Acoel development indicates the independent evolution of the bilaterian mouth and anus

The Acoel nervous system: morphology and development

Acoel flatworms have played a relevant role in classical (and current) discussions on the evolutionary origin of bilaterian animals. This is mostly derived from the apparent simplicity of their body architectures. This tenet has been challenged over the last couple of decades, mostly because detailed studies of their morphology and the introduction of multiple genomic technologies have unveiled a complexity of cell types, tissular arrangements and patterning mechanisms that were hidden below this 'superficial' simplicity. One tissue that has received a particular attention has been the nervous system (NS). The combination of ultrastructural and single cell methodologies has revealed unique cellular diversity and developmental trajectories for most of their neurons and associated sensory systems. Moreover, the great diversity in NS architectures shown by different acoels offers us with a unique group of animals where to study key aspects of neurogenesis and diversification od neural systems over evolutionary time.In this review we revisit some recent developments in the characterization of the acoel nervous system structure and the regulatory mechanisms that contribute to their embryological development. We end up by suggesting some promising avenues to better understand how this tissue is organized in its finest cellular details and how to achieve a deeper knowledge of the functional roles that genes and gene networks play in its construction.

Highly conserved and extremely evolvable: BMP signalling in secondary axis patterning of Cnidaria and Bilateria

Bilateria encompass the vast majority of the animal phyla. As the name states, they are bilaterally symmetric, that is with a morphologically clear main body axis connecting their anterior and posterior ends, a second axis running between their dorsal and ventral surfaces, and with a left side being roughly a mirror image of their right side. Bone morphogenetic protein (BMP) signalling has widely conserved functions in the formation and patterning of the second, dorso-ventral (DV) body axis, albeit to different extents in different bilaterian species. Whilst initial findings in the fruit fly Drosophila and the frog Xenopus highlighted similarities amongst these evolutionarily very distant species, more recent analyses featuring other models revealed considerable diversity in the mechanisms underlying dorsoventral patterning. In fact, as phylogenetic sampling becomes broader, we find that this axis patterning system is so evolvable that even its core components can be deployed differently or lost in different model organisms. In this review, we will try to highlight the diversity of ways by which BMP signalling controls bilaterality in different animals, some of which do not belong to Bilateria. Future research combining functional analyses and modelling is bound to give us some understanding as to where the limits to the extent of the evolvability of BMP-dependent axial patterning may lie.

Coordinated wound responses in a regenerative animal-algal holobiont

Animal regeneration involves coordinated responses across cell types throughout the animal body. In endosymbiotic animals, whether and how symbionts react to host injury and how cellular responses are integrated across species remain unexplored. Here, we study the acoel Convolutriloba longifissura, which hosts symbiotic Tetraselmis sp. green algae and can regenerate entire bodies from tissue fragments. We show that animal injury causes a decline in the photosynthetic efficiency of the symbiotic algae, alongside two distinct, sequential waves of transcriptional responses in acoel and algal cells. The initial algal response is characterized by the upregulation of a cohort of photosynthesis-related genes, though photosynthesis is not necessary for regeneration. A conserved animal transcription factor, runt, is induced after injury and required for acoel regeneration. Knockdown of Cl-runt dampens transcriptional responses in both species and further reduces algal photosynthetic efficiency post-injury. Our results suggest that the holobiont functions as an integrated unit of biological organization by coordinating molecular networks across species through the runt-dependent animal regeneration program.

Decline and fall of the Ediacarans: late-Neoproterozoic extinctions and the rise of the modern biosphere

The end-Neoproterozoic transition marked a gradual but permanent shift between distinct configurations of Earth's biosphere. This interval witnessed the demise of the enigmatic Ediacaran Biota, ushering in the structured trophic webs and disparate animal body plans of Phanerozoic ecosystems. However, little consensus exists on the reality, drivers, and macroevolutionary implications of end-Neoproterozoic extinctions. Here we evaluate potential drivers of late-Neoproterozoic turnover by addressing recent findings on Ediacaran geochronology, the persistence of classical Ediacaran macrobionts into the Cambrian, and the existence of Ediacaran crown-group eumetazoans. Despite renewed interest in the possibility of Phanerozoic-style 'mass extinctions' in the latest Neoproterozoic, our synthesis of the available evidence does not support extinction models based on episodic geochemical triggers, nor does it validate simple ecological interpretations centred on direct competitive displacement. Instead, we argue that the protracted and indirect effects of early bilaterian innovations, including escalations in sediment engineering, predation, and the largely understudied impacts of reef-building, may best account for the temporal structure and possible selectivity of late-Neoproterozoic extinctions. We integrate these processes into a generalised model of early eumetazoan-dominated ecologies, charting the disruption of spatial and temporal isotropy on the Ediacaran benthos as a consequence of diversifying macrofaunal interactions. Given the nature of resource distribution in Ediacaran ecologies, the continuities among Ediacaran and Cambrian faunas, and the convergent origins of ecologically disruptive innovations among bilaterians we suggest that the rise of Phanerozoic-type biotas may have been unstoppable.

A common modular design of nervous systems originating in soft-bodied invertebrates

Nervous systems of vertebrates and invertebrates show a common modular theme in the flow of information for cost-benefit decisions. Sensory inputs are incentivized by integrating stimulus qualities with motivation and memory to affect appetitive state, a system of homeostatic drives, and labelled for directionality. Appetitive state determines action responses from a repertory of possibles and transmits the decision to a premotor system that frames the selected action in motor arousal and appropriate postural and locomotion commands. These commands are then sent to the primary motor pattern generators controlling the motorneurons, with feedback at each stage. In the vertebrates, these stages are mediated by forebrain pallial derivatives for incentive and directionality (olfactory bulb, cerebral cortex, pallial amygdala, etc.) interacting with hypothalamus (homeostasis, motivation, and reward) for action selection in the forebrain basal ganglia, the mid/hindbrain reticular formation as a premotor translator for posture, locomotion, and arousal state, and the spinal cord and cranial nuclei as primary motor pattern generators. Gastropods, like the predatory sea slug Pleurobranchaea californica, show a similar organization but with differences that suggest how complex brains evolved from an ancestral soft-bodied bilaterian along with segmentation, jointed skeletons, and complex exteroceptors. Their premotor feeding network combines functions of hypothalamus and basal ganglia for homeostasis, motivation, presumed reward, and action selection for stimulus approach or avoidance. In Pleurobranchaea, the premotor analogy to the vertebrate reticular formation is the bilateral "A-cluster" of cerebral ganglion neurons that controls posture, locomotion, and serotonergic motor arousal. The A-cluster transmits motor commands to the pedal ganglia analogs of the spinal cord, for primary patterned motor output. Apparent pallial precursors are not immediately evident in Pleurobranchaea's central nervous system, but a notable candidate is a subepithelial nerve net in the peripheral head region that integrates chemotactile stimuli for incentive and directionality. Evolutionary centralization of its computational functions may have led to the olfaction-derived pallial forebrain in the ancestor's vertebrate descendants and their analogs in arthropods and annelids.

Transcriptomic evidence for Brachyury expression in the caudal tip region of adult Ptychodera flava (Hemichordata)

Most metazoans have a single copy of the T-box transcription factor gene Brachyury. This gene is expressed in cells of the blastopore of late blastulae and the archenteron invagination region of gastrulae. It appears to be crucial for gastrulation and mesoderm differentiation of embryos. Although this expression pattern is shared by most deuterostomes, Brachyury expression has not been reported in adult stages. Here we show that Brachyury of an indirect developer, the hemichordate acorn worm Ptychodera flava, is expressed not only in embryonic cells, but also in cells of the caudal tip (anus) region of adults. This spatially restricted expression, shown by whole-mount in situ hybridization, was confirmed by Iso-Seq RNA sequencing and single-cell RNA-seq (scRNA-seq) analysis. Iso-Seq analysis showed that gene expression occurs only in the caudal region of adults, but not in anterior regions, including the stomochord. scRNA-seq analysis showed a cluster that contained Brachyury-expressing cells comprising epidermis- and mesoderm-related cells, but which is unlikely to be associated with the nervous system or muscle. Although further investigation is required to examine the roles of Brachyury in adults, this study provides important clues for extending studies on Brachyury expression involved in development of the most posterior region of deuterostomes.

The brain regulatory program predates central nervous system evolution

Understanding how brains evolved is critical to determine the origin(s) of centralized nervous systems. Brains are patterned along their anteroposterior axis by stripes of gene expression that appear to be conserved, suggesting brains are homologous. However, the striped expression is also part of the deeply conserved anteroposterior axial program. An emerging hypothesis is that similarities in brain patterning are convergent, arising through the repeated co-option of axial programs. To resolve whether shared brain neuronal programs likely reflect convergence or homology, we investigated the evolution of axial programs in neurogenesis. We show that the bilaterian anteroposterior program patterns the nerve net of the cnidarian Nematostella along the oral-aboral axis arguing that anteroposterior programs regionalized developing nervous systems in the cnidarian-bilaterian common ancestor prior to the emergence of brains. This finding rejects shared patterning as sufficient evidence to support brain homology and provides functional support for the plausibility that axial programs could be co-opted if nervous systems centralized in multiple lineages.

Acoel single-cell atlas reveals expression dynamics and heterogeneity of adult pluripotent stem cells

Adult pluripotent stem cell (aPSC) populations underlie whole-body regeneration in many distantly-related animal lineages, but how the underlying cellular and molecular mechanisms compare across species is unknown. Here, we apply single-cell RNA sequencing to profile transcriptional cell states of the acoel worm Hofstenia miamia during postembryonic development and regeneration. We identify cell types shared across stages and their associated gene expression dynamics during regeneration. Functional studies confirm that the aPSCs, also known as neoblasts, are the source of differentiated cells and reveal transcription factors needed for differentiation. Subclustering of neoblasts recovers transcriptionally distinct subpopulations, the majority of which are likely specialized to differentiated lineages. One neoblast subset, showing enriched expression of the histone variant H3.3, appears to lack specialization. Altogether, the cell states identified in this study facilitate comparisons to other species and enable future studies of stem cell fate potentials.

Saccorhytus is an early ecdysozoan and not the earliest deuterostome

The early history of deuterostomes, the group composed of the chordates, echinoderms and hemichordates¹, is still controversial, not least because of a paucity of stem representatives of these clades^2-5. The early Cambrian microscopic animal Saccorhytus coronarius was interpreted as an early deuterostome on the basis of purported pharyngeal openings, providing evidence for a meiofaunal ancestry⁶ and an explanation for the temporal mismatch between palaeontological and molecular clock timescales of animal evolution^6-8. Here we report new material of S. coronarius, which is reconstructed as a millimetric and ellipsoidal meiobenthic animal with spinose armour and a terminal mouth but no anus. Purported pharyngeal openings in support of the deuterostome hypothesis⁶ are shown to be taphonomic artefacts. Phylogenetic analyses indicate that S. coronarius belongs to total-group Ecdysozoa, expanding the morphological disparity and ecological diversity of early Cambrian ecdysozoans.

A novel endocast technique providing a 3D quantitative analysis of the gastrovascular system in Rhizostoma pulmo: An unexpected through-gut in cnidaria

The investigation of jellyfish gastrovascular systems mainly focused on stain injections and dissections, negatively affected by thickness and opacity of the mesoglea. Therefore, descriptions are incomplete and data about tridimensional structures are scarce. In this work, morphological and functional anatomy of the gastrovascular system of Rhizostoma pulmo (Macri 1778) was investigated in detail with innovative techniques: resin endocasts and 3D X-ray computed microtomography. The gastrovascular system consists of a series of branching canals ending with numerous openings within the frilled margins of the oral arms. Canals presented a peculiar double hemi-canal structure with a medial adhesion area which separates centrifugal and centripetal flows. The inward flow involves only the “mouth” openings on the internal wing of the oral arm and relative hemi-canals, while the outward flow involves only the two outermost wings’ hemi-canals and relative “anal” openings on the external oral arm. The openings differentiation recalls the functional characteristics of a through-gut apparatus. We cannot define the gastrovascular system in Rhizostoma pulmo as a traditional through-gut, rather an example of adaptive convergence, that partially invalidates the paradigm of a single oral opening with both the uptake and excrete function.

A mosaic of conserved and novel modes of gene expression and morphogenesis in mesoderm and muscle formation of a larval bivalve

The mesoderm gives rise to several key morphological features of bilaterian animals including endoskeletal elements and the musculature. A number of regulatory genes involved in mesoderm and/or muscle formation (e.g., Brachyury (Bra), even-skipped (eve), Mox, myosin II heavy chain (mhc)) have been identified chiefly from chordates and the ecdysozoans Drosophila and Caenorhabditis elegans, but data for non-model protostomes, especially those belonging to the ecdysozoan sister clade, Lophotrochozoa (e.g., flatworms, annelids, mollusks), are only beginning to emerge. Within the lophotrochozoans, Mollusca constitutes the most speciose and diverse phylum. Interestingly, however, information on the morphological and molecular underpinnings of key ontogenetic processes such as mesoderm formation and myogenesis remains scarce even for prominent molluscan sublineages such as the bivalves. Here, we investigated myogenesis and developmental expression of Bra, eve, Mox, and mhc in the quagga mussel Dreissena rostriformis, an invasive freshwater bivalve and an emerging model in invertebrate evodevo. We found that all four genes are expressed during mesoderm formation, but some show additional, individual sites of expression during ontogeny. While Mox and mhc are involved in early myogenesis, eve is also expressed in the embryonic shell field and Bra is additionally present in the foregut. Comparative analysis suggests that Mox has an ancestral role in mesoderm and possibly muscle formation in bilaterians, while Bra and eve are conserved regulators of mesoderm development of nephrozoans (protostomes and deuterostomes). The fully developed Dreissena veliger larva shows a highly complex muscular architecture, supporting a muscular ground pattern of autobranch bivalve larvae that includes at least a velum muscle ring, three or four pairs of velum retractors, one or two pairs of larval retractors, two pairs of foot retractors, a pedal plexus, possibly two pairs of mantle retractors, and the muscles of the pallial line, as well as an anterior and a posterior adductor. As is typical for their molluscan kin, remodelling and loss of prominent larval features such as the velum musculature and various retractor systems appear to be also common in bivalves.

Supplementary information

The online version contains supplementary material available at 10.1007/s13127-022-00569-5.

Studying development, regeneration, stem cells, and more in the acoel Hofstenia miamia

Acoel worms represent an enigmatic lineage of animals (Acoelomorpha) that has danced around the tree of animal life. Morphology-based classification placed them as flatworms (Phylum Platyhelminthes), with much of their biology being interpreted as a variation on what is observed in better-studied members of that phylum. However, molecular phylogenies suggest that acoels belong to a clade (Xenacoelomorpha) that could be a sister group to other animals with bilateral symmetry (Bilateria) or could belong within deuterostomes, closely related to a group that includes sea stars (Ambulacraria). This change in phylogenetic position has led to renewed interest in the biology of acoels, which can now offer insights into the evolution of many bilaterian traits. The acoel Hofstenia miamia has emerged as a powerful new research organism that enables mechanistic studies of xenacoelomorph biology, especially of developmental and regenerative processes. This article explains the motivation for developing Hofstenia as a new model system, describes Hofstenia biology, highlights the tools and resources that make Hofstenia a good research organism, and considers the questions that Hofstenia is well-positioned to answer. Finally, looking to the future, this article serves as an invitation to new and established scientists to join the growing community of researchers studying this exciting model system.

Acoel Single-Cell Transcriptomics: Cell Type Analysis of a Deep Branching Bilaterian

Bilaterian animals display a wide variety of cell types, organized into defined anatomical structures and organ systems, which are mostly absent in prebilaterian animals. Xenacoelomorpha are an early-branching bilaterian phylum displaying an apparently relatively simple anatomical organization that have greatly diverged from other bilaterian clades. In this study, we use whole-body single-cell transcriptomics on the acoel Isodiametra pulchra to identify and characterize different cell types. Our analysis identifies the existence of ten major cell type categories in acoels all contributing to main biological functions of the organism: metabolism, locomotion and movements, behavior, defense, and development. Interestingly, although most cell clusters express core fate markers shared with other animal clades, we also describe a surprisingly large number of clade-specific marker genes, suggesting the emergence of clade-specific common molecular machineries functioning in distinct cell types. Together, these results provide novel insight into the evolution of bilaterian cell types and open the door to a better understanding of the origins of the bilaterian body plan and their constitutive cell types.

Embryonic development in the acoel Hofstenia miamia

Acoels are marine worms that belong to the phylum Xenacoelomorpha, a deep-diverging bilaterian lineage. This makes acoels an attractive system for studying the evolution of major bilaterian traits. Thus far, acoel development has not been described in detail at the morphological and transcriptomic levels in a species in which functional genetic studies are possible. We present a set of developmental landmarks for embryogenesis in the highly regenerative acoel Hofstenia miamia. We generated a developmental staging atlas from zygote to hatched worm based on gross morphology, with accompanying bulk transcriptome data. Hofstenia embryos undergo a stereotyped cleavage program known as duet cleavage, which results in two large vegetal pole 'macromeres' and numerous small animal pole 'micromeres'. These macromeres become internalized as micromere progeny proliferate and move vegetally. We also noted a second, previously undescribed, cell-internalization event at the animal pole, following which we detected major body axes and tissues corresponding to all three germ layers. Our work on Hofstenia embryos provides a resource for mechanistic investigations of acoel development, which will yield insights into the evolution of bilaterian development and regeneration.

Lack of support for Deuterostomia prompts reinterpretation of the first Bilateria

The bilaterally symmetric animals (Bilateria) are considered to comprise two monophyletic groups, Protostomia (Ecdysozoa and the Lophotrochozoa) and Deuterostomia (Chordata and the Xenambulacraria). Recent molecular phylogenetic studies have not consistently supported deuterostome monophyly. Here, we compare support for Protostomia and Deuterostomia using multiple, independent phylogenomic datasets. As expected, Protostomia is always strongly supported, especially by longer and higher-quality genes. Support for Deuterostomia, however, is always equivocal and barely higher than support for paraphyletic alternatives. Conditions that cause tree reconstruction errors-inadequate models, short internal branches, faster evolving genes, and unequal branch lengths-coincide with support for monophyletic deuterostomes. Simulation experiments show that support for Deuterostomia could be explained by systematic error. The branch between bilaterian and deuterostome common ancestors is, at best, very short, supporting the idea that the bilaterian ancestor may have been deuterostome-like. Our findings have important implications for the understanding of early animal evolution.

The Caudal ParaHox gene is required for hindgut development in the mollusc Tritia (a.k.a. Ilyanassa)

Caudal homeobox genes are found across animals, typically linked to two other homeobox genes in what has been called the ParaHox cluster. These genes have been proposed to pattern the anterior-posterior axis of the endoderm ancestrally, but the expression of Caudal in extant groups is varied and often occurs in other germ layers. Here we examine the role of Caudal in the embryo of the mollusc Tritia (Ilyanassa) obsoleta. ToCaudal expression is initially broad, then becomes progressively restricted and is finally only in the developing hindgut (a.k.a. intestine). Knockdown of ToCaudal using morpholino oligonucleotides specifically blocks hindgut development, indicating that despite its initially broad expression, the functional role of ToCaudal is in hindgut patterning. This is the first functional characterization of Caudal in an animal with spiralian development, which is an ancient mode of embryogenesis that arose early in bilaterian animal evolution. These results are consistent with the hypothesis that the ancestral role of the ParaHox genes was anterior-posterior patterning of the endoderm.

Conserved Patterns in Developmental Processes and Phases, Rather than Genes, Unite the Highly Divergent Bilateria

Bilateria are the predominant clade of animals on Earth. Despite having evolved a wide variety of body plans and developmental modes, they are characterized by common morphological traits. By default, researchers have tried to link clade-specific genes to these traits, thus distinguishing bilaterians from non-bilaterians, by their gene content. Here we argue that it is rather biological processes that unite Bilateria and set them apart from their non-bilaterian sisters, with a less complex body morphology. To test this hypothesis, we compared proteomes of bilaterian and non-bilaterian species in an elaborate computational pipeline, aiming to search for a set of bilaterian-specific genes. Despite the limited confidence in their bilaterian specificity, we nevertheless detected Bilateria-specific functional and developmental patterns in the sub-set of genes conserved in distantly related Bilateria. Using a novel multi-species GO-enrichment method, we determined the functional repertoire of genes that are widely conserved among Bilateria. Analyzing expression profiles in three very distantly related model species-D. melanogaster, D. rerio and C. elegans-we find characteristic peaks at comparable stages of development and a delayed onset of expression in embryos. In particular, the expression of the conserved genes appears to peak at the phylotypic stage of different bilaterian phyla. In summary, our study illustrate how development connects distantly related Bilateria after millions of years of divergence, pointing to processes potentially separating them from non-bilaterians. We argue that evolutionary biologists should return from a purely gene-centric view of evolution and place more focus on analyzing and defining conserved developmental processes and periods.

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.

A technical review and guide to RNA fluorescence in situ hybridization

RNA-fluorescence in situ hybridization (FISH) is a powerful tool to visualize target messenger RNA transcripts in cultured cells, tissue sections or whole-mount preparations. As the technique has been developed over time, an ever-increasing number of divergent protocols have been published. There is now a broad selection of options available to facilitate proper tissue preparation, hybridization, and post-hybridization background removal to achieve optimal results. Here we review the technical aspects of RNA-FISH, examining the most common methods associated with different sample types including cytological preparations and whole-mounts. We discuss the application of commonly used reagents for tissue preparation, hybridization, and post-hybridization washing and provide explanations of the functional roles for each reagent. We also discuss the available probe types and necessary controls to accurately visualize gene expression. Finally, we review the most recent advances in FISH technology that facilitate both highly multiplexed experiments and signal amplification for individual targets. Taken together, this information will guide the methods development process for investigators that seek to perform FISH in organisms that lack documented or optimized protocols.

Horizontal Transfer and Gene Loss Shaped the Evolution of Alpha-Amylases in Bilaterians

The subfamily GH13_1 of alpha-amylases is typical of Fungi, but it is also found in some unicellular eukaryotes (e.g., Amoebozoa, choanoflagellates) and non-bilaterian Metazoa. Since a previous study in 2007, GH13_1 amylases were considered ancestral to the Unikonts, including animals, except Bilateria, such that it was thought to have been lost in the ancestor of this clade. The only alpha-amylases known to be present in Bilateria so far belong to the GH13_15 and 24 subfamilies (commonly called bilaterian alpha-amylases) and were likely acquired by horizontal transfer from a proteobacterium. The taxonomic scope of Eukaryota genomes in databases has been greatly increased ever since 2007. We have surveyed GH13_1 sequences in recent data from ca. 1600 bilaterian species, 60 non-bilaterian animals and also in unicellular eukaryotes. As expected, we found a number of those sequences in non-bilaterians: Anthozoa (Cnidaria) and in sponges, confirming the previous observations, but none in jellyfishes and in Ctenophora. Our main and unexpected finding is that such fungal (also called Dictyo-type) amylases were also consistently retrieved in several bilaterian phyla: hemichordates (deuterostomes), brachiopods and related phyla, some molluscs and some annelids (protostomes). We discuss evolutionary hypotheses possibly explaining the scattered distribution of GH13_1 across bilaterians, namely, the retention of the ancestral gene in those phyla only and/or horizontal transfers from non-bilaterian donors.

Molecular patterning during the development of Phoronopsis harmeri reveals similarities to rhynchonelliform brachiopods

BACKGROUND

Phoronids, rhynchonelliform and linguliform brachiopods show striking similarities in their embryonic fate maps, in particular in their axis specification and regionalization. However, although brachiopod development has been studied in detail and demonstrated embryonic patterning as a causal factor of the gastrulation mode (protostomy vs deuterostomy), molecular descriptions are still missing in phoronids. To understand whether phoronids display underlying embryonic molecular mechanisms similar to those of brachiopods, here we report the expression patterns of anterior (otx, gsc, six3/6, nk2.1), posterior (cdx, bra) and endomesodermal (foxA, gata4/5/6, twist) markers during the development of the protostomic phoronid Phoronopsis harmeri.

RESULTS

The transcription factors foxA, gata4/5/6 and cdx show conserved expression in patterning the development and regionalization of the phoronid embryonic gut, with foxA expressed in the presumptive foregut, gata4/5/6 demarcating the midgut and cdx confined to the hindgut. Furthermore, six3/6, usually a well-conserved anterior marker, shows a remarkably dynamic expression, demarcating not only the apical organ and the oral ectoderm, but also clusters of cells of the developing midgut and the anterior mesoderm, similar to what has been reported for brachiopods, bryozoans and some deuterostome Bilateria. Surprisingly, brachyury, a transcription factor often associated with gastrulation movements and mouth and hindgut development, seems not to be involved with these patterning events in phoronids.

CONCLUSIONS

Our description and comparison of gene expression patterns with other studied Bilateria reveals that the timing of axis determination and cell fate distribution of the phoronid shows highest similarity to that of rhynchonelliform brachiopods, which is likely related to their shared protostomic mode of development. Despite these similarities, the phoronid Ph. harmeri also shows particularities in its development, which hint to divergences in the arrangement of gene regulatory networks responsible for germ layer formation and axis specification.

A small set of conserved genes, including sp5 and Hox, are activated by Wnt signaling in the posterior of planarians and acoels

Wnt signaling regulates primary body axis formation across the Metazoa, with high Wnt signaling specifying posterior identity. Whether a common Wnt-driven transcriptional program accomplishes this broad role is poorly understood. We identified genes acutely affected after Wnt signaling inhibition in the posterior of two regenerative species, the planarian Schmidtea mediterranea and the acoel Hofstenia miamia, which are separated by >550 million years of evolution. Wnt signaling was found to maintain positional information in muscle and regional gene expression in multiple differentiated cell types. sp5, Hox genes, and Wnt pathway components are down-regulated rapidly after β-catenin RNAi in both species. Brachyury, a vertebrate Wnt target, also displays Wnt-dependent expression in Hofstenia. sp5 inhibits trunk gene expression in the tail of planarians and acoels, promoting separate tail-trunk body domains. A planarian posterior Hox gene, Post-2d, promotes normal tail regeneration. We propose that common regulation of a small gene set–Hox, sp5, and Brachyury–might underlie the widespread utilization of Wnt signaling in primary axis patterning across the Bilateria.

How animals form and maintain their body axes is a fundamental topic in developmental biology. Wnt signaling is an important regulator of head-tail axis formation across animals, with high Wnt signaling specifying tail identity. In this study, we use two species that are separated by more than 550 million years of evolution, planarians and acoels, to find genes regulated by Wnt signaling in the tail broadly in the Bilateria. We identified a small conserved set of Wnt-regulated genes, including the transcription factor-encoding genes sp5 and Hox. This suggests that regulation of this gene set might be a key function of Wnt signaling in the tails of bilaterally symmetric animals. Inhibition of a planarian posterior Hox gene, Post-2d, by RNAi caused tail-regeneration defects. Inhibition of sp5 by RNAi revealed that it functions to restrict the expression of trunk genes in the tail of planarians and acoels. Since Wnt signaling activates both trunk and tail patterning gene expression in planarians, this suggests a mechanism by which Wnt signaling can establish separate trunk-tail body domains through regulation of sp5.

A developmental perspective on the evolution of the nervous system

The evolution of nervous systems in animals has always fascinated biologists, and thus multiple evolutionary scenarios have been proposed to explain the appearance of neurons and complex neuronal centers. However, the absence of a robust phylogenetic framework for animal interrelationships, the lack of a mechanistic understanding of development, and a recapitulative view of animal ontogeny have traditionally limited these scenarios. Only recently, the integration of advanced molecular and morphological studies in a broad range of animals has allowed to trace the evolution of developmental and neuronal characters on a better-resolved animal phylogeny. This has falsified most traditional scenarios for nervous system evolution, paving the way for the emergence of new testable hypotheses. Here we summarize recent progress in studies of nervous system development in major animal lineages and formulate some of the arising questions. In particular, we focus on how lineage analyses of nervous system development and a comparative study of the expression of neural-related genes has influenced our understanding of the evolution of an elaborated central nervous system in Bilateria. We argue that a phylogeny-guided study of neural development combining thorough descriptive and functional analyses is key to establish more robust scenarios for the origin and evolution of animal nervous systems.

Active mode of excretion across digestive tissues predates the origin of excretory organs

Most bilaterian animals excrete toxic metabolites through specialized organs, such as nephridia and kidneys, which share morphological and functional correspondences. In contrast, excretion in non-nephrozoans is largely unknown, and therefore the reconstruction of ancestral excretory mechanisms is problematic. Here, we investigated the excretory mode of members of the Xenacoelomorpha, the sister group to Nephrozoa, and Cnidaria, the sister group to Bilateria. By combining gene expression, inhibitor experiments, and exposure to varying environmental ammonia conditions, we show that both Xenacoelomorpha and Cnidaria are able to excrete across digestive-associated tissues. However, although the cnidarian Nematostella vectensis seems to use diffusion as its main excretory mode, the two xenacoelomorphs use both active transport and diffusion mechanisms. Based on these results, we propose that digestive-associated tissues functioned as excretory sites before the evolution of specialized organs in nephrozoans. We conclude that the emergence of a compact, multiple-layered bilaterian body plan necessitated the evolution of active transport mechanisms, which were later recruited into the specialized excretory organs.

Mitigating Anticipated Effects of Systematic Errors Supports Sister-Group Relationship between Xenacoelomorpha and Ambulacraria

Xenoturbella and the acoelomorph worms (Xenacoelomorpha) are simple marine animals with controversial affinities. They have been placed as the sister group of all other bilaterian animals (Nephrozoa hypothesis), implying their simplicity is an ancient characteristic [1, 2]; alternatively, they have been linked to the complex Ambulacraria (echinoderms and hemichordates) in a clade called the Xenambulacraria [3-5], suggesting their simplicity evolved by reduction from a complex ancestor. The difficulty resolving this problem implies the phylogenetic signal supporting the correct solution is weak and affected by inadequate modeling, creating a misleading non-phylogenetic signal. The idea that the Nephrozoa hypothesis might be an artifact is prompted by the faster molecular evolutionary rate observed within the Acoelomorpha. Unequal rates of evolution are known to result in the systematic artifact of long branch attraction, which would be predicted to result in an attraction between long-branch acoelomorphs and the outgroup, pulling them toward the root [6]. Other biases inadequately accommodated by the models used can also have strong effects, exacerbated in the context of short internal branches and long terminal branches [7]. We have assembled a large and informative dataset to address this problem. Analyses designed to reduce or to emphasize misleading signals show the Nephrozoa hypothesis is supported under conditions expected to exacerbate errors, and the Xenambulacraria hypothesis is preferred in conditions designed to reduce errors. Our reanalyses of two other recently published datasets [1, 2] produce the same result. We conclude that the Xenacoelomorpha are simplified relatives of the Ambulacraria.

FoxA expression pattern in two polychaete species, Alitta virens and Platynereis dumerilii: Examination of the conserved key regulator of the gut development from cleavage through larval life, postlarval growth, and regeneration

BACKGROUND

foxA orthologs are involved in various processes from embryo patterning to regulation of metabolism. Since foxA conserved role in the development of the gut of errant annelids has never been thoroughly studied, we used a candidate gene approach to unravel the molecular profile of the alimentary canal in two closely related nereid worms with a trochophore-type lecithotrophic larva.

RESULTS

The character of foxA expression in the two polychaetes was similar but not identical. The genes were successively activated first in blastoporal cells, then in the stomodeum, the midgut, and hindgut primordia, and in the cells of central and peripheral nervous system. Before the start of active feeding of nectochaetes, we observed a short phase of foxA expression in the entire digestive tract. After amputation of posterior segments, foxA expression was established de novo in the new terminal part of the intestine, and then in the developing hindgut and the anus.

CONCLUSIONS

We discovered an early marker of endoderm formation previously unknown in errant annelids. Its expression dynamics provided valuable insights into the gut development. Comparative analysis of foxA activity suggests its primary role in gastrulation morphogenesis independently of its type and in midgut and foregut specification. Developmental Dynamics 248:728-743, 2019. © 2018 Wiley Periodicals, Inc.

Support for a clade of Placozoa and Cnidaria in genes with minimal compositional bias

The phylogenetic placement of the morphologically simple placozoans is crucial to understanding the evolution of complex animal traits. Here, we examine the influence of adding new genomes from placozoans to a large dataset designed to study the deepest splits in the animal phylogeny. Using site-heterogeneous substitution models, we show that it is possible to obtain strong support, in both amino acid and reduced-alphabet matrices, for either a sister-group relationship between Cnidaria and Placozoa, or for Cnidaria and Bilateria as seen in most published work to date, depending on the orthologues selected to construct the matrix. We demonstrate that a majority of genes show evidence of compositional heterogeneity, and that support for the Cnidaria + Bilateria clade can be assigned to this source of systematic error. In interpreting these results, we caution against a peremptory reading of placozoans as secondarily reduced forms of little relevance to broader discussions of early animal evolution.

Evolution of the bilaterian mouth and anus

It is widely held that the bilaterian tubular gut with mouth and anus evolved from a simple gut with one major gastric opening. However, there is no consensus on how this happened. Did the single gastric opening evolve into a mouth, with the anus forming elsewhere in the body (protostomy), or did it evolve into an anus, with the mouth forming elsewhere (deuterostomy), or did it evolve into both mouth and anus (amphistomy)? These questions are addressed by the comparison of developmental fates of the blastopore, the opening of the embryonic gut, in diverse animals that live today. Here we review comparative data on the identity and fate of blastoporal tissue, investigate how the formation of the through-gut relates to the major body axes, and discuss to what extent evolutionary scenarios are consistent with these data. Available evidence indicates that stem bilaterians had a slit-like gastric opening that was partially closed in subsequent evolution, leaving open the anus and most likely also the mouth, which would favour amphistomy. We discuss remaining difficulties, and outline directions for future research.

Reactive oxygen species may play an essential role in driving biological evolution: The Cambrian Explosion as an example

The Cambrian Explosion is one of the most significant events in the history of life; essentially all easily fossilizable animal body plans first evolved during this event. Although many theories have been proposed to explain this event, its cause remains unresolved. Here, we propose that the elevated level of oxygen, in combination with the increased mobility and food intake of metazoans, led to increased cellular levels of reactive oxygen species (ROS), which drove evolution by enhancing mutation rates and providing new regulatory mechanisms. Our hypothesis may provide a unified explanation for the Cambrian Explosion as it incorporates both environmental and developmental factors and is also consistent with ecological explanations for animal radiation. Future studies should focus on testing this hypothesis, and may lead to important insights into evolution.

Acquisition of the dorsal structures in chordate amphioxus

Acquisition of dorsal structures, such as notochord and hollow nerve cord, is likely to have had a profound influence upon vertebrate evolution. Dorsal formation in chordate development thus has been intensively studied in vertebrates and ascidians. However, the present understanding does not explain how chordates acquired dorsal structures. Here we show that amphioxus retains a key clue to answer this question. In amphioxus embryos, maternal nodal mRNA distributes asymmetrically in accordance with the remodelling of the cortical cytoskeleton in the fertilized egg, and subsequently lefty is first expressed in a patch of blastomeres across the equator where wnt8 is expressed circularly and which will become the margin of the blastopore. The lefty domain co-expresses zygotic nodal by the initial gastrula stage on the one side of the blastopore margin and induces the expression of goosecoid, not-like, chordin and brachyury1 genes in this region, as in the oral ectoderm of sea urchin embryos, which provides a basis for the formation of the dorsal structures. The striking similarity in the gene regulations and their respective expression domains when comparing dorsal formation in amphioxus and the determination of the oral ectoderm in sea urchin embryos suggests that chordates derived from an ambulacrarian-type blastula with dorsoventral inversion.

On the evolution of bilaterality

Bilaterality – the possession of two orthogonal body axes – is the name-giving trait of all bilaterian animals. These body axes are established during early embryogenesis and serve as a three-dimensional coordinate system that provides crucial spatial cues for developing cells, tissues, organs and appendages. The emergence of bilaterality was a major evolutionary transition, as it allowed animals to evolve more complex body plans. Therefore, how bilaterality evolved and whether it evolved once or several times independently is a fundamental issue in evolutionary developmental biology. Recent findings from non-bilaterian animals, in particular from Cnidaria, the sister group to Bilateria, have shed new light into the evolutionary origin of bilaterality. Here, we compare the molecular control of body axes in radially and bilaterally symmetric cnidarians and bilaterians, identify the minimal set of traits common for Bilateria, and evaluate whether bilaterality arose once or more than once during evolution.

Clustered brachiopod Hox genes are not expressed collinearly and are associated with lophotrochozoan novelties

Temporal collinearity is often considered the main force preserving Hox gene clusters in animal genomes. Studies that combine genomic and gene expression data are scarce, however, particularly in invertebrates like the Lophotrochozoa. As a result, the temporal collinearity hypothesis is currently built on poorly supported foundations. Here we characterize the complement, cluster, and expression of Hox genes in two brachiopod species, Terebratalia transversa and Novocrania anomalaT. transversa has a split cluster with 10 genes (lab, pb, Hox3, Dfd, Scr, Lox5, Antp, Lox4, Post2, and Post1), whereas N. anomala has 9 genes (apparently missing Post1). Our in situ hybridization, real-time quantitative PCR, and stage-specific transcriptomic analyses show that brachiopod Hox genes are neither strictly temporally nor spatially collinear; only pb (in T. transversa), Hox3 (in both brachiopods), and Dfd (in both brachiopods) show staggered mesodermal expression. Thus, our findings support the idea that temporal collinearity might contribute to keeping Hox genes clustered. Remarkably, expression of the Hox genes in both brachiopod species demonstrates cooption of Hox genes in the chaetae and shell fields, two major lophotrochozoan morphological novelties. The shared and specific expression of Hox genes, together with Arx, Zic, and Notch pathway components in chaetae and shell fields in brachiopods, mollusks, and annelids provide molecular evidence supporting the conservation of the molecular basis for these lophotrochozoan hallmarks.

The developmental basis for the recurrent evolution of deuterostomy and protostomy

The mouth opening of bilaterian animals develops either separate from (deuterostomy) or connected to (protostomy) the embryonic blastopore, the site of endomesoderm internalization. Although this distinction preluded the classification of bilaterian animals in Deuterostomia and Protostomia, and has influenced major scenarios of bilaterian evolution, the developmental basis for the appearance of these different embryonic patterns remains unclear. To identify the underlying mechanisms, we compared the development of two brachiopod species that show deuterostomy (Novocrania anomala) and protostomy (Terebratalia transversa), respectively. We show that the differential activity of Wnt signalling, together with the timing and location of mesoderm formation, correlate with the differential behaviour and fate of the blastopore. We further assess these principles in the spiral-cleaving group Annelida, and propose that the developmental relationships of mouth and blastoporal openings are secondary by-products of variations in axial and mesoderm development. This challenges the previous evolutionary emphasis on extant blastoporal behaviours to explain the origin and diversification of bilaterian animals.

The role of fluency in preferences for inward over outward words

The present studies examined a novel explanation for the in-out effect, the phenomenon that words with inward wanderings of consonantal articulation spots are preferred over words with outward wanderings. We hypothesized that processing fluency might account for the in-out effect instead of, or in addition to, the originally proposed mechanism of motor-associated motivational states. Inward words could be more fluently processed than outward words, which could lead to the preference effect. Corpus analyses (Studies 1a and 1b) revealed more inward than outward words in English and German, which could account for their differing fluency. Additionally, inward compared to outward words were pronounced faster (Study 2) and were rated as being easier to pronounce (Studies 3a and 3b), indicating greater fluency. Crucially, a mediation analysis (Study 4) suggests that the influence of consonantal direction on preference was partially mediated by fluency. However, accounting for the influence of fluency still left a significant residual in-out effect, not accounted for by our fluency measure. This evidence supports a partial causal contribution of articulation fluency to the in-out effect.

Xenacoelomorpha: a case of independent nervous system centralization?

Centralized nervous systems (NSs) and complex brains are among the most important innovations in the history of life on our planet. In this context, two related questions have been formulated: How did complex NSs arise in evolution, and how many times did this occur? As a step towards finding an answer, we describe the NS of several representatives of the Xenacoelomorpha, a clade whose members show different degrees of NS complexity. This enigmatic clade is composed of three major taxa: acoels, nemertodermatids and xenoturbellids. Interestingly, while the xenoturbellids seem to have a rather 'simple' NS (a nerve net), members of the most derived group of acoel worms clearly have ganglionic brains. This interesting diversity of NS architectures (with different degrees of compaction) provides a unique system with which to address outstanding questions regarding the evolution of brains and centralized NSs. The recent sequencing of xenacoelomorph genomes gives us a privileged vantage point from which to analyse neural evolution, especially through the study of key gene families involved in neurogenesis and NS function, such as G protein-coupled receptors, helix-loop-helix transcription factors and Wnts. We finish our manuscript proposing an adaptive scenario for the origin of centralized NSs (brains).

Phylogenomic Insights into Animal Evolution

Animals make up only a small fraction of the eukaryotic tree of life, yet, from our vantage point as members of the animal kingdom, the evolution of the bewildering diversity of animal forms is endlessly fascinating. In the century following the publication of Darwin's Origin of Species, hypotheses regarding the evolution of the major branches of the animal kingdom - their relationships to each other and the evolution of their body plans - was based on a consideration of the morphological and developmental characteristics of the different animal groups. This morphology-based approach had many successes but important aspects of the evolutionary tree remained disputed. In the past three decades, molecular data, most obviously primary sequences of DNA and proteins, have provided an estimate of animal phylogeny largely independent of the morphological evolution we would ultimately like to understand. The molecular tree that has evolved over the past three decades has drastically altered our view of animal phylogeny and many aspects of the tree are no longer contentious. The focus of molecular studies on relationships between animal groups means, however, that the discipline has become somewhat divorced from the underlying biology and from the morphological characteristics whose evolution we aim to understand. Here, we consider what we currently know of animal phylogeny; what aspects we are still uncertain about and what our improved understanding of animal phylogeny can tell us about the evolution of the great diversity of animal life.

Deployment of regulatory genes during gastrulation and germ layer specification in a model spiralian mollusc Crepidula

BACKGROUND

During gastrulation, endoderm and mesoderm are specified from a bipotential precursor (endomesoderm) that is argued to be homologous across bilaterians. Spiralians also generate mesoderm from ectodermal precursors (ectomesoderm), which arises near the blastopore. While a conserved gene regulatory network controls specification of endomesoderm in deuterostomes and ecdysozoans, little is known about genes controlling specification or behavior of either source of spiralian mesoderm or the digestive tract.

RESULTS

Using the mollusc Crepidula, we examined conserved regulatory factors and compared their expression to fate maps to score expression in the germ layers, blastopore lip, and digestive tract. Many genes were expressed in both ecto- and endomesoderm, but only five were expressed in ectomesoderm exclusively. The latter may contribute to epithelial-to-mesenchymal transition seen in ectomesoderm.

CONCLUSIONS

We present the first comparison of genes expressed during spiralian gastrulation in the context of high-resolution fate maps. We found variation of genes expressed in the blastopore lip, mouth, and cells that will form the anus. Shared expression of many genes in both mesodermal sources suggests that components of the conserved endomesoderm program were either co-opted for ectomesoderm formation or that ecto- and endomesoderm are derived from a common mesodermal precursor that became subdivided into distinct domains during evolution.

Xenacoelomorpha's significance for understanding bilaterian evolution

The Xenacoelomorpha, with its phylogenetic position as sister group of the Nephrozoa (Protostomia+Deuterostomia), plays a key-role in understanding the evolution of bilaterian cell types and organ systems. Current studies of the morphological and developmental diversity of this group allow us to trace the evolution of different organ systems within the group and to reconstruct characters of the most recent common ancestor of Xenacoelomorpha. The disparity of the clade shows that there cannot be a single xenacoelomorph 'model' species and strategic sampling is essential for understanding the evolution of major traits. With this strategy, fundamental insights into the evolution of molecular mechanisms and their role in shaping animal organ systems can be expected in the near future.

HOX gene complement and expression in the planarian Schmidtea mediterranea

Background

Freshwater planarians are well known for their regenerative abilities. Less well known is how planarians maintain spatial patterning in long-lived adult animals or how they re-pattern tissues during regeneration. HOX genes are good candidates to regulate planarian spatial patterning, yet the full complement or genomic clustering of planarian HOX genes has not yet been described, primarily because only a few have been detectable by in situ hybridization, and none have given morphological phenotypes when knocked down by RNAi.

Results

Because the planarian Schmidteamediterranea (S. mediterranea) is unsegmented, appendage less, and morphologically simple, it has been proposed that it may have a simplified HOX gene complement. Here, we argue against this hypothesis and show that S. mediterranea has a total of 13 HOX genes, which represent homologs to all major axial categories, and can be detected by whole-mount in situ hybridization using a highly sensitive method. In addition, we show that planarian HOX genes do not cluster in the genome, yet 5/13 have retained aspects of axially restricted expression. Finally, we confirm HOX gene axial expression by RNA deep-sequencing 6 anterior–posterior “zones” of the animal, which we provide as a dataset to the community to discover other axially restricted transcripts.

Conclusions

Freshwater planarians have an unappreciated HOX gene complexity, with all major axial categories represented. However, we conclude based on adult expression patterns that planarians have a derived body plan and their asexual lifestyle may have allowed for large changes in HOX expression from the last common ancestor between arthropods, flatworms, and vertebrates. Using our in situ method and axial zone RNAseq data, it should be possible to further understand the pathways that pattern the anterior–posterior axis of adult planarians.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-016-0044-8) contains supplementary material, which is available to authorized users.

Amphioxus mouth after dorso-ventral inversion

INTRODUCTION

Deuterostomes (animals with 'secondary mouths') are generally accepted to develop the mouth independently of the blastopore. However, it remains largely unknown whether mouths are homologous among all deuterostome groups. Unlike other bilaterians, in amphioxus the mouth initially opens on the left lateral side. This peculiar morphology has not been fully explained in the evolutionary developmental context. We studied the developmental process of the amphioxus mouth to understand whether amphioxus acquired a new mouth, and if so, how it is related to or differs from mouths in other deuterostomes.

RESULTS

The left first somite in amphioxus produces a coelomic vesicle between the epidermis and pharynx that plays a crucial role in the mouth opening. The vesicle develops in association with the amphioxus-specific Hatschek nephridium, and first opens into the pharynx and then into the exterior as a mouth. This asymmetrical development of the anterior-most somites depends on the Nodal-Pitx signaling unit, and the perturbation of laterality-determining Nodal signaling led to the disappearance of the vesicle, producing a symmetric pair of anterior-most somites that resulted in larvae lacking orobranchial structures. The vesicle expressed bmp2/4, as seen in ambulacrarian coelomic pore-canals, and the mouth did not open when Bmp2/4 signaling was blocked.

CONCLUSIONS

We conclude that the amphioxus mouth, which uniquely involves a mesodermal coelomic vesicle, shares its evolutionary origins with the ambulacrarian coelomic pore-canal. Our observations suggest that there are at least three types of mouths in deuterostomes, and that the new acquisition of chordate mouths was likely related to the dorso-ventral inversion that occurred in the last common ancestor of chordates.

Embracing the comparative approach: how robust phylogenies and broader developmental sampling impacts the understanding of nervous system evolution

Molecular biology has provided a rich dataset to develop hypotheses of nervous system evolution. The startling patterning similarities between distantly related animals during the development of their central nervous system (CNS) have resulted in the hypothesis that a CNS with a single centralized medullary cord and a partitioned brain is homologous across bilaterians. However, the ability to precisely reconstruct ancestral neural architectures from molecular genetic information requires that these gene networks specifically map with particular neural anatomies. A growing body of literature representing the development of a wider range of metazoan neural architectures demonstrates that patterning gene network complexity is maintained in animals with more modest levels of neural complexity. Furthermore, a robust phylogenetic framework that provides the basis for testing the congruence of these homology hypotheses has been lacking since the advent of the field of 'evo-devo'. Recent progress in molecular phylogenetics is refining the necessary framework to test previous homology statements that span large evolutionary distances. In this review, we describe recent advances in animal phylogeny and exemplify for two neural characters-the partitioned brain of arthropods and the ventral centralized nerve cords of annelids-a test for congruence using this framework. The sequential sister taxa at the base of Ecdysozoa and Spiralia comprise small, interstitial groups. This topology is not consistent with the hypothesis of homology of tripartitioned brain of arthropods and vertebrates as well as the ventral arthropod and rope-like ladder nervous system of annelids. There can be exquisite conservation of gene regulatory networks between distantly related groups with contrasting levels of nervous system centralization and complexity. Consequently, the utility of molecular characters to reconstruct ancestral neural organization in deep time is limited.

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.