Evolution of striated muscle: Jellyfish and the origin of triploblasty

How animals move: comparative lessons on animal locomotion

Comparative physiology often provides unique insights in animal structure and function. It is specifically through this lens that we discuss the fundamental properties of skeletal muscle and animal locomotion, incorporating variation in body size and evolved difference among species. For example, muscle frequencies in vivo are highly constrained by body size, which apparently tunes muscle use to maximize recovery of elastic recoil potential energy. Secondary to this constraint, there is an expected linking of skeletal muscle structural and functional properties. Muscle is relatively simple structurally, but by changing proportions of the few muscle components, a diverse range of functional outputs is possible. Thus, there is a consistent and predictable relation between muscle function and myocyte composition that illuminates animal locomotion. When animals move, the mechanical properties of muscle diverge from the static textbook force-velocity relations described by A. V. Hill, as recovery of elastic potential energy together with force and power enhancement with activation during stretch combine to modulate performance. These relations are best understood through the tool of work loops. Also, when animals move, locomotion is often conveniently categorized energetically. Burst locomotion is typified by high-power outputs and short durations while sustained, cyclic, locomotion engages a smaller fraction of the muscle tissue, yielding lower force and power. However, closer examination reveals that rather than a dichotomy, energetics of locomotion is a continuum. There is a remarkably predictable relationship between duration of activity and peak sustainable performance.

A detailed description of the development of the hemichordate Saccoglossus kowalevskii using SEM, TEM, Histology and 3D-reconstructions

INTRODUCTION

Traditionally, the origin of the third germ layer and its special formation of coelomic cavities by enterocoely is regarded to be an informative character in phylogenetic analyses. In early deuterostomes such as sea urchins, the mesoderm forms through a single evagination pinching off from the apical end of the archenteron which then gives off mesocoela and metacoela on each side. This echinoid-type coelom formation has conventionally been assumed to be ancestral for Deuterostomia. However, recent phylogenetic analyses show that Echinodermata hold a more derived position within Deuterostomia. In this regard a subgroup of Hemichordata, namely enteropneusts, seem to host promising candidates, because they are supposed to have retained many ancestral deuterostome features on the one hand, and furthermore share some characteristics with chordates on the other hand. In enteropneusts a wide range of different modes of coelom formation has been reported and in many cases authors of the original observations carefully detailed the limitations of their descriptions, while these doubts disappeared in subsequent reviews. In the present study, we investigated the development of all tissues in an enteropneust, Saccoglossus kowalevskii by using modern morphological techniques such as complete serial sectioning for LM and TEM, and 3D-reconstructions, in order to contribute new data to the elucidation of deuterostome evolution.

RESULTS

Our data show that in the enteropneust S. kowalevskii all main coelomic cavities (single protocoel, paired mesocoela and metacoela) derive from the endoderm via enterocoely as separate evaginations, in contrast to the aforementioned echinoid-type. The anlagen of the first pair of gill slits emerge at the late kink stage (~96 h pf). From that time onwards, we documented a temporal left-first development of the gill slits and skeletal gill rods in S. kowalevskii until the 2 gill slit juvenile stage.

CONCLUSIONS

The condition of coelom formation from separate evaginations is recapitulated in the larva of amphioxus and can be observed in crinoid echinoderms in a similar way. Therefore, coelom formation from separated pouches, rather than from a single apical pouch with eventual subdivision is suggested as the ancestral type of coelom formation for Deuterostomia. Left-right asymmetries are also present in echinoderms (rudiment formation), cephalochordates (larval development), tunicates (gut coiling) and vertebrates (visceral organs), and it is known from other studies applying molecular genetic analyses that genes such as nodal, lefty and pitx are involved during development. We discuss our findings in S. kowalevskii in the light of morphological as well as molecular genetic data.

Mesodermal gene expression in the acoel Isodiametra pulchra indicates a low number of mesodermal cell types and the endomesodermal origin of the gonads

Acoelomorphs are bilaterally symmetric small marine worms that lack a coelom and possess a digestive system with a single opening. Two alternative phylogenetic positions of this group within the animal tree are currently debated. In one view, Acoelomorpha is the sister group to all remaining Bilateria and as such, is a morphologically simple stepping stone in bilaterian evolution. In the other, the group is a lineage within the Deuterostomia, and therefore, has derived a simple morphology from a more complex ancestor. Acoels and the closely related Nemertodermatida and Xenoturbellida, which together form the Acoelomorpha, possess a very limited number of cell types. To further investigate the diversity and origin of mesodermal cell types we describe the expression pattern of 12 orthologs of bilaterian mesodermal markers including Six1/2, Twist, FoxC, GATA4/5/6, in the acoel Isodiametra pulchra. All the genes are expressed in stem cells (neoblasts), gonads, and at least subsets of the acoel musculature. Most are expressed in endomesodermal compartments of I. pulchra developing embryos similar to what has been described in cnidarians. Our molecular evidence indicates a very limited number of mesodermal cell types and suggests an endomesodermal origin of the gonads and the stem cell system. We discuss our results in light of the two prevailing phylogenetic positions of Acoelomorpha.

Short toxin-like proteins abound in Cnidaria genomes

Cnidaria is a rich phylum that includes thousands of marine species. In this study, we focused on Anthozoa and Hydrozoa that are represented by the Nematostella vectensis (Sea anemone) and Hydra magnipapillata genomes. We present a method for ranking the toxin-like candidates from complete proteomes of Cnidaria. Toxin-like functions were revealed using ClanTox, a statistical machine-learning predictor trained on ion channel inhibitors from venomous animals. Fundamental features that were emphasized in training ClanTox include cysteines and their spacing along the sequences. Among the 83,000 proteins derived from Cnidaria representatives, we found 170 candidates that fulfill the properties of toxin-like-proteins, the vast majority of which were previously unrecognized as toxins. An additional 394 short proteins exhibit characteristics of toxin-like proteins at a moderate degree of confidence. Remarkably, only 11% of the predicted toxin-like proteins were previously classified as toxins. Based on our prediction methodology and manual annotation, we inferred functions for over 400 of these proteins. Such functions include protease inhibitors, membrane pore formation, ion channel blockers and metal binding proteins. Many of the proteins belong to small families of paralogs. We conclude that the evolutionary expansion of toxin-like proteins in Cnidaria contributes to their fitness in the complex environment of the aquatic ecosystem.

Independent evolution of striated muscles in cnidarians and bilaterians

Striated muscles are present in bilaterian animals (for example, vertebrates, insects and annelids) and some non-bilaterian eumetazoans (that is, cnidarians and ctenophores). The considerable ultrastructural similarity of striated muscles between these animal groups is thought to reflect a common evolutionary origin. Here we show that a muscle protein core set, including a type II myosin heavy chain (MyHC) motor protein characteristic of striated muscles in vertebrates, was already present in unicellular organisms before the origin of multicellular animals. Furthermore, 'striated muscle' and 'non-muscle' myhc orthologues are expressed differentially in two sponges, compatible with a functional diversification before the origin of true muscles and the subsequent use of striated muscle MyHC in fast-contracting smooth and striated muscle. Cnidarians and ctenophores possess striated muscle myhc orthologues but lack crucial components of bilaterian striated muscles, such as genes that code for titin and the troponin complex, suggesting the convergent evolution of striated muscles. Consistently, jellyfish orthologues of a shared set of bilaterian Z-disc proteins are not associated with striated muscles, but are instead expressed elsewhere or ubiquitously. The independent evolution of eumetazoan striated muscles through the addition of new proteins to a pre-existing, ancestral contractile apparatus may serve as a model for the evolution of complex animal cell types.

Independent specialisation of myosin II paralogues in muscle vs. non-muscle functions during early animal evolution: a ctenophore perspective

Background

Myosin II (or Myosin Heavy Chain II, MHCII) is a family of molecular motors involved in the contractile activity of animal muscle cells but also in various other cellular processes in non-muscle cells. Previous phylogenetic analyses of bilaterian MHCII genes identified two main clades associated respectively with smooth/non-muscle cells (MHCIIa) and striated muscle cells (MHCIIb). Muscle cells are generally thought to have originated only once in ancient animal history, and decisive insights about their early evolution are expected to come from expression studies of Myosin II genes in the two non-bilaterian phyla that possess muscles, the Cnidaria and Ctenophora.

Results

We have uncovered three MHCII paralogues in the ctenophore species Pleurobrachia pileus. Phylogenetic analyses indicate that the MHCIIa / MHCIIb duplication is more ancient than the divergence between extant metazoan lineages. The ctenophore MHCIIa gene (PpiMHCIIa) has an expression pattern akin to that of "stem cell markers" (Piwi, Vasa…) and is expressed in proliferating cells. We identified two MHCIIb genes that originated from a ctenophore-specific duplication. PpiMHCIIb1 represents the exclusively muscular form of myosin II in ctenophore, while PpiMHCIIb2 is expressed in non-muscle cells of various types. In parallel, our phalloidin staining and TEM observations highlight the structural complexity of ctenophore musculature and emphasize the experimental interest of the ctenophore tentacle root, in which myogenesis is spatially ordered and strikingly similar to striated muscle formation in vertebrates.

Conclusion

MHCIIa expression in putative stem cells/proliferating cells probably represents an ancestral trait, while specific involvement of some MHCIIa genes in smooth muscle fibres is a uniquely derived feature of the vertebrates. That one ctenophore MHCIIb paralogue (PpiMHCIIb2) has retained MHCIIa-like expression features furthermore suggests that muscular expression of the other paralogue, PpiMHCIIb1, was the result of neofunctionalisation within the ctenophore lineage, making independent origin of ctenophore muscle cells a likely option.

Aurelia aurita-Cnidarian with a prominent medusiod stage

Aurelia aurita has a complex life cycle that consists of several stages including alternating generations of medusa and polyps, huge sexual, and tiny asexual stages. Cnidarian is thought to possess two tissue layers: endoderm (gastroderm) and ectoderm, which are separated by mesoglea in medusa. The determination of the composition of the A. aurita jellyfish mesoglea was performed. New protein "mesoglein" was determined as one of the main components of mesoglea. Mesoglein is synthesized by mesogleal cells (Mc), which are populated A. aurita mesoglea as a high molecular mass precursor. Mc are involved in the formation of noncollagenous "elastic" fibers. Deduced amino acid sequence of mesoglein contains Zona Pellucida (ZP) domain and Delta/Serrate/Lag-2 domain. According to reverse transcription PCR, mesoglein is expressed in the mature medusa exclusively in the Mc. The sperm binding to the ZP is particularly important for successful fertilization. Antibodies against mesoglein stain the plate in the place of contact of germinal epithelium and oocyte. The structure found was named the "contact plate." The contact plate could be the precursor of the ZP. All our data suggest that Mc and, probably, the whole mesoglea originate from the epidermis (ectoderm). Computer search for mesoglein relatives reveals Nematostella and Trichoplax proteins as predicted ORFs, indicating that ZP proteins are quite ancient purchase in the evolution.

Molecular architecture of muscles in an acoel and its evolutionary implications

We have characterized the homologs of an actin, a troponin I, and a tropomyosin gene in the acoel Symsagittifera roscoffensis. These genes are expressed in muscles and most likely coexpressed in at least a subset of them. In addition, and for the first time for Acoela, we have produced a species-specific muscular marker, an antibody against the tropomyosin protein. We have followed tropomyosin gene and protein expression during postembryonic development and during the posterior regeneration of amputated adults, showing that preexisting muscle fibers contribute to the wound closure. The three genes characterized in this study interact in the striated muscles of vertebrates and invertebrates, where troponin I and tropomyosin are key regulators of the contraction of the sarcomere. S. roscoffensis and all other acoels so far described have only smooth muscles, but the molecular architecture of these is the same as that of striated fibers of other bilaterians. Given the proposed basal position of acoels within the Bilateria, we suggest that sarcomeric muscles arose from a smooth muscle type, which had the molecular repertoire of striated musculature already in place. We discuss this model in a broad comparative perspective.

Transphyletic conservation of developmental regulatory state in animal evolution

Specific regulatory states, i.e., sets of expressed transcription factors, define the gene expression capabilities of cells in animal development. Here we explore the functional significance of an unprecedented example of regulatory state conservation from the cnidarian Nematostella to Drosophila, sea urchin, fish, and mammals. Our probe is a deeply conserved cis-regulatory DNA module of the SRY-box B2 (soxB2), recognizable at the sequence level across many phyla. Transphyletic cis-regulatory DNA transfer experiments reveal that the plesiomorphic control function of this module may have been to respond to a regulatory state associated with neuronal differentiation. By introducing expression constructs driven by this module from any phyletic source into the genomes of diverse developing animals, we discover that the regulatory state to which it responds is used at different levels of the neurogenic developmental process, including patterning and development of the vertebrate forebrain and neurogenesis in the Drosophila optic lobe and brain. The regulatory state recognized by the conserved DNA sequence may have been redeployed to different levels of the developmental regulatory program during evolution of complex central nervous systems.

Evolution of gene regulatory networks controlling body plan development

Evolutionary change in animal morphology results from alteration of the functional organization of the gene regulatory networks (GRNs) that control development of the body plan. A major mechanism of evolutionary change in GRN structure is alteration of cis-regulatory modules that determine regulatory gene expression. Here we consider the causes and consequences of GRN evolution. Although some GRN subcircuits are of great antiquity, other aspects are highly flexible and thus in any given genome more recent. This mosaic view of the evolution of GRN structure explains major aspects of evolutionary process, such as hierarchical phylogeny and discontinuities of paleontological change.

The contractile sponge epithelium sensu lato – body contraction of the demosponge Tethya wilhelma is mediated by the pinacoderm

Sponges constitute one of the two metazoan phyla that are able to contract their bodies despite a complete lack of muscle cells. Two competing hypotheses on the mechanisms behind this have been postulated to date: (1) mesohyl-mediated contraction originating from fusiform smooth muscle-like actinocytes (‘myocytes’) and (2) epidermal contraction originating in pinacocytes. No direct support exists for either hypothesis. The question of agonist–antagonist interaction in sponge contraction seems to have been completely neglected so far. In the present study we addressed this by studying sponge contraction kinetics. We also tested both hypotheses by carrying out volumetric studies of 3D synchrotron radiation-based x-ray microtomography data obtained from contracted and expanded specimens of Tethya wilhelma. Our results support the pinacoderm contraction hypothesis. Should mesohyl contraction be present, it is likely to be part of the antagonist system. We conclude that epithelial contraction plays a major role in sponges. Contractile epithelia sensu lato may be regarded as part of the ground pattern of the Metazoa.

Origin and early evolution of neural circuits for the control of ciliary locomotion

Behaviour evolved before nervous systems. Various single-celled eukaryotes (protists) and the ciliated larvae of sponges devoid of neurons can display sophisticated behaviours, including phototaxis, gravitaxis or chemotaxis. In single-celled eukaryotes, sensory inputs directly influence the motor behaviour of the cell. In swimming sponge larvae, sensory cells influence the activity of cilia on the same cell, thereby steering the multicellular larva. In these organisms, the efficiency of sensory-to-motor transformation (defined as the ratio of sensory cells to total cell number) is low. With the advent of neurons, signal amplification and fast, long-range communication between sensory and motor cells became possible. This may have first occurred in a ciliated swimming stage of the first eumetazoans. The first axons may have had en passant synaptic contacts to several ciliated cells to improve the efficiency of sensory-to-motor transformation, thereby allowing a reduction in the number of sensory cells tuned for the same input. This could have allowed the diversification of sensory modalities and of the behavioural repertoire. I propose that the first nervous systems consisted of combined sensory-motor neurons, directly translating sensory input into motor output on locomotor ciliated cells and steering muscle cells. Neuronal circuitry with low levels of integration has been retained in cnidarians and in the ciliated larvae of some marine invertebrates. This parallel processing stage could have been the starting point for the evolution of more integrated circuits performing the first complex computations such as persistence or coincidence detection. The sensory-motor nervous systems of cnidarians and ciliated larvae of diverse phyla show that brains, like all biological structures, are not irreducibly complex.

Trichobilharzia regenti (Digenea: Schistosomatidae): changes of body wall musculature during the development from miracidium to adult worm

Trichobilharzia regenti (Schistosomatidae, Digenea), a parasite of birds, exhibits a unique strategy among schistosomes, having affinity to the nervous system of vertebrate hosts. Migration of parasitic stages within hosts and/or swimming of non-parasitic larvae in water environment depend on the action of body wall muscles which were studied with confocal and electron microscopy. In all stages, body wall musculature is comprised of differently organized circular and longitudinal muscles. During the development, an extensive change of musculature characteristics and/or formation of new muscle structures were recorded; cercariae, schistosomula and adult worms produce additional underlying diagonal muscle fibers and inner plexus of radial musculature. Substantial changes of the outer environment during penetration of a host (osmotic values of water vs. host tissues) are accompanied by surface transformation of miracidia/mother sporocysts and cercariae/schistosomula. Contrary to that, changes of body musculature in these stages are characterized only by growth and re-organization of existing structures, and never by formation of new components of body musculature. Future studies in this field may contribute to a better knowledge of morphology and function of trematode muscles, including those of schistosomes that are important pathogens of humans and animals.

Highly conserved functions of the Brachyury gene on morphogenetic movements: insight from the early-diverging phylum Ctenophora

Brachyury, a member of the T-box transcription family identified in a diverse array of metazoans, was initially recognized for its function in mesoderm formation and notochord differentiation in vertebrates; however, its ancestral role has been suggested to be in control of morphogenetic movements. Here, we show that morpholino oligonucleotide knockdown of Brachyury (MlBra) in embryos of a ctenophore, one of the most ancient groups of animals, prevents the invagination of MlBra expressing stomodeal cells and is rescued with corresponding RNA injections. Injection of RNA encoding a dominant-interfering construct of MlBra causes identical phenotypes to that of RNA encoding a dominant-interfering form of Xenopus Brachyury (Xbra) in Xenopus embryos. Both injected embryos down-regulate Xbra downstream genes, Xbra itself and Xwnt11 but not axial mesodermal markers, resulting in failure to complete gastrulation due to loss of convergent extension movements. Moreover, animal cap assay reveals that MlBra induces Xwnt11 like Xbra. Overall results using Xenopus embryos show that these two genes are functionally interchangeable. These functional experiments demonstrate for the first time in a basal metazoan that the primitive role of Brachyury is to regulate morphogenetic movements, rather than to specify endomesodermal fates, and the role is conserved between non-bilaterian metazoans and vertebrates.

On the independent origins of complex brains and neurons

Analysis of the origin and evolution of neurons is crucial for revealing principles of organization of neural circuits with unexpected implications for genomic sciences, biomedical applications and regenerative medicine. This article presents an overview of some controversial ideas about the origin and evolution of neurons and nervous systems, focusing on the independent origin of complex brains and possible independent origins of neurons. First, earlier hypotheses related to the origin of neurons are summarized. Second, the diversity of nervous systems and convergent evolution of complex brains in relation to current views about animal phylogeny is discussed. Third, the lineages of molluscs and basal metazoans are used as illustrated examples of multiple origins of complex brains and neurons. Finally, a hypothesis about the independent origin of complex brains, centralized nervous systems and neurons is outlined. Injury-associated mechanisms leading to secretion of signal peptides (and related molecules) can be considered as evolutionary predecessors of inter-neuronal signaling and the major factors in the appearance of neurons in the first place.

Early origin of the bilaterian developmental toolkit

Whole-genome sequences from the choanoflagellate Monosiga brevicollis, the placozoan Trichoplax adhaerens and the cnidarian Nematostella vectensis have confirmed results from comparative evolutionary developmental studies that much of the developmental toolkit once thought to be characteristic of bilaterians appeared much earlier in the evolution of animals. The diversity of transcription factors and signalling pathway genes in animals with a limited number of cell types and a restricted developmental repertoire is puzzling, particularly in light of claims that such highly conserved elements among bilaterians provide evidence of a morphologically complex protostome-deuterostome ancestor. Here, I explore the early origination of elements of what became the bilaterian toolkit, and suggest that placozoans and cnidarians represent a depauperate residue of a once more diverse assemblage of early animals, some of which may be represented in the Ediacaran fauna (c. 585-542 Myr ago).

Concatenated analysis sheds light on early metazoan evolution and fuels a modern "urmetazoon" hypothesis

For more than a century, the origin of metazoan animals has been debated. One aspect of this debate has been centered on what the hypothetical “urmetazoon” bauplan might have been. The morphologically most simply organized metazoan animal, the placozoan Trichoplax adhaerens, resembles an intriguing model for one of several “urmetazoon” hypotheses: the placula hypothesis. Clear support for a basal position of Placozoa would aid in resolving several key issues of metazoan-specific inventions (including, for example, head–foot axis, symmetry, and coelom) and would determine a root for unraveling their evolution. Unfortunately, the phylogenetic relationships at the base of Metazoa have been controversial because of conflicting phylogenetic scenarios generated while addressing the question. Here, we analyze the sum of morphological evidence, the secondary structure of mitochondrial ribosomal genes, and molecular sequence data from mitochondrial and nuclear genes that amass over 9,400 phylogenetically informative characters from 24 to 73 taxa. Together with mitochondrial DNA genome structure and sequence analyses and Hox-like gene expression patterns, these data (1) provide evidence that Placozoa are basal relative to all other diploblast phyla and (2) spark a modernized “urmetazoon” hypothesis.

Following one of the basic principles in evolutionary biology that complex life forms derive from more primitive ancestors, it has long been believed that the higher animals, the Bilateria, arose from simpler (diploblastic) organisms such as the cnidarians (corals, polyps, and jellyfishes). A large number of studies, using different datasets and different methods, have tried to determine the most ancestral animal group as well as the ancestor of the higher animals. Here, we use “total evidence” analysis, which incorporates all available data (including morphology, genome, and gene expression data) and come to a surprising conclusion. The Bilateria and Cnidaria (together with the other diploblastic animals) are in fact sister groups: that is, they evolved in parallel from a very simple common ancestor. We conclude that the higher animals (Bilateria) and lower animals (diploblasts), probably separated very early, at the very beginning of metazoan animal evolution and independently evolved their complex body plans, including body axes, nervous system, sensory organs, and other characteristics. The striking similarities in several complex characters (such as the eyes) resulted from both lineages using the same basic genetic tool kit, which was already present in the common ancestor. The study identifies Placozoa as the most basal diploblast group and thus a living fossil genome that nicely demonstrates, not only that complex genetic tool kits arise before morphological complexity, but also that these kits may form similar morphological structures in parallel.

Total evidence analyses reveal a surprise: Higher animals did not evolve from any known lower animal group.

The initial steps of myofibril assembly: integrins pave the way

Myofibril assembly results in a regular array of identical sarcomeres in striated muscle. Sarcomere structure is conserved across the animal kingdom, which implies that the mechanisms of myofibril assembly are also likely to be conserved. Recent advances from model genetic systems and insights from stress fibre cell biology have shed light on the mechanisms that set sarcomere spacing and the initial assembly of sarcomere arrays. We propose a model of integrin-dependent cell-matrix adhesion as the starting point for myofibrillogenesis.

Heart and craniofacial muscle development: a new developmental theme of distinct myogenic fields

Head muscle development has been studied less intensively than myogenesis in the trunk, although this situation is gradually changing, as embryological and genetic insights accumulate. This review focuses on novel studies of the origins, composition and evolution of distinct craniofacial muscles. Cellular and molecular parallels are drawn between cardiac and branchiomeric muscle developmental programs, both of which utilize multiple lineages with distinct developmental histories, and argue for the tissues' common evolutionary origin. In addition, there is increasing evidence that the specification of skeletal muscles in the head appears to be distinct from that operating in the trunk: considerable variation among the different craniofacial muscle groups is seen, in a manner resembling myogenic specification in lower organisms.

The evolution of cell types in animals: emerging principles from molecular studies

Cell types are fundamental units of multicellular life but their evolution is obscure. How did the first cell types emerge and become distinct in animal evolution? What were the sets of cell types that existed at important evolutionary nodes that represent eumetazoan or bilaterian ancestors? How did these ancient cell types diversify further during the evolution of organ systems in the descending evolutionary lines? The recent advent of cell type molecular fingerprinting has yielded initial insights into the evolutionary interrelationships of cell types between remote animal phyla and has allowed us to define some first principles of cell type diversification in animal evolution.

Back in time: a new systematic proposal for the Bilateria

Conventional wisdom suggests that bilateral organisms arose from ancestors that were radially, rather than bilaterally, symmetrical and, therefore, had a single body axis and no mesoderm. The two main hypotheses on how this transformation took place consider either a simple organism akin to the planula larva of extant cnidarians or the acoel Platyhelminthes (planuloid-acoeloid theory), or a rather complex organism bearing several or most features of advanced coelomate bilaterians (archicoelomate theory). We report phylogenetic analyses of bilaterian metazoans using quantitative (ribosomal, nuclear and expressed sequence tag sequences) and qualitative (HOX cluster genes and microRNA sets) markers. The phylogenetic trees obtained corroborate the position of acoel and nemertodermatid flatworms as the earliest branching extant members of the Bilateria. Moreover, some acoelomate and pseudocoelomate clades appear as early branching lophotrochozoans and deuterostomes. These results strengthen the view that stem bilaterians were small, acoelomate/pseudocoelomate, benthic organisms derived from planuloid-like organisms. Because morphological and recent gene expression data suggest that cnidarians are actually bilateral, the origin of the last common bilaterian ancestor has to be put back in time earlier than the cnidarian-bilaterian split in the form of a planuloid animal. A new systematic scheme for the Bilateria that includes the Cnidaria is suggested and its main implications discussed.

Sacculogenesis of Buddenbrockia plumatellae (Myxozoa) within the invertebrate host Plumatella repens (Bryozoa) with comments on the evolutionary relationships of the Myxozoa

Members of the phylum Myxozoa are obligate parasites, primarily of aquatic organisms. Their phylogeny has remained problematic, with studies placing them within either the Bilateria or Cnidaria. The discovery that the enigmatic Buddenbrockia plumatellae is a myxozoan that possesses distinct bilaterian features appeared to have finally resolved the debate. B. plumatellae is described as a triploblastic 'worm-like' organism, within which typical myxozoan malacospores form. Using EM we examined the early development of the B. plumatellae 'worms' within the bryozoan host Plumatella repens. The initial development involved numerous unicellular, amoeboid pre-saccular stages that were present within the basal lamina of the host's body wall. These stages migrate immediately beneath the peritoneum where a significant host tissue reaction occurs. The stages aggregate, initiating the formation of a 'worm'. The base of a developing 'worm' forms a pseudosyncytium which resolves into an ectoderm surrounding a mesendoderm. The pseudosyncytium is directly anchored into neighbouring host cells via masses of striated fibres. The replication of the ectodermal and mesendodermal cells extends the developing 'worm' into the coelom of the host. The mesendoderm resolves to form a mesoderm and an endoderm. Myogenesis appears to be initiated from the anchored end of the 'worm' and develops along the mesoderm. The aggregation and differentiation of amoeboid pre-saccular stages to initiate the 'worm' draws analogies to the sacculogenesis observed for Tetracapsuloides bryosalmonae, B. plumatellae's sister taxon within the class Malacosporea. The development of a multicellular, spore forming organism, from single cells does not correlate to any bilaterian or cnidarian species. Current phylogenies indicate the Myxozoa are basal bilaterians along with the Acoela and Mesozoa. Comparison with these other basal groups may help to resolve the placement of Myxozoa within the tree of life.

Sea anemone genome reveals ancestral eumetazoan gene repertoire and genomic organization

Sea anemones are seemingly primitive animals that, along with corals, jellyfish, and hydras, constitute the oldest eumetazoan phylum, the Cnidaria. Here, we report a comparative analysis of the draft genome of an emerging cnidarian model, the starlet sea anemone Nematostella vectensis. The sea anemone genome is complex, with a gene repertoire, exon-intron structure, and large-scale gene linkage more similar to vertebrates than to flies or nematodes, implying that the genome of the eumetazoan ancestor was similarly complex. Nearly one-fifth of the inferred genes of the ancestor are eumetazoan novelties, which are enriched for animal functions like cell signaling, adhesion, and synaptic transmission. Analysis of diverse pathways suggests that these gene "inventions" along the lineage leading to animals were likely already well integrated with preexisting eukaryotic genes in the eumetazoan progenitor.

Ancient connection between NKL genes and the mesoderm? Insights from Tlx expression in a ctenophore

In recent years, evo-devo studies on non-bilaterian metazoans have improved our understanding of the early evolution of animal body plans. In particular, works on cnidarians suggested that contrary to classical views, the mesoderm originated far before the emergence of the Bilateria. In this context, a synthesis of genomic and functional data concerning the Antennapedia (Antp) superclass of homeobox genes suggested that early in animal evolution, each of the three germ layers was under the control of one cluster of Antp genes. In particular, the patterning and differentiation of the mesoderm was under the control of the NKL cluster. The ctenophores stand as a key taxon with respect to such issues because unlike other non-bilaterian phyla, their intermediate germ layer satisfies the strict embryological definition of a mesoderm. For that reason, we investigated the only known member of the NKL group in Ctenophora, a gene previously isolated from Pleurobrachia and attributed to the Tlx family. In our analysis of the NKL group, this ctenophore gene branches as the sister-group of bilaterian Tlx genes, but without statistical support. The expression pattern of this gene was revealed by in situ hybridisation in the adult ctenophore. The expression territories of PpiTlx are predominantly ectodermal, in two distinct types of ciliated epidermal cells and in one category of gland cells. We also identified a probable endodermal site of expression. Because we failed to detect any mesodermal expression, the results do not provide support to the hypothesis of an ancient functional association between the NKL group and the mesoderm.

Head regeneration in wild-type hydra requires de novo neurogenesis

Because head regeneration occurs in nerve-free hydra mutants, neurogenesis was regarded as dispensable for this process. Here, in wild-type hydra, we tested the function of the ParaHox gsx homolog gene, cnox-2,which is a specific marker for bipotent neuronal progenitors, expressed in cycling interstitial cells that give rise to apical neurons and gastric nematoblasts (i.e. sensory mechanoreceptor precursors). cnox-2 RNAi silencing leads to a dramatic downregulation of hyZic, prdl-a, gscand cnASH, whereas hyCOUP-TF is upregulated. cnox-2indeed acts as an upstream regulator of the neuronal and nematocyte differentiation pathways, as cnox-2(-) hydra display a drastic reduction in apical neurons and gastric nematoblasts, a disorganized apical nervous system and a decreased body size. During head regeneration, the locally restricted de novo neurogenesis that precedes head formation is cnox-2 dependent: cnox-2 expression is induced in neuronal precursors and differentiating neurons that appear in the regenerating tip; cnox-2 RNAi silencing reduces this de novo neurogenesis and delays head formation. Similarly, the disappearance of cnox-2+cells in sf-1 mutants also correlates with head regeneration blockade. Hence in wild-type hydra, head regeneration requires the cnox-2 neurogenic function. When neurogenesis is missing, an alternative, slower and less efficient, head developmental program is possibly activated.

Muscular system of a peculiar parasitic cnidarian Polypodium hydriforme: a phalloidin fluorescence study

Musculature of the free-living stages of Polypodium hydriforme has been studied using phalloidin fluorescence method and confocal microscopy. P. hydriforme is a unique cnidarian possessing only smooth muscle cells situated within the mesoglea, not epithelial muscle cells, like the rest of cnidarians. Phalloidin fluorescence on whole mount preparations demonstrates an extensively developed subepidermal muscle system mostly consisting of long parallel fibers running along the tentacles. For the first time along with contracted muscle fibers we could clearly demonstrate relaxed fibers looking as long spirals. System of thin parallel circular F-actin positive fibers has been discovered outside of longitudinal muscles. The body of the animal and the mouth cone contain weakly developed parallel muscles. No special attachment of the muscle fibers to the tips of the tentacles or to the rim of the mouth has been observed. The results are discussed in connection with the "triploblastic" organization of P. hydriforme and its phylogenetic position.

Generation of bilateral symmetry in Anthozoa: a model

Polyps of Anthozoa usually display bilateral symmetry with respect to their mouth opening, to their pharynx, and in particular to the arrangement of their mesenteries. Mesenteries, which are endodermal folds running from the apical to the basal end of the body, subdivide the gastric cavity into pouches. They form in a bilateral symmetric sequence. In this article I propose that early in polyp development the endoderm subdivides successively into three different types of compartments. A mesentery forms at the border between compartments. Two of the compartments are homologous to those of Scyphozoa. They form by mutual activation of cell states that locally exclude each other. The third compartment leads to siphonoglyph formation and is an evolutionary innovation of the Anthozoa. The mechanism that controls the number and spatial arrangement of the third type of compartment changes the radial symmetry into a bilateral one and occasionally into a different one. The dynamics of its formation indicate an activator-inhibitor mechanism. Computer models are provided that reproduce decision steps in the generation of the mesenteries.

Insights from diploblasts; the evolution of mesoderm and muscle

The origin of both mesoderm and muscle are central questions in metazoan evolution. The majority of metazoan phyla are triploblasts, possessing three discrete germ layers. Attention has therefore been focused on two outgroups to triploblasts, Cnidaria and Ctenophora. Modern texts describe these taxa as diploblasts, lacking a mesodermal germ layer. However, some members of Medusozoa, one of two subphyla within Cnidaria, possess tissue independent of either the ectoderm or endoderm referred to as the entocodon. Furthermore, members of both Cnidaria and Ctenophora have been described as possessing striated muscle, a mesodermal derivative. While it is widely accepted that the ancestor of Eumetazoa was diploblastic, homology of the entocodon and mesoderm as well as striated muscle within Eumetazoa has been suggested. This implies a potential triploblastic ancestor of Eumetazoa possessing striated muscle. In the following review, I examine the evidence for homology of both muscle and mesoderm. Current data support a diploblastic ancestor of cnidarians, ctenophores, and triploblasts lacking striated muscle.

A conserved role for the nodal signaling pathway in the establishment of dorso-ventral and left–right axes in deuterostomes

Nodal factors play crucial roles during embryogenesis of chordates. They have been implicated in a number of developmental processes, including mesoderm and endoderm formation and patterning of the embryo along the anterior-posterior and left-right axes. We have analyzed the function of the Nodal signaling pathway during the embryogenesis of the sea urchin, a non-chordate organism. We found that Nodal signaling plays a central role in axis specification in the sea urchin, but surprisingly, its first main role appears to be in ectoderm patterning and not in specification of the endoderm and mesoderm germ layers as in vertebrates. Starting at the early blastula stage, sea urchin nodal is expressed in the presumptive oral ectoderm where it controls the formation of the oral-aboral axis. A second conserved role for nodal signaling during vertebrate evolution is its involvement in the establishment of left-right asymmetries. Sea urchin larvae exhibit profound left-right asymmetry with the formation of the adult rudiment occurring only on the left side. We found that a nodal/lefty/pitx2 gene cassette regulates left-right asymmetry in the sea urchin but that intriguingly, the expression of these genes is reversed compared to vertebrates. We have shown that Nodal signals emitted from the right ectoderm of the larva regulate the asymmetrical morphogenesis of the coelomic pouches by inhibiting rudiment formation on the right side of the larva. This result shows that the mechanisms responsible for patterning the left-right axis are conserved in echinoderms and that this role for nodal is conserved among the deuterostomes. We will discuss the implications regarding the reference axes of the sea urchin and the ancestral function of the nodal gene in the last section of this review.

High regulatory gene use in sea urchin embryogenesis: Implications for bilaterian development and evolution

A global scan of transcription factor usage in the sea urchin embryo was carried out in the context of the Strongylocentrotus purpuratus genome sequencing project, and results from six individual studies are here considered. Transcript prevalence data were obtained for over 280 regulatory genes encoding sequence-specific transcription factors of every known family, but excluding genes encoding zinc finger proteins. This is a statistically inclusive proxy for the total "regulome" of the sea urchin genome. Close to 80% of the regulome is expressed at significant levels by the late gastrula stage. Most regulatory genes must be used repeatedly for different functions as development progresses. An evolutionary implication is that animal complexity at the stage when the regulome first evolved was far simpler than even the last common bilaterian ancestor, and is thus of deep antiquity.

Polarised expression of FoxB and FoxQ2 genes during development of the hydrozoan Clytia hemisphaerica

We have characterised the expression of four genes coding for Forkhead box-containing ('Fox') transcription factors identified from the hydrozoan (Leptomedusa) Clytia hemisphaerica. Phylogenetic analyses including all available non-bilaterian Fox sequences placed these genes in subfamilies B, Q2 (two genes) and O, and indicated that at least 17 Fox subfamilies were present in the common cnidarian/bilaterian ancestor, with multiple subsequent losses in cnidarian lineages. Chordate FoxB and FoxQ2A subfamily genes show polarised expression in early embryos. Correspondingly, Clytia CheFoxB expression was localised around the gastrulation site (future oral pole) at blastula and gastrula stages, with CheFoxQ2a expressed in a complementary aboral domain, maintained through larval development. Distinct later expression domains were observed for CheFoxB in the larval endoderm region, and in the statocyst, gonad and tentacle bulb of the medusa. A second Clytia FoxQ2 gene, CheFoxQ2b, not expressed in the embryo, larva or polyp, was detected uniquely in the gonads of the medusa. In contrast, CheFoxO, whose sequence indicates regulation by the PI3-Kinase/PKB signalling pathway consistent with known roles in bilaterian developmental regulation, was detected throughout the Clytia life cycle. CheFoxO expression was enhanced in regions associated with growth control including larval poles, gonad and the margin of the medusa bell. These results support the idea that an early embryonic patterning system involving FoxB and FoxQ2 family genes has been evolutionary conserved and indicate that Fox family genes have also acquired distinct roles during other phases of the hydrozoan life cycle.

Minimal ProtoHox cluster inferred from bilaterian and cnidarian Hox complements

Bilaterian animals have a Hox gene cluster essential for patterning the main body axis, and a ParaHox gene cluster. Comparison of Hox and ParaHox genes has led workers to postulate that both clusters originated from the duplication of an ancient cluster named ProtoHox, which contained up to four genes with at least the precursors of anterior and posterior Hox/ParaHox genes. However, the way in which genes diversified within the ProtoHox, Hox and ParaHox clusters remains unclear because no systematic study of non-bilaterian animals exists. Here we characterize the full Hox/ParaHox gene complements and genomic organization in two cnidarian species (Nematostella vectensis and Hydra magnipapillata), and suggest a ProtoHox cluster simpler than originally thought on the basis of three arguments. First, both species possess bilaterian-like anterior Hox genes, but their non-anterior genes do not appear as counterparts of either bilaterian central or posterior genes; second, two clustered ParaHox genes, Gsx and a gene related to Xlox and Cdx, are found in Nematostella vectensis; and third, we do not find clear phylogenetic support for a common origin of bilaterian Cdx and posterior genes, which might therefore have appeared after the ProtoHox cluster duplication. Consequently, the ProtoHox cluster might have consisted of only two anterior genes. Non-anterior genes could have appeared independently in the Hox and ParaHox clusters, possibly after the separation of bilaterians and cnidarians.

Medusozoan phylogeny and character evolution clarified by new large and small subunit rDNA data and an assessment of the utility of phylogenetic mixture models

A newly compiled data set of nearly complete sequences of the large subunit of the nuclear ribosome (LSU or 28S) sampled from 31 diverse medusozoans greatly clarifies the phylogenetic history of Cnidaria. These data have substantial power to discern among many of the competing hypotheses of relationship derived from prior work. Moreover, LSU data provide strong support at key nodes that were equivocal based on other molecular markers. Combining LSU sequences with those of the small subunit of the nuclear ribosome (SSU or 18S), we present a detailed working hypothesis of medusozoan relationships and discuss character evolution within this diverse clade. Stauromedusae, comprising the benthic, so-called stalked jellyfish, appears to be the sister group of all other medusozoans, implying that the free-swimming medusa stage, the motor nerve net, and statocysts of ecto-endodermal origin are features derived within Medusozoa. Cubozoans, which have had uncertain phylogenetic affinities since the elucidation of their life cycles, form a clade-named Acraspeda-with the scyphozoan groups Coronatae, Rhizostomeae, and Semaeostomeae. The polyps of both cubozoans and hydrozoans appear to be secondarily simplified. Hydrozoa is comprised by two well-supported clades, Trachylina and Hydroidolina. The position of Limnomedusae within Trachylina indicates that the ancestral hydrozoan had a biphasic life cycle and that the medusa was formed via an entocodon. Recently hypothesized homologies between the entocodon and bilaterian mesoderm are therefore suspect. Laingiomedusae, which has often been viewed as a close ally of the trachyline group Narcomedusae, is instead shown to be unambiguously a member of Hydroidolina. The important model organisms of the Hydra species complex are part of a clade, Aplanulata, with other hydrozoans possessing direct development not involving a ciliated planula stage. Finally, applying phylogenetic mixture models to our data proved to be of little additional value over a more traditional phylogenetic approach involving explicit hypothesis testing and bootstrap analyses under multiple optimality criteria. [18S; 28S; Cubozoa; Hydrozoa; medusa; molecular systematics; polyp; Scyphozoa; Staurozoa.].

Silencing of the hydra serine protease inhibitorKazal1gene mimics the humanSPINK1pancreatic phenotype

In hydra, the endodermal epithelial cells carry out the digestive function together with the gland cells that produce zymogens and express the evolutionarily conserved gene Kazal1. To assess the hydra Kazal1 function, we silenced gene expression through double-stranded RNA feeding. A progressive Kazal1 silencing affected homeostatic conditions as evidenced by the low budding rate and the induced animal death. Concomitantly, a dramatic disorganization followed by a massive death of gland cells was observed, whereas the cytoplasm of digestive cells became highly vacuolated. The presence of mitochondria and late endosomes within those vacuoles assigned them as autophagosomes. The enhanced Kazal1 expression in regenerating tips was strongly diminished in Kazal1(–) hydra, and the amputation stress led to an immediate disorganization of the gland cells, vacuolization of the digestive cells and death after prolonged silencing. This first cellular phenotype resulting from a gene knock-down in cnidarians suggests that the Kazal1 serine-protease-inhibitor activity is required to prevent excessive autophagy in intact hydra and to exert a cytoprotective function to survive the amputation stress. Interestingly, these functions parallel the pancreatic autophagy phenotype observed upon mutation within the Kazal domain of the SPINK1 and SPINK3 genes in human and mice, respectively.

The genesis and evolution of homeobox gene clusters

Once called the 'Rosetta stone' of developmental biology, the homeobox continues to fascinate both evolutionary and developmental biologists. The birth of the homeotic, or Hox, gene cluster, and its subsequent evolution, has been crucial in mediating the major transitions in metazoan body plan. Comparative genomics studies indicate that the more recently discovered ParaHox and NK clusters were linked to the Hox cluster early in evolution, and that together they constituted a 'megacluster' of homeobox genes that conspicuously contributed to body-plan evolution.

T-box and homeobox genes from the ctenophore Pleurobrachia pileus: comparison of Brachyury, Tbx2/3 and Tlx in basal metazoans and bilaterians

Most animals are classified as Bilateria and only four phyla are still extant as outgroups, namely Porifera, Placozoa, Cnidaria and Ctenophora. These non-bilaterians were not considered to have a mesoderm and hence mesoderm-specific genes. However, the T-box gene Brachyury could be isolated from sponges, placozoans and cnidarians. Here, we describe the first Brachyury and a Tbx2/3 homologue from a ctenophore. In addition, analysing T-box and homeobox genes under comparable conditions in all four basal phyla lead to the discovery of novel T-box genes in sponges and cnidarians and a Tlx homeobox gene in the ctenophore Pleurobrachia pileus. The conservation of the T-box and the homeobox genes suggest that distinct subfamilies with different roles in bilaterians were already split in non-bilaterians.