As a sister group to Bilateria, Cnidaria is important for understanding early nervous system evolution. Here we examine neural development in the anthozoan cnidarian Nematostella vectensis in order to better understand whether similar developmental mechanisms are utilized to establish the strikingly different overall organization of bilaterian and cnidarian nervous systems. We generated a neuron-specific transgenic NvElav1 reporter line of N. vectensis and used it in combination with immunohistochemistry against neuropeptides, in situ hybridization and confocal microscopy to analyze nervous system formation in this cnidarian model organism in detail. We show that the development of neurons commences in the ectoderm during gastrulation and involves interkinetic nuclear migration. Transplantation experiments reveal that sensory and ganglion cells are autonomously generated by the ectoderm. In contrast to bilaterians, neurons are also generated throughout the endoderm during planula stages. Morpholino-mediated gene knockdown shows that the development of a subset of ectodermal neurons requires NvElav1, the ortholog to bilaterian neural elav1 genes. The orientation of ectodermal neurites changes during planula development from longitudinal (in early-born neurons) to transverse (in late-born neurons), whereas endodermal neurites can grow in both orientations at any stage. Our findings imply that elav1-dependent ectodermal neurogenesis evolved prior to the divergence of Cnidaria and Bilateria. Moreover, they suggest that, in contrast to bilaterians, almost the entire ectoderm and endoderm of the body column of Nematostella planulae have neurogenic potential and that the establishment of connectivity in its seemingly simple nervous system involves multiple neurite guidance systems.

https://espace.library.uq.edu.au/view/UQ:316917/UQ316917_OA.pdf

Nervous systems of the sea anemone Nematostella vectensis are generated by ectoderm and endoderm and shaped by distinct mechanisms

Life-cycle transitions connecting larval and juvenile stages in metazoans are orchestrated by neuroendocrine signals including neuropeptides and hormones. In marine invertebrate life cycles, which often consist of planktonic larval and benthic adult stages, settlement of the free-swimming larva to the sea floor in response to environmental cues is a key life cycle transition. Settlement is regulated by a specialized sensory–neurosecretory system, the larval apical organ. The neuroendocrine mechanisms through which the apical organ transduces environmental cues into behavioral responses during settlement are not yet understood. Here we show that myoinhibitory peptide (MIP)/allatostatin-B, a pleiotropic neuropeptide widespread among protostomes, regulates larval settlement in the marine annelid Platynereis dumerilii. MIP is expressed in chemosensory–neurosecretory cells in the annelid larval apical organ and signals to its receptor, an orthologue of the Drosophila sex peptide receptor, expressed in neighboring apical organ cells. We demonstrate by morpholino-mediated knockdown that MIP signals via this receptor to trigger settlement. These results reveal a role for a conserved MIP receptor–ligand pair in regulating marine annelid settlement.

https://www.pnas.org/content/pnas/110/20/8224.full.pdf

Conserved MIP receptor–ligand pair regulates Platynereis larval settlement

The origin of nervous systems has traditionally been discussed within two conceptual frameworks. Input–output models stress the sensory-motor aspects of nervous systems, while internal coordination models emphasize the role of nervous systems in coordinating multicellular activity, especially muscle-based motility. Here we consider both frameworks and apply them to describe aspects of each of three main groups of phenomena that nervous systems control: behaviour, physiology and development. We argue that both frameworks and all three aspects of nervous system function need to be considered for a comprehensive discussion of nervous system origins. This broad mapping of the option space enables an overview of the many influences and constraints that may have played a role in the evolution of the first nervous systems.

/pdf/an-option-space-for-early-neural-evolution-dht65cxq92.pdf

An option space for early neural evolution.

The evolution of early neurogenesis.

Neurogenesis initiates during early development and it continues through later developmental stages and in adult animals to enable expansion, remodeling, and homeostasis of the nervous system. The generation of nerve cells has been analyzed in detail in few bilaterian model organisms, leaving open many questions about the evolution of this process. As the sister group to bilaterians, cnidarians occupy an informative phylogenetic position to address the early evolution of cellular and molecular aspects of neurogenesis and to understand common principles of neural development. Here we review studies in several cnidarian model systems that have revealed significant similarities and interesting differences compared to neurogenesis in bilaterian species, and between different cnidarian taxa. Cnidarian neurogenesis is currently best understood in the sea anemone Nematostella vectensis, where it includes epithelial neural progenitor cells that express transcription factors of the soxB and atonal families. Notch signaling regulates the number of these neural progenitor cells, achaete-scute and dmrt genes are required for their further development and Wnt and BMP signaling appear to be involved in the patterning of the nervous system. In contrast to many vertebrates and Drosophila, cnidarians have a high capacity to generate neurons throughout their lifetime and during regeneration. Utilizing this feature of cnidarian biology will likely allow gaining new insights into the similarities and differences of embryonic and regenerative neurogenesis. The use of different cnidarian model systems and their expanding experimental toolkits will thus continue to provide a better understanding of evolutionary and developmental aspects of nervous system formation. WIREs Dev Biol 2017, 6:e257. doi: 10.1002/wdev.257 For further resources related to this article, please visit the WIREs website.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/wdev.257

The cellular and molecular basis of cnidarian neurogenesis.

The organization of the cnidarian nervous system has been widely documented in polyps and medusae, but little is known about the nervous system of planula larvae, which give rise to adult forms after settling and metamorphosis. We describe histological and cytological features of the nervous system in planulae of the hydrozoan Clava multicornis. These planulae do not swim freely in the water column but rather crawl on the substrate by means of directional, coordinated ciliary movement coupled to lateral muscular bending movements associated with positive phototaxis. Histological analysis shows pronounced anteroposterior regionalization of the planula’s nervous system, with different neural cell types highly concentrated at the anterior pole. Transmission electron microscopy of planulae shows the nervous system to be unusually complex, with a large, orderly array of sensory cells at the anterior pole. In the anterior half of the planula, the basiectodermal plexus of neurites forms an extensive orthogonal network, whereas more posteriorly neurites extend longitudinally along the body axis. Additional levels of nervous system complexity are uncovered by neuropeptide-specific immunocytochemistry, which reveals distinct neural subsets having specific molecular phenotypes. Together these observations imply that the nervous system of the planula of Clava multicornis manifests a remarkable level of histological, cytological, and functional organization, the features of which may be reminiscent of those present in early bilaterian animals. J. Comp. Neurol. 519:1931–1951, 2011.

Complex neural architecture in the diploblastic larva of Clava multicornis (Hydrozoa, Cnidaria)

Thalia democratica is a cosmopolitan tunicate belonging to the Thaliacea class. To further investigate the anatomy of this species, immunohistochemical labelling was performed using anti-tubulin and anti-serotonin antibodies on specimens collected in the Mediterranean Sea. The anti-tubulin antibody stained the cilia of the endostyle, the pericoronal bands and of the gill bar, enabling a detailed description of these structures. Moreover, immunolabelling of the nervous system showed the presence of eight pairs of nerve fibres emerging from the neural ganglion. Serotonergic cells were observed in the distal tract of the intestine, along the pericoronal bands, and in the placenta of gravid blastozooids, as well as in the neural ganglion. The presence of serotonin in the central nervous system has also been reported in the larvae of ascidians and may be linked to the planktonic life of these animals, a condition shared by adult thaliaceans and ascidian larvae. This work improves our knowledge of the anatomy of T. democratica and demonstrates the presence of a complex serotonergic system.

/pdf/immunohistochemical-study-of-the-nervous-system-of-the-2pfzdvviky.pdf

Immunohistochemical study of the nervous system of the tunicate Thalia democratica (Forsskal, 1775)

The planula larva of the hydroid Clava multicornis (Forskal, 1775) has a complex nervous system, characterized by the presence of distinct, anteriorly concentrated peptidergic populations of amidated neurons, presumably involved in the detection of environmental stimuli and metamorphic signals. Differently from other hydrozoan larvae in C. multicornis planulae GLW-positive cells with putative sensory role have a peculiar dome-shaped forefront organization, followed by a belt of RF-positive nerve cells. By immunohistochemistry, we investigated the transformation of the peptidergic (GLW-amide and RF-amide) larval neuroanatomy at different stages of metamorphosis and the subsequent development of the primary polyp nervous system. By terminal transferase-mediated dUTP nick end-labeling assay, apoptotic nuclei were first identified in the anterior pole of the settled larva, in the same region occupied by GLW-amide positive putative sensory cells. In primary polyps, GLW-amide positive signals first encircled the hypostome area, later extending downwards along the polyp column or upwards over the hypostome dome, whereas RF-amide positive sensory cells initially appeared at the tentacles base to later extend in the tentacles and the polyp column. In spite of the possession of distinct neuroanatomies, different cnidarian planulae may share common developmental mechanisms underlying metamorphosis, including apoptosis and de novo differentiation. Our data confirm the hypothesis that the developmental dynamics of tissue rearrangements may be not uniform across different taxa.

Neural system reorganization during metamorphosis in the planula larva of Clava multicornis (Hydrozoa, Cnidaria)

Retinoic acid (RA) is a derivative of vitamin A known to be involved in the regulation of many developmental and physiological processes in chordates. Recently, evidence showing its presence and function in invertebrate deuterostomes and protostomes indicated the early evolutionary origin and conservation of the RA developmental machinery, with a key role also in neural differentiation. Moreover, it is known that retinoids can influence pattern specification in polyps of the hydroid Hydractinia echinata. The planula larvae of the hydrozoan Clava multicornis are characterized by a complex nervous system formed by a frontal neural plexus composed by two distinct peptidergic cell populations with antero-posterior organization: a first, anterior-most GLWamide immunoreactive (GLW-IR) population and an RFamide immunoreactive (RF-IR) cell belt just posterior to the first population. In contrast to most swimming planulae, C. multicornis larvae display a smooth gliding movement on the substrate, characterized by alternate bending of the anterior pole. We tested the effects of RA and of a RA antagonist on C. multicornis development by analyzing the nervous system organization and the photoresponsive behavior of larvae exposed to RA during their embryogenesis. After the exposure, the nervous system became completely disorganized, leading to the displacement of RFamide-IR cells. Moreover, RA-treated larvae did not exhibit the typical phototropic behavior of control specimens. Our results suggest that RA can alter the normal development of nervous elements in this hydroid species, supporting the hypothesis that the morphogenetic activity of RA predates the origin of chordates.

Alessandro Dell'Anna

Papers

Complex neural architecture in the diploblastic larva of Clava multicornis (Hydrozoa, Cnidaria)

Immunohistochemical study of the nervous system of the tunicate Thalia democratica (Forsskal, 1775)

Neural system reorganization during metamorphosis in the planula larva of Clava multicornis (Hydrozoa, Cnidaria)

Retinoic acid influences antero-posterior positioning of peptidergic neurons in the planula larva of the hydrozoan Clava multicornis