Evolution of the nervous system in Paraphanostoma (Acoela)

Biodiversity of symbiotic microalgae associated with meiofaunal marine acoels in Southern Japan

Acoels in the family Convolutidae are commonly found with microalgal symbionts. Convolutids can host green algal Tetraselmis and dinoflagellates within the family Symbiodiniaceae and the genus Amphidinium. The diversity of these microalgae has not been well surveyed. In this study, we used PCR and culture techniques to demonstrate the biodiversity of Tetraselmis and dinoflagellates in symbiosis with meiofaunal acoels. Here, 66 acoels were collected from seven localities around Okinawa, Ishigaki, and Kochi, Japan. While convolutids were heavily represented in this sampling, some acoels formed a clade outside Convolutidae and are potentially a new family of acoels harboring symbiotic microalgae. From the acoels collected, a total of 32 Tetraselmis and 26 Symbiodiniaceae cultures were established. Molecular phylogenies were constructed from cultured material (and from total host DNA) using the 18S rRNA gene (Tetraselmis) and 28S rRNA gene (dinoflagellates). The majority of Tetraselmis sequences grouped within the T. astigmatica clade but strains closely related to T. convolutae, T. marina, and T. gracilis were also observed. This is the first report of Tetraselmis species, other than T. convolutae, naturally associating with acoels. For dinoflagellates, members of Cladocopium and Miliolidium were observed, but most Symbiodiniaceae sequences formed clusters within Symbiodinium, grouping with S. natans, or sister to S. tridacnidorum. Several new Symbiodinium sequences from this study may represent novel species. This is the first molecular record of Miliolidium and Symbiodinium from acoels. Microalgal strains from this study will provide a necessary framework for future taxonomic studies and research on symbiotic relationships between acoels and microalgae.

Phylogenetic assessment and systematic revision of the acoel family Isodiametridae

Isodiametridae is a large family of Acoela with 22 nominal genera and nearly 100 species. Unfortunately, systematics of Isodiametridae, as it stands, is highly problematic. Genera frequently have been proposed without reference to an explicit phylogenetic hypothesis, such that the current classification system holds little or no predictive power. Many taxa do not fit with the family diagnosis, and it is increasingly difficult to determine in which taxon a new species should be described. Herein, we reconstruct the phylogenetic relationships of Acoela with a focus on Isodiametridae using both previously published and new ribosomal and mitochondrial sequence data. Our dataset comprises sequences from 45 species representing 16 of the 22 isodiametrid genera. Our results recovered a well-supported Isodiametridae, but provided further evidence that the family and several genera within require revision. We have updated the classification system of Isodiametridae to be consistent with its phylogeny, including the transference of Otocelis to Otocelididae, Postaphanostoma and Faerlea to Mecynostomidae and Alluna to Actinoposthiidae. Six other genera are placed in synonymy. We review the morphological taxonomy and provide an identification key of the genera in the revised family.

Neural architecture and regeneration in the acoel Hofstenia miamia

The origin of bilateral symmetry, a major transition in animal evolution, coincided with the evolution of organized nervous systems that show regionalization along major body axes. Studies of Xenacoelomorpha, the likely outgroup lineage to all other animals with bilateral symmetry, can inform the evolutionary history of animal nervous systems. Here, we characterized the neural anatomy of the acoel Hofstenia miamia. Our analysis of transcriptomic data uncovered orthologues of enzymes for all major neurotransmitter synthesis pathways. Expression patterns of these enzymes revealed the presence of a nerve net and an anterior condensation of neural cells. The anterior condensation was layered, containing several cell types with distinct molecular identities organized in spatially distinct territories. Using these anterior cell types and structures as landmarks, we obtained a detailed timeline for regeneration of the H. miamia nervous system, showing that the anterior condensation is restored by eight days after amputation. Our work detailing neural anatomy in H. miamia will enable mechanistic studies of neural cell type diversity and regeneration and provide insight into the evolution of these processes.

SALMFamide2 and serotonin immunoreactivity in the nervous system of some acoels (Xenacoelomorpha)

Acoel worms are simple, often microscopic animals with direct development, a multiciliated epidermis, a statocyst, and a digestive parenchyma instead of a gut epithelium. Morphological characters of acoels have been notoriously difficult to interpret due to their relative scarcity. The nervous system is one of the most accessible and widely used comparative features in acoels, which have a so-called commissural brain without capsule and several major longitudinal neurite bundles. Here, we use the selective binding properties of a neuropeptide antibody raised in echinoderms (SALMFamide2, or S2), and a commercial antibody against serotonin (5-HT) to provide additional characters of the acoel nervous system. We have prepared whole-mount immunofluorescent stainings of three acoel species: Symsagittifera psammophila (Convolutidae), Aphanostoma pisae, and the model acoel Isodiametra pulchra (both Isodiametridae). The commissural brain of all three acoels is delimited anteriorly by the ventral anterior commissure, and posteriorly by the dorsal posterior commissure. The dorsal anterior commissure is situated between the ventral anterior commissure and the dorsal posterior commissure, while the statocyst lies between dorsal anterior and dorsal posterior commissure. S2 and serotonin do not co-localise, and they follow similar patterns to each other within an animal. In particular, S2, but not 5-HT, stains a prominent commissure posterior to the main (dorsal) posterior commissure. We have for the first time observed a closed posterior loop of the main neurite bundles in S. psammophila for both the amidergic and the serotonergic nervous system. In I. pulchra, the lateral neurite bundles also form a posterior loop in our serotonergic nervous system stainings.

The nervous system of Xenacoelomorpha: a genomic perspective

Xenacoelomorpha is, most probably, a monophyletic group that includes three clades: Acoela, Nemertodermatida and Xenoturbellida. The group still has contentious phylogenetic affinities; though most authors place it as the sister group of the remaining bilaterians, some would include it as a fourth phylum within the Deuterostomia. Over the past few years, our group, along with others, has undertaken a systematic study of the microscopic anatomy of these worms; our main aim is to understand the structure and development of the nervous system. This research plan has been aided by the use of molecular/developmental tools, the most important of which has been the sequencing of the complete genomes and transcriptomes of different members of the three clades. The data obtained has been used to analyse the evolutionary history of gene families and to study their expression patterns during development, in both space and time. A major focus of our research is the origin of 'cephalized' (centralized) nervous systems. How complex brains are assembled from simpler neuronal arrays has been a matter of intense debate for at least 100 years. We are now tackling this issue using Xenacoelomorpha models. These represent an ideal system for this work because the members of the three clades have nervous systems with different degrees of cephalization; from the relatively simple sub-epithelial net of Xenoturbella to the compact brain of acoels. How this process of 'progressive' cephalization is reflected in the genomes or transcriptomes of these three groups of animals is the subject of this paper.

The nervous system of Isodiametra pulchra (Acoela) with a discussion on the neuroanatomy of the Xenacoelomorpha and its evolutionary implications

Introduction

Acoels are microscopic marine worms that have become the focus of renewed debate and research due to their placement at the base of the Bilateria by molecular phylogenies. To date, Isodiametra pulchra is the most promising “model acoel” as it can be cultured and gene knockdown can be performed with double-stranded RNA. Despite its well-known morphology data on the nervous system are scarce. Therefore we examined this organ using various microscopic techniques, including histology, conventional histochemistry, electron microscopy, and immunocytochemistry in combination with CLSM and discuss our results in light of recently established phylogenies.

Results

The nervous system of Isodiametra pulchra consists of a bilobed brain with a dorsal posterior commissure, a frontal ring and tracts, four pairs of longitudinal neurite bundles, as well as a supramuscular and submuscular plexus. Serotonin-like immunoreactivity (SLI) is displayed in parts of the brain, the longitudinal neurite bundles and a large part of the supramuscular plexus, while FMRFamide-like immunoreactivity (RFLI) is displayed in parts of the brain and a distinct set of neurons, the longitudinal neurite bundles and the submuscular plexus. Despite this overlap SLI and RFLI are never colocalized. Most remarkable though is the presence of a distinct functional neuro-muscular system consisting of the statocyst, tracts, motor neurons and inner muscles, as well as the presence of various muscles that differ with regard to their ultrastructure and innervation.

Conclusions

The nervous system of Isodiametra pulchra consists of an insunk, bilobed brain, a peripheral part for perception and innervation of the smooth body-wall musculature as well as tracts and motor neurons that together with pseudostriated inner muscles are responsible for steering and quick movements. The insunk, bilobed brains with two to three commissures found in numerous acoels are homologous and evolved from a ring-commissural brain that was present in the stem species of acoelomorphs. The acoelomorph brain is bipartite, consisting of a Six3/6-dependend animal pole nervous system that persists throughout adulthood and an axial nervous system that does not develop by exhibiting a staggered pattern of conserved regulatory genes as in other bilaterians but by a nested pattern of these genes. This indicates that acoelomorphs stem from an ancestor with a simple brain or with a biphasic life cycle.

The Acoela: on their kind and kinships, especially with nemertodermatids and xenoturbellids (Bilateria incertae sedis)

Acoels are among the simplest worms and therefore have often been pivotal in discussions of the origin of the Bilateria. Initially thought primitive because of their “planula-like” morphology, including their lumenless digestive system, they were subsequently dismissed by many morphologists as a specialized clade of the Platyhelminthes. However, since molecular phylogenies placed them outside the Platyhelminthes and outside all other phyla at the base of the Bilateria, they became the focus of renewed debate and research. We review what is currently known of acoels, including information regarding their morphology, development, systematics, and phylogenetic relationships, and put some of these topics in a historical perspective to show how the application of new methods contributed to the progress in understanding these animals. Taking all available data into consideration, clear-cut conclusions cannot be made; however, in our view it becomes successively clearer that acoelomorphs are a “basal” but “divergent” branch of the Bilateria.

Steps towards a centralized nervous system in basal bilaterians: insights from neurogenesis of the acoel Symsagittifera roscoffensis

Due to its proposed basal position in the bilaterian Tree of Life, Acoela may hold the key to our understanding of the evolution of a number of bodyplan features including the central nervous system. In order to contribute novel data to this discussion we investigated the distribution of α-tubulin and the neurotransmitters serotonin and RFamide in juveniles and adults of the sagittiferid Symsagittifera roscoffensis. In addition, we present the expression pattern of the neuropatterning gene SoxB1. Adults and juveniles exhibit six serotonergic longitudinal neurite bundles and an anterior concentration of serotonergic sensory cells. While juveniles show an "orthogon-like" arrangement of longitudinal neurite bundles along the anterior-posterior axis, it appears more diffuse in the posterior region of adults. Commissures between the six neurite bundles are present only in the anterior body region of adults, while irregularly distributed individual neurites, often interconnected by serotonergic nerve cells, are found in the posterior region. Anti-RFamide staining shows numerous individual neurites around the statocyst. The orthogon-like nervous system of S. roscoffensis is confirmed by α-tubulin immunoreactivity. In the region of highest neurotransmitter density (i.e., anterior), the HMG-box gene SrSoxB1, a transcription factor known to be involved in neurogenesis in other bilaterians, is expressed in juvenile specimens. Accordingly, SoxB1 expression in S. roscoffensis follows the typical pattern of higher bilaterians that have a brain. Thus, our data support the notion that Urbilateria already had the genetic toolkit required to form brain-like neural structures, but that its morphological degree of neural concentration was still low.

Three-dimensional reconstruction and neural map of the serotonergic brain of Asplanchna brightwellii (Rotifera, Monogononta)

The basic organization of the rotifer brain has been known for nearly a century; yet, fine details on its structure and organization remain limited despite the importance of rotifers in studies of evolution and population biology. To gain insight into the structure of the rotifer brain, and provide a foundation for future neurophysiologic and neurophylogenetic research, the brain of Asplanchna brightwellii was studied with immunohistochemistry, confocal laser scanning microscopy, and computer modeling. A three-dimensional map of serotonergic connections reveals a complex network of approximately 28 mostly unipolar, cerebral perikarya and associated neurites. Cells and their projections display symmetry in quantity, size, connections, and pathways between cerebral hemispheres within and among individuals. Most immunopositive cells are distributed close to the brain midline. Three pairs of neurites form decussations at the brain midline and may innervate sensory receptors in the corona. A single neuronal pathway appears to connect both the lateral horns and dorsolateral apical receptors, suggesting that convergence of synaptic connections may be common in the afferent sensory systems of rotifers. Results show that the neural map of A. brightwellii is much more intricate than that of other monogonont rotifers; nevertheless, the consistency in neural circuits provides opportunities to identify homologous neurons, distinguish functional regions based on neurotransmitter phenotype, and explore new avenues of neurophylogeny in Rotifera.

Assembling the lophotrochozoan (=spiralian) tree of life

The advent of numerical methods for analysing phylogenetic relationships, along with the study of morphology and molecular data, has driven our understanding of animal relationships for the past three decades. Within the protostome branch of the animal tree of life, these data have sufficed to establish its two main side branches, the moulting Ecdysozoa and the non-moulting Lophotrochozoa. In this review, I explore our current knowledge of protostome relationships and discuss progress and future perspectives and strategies to increase resolution within the main lophotrochozoan clades. Novel approaches to coding morphological characters are needed by scoring real observations on species selected as terminals. Still, methodological issues, for example, how to deal with inapplicable characters or the coding of absences, may require novel algorithmic developments. Taxon sampling is another key issue, as phyla should include enough species so as to represent their span of anatomical disparity. On the molecular side, phylogenomics is playing an increasingly important role in elucidating animal relationships, but genomic sampling is still fairly limited within the lophotrochozoan protostomes, for which only three phyla are represented in currently available phylogenies. Future work should therefore concentrate on generating novel morphological observations and on producing genomic data for the lophotrochozoan side of the animal tree of life.

Central nervous system of Chaetoderma japonicum (Caudofoveata, Aplacophora): implications for diversified ganglionic plans in early molluscan evolution

The organization of the central nervous system of an "aplacophoran" mollusc, Chaetoderma japonicum, is described as a means to understand a primitive condition in highly diversified molluscan animals. This histological and immunocytochemical study revealed that C. japonicum still retains a conservative molluscan tetra-neural plan similar to those of neomenioids, polyplacophorans, and tryblidiids. However, the ventral and lateral nerve cords of C. japonicum are obviously ganglionated to various degrees, and the cerebral cord-like ganglia display a lobular structure. The putative chemosensory networks are developed, being composed of sensory cells of the oral shield, eight precerebral ganglia, and eight neuropil compartments that form distinct masses of neurites. In the cerebral cord-like ganglia, three anterior, posterior, and dorsal lobes are distinguished with well-fasciculated tracts in their neuropils. Most neuronal somata are uniform in size, and no small globuli-like cell clusters are found; however, localized serotonin-like immunoreactivity and acetylated tubulin-containing tracts suggest the presence of functional subdivisions. These complicated morphological features may be adaptive structures related to the specialized foraminiferan food in muddy bottoms. Based on a comparative scheme in basal molluscan groups, we characterize an independent evolutionary process for the unique characters of the central nervous systems of chaetoderms.

Copulatory organ musculature in Childia (Acoela) as revealed by phalloidin fluorescence and confocal microscopy

Copulatory organs of eight species of the monophyletic taxon Childia were investigated in detail, using phalloidin fluorescence method and confocal microscopy. Childia species were shown to have one, two or several tubular stylets, conical to cylindrical in shape, composed of few to numerous needles. The musculature varied greatly, from the absence of seminal vesicle to extensively developed seminal vesicles with several additional types of specialized muscles. Ten copulatory organ characters were coded and mapped on the total evidence tree. The data obtained permitted to follow the evolution of the Childia stylet and to demonstrate that the structure of the stylet apparatus is largely consistent with the phylogeny of the group (CI=0.75). Possible function of different muscle specializations was discussed.

Trochophora larvae: cell-lineages, ciliary bands and body regions. 2. Other groups and general discussion

The embryology of sipunculans, entoprocts, nemertines, platyhelminths (excluding acoelomorphs), rotifers, ectoprocts, phoronids, brachiopods, echinoderms and enteropneusts is reviewed with special emphasis on cell-lineage and differentiation of ectodermal structures. A group Spiralia comprising the four first-mentioned phyla plus annelids and molluscs seems well defined through the presence of spiral cleavage with early blastomere specification, prototroch with characteristic cell-lineage, cerebral ganglia developing from cells of the first micromere quartet (i.e., the episphere) and a ventral nervous system developing from the hyposphere. The planktotrophic trochophore was probably the larval type of the ancestor of this group. Another group comprising phoronids, brachiopods, echinoderms and enteropneusts appears equally well delimited. It is characterized by radial cleavage with late blastomere specification, possibly by the presence of a neotroch consisting of monociliate cells, by the absence of cerebral ganglia and of a well-defined brain and paired longitudinal nerve cords developing in connection with the blastopore, and by coelomic organization. Its ancestral larval type was probably a dipleurula. Several characters link rotifers with the spiralians, although they do not show the spiral pattern in the cleavage. Ectoprocts are still a problematic group, but some characters indicate spiralian affinities.

Basiepidermal nervous system in Nemertoderma westbladi (Nemertodermatida): GYIRFamide immunoreactivity

The Nemertodermatida are a small group of microscopic marine worms. Recent molecular studies have demonstrated that they are likely to be the earliest extant bilaterian animals. What was the nervous system (NS) of a bilaterian ancestor like? In order to answer that question, the NS of Nemertoderma westbladi was investigated by means of indirect immunofluorescence technique and confocal scanning laser microscopy. The antibodies to a flatworm neuropeptide GYIRFamide were used in combination with anti-serotonin antibodies and phalloidin-TRITC staining. The immunostaining revealed an entirely basiepidermal NS. A ring lying outside the body wall musculature at the level of the statocyst forms the only centralisation, the "brain". No stomatogastric NS has been observed. The GYIRFamide immunoreactive part of the "brain" is formed of loosely packed nerve fibres with multiple small neurones and a few large ones. The peptidergic and aminergic patterns of the NS do not correspond to each other: the former is more developed on the ventral side, the latter is more pronounced on the dorsal side. A pair of GYIRFamide immunoreactive nerve cords innervates the ventral side of the animal, the mouth and the male genital opening. The nemertodermatids studied to-date display no common NS pattern. Possible synapomorphies of the Acoelomorpha are discussed. The study demonstrates that the nemertodermatid NS possesses a number of plesiomorphic features and appears more primitive than the NS in other worms, except the Xenoturbellida. The bilaterian ancestor supposedly possessed only a basiepidermal nerve net and had no centralised brain-like structures and no stomatogastric NS.