The diversification and lineage-specific expansion of nitric oxide signaling in Placozoa: insights in the evolution of gaseous transmission

Electrical Signaling Beyond Neurons

Neural action potentials (APs) are difficult to interpret as signal encoders and/or computational primitives. Their relationships with stimuli and behaviors are obscured by the staggering complexity of nervous systems themselves. We can reduce this complexity by observing that "simpler" neuron-less organisms also transduce stimuli into transient electrical pulses that affect their behaviors. Without a complicated nervous system, APs are often easier to understand as signal/response mechanisms. We review examples of nonneural stimulus transductions in domains of life largely neglected by theoretical neuroscience: bacteria, protozoans, plants, fungi, and neuron-less animals. We report properties of those electrical signals-for example, amplitudes, durations, ionic bases, refractory periods, and particularly their ecological purposes. We compare those properties with those of neurons to infer the tasks and selection pressures that neurons satisfy. Throughout the tree of life, nonneural stimulus transductions time behavioral responses to environmental changes. Nonneural organisms represent the presence or absence of a stimulus with the presence or absence of an electrical signal. Their transductions usually exhibit high sensitivity and specificity to a stimulus, but are often slow compared to neurons. Neurons appear to be sacrificing the specificity of their stimulus transductions for sensitivity and speed. We interpret cellular stimulus transductions as a cell's assertion that it detected something important at that moment in time. In particular, we consider neural APs as fast but noisy detection assertions. We infer that a principal goal of nervous systems is to detect extremely weak signals from noisy sensory spikes under enormous time pressure. We discuss neural computation proposals that address this goal by casting neurons as devices that implement online, analog, probabilistic computations with their membrane potentials. Those proposals imply a measurable relationship between afferent neural spiking statistics and efferent neural membrane electrophysiology.

Parallel evolution of gravity sensing

Omnipresent gravity affects all living organisms; it was a vital factor in the past and the current bottleneck for future space exploration. However, little is known about the evolution of gravity sensing and the comparative biology of gravity reception. Here, by tracing the parallel evolution of gravity sensing, we encounter situations when assemblies of homologous modules result in the emergence of non-homologous structures with similar systemic properties. This is a perfect example to study homoplasy at all levels of biological organization. Apart from numerous practical implementations for bioengineering and astrobiology, the diversity of gravity signaling presents unique reference paradigms to understand hierarchical homology transitions to the convergent evolution of integrative systems. Second, by comparing gravisensory systems in major superclades of basal metazoans (ctenophores, sponges, placozoans, cnidarians, and bilaterians), we illuminate parallel evolution and alternative solutions implemented by basal metazoans toward spatial orientation, focusing on gravitational sensitivity and locomotory integrative systems.

Long-Term Culturing of Placozoans (Trichoplax and Hoilungia)

The phylum Placozoa remains one of the least explored among early-branching metazoan lineages. For over 130 years, this phylum had been represented by the single species Trichoplax adhaerens-an animal with the simplest known body plan (three cell layers without any organs) but complex behaviors. Recently, extensive sampling of placozoans across the globe and their subsequent genetic analysis have revealed incredible biodiversity with numerous cryptic species worldwide. However, only a few culture protocols are available to date, and all are for one species only. Here, we describe the breeding of four different species representing two placozoan genera: Trichoplax adhaerens, Trichoplax sp. H2, Hoilungia sp. H4, and Hoilungia hongkongensis originating from diverse biotopes. Our protocols allow to culture all species under comparable conditions. Next, we outlined various food sources and optimized strain-specific parameters enabling long-term culturing. These protocols can facilitate comparative analyses of placozoan biology and behaviors, which together will contribute to deciphering general principles of animal organization.

Brief History of Placozoa

Placozoans are morphologically the simplest free-living animals. They represent a unique window of opportunities to understand both the origin of the animal organization and the rules of life for the system and synthetic biology of the future. However, despite more than 100 years of their investigations, we know little about their organization, natural habitats, and life strategies. Here, we introduce this unique animal phylum and highlight some directions vital to broadening the frontiers of the biomedical sciences. In particular, understanding the genomic bases of placozoan biodiversity, cell identity, connectivity, reproduction, and cellular bases of behavior are critical hot spots for future studies.

Neuronal gene expression in two generations of the marine parasitic worm, Cryptocotyle lingua

Trematodes, or flukes, undergo intricate anatomical and behavioral transformations during their life cycle, yet the functional changes in their nervous system remain poorly understood. We investigated the molecular basis of nervous system function in Cryptocotyle lingua, a species of relevance for fisheries. Transcriptomic analysis revealed a streamlined molecular toolkit with the absence of key signaling pathways and ion channels. Notably, we observed the loss of nitric oxide synthase across the Platyhelminthes. Furthermore, we identified upregulated neuronal genes in dispersal larvae, including those involved in aminergic pathways, synaptic vesicle trafficking, TRPA channels, and surprisingly nitric oxide receptors. Using neuronal markers and in situ hybridization, we hypothesized their functional relevance to larval adaptations and host-finding strategies. Additionally, employing a behavior quantification toolkit, we assessed cercaria motility, facilitating further investigations into the behavior and physiology of parasitic flatworms. This study enhances our understanding of trematode neurobiology and provides insights for targeted antiparasitic strategies.

Exaptation and Evolutionary Adaptation in Nociceptor Mechanisms Driving Persistent Pain

BACKGROUND

Several evolutionary explanations have been proposed for why chronic pain is a major clinical problem. One is that some mechanisms important for driving chronic pain, while maladaptive for modern humans, were adaptive because they enhanced survival. Evidence is reviewed for persistent nociceptor hyperactivity (PNH), known to promote chronic pain in rodents and humans, being an evolutionarily adaptive response to significant bodily injury, and primitive molecular mechanisms related to cellular injury and stress being exapted (co-opted or repurposed) to drive PNH and consequent pain.

SUMMARY

PNH in a snail (Aplysia californica), squid (Doryteuthis pealeii), fruit fly (Drosophila melanogaster), mice, rats, and humans has been documented as long-lasting enhancement of action potential discharge evoked by peripheral stimuli, and in some of these species as persistent extrinsically driven ongoing activity and/or intrinsic spontaneous activity (OA and SA, respectively). In mammals, OA and SA are often initiated within the protected nociceptor soma long after an inducing injury. Generation of OA or SA in nociceptor somata may be very rare in invertebrates, but prolonged afterdischarge in nociceptor somata readily occurs in sensitized Aplysia. Evidence for the adaptiveness of injury-induced PNH has come from observations of decreased survival of injured squid exposed to predators when PNH is blocked, from plausible survival benefits of chronic sensitization after severe injuries such as amputation, and from the functional coherence and intricacy of mammalian PNH mechanisms. Major contributions of cAMP-PKA signaling (with associated calcium signaling) to the maintenance of PNH both in mammals and molluscs suggest that this ancient stress signaling system was exapted early during the evolution of nociceptors to drive hyperactivity following bodily injury. Vertebrates have retained core cAMP-PKA signaling modules for PNH while adding new extracellular modulators (e.g., opioids) and cAMP-regulated ion channels (e.g., TRPV1 and Nav1.8 channels).

KEY MESSAGES

Evidence from multiple phyla indicates that PNH is a physiological adaptation that decreases the risk of attacks on injured animals. Core cAMP-PKA signaling modules make major contributions to the maintenance of PNH in molluscs and mammals. This conserved signaling has been linked to ancient cellular responses to stress, which may have been exapted in early nociceptors to drive protective hyperactivity that can persist while bodily functions recover after significant injury.

What is it like to be a choanoflagellate? Sensation, processing and behavior in the closest unicellular relatives of animals

All animals evolved from a single lineage of unicellular precursors more than 600 million years ago. Thus, the biological and genetic foundations for animal sensation, cognition and behavior must necessarily have arisen by modifications of pre-existing features in their unicellular ancestors. Given that the single-celled ancestors of the animal kingdom are extinct, the only way to reconstruct how these features evolved is by comparing the biology and genomic content of extant animals to their closest living relatives. Here, we reconstruct the Umwelt (the subjective, perceptive world) inhabited by choanoflagellates, a group of unicellular (or facultatively multicellular) aquatic microeukaryotes that are the closest living relatives of animals. Although behavioral research on choanoflagellates remains patchy, existing evidence shows that they are capable of chemosensation, photosensation and mechanosensation. These processes often involve specialized sensorimotor cellular appendages (cilia, microvilli, and/or filopodia) that resemble those that underlie perception in most animal sensory cells. Furthermore, comparative genomics predicts an extensive "sensory molecular toolkit" in choanoflagellates, which both provides a potential basis for known behaviors and suggests the existence of a largely undescribed behavioral complexity that presents exciting avenues for future research. Finally, we discuss how facultative multicellularity in choanoflagellates might help us understand how evolution displaced the locus of decision-making from a single cell to a collective, and how a new space of behavioral complexity might have become accessible in the process.

Chemical cognition: chemoconnectomics and convergent evolution of integrative systems in animals

Neurons underpin cognition in animals. However, the roots of animal cognition are elusive from both mechanistic and evolutionary standpoints. Two conceptual frameworks both highlight and promise to address these challenges. First, we discuss evidence that animal neural and other integrative systems evolved more than once (convergent evolution) within basal metazoan lineages, giving us unique experiments by Nature for future studies. The most remarkable examples are neural systems in ctenophores and neuroid-like systems in placozoans and sponges. Second, in addition to classical synaptic wiring, a chemical connectome mediated by hundreds of signal molecules operates in tandem with neurons and is the most information-rich source of emerging properties and adaptability. The major gap-dynamic, multifunctional chemical micro-environments in nervous systems-is not understood well. Thus, novel tools and information are needed to establish mechanistic links between orchestrated, yet cell-specific, volume transmission and behaviors. Uniting what we call chemoconnectomics and analyses of the cellular bases of behavior in basal metazoan lineages arguably would form the foundation for deciphering the origins and early evolution of elementary cognition and intelligence.

Nitric Oxide Function and Nitric Oxide Synthase Evolution in Aquatic Chordates

Nitric oxide (NO) is a key signaling molecule in almost all organisms and is active in a variety of physiological and pathological processes. Our understanding of the peculiarities and functions of this simple gas has increased considerably by extending studies to non-mammal vertebrates and invertebrates. In this review, we report the nitric oxide synthase (Nos) genes so far characterized in chordates and provide an extensive, detailed, and comparative analysis of the function of NO in the aquatic chordates tunicates, cephalochordates, teleost fishes, and amphibians. This comprehensive set of data adds new elements to our understanding of Nos evolution, from the single gene commonly found in invertebrates to the three genes present in vertebrates.

Recording cilia activity in ctenophores: effects of nitric oxide and low molecular weight transmitters

Cilia are the major effectors in Ctenophores, but very little is known about their transmitter control and integration. Here, we present a simple protocol to monitor and quantify cilia activity and provide evidence for polysynaptic control of cilia coordination in ctenophores. We also screened the effects of several classical bilaterian neurotransmitters (acetylcholine, dopamine, L-DOPA, serotonin, octopamine, histamine, gamma-aminobutyric acid (GABA), L-aspartate, L-glutamate, glycine), neuropeptide (FMRFamide), and nitric oxide (NO) on cilia beating in Pleurobrachia bachei and Bolinopsis infundibulum. NO and FMRFamide produced noticeable inhibitory effects on cilia activity, whereas other tested transmitters were ineffective. These findings further suggest that ctenophore-specific neuropeptides could be major candidates for signal molecules controlling cilia activity in representatives of this early-branching metazoan lineage.

Nitric oxide suppresses cilia activity in ctenophores

Cilia are the major effectors in Ctenophores, but very little is known about their transmitter control and integration. Here, we present a simple protocol to monitor and quantify cilia activity in semi-intact preparations and provide evidence for polysynaptic control of cilia coordination in ctenophores. Next, we screen the effects of several classical bilaterian neurotransmitters (acetylcholine, dopamine, L-DOPA, serotonin, octopamine, histamine, gamma-aminobutyric acid (GABA), L-aspartate, L-glutamate, glycine), neuropeptides (FMRFamide), and nitric oxide (NO) on cilia beating in Pleurobrachia bachei and Bolinopsis infundibulum . Only NO inhibited cilia beating, whereas other tested transmitters were ineffective. These findings further suggest that ctenophore-specific neuropeptides could be major candidate signaling molecules controlling cilia activity in representatives of this early-branching metazoan lineage.

Study of NIT domain-containing chemoreceptors from two global phytopathogens and identification of NIT domains in eukaryotes

Bacterial signal transduction systems are typically activated by the binding of signal molecules to receptor ligand binding domains (LBDs), such as the NIT LBD. We report here the identification of the NIT domain in more than 15,000 receptors that were present in 30 bacterial phyla, but also in 19 eukaryotic phyla, expanding its known phylogenetic distribution. The NIT domain formed part of seven receptor families that either control transcription, mediate chemotaxis or regulate second messenger levels. We have produced the NIT domains from chemoreceptors of the bacterial phytopathogens Pectobacterium atrosepticum (PacN) and Pseudomonas savastanoi (PscN) as individual purified proteins. High-throughput ligand screening using compound libraries revealed a specificity for nitrate and nitrite binding. Isothermal titration calorimetry experiments showed that PacN-LBD bound preferentially nitrate ( K _D  = 1.9 μM), whereas the affinity of PscN-LBD for nitrite ( K _D  = 2.1 μM) was 22 times higher than that for nitrate. Analytical ultracentrifugation experiments indicated that PscN-LBD is monomeric in the presence and absence of ligands. The R182A mutant of PscN did not bind nitrate or nitrite. This residue is not conserved in the NIT domain of the Pseudomonas aeruginosa chemoreceptor PA4520, which may be related to its failure to bind nitrate/nitrite. The magnitude of P. atrosepticum chemotaxis towards nitrate was significantly greater than that of nitrite and pacN deletion almost abolished responses to both compounds. This study highlights the important role of nitrate and nitrite as signal molecules in life and advances our knowledge on the NIT domain as universal nitrate/nitrite sensor module.

Amino acids integrate behaviors in nerveless placozoans

Placozoans are the simplest known free-living animals without recognized neurons and muscles but a complex behavioral repertoire. However, mechanisms and cellular bases of behavioral coordination are unknown. Here, using Trichoplax adhaerens as a model, we described 0.02-0.002 Hz oscillations in locomotory and feeding patterns as evidence of complex multicellular integration; and showed their dependence on the endogenous secretion of signal molecules. Evolutionary conserved low-molecular-weight transmitters (glutamate, aspartate, glycine, GABA, and ATP) acted as coordinators of distinct locomotory and feeding patterns. Specifically, L-glutamate induced and partially mimicked endogenous feeding cycles, whereas glycine and GABA suppressed feeding. ATP-modified feeding is complex, first causing feeding-like cycles and then suppressing feeding. Trichoplax locomotion was modulated by glycine, GABA, and, surprisingly, by animals' own mucus trails. Mucus triples locomotory speed compared to clean substrates. Glycine and GABA increased the frequency of turns. The effects of the amino acids are likely mediated by numerous receptors (R), including those from ionotropic GluRs, metabotropic GluRs, and GABA-BR families. Eighty-five of these receptors are encoded in the Trichoplax genome, more than in any other animal sequenced. Phylogenetic reconstructions illuminate massive lineage-specific expansions of amino acid receptors in Placozoa, Cnidaria, and Porifera and parallel evolution of nutritional sensing. Furthermore, we view the integration of feeding behaviors in nerveless animals by amino acids as ancestral exaptations that pave the way for co-options of glutamate, glycine, GABA, and ATP as classical neurotransmitters in eumetazoans.

Nitric oxide signaling in ctenophores

Nitric oxide (NO) is one of the most ancient and versatile signal molecules across all domains of life. NO signaling might also play an essential role in the origin of animal organization. Yet, practically nothing is known about the distribution and functions of NO-dependent signaling pathways in representatives of early branching metazoans such as Ctenophora. Here, we explore the presence and organization of NO signaling components using Mnemiopsis and kin as essential reference species. We show that NO synthase (NOS) is present in at least eight ctenophore species, including Euplokamis and Coeloplana, representing the most basal ctenophore lineages. However, NOS could be secondarily lost in many other ctenophores, including Pleurobrachia and Beroe. In Mnemiopsis leidyi, NOS is present both in adult tissues and differentially expressed in later embryonic stages suggesting the involvement of NO in developmental mechanisms. Ctenophores also possess soluble guanylyl cyclases as potential NO receptors with weak but differential expression across tissues. Combined, these data indicate that the canonical NO-cGMP signaling pathways existed in the common ancestor of animals and could be involved in the control of morphogenesis, cilia activities, feeding and different behaviors.

Alternative neural systems: What is a neuron? (Ctenophores, sponges and placozoans)

How to make a neuron, a synapse, and a neural circuit? Is there only one 'design' for a neural architecture with a universally shared genomic blueprint across species? The brief answer is "No." Four early divergent lineages from the nerveless common ancestor of all animals independently evolved distinct neuroid-type integrative systems. One of these is a subset of neural nets in comb jellies with unique synapses; the second lineage is the well-known Cnidaria + Bilateria; the two others are non-synaptic neuroid systems in sponges and placozoans. By integrating scRNA-seq and microscopy data, we revise the definition of neurons as synaptically-coupled polarized and highly heterogenous secretory cells at the top of behavioral hierarchies with learning capabilities. This physiological (not phylogenetic) definition separates 'true' neurons from non-synaptically and gap junction-coupled integrative systems executing more stereotyped behaviors. Growing evidence supports the hypothesis of multiple origins of neurons and synapses. Thus, many non-bilaterian and bilaterian neuronal classes, circuits or systems are considered functional rather than genetic categories, composed of non-homologous cell types. In summary, little-explored examples of convergent neuronal evolution in representatives of early branching metazoans provide conceptually novel microanatomical and physiological architectures of behavioral controls in animals with prospects of neuro-engineering and synthetic biology.

The Evolution of Nitric Oxide Function: From Reactivity in the Prebiotic Earth to Examples of Biological Roles and Therapeutic Applications

Nitric oxide was once considered to be of marginal interest to the biological sciences and medicine; however, there is now wide recognition, but not yet a comprehensive understanding, of its functions and effects. NO is a reactive, toxic free radical with numerous biological targets, especially metal ions. However, NO and its reaction products also play key roles as reductant and oxidant in biological redox processes, in signal transduction, immunity and infection, as well as other roles. Consequently, it can be sensed, metabolized and modified in biological systems. Here, we present a brief overview of the chemistry and biology of NO—in particular, its origins in geological time and in contemporary biology, its toxic consequences and its critical biological functions. Given that NO, with its intrinsic reactivity, appeared in the early Earth’s atmosphere before the evolution of complex lifeforms, we speculate that the potential for toxicity preceded biological function. To examine this hypothesis, we consider the nature of non-biological and biological targets of NO, the evolution of biological mechanisms for NO detoxification, and how living organisms generate this multifunctional gas.

1,2,3-Triazine formation mechanism of the fairy chemical 2-azahypoxanthine in the fairy ring-forming fungus Lepista sordida

2-Azahypoxanthine (AHX) was first isolated from the culture broth of the fungus Lepista sordida as a fairy ring-inducing compound. It has since been found that a large number of plants and mushrooms produce AHX endogenously and that AHX has beneficial effects on plant growth. The AHX molecule has an unusual, nitrogen-rich 1,2,3-triazine moiety of unknown biosynthetic origin. Here, we establish the biosynthetic pathway for AHX formation in L. sordida. Our results reveal that the key nitrogen sources that are responsible for the 1,2,3-triazine formation are reactive nitrogen species (RNS), which are derived from nitric oxide (NO) produced by NO synthase (NOS). Furthermore, RNS are also involved in the biochemical conversion of 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranosyl 5'-monophosphate (AICAR) to AHX-ribotide (AHXR), suggesting that a novel biosynthetic route that produces AHX exists in the fungus. These findings demonstrate a physiological role for NOS in AHX biosynthesis as well as in biosynthesis of other natural products containing a nitrogen-nitrogen bond.

Novel Insights on Nitric Oxide Synthase and NO Signaling in Ascidian Metamorphosis

Nitric oxide (NO) is a pivotal signaling molecule involved in a wide range of physiological and pathological processes. We investigated NOS/NO localization patterns during the different stages of larval development in the ascidia Ciona robusta and evidenced a specific and temporally controlled pattern. NOS/NO expression starts in the most anterior sensory structures of the early larva and progressively moves towards the caudal portion as larval development and metamorphosis proceeds. We here highlight the pattern of NOS/NO expression in the central and peripheral nervous system of Ciona larvae which precisely follows the progression of neural signals of the central pattern generator necessary for the control of the movements of the larva towards the substrate. This highly dynamic localization profile perfectly matches with the central role played by NO from the first phase of settlement induction to the next control of swimming behavior, adhesion to substrate and progressive tissue resorption and reorganization of metamorphosis itself.

Expanding of Life Strategies in Placozoa: Insights From Long-Term Culturing of Trichoplax and Hoilungia

Placozoans are essential reference species for understanding the origins and evolution of animal organization. However, little is known about their life strategies in natural habitats. Here, by maintaining long-term culturing for four species of Trichoplax and Hoilungia, we extend our knowledge about feeding and reproductive adaptations relevant to the diversity of life forms and immune mechanisms. Three modes of population dynamics depended upon feeding sources, including induction of social behaviors, morphogenesis, and reproductive strategies. In addition to fission, representatives of all species produced “swarmers” (a separate vegetative reproduction stage), which could also be formed from the lower epithelium with greater cell-type diversity. We monitored the formation of specialized spheroid structures from the upper cell layer in aging culture. These “spheres” could be transformed into juvenile animals under favorable conditions. We hypothesize that spheroid structures represent a component of the innate immune defense response with the involvement of fiber cells. Finally, we showed that regeneration could be a part of the adaptive reproductive strategies in placozoans and a unique experimental model for regenerative biology.

Evolution of synapses and neurotransmitter systems: The divide-and-conquer model for early neural cell-type evolution

Nervous systems evolved around 560 million years ago to coordinate and empower animal bodies. Ctenophores - one of the earliest-branching lineages - are thought to share a few neuronal genes with bilaterians and may have evolved neurons convergently. Here we review our current understanding of the evolution of neuronal molecules in nonbilaterians. We also reanalyse single-cell sequencing data in light of new cell-cluster identities from a ctenophore and uncover evidence supporting the homology of one ctenophore neuron-type with neurons in Bilateria. The specific coexpression of the presynaptic proteins Unc13 and RIM with voltage-gated channels, neuropeptides and homeobox genes pinpoint a spiking sensory-peptidergic cell in the ctenophore mouth. Similar Unc13-RIM neurons may have been present in the first eumetazoans to rise to dominance only in stem Bilateria. We hypothesise that the Unc13-RIM lineage ancestrally innervated the mouth and conquered other parts of the body with the rise of macrophagy and predation during the Cambrian explosion.

Evolution of glutamatergic signaling and synapses

Glutamate (Glu) is the primary excitatory transmitter in the mammalian brain. But, we know little about the evolutionary history of this adaptation, including the selection of l-glutamate as a signaling molecule in the first place. Here, we used comparative metabolomics and genomic data to reconstruct the genealogy of glutamatergic signaling. The origin of Glu-mediated communications might be traced to primordial nitrogen and carbon metabolic pathways. The versatile chemistry of L-Glu placed this molecule at the crossroad of cellular biochemistry as one of the most abundant metabolites. From there, innovations multiplied. Many stress factors or injuries could increase extracellular glutamate concentration, which led to the development of modular molecular systems for its rapid sensing in bacteria and archaea. More than 20 evolutionarily distinct families of ionotropic glutamate receptors (iGluRs) have been identified in eukaryotes. The domain compositions of iGluRs correlate with the origins of multicellularity in eukaryotes. Although L-Glu was recruited as a neuro-muscular transmitter in the early-branching metazoans, it was predominantly a non-neuronal messenger, with a possibility that glutamatergic synapses evolved more than once. Furthermore, the molecular secretory complexity of glutamatergic synapses in invertebrates (e.g., Aplysia) can exceed their vertebrate counterparts. Comparative genomics also revealed 15+ subfamilies of iGluRs across Metazoa. However, most of this ancestral diversity had been lost in the vertebrate lineage, preserving AMPA, Kainate, Delta, and NMDA receptors. The widespread expansion of glutamate synapses in the cortical areas might be associated with the enhanced metabolic demands of the complex brain and compartmentalization of Glu signaling within modular neuronal ensembles.

The enigmatic Placozoa part 1: Exploring evolutionary controversies and poor ecological knowledge

The placozoan Trichoplax adhaerens is a tiny hairy plate and more simply organized than any other living metazoan. After its original description by F.E. Schulze in 1883, it attracted attention as a potential model for the ancestral state of metazoan organization, the "Urmetazoon". Trichoplax lacks any kind of symmetry, organs, nerve cells, muscle cells, basal lamina, and extracellular matrix. Furthermore, the placozoan genome is the smallest (not secondarily reduced) genome of all metazoan genomes. It harbors a remarkably rich diversity of genes and has been considered the best living surrogate for a metazoan ancestor genome. The phylum Placozoa presently harbors three formally described species, while several dozen "cryptic" species are yet awaiting their description. The phylogenetic position of placozoans has recently become a contested arena for modern phylogenetic analyses and view-driven claims. Trichoplax offers unique prospects for understanding the minimal requirements of metazoan animal organization and their corresponding malfunctions.

Hidden cell diversity in Placozoa: ultrastructural insights from Hoilungia hongkongensis

From a morphological point of view, placozoans are among the most simple free-living animals. This enigmatic phylum is critical for our understanding of the evolution of animals and their cell types. Their millimeter-sized, disc-like bodies consist of only three cell layers that are shaped by roughly seven major cell types. Placozoans lack muscle cells and neurons but are able to move using their ciliated lower surface and take up food in a highly coordinated manner. Intriguingly, the genome of Trichoplax adhaerens, the founding member of the enigmatic phylum, has disclosed a surprising level of genetic complexity. Moreover, recent molecular and functional investigations have uncovered a much larger, so-far hidden cell-type diversity. Here, we have extended the microanatomical characterization of a recently described placozoan species-Hoilungia hongkongensis. In H. hongkongensis, we recognized the established canonical three-layered placozoan body plan but also came across several morphologically distinct and potentially novel cell types, among them novel gland cells and "shiny spheres"-bearing cells at the upper epithelium. Thus, the diversity of cell types in placozoans is indeed higher than anticipated.

The American lobster genome reveals insights on longevity, neural, and immune adaptations

The American lobster, Homarus americanus, is integral to marine ecosystems and supports an important commercial fishery. This iconic species also serves as a valuable model for deciphering neural networks controlling rhythmic motor patterns and olfaction. Here, we report a high-quality draft assembly of the H. americanus genome with 25,284 predicted gene models. Analysis of the neural gene complement revealed extraordinary development of the chemosensory machinery, including a profound diversification of ligand-gated ion channels and secretory molecules. The discovery of a novel class of chimeric receptors coupling pattern recognition and neurotransmitter binding suggests a deep integration between the neural and immune systems. A robust repertoire of genes involved in innate immunity, genome stability, cell survival, chemical defense, and cuticle formation represents a diversity of defense mechanisms essential to thrive in the benthic marine environment. Together, these unique evolutionary adaptations contribute to the longevity and ecological success of this long-lived benthic predator.

Motility disorders and disintegration into separate cells of Trichoplax sp. H2 in the presence of Zn2+ ions and L-cysteine molecules: A systems approach

Placozoa remain an ancient multicellular system with a dynamic body structure where calcium ions carry out a primary role in maintaining the integrity of the entire animal. Zinc ions can compete with calcium ions adsorption. We studied the effect of zinc ions and l-cysteine molecules on the interaction of Trichoplax sp. H2 cells. The regularity of formless motion was diminished in the presence of 20-25 μM of Zn²⁺ ions leading to the formation of branching animal forms. Locomotor ciliated cells moved chaotically and independently of each other leaving the Trichoplax body and opening a network of fiber cells. Application of 100 μM cysteine resulted in dissociation of the plate into separate cells. The combined chemical treatment shifted the effect in a random sample of animals toward disintegration, i.e. initially leading to disorder of collective cell movement and then to total body fragmentation. Two dissociation patterns of Trichoplax plate as "expanding ring" and "bicycle wheel" were revealed. Analysis of the interaction of Ca²⁺ and Zn²⁺ ions with cadherin showed that more than half (54%) of the amino acid residues with which Ca²⁺ and Zn²⁺ ions bind are common. The contact interaction of cells covered by the cadherin molecules is important for the coordinated movements of Trichoplax organism, while zinc ions are capable to break junctions between the cells. The involvement of other players, for example, l-cysteine in the regulation of Ca²⁺-dependent adhesion may be critical leading to the typical dissociation of Trichoplax body like in a calcium-free environment. A hypothesis about the essential role of calcium ions in the emergence of Metazoa ancestor is proposed.

Neural versus alternative integrative systems: molecular insights into origins of neurotransmitters

Transmitter signalling is the universal chemical language of any nervous system, but little is known about its early evolution. Here, we summarize data about the distribution and functions of neurotransmitter systems in basal metazoans as well as outline hypotheses of their origins. We explore the scenario that neurons arose from genetically different populations of secretory cells capable of volume chemical transmission and integration of behaviours without canonical synapses. The closest representation of this primordial organization is currently found in Placozoa, disk-like animals with the simplest known cell composition but complex behaviours. We propose that injury-related signalling was the evolutionary predecessor for integrative functions of early transmitters such as nitric oxide, ATP, protons, glutamate and small peptides. By contrast, acetylcholine, dopamine, noradrenaline, octopamine, serotonin and histamine were recruited as canonical neurotransmitters relatively later in animal evolution, only in bilaterians. Ligand-gated ion channels often preceded the establishment of novel neurotransmitter systems. Moreover, lineage-specific diversification of neurotransmitter receptors occurred in parallel within Cnidaria and several bilaterian lineages, including acoels. In summary, ancestral diversification of secretory signal molecules provides unique chemical microenvironments for behaviour-driven innovations that pave the way to complex brain functions and elementary cognition. This article is part of the theme issue 'Basal cognition: multicellularity, neurons and the cognitive lens'.

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'.

Nitric Oxide Signaling in the Auditory Pathway

Nitric oxide (NO) is of fundamental importance in regulating immune, cardiovascular, reproductive, neuromuscular, and nervous system function. It is rapidly synthesized and cannot be confined, it is highly reactive, so its lifetime is measured in seconds. These distinctive properties (contrasting with classical neurotransmitters and neuromodulators) give rise to the concept of NO as a "volume transmitter," where it is generated from an active source, diffuses to interact with proteins and receptors within a sphere of influence or volume, but limited in distance and time by its short half-life. In the auditory system, the neuronal NO-synthetizing enzyme, nNOS, is highly expressed and tightly coupled to postsynaptic calcium influx at excitatory synapses. This provides a powerful activity-dependent control of postsynaptic intrinsic excitability via cGMP generation, protein kinase G activation and modulation of voltage-gated conductances. NO may also regulate vesicle mobility via retrograde signaling. This Mini Review focuses on the auditory system, but highlights general mechanisms by which NO mediates neuronal intrinsic plasticity and synaptic transmission. The dependence of NO generation on synaptic and sound-evoked activity has important local modulatory actions and NO serves as a "volume transmitter" in the auditory brainstem. It also has potentially destructive consequences during intense activity or on spill-over from other NO sources during pathological conditions, when aberrant signaling may interfere with the precisely timed and tonotopically organized auditory system.

Sodium action potentials in placozoa: Insights into behavioral integration and evolution of nerveless animals

Placozoa are small disc-shaped animals, representing the simplest known, possibly ancestral, organization of free-living animals. With only six morphological distinct cell types, without any recognized neurons or muscle, placozoans exhibit fast effector reactions and complex behaviors. However, little is known about electrogenic mechanisms in these animals. Here, we showed the presence of rapid action potentials in four species of placozoans (Trichoplax adhaerens [H1 haplotype], Trichoplax sp.[H2], Hoilungia hongkongensis [H13], and Hoilungia sp. [H4]). These action potentials are sodium-dependent and can be inducible. The molecular analysis suggests the presence of 5-7 different types of voltage-gated sodium channels, which showed substantial evolutionary radiation compared to many other metazoans. Such unexpected diversity of sodium channels in early-branched metazoan lineages reflect both duplication events and parallel evolution of unique behavioral integration in these nerveless animals.