Diversity of Cnidarian Muscles: Function, Anatomy, Development and Regeneration

Jellyfish for the study of nervous system evolution and function

Jellyfish comprise a diverse clade of free-swimming predators that arose prior to the Cambrian explosion. They play major roles in ocean ecosystems via a suite of complex foraging, reproductive, and defensive behaviors. These behaviors arise from decentralized, regenerative nervous systems composed of body parts that generate the appropriate part-specific behaviors autonomously following excision. Here, we discuss the organization of jellyfish nervous systems and opportunities afforded by the recent development of a genetically tractable jellyfish model for systems and evolutionary neuroscience.

The Palaeobiology of Two Crown Group Cnidarians: Haootia quadriformis and Mamsetia manunis gen. et sp. nov. from the Ediacaran of Newfoundland, Canada

The Ediacaran of eastern Newfoundland preserves the world's oldest known eumetazoan body fossils, as well as the earliest known record of fossilized muscular tissue. Re-examination of the holotype of the eight-armed Haootia quadriformis in terms of its morphology, the arrangement of its muscle filament bundles, and hitherto undescribed aspects of its anatomy support its interpretation as a crown staurozoan. We also document several new fossils preserving muscle tissue with a different muscular architecture to Haootia, but with only four arms. This new material allows us to describe a new crown group staurozoan, Mamsetia manunis gen. et sp. nov. This work confirms the presence of crown group medusozoan cnidarians of the Staurozoa in the Ediacaran of Newfoundland circa 565 Ma.

Homeostasis and evolution in relation to regeneration and repair

Homeostasis constitutes a key concept in physiology and refers to self-regulating processes that maintain internal stability when adjusting to changing external conditions. It diminishes internal entropy constituting a driving force behind evolution. Natural selection might act on homeostatic regulatory mechanisms and control mechanisms including homeodynamics, allostasis, hormesis and homeorhesis, where different stable stationary states are reached. Regeneration is under homeostatic control through hormesis. Damage to tissues initiates a response to restore the impaired equilibrium caused by mild stress using cell proliferation, cell differentiation and cell death to recover structure and function. Repair is a homeorhetic change leading to a new stable stationary state with decreased functionality and fibrotic scarring without reconstruction of the 3-D pattern. Mechanisms determining entrance of the tissue or organ to regeneration or repair include the balance between innate and adaptive immune cells in relation to cell plasticity and stromal stem cell responses, and redox balance. The regenerative and reparative capacities vary in different species, distinct tissues and organs, and at different stages of development including ageing. Many cell signals and pathways play crucial roles determining regeneration or repair by regulating protein synthesis, cellular growth, inflammation, proliferation, autophagy, lysosomal function, metabolism and metalloproteinase cell signalling. Attempts to favour the entrance of damaged tissues to regeneration in those with low proliferative rates have been made; however, there are evolutionary constraint mechanisms leading to poor proliferation of stem cells in unfavourable environments or tumour development. More research is required to better understand the regulatory processes of these mechanisms.

Insights and perspectives on the enigmatic alary muscles of arthropods

Three types of muscles, cardiac, smooth and skeletal muscles are classically distinguished in eubilaterian animals. The skeletal, striated muscles are innervated multinucleated syncytia, which, together with bones and tendons, carry out voluntary and reflex body movements. Alary muscles (AMs) are another type of striated syncytial muscles, which connect the exoskeleton to the heart in adult arthropods and were proposed to control hemolymph flux. Developmental studies in Drosophila showed that larval AMs are specified in embryos under control of conserved myogenic transcription factors and interact with excretory, respiratory and hematopoietic tissues in addition to the heart. They also revealed the existence of thoracic AMs (TARMs) connecting to specific gut regions. Their asymmetric attachment sites, deformation properties in crawling larvae and ablation-induced phenotypes, suggest that AMs and TARMs could play both architectural and signalling functions. During metamorphosis, and heart remodelling, some AMs trans-differentiate into another type of muscles. Remaining critical questions include the enigmatic modes and roles of AM innervation, mechanical properties of AMs and TARMS and their evolutionary origin. The purpose of this review is to consolidate facts and hypotheses surrounding AMs/TARMs and underscore the need for further detailed investigation into these atypical muscles.

Cell contractility in early animal evolution

Tissue deformation mediated by collective cell contractility is a signature characteristic of animals. In most animals, fast and reversible contractions of muscle cells mediate behavior, while slow and irreversible contractions of epithelial or mesenchymal cells play a key role in morphogenesis. Animal tissue contractility relies on the activity of the actin/myosin II complex (together referred to as 'actomyosin'), an ancient and versatile molecular machinery that performs a broad range of functions in development and physiology. This review synthesizes emerging insights from morphological and molecular studies into the evolutionary history of animal contractile tissue. The most ancient functions of actomyosin are cell crawling and cytokinesis, which are found in a wide variety of unicellular eukaryotes and in individual metazoan cells. Another contractile functional module, apical constriction, is universal in metazoans and shared with choanoflagellates, their closest known living relatives. The evolution of animal contractile tissue involved two key innovations: firstly, the ability to coordinate and integrate actomyosin assembly across multiple cells, notably to generate supracellular cables, which ensure tissue integrity but also allow coordinated morphogenesis and movements at the organism scale; and secondly, the evolution of dedicated contractile cell types for adult movement, belonging to two broad categories respectively defined by the expression of the fast (striated-type) and slow (smooth/non-muscle-type) myosin II paralogs. Both contractile cell types ancestrally resembled generic contractile epithelial or mesenchymal cells and might have played a versatile role in both behavior and morphogenesis. Modern animal contractile cells span a continuum between unspecialized contractile epithelia (which underlie behavior in modern placozoans), epithelia with supracellular actomyosin cables (found in modern sponges), epitheliomuscular tissues (with a concentration of actomyosin cables in basal processes, for example in sea anemones), and specialized muscle tissue that has lost most or all epithelial properties (as in ctenophores, jellyfish and bilaterians). Recent studies in a broad range of metazoans have begun to reveal the molecular basis of these transitions, powered by the elaboration of the contractile apparatus and the evolution of 'core regulatory complexes' of transcription factors specifying contractile cell identity.

Clade-specific genes and the evolutionary origin of novelty; new tools in the toolkit

Clade-specific (a.k.a. lineage-specific) genes are very common and found at all taxonomic levels and in all clades examined. They can arise by duplication of previously existing genes, which can involve partial truncations or combinations with other protein domains or regulatory sequences. They can also evolve de novo from non-coding sequences, leading to potentially truly novel protein domains. Finally, since clade-specific genes are generally defined by lack of sequence homology with other proteins, they can also arise by sequence evolution that is rapid enough that previous sequence homology can no longer be detected. In such cases, where the rapid evolution is followed by constraint, we consider them to be ontologically non-novel but likely novel at a functional level. In general, clade-specific genes have received less attention from biologists but there are increasing numbers of fascinating examples of their roles in important traits. Here we review some selected recent examples, and argue that attention to clade-specific genes is an important corrective to the focus on the conserved developmental regulatory toolkit that has been the habit of evo-devo as a field. Finally, we discuss questions that arise about the evolution of clade-specific genes, and how these might be addressed by future studies. We highlight the hypothesis that clade-specific genes are more likely to be involved in synapomorphies that arose in the stem group where they appeared, compared to other genes.

Muscle cell-type diversification is driven by bHLH transcription factor expansion and extensive effector gene duplications

Animals are typically composed of hundreds of different cell types, yet mechanisms underlying the emergence of new cell types remain unclear. Here we address the origin and diversification of muscle cells in the non-bilaterian, diploblastic sea anemone Nematostella vectensis. We discern two fast and two slow-contracting muscle cell populations, which differ by extensive sets of paralogous structural protein genes. We find that the regulatory gene set of the slow cnidarian muscles is remarkably similar to the bilaterian cardiac muscle, while the two fast muscles differ substantially from each other in terms of transcription factor profiles, though driving the same set of structural protein genes and having similar physiological characteristics. We show that anthozoan-specific paralogs of Paraxis/Twist/Hand-related bHLH transcription factors are involved in the formation of fast and slow muscles. Our data suggest that the subsequent recruitment of an entire effector gene set from the inner cell layer into the neural ectoderm contributes to the evolution of a novel muscle cell type. Thus, we conclude that extensive transcription factor gene duplications and co-option of effector modules act as an evolutionary mechanism underlying cell type diversification during metazoan evolution.

A complete biomechanical model of Hydra contractile behaviors, from neural drive to muscle to movement

How does neural activity drive muscles to produce behavior? The recent development of genetic lines in Hydra that allow complete calcium imaging of both neuronal and muscle activity, as well as systematic machine learning quantification of behaviors, makes this small cnidarian an ideal model system to understand and model the complete transformation from neural firing to body movements. To achieve this, we have built a neuromechanical model of Hydra's fluid-filled hydrostatic skeleton, showing how drive by neuronal activity activates distinct patterns of muscle activity and body column biomechanics. Our model is based on experimental measurements of neuronal and muscle activity and assumes gap junctional coupling among muscle cells and calcium-dependent force generation by muscles. With these assumptions, we can robustly reproduce a basic set of Hydra's behaviors. We can further explain puzzling experimental observations, including the dual timescale kinetics observed in muscle activation and the engagement of ectodermal and endodermal muscles in different behaviors. This work delineates the spatiotemporal control space of Hydra movement and can serve as a template for future efforts to systematically decipher the transformations in the neural basis of behavior.

Cellular adaptations leading to coral fragment attachment on artificial substrates in Acropora millepora (Am-CAM)

Reproductive propagation by asexual fragmentation in the reef-building coral Acropora millepora depends on (1) successful attachment to the reef substrate through modification of soft tissues and (2) a permanent bond with skeletal encrustation. Despite decades of research examining asexual propagation in corals, the initial response, cellular reorganisation, and development leading to fragment substrate attachment via a newly formed skeleton has not been documented in its entirety. Here, we establish the first "coral attachment model" for this species ("Am-CAM") by developing novel methods that allow correlation of fluorescence and electron microscopy image data with in vivo microscopic time-lapse imagery. This multi-scale imaging approach identified three distinct phases involved in asexual propagation: (1) the contact response of the coral fragment when contact with the substrate, followed by (2) fragment stabilisation through anchoring by the soft tissue, and (3) formation of a "lappet-like appendage" structure leading to substrate bonding of the tissue for encrustation through the onset of skeletal calcification. In developing Am-CAM, we provide new biological insights that can enable reef researchers, managers and coral restoration practitioners to begin evaluating attachment effectiveness, which is needed to optimise species-substrate compatibility and achieve effective outplanting.

siRNA-mediated gene knockdown via electroporation in hydrozoan jellyfish embryos

As the sister group to bilaterians, cnidarians stand in a unique phylogenetic position that provides insight into evolutionary aspects of animal development, physiology, and behavior. While cnidarians are classified into two types, sessile polyps and free-swimming medusae, most studies at the cellular and molecular levels have been conducted on representative polyp-type cnidarians and have focused on establishing techniques of genetic manipulation. Recently, gene knockdown by delivery of short hairpin RNAs into eggs via electroporation has been introduced in two polyp-type cnidarians, Nematostella vectensis and Hydractinia symbiolongicarpus, enabling systematic loss-of-function experiments. By contrast, current methods of genetic manipulation for most medusa-type cnidarians, or jellyfish, are quite limited, except for Clytia hemisphaerica, and reliable techniques are required to interrogate function of specific genes in different jellyfish species. Here, we present a method to knock down target genes by delivering small interfering RNA (siRNA) into fertilized eggs via electroporation, using the hydrozoan jellyfish, Clytia hemisphaerica and Cladonema paciificum. We show that siRNAs targeting endogenous GFP1 and Wnt3 in Clytia efficiently knock down gene expression and result in known planula phenotypes: loss of green fluorescence and defects in axial patterning, respectively. We also successfully knock down endogenous Wnt3 in Cladonema by siRNA electroporation, which circumvents the technical difficulty of microinjecting small eggs. Wnt3 knockdown in Cladonema causes gene expression changes in axial markers, suggesting a conserved Wnt/β-catenin-mediated pathway that controls axial polarity during embryogenesis. Our gene-targeting siRNA electroporation method is applicable to other animals, including and beyond jellyfish species, and will facilitate the investigation and understanding of myriad aspects of animal development.

Reproducible propagation technique for the symbiotic cnidarian model system Cassiopea xamachana

The phylum Cnidaria is composed of corals, jellyfish, hydras, and sea anemones. Cnidarians are well-known for their regenerative capability, with many species maintaining the ability to regenerate complete structures. This regenerative capacity has been used casually for propagation purposes (via dissection) for some cnidarians used in laboratory research but has yet been documented in a manner meant to be reproducible. One such cnidarian model system is the scyphozoan jellyfish Cassiopea xamachana. C. xamachana has become an emerging model system for studying the cnidarian-algal symbiotic relationship, so determining a reliable and fast method for expansion of laboratory animals is crucial. Here we outline a reproducible propagation method for continued generation and growth of C. xamachana polyps.

This article has an associated First Person interview with the first author of the paper.

Summary: This manuscript outlines a dissection protocol to propagate the upside-down jellyfish, Cassiopea xamachana.

Renewed perspectives on the sedentary-pelagic last common bilaterian ancestor

Various evaluations of the last common bilaterian ancestor (lcba) currently suggest that it resembled either a microscopic, non-segmented motile adult; or, on the contrary, a complex segmented adult motile urbilaterian. These fundamental inconsistencies remain largely unexplained. A majority of multidisciplinary data regarding sedentary adult ancestral bilaterian organization is overlooked. The sedentary-pelagic model is supported now by a number of novel developmental, paleontological and molecular phylogenetic data: (1) data in support of sedentary sponges, in the adult stage, as sister to all other Metazoa; (2) a similarity of molecular developmental pathways in both adults and larvae across sedentary sponges, cnidarians, and bilaterians; (3) a cnidarian-bilaterian relationship, including a unique sharing of a bona fide Hox-gene cluster, of which the evolutionary appearance does not connect directly to a bilaterian motile organization; (4) the presence of sedentary and tube-dwelling representatives of the main bilaterian clades in the early Cambrian; (5) an absence of definite taxonomic attribution of Ediacaran taxa reconstructed as motile to any true bilaterian phyla; (6) a similarity of tube morphology (and the clear presence of a protoconch-like apical structure of the Ediacaran sedentary Cloudinidae) among shells of the early Cambrian, and later true bilaterians, such as semi-sedentary hyoliths and motile molluscs; (7) recent data that provide growing evidence for a complex urbilaterian, despite a continuous molecular phylogenetic controversy. The present review compares the main existing models and reconciles the sedentary model of an urbilaterian and the model of a larva-like lcba with a unified sedentary(adult)-pelagic(larva) model of the lcba.

MRTF specifies a muscle-like contractile module in Porifera

Muscle-based movement is a hallmark of animal biology, but the evolutionary origins of myocytes are unknown. Although believed to lack muscles, sponges (Porifera) are capable of coordinated whole-body contractions that purge debris from internal water canals. This behavior has been observed for decades, but their contractile tissues remain uncharacterized with respect to their ultrastructure, regulation, and development. We examine the sponge Ephydatia muelleri and find tissue-wide organization of a contractile module composed of actin, striated-muscle myosin II, and transgelin, and that contractions are regulated by the release of internal Ca²⁺ stores upstream of the myosin-light-chain-kinase (MLCK) pathway. The development of this contractile module appears to involve myocardin-related transcription factor (MRTF) as part of an environmentally inducible transcriptional complex that also functions in muscle development, plasticity, and regeneration. As an actin-regulated force-sensor, MRTF-activity offers a mechanism for how the contractile tissues that line water canals can dynamically remodel in response to flow and can re-form normally from stem-cells in the absence of the intrinsic spatial cues typical of animal embryogenesis. We conclude that the contractile module of sponge tissues shares elements of homology with contractile tissues in other animals, including muscles, indicating descent from a common, multifunctional tissue in the animal stem-lineage.

Myocytes are a key cell type that enable animal movement, but their evolutionary origins remain unclear. Colgren and Nichols describe molecular and functional similarities between a contractile module in tissues of a sponge and muscle tissues in other animals, indicating a common evolutionary origin.

How to build single-celled cnidarians with worm-like motility: Lessons from Myxozoa

Metazoans with worm-like morphologies across diverse and disparate groups typically demonstrate motility generated by hydrostatic skeletons involving tissue layers (muscles and epithelia). Here we present representative morphological, behavioural and molecular data for parasitic cnidarians (myxozoans) that demonstrate unprecedented variation in form and function, developing as cellular hydrostats. Motile elongate plasmodia characterise a remarkable radiation of species in the genus Ceratomyxa. The vermiform plasmodia inhabit gall bladders of a range of South American freshwater fish and exhibit undulatory motility reminiscent of nematodes but achieved at the cellular level. Collective insights from ultrastructure, confocal and light microscopy along with videos depicting movements highlight key features that we propose explain the unique motility of the plasmodia. These features include cytoskeletal elements (net forming microfilaments and microtubules), a large internal vacuole, a relatively rigid outer glycocalyx and peripherally arranged mitochondria. These constituents provide collective evidence for repurposing of the cnidarian epitheliomuscular cell to support worm-like motility at the cellular level. The apparent restriction of vermiform ceratomyxids to South American freshwaters suggests an origination via Cretaceous or Miocene marine transgressions and subsequent radiation.

Studying of Molecular Regulation of Developmental Processes of Lower Metazoans Exemplified by Cnidaria Using High-Throughput Sequencing

A unique set of features and characteristics of species of the Cnidaria phylum is the one reason that makes them a model for a various studies. The plasticity of a life cycle and the processes of cell differentiation and development of an integral multicellular organism associated with it are of a specific scientific interest. A new stage of development of molecular genetic methods, including methods for high-throughput genome, transcriptome, and epigenome sequencing, both at the level of the whole organism and at the level of individual cells, makes it possible to obtain a detailed picture of the development of these animals. This review examines some modern approaches and advances in the reconstruction of the processes of ontogenesis of cnidarians by studying the regulatory signal transduction pathways and their interactions.

A myxozoan genome reveals mosaic evolution in a parasitic cnidarian

BACKGROUND

Parasite evolution has been conceptualized as a process of genetic loss and simplification. Contrary to this model, there is evidence of expansion and conservation of gene families related to essential functions of parasitism in some parasite genomes, reminiscent of widespread mosaic evolution-where subregions of a genome have different rates of evolutionary change. We found evidence of mosaic genome evolution in the cnidarian Myxobolus honghuensis, a myxozoan parasite of fish, with extremely simple morphology.

RESULTS

We compared M. honghuensis with other myxozoans and free-living cnidarians, and determined that it has a relatively larger myxozoan genome (206 Mb), which is less reduced and less compact due to gene retention, large introns, transposon insertion, but not polyploidy. Relative to other metazoans, the M. honghuensis genome is depleted of neural genes and has only the simplest animal immune components. Conversely, it has relatively more genes involved in stress resistance, tissue invasion, energy metabolism, and cellular processes compared to other myxozoans and free-living cnidarians. We postulate that the expansion of these gene families is the result of evolutionary adaptations to endoparasitism. M. honghuensis retains genes found in free-living Cnidaria, including a reduced nervous system, myogenic components, ANTP class Homeobox genes, and components of the Wnt and Hedgehog pathways.

CONCLUSIONS

Our analyses suggest that the M. honghuensis genome evolved as a mosaic of conservative, divergent, depleted, and enhanced genes and pathways. These findings illustrate that myxozoans are not as genetically simple as previously regarded, and the evolution of some myxozoans is driven by both genomic streamlining and expansion.

Muscle systems and motility of early animals highlighted by cnidarians from the basal Cambrian

Although fossil evidence suggests that various animal groups were able to move actively through their environment in the early stages of their evolution, virtually no direct information is available on the nature of their muscle systems. The origin of jellyfish swimming, for example, is of great interest to biologists. Exceptionally preserved muscles are described here in benthic peridermal olivooid medusozoans from the basal Cambrian of China (Kuanchuanpu Formation, ca. 535 Ma) that have direct equivalent in modern medusozoans. They consist of circular fibers distributed over the bell surface (subumbrella) and most probably have a myoepithelial origin. This is the oldest record of a muscle system in cnidarians and more generally in animals. This basic system was probably co-opted by early Cambrian jellyfish to develop capacities for jet-propelled swimming within the water column. Additional lines of fossil evidence obtained from ecdysozoans (worms and panarthropods) show that the muscle systems of early animals underwent a rapid diversification through the early Cambrian and increased their capacity to colonize a wide range of habitats both within the water column and sediment at a critical time of their evolutionary radiation.

Whole-animal multiplexed single-cell RNA-seq reveals transcriptional shifts across Clytia medusa cell types

We present an organism-wide, transcriptomic cell atlas of the hydrozoan medusa Clytia hemisphaerica and describe how its component cell types respond to perturbation. Using multiplexed single-cell RNA sequencing, in which individual animals were indexed and pooled from control and perturbation conditions into a single sequencing run, we avoid artifacts from batch effects and are able to discern shifts in cell state in response to organismal perturbations. This work serves as a foundation for future studies of development, function, and regeneration in a genetically tractable jellyfish species. Moreover, we introduce a powerful workflow for high-resolution, whole-animal, multiplexed single-cell genomics that is readily adaptable to other traditional or nontraditional model organisms.

Nematostella vectensis, an Emerging Model for Deciphering the Molecular and Cellular Mechanisms Underlying Whole-Body Regeneration

The capacity to regenerate lost or injured body parts is a widespread feature within metazoans and has intrigued scientists for centuries. One of the most extreme types of regeneration is the so-called whole body regenerative capacity, which enables regeneration of fully functional organisms from isolated body parts. While not exclusive to this habitat, whole body regeneration is widespread in aquatic/marine invertebrates. Over the past decade, new whole-body research models have emerged that complement the historical models Hydra and planarians. Among these, the sea anemone Nematostella vectensis has attracted increasing interest in regard to deciphering the cellular and molecular mechanisms underlying the whole-body regeneration process. This manuscript will present an overview of the biological features of this anthozoan cnidarian as well as the available tools and resources that have been developed by the scientific community studying Nematostella. I will further review our current understanding of the cellular and molecular mechanisms underlying whole-body regeneration in this marine organism, with emphasis on how comparing embryonic development and regeneration in the same organism provides insight into regeneration specific elements.

Regeneration Potential of Jellyfish: Cellular Mechanisms and Molecular Insights

Medusozoans, the Cnidarian subphylum, have multiple life stages including sessile polyps and free-swimming medusae or jellyfish, which are typically bell-shaped gelatinous zooplanktons that exhibit diverse morphologies. Despite having a relatively complex body structure with well-developed muscles and nervous systems, the adult medusa stage maintains a high regenerative ability that enables organ regeneration as well as whole body reconstitution from the part of the body. This remarkable regeneration potential of jellyfish has long been acknowledged in different species; however, recent studies have begun dissecting the exact processes underpinning regeneration events. In this article, we introduce the current understanding of regeneration mechanisms in medusae, particularly focusing on cellular behaviors during regeneration such as wound healing, blastema formation by stem/progenitor cells or cell fate plasticity, and the organism-level patterning that restores radial symmetry. We also discuss putative molecular mechanisms involved in regeneration processes and introduce a variety of novel model jellyfish species in the effort to understand common principles and diverse mechanisms underlying the regeneration of complex organs and the entire body.

Pattern regulation in a regenerating jellyfish

Jellyfish, with their tetraradial symmetry, offer a novel paradigm for addressing patterning mechanisms during regeneration. Here we show that an interplay between mechanical forces, cell migration and proliferation allows jellyfish fragments to regain shape and functionality rapidly, notably by efficient restoration of the central feeding organ (manubrium). Fragmentation first triggers actomyosin-powered remodeling that restores body umbrella shape, causing radial smooth muscle fibers to converge around 'hubs' which serve as positional landmarks. Stabilization of these hubs, and associated expression of Wnt6, depends on the configuration of the adjoining muscle fiber 'spokes'. Stabilized hubs presage the site of the manubrium blastema, whose growth is Wnt/β-catenin dependent and fueled by both cell proliferation and long-range cell recruitment. Manubrium morphogenesis is modulated by its connections with the gastrovascular canal system. We conclude that body patterning in regenerating jellyfish emerges mainly from local interactions, triggered and directed by the remodeling process.

The Diversity of Muscles and Their Regenerative Potential across Animals

Cells with contractile functions are present in almost all metazoans, and so are the related processes of muscle homeostasis and regeneration. Regeneration itself is a complex process unevenly spread across metazoans that ranges from full-body regeneration to partial reconstruction of damaged organs or body tissues, including muscles. The cellular and molecular mechanisms involved in regenerative processes can be homologous, co-opted, and/or evolved independently. By comparing the mechanisms of muscle homeostasis and regeneration throughout the diversity of animal body-plans and life cycles, it is possible to identify conserved and divergent cellular and molecular mechanisms underlying muscle plasticity. In this review we aim at providing an overview of muscle regeneration studies in metazoans, highlighting the major regenerative strategies and molecular pathways involved. By gathering these findings, we wish to advocate a comparative and evolutionary approach to prompt a wider use of “non-canonical” animal models for molecular and even pharmacological studies in the field of muscle regeneration.

Regulation of growth-related genes by nutrition in paralarvae of the common octopus (Octopus vulgaris)

The common octopus (Octopus vulgaris) is a species of great interest to the aquaculture industry. However, the high mortalities registered during different phases of the octopus lifecycle, particularly the paralarvae stage, present a challenge for commercial aquaculture. Improvement of diet formulation is seen as one way to reduce mortality and improve growth. Molecular growth-markers could help to improve rearing protocols and increase survival and growth performance; therefore, over a hundred orthologous genes related to protein balance and muscle growth in vertebrates were identified for the common octopus and their suitability as molecular markers for growth in octopus paralarvae explored. We successfully amplified 14 of those genes and studied their transcription in paralarvae either fed with artemia, artemia + zoea diets or submitted to a short fasting-refeeding procedure. Paralarvae fed with artemia + zoea had higher growth rates compared to those fed only with artemia, as well as a significant increase in octopus mtor (mtor-L) and hsp90 (hsp90-L) transcription, with both genes also up-regulated during refeeding. Our results suggest that at least mtor-L and hsp90-L are likely linked to somatic growth in octopus paralarvae. Conversely, ckip1-L, crk-L, src-L and srf-L had expression patterns that did not match to periods of growth as would be expected based on similar studies in vertebrates, indicating that further research is needed to understand their function during growth and in a muscle specific context.

Harnessing hypoxia as an evolutionary driver of complex multicellularity

Animal tissue requires low-oxygen conditions for its maintenance. The need for low-oxygen conditions contrasts with the idea of an evolutionary leap in animal diversity as a result of expanding oxic conditions. To accommodate tissue renewal at oxic conditions, however, vertebrate animals and vascular plants demonstrate abilities to access hypoxia. Here, I argue that multicellular organisms sustain oxic conditions first after internalizing hypoxic conditions. The 'harnessing' of hypoxia has allowed multicellular evolution to leave niches that were stable in terms of oxygen concentrations for those where oxygen fluctuates. Since oxygen fluctuates in most settings on Earth's surface, the ancestral niche would have been a deep marine setting. The hypothesis that 'large life' depends on harnessing hypoxia is illustrated in the context of conditions that promote the immature cell phenotype (stemness) in animal physiology and tumour biology and offers one explanation for the general rarity of diverse multicellularity over most of Earth's history.

Calcium signalling in early divergence of Metazoa: mechanisms involved in the control of muscle-like cell contraction in Hydra plagiodesmica

Our laboratory has previously examined the effect of neuropeptides on the activity of the hypostome of the hydra Hydra plagiodesmica Dioni, 1968 (Cnidaria: Hydrozoa). These results showed that the hypostome, a structure extruded during feeding, responds to myoregulatory peptides and that this mechanism might be regulated by changes in the cytosolic levels of calcium (Ca2+). We analyse now the ways in which Ca2+ modulates hypostome activity during feeding. The use of calcium chelators confirms that Ca2+ is relevant in inducing hypostome extrusion. The assay of compounds that modulate the activity of Ca2+ channels in the endoplasmic reticulum suggests that, beyond the extracellular influx of calcium, intracellular sources of the ion are involved and might include both ryanodine receptors (RyR) and the inositol 1,4,5-trisphosphate receptor (IP3R). Bioinformatic searches based on sequences of RyR and IP3R of humans (Homo sapiens Linnaeus, 1758) show that IP3Rs are present in all groups analysed, including Fungi and Choanoflagellata. Although H. plagiodesmica responds to caffeine and ryanodine, which are known to modulate RyRs, this family of receptors seems not to be predicted in Cnidaria, suggesting that this phylum either lacks these kinds of channels or that they possess a different structure compared with those possessed by other Metazoa.

The genome of the giant Nomura's jellyfish sheds light on the early evolution of active predation

BACKGROUND

Unique among cnidarians, jellyfish have remarkable morphological and biochemical innovations that allow them to actively hunt in the water column and were some of the first animals to become free-swimming. The class Scyphozoa, or true jellyfish, are characterized by a predominant medusa life-stage consisting of a bell and venomous tentacles used for hunting and defense, as well as using pulsed jet propulsion for mobility. Here, we present the genome of the giant Nomura's jellyfish (Nemopilema nomurai) to understand the genetic basis of these key innovations.

RESULTS

We sequenced the genome and transcriptomes of the bell and tentacles of the giant Nomura's jellyfish as well as transcriptomes across tissues and developmental stages of the Sanderia malayensis jellyfish. Analyses of the Nemopilema and other cnidarian genomes revealed adaptations associated with swimming, marked by codon bias in muscle contraction and expansion of neurotransmitter genes, along with expanded Myosin type II family and venom domains, possibly contributing to jellyfish mobility and active predation. We also identified gene family expansions of Wnt and posterior Hox genes and discovered the important role of retinoic acid signaling in this ancient lineage of metazoans, which together may be related to the unique jellyfish body plan (medusa formation).

CONCLUSIONS

Taken together, the Nemopilema jellyfish genome and transcriptomes genetically confirm their unique morphological and physiological traits, which may have contributed to the success of jellyfish as early multi-cellular predators.

How does the environment affect fighting? The interaction between extrinsic fighting ability and resource value during contests

An individual's performance during a fight is influenced by a combination of their capacity and willingness to compete. While willingness to fight is known to be determined by both intrinsic and extrinsic drivers, an individual's capacity to fight is generally thought of as solely intrinsic, being driven by a host of physiological factors. However, evidence indicates that variation in fighting ability can also be generated through exposure to different environmental conditions. Environmental contributions to fighting ability may be particularly important for animals living in spatially and temporally heterogeneous habitats, in which fights can occur between rivals recently exposed to different environmental conditions. The rapidly changing environment experienced within intertidal zones, for example, means that seawater parameters, including dissolved oxygen content and temperature, can vary across small spatial and temporal scales. Here, we investigated the relative importance of these extrinsic contributions to fighting ability and resource value on contest dynamics in the beadlet sea anemone Actinia equina We manipulated the extrinsic fighting ability of both opponents (through dissolved oxygen concentration prior to fights) and resource value (through seawater flow rate during the fight). Our results indicate that the extrinsic fighting ability of both opponents can interact with resource value to drive escalation patterns and that extrinsic drivers can be more important in determining contest dynamics than the intrinsic traits commonly studied. Our study highlights the need to combine data on intrinsic state and extrinsic conditions in order to gain a more holistic view of the factors driving contest behaviour.

Self/Non-Self Recognition Affects Cnida Discharge and Tentacle Contraction in the Sea Anemone Haliplanella luciae

Certain species of sea anemone live in tightly packed communities, among clonemates and non-clonemates. Competition for space leads to intraspecific and interspecific aggressive interactions among anemones. The initial aggressive interactions appear to involve reciprocal discharge of cnidae triggered by contact with non-self feeding tentacles. We asked whether molecules contained in anemone-derived mucus constituted an important cue alone or in combination with cell surface molecules in stimulating aggressive or avoidance behaviors. In this study, we found that self and non-self stimuli differentially influenced two effector systems: cnida discharge and tentacle contraction. Interspecific mucus enhanced nematocyst discharge by 44% and spirocyst discharge by 90%, as compared to baseline discharge obtained in seawater alone. Conspecific stimuli accompanying touch inhibited specific tentacle contractions occurring on the far side of anemones relative to the site of contact. The greatest tentacle contractions occurred with exposure to interspecific mucus and tissue. Thus, several receptor systems are involved that integrate chemical and mechanical cues in order to initiate appropriate and graded effector responses during competition for space.

Smooth muscle-like Ca2+-regulation of actin-myosin interaction in adult jellyfish striated muscle

Cnidaria is an animal phylum, whose members probably have the most ancestral musculature. We prepared and characterized, for the first time to our knowledge, native actomyosin from the striated myoepithelium of the adult moon jelly Aurelia sp. The actomyosin contained myosin, paramyosin-like protein, Ser/Thr-kinase, actin, and two isoforms of tropomyosin, but not troponin, which is known to activate contraction dependent on intracellular Ca²⁺ signaling in almost all striated muscles of bilaterians. Notably, the myosin comprised striated muscle-type heavy chain and smooth muscle-type regulatory light chains. In the presence of Ca²⁺, the Mg-ATPase activity of actomyosin was stimulated and Ser21 of the regulatory light chain was concomitantly phosphorylated by the addition of calmodulin and myosin light chain kinase prepared from chicken smooth muscle. Collectively, these results suggest that, similar to smooth muscle, the contraction of jellyfish striated muscle is regulated by Ca²⁺-dependent phosphorylation of the myosin light chain.

A gonad-expressed opsin mediates light-induced spawning in the jellyfish Clytia

Across the animal kingdom, environmental light cues are widely involved in regulating gamete release, but the molecular and cellular bases of the photoresponsive mechanisms are poorly understood. In hydrozoan jellyfish, spawning is triggered by dark-light or light-dark transitions acting on the gonad, and is mediated by oocyte maturation-inducing neuropeptide hormones (MIHs) released from the ectoderm. We determined in Clytia hemisphaerica that blue-cyan light triggers spawning in isolated gonads. A candidate opsin (Opsin9) was found co-expressed with MIH within specialised ectodermal cells. Opsin9 knockout jellyfish generated by CRISPR/Cas9 failed to undergo oocyte maturation and spawning, a phenotype reversible by synthetic MIH. Gamete maturation and release in Clytia is thus regulated by gonadal photosensory-neurosecretory cells that secrete MIH in response to light via Opsin9. Similar cells in ancestral eumetazoans may have allowed tissue-level photo-regulation of diverse behaviours, a feature elaborated in cnidarians in parallel with expansion of the opsin gene family.

Many animals living in the sea reproduce by releasing sperm and egg cells at the same time into the surrounding water. Animals often use changes in ambient light at dawn and dusk as reliable daily cues to coordinate this spawning behavior between individuals. For example, jellyfish of the species Clytia hemisphaerica, which can easily be raised in the laboratory, spawn exactly two hours after the light comes on.

Researchers recently discovered that spawning in Clytia and other related jellyfish species is coordinated by a hormone called ‘oocyte maturation-inducing hormone’, or MIH for short. This hormone is produced by a cell layer that surrounds the immature eggs and sperm within each reproductive organ, and is secreted in response to light cues. It then diffuses both inside and outside of the jellyfish, and triggers the production of mature eggs and sperm, followed by their release into the ocean. However, until now it was not known which cells and molecules are responsible for detecting light to initiate the secretion of MIH.

Quiroga Artigas et al. – including some of the researchers involved in the MIH work – now discovered that a single specialised cell type in the reproductive organs of Clytia responds to light and secretes MIH. These cells contain a light-sensitive protein called Opsin9, which is closely related to the opsin proteins in the human eye well known for their role in vision. When Opsin9 was experimentally mutated, Clytia cells could not secrete MIH in response to light, and the jellyfish failed to spawn. This opsin protein is thus necessary to detect light in order to trigger spawning in jellyfish.

A next step will be to examine and compare whether other proteins of the opsin family and hormones related to MIH also regulate spawning in other marine animals. This could have practical benefits for raising marine animals in aquariums and as food resources, and in initiatives to protect the environment. More widely, these findings could help unravel how sexual reproduction has evolved within the animal kingdom.

Mef2 and the skeletal muscle differentiation program

Mef2 is a conserved and significant transcription factor in the control of muscle gene expression. In cell culture Mef2 synergises with MyoD-family members in the activation of gene expression and in the conversion of fibroblasts into myoblasts. Amongst its in vivo roles, Mef2 is required for both Drosophila muscle development and mammalian muscle regeneration. Mef2 has functions in other cell-types too, but this review focuses on skeletal muscle and surveys key findings on Mef2 from its discovery, shortly after that of MyoD, up to the present day. In particular, in vivo functions, underpinning mechanisms and areas of uncertainty are highlighted. We describe how Mef2 sits at a nexus in the gene expression network that controls the muscle differentiation program, and how Mef2 activity must be regulated in time and space to orchestrate specific outputs within the different aspects of muscle development. A theme that emerges is that there is much to be learnt about the different Mef2 proteins (from different paralogous genes, spliced transcripts and species) and how the activity of these proteins is controlled.