropeptide precursors profile at different embryonic and larval developmental stages. This analysis showed that 40 neuropeptide precursors present in adult sea cucumbers were also identified at different developmental stages of embryos and larvae, and only four neuropeptide precursors (SWYG precursor 2, GYWKDLDNYVKAHKT precursor, Neuropeptide precursor 14-like precursor, GLRFAmprecursor-like precursor) predicted in adults were absent in embryos and larvae. Combining the quantitative expression of ten specific neuropeptide precursor genes (NPs) by qRT-PCR, we revealed the potential important roles of neuropeptides in embryo development, feeding and attachment in A. japonicus larvae. In conclusion, this work provides novel perspectives on the diverse physiological functions of neuropeptides and contributes to understanding the evolution of neuropeptidergic systems in echinoderm embryos and larvae.

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.

Nemertean, brachiopod and phoronid neuropeptidomics reveals ancestral spiralian signalling systems

Neuropeptides are diverse signaling molecules in animals commonly acting through G-protein coupled receptors (GPCRs). Neuropeptides and their receptors underwent extensive diversification in bilaterians and the relationships of many peptide–receptor systems have been clarified. However, we lack a detailed picture of neuropeptide evolution in lophotrochozoans as in-depth studies only exist for mollusks and annelids. Here, we analyze peptidergic systems in Nemertea, Brachiopoda, and Phoronida. We screened transcriptomes from 13 nemertean, 6 brachiopod, and 4 phoronid species for proneuropeptides and neuropeptide GPCRs. With mass spectrometry from the nemertean Lineus longissimus, we validated several predicted peptides and identified novel ones. Molecular phylogeny combined with peptide-sequence and gene-structure comparisons allowed us to comprehensively map spiralian neuropeptide evolution. We found most mollusk and annelid peptidergic systems also in nemerteans, brachiopods, and phoronids. We uncovered previously hidden relationships including the orthologies of spiralian CCWamides to arthropod agatoxin-like peptides and of mollusk APGWamides to RGWamides from annelids, with ortholog systems in nemerteans, brachiopods, and phoronids. We found that pleurin neuropeptides previously only found in mollusks are also present in nemerteans and brachiopods. We also identified cases of gene family duplications and losses. These include a protostome-specific expansion of RFamide/Wamide signaling, a spiralian expansion of GnRH-related peptides, and duplications of vasopressin/oxytocin before the divergence of brachiopods, phoronids, and nemerteans. This analysis expands our knowledge of peptidergic signaling in spiralians and other protostomes. Our annotated data set of nearly 1,300 proneuropeptide sequences and 600 GPCRs presents a useful resource for further studies of neuropeptide signaling.

The chemical brain hypothesis for the origin of nervous systems

In nervous systems, there are two main modes of transmission for the propagation of activity between cells. Synaptic transmission relies on close contact at chemical or electrical synapses while volume transmission is mediated by diffusible chemical signals and does not require direct contact. It is possible to wire complex neuronal networks by both chemical and synaptic transmission. Both types of networks are ubiquitous in nervous systems, leading to the question which of the two appeared first in evolution. This paper explores a scenario where chemically organized cellular networks appeared before synapses in evolution, a possibility supported by the presence of complex peptidergic signalling in all animals except sponges. Small peptides are ideally suited to link up cells into chemical networks. They have unlimited diversity, high diffusivity and high copy numbers derived from repetitive precursors. But chemical signalling is diffusion limited and becomes inefficient in larger bodies. To overcome this, peptidergic cells may have developed projections and formed synaptically connected networks tiling body surfaces and displaying synchronized activity with pulsatile peptide release. The advent of circulatory systems and neurohemal organs further reduced the constraint imposed on chemical signalling by diffusion. This could have contributed to the explosive radiation of peptidergic signalling systems in stem bilaterians. Neurosecretory centres in extant nervous systems are still predominantly chemically wired and coexist with the synaptic brain. This article is part of the theme issue 'Basal cognition: multicellularity, neurons and the cognitive lens'.

Interstitial Annelida

Members of the following marine annelid families are found almost exclusively in the interstitial environment and are highly adapted to move between sand grains, relying mostly on ciliary locomotion: Apharyngtidae n. fam., Dinophilidae, Diurodrilidae, Nerillidae, Lobatocerebridae, Parergodrilidae, Polygordiidae, Protodrilidae, Protodriloididae, Psammodrilidae and Saccocirridae. This article provides a review of the evolution, systematics, and diversity of these families, with the exception of Parergodrilidae, which was detailed in the review of Orbiniida by Meca, Zhadan, and Struck within this Special Issue. While several of the discussed families have previously only been known by a few described species, recent surveys inclusive of molecular approaches have increased the number of species, showing that all of the aforementioned families exhibit a high degree of cryptic diversity shadowed by a limited number of recognizable morphological traits. This is a challenge for studies of the evolution, taxonomy, and diversity of interstitial families as well as for their identification and incorporation into ecological surveys. By compiling a comprehensive and updated review on these interstitial families, we hope to promote new studies on their intriguing evolutionary histories, adapted life forms and high and hidden diversity.

Conservative route to genome compaction in a miniature annelid

The causes and consequences of genome reduction in animals are unclear because our understanding of this process mostly relies on lineages with often exceptionally high rates of evolution. Here, we decode the compact 73.8-megabase genome of Dimorphilus gyrociliatus, a meiobenthic segmented worm. The D. gyrociliatus genome retains traits classically associated with larger and slower-evolving genomes, such as an ordered, intact Hox cluster, a generally conserved developmental toolkit and traces of ancestral bilaterian linkage. Unlike some other animals with small genomes, the analysis of the D. gyrociliatus epigenome revealed canonical features of genome regulation, excluding the presence of operons and trans-splicing. Instead, the gene-dense D. gyrociliatus genome presents a divergent Myc pathway, a key physiological regulator of growth, proliferation and genome stability in animals. Altogether, our results uncover a conservative route to genome compaction in annelids, reminiscent of that observed in the vertebrate Takifugu rubripes.

This study reports the genome of the miniature segmented annelid Dimorphilus gyrociliatus and reveals no drastic changes in genome architecture and regulation, unlike other cases of genome miniaturization.

Broad North Atlantic distribution of a meiobenthic annelid - against all odds

DNA barcoding and population genetic studies have revealed an unforeseen hidden diversity of cryptic species among microscopic marine benthos, otherwise exhibiting highly similar and simple morphologies. This has led to a paradigm shift, rejecting cosmopolitism of marine meiofauna until genetically proven and challenging the "Everything is Everywhere, but the environment selects" hypothesis that claims ubiquitous distribution of microscopic organisms. With phylogenetic and species delimitation analyses of worldwide genetic samples of the meiofaunal family Dinophilidae (Annelida) we here resolve three genera within the family and showcase an exceptionally broad, boreal, North Atlantic distribution of a single microscopic marine species with no obvious means of dispersal besides vicariance. With its endobenthic lifestyle, small size, limited migratory powers and lack of pelagic larvae, the broad distribution of Dinophilus vorticoides seems to constitute a "meiofaunal paradox". This species feasts in the biofilm among sand grains, but also on macroalgae and ice within which it can likely survive long-distance rafting dispersal due to its varying lifecycle stages; eggs encapsulated in cocoons and dormant encystment stages. Though often neglected and possibly underestimated among marine microscopic species, dormancy may be a highly significant factor for explaining wide distribution patterns and a key to solving this meiofaunal paradox.

Nerves innervating copulatory organs show common FMRFamide, FVRIamide, MIP and serotonin immunoreactivity patterns across Dinophilidae (Annelida) indicating their conserved role in copulatory behaviour

Background

Males of the microscopic annelid family Dinophilidae use their prominent glandomuscular copulatory organ (penis) to enzymatically dissolve the female’s epidermis and thereafter inject sperm. In order to test for putative conserved copulatory structures and neural orchestration across three dinophilid species, we reconstructed the reproductive myo- and neuroanatomy and mapped immunoreactivity patterns against two specific neurotransmitter markers with reported roles in invertebrate male mating behaviour (FVRIamide, MIP) and three general neural markers (acetylated α-tubulin, serotonin, FMRFamide).

Results

Seminal vesicles (one or two pairs), surrounded by a thin layer of longitudinal and circular muscles and innervated by neurites, are found between testes and copulatory organ in the larger males of Dinophilus vorticoides and Trilobodrilus axi, but are missing in the only 0.05 mm long D. gyrociliatus dwarf males. The midventral copulatory organ is in all species composed of an outer muscular penis sheath and an inner penis cone. Neurites encircle the organ equatorially, either as a ring-shaped circumpenial fibre mass or as dorsal and ventral commissures, which are connected to the ventrolateral nerve cords. All three examined dinophilids show similar immunoreactivity patterns against serotonin, FMRFamide, and FVRIamide in the neurons surrounding the penis, supporting the hypotheses about the general involvement of these neurotransmitters in copulatory behaviour in dinophilids. Immunoreactivity against MIP is restricted to the circumpenial fibre mass in D. gyrociliatus and commissures around the penis in T. axi (but not found in D. vorticoides), indicating its role in controlling the copulatory organ.

Conclusions

The overall myo- and neuroanatomy of the reproductive organs is rather similar in the three studied species, suggesting a common ancestry of the unpaired glandomuscular copulatory organ and its innervation in Dinophilidae. This is furthermore supported by the similar immunoreactivity patterns against the tested neurotransmitters around the penis. Smaller differences in the immunoreactivity patterns around the seminal vesicles and spermioducts might account for additional, individual traits. We thus show morphological support for the putatively conserved role of FMRFamide, FVRIamide, MIP and serotonin in dinophilid copulatory behaviour.

A nemertean excitatory peptide/CCHamide regulates ciliary swimming in the larvae of Lineus longissimus

Background

The trochozoan excitatory peptide (EP) and its ortholog, the arthropod CCHamide, are neuropeptides that are only investigated in very few animal species. Previous studies on different trochozoan species focused on their physiological effect in adult specimens, demonstrating a myo-excitatory effect, often on tissues of the digestive system. The function of EP in the planktonic larvae of trochozoans has not yet been studied.

Results

We surveyed transcriptomes from species of various spiralian (Orthonectida, Nemertea, Brachiopoda, Entoprocta, Rotifera) and ecdysozoan taxa (Tardigrada, Onychophora, Priapulida, Loricifera, Nematomorpha) to investigate the evolution of EPs/CCHamides in protostomes. We found that the EPs of several pilidiophoran nemerteans show a characteristic difference in their C-terminus. Deorphanization of a pilidiophoran EP receptor showed, that the two splice variants of the nemertean Lineus longissimus EP activate a single receptor. We investigated the expression of EP in L. longissimus larvae and juveniles with customized antibodies and found that EP positive nerves in larvae project from the apical organ to the ciliary band and that EP is expressed more broadly in juveniles in the neuropil and the prominent longitudinal nerve cords. While exposing juvenile L. longissimus specimens to synthetic excitatory peptides did not show any obvious effect, exposure of larvae to either of the two EPs increased the beat frequency of their locomotory cilia and shifted their vertical swimming distribution in a water column upwards.

Conclusion

Our results show that EP/CCHamide peptides are broadly conserved in protostomes. We show that the EP increases the ciliary beat frequency of L. longissimus larvae, which shifts their vertical distribution in a water column upwards. Endogenous EP may be released at the ciliary band from the projections of apical organ EP positive neurons to regulate ciliary beating. This locomotory function of EP in L. longissimus larvae stands in contrast to the repeated association of EP/CCHamides with its myo-excitatory effect in adult trochozoans and the general association with the digestive system in many protostomes.

Electronic supplementary material

The online version of this article (10.1186/s12983-019-0326-9) contains supplementary material, which is available to authorized users.

Xenacoelomorph Neuropeptidomes Reveal a Major Expansion of Neuropeptide Systems during Early Bilaterian Evolution

Neuropeptides are neurosecretory signaling molecules in protostomes and deuterostomes (together Nephrozoa). Little, however, is known about the neuropeptide complement of the sister group of Nephrozoa, the Xenacoelomorpha, which together form the Bilateria. Because members of the xenacoelomorph clades Xenoturbella, Nemertodermatida, and Acoela differ extensively in their central nervous system anatomy, the reconstruction of the xenacoelomorph and bilaterian neuropeptide complements may provide insights into the relationship between nervous system evolution and peptidergic signaling. Here, we analyzed transcriptomes of seven acoels, four nemertodermatids, and two Xenoturbella species using motif searches, similarity searches, mass spectrometry and phylogenetic analyses to characterize neuropeptide precursors and neuropeptide receptors. Our comparison of these repertoires with previously reported nephrozoan and cnidarian sequences shows that the majority of annotated neuropeptide GPCRs in cnidarians are not orthologs of specific bilaterian neuropeptide receptors, which suggests that most of the bilaterian neuropeptide systems evolved after the cnidarian–bilaterian evolutionary split. This expansion of more than 20 peptidergic systems in the stem leading to the Bilateria predates the evolution of complex nephrozoan organs and nervous system architectures. From this ancient set of neuropeptides, acoels show frequent losses that correlate with their divergent central nervous system anatomy. We furthermore detected the emergence of novel neuropeptides in xenacoelomorphs and their expansion along the nemertodermatid and acoel lineages, the two clades that evolved nervous system condensations. Together, our study provides fundamental insights into the early evolution of the bilaterian peptidergic systems, which will guide future functional and comparative studies of bilaterian nervous systems.