Polychaete trunk neuroectoderm converges and extends by mediolateral cell intercalation

Dynamic microvilli sculpt bristles at nanometric scale

Organisms generate shapes across size scales. Whereas patterning and morphogenesis of macroscopic tissues has been extensively studied, the principles underlying the formation of micrometric and submicrometric structures remain largely enigmatic. Individual cells of polychaete annelids, so-called chaetoblasts, are associated with the generation of chitinous bristles of highly stereotypic geometry. Here we show that bristle formation requires a chitin-producing enzyme specifically expressed in the chaetoblasts. Chaetoblasts exhibit dynamic cell surfaces with stereotypical patterns of actin-rich microvilli. These microvilli can be matched with internal and external structures of bristles reconstructed from serial block-face electron micrographs. Individual chitin teeth are deposited by microvilli in an extension-disassembly cycle resembling a biological 3D printer. Consistently, pharmacological interference with actin dynamics leads to defects in tooth formation. Our study reveals that both material and shape of bristles are encoded by the same cell, and that microvilli play a role in micro- to submicrometric sculpting of biomaterials.

JNK Mediates Differentiation, Cell Polarity and Apoptosis During Amphioxus Development by Regulating Actin Cytoskeleton Dynamics and ERK Signalling

c-Jun N-terminal kinase (JNK) is a multi-functional protein involved in a diverse array of context-dependent processes, including apoptosis, cell cycle regulation, adhesion, and differentiation. It is integral to several signalling cascades, notably downstream of non-canonical Wnt and mitogen activated protein kinase (MAPK) signalling pathways. As such, it is a key regulator of cellular behaviour and patterning during embryonic development across the animal kingdom. The cephalochordate amphioxus is an invertebrate chordate model system straddling the invertebrate to vertebrate transition and is thus ideally suited for comparative studies of morphogenesis. However, next to nothing is known about JNK signalling or cellular processes in this lineage. Pharmacological inhibition of JNK signalling using SP600125 during embryonic development arrests gastrula invagination and causes convergence extension-like defects in axial elongation, particularly of the notochord. Pharynx formation and anterior oral mesoderm derivatives like the preoral pit are also affected. This is accompanied by tissue-specific transcriptional changes, including reduced expression of six3/6 and wnt2 in the notochord, and ectopic wnt11 in neurulating embryos treated at late gastrula stages. Cellular delamination results in accumulation of cells in the gut cavity and a dorsal fin-like protrusion, followed by secondary Caspase-3-mediated apoptosis of polarity-deficient cells, a phenotype only partly rescued by co-culture with the pan-Caspase inhibitor Z-VAD-fmk. Ectopic activation of extracellular signal regulated kinase (ERK) signalling in the neighbours of extruded notochord and neural cells, possibly due to altered adhesive and tensile properties, as well as defects in cellular migration, may explain some phenotypes caused by JNK inhibition. Overall, this study supports conserved functions of JNK signalling in mediating the complex balance between cell survival, apoptosis, differentiation, and cell fate specification during cephalochordate morphogenesis.

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.

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.

Rapid and gentle hydrogel encapsulation of living organisms enables long-term microscopy over multiple hours

Imaging living organisms at high spatial resolution requires effective and innocuous immobilization. Long-term imaging places further demands on sample mounting with minimal perturbation of the organism. Here we present a simple, inexpensive method for rapid encapsulation of small animals of any developmental stage within a photo-crosslinked polyethylene glycol (PEG) hydrogel, gently restricting movement within their confined spaces. Immobilized animals maintain their original morphology in a hydrated environment compatible with chemical treatment, optical stimulation, and light-sheet microscopy. We demonstrate prolonged three-dimensional imaging of neural responses in the nematode Caenorhabditis elegans, recovery of viable organisms after 24 h, and imaging of larger squid hatchlings. We characterize a range of hydrogel and illumination conditions for immobilization quality, and identify paralytic-free conditions suitable for high-resolution single-cell imaging. Overall, PEG hydrogel encapsulation provides fast, versatile, and gentle mounting of small living organisms, from yeast to zebrafish, for continuous observation over hours.

Kyra Burnett et al. present a simple and economical method to encapsulate small living organisms for long-term microscopy in a photo-crosslinked polyethylene glycol hydrogel. This method provides a fast and gentle mounting for continuous imaging over hours, and works with light-sheet microscopy and optogenetic stimulation.

Expression of six3 and otx in Solenogastres (Mollusca) supports an ancestral role in bilaterian anterior-posterior axis patterning

The homeodomain transcription factors six3 and otx are involved in patterning the anterior body and parts of the central nervous system (CNS) in bilaterians. Their similar expression patterns have been used as an argument for homology of heads, brains, segmentation, and ciliated larvae. We investigated the developmental expression of six3 and otx in the aplacophoran mollusk Wirenia argentea. Six3 is expressed in subepithelial cells delimiting the apical organ of the solenogaster pericalymma larva. Otx is expressed in cells of the prototroch and adjacent regions as well as in posterior extensions of the prototrochal expression domain. Advanced larvae also show pretrochal otx expression in the developing CNS. Comparative analysis of six3 and otx expression in bilaterians argues for an ancestral function in anterior-posterior body axis patterning but, due to its presence in animals lacking a head and/or a brain, not necessarily for the presence of these morphological structures in the last common ancestor (LCA) of bilaterians. Likewise, the hypothesis that the posterior border of otx expression corresponds to the border between the unsegmented head and the segmented trunk of the LCA of protostomes is not supported, since otx is extensively expressed in the trunk in W. argentea and numerous other protostomes.

Brain regionalization genes are co-opted into shell field patterning in Mollusca

The 'brain regionalization genes' Six3/6, Otx, Pax2/5/8, Gbx, and Hox1 are expressed in a similar fashion in the deuterostome, ecdysozoan, and the cephalopod brain, questioning whether this holds also true for the remaining Mollusca. We investigated developmental Gbx-expression in representatives of both molluscan sister groups, the Aculifera and Conchifera. Gbx is expressed in the posterior central nervous system of an aculiferan polyplacophoran and solenogaster but not in a conchiferan bivalve suggesting that Gbx, together with Six3/6, Otx, Pax2/5/8, and Hox1, is involved in central nervous system regionalization as reported for other bilaterians. Gbx is, however, also expressed in the anterior central nervous system, i.e. the anlagen of the cerebral ganglia, in the solenogaster, a condition not reported for any other bilaterian so far. Strikingly, all Gbx-orthologs and the other 'posterior brain regionalization genes' such as Pax2/5/8 and Hox1 are expressed in the mantle that secretes shell(s) and spicules of mollusks (except cephalopods). In bivalves, the ancestral condition has even been lost, with Gbx and Pax2/5/8 not being expressed in the developing central nervous system anymore. This suggests an additional role in the formation of the molluscan shell field(s) and spicule-bearing cells, key features of mollusks.

Development of the nervous system in Platynereis dumerilii (Nereididae, Annelida)

Background

The structure and development of the nervous system in Lophotrochozoa has long been recognized as one of the most important subjects for phylogenetic and evolutionary discussion. Many recent papers have presented comprehensive data on the structure and development of catecholaminergic, serotonergic and FMRFamidergic parts of the nervous system. However, relatively few papers contain detailed descriptions of the nervous system in Annelida, one of the largest taxa of Lophotrochozoa. The polychaete species Platynereis dumerilii has recently become one of the more popular model animals in evolutionary and developmental biology. The goal of the present study was to provide a detailed description of its neuronal development. The data obtained will contribute to a better understanding of the basic features of neuronal development in polychaetes.

Results

We have studied the development of the nervous system in P. dumerilii utilizing histo- and immunochemical labelling of catecholamines, serotonin, FMRFamide related peptides, and acetylated tubulin. The first neuron differentiates at the posterior extremity of the protrochophore, reacts to the antibodies against both serotonin and FMRFamide. Then its fibres run forwards along the ventral side. Soon, more neurons appear at the apical extreme, and their basal neurites form the basel structure of the developing brain (cerebral neuropil and circumesophageal connectives). Initial development of the nervous system starts in two rudiments: anterior and posterior. At the nectochaete stage, segmental ganglia start to differentiate in the anterior-to-posterior direction, and the first structures of the stomatogastric and peripheral nervous system appear. All connectives including the unpaired ventral cord develop from initially paired nerves.

Conclusions

We present a detailed description of Platynereis dumerilii neuronal development based on anti-acetylated tubulin, serotonin, and FMRFamide-like immunostaining as well as catecholamine histofluorescence. The development of the nervous system starts from peripheral pioneer neurons at both the posterior and anterior poles of the larva, and their neurites form a scaffold upon which the adult central nervous system develops. The anterior-to-posterior mode of the ventral ganglia development challenges the primary heteronomy concept. Comparison with the development of Mollusca reveals substantial similarities with early neuronal development in larval Solenogastres.

Electronic supplementary material

The online version of this article (doi:10.1186/s12983-017-0211-3) contains supplementary material, which is available to authorized users.

Mechano-sensitive regulation of gene expression during the embryonic development

Cell movements during embryogenesis produce mechanical tensions that shape the embryo and can also regulate gene expression, thereby affecting cell differentiation. Increasing evidence indicates that mechanosensitive regulation of gene expression plays important roles during embryogenesis by coupling the processes of morphogenesis and differentiation. However, the molecular mechanisms of this phenomenon remain poorly understood. This review focuses on the molecular mechanisms that "translate" mechanical stimuli into gene expression.

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.

Molluscan models: Crepidula fornicata

Gastropod snails in the genus Crepidula have emerged as model systems for studying a metazoan super clade, the Spiralia. Recent work on one species in particular, Crepidula fornicata, has produced high-resolution cell lineage fate maps, details of morphogenetic events during gastrulation, key insights into the molecular underpinnings of early development, and the first demonstration of CRISPR/Cas9 genome editing in the Spiralia. Furthermore, invasive species of Crepidula are a significant ecological threat, while one of these, C. fornicata, is also being harvested for food. This review highlights progress towards developing these animals as models for evolutionary, developmental, and ecological studies. Such studies have contributed greatly to our understanding of biology in a major clade of bilaterians. This information may also help us to control and cultivate these snails.

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.

From nerve net to nerve ring, nerve cord and brain--evolution of the nervous system

The puzzle of how complex nervous systems emerged remains unsolved. Comparative studies of neurodevelopment in cnidarians and bilaterians suggest that this process began with distinct integration centres that evolved on opposite ends of an initial nerve net. The 'apical nervous system' controlled general body physiology, and the 'blastoporal nervous system' coordinated feeding movements and locomotion. We propose that expansion, integration and fusion of these centres gave rise to the bilaterian nerve cord and brain.

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.

Spiralian gastrulation: germ layer formation, morphogenesis, and fate of the blastopore in the slipper snail Crepidula fornicata

Background

Gastrulation is a critical step in bilaterian development, directly linked to the segregation of germ layers, establishment of axes, and emergence of the through-gut. Theories about the evolution of gastrulation often concern the fate of the blastopore (site of endomesoderm internalization), which varies widely in a major branch of bilaterians, the Spiralia. In this group, the blastopore has been said to become the mouth, the anus, both, or neither. Different developmental explanations for this variation exist, yet no modern lineage tracing study has ever correlated the position of cells surrounding the blastopore with their contribution to tissues of the mouth, foregut, and anus in a spiralian. This is the first study to do so, using the gastropod Crepidula fornicata.

Results

Crepidula gastrulation occurs by epiboly: the first through third quartet micromeres form an epithelial animal cap that expands to cover vegetal endomesodermal precursors. Initially, descendants of the second and third quartet micromeres (2a–2d, 3a–3d) occupy a portion of the blastopore lip. As the blastopore narrows, the micromeres’ progeny exhibit lineage-specific behaviors that result in certain sublineages leaving the lip’s edge. Anteriorly, cells derived from 3a² and 3b² undergo a unique epithelial-to-mesenchymal transition involving proliferation and a collective movement of cells into the archenteron. These cells make a novel spiralian germ layer, the ectomesoderm. Posteriorly, cells derived from 3c² and 3d² undergo a form of convergence and extension that involves zippering of cells and their intercalation across the ventral midline. During this process, several of these cells, as well as the 2d clone, become displaced posteriorly, away from the blastopore. Progeny of 2a-2c and 3a-3d make the mouth and foregut, and the blastopore becomes the opening to the mouth. The anus forms days later, as a secondary opening within the 2d² clone, and not from the classically described “anal cells”, which we identify as the 3c²²¹ and 3d²²¹ cells.

Conclusions

Our analysis of Crepidula gastrulation constitutes the first description of blastopore lip morphogenesis and fates using lineage tracing and live imaging. These data have profound implications for hypotheses about the evolution of the bilaterian gut and help explain observed variation in blastopore morphogenesis among spiralians.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-015-0019-1) contains supplementary material, which is available to authorized users.

The study of Priapulus caudatus reveals conserved molecular patterning underlying different gut morphogenesis in the Ecdysozoa

Background

The digestive systems of animals can become highly specialized in response to their exploration and occupation of new ecological niches. Although studies on different animals have revealed commonalities in gut formation, the model systems Caenorhabditis elegans and Drosophila melanogaster, which belong to the invertebrate group Ecdysozoa, exhibit remarkable deviations in how their intestines develop. Their morphological and developmental idiosyncrasies have hindered reconstructions of ancestral gut characters for the Ecdysozoa, and limit comparisons with vertebrate models. In this respect, the phylogenetic position, and slow evolving morphological and molecular characters of marine priapulid worms advance them as a key group to decipher evolutionary events that occurred in the lineages leading to C. elegans and D. melanogaster.

Results

In the priapulid Priapulus caudatus, the gut consists of an ectodermal foregut and anus, and a mid region of at least partial endodermal origin. The inner gut develops into a 16-cell primordium devoid of visceral musculature, arranged in three mid tetrads and two posterior duplets. The mouth invaginates ventrally and shifts to a terminal anterior position as the ventral anterior ectoderm differentially proliferates. Contraction of the musculature occurs as the head region retracts into the trunk and resolves the definitive larval body plan. Despite obvious developmental differences with C. elegans and D. melanogaster, the expression in P. caudatus of the gut-related candidate genes NK2.1, foxQ2, FGF8/17/18, GATA456, HNF4, wnt1, and evx demonstrate three distinct evolutionarily conserved molecular profiles that correlate with morphologically identified sub-regions of the gut.

Conclusions

The comparative analysis of priapulid development suggests that a midgut formed by a single endodermal population of vegetal cells, a ventral mouth, and the blastoporal origin of the anus are ancestral features in the Ecdysozoa. Our molecular data on P. caudatus reveal a conserved ecdysozoan gut-patterning program and demonstrates that extreme morphological divergence has not been accompanied by major molecular innovations in transcriptional regulators during digestive system evolution in the Ecdysozoa. Our data help us understand the origins of the ecdysozoan body plan, including those of C. elegans and D. melanogaster, and this is critical for comparisons between these two prominent model systems and their vertebrate counterparts.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-015-0139-z) contains supplementary material, which is available to authorized users.

Genetic and genomic tools for the marine annelid Platynereis dumerilii

The bristle worm Platynereis dumerilii displays many interesting biological characteristics. These include its reproductive timing, which is synchronized to the moon phase, its regenerative capacity that is hormonally controlled, and a slow rate of evolution, which permits analyses of ancestral genes and cell types. As a marine annelid, Platynereis is also representative of the marine ecosystem, as well as one of the three large animal subphyla, the Lophotrochozoa. Here, we provide an overview of the molecular resources, functional techniques, and behavioral assays that have recently been established for the bristle worm. This combination of tools now places Platynereis in an excellent position to advance research at the frontiers of neurobiology, chronobiology, evo-devo, and marine biology.

The bilaterian forebrain: an evolutionary chimaera

The insect, annelid and vertebrate forebrains harbour two major centres of output control, a sensory-neurosecretory centre releasing hormones and a primordial locomotor centre that controls the initiation of muscular body movements. In vertebrates, both reside in the hypothalamus. Here, we review recent comparative neurodevelopmental evidence indicating that these centres evolved from separate condensations of neurons on opposite body sides ('apical nervous system' versus 'blastoporal nervous system') and that their developmental specification involved distinct regulatory networks (apical six3 and rx versus mediolateral nk and pax gene-dependent patterning). In bilaterian ancestors, both systems approached each other and became closely intermingled, physically, functionally and developmentally. Our 'chimeric brain hypothesis' sheds new light on the vast success and rapid diversification of bilaterian animals in the Cambrian and revises our understanding of brain architecture.

Cell and tissue dynamics during Tribolium embryogenesis revealed by versatile fluorescence labeling approaches

Studies on new arthropod models such as the beetle Tribolium castaneum are shifting our knowledge of embryonic patterning and morphogenesis beyond the Drosophila paradigm. In contrast to Drosophila, Tribolium embryos exhibit the short-germ type of development and become enveloped by extensive extra-embryonic membranes, the amnion and serosa. The genetic basis of these processes has been the focus of active research. Here, we complement genetic approaches with live fluorescence imaging of Tribolium embryos to make the link between gene function and morphogenetic cell behaviors during blastoderm formation and differentiation, germband condensation and elongation, and extra-embryonic development. We first show that transient labeling methods result in strong, homogeneous and persistent expression of fluorescent markers in Tribolium embryos, labeling the chromatin, membrane, cytoskeleton or combinations thereof. We then use co-injection of fluorescent markers with dsRNA for live imaging of embryos with disrupted caudal gene function caused by RNA interference. Using these approaches, we describe and compare cell and tissue dynamics in Tribolium embryos with wild-type and altered fate maps. We find that Tribolium germband condensation is effected by cell contraction and intercalation, with the latter being dependent on the anterior-posterior patterning system. We propose that germband condensation drives initiation of amnion folding, whereas expansion of the amniotic fold and closure of the amniotic cavity are likely driven by contraction of an actomyosin cable at the boundary between the amnion and serosa. Our methodology provides a comprehensive framework for testing quantitative models of patterning, growth and morphogenetic mechanisms in Tribolium and other arthropod species.

Mutual repression between Gbx2 and Otx2 in sensory placodes reveals a general mechanism for ectodermal patterning

In the vertebrate head, central and peripheral components of the sensory nervous system have different embryonic origins, the neural plate and sensory placodes. This raises the question of how they develop in register to form functional sense organs and sensory circuits. Here we show that mutual repression between the homeobox transcription factors Gbx2 and Otx2 patterns the placode territory by influencing regional identity and by segregating inner ear and trigeminal progenitors. Activation of Otx2 targets is necessary for anterior olfactory, lens and trigeminal character, while Gbx2 function is required for the formation of the posterior otic placode. Thus, like in the neural plate antagonistic interaction between Otx2 and Gbx2 establishes positional information thus providing a general mechanism for rostro-caudal patterning of the ectoderm. Our findings support the idea that the Otx/Gbx boundary has an ancient evolutionary origin to which different modules were recruited to specify cells of different fates.

The segmental pattern of otx, gbx, and Hox genes in the annelid Platynereis dumerilii

SUMMARY Annelids and arthropods, despite their distinct classification as Lophotrochozoa and Ecdysozoa, present a morphologically similar, segmented body plan. To elucidate the evolution of segmentation and, ultimately, to align segments across remote phyla, we undertook a refined expression analysis to precisely register the expression of conserved regionalization genes with morphological boundaries and segmental units in the marine annelid Platynereis dumerilii. We find that Pdu-otx defines a brain region anterior to the first discernable segmental entity that is delineated by a stripe of engrailed-expressing cells. The first segment is a "cryptic" segment that lacks chaetae and parapodia. This and the subsequent three chaetigerous larval segments harbor the anterior expression boundary of gbx, hox1, hox4, and lox5 genes, respectively. This molecular segmental topography matches the segmental pattern of otx, gbx, and Hox gene expression in arthropods. Our data thus support the view that an ancestral ground pattern of segmental identities existed in the trunk of the last common protostome ancestor that was lost or modified in protostomes lacking overt segmentation.

The normal development of Platynereis dumerilii (Nereididae, Annelida)

BACKGROUND

The polychaete annelid Platynereis dumerilii is an emerging model organism for the study of molecular developmental processes, evolution, neurobiology and marine biology. Annelids belong to the Lophotrochozoa, the so far understudied third major branch of bilaterian animals besides deuterostomes and ecdysozoans. P. dumerilii has proven highly relevant to explore ancient bilaterian conditions via comparison to the deuterostomes, because it has accumulated less evolutionary change than conventional ecdysozoan models. Previous staging was mainly referring to hours post fertilization but did not allow matching stages between studies performed at (even slightly) different temperatures. To overcome this, and to provide a first comprehensive description of P. dumerilii normal development, a temperature-independent staging system is needed.

RESULTS

Platynereis dumerilii normal development is subdivided into 16 stages, starting with the zygote and ending with the death of the mature worms after delivering their gametes. The stages described can be easily identified by conventional light microscopy or even by dissecting scope. Developmental landmarks such as the beginning of phototaxis, the visibility of the stomodeal opening and of the chaetae, the first occurrence of the ciliary bands, the formation of the parapodia, the extension of antennae and cirri, the onset of feeding and other characteristics are used to define different developmental stages. The morphology of all larval stages as well as of juveniles and adults is documented by light microscopy. We also provide an overview of important steps in the development of the nervous system and of the musculature, using fluorescent labeling techniques and confocal laser-scanning microscopy. Timing of each developmental stage refers to hours post fertilization at 18 ± 0.1°C. For comparison, we determined the pace of development of larvae raised at 14°C, 16°C, 20°C, 25°C, 28°C and 30°C. A staging ontology representing the comprehensive list of developmental stages of P. dumerilii is available online.

CONCLUSIONS

Our atlas of Platynereis dumerilii normal development represents an important resource for the growing Platynereis community and can also be applied to other nereidid annelids.

Six3 demarcates the anterior-most developing brain region in bilaterian animals

Background

The heads of annelids (earthworms, polychaetes, and others) and arthropods (insects, myriapods, spiders, and others) and the arthropod-related onychophorans (velvet worms) show similar brain architecture and for this reason have long been considered homologous. However, this view is challenged by the 'new phylogeny' placing arthropods and annelids into distinct superphyla, Ecdysozoa and Lophotrochozoa, together with many other phyla lacking elaborate heads or brains. To compare the organisation of annelid and arthropod heads and brains at the molecular level, we investigated head regionalisation genes in various groups. Regionalisation genes subdivide developing animals into molecular regions and can be used to align head regions between remote animal phyla.

Results

We find that in the marine annelid Platynereis dumerilii, expression of the homeobox gene six3 defines the apical region of the larval body, peripherally overlapping the equatorial otx+ expression. The six3+ and otx+ regions thus define the developing head in anterior-to-posterior sequence. In another annelid, the earthworm Pristina, as well as in the onychophoran Euperipatoides, the centipede Strigamia and the insects Tribolium and Drosophila, a six3/optix+ region likewise demarcates the tip of the developing animal, followed by a more posterior otx/otd+ region. Identification of six3+ head neuroectoderm in Drosophila reveals that this region gives rise to median neurosecretory brain parts, as is also the case in annelids. In insects, onychophorans and Platynereis, the otx+ region instead harbours the eye anlagen, which thus occupy a more posterior position.

Conclusions

These observations indicate that the annelid, onychophoran and arthropod head develops from a conserved anterior-posterior sequence of six3+ and otx+ regions. The six3+ anterior pole of the arthropod head and brain accordingly lies in an anterior-median embryonic region and, in consequence, the optic lobes do not represent the tip of the neuraxis. These results support the hypothesis that the last common ancestor of annelids and arthropods already possessed neurosecretory centres in the most anterior region of the brain. In light of its broad evolutionary conservation in protostomes and, as previously shown, in deuterostomes, the six3-otx head patterning system may be universal to bilaterian animals.

Hedgehog signaling regulates segment formation in the annelid Platynereis

Annelids and arthropods share a similar segmented organization of the body whose evolutionary origin remains unclear. The Hedgehog signaling pathway, prominent in arthropod embryonic segment patterning, has not been shown to have a similar function outside arthropods. We show that the ligand Hedgehog, the receptor Patched, and the transcription factor Gli are all expressed in striped patterns before the morphological appearance of segments in the annelid Platynereis dumerilii. Treatments with small molecules antagonistic to Hedgehog signaling disrupt segment formation. Platynereis Hedgehog is not necessary to establish early segment patterns but is required to maintain them. The molecular similarity of segment patterning functions of the Hedgehog pathway in an annelid and in arthropods supports a common origin of segmentation in protostomes.

Fate map and morphogenesis of presumptive neural crest and dorsal neural tube

In contrast to the classical assumption that neural crest cells are induced in chick as the neural folds elevate, recent data suggest that they are already specified during gastrulation. This prompted us to map the origin of the neural crest and dorsal neural tube in the early avian embryo. Using a combination of focal dye injections and time-lapse imaging, we find that neural crest and dorsal neural tube precursors are present in a broad, crescent-shaped region of the gastrula. Surprisingly, static fate maps together with dynamic confocal imaging reveal that the neural plate border is considerably broader and extends more caudally than expected. Interestingly, we find that the position of the presumptive neural crest broadly correlates with the BMP4 expression domain from gastrula to neurula stages. Some degree of rostrocaudal patterning, albeit incomplete, is already evident in the gastrula. Time-lapse imaging studies show that the neural crest and dorsal neural tube precursors undergo choreographed movements that follow a spatiotemporal progression and include convergence and extension, reorientation, cell intermixing, and motility deep within the embryo. Through these rearrangement and reorganization movements, the neural crest and dorsal neural tube precursors become regionally segregated, coming to occupy predictable rostrocaudal positions along the embryonic axis. This regionalization occurs progressively and appears to be complete in the neurula by stage 7 at levels rostral to Hensen's node.

Wnt/beta-catenin and noncanonical Wnt signaling interact in tissue evagination in the simple eumetazoan Hydra

In and evaginations of 2D cell sheets are major shape generating processes in animal development. They result from directed movement and intercalation of polarized cells associated with cell shape changes. Work on several bilaterian model organisms has emphasized the role of noncanonical Wnt signaling in cell polarization and movement. However, the molecular processes responsible for generating tissue and body shape in ancestral, prebilaterian animals are unknown. We show that noncanonical Wnt signaling acts in mass tissue movements during bud and tentacle evagination and regeneration in the cnidarian polyp Hydra. The wnt5, wnt8, frizzled2 (fz2), and dishevelled-expressing cell clusters define the positions, where bud and tentacle evaginations are initiated; wnt8, fz2, and dishevelled remain up-regulated in those epithelial cells, undergoing cell shape changes during the entire evagination process. Downstream of wnt and dsh expression, JNK activity is required for the evagination process. Multiple ectopic wnt5, wnt8, fz2, and dishevelled-expressing centers and the subsequent evagination of ectopic tentacles are induced throughout the body column by activation of Wnt/beta-Catenin signaling. Our results indicate that integration of axial patterning and tissue morphogenesis by the coordinated action of canonical and noncanonical Wnt pathways was crucial for the evolution of eumetazoan body plans.

Large-scale clonal analysis reveals unexpected complexity in surface ectoderm morphogenesis

BACKGROUND

Understanding the series of morphogenetic processes that underlie the making of embryo structures is a highly topical issue in developmental biology, essential for interpreting the massive molecular data currently available. In mouse embryo, long-term in vivo analysis of cell behaviours and movements is difficult because of the development in utero and the impossibility of long-term culture.

METHODOLOGY/PRINCIPAL FINDINGS

We improved and combined two genetic methods of clonal analysis that together make practicable large-scale production of labelled clones. Using these methods we performed a clonal analysis of surface ectoderm (SE), a poorly understood structure, for a period that includes gastrulation and the establishment of the body plan. We show that SE formation starts with the definition at early gastrulation of a pool of founder cells that is already dorso-ventrally organized. This pool is then regionalized antero-posteriorly into three pools giving rise to head, trunk and tail. Each pool uses its own combination of cell rearrangements and mode of proliferation for elongation, despite a common clonal strategy that consists in disposing along the antero-posterior axis precursors of dorso-ventrally-oriented stripes of cells.

CONCLUSIONS/SIGNIFICANCE

We propose that these series of morphogenetic processes are organized temporally and spatially in a posterior zone of the embryo crucial for elongation. The variety of cell behaviours used by SE precursor cells indicates that these precursors are not equivalent, regardless of a common clonal origin and a common clonal strategy. Another major result is the finding that there are founder cells that contribute only to the head and tail. This surprising observation together with others can be integrated with ideas about the origin of axial tissues in bilaterians.

Convergent extension by intercalation without mediolaterally fixed cell motion

We construct and implement a stochastic model of convergent extension, using a minimal set of assumptions on cell behavior. In addition to the basic assumptions of volume conservation, random cell motion, and cell-cell and cell-ECM adhesion, and a non-standard assumption that cytoskeletal polymerization generates an internal pressure tending to keep cells convex, we find that we need only two conditions for convergent extension. (1) Each cell type has a particular aspect ratio towards which it regulates its geometry. We do not require that cells align in a specific orientation, e.g. to be oriented mediolaterally. (2) The elongating tissue is composed of cells that prefer to be elongated, and these cells must be accompanied by cells which prefer to be round. The latter effectively provide a boundary to capture. In simulations, our model tissue extends and converges to a stacked arrangement of elongated cells one cell wide, an arrangement which is seen in ascidian notochords, but which has not been observed in other models. This arrangement is achieved without any direct mediolateral bias other than that which is provided by the physical edge of the adjacent tissue.

Acoel development supports a simple planula-like urbilaterian

Molecular approaches to the study of development and evolution have had profound effects on our understanding of the nature of the evolutionary process. Developmental biologists became intoxicated with fanciful notions of reconstructing genetic pathways of morphogenesis while evolutionary biologists were sobered by the fallacy of reconstructing organismal relationships along increasing grades of morphological complexity. Increased taxon sampling and improvements in analytical techniques are providing a new approach and are forcing biologists to move past historical biases to allow more accurate mapping of morphological and developmental characters through evolutionary time. Here, we discuss the possible developmental and morphological features of the 'urbilaterian', the triploblastic animal with anterior-posterior and dorsoventral axes and predecessor of the protostome-deuterostome ancestor. We argue that this animal, with features resembling acoelomorph flatworms, was far simpler morphologically than the protostome-deuterostome ancestor despite possessing a nearly complete eubilaterian genome. We show that the deployment of some genes expected to pattern the protostome-deuterostome ancestor is not deployed in acoels in the predicted manner and thus might have been co-opted after the evolution of the urbilaterian. We also identify the developmental changes related to gastrulation that gave rise to the urbilaterian from a simpler cnidarian-like ancestor.

A procephalic territory in Drosophila exhibiting similarities and dissimilarities compared to the vertebrate midbrain/hindbrain boundary region

Background

In vertebrates, the primordium of the brain is subdivided by the expression of Otx genes (forebrain/anterior midbrain), Hox genes (posterior hindbrain), and the genes Pax2, Pax5 and Pax8 (intervening region). The latter includes the midbrain/hindbrain boundary (MHB), which acts as a key organizer during brain patterning. Recent studies in Drosophila revealed that orthologous sets of genes are expressed in a similar tripartite pattern in the late embryonic brain, which suggested correspondence between the Drosophila deutocerebral/tritocerebral boundary region and the vertebrate MHB. To gain more insight into the evolution of brain regions, and particularly the MHB, I examined the expression of a comprehensive array of MHB-specific gene orthologs in the procephalic neuroectoderm and in individually identified neuroblasts during early embryonic stages 8–11, at which the segmental organization of the brain is most clearly displayed.

Results and conclusion

I show that the early embryonic brain exhibits an anterior Otx/otd domain and a posterior Hox1/lab domain, but that Pax2/5/8 orthologs are not expressed in the neuroectoderm and neuroblasts of the intervening territory. Furthermore, the expression domains of Otx/otd and Gbx/unpg exhibit a small common interface within the anterior deutocerebrum. In contrast to vertebrates, Fgf8-related genes are not expressed posterior to the otd/unpg interface. However, at the otd/unpg interface the early expression of other MHB-specific genes (including btd, wg, en), and of dorsoventral patterning genes, closely resembles the situation at the vertebrate MHB. Altogether, these results suggest the existence of an ancestral territory within the primordium of the deutocerebrum and adjacent protocerebrum, which might be the evolutionary equivalent of the region of the vertebrate MHB. However, lack of expression of Pax2/5/8 and Fgf8-related genes, and significant differences in the expression onset of other key regulators at the otd/unpg interface, imply that genetic interactions crucial for the vertebrate organizer activity are absent in the early embryonic brain of Drosophila.