Beta-catenin is required for the establishment of vegetal embryonic fates in the nemertean, Cerebratulus lacteus

Emerging trends in the study of spiralian larvae

Many animals undergo indirect development, where their embryogenesis produces an intermediate life stage, or larva, that is often free-living and later metamorphoses into an adult. As their adult counterparts, larvae can have unique and diverse morphologies and occupy various ecological niches. Given their broad phylogenetic distribution, larvae have been central to hypotheses about animal evolution. However, the evolution of these intermediate forms and the developmental mechanisms diversifying animal life cycles are still debated. This review focuses on Spiralia, a large and diverse clade of bilaterally symmetrical animals with a fascinating array of larval forms, most notably the archetypical trochophore larva. We explore how classic research and modern advances have improved our understanding of spiralian larvae, their development, and evolution. Specifically, we examine three morphological features of spiralian larvae: the anterior neural system, the ciliary bands, and the posterior hyposphere. The combination of molecular and developmental evidence with modern high-throughput techniques, such as comparative genomics, single-cell transcriptomics, and epigenomics, is a promising strategy that will lead to new testable hypotheses about the mechanisms behind the evolution of larvae and life cycles in Spiralia and animals in general. We predict that the increasing number of available genomes for Spiralia and the optimization of genome-wide and single-cell approaches will unlock the study of many emerging spiralian taxa, transforming our views of the evolution of this animal group and their larvae.

Role of maternal spiralian-specific homeobox gene SPILE-E in the specification of blastomeres along the animal-vegetal axis during the early cleavage stages of mollusks

Spiralians, one of the major clades of bilaterians, share a unique development known as spiralian development, characterized by the formation of tiers of cells called quartets, which exhibit different developmental potentials along the animal-vegetal axis. Recently, spiralian-specific TALE-type homeobox genes (SPILE) have been identified, some of which show zygotic and staggered expression patterns along the animal-vegetal axis and function in quartet specification in mollusks. However, it is unclear which maternal molecular components control the zygotic expression of these transcription factors. In this study, we focused on SPILE-E, a maternal transcription factor, and investigated its expression and function in mollusks. We found that the maternal and ubiquitous expression of SPILE-E in the cleavage stages is conserved in molluskan species, including limpets, mussels, and chitons. We knocked down SPILE-E in limpets and revealed that the expression of transcription factors specifically expressed in the first quartet (1q2 ; foxj1b) and second quartet (2q; SPILE-B) was abolished, whereas the macromere-quartet marker (SPILE-C) was ectopically expressed in 1q2 in SPILE-E morphants. Moreover, we showed that the expression of SPILE-A, which upregulates SPILE-B but represses SPILE-C expression, decreased in SPILE-E morphants. Consistent with changes in the expression pattern of the above transcription factors, SPILE-E-morphant larvae exhibited patchy or complete loss of expression of marker genes of ciliated cells and shell fields, possibly reflecting incomplete specification of 1q2 and 2q. Our results provide a molecular framework for quartet specification and highlight the importance of maternal lineage-specific transcription factors in the development and evolution of spiralians.

Evolution and loss of ß-catenin and TCF-dependent axis specification in insects

Mechanisms and evolution of primary axis specification in insects are discussed in the context of the roles of ß-catenin and TCF in polarizing metazoan embryos. Three hypotheses are presented. First, insects with sequential segmentation and posterior growth use cell-autonomous mechanisms for establishing embryo polarity via the nuclear ratio of ß-catenin and TCF. Second, TCF homologs establish competence for anterior specification. Third, the evolution of simultaneous segmentation mechanisms, also known as long-germ development, resulted in primary axis specification mechanisms that are independent of ß-catenin but reliant on TCF, a condition that preceded the frequent replacement of anterior determinants in long germ insects.

Distinct Frizzled receptors independently mediate endomesoderm specification and primary archenteron invagination during gastrulation in Nematostella

Endomesodermal cell fate specification and archenteron formation during gastrulation are tightly linked developmental processes in most metazoans. However, studies have shown that in the anthozoan cnidarian Nematostella vectensis, Wnt/β-catenin (cWnt) signalling-mediated endomesodermal cell fate specification can be experimentally uncoupled from Wnt/Planar Cell Polarity (PCP) signalling-mediated primary archenteron invagination. The upstream signalling mechanisms regulating cWnt signalling-dependent endomesoderm cell fate specification and Wnt/PCP signalling-mediated primary archenteron invagination in Nematostella embryos are not well understood. By screening for potential upstream mediators of cWnt and Wnt/PCP signalling, we identified two Nematostella Frizzled homologs that are expressed early in development. NvFzd1 is expressed maternally and in a broad pattern during early development while NvFzd10 is zygotically expressed at the animal pole in blastula stage embryos and is restricted to the invaginating cells of the presumptive endomesoderm. Molecular and morphological characterization of NvFzd1 and NvFzd10 knock-down phenotypes provide evidence for distinct regulatory roles for the two receptors in endomesoderm cell fate specification and primary archenteron invagination. These results provide further experimental evidence for the independent regulation of endomesodermal cell fate specification and primary archenteron invagination during gastrulation in Nematostella. Moreover, these results provide additional support for the previously proposed two-step model for the independent evolution of cWnt-mediated cell fate specification and Wnt/PCP-mediated primary archenteron invagination.

Cnidarian-bilaterian comparison reveals the ancestral regulatory logic of the β-catenin dependent axial patterning

In animals, body axis patterning is based on the concentration-dependent interpretation of graded morphogen signals, which enables correct positioning of the anatomical structures. The most ancient axis patterning system acting across animal phyla relies on β-catenin signaling, which directs gastrulation, and patterns the main body axis. However, within Bilateria, the patterning logic varies significantly between protostomes and deuterostomes. To deduce the ancestral principles of β-catenin-dependent axial patterning, we investigate the oral–aboral axis patterning in the sea anemone Nematostella—a member of the bilaterian sister group Cnidaria. Here we elucidate the regulatory logic by which more orally expressed β-catenin targets repress more aborally expressed β-catenin targets, and progressively restrict the initially global, maternally provided aboral identity. Similar regulatory logic of β-catenin-dependent patterning in Nematostella and deuterostomes suggests a common evolutionary origin of these processes and the equivalence of the cnidarian oral–aboral and the bilaterian posterior–anterior body axes.

The authors show in Nematostella that the more orally expressed β-catenin targets repress the more aborally expressed β-catenin targets, thus patterning the oral-aboral axis. This likely represents the common mechanism of β-catenin-dependent axial patterning shared by Cnidaria and Bilateria.

Molecular insights into deuterostome evolution from hemichordate developmental biology

Hemichordates, along with echinoderms and chordates, belong to the lineage of bilaterians called the deuterostomes. Their phylogenetic position as an outgroup to chordates provides an opportunity to investigate the evolutionary origins of the chordate body plan and reconstruct ancestral deuterostome characters. The body plans of the hemichordates and chordates are organizationally divergent making anatomical comparisons very challenging. The developmental underpinnings of animal body plans are often more conservative than the body plans they regulate, and offer a novel data set for making comparisons between morphologically divergent body architectures. Here I review the hemichordate developmental data generated over the past 20 years that further test hypotheses of proposed morphological affinities between the two taxa, but also compare the conserved anteroposterior, dorsoventral axial patterning programs and germ layer specification programs. These data provide an opportunity to determine which developmental programs are ancestral deuterostome or bilaterian innovations, and which ones occurred in stem chordates or vertebrates representing developmental novelties of the chordate body plan.

Apcdd1 is a dual BMP/Wnt inhibitor in the developing nervous system and skin

Animal development and homeostasis depend on precise temporal and spatial intercellular signaling. Components shared between signaling pathways, generally thought to decrease specificity, paradoxically can also provide a solution to pathway coordination. Here we show that the Bone Morphogenetic Protein (BMP) and Wnt signaling pathways share Apcdd1 as a common inhibitor and that Apcdd1 is a taxon-restricted gene with novel domains and signaling functions. Previously, we showed that Apcdd1 inhibits Wnt signaling (Shimomura et al., 2010), here we find that Apcdd1 potently inhibits BMP signaling in body axis formation and neural differentiation in chicken, frog, zebrafish. Furthermore, we find that Apcdd1 has an evolutionarily novel protein domain. Our results from experiments and modeling suggest that Apcdd1 may coordinate the outputs of two signaling pathways that are central to animal development and human disease.

Wnt/β-catenin signaling is an evolutionarily conserved determinant of chordate dorsal organizer

Deciphering the mechanisms of axis formation in amphioxus is a key step to understanding the evolution of chordate body plan. The current view is that Nodal signaling is the only factor promoting the dorsal axis specification in the amphioxus, whereas Wnt/β-catenin signaling plays no role in this process. Here, we re-examined the role of Wnt/βcatenin signaling in the dorsal/ventral patterning of amphioxus embryo. We demonstrated that the spatial activity of Wnt/β-catenin signaling is located in presumptive dorsal cells from cleavage to gastrula stage, and provided functional evidence that Wnt/β-catenin signaling is necessary for the specification of dorsal cell fate in a stage-dependent manner. Microinjection of Wnt8 and Wnt11 mRNA induced ectopic dorsal axis in neurulae and larvae. Finally, we demonstrated that Nodal and Wnt/β-catenin signaling cooperate to promote the dorsal-specific gene expression in amphioxus gastrula. Our study reveals high evolutionary conservation of dorsal organizer formation in the chordate lineage.

Unravelling spiral cleavage

Snails, earthworms and flatworms are remarkably different animals, but they all exhibit a very similar mode of early embryogenesis: spiral cleavage. This is one of the most widespread developmental programs in animals, probably ancestral to almost half of the animal phyla, and therefore its study is essential for understanding animal development and evolution. However, our knowledge of spiral cleavage is still in its infancy. Recent technical and conceptual advances, such as the establishment of genome editing and improved phylogenetic resolution, are paving the way for a fresher and deeper look into this fascinating early cleavage mode.

Antero-posterior ectoderm patterning by canonical Wnt signaling during ascidian development

Wnt/β-catenin signaling is an ancient pathway in metazoans and controls various developmental processes, in particular the establishment and patterning of the embryonic primary axis. In vertebrates, a graded Wnt activity from posterior to anterior endows cells with positional information in the central nervous system. Recent studies in hemichordates support a conserved role for Wnt/β-catenin in ectoderm antero-posterior patterning at the base of the deuterostomes. Ascidians are marine invertebrates and the closest relatives of vertebrates. By combining gain- and loss-of-function approaches, we have determined the role of Wnt/β-catenin in patterning the three ectoderm derivatives of the ascidian Ciona intestinalis, central nervous system, peripheral nervous system and epidermis. Activating Wnt/β-catenin signaling from gastrulation led to a dramatic transformation of the ectoderm with a loss of anterior identities and a reciprocal anterior extension of posterior identities, consistent with studies in other metazoans. Surprisingly, inhibiting Wnt signaling did not produce a reciprocal anteriorization of the embryo with a loss of more posterior identities like in vertebrates and hemichordate. Epidermis patterning was overall unchanged. Only the identity of two discrete regions of the central nervous system, the anteriormost and the posteriormost regions, were under the control of Wnt. Finally, the caudal peripheral nervous system, while being initially Wnt dependent, formed normally. Our results show that the Ciona embryonic ectoderm responds to Wnt activation in a manner that is compatible with the proposed function for this pathway at the base of the deuterostomes. However, possibly because of its fast and divergent mode of development that includes extensive use of maternal determinants, the overall antero-posterior patterning of the Ciona ectoderm is Wnt independent, and Wnt/β-catenin signaling controls the formation of some sub-domains. Our results thus indicate that there has likely been a drift in the developmental systems controlling ectoderm patterning in the lineage leading to ascidians.

The Wnt/β-catenin pathway is a system of cell-cell communication. It has an ancient origin in animals and plays multiple roles during embryogenesis and adult life. In particular, it is involved in determining, in the vertebrate embryo, the identity of the different parts of the body and their relative positions along the antero-posterior axis. We have investigated in an ascidian (or sea squirt) species, a marine invertebrate that is closely related to vertebrates, whether this pathway had a similar role. Like in vertebrates, activating Wnt/β-catenin led to a posteriorization of the embryo with a loss of anterior structures. By contrast, unlike vertebrates, ascidian embryos formed rather normally following Wnt/β-catenin inactivation. Since hemichordates (or acorn worms), earlier divergent invertebrates, use Wnt/β-catenin in a manner comparable to vertebrates, it is in the ascidian lineage that changes have occurred. Consequently, ascidians build an antero-posterior axis, very similarly organized to that of vertebrates, but in a different way.

GSK3β controls the timing and pattern of the fifth spiral cleavage at the 2-4 cell stage in Lymnaea stagnalis

Establishment of the body plan of multicellular organisms by the primary body axis determination and cell-fate specification is a key issue in biology. We have examined the mRNA localization of three Wnt pathway components gsk3β, β-catenin, and disheveled and investigated the effects of four selective inhibitors of these proteins on the early developmental stages of the spiral cleavage embryo of the fresh water snail Lymnaea (L.) stagnalis. mRNAs for gsk3β and β-catenin were distributed uniformly throughout the embryo during development whereas disheveled mRNA showed specific localization with intra- and inter-blastomere differences in concentration along the A-V axis during spiral cleavages. Remarkably, through inhibitor studies, we identified a short sensitive period from the 2- to 4-cell stage in which GSK3β inhibition by the highly specific 1-azakenpaullone (AZ) and by LiCl induced a subsequent dramatic developmental delay and alteration of the cleavage patterns of blastomeres at the fifth cleavage (16- to 24-cell stage) resulting in exogastrulation and other abnormalities in later stages. Inhibition of β-Catenin or Disheveled had no effect. Our inhibitor experiments establish a novel role for GSK3β in the developmental timing and orientated cell division of the snail embryo. Further work will be needed to identify the downstream targets of the kinase.

Canonical Wnt/β-catenin and Notch signaling regulate animal/vegetal axial patterning in the cephalochordate amphioxus

In bilaterians, animal/vegetal axial (A/V) patterning is a fundamental early developmental event for establishment of animal/vegetal polarity and following specification of the germ layers (ectoderm, mesoderm, endoderm), of which the evolutionary origin is enigmatic. Understanding A/V axial patterning in a basal animal from each phylum would help to reconstruct the ancestral state of germ layer specification in bilaterians and thus, the evolution of mesoderm, the third intermediate cell layer. Herein, data show that the canonical Wnt/β-catenin (cWnt) and Notch signaling pathways control mesoderm specification from the early endomesoderm in the basal chordate amphioxus. Amphioxus belongs to the deuterostome, one of the main superphyla in Bilateria. In the present study, genes (tcf, dsh, axin, gsk3β) encoding cWnt components were expressed in the endomesoderm during the gastrula stages. Excess cWnt signaling by BIO, a GSK3 inhibitor, expanded the expression domains of outer endomesodermal genes that include future mesodermal ones and suppressed inner endomesodermal and ectodermal genes. Interfering Notch signaling by DAPT, a γ-secretase inhibitor, resulted in decreased expression of ectodermal and endomesodermal markers. These results suggest that cWnt and Notch have important roles in mesoderm specification in amphioxus embryos. The evolution of the mesoderm is also discussed.

Wnt evolution and function shuffling in liberal and conservative chordate genomes

BACKGROUND

What impact gene loss has on the evolution of developmental processes, and how function shuffling has affected retained genes driving essential biological processes, remain open questions in the fields of genome evolution and EvoDevo. To investigate these problems, we have analyzed the evolution of the Wnt ligand repertoire in the chordate phylum as a case study.

RESULTS

We conduct an exhaustive survey of Wnt genes in genomic databases, identifying 156 Wnt genes in 13 non-vertebrate chordates. This represents the most complete Wnt gene catalog of the chordate subphyla and has allowed us to resolve previous ambiguities about the orthology of many Wnt genes, including the identification of WntA for the first time in chordates. Moreover, we create the first complete expression atlas for the Wnt family during amphioxus development, providing a useful resource to investigate the evolution of Wnt expression throughout the radiation of chordates.

CONCLUSIONS

Our data underscore extraordinary genomic stasis in cephalochordates, which contrasts with the liberal and dynamic evolutionary patterns of gene loss and duplication in urochordate genomes. Our analysis has allowed us to infer ancestral Wnt functions shared among all chordates, several cases of function shuffling among Wnt paralogs, as well as unique expression domains for Wnt genes that likely reflect functional innovations in each chordate lineage. Finally, we propose a potential relationship between the evolution of WntA and the evolution of the mouth in chordates.

A novel gene's role in an ancient mechanism: secreted Frizzled-related protein 1 is a critical component in the anterior-posterior Wnt signaling network that governs the establishment of the anterior neuroectoderm in sea urchin embryos

The anterior neuroectoderm (ANE) in many deuterostome embryos (echinoderms, hemichordates, urochordates, cephalochordates, and vertebrates) is progressively restricted along the anterior-posterior axis to a domain around the anterior pole. In the sea urchin embryo, three integrated Wnt signaling branches (Wnt/β-catenin, Wnt/JNK, and Wnt/PKC) govern this progressive restriction process, which begins around the 32- to 60-cell stage and terminates by the early gastrula stage. We previously have established that several secreted Wnt modulators of the Dickkopf and secreted Frizzled-related protein families (Dkk1, Dkk3, and sFRP-1/5) are expressed within the ANE and play important roles in modulating the Wnt signaling network during this process. In this study, we use morpholino and dominant-negative interference approaches to characterize the function of a novel Frizzled-related protein, secreted Frizzled-related protein 1 (sFRP-1), during ANE restriction. sFRP-1 appears to be related to a secreted Wnt modulator, sFRP3/4, that is essential to block Wnt signaling and establish the ANE in vertebrates. Here, we show that the sea urchin sFRP3/4 orthologue is not expressed during ANE restriction in the sea urchin embryo. Instead, our results indicate that ubiquitously expressed maternal sFRP-1 and Fzl1/2/7 signaling act together as early as the 32- to 60-cell stage to antagonize the ANE restriction mechanism mediated by Wnt/β-catenin and Wnt/JNK signaling. Then, starting from the blastula stage, Fzl5/8 signaling activates zygotic sFRP-1 within the ANE territory, where it works with the secreted Wnt antagonist Dkk1 (also activated by Fzl5/8 signaling) to antagonize Wnt1/Wnt8-Fzl5/8-JNK signaling in a negative feedback mechanism that defines the outer ANE territory boundary. Together, these data indicate that maternal and zygotic sFRP-1 protects the ANE territory by antagonizing the Wnt1/Wnt8-Fzl5/8-JNK signaling pathway throughout ANE restriction, providing precise spatiotemporal control of the mechanism responsible for the establishment of the ANE territory around the anterior pole of the sea urchin embryo.

Anteroposterior axis patterning by early canonical Wnt signaling during hemichordate development

The Wnt family of secreted proteins has been proposed to play a conserved role in early specification of the bilaterian anteroposterior (A/P) axis. This hypothesis is based predominantly on data from vertebrate embryogenesis as well as planarian regeneration and homeostasis, indicating that canonical Wnt (cWnt) signaling endows cells with positional information along the A/P axis. Outside of these phyla, there is strong support for a conserved role of cWnt signaling in the repression of anterior fates, but little comparative support for a conserved role in promotion of posterior fates. We further test the hypothesis by investigating the role of cWnt signaling during early patterning along the A/P axis of the hemichordate Saccoglossus kowalevskii. We have cloned and investigated the expression of the complete Wnt ligand and Frizzled receptor complement of S. kowalevskii during early development along with many secreted Wnt modifiers. Eleven of the 13 Wnt ligands are ectodermally expressed in overlapping domains, predominantly in the posterior, and Wnt antagonists are localized predominantly to the anterior ectoderm in a pattern reminiscent of their distribution in vertebrate embryos. Overexpression and knockdown experiments, in combination with embryological manipulations, establish the importance of cWnt signaling for repression of anterior fates and activation of mid-axial ectodermal fates during the early development of S. kowalevskii. However, surprisingly, terminal posterior fates, defined by posterior Hox genes, are unresponsive to manipulation of cWnt levels during the early establishment of the A/P axis at late blastula and early gastrula. We establish experimental support for a conserved role of Wnt signaling in the early specification of the A/P axis during deuterostome body plan diversification, and further build support for an ancestral role of this pathway in early evolution of the bilaterian A/P axis. We find strong support for a role of cWnt in suppression of anterior fates and promotion of mid-axial fates, but we find no evidence that cWnt signaling plays a role in the early specification of the most posterior axial fates in S. kowalevskii. This posterior autonomy may be a conserved feature of early deuterostome axis specification.

Expansion of TALE homeobox genes and the evolution of spiralian development

Spiralians, including molluscs, annelids and platyhelminths, share a unique development process that includes the typical geometry of early cleavage and early segregation of cell fate in blastomeres along the animal-vegetal axis. However, the molecular mechanisms underlying this early cell fate segregation are largely unknown. Here, we report spiralian-specific expansion of the three-amino-acid loop extension (TALE) class of homeobox genes. During early development, some of these TALE genes are expressed in staggered domains along the animal-vegetal axis in the limpet Nipponacmea fuscoviridis and the polychaete Spirobranchus kraussii. Inhibition or overexpression of these genes alters the developmental fate of blastomeres, as predicted by the gene expression patterns. These results suggest that the expansion of novel TALE genes plays a critical role in the establishment of a novel cell fate segregation mechanism in spiralians.

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.

Morphogenesis along the animal-vegetal axis: fates of primary quartet micromere daughters in the gastropod Crepidula fornicata

Background

The Spiralia are a large, morphologically diverse group of protostomes (e.g. molluscs, annelids, nemerteans) that share a homologous mode of early development called spiral cleavage. One of the most highly-conserved features of spiralian development is the contribution of the primary quartet cells, 1a-1d, to the anterior region of the embryo (including the brain, eyes, and the anterior ciliary band, called the prototroch). Yet, very few studies have analyzed the ultimate fates of primary quartet sub-lineages, or examined the morphogenetic events that take place in the anterior region of the embryo.

Results

This study focuses on the caenogastropod slipper snail, Crepidula fornicata, a model for molluscan developmental biology. Through direct lineage tracing of primary quartet daughter cells, and examination of these cells during gastrulation and organogenesis stages, we uncovered behaviors never described before in a spiralian. For the first time, we show that the 1a²-1d² cells do not contribute to the prototroch (as they do in other species) and are ultimately lost before hatching. During gastrulation and anterior-posterior axial elongation stages, these cells cleavage-arrest and spread dramatically, contributing to a thin provisional epidermis on the dorsal side of the embryo. This spreading is coupled with the displacement of the animal pole, and other pretrochal cells, closer to the ventrally-positioned mouth, and the vegetal pole.

Conclusions

This is the first study to document the behavior and fate of primary quartet sub-lineages among molluscs. We speculate that the function of 1a²-1d² cells (in addition to two cells derived from 1d¹², and the 2b lineage) is to serve as a provisional epithelium that allows for anterior displacement of the other progeny of the primary quartet towards the anterior-ventral side of the embryo. These data support a new and novel mechanism for axial bending, distinct from canonical models in which axial bending is suggested to be driven primarily by differential proliferation of posterior dorsal cells. These data suggest also that examining sub-lineages in other spiralians will reveal greater variation than previously assumed.

Antagonistic BMP-cWNT signaling in the cnidarian Nematostella vectensis reveals insight into the evolution of mesoderm

Gastrulation was arguably the key evolutionary innovation that enabled metazoan diversification, leading to the formation of distinct germ layers and specialized tissues. Differential gene expression specifying cell fate is governed by the inputs of intracellular and/or extracellular signals. Beta-catenin/Tcf and the TGF-beta bone morphogenetic protein (BMP) provide critical molecular signaling inputs during germ layer specification in bilaterian metazoans, but there has been no direct experimental evidence for a specific role for BMP signaling during endomesoderm specification in the early branching metazoan Nematostella vectensis (an anthozoan cnidarian). Using forward transcriptomics, we show that beta-catenin/Tcf signaling and BMP2/4 signaling provide differential inputs into the cnidarian endomesodermal gene regulatory network (GRN) at the onset of gastrulation (24 h postfertilization) in N. vectensis Surprisingly, beta-catenin/Tcf signaling and BMP2/4 signaling regulate a subset of common downstream target genes in the GRN in opposite ways, leading to the spatial and temporal differentiation of fields of cells in the developing embryo. Thus, we show that regulatory interactions between beta-catenin/Tcf signaling and BMP2/4 signaling are required for the specification and determination of different embryonic regions and the patterning of the oral-aboral axis in Nematostella We also show functionally that the conserved "kernel" of the bilaterian heart mesoderm GRN is operational in N. vectensis, which reinforces the hypothesis that the endoderm and mesoderm in triploblastic bilaterians evolved from the bifunctional endomesoderm (gastrodermis) of a diploblastic ancestor, and that slow rhythmic contractions might have been one of the earliest functions of mesodermal tissue.

Expression analysis of eight amphioxus genes involved in the Wnt/β-catenin signaling pathway

The Wnt/β-catenin signaling pathway plays a crucial role in the embryonic development of metazoans. Although the pathway has been studied extensively in many model animals, its function in amphioxus, the most primitive chordate, remains largely uncharacterized. To obtain basic data for functional analysis, we identified and isolated seven genes (Lrp5/6, Dvl, APC, CkIα, CkIδ, Gsk3β, and Gro) of the Wnt/β-catenin signaling pathway from the amphioxus (Branchiostoma floridae) genome. Phylogenetic analysis revealed that amphioxus had fewer members of each gene family than that found in vertebrates. Whole-mount in situ hybridization showed that the genes were maternally expressed and broadly distributed throughout the whole embryo at the cleavage and blastula stages. Among them, Dvl was expressed asymmetrically towards the animal pole, while the others were evenly distributed in all blastomeres. At the mid-gastrula stage, the genes were specifically expressed in the primitive endomesoderm, but displayed different patterns. When the embryo developed into the neurula stage, the gene expressions were mainly detected in either paraxial somites or the tail bud. With the development of the embryo, the expression levels further decreased gradually and remained only in some pharyngeal regions or the tail bud at the larva stage. Our results suggest that the Wnt/β-catenin pathway might be involved in amphioxus somite formation and posterior growth, but not in endomesoderm specification.

Localization of planarian β-CATENIN-1 reveals multiple roles during anterior-posterior regeneration and organogenesis

The β-catenin-dependent Wnt pathway exerts multiple context-dependent roles in embryonic and adult tissues. In planarians, β-catenin-1 is thought to specify posterior identities through the generation of an anteroposterior gradient. However, the existence of such a gradient has not been directly demonstrated. Here, we use a specific polyclonal antibody to demonstrate that nuclear β-CATENIN-1 exists as an anteroposterior gradient from the pre-pharyngeal region to the tail of the planarian Schmidtea polychroa High levels in the posterior region steadily decrease towards the pre-pharyngeal region but then increase again in the head region. During regeneration, β-CATENIN-1 is nuclearized in both anterior and posterior blastemas, but the canonical WNT1 ligand only influences posterior nuclearization. Additionally, β-catenin-1 is required for proper anterior morphogenesis, consistent with the high levels of nuclear β-CATENIN-1 observed in this region. We further demonstrate that β-CATENIN-1 is abundant in developing and differentiated organs, and is particularly required for the specification of the germline. Altogether, our findings provide the first direct evidence of an anteroposterior nuclear β-CATENIN-1 gradient in adult planarians and uncover novel, context-dependent roles for β-catenin-1 during anterior regeneration and organogenesis.

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.

Co-expression of Foxa.a, Foxd and Fgf9/16/20 defines a transient mesendoderm regulatory state in ascidian embryos

In many bilaterian embryos, nuclear β-catenin (nβ-catenin) promotes mesendoderm over ectoderm lineages. Although this is likely to represent an evolutionary ancient developmental process, the regulatory architecture of nβ-catenin-induced mesendoderm remains elusive in the majority of animals. Here, we show that, in ascidian embryos, three nβ-catenin transcriptional targets, Foxa.a, Foxd and Fgf9/16/20, are each required for the correct initiation of both the mesoderm and endoderm gene regulatory networks. Conversely, these three factors are sufficient, in combination, to produce a mesendoderm ground state that can be further programmed into mesoderm or endoderm lineages. Importantly, we show that the combinatorial activity of these three factors is sufficient to reprogramme developing ectoderm cells to mesendoderm. We conclude that in ascidian embryos, the transient mesendoderm regulatory state is defined by co-expression of Foxa.a, Foxd and Fgf9/16/20.

DOI:http://dx.doi.org/10.7554/eLife.14692.001

Pre-bilaterian origin of the blastoporal axial organizer

The startling capacity of the amphibian Spemann organizer to induce naïve cells to form a Siamese twin embryo with a second set of body axes is one of the hallmarks of developmental biology. However, the axis-inducing potential of the blastopore-associated tissue is commonly regarded as a chordate feature. Here we show that the blastopore lip of a non-bilaterian metazoan, the anthozoan cnidarian Nematostella vectensis, possesses the same capacity and uses the same molecular mechanism for inducing extra axes as chordates: Wnt/β-catenin signaling. We also demonstrate that the establishment of the secondary, directive axis in Nematostella by BMP signaling is sensitive to an initial Wnt signal, but once established the directive axis becomes Wnt-independent. By combining molecular analysis with experimental embryology, we provide evidence that the emergence of the Wnt/β-catenin driven blastopore-associated axial organizer predated the cnidarian-bilaterian split over 600 million years ago.

Development of the aboral domain in Nematostella requires β-catenin and the opposing activities of Six3/6 and Frizzled5/8

The development of the oral pole in cnidarians and the posterior pole in bilaterians is regulated by canonical Wnt signaling, whereas a set of transcription factors, including Six3/6 and FoxQ2, controls aboral development in cnidarians and anterior identity in bilaterians. However, it is poorly understood how these two patterning systems are initially set up in order to generate correct patterning along the primary body axis. Investigating the early steps of aboral pole formation in the sea anemone Nematostella vectensis, we found that, at blastula stage, oral genes are expressed before aboral genes and that Nvβ-catenin regulates both oral and aboral development. In the oral hemisphere, Nvβ-catenin specifies all subdomains except the oral-most, NvSnailA-expressing domain, which is expanded upon Nvβ-catenin knockdown. In addition, Nvβ-catenin establishes the aboral patterning system by promoting the expression of NvSix3/6 at the aboral pole and suppressing the Wnt receptor NvFrizzled5/8 at the oral pole. NvFrizzled5/8 expression thereby gets restricted to the aboral domain. At gastrula stage, NvSix3/6 and NvFrizzled5/8 are both expressed in the aboral domain, but they have opposing activities, with NvSix3/6 maintaining and NvFrizzled5/8 restricting the size of the aboral domain. At planula stage, NvFrizzled5/8 is required for patterning within the aboral domain and for regulating the size of the apical organ by modulation of a previously characterized FGF feedback loop. Our findings suggest conserved roles for Six3/6 and Frizzled5/8 in aboral/anterior development and reveal key functions for Nvβ-catenin in the patterning of the entire oral-aboral axis of Nematostella.

Lens regeneration from the cornea requires suppression of Wnt/β-catenin signaling

The frog, Xenopus laevis, possesses a high capacity to regenerate various larval tissues, including the lens, which is capable of complete regeneration from the cornea epithelium. However, the molecular signaling mechanisms of cornea-lens regeneration are not fully understood. Previous work has implicated the involvement of the Wnt signaling pathway, but molecular studies have been very limited. Iris-derived lens regeneration in the newt (Wolffian lens regeneration) has shown a necessity for active Wnt signaling in order to regenerate a new lens. Here we provide evidence that the Wnt signaling pathway plays a different role in the context of cornea-lens regeneration in Xenopus. We examined the expression of frizzled receptors and wnt ligands in the frog cornea epithelium. Numerous frizzled receptors (fzd1, fzd2, fzd3, fzd4, fzd6, fzd7, fzd8, and fzd10) and wnt ligands (wnt2b.a, wnt3a, wnt4, wnt5a, wnt5b, wnt6, wnt7b, wnt10a, wnt11, and wnt11b) are expressed in the cornea epithelium, demonstrating that this tissue is transcribing many of the ligands and receptors of the Wnt signaling pathway. When compared to flank epithelium, which is lens regeneration incompetent, only wnt11 and wnt11b are different (present only in the cornea epithelium), identifying them as potential regulators of cornea-lens regeneration. To detect changes in canonical Wnt/β-catenin signaling occurring within the cornea epithelium, axin2 expression was measured over the course of regeneration. axin2 is a well-established reporter of active Wnt/β-catenin signaling, and its expression shows a significant decrease at 24 h post-lentectomy. This decrease recovers to normal endogenous levels by 48 h. To test whether this signaling decrease was necessary for lens regeneration to occur, regenerating eyes were treated with either 6-bromoindirubin-3'-oxime (BIO) or 1-azakenpaullone - both activators of Wnt signaling - resulting in a significant reduction in the percentage of cases with successful regeneration. In contrast, inhibition of Wnt signaling using either the small molecule IWR-1, treatment with recombinant human Dickkopf-1 (rhDKK1) protein, or transgenic expression of Xenopus DKK1, did not significantly affect the percentage of successful regeneration. Together, these results suggest a model where Wnt/β-catenin signaling is active in the cornea epithelium and needs to be suppressed during early lens regeneration in order for these cornea cells to give rise to a new lentoid. While this finding differs from what has been described in the newt, it closely resembles the role of Wnt signaling during the initial formation of the lens placode from the surface ectoderm during early embryogenesis.

Evolution and development of the adelphophagic, intracapsular Schmidt's larva of the nemertean Lineus ruber

Background

The life cycle of many animals includes a larval stage, which has diversified into an astonishing variety of ecological strategies. The Nemertea is a group of spiralians that exhibits a broad diversity of larval forms, including the iconic pilidium. A pelagic planktotrophic pilidium is the ancestral form in the Pilidiophora, but several lineages exhibit deviations of this condition, mostly as a transition to pelagic lecithotrophy. The most extreme case occurs, however, in the Pilidiophoran Lineus ruber, which exhibits an adelphophagic intracapsular pilidium, the so-called Schmidt’s larva.

Results

We combined confocal laser scanning microscopy and gene expression studies to characterize the development and metamorphosis of the Schmidt’s larva of L. ruber. The larva forms after gastrulation, and comprises a thin epidermis, a proboscis rudiment and two pairs of imaginal discs from which the juvenile will develop. The cells internalized during gastrulation form a blind gut and the blastopore gives rise to the mouth of the larva and juvenile. The Schmidt’s larva eats other siblings that occupy the same egg capsule, accumulating nutrients for the juvenile. A gradual metamorphosis involves the differentiation of the juvenile cell types from the imaginal discs and the shedding of the larval epidermis. The expression of evolutionarily conserved anterior (foxQ2, six3/6, gsc, otx), endomesodermal (foxA, GATA456-a, twi-a) and posterior (evx, cdx) markers demonstrate that the juvenile retains the molecular patterning of the Schmidt’s larva. After metamorphosis, the juveniles stay over 20 days within the egg masses, until they are fully mature and hatch.

Conclusions

The evolution of the intracapsular Schmidt’s larva involved the loss of the typical feeding structures of the planktotrophic pilidium and a precocious formation of the imaginal discs, as also observed in other pelagic lecithotrophic forms. However, no special adaptations are observed related to adelphophagy. As in planktotrophic pilidium, the molecular mechanism patterning the juvenile is only active in the imaginal discs and not during the early development of the larva, suggesting two separate molecular programs during nemertean embryogenesis. Our results illuminate the diversification of larval forms in the Pilidiophora and Nemertea, and thus on the developmental mechanisms underlying metazoan larval evolution.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-015-0023-5) contains supplementary material, which is available to authorized users.

The deuterostome context of chordate origins

Our understanding of vertebrate origins is powerfully informed by comparative morphology, embryology and genomics of chordates, hemichordates and echinoderms, which together make up the deuterostome clade. Striking body-plan differences among these phyla have historically hindered the identification of ancestral morphological features, but recent progress in molecular genetics and embryology has revealed deep similarities in body-axis formation and organization across deuterostomes, at stages before morphological differences develop. These developmental genetic features, along with robust support of pharyngeal gill slits as a shared deuterostome character, provide the foundation for the emergence of chordates.

Hox genes pattern the anterior-posterior axis of the juvenile but not the larva in a maximally indirect developing invertebrate, Micrura alaskensis (Nemertea)

BACKGROUND

The pilidium larva is a novel body plan that arose within a single clade in the phylum Nemertea - the Pilidiophora. While the sister clade of the Pilidiophora and the basal nemerteans develop directly, pilidiophorans have a long-lived planktotrophic larva with a body plan distinctly different from that of the juvenile. Uniquely, the pilidiophoran juvenile develops inside the larva from several discrete rudiments. The orientation of the juvenile with respect to the larval body varies within the Pilidiophora, which suggests that the larval and juvenile anteroposterior (AP) axes are patterned differently. In order to gain insight into the evolutionary origins of the pilidium larva and the mechanisms underlying this implied axial uncoupling, we examined the expression of the Hox genes during development of the pilidiophoran Micrura alaskensis.

RESULTS

We identified sequences of nine Hox genes and the ParaHox gene caudal through a combination of transcriptome analysis and molecular cloning, and determined their expression pattern during development using in situ hybridization in whole-mounted larvae. We found that Hox genes are first expressed long after the pilidium is fully formed and functional. The Hox genes are expressed in apparently overlapping domains along the AP axis of the developing juvenile in a subset of the rudiments that give rise to the juvenile trunk. Hox genes are not expressed in the larval body at any stage of development.

CONCLUSIONS

While the Hox genes pattern the juvenile pilidiophoran, the pilidial body, which appears to be an evolutionary novelty, must be patterned by some mechanism other than the Hox genes. Although the pilidiophoran juvenile develops from separate rudiments with no obvious relationship to the embryonic formation of the larva, the Hox genes appear to exhibit canonical expression along the juvenile AP axis. This suggests that the Hox patterning system can maintain conserved function even when widely decoupled from early polarity established in the egg.

Dose-dependent nuclear β-catenin response segregates endomesoderm along the sea star primary axis

In many invertebrates, the nuclearization of β-catenin at one pole of the embryo initiates endomesoderm specification. An intriguing possibility is that a gradient of nuclear β-catenin (nβ-catenin), similar to that operating in vertebrate neural tube patterning, functions to distinguish cell fates in invertebrates. To test this hypothesis, we determined the function of nβ-catenin during the early development of the sea star, which undergoes a basal deuterostomal mode of embryogenesis. We show that low levels of nβ-catenin activity initiate bra, which is expressed in the future posterior endoderm-fated territory; intermediate levels are required for expression of foxa and gata4/5/6, which are later restricted to the endoderm; and activation of ets1 and erg in the mesoderm-fated territory requires the highest nβ-catenin activity. Transcription factors acting downstream of high nβ-catenin segregate the endoderm/mesoderm boundary, which is further reinforced by Delta/Notch signaling. Significantly, therefore, in sea stars, endomesoderm segregation arises through transcriptional responses to levels of nβ-catenin activity. Here, we describe the first empirical evidence of a dose-dependent response to a dynamic spatiotemporal nβ-catenin activity that patterns cell fates along the primary axis in an invertebrate.

Animal development: an ancient β-catenin switch?

High and low nuclear levels of the conserved transcriptional regulator β-catenin distinguish multiple sister cell fates to specify endoderm and mesoderm during early embryogenesis in a chordate embryo.

Differential regulation of disheveled in a novel vegetal cortical domain in sea urchin eggs and embryos: implications for the localized activation of canonical Wnt signaling

Pattern formation along the animal-vegetal (AV) axis in sea urchin embryos is initiated when canonical Wnt (cWnt) signaling is activated in vegetal blastomeres. The mechanisms that restrict cWnt signaling to vegetal blastomeres are not well understood, but there is increasing evidence that the egg's vegetal cortex plays a critical role in this process by mediating localized "activation" of Disheveled (Dsh). To investigate how Dsh activity is regulated along the AV axis, sea urchin-specific Dsh antibodies were used to examine expression, subcellular localization, and post-translational modification of Dsh during development. Dsh is broadly expressed during early sea urchin development, but immunolocalization studies revealed that this protein is enriched in a punctate pattern in a novel vegetal cortical domain (VCD) in the egg. Vegetal blastomeres inherit this VCD during embryogenesis, and at the 60-cell stage Dsh puncta are seen in all cells that display nuclear β-catenin. Analysis of Dsh post-translational modification using two-dimensional Western blot analysis revealed that compared to Dsh pools in the bulk cytoplasm, this protein is differentially modified in the VCD and in the 16-cell stage micromeres that partially inherit this domain. Dsh localization to the VCD is not directly affected by disruption of microfilaments and microtubules, but unexpectedly, microfilament disruption led to degradation of all the Dsh pools in unfertilized eggs over a period of incubation suggesting that microfilament integrity is required for maintaining Dsh stability. These results demonstrate that a pool of differentially modified Dsh in the VCD is selectively inherited by the vegetal blastomeres that activate cWnt signaling in early embryos, and suggests that this domain functions as a scaffold for localized Dsh activation. Localized cWnt activation regulates AV axis patterning in many metazoan embryos. Hence, it is possible that the VCD is an evolutionarily conserved cytoarchitectural domain that specifies the AV axis in metazoan ova.

Nuclearization of β-catenin in ectodermal precursors confers organizer-like ability to induce endomesoderm and pattern a pluteus larva

BACKGROUND

In many bilaterians, asymmetric activation of canonical Wnt (cWnt) signaling at the posterior pole is critical for anterior-posterior (AP) body axis formation. In 16-cell stage sea urchins, nuclearization of β-catenin in micromeres activates a gene regulatory network that defines body axes and induces endomesoderm. Transplanting micromeres to the animal pole of a host embryo induces ectopic endomesoderm in the mesomeres (ectoderm precursors) whereas inhibiting cWnt signaling blocks their endomesoderm-inducing activity and the micromeres become ectoderm-like. We have tested whether ectopic activation of cWnt signaling in mesomeres is sufficient to impart the cells with organizer-like abilities, allowing them to pattern normal embryonic body axes when recombined with a field of mesomeres.

RESULTS

Fertilized eggs were microinjected with constitutively active Xenopus β-catenin (actβ-cat) mRNA and allowed to develop until the 16-cell stage. Two mesomeres from injected embryos were then recombined with isolated animal halves (AH) from uninjected 16-cell stage embryos. Control chimeras produced animalized phenotypes (hollow balls of ectoderm) and rarely formed skeletogenic mesoderm (SM)-derived spicules, endoderm or pigment cells, a type of non-skeletogenic mesoderm (NSM). In contrast, over half of the 0.5 pg/pL actβ-cat mesomere/AH chimeras formed a partial or complete gut (exhibiting AP polarity), contained mesenchyme-like cells similar to SM, and produced pigment cells. At three days, chimeras formed plutei with normal embryonic body axes. When fates of the actβ-cat mRNA-injected mesomeres were tracked, we found that injected mesomeres formed mesenchyme-like and pigment cells, but endoderm was induced. Higher concentrations of actβ-cat mRNA were less likely to induce endoderm or pigment cells, but had similar mesenchyme-like cell production to 0.5 pg/pL actβ-cat mesomere/AH chimeras.

CONCLUSIONS

Our results show that nuclear β-catenin is sufficient to endow naïve cells with the ability to act as an organizing center and that β-catenin has both cell-autonomous and non-autonomous effects on cell fate specification in a concentration-dependent manner. These results are consistent with the hypothesis that a shift in the site of early cWnt signaling in cleaving embryos could have modified polarity of the main body axes during metazoan evolution.

Expression of wnt and frizzled genes during early sea star development

The Wnt signaling pathway is highly conserved across metazoa and has pleiotropic functions in the development of many animals. Binding of a secreted Wnt ligand to its Frizzled (Fz) receptor activates Dishevelled, which then drives one of three major signaling cascades, canonical (β-catenin), calcium, or planar cell polarity signaling. These pathways have distinct developmental effects and function in different processes in different organisms. Here we report the expression of six wnt and three fz genes during embryogenesis of the sea star, Patiria miniata, as a first step in uncovering the roles of Wnt signaling in the development of this organism. wnt3, wnt4, wnt8, and wnt16 are expressed in nested domains in the endoderm and lateral ectoderm from blastula through late gastrula stages; wnt2 and wnt5 are expressed in the mesoderm and anterior endoderm. Expression of different fz paralogs is detected in the mesoderm; posterior endoderm and ectoderm; and anterior ectoderm. Taken together, this suggests that Wnt signaling can occur throughout most of the embryo and may therefore play multiple roles during sea star development.

The bilaterian head patterning gene six3/6 controls aboral domain development in a cnidarian

The origin of the bilaterian head is a fundamental question for the evolution of animal body plans. The head of bilaterians develops at the anterior end of their primary body axis and is the site where the brain is located. Cnidarians, the sister group to bilaterians, lack brain-like structures and it is not clear whether the oral, the aboral, or none of the ends of the cnidarian primary body axis corresponds to the anterior domain of bilaterians. In order to understand the evolutionary origin of head development, we analysed the function of conserved genetic regulators of bilaterian anterior development in the sea anemone Nematostella vectensis. We show that orthologs of the bilaterian anterior developmental genes six3/6, foxQ2, and irx have dynamic expression patterns in the aboral region of Nematostella. Functional analyses reveal that NvSix3/6 acts upstream of NvFoxQ2a as a key regulator of the development of a broad aboral territory in Nematostella. NvSix3/6 initiates an autoregulatory feedback loop involving positive and negative regulators of FGF signalling, which subsequently results in the downregulation of NvSix3/6 and NvFoxQ2a in a small domain at the aboral pole, from which the apical organ develops. We show that signalling by NvFGFa1 is specifically required for the development of the apical organ, whereas NvSix3/6 has an earlier and broader function in the specification of the aboral territory. Our functional and gene expression data suggest that the head-forming region of bilaterians is derived from the aboral domain of the cnidarian-bilaterian ancestor.

Integration of canonical and noncanonical Wnt signaling pathways patterns the neuroectoderm along the anterior-posterior axis of sea urchin embryos

Three different Wnt signaling pathways function to restrict the anterior neuroectoderm state to the anterior end of the sea urchin embryo, a mechanism of anterior fate restriction that could be conserved among deuterostomes.

Patterning the neuroectoderm along the anterior–posterior (AP) axis is a critical event in the early development of deuterostome embryos. However, the mechanisms that regulate the specification and patterning of the neuroectoderm are incompletely understood. Remarkably, the anterior neuroectoderm (ANE) of the deuterostome sea urchin embryo expresses many of the same transcription factors and secreted modulators of Wnt signaling, as does the early vertebrate ANE (forebrain/eye field). Moreover, as is the case in vertebrate embryos, confining the ANE to the anterior end of the embryo requires a Wnt/β-catenin-dependent signaling mechanism. Here we use morpholino- or dominant negative-mediated interference to demonstrate that the early sea urchin embryo integrates information not only from Wnt/β-catenin but also from Wnt/Fzl5/8-JNK and Fzl1/2/7-PKC pathways to provide precise spatiotemporal control of neuroectoderm patterning along its AP axis. Together, through the Wnt1 and Wnt8 ligands, they orchestrate a progressive posterior-to-anterior wave of re-specification that restricts the initial, ubiquitous, maternally specified, ANE regulatory state to the most anterior blastomeres. There, the Wnt receptor antagonist, Dkk1, protects this state through a negative feedback mechanism. Because these different Wnt pathways converge on the same cell fate specification process, our data suggest they may function as integrated components of an interactive Wnt signaling network. Our findings provide strong support for the idea that the sea urchin ANE regulatory state and the mechanisms that position and define its borders represent an ancient regulatory patterning system that was present in the common echinoderm/vertebrate ancestor.

The initial regulatory state of most cells in many deuterostome embryos, including those of vertebrates and sea urchins, supports anterior neural fate specification. It is important to restrict this neurogenic potential to the anterior end of the embryo during early embryogenesis, but the molecular mechanisms by which this re-specification of posterior fate occurs are incompletely understood in any embryo. The sea urchin embryo is ideally suited to study this process because, in contrast to vertebrates, anterior–posterior neuroectoderm patterning occurs independently of dorsal-ventral axis patterning and takes place before the complex cell movements of gastrulation. In this study, we show that a linked, three-step process involving at least three different Wnt signaling pathways provides precise spatiotemporal restriction of the anterior neuroectoderm regulatory state to the anterior end of the sea urchin embryo. Because these three pathways impinge on the same developmental process, they could be functioning as an integrated Wnt signaling network. Moreover, striking parallels among gene expression patterns and functional studies suggest that this mechanism of anterior fate restriction could be highly conserved among deuterostomes.

Stem cells and lineages of the intestine: a developmental and evolutionary perspective

The intestine consists of epithelial cells that secrete digestive enzymes and mucus (gland cells), absorb food particles (enterocytes), and produce hormones (endocrine cells). Intestinal cells are rapidly turned over and need to be replaced. In cnidarians, mitosis of differentiated intestinal cells accounts for much of the replacement; in addition, migratory, multipotent stem cells (interstitial cells) contribute to the production of intestinal cells. In other phyla, intestinal cell replacement is solely the function of stem cells entering the gut from the outside (such as in case of the neoblasts of platyhelminths) or intestinal stem cells located within the midgut epithelium (as in both vertebrates or arthropods). We will attempt in the following to review important aspects of midgut stem cells in different animal groups: where are they located, what types of lineages do they produce, and how do they develop. We will start out with a comparative survey of midgut cell types found across the animal kingdom; then briefly look at the specification of these cells during embryonic development; and finally focus on the stem cells that regenerate midgut cells during adult life. In a number of model systems, including mouse, zebrafish and Drosophila, the molecular pathways controlling intestinal stem cells proliferation and the specification of intestinal cell types are under intensive investigation. We will highlight findings of the recent literature, focusing on aspects that are shared between the different models and that point at evolutionary ancient mechanisms of intestinal cell formation.

A conserved function for Strabismus in establishing planar cell polarity in the ciliated ectoderm during cnidarian larval development

Functional and morphological planar cell polarity (PCP) oriented along the oral-aboral body axis is clearly evident in the ectoderm of torpedo-shaped planula larvae of hydrozoan cnidarians such as Clytia hemisphaerica. Ectodermal epithelial cells bear a single motile cilium the beating of which is coordinated between cells, causing directional swimming towards the blunt, aboral pole. We have characterised PCP during Clytia larval development and addressed its molecular basis. PCP is first detectable in ectodermal cells during gastrulation as coordinated basal body positioning, the ciliary root becoming consistently positioned on the oral side of the apical surface of the cell. At later stages, more pronounced structural polarity develops around the base of each cilium in relation to the cilia beating direction, including a characteristic asymmetric cortical actin organisation. Morpholino antisense oligonucleotide and mRNA injection studies showed that PCP development requires the Clytia orthologues of the core Fz-PCP pathway components Strabismus (CheStbm), Frizzled (CheFz1) and Dishevelled (CheDsh). Morpholinos targeting any of these components prevented ectodermal PCP, disrupted ciliogenesis and inhibited embryo elongation during gastrulation, which involves cell intercalation. We show that YFP-tagged CheStbm adopts a polarised intracellular distribution, localising preferentially to the aboral boundary of each cell, as has been demonstrated in Drosophila and some vertebrate PCP studies. Our findings in a cnidarian strongly suggest that the Fz-PCP pathway is a highly conserved and evolutionary ancient metazoan feature that is probably widely responsible for oriented swimming and/or feeding in relation to body axis in the many ciliated larval types found throughout the animal kingdom.

Evolutionary crossroads in developmental biology: hemichordates

Hemichordates are a deuterostome phylum, the sister group to echinoderms, and closely related to chordates. They have thus been used to gain insights into the origins of deuterostome and chordate body plans. Developmental studies of this group have a long and distinguished history. Recent improvements in animal husbandry, functional tool development and genomic resources have resulted in novel developmental data from several species in this group. In this Primer, we introduce representative hemichordate species with contrasting modes of development and summarize recent findings that are beginning to yield important insights into deuterostome developmental mechanisms.

Frizzled1/2/7 signaling directs β-catenin nuclearisation and initiates endoderm specification in macromeres during sea urchin embryogenesis

In sea urchins, the nuclear accumulation of β-catenin in micromeres and macromeres at 4th and 5th cleavage activates the developmental gene regulatory circuits that specify all of the vegetal tissues (i.e. skeletogenic mesoderm, endoderm and non-skeletogenic mesoderm). Here, through the analysis of maternal Frizzled receptors as potential contributors to these processes, we found that, in Paracentrotus lividus, the receptor Frizzled1/2/7 is required by 5th cleavage for β-catenin nuclearisation selectively in macromere daughter cells. Perturbation analyses established further that Frizzled1/2/7 signaling is required subsequently for the specification of the endomesoderm and then the endoderm but not for that of the non-skeletogenic mesoderm, even though this cell type also originates from the endomesoderm lineage. Complementary analyses on Wnt6 showed that this maternal ligand is similarly required at 5th cleavage for the nuclear accumulation of β-catenin exclusively in the macromeres and for endoderm but not for non-skeletogenic mesoderm specification. In addition, Wnt6 misexpression reverses Frizzled1/2/7 downregulation-induced phenotypes. Thus, the results indicate that Wnt6 and Frizzled1/2/7 are likely to behave as the ligand-receptor pair responsible for initiating β-catenin nuclearisation in macromeres at 5th cleavage and that event is necessary for endoderm specification. They show also that β-catenin nuclearisation in micromeres and macromeres takes place through a different mechanism, and that non-skeletogenic mesoderm specification occurs independently of the nuclear accumulation of β-catenin in macromeres at the 5th cleavage. Evolutionarily, this analysis outlines further the conserved involvement of the Frizzled1/2/7 subfamily, but not of specific Wnts, in the activation of canonical Wnt signaling during early animal development.

β-catenin specifies the endomesoderm and defines the posterior organizer of the hemichordate Saccoglossus kowalevskii

The canonical Wnt/β-catenin pathway is a key regulator of body plan organization and axis formation in metazoans, being involved in germ layer specification, posterior growth and patterning of the anteroposterior axis. Results from animals spanning a wide phylogenetic range suggest that a unifying function of β-catenin in metazoans is to define the posterior/vegetal part of the embryo. Although the specification of vegetal territories (endoderm) by β-catenin has been demonstrated in distantly related animals (cnidarians, a protostome, echinoderms and ascidians), the definition of the posterior part of the embryo is well supported only for vertebrates and planarians. To gain insights into β-catenin functions during deuterostome evolution, we have studied the early development of the direct developing hemichordate Saccoglossus kowalevskii. We show that the zygote is polarized after fertilization along the animal-vegetal axis by cytoplasmic rearrangements resembling the ascidian vegetal contraction. This early asymmetry is translated into nuclear accumulation of β-catenin at the vegetal pole, which is necessary and sufficient to specify endomesoderm. We show that endomesoderm specification is crucial for anteroposterior axis establishment in the ectoderm. The endomesoderm secretes as yet unidentified signals that posteriorize the ectoderm, which would otherwise adopt an anterior fate. Our results point to a conserved function at the base of deuterostomes for β-catenin in germ layer specification and to a causal link in the definition of the posterior part of the embryonic ectoderm by way of activating posteriorizing endomesodermal factors. Consequently, the definition of the vegetal and the posterior regions of the embryo by β-catenin should be distinguished and carefully re-examined.

Strabismus-mediated primary archenteron invagination is uncoupled from Wnt/β-catenin-dependent endoderm cell fate specification in Nematostella vectensis (Anthozoa, Cnidaria): Implications for the evolution of gastrulation

BACKGROUND

Gastrulation is a uniquely metazoan character, and its genesis was arguably the key step that enabled the remarkable diversification within this clade. The process of gastrulation involves two tightly coupled events during embryogenesis of most metazoans. Morphogenesis produces a distinct internal epithelial layer in the embryo, and this epithelium becomes segregated as an endoderm/endomesodermal germ layer through the activation of a specific gene regulatory program. The developmental mechanisms that induced archenteron formation and led to the segregation of germ layers during metazoan evolution are unknown. But an increased understanding of development in early diverging taxa at the base of the metazoan tree may provide insights into the origins of these developmental mechanisms.

RESULTS

In the anthozoan cnidarian Nematostella vectensis, initial archenteron formation begins with bottle cell-induced buckling of the blastula epithelium at the animal pole. Here, we show that bottle cell formation and initial gut invagination in Nematostella requires NvStrabismus (NvStbm), a maternally-expressed core component of the Wnt/Planar Cell Polarity (PCP) pathway. The NvStbm protein is localized to the animal pole of the zygote, remains asymmetrically expressed through the cleavage stages, and becomes restricted to the apical side of invaginating bottle cells at the blastopore. Antisense morpholino-mediated NvStbm-knockdown blocks bottle cell formation and initial archenteron invagination, but it has no effect on Wnt/ß-catenin signaling-mediated endoderm cell fate specification. Conversely, selectively blocking Wnt/ß-catenin signaling inhibits endoderm cell fate specification but does not affect bottle cell formation and initial archenteron invagination.

CONCLUSIONS

Our results demonstrate that Wnt/PCP-mediated initial archenteron invagination can be uncoupled from Wnt/ß-catenin-mediated endoderm cell fate specification in Nematostella, and provides evidence that these two processes could have evolved independently during metazoan evolution. We propose a two-step model for the evolution of an archenteron and the evolution of endodermal germ layer segregation. Asymmetric accumulation and activation of Wnt/PCP components at the animal pole of the last common ancestor to the eumetazoa may have induced the cell shape changes that led to the initial formation of an archenteron. Activation of Wnt/ß-catenin signaling at the animal pole may have led to the activation of a gene regulatory network that specified an endodermal cell fate in the archenteron.

Cellular symmetry breaking during Caenorhabditis elegans development

The nematode worm Caenorhabditis elegans has produced a wellspring of insights into mechanisms that govern cellular symmetry breaking during animal development. Here we focus on two highly conserved systems that underlie many of the key symmetry-breaking events that occur during embryonic and larval development in the worm. One involves the interplay between Par proteins, Rho GTPases, and the actomyosin cytoskeleton and mediates asymmetric cell divisions that establish the germline. The other uses elements of the Wnt signaling pathway and a highly reiterative mechanism that distinguishes anterior from posterior daughter cell fates. Much of what we know about these systems comes from intensive study of a few key events-Par/Rho/actomyosin-mediated polarization of the zygote in response to a sperm-derived cue and the Wnt-mediated induction of endoderm at the four-cell stage. However, a growing body of work is revealing how C. elegans exploits elements/variants of these systems to accomplish a diversity of symmetry-breaking tasks throughout embryonic and larval development.

The slipper snail, Crepidula: an emerging lophotrochozoan model system

Recent developmental and genomic research focused on "slipper snails" in the genus Crepidula has positioned Crepidula fornicata as a de facto model system for lophotrochozoan development. Here we review recent developments, as well as earlier reports demonstrating the widespread use of this system in studies of development and life history. Recent studies have resulted in a well-resolved fate map of embryonic cell lineage, documented mechanisms for axis determination and D quadrant specification, preliminary gene expression patterns, and the successful application of loss- and gain-of-function assays. The recent development of expressed sequence tags and preliminary genomics work will promote the use of this system, particularly in the area of developmental biology. A wealth of comparative information on phylogenetic relationships, variation in mode of development within the family, and numerous studies on larval biology and metamorphosis, primarily in Crepidula fornicata, make these snails a powerful tool for studies of the evolution of the mechanisms of development in the Mollusca and Lophotrochozoa. By bringing a review of the current state of knowledge of Crepidula life histories and development together with some detailed experimental methods, we hope to encourage further use of this system in various fields of investigation.

Developmental patterns in spiralian embryos

At least five animal phyla exhibit spiralian development, which is characterized by striking similarities in the geometry of the early cleavage pattern and the fate map of the blastula, along with similarities in larval morphology. Recent advances in reconstructing the phylogeny of spiralians and their relatives suggest that the common ancestor of a large clade of protostome phyla known as the Lophotrochozoa had spiralian development. In this minireview, I describe characteristics of spiralian development and some recent insights into its mechanisms and evolution.