Presumptive Endoderm Research Articles

The Degenerate Tale of Ascidian Tails.

Ascidians are invertebrate chordates, with swimming chordate tadpole larvae that have distinct heads and tails. The head contains the small brain, sensory organs, including the ocellus (light) and otolith (gravity) and the presumptive endoderm, while the tail has a notochord surrounded by muscle cells and a dorsal nerve cord. One of the chordate features is a post-anal tail. Ascidian tadpoles are nonfeeding, and their tails are critical for larval locomotion. After hatching the larvae swim up toward light and are carried by the tide and ocean currents. When competent to settle, ascidian tadpole larvae swim down, away from light, to settle and metamorphose into a sessile adult. Tunicates are classified as chordates because of their chordate tadpole larvae; in contrast, the sessile adult has a U-shaped gut and very derived body plan, looking nothing like a chordate. There is one group of ascidians, the Molgulidae, where many species are known to have tailless larvae. The Swalla Lab has been studying the evolution of tailless ascidian larvae in this clade for over 30 years and has shown that tailless larvae have evolved independently several times in this clade. Comparison of the genomes of two closely related species, the tailed Molgula oculata and tailless Molgula occulta reveals much synteny, but there have been multiple insertions and deletions that have disrupted larval genes in the tailless species. Genomics and transcriptomics have previously shown that there are pseudogenes expressed in the tailless embryos, suggesting that the partial rescue of tailed features in their hybrid larvae is due to the expression of intact genes from the tailed parent. Yet surprisingly, we find that the notochord gene regulatory network is mostly intact in the tailless M. occulta, although the notochord does not converge and extend and remains as an aggregate of cells we call the "notoball." We expect that eventually many of the larval gene networks will become evolutionarily lost in tailless ascidians and the larval body plan abandoned, with eggs developing directly into an adult. Here we review the current evolutionary and developmental evidence on how the molgulids lost their tails.

A cell-based boundary model of gastrulation by unipolar ingression in the hydrozoan cnidarian Clytia hemisphaerica

In Cnidaria, modes of gastrulation to produce the two body layers vary greatly between species. In the hydrozoan species Clytia hemisphaerica gastrulation involves unipolar ingression of presumptive endoderm cells from an oral domain of the blastula, followed by migration of these cells to fill the blastocoel with concomitant narrowing of the gastrula and elongation along the oral-aboral axis. We developed a 2D computational boundary model capable of simulating the morphogenetic changes during embryonic development from early blastula stage to the end of gastrulation. Cells are modeled as polygons with elastic membranes and cytoplasm, colliding and adhering to other cells, and capable of forming filopodia. With this model we could simulate compaction of the embryo preceding gastrulation, bottle cell formation, ingression, and intercalation between cells of the ingressing presumptive endoderm. We show that embryo elongation is dependent on the number of endodermal cells, low endodermal cell-cell adhesion, and planar cell polarity (PCP). When the strength of PCP is reduced in our model, resultant embryo morphologies closely resemble those reported previously following morpholino-mediated knockdown of the core PCP proteins Strabismus and Frizzled. Based on our results, we postulate that cellular processes of apical constriction, compaction, ingression, and then reduced cell-cell adhesion and mediolateral intercalation in the presumptive endoderm, are required and when combined, sufficient for Clytia gastrulation.

Isolated Intraspinal Neurenteric Cyst, Mimicking Arachnoid Cyst: A Case Report

Neurenteric cysts account for 0.7-1.3% of spinal axis tumors. These uncommon lesions results from the inappropriate partitioning of the embryonic notochordal plate and presumptive endoderm during the third week of human development. Heterotopic nests of epithelium reminiscent of gastrointestinal and respiratory tissue lead to eventual formation of compressive cystic lesions of the pediatric and adult spine. We report a 40-year-old male presenting with slow-progressive myelopathic manifestation and ovoid non-enhancing cysticintradural extramedullary lesion at C7 level on MRI, mimicking intraspinal arachnoid cyst, who underwent successful surgical excision. The histopathological examination prove it to be neurenteric cyst.

Ingression-type cell migration drives vegetal endoderm internalisation in the Xenopus gastrula.

During amphibian gastrulation, presumptive endoderm is internalised as part of vegetal rotation, a large-scale movement that encompasses the whole vegetal half of the embryo. It has been considered a gastrulation process unique to amphibians, but we show that at the cell level, endoderm internalisation exhibits characteristics reminiscent of bottle cell formation and ingression, known mechanisms of germ layer internalisation. During ingression proper, cells leave a single-layered epithelium. In vegetal rotation, the process occurs in a multilayered cell mass; we refer to it as ingression-type cell migration. Endoderm cells move by amoeboid shape changes, but in contrast to other instances of amoeboid migration, trailing edge retraction involves ephrinB1-dependent macropinocytosis and trans-endocytosis. Moreover, although cells are separated by wide gaps, they are connected by filiform protrusions, and their migration depends on C-cadherin and the matrix protein fibronectin. Cells move in the same direction but at different velocities, to rearrange by differential migration.

Transcription factors Mix1 and VegT, relocalization of vegt mRNA, and conserved endoderm and dorsal specification in frogs

Protein expression of the transcription factor genes mix1 and vegt characterized the presumptive endoderm in embryos of the frogs Engystomops randi, Epipedobates machalilla, Gastrotheca riobambae, and Eleutherodactylus coqui, as in Xenopus laevis embryos. Protein VegT was detected in the animal hemisphere of the early blastula in all frogs, and only the animal pole was VegT-negative. This finding stimulated a vegt mRNA analysis in X. laevis eggs and embryos. vegt mRNA was detected in the animal region of X. laevis eggs and early embryos, in agreement with the VegT localization observed in the analyzed frogs. Moreover, a dorso-animal relocalization of vegt mRNA occurred in the egg at fertilization. Thus, the comparative analysis indicated that vegt may participate in dorsal development besides its known roles in endoderm development, and germ-layer specification. Zygotic vegt (zvegt) mRNA was detected as a minor isoform besides the major maternal (mvegt) isoform of the X. laevis egg. In addition, α-amanitin-insensitive vegt transcripts were detected around vegetal nuclei of the blastula. Thus, accumulation of vegt mRNA around vegetal nuclei was caused by relocalization rather than new mRNA synthesis. The localization of vegt mRNA around vegetal nuclei may contribute to the identity of vegetal blastomeres. These and previously reportedly localization features of vegt mRNA and protein derive from the master role of vegt in the development of frogs. The comparative analysis indicated that the strategies for endoderm, and dorsal specification, involving vegt and mix1, have been evolutionary conserved in frogs.

Molecular conservation of metazoan gut formation: evidence from expression of endomesoderm genes in Capitella teleta (Annelida).

BackgroundMetazoan digestive systems develop from derivatives of ectoderm, endoderm and mesoderm, and vary in the relative contribution of each germ layer across taxa and between gut regions. In a small number of well-studied model systems, gene regulatory networks specify endoderm and mesoderm of the gut within a bipotential germ layer precursor, the endomesoderm. Few studies have examined expression of endomesoderm genes outside of those models, and thus, it is unknown whether molecular specification of gut formation is broadly conserved. In this study, we utilize a sequenced genome and comprehensive fate map to correlate the expression patterns of six transcription factors with embryonic germ layers and gut subregions during early development in Capitella teleta.ResultsThe genome of C. teleta contains the five core genes of the sea urchin endomesoderm specification network. Here, we extend a previous study and characterize expression patterns of three network orthologs and three additional genes by in situ hybridization during cleavage and gastrulation stages and during formation of distinct gut subregions. In cleavage stage embryos, Ct-otx, Ct-blimp1, Ct-bra and Ct-nkx2.1a are expressed in all four macromeres, the endoderm precursors. Ct-otx, Ct-blimp1, and Ct-nkx2.1a are also expressed in presumptive endoderm of gastrulae and later during midgut development. Additional gut-specific expression patterns include Ct-otx, Ct-bra, Ct-foxAB and Ct-gsc in oral ectoderm; Ct-otx, Ct-blimp1, Ct-bra and Ct-nkx2.1a in the foregut; and both Ct- bra and Ct-nkx2.1a in the hindgut.ConclusionsIdentification of core sea urchin endomesoderm genes in C. teleta indicates they are present in all three bilaterian superclades. Expression of Ct-otx, Ct-blimp1 and Ct-bra, combined with previously published Ct-foxA and Ct-gataB1 patterns, provide the most comprehensive comparison of these five orthologs from a single species within Spiralia. Each ortholog is likely involved in endoderm specification and midgut development, and several may be essential for establishment of the oral ectoderm, foregut and hindgut, including specification of ectodermal and mesodermal contributions. When the five core genes are compared across the Metazoa, their conserved expression patterns suggest that ‘gut gene’ networks evolved to specify distinct digestive system subregions, regardless of species-specific differences in gut architecture or germ layer contributions within each subregion.Electronic supplementary materialThe online version of this article (doi:10.1186/2041-9139-5-39) contains supplementary material, which is available to authorized users.

External morphological characteristics during the embryonic development of Portunus pelagicus

The embryonic development of Portunus pelagicus was studied under laboratory conditions at a water temperature of 25-26 Degrees Celsius, salinity of 30, and pH of 7.8-8.4. The embryogenesis of Portunus pelagicus was divided into six stages: cleavage, blastula, gastrula, nauplius, metanauplius, and protozoea. Embryogenesis lasted about 300 h post spawning. Eggs began superficial cleavage about 28 h after spawning when the nucleus appeared at the surface of the egg till the egg divided into 16 cells. The blastula stage was observed about 40 h post spawning and gastrula stage appeared when the presumptive endoderm and other cells near them invaginated. The fourth-stage of embryogenesis, nauplius, was characterized by three pairs of appendages appearing about 90 h post spawning, while metanauplius, the fifth-stage of embryogenesis, was characterized by five pairs of appendages, which appeared about 110 h post spawning. The sixth stage of embryogenesis was protozoea, which was characterized by seven pairs of appendages appearing about 140 h post spawning. The compound eye, heart and pigment cells were also found in the protozoea stage. After the natatory seta formed on the top of maxilliped, the protozoea developed into the zoea at the time of hatching (about 300 h post spawning).

Neurenteric cysts of the spine

Neurenteric cysts account for 0.7-1.3% of spinal axis tumors. These rare lesions result from the inappropriate partitioning of the embryonic notochordal plate and presumptive endoderm during the third week of human development. Heterotopic rests of epithelium reminiscent of gastrointestinal and respiratory tissue lead to eventual formation of compressive cystic lesions of the pediatric and adult spine. Histopathological analysis of neurenteric tissue reveals a highly characteristic structure of columnar or cuboidal epithelium with or without cilia and mucus globules. Patients with symptomatic neurenteric cysts typically present in the second and third decades of life with size-dependent myelopathic and/or radicular signs. Magnetic resonance imaging and computed tomography are essential diagnostic tools for the delineation of cyst form and overlying osseous architecture. A variety of approaches have been employed in the treatment of neurenteric cysts each with a goal of total surgical resection. Although long-term outcome analyses are limited, data available indicate that surgical intervention in the case of neurenteric cysts results in a high frequency of resolution of neurological deficit with minimal morbidity. However, recurrence rates as high as 37% have been reported with incomplete resection secondary to factors such as cyst adhesion to surrounding structure and unclear dissection planes. Here we present a systematic review of English language literature from January 1966 to December 2009 utilizing MEDLINE with the following search terminology: neurenteric cyst, enterogenous cyst, spinal cord tumor, spinal dysraphism, intraspinal cyst, intramedullary cyst, and intradural cyst. In addition, the references of publications returned from the MEDLINE search criteria were surveyed in order to examine other pertinent reports.

Expression of FoxA and GATA transcription factors correlates with regionalized gut development in two lophotrochozoan marine worms: Chaetopterus (Annelida) and Themiste lageniformis (Sipuncula)

BackgroundA through gut is present in almost all metazoans, and most likely represents an ancient innovation that enabled bilaterian animals to exploit a wide range of habitats. Molecular developmental studies indicate that Fox and GATA regulatory genes specify tissue regions along the gut tube in a broad diversity of taxa, although little is known about gut regionalization within the Lophotrochozoa. In this study, we isolated FoxA and GATA456 orthologs and used whole mount in situ hybridization during larval gut formation in two marine worms: the segmented, polychaete annelid Chaetopterus, which develops a planktotrophic larva with a tripartite gut, and the non-segmented sipunculan Themiste lageniformis, which develops a lecithotrophic larva with a U-shaped gut.ResultsFoxA and GATA456 transcripts are predominantly restricted to gut tissue, and together show regional expression spanning most of the alimentary canal in each of these lophotrochozoans, although neither FoxA nor GATA456 is expressed in the posterior intestine of Chaetopterus. In both species, FoxA is expressed at the blastula stage, transiently in presumptive endoderm before formation of a definitive gut tube, and throughout early larval development in discrete foregut and hindgut domains. GATA456 genes are expressed during endoderm formation, and in endoderm and mesoderm associated with the midgut in each species. Several species-specific differences were detected, including an overlap of FoxA and GATA456 expression in the intestinal system of Themiste, which is instead complimentary in Chaetopterus. Other differences include additional discrete expression domains of FoxA in ectodermal trunk cells in Themiste but not Chaetopterus, and expression of GATA456 in anterior ectoderm and midgut cells unique to Chaetopterus.ConclusionsThis study of gene expression in a sipunculan contributes new comparative developmental insights from lophotrochozoans, and shows that FoxA and GATA456 transcription factors are part of an ancient patterning mechanism that was deployed during early evolution of the metazoan through gut. The common utilization of FoxA and GATA456 throughout gut formation by species with contrasting life history modes indicates that both genes are core components of a gut-specific gene regulatory network in spiralians. Despite a highly conserved pattern of early development, and probably similar ontogenic origins of gut tissue, there are molecular differences in gut regionalization between lophotrochozoan species.

Spatio-temporal intersection of Lhx3 and Tbx6 defines the cardiac field through synergistic activation of Mesp

Mesp encodes a bHLH transcription factor required for specification of the cardiac mesoderm in Ciona embryos. The activities of Macho-1 and β-catenin, two essential maternal determinants, are required for Mesp expression in the B7.5 blastomeres, which constitute the heart field. The T-box transcription factor Tbx6 functions downstream of Macho-1 as a direct activator of Mesp expression. However, Tbx6 cannot account for the restricted expression of Mesp in the B7.5 lineage since it is expressed throughout the presumptive tail muscles. Here we present evidence that the LIM-homeobox gene Lhx3, a direct target of β-catenin, is essential for localized Mesp expression. Lhx3 is expressed throughout the presumptive endoderm and B7.5 blastomeres. Thus, the B7.5 blastomeres are the only cells to express sustained levels of the Tbx6 and Lhx3 activators. Like mammalian Lhx3 genes, Ci-Lhx3 encodes two isoforms with distinct N-terminal peptides. The Lhx3a isoform appears to be expressed both maternally and zygotically, while the Lhx3b isoform is exclusively zygotic. Misexpression of Lhx3b is sufficient to induce ectopic Mesp activation in cells expressing Tbx6b. Injection of antisense morpholino oligonucleotides showed that the Lhx3b isoform is required for endogenous Mesp expression. Mutations in the Lhx3 half-site of Tbx6/Lhx3 composite elements strongly reduced the activity of a minimal Mesp enhancer. We discuss the delineation of the heart field by the synergistic action of muscle and gut determinants.

Sox genes in the coral Acropora millepora: divergent expression patterns reflect differences in developmental mechanisms within the Anthozoa

BackgroundSox genes encode transcription factors that function in a wide range of developmental processes across the animal kingdom. To better understand both the evolution of the Sox family and the roles of these genes in cnidarians, we are studying the Sox gene complement of the coral, Acropora millepora (Class Anthozoa).ResultsBased on overall domain structures and HMG box sequences, the Acropora Sox genes considered here clearly fall into four of the five major Sox classes. AmSoxC is expressed in the ectoderm during development, in cells whose morphology is consistent with their assignment as sensory neurons. The expression pattern of the Nematostella ortholog of this gene is broadly similar to that of AmSoxC, but there are subtle differences – for example, expression begins significantly earlier in Acropora than in Nematostella. During gastrulation, AmSoxBb and AmSoxB1 transcripts are detected only in the presumptive ectoderm while AmSoxE1 transcription is restricted to the presumptive endoderm, suggesting that these Sox genes might play roles in germ layer specification. A third type B Sox gene, AmSoxBa, and a Sox F gene AmSoxF also have complex and specific expression patterns during early development. Each of these genes has a clear Nematostella ortholog, but in several cases the expression pattern observed in Acropora differs significantly from that reported in Nematostella.ConclusionThese differences in expression patterns between Acropora and Nematostella largely reflect fundamental differences in developmental processes, underscoring the diversity of mechanisms within the anthozoan Sub-Class Hexacorallia (Zoantharia).

Unexpected diversity of cnidarian integrins: expression during coral gastrulation

BackgroundAdhesion mediated through the integrin family of cell surface receptors is central to early development throughout the Metazoa, playing key roles in cell-extra cellular matrix adhesion and modulation of cadherin activity during the convergence and extension movements of gastrulation. It has been suggested that Caenorhabditis elegans, which has a single β and two α integrins, might reflect the ancestral integrin complement. Investigation of the integrin repertoire of anthozoan cnidarians such as the coral Acropora millepora is required to test this hypothesis and may provide insights into the original roles of these molecules.ResultsTwo novel integrins were identified in Acropora. AmItgα1 shows features characteristic of α integrins lacking an I-domain, but phylogenetic analysis gives no clear indication of its likely binding specificity. AmItgβ2 lacks consensus cysteine residues at positions 8 and 9, but is otherwise a typical β integrin. In situ hybridization revealed that AmItgα1, AmItgβ1, and AmItgβ2 are expressed in the presumptive endoderm during gastrulation. A second anthozoan, the sea anemone Nematostella vectensis, has at least four β integrins, two resembling AmItgβ1 and two like AmItgβ2, and at least three α integrins, based on its genomic sequence.ConclusionIn two respects, the cnidarian data do not fit expectations. First, the cnidarian integrin repertoire is more complex than predicted: at least two βs in Acropora, and at least three αs and four βs in Nematostella. Second, whereas the bilaterian αs resolve into well-supported groups corresponding to those specific for RGD-containing or laminin-type ligands, the known cnidarian αs are distinct from these. During early development in Acropora, the expression patterns of the three known integrins parallel those of amphibian and echinoderm integrins.

Xenopus laevis Animal Cap/Vegetal Endoderm Conjugates: Figure 1.

INTRODUCTIONIn Xenopus laevis, mesoderm can be induced in animal cap cells by contact with vegetal cells (presumptive endoderm). This juxtaposition of tissues was the first indication that the process of mesoderm induction could be separated from neural induction. This protocol describes how to set up a conjugate using Xenopus animal cap and vegetal tissue. In theory, a single embryo can provide both animal and vegetal cells; however, in practice, this is difficult to achieve.

Regional specification of the endoderm in the early chick embryo

In the avian embryo, the endoderm, which forms a simple flat-sheet structure after gastrulation, is regionally specified in a gradual manner along the antero-posterior and dorso-ventral axes, and eventually differentiates into specific organs with defined morphologies and gene expression profiles. In our study, we carried out transplantation experiments using early chick embryos to elucidate the timing of fate establishment in the endoderm. We showed that at stage 5, posteriorly grafted presumptive foregut endoderm expressed CdxA, a posterior endoderm marker, but not cSox2, an anterior endoderm marker. Conversely, anteriorly grafted presumptive mid-hindgut endoderm expressed cSox2 but not CdxA. At stage 8, posteriorly grafted presumptive foregut endoderm also expressed CdxA and not cSox2, but anteriorly grafted presumptive mid-hindgut endoderm showed no changes in its posterior-specific gene expression pattern. At stage 10, both posteriorly grafted foregut endoderm and anteriorly grafted mid-hindgut endoderm maintain their original gene expression patterns. These results suggest that the regional specification of the endoderm occurs between stages 8 and 10 in the foregut, and between stages 5 and 8 in the mid-hindgut.

Specification of germ layer identity in the chick gastrula

BackgroundChick definitive endoderm is an important source of signals that pattern the early embryo forming a central structure around which the body plan is constructed. Although the origin of definitive endoderm has been mapped in the chick, arising principally from rostral streak at elongating streak stages, it is not known when this layer first becomes fully committed to its germ layer fate, an important issue to resolve in light of its critical role in subsequent patterning of the early embryo.ResultsThrough gene expression screening of chick gastrula, we identified molecular markers of definitive endoderm restricted to rostral (Sox17) and caudal (Gata5/6) regions, suggesting that at least two subpopulations of definitive endodermal cells exist during ingression. We show (1) that presumptive mesoderm cells migrate to the middle layer and remain mesenchymal when transplanted to rostral primitive streak, and prospective endoderm cells enter the lower layer and become epithelial when transplanted to caudal primitive streak; and (2) that presumptive endoderm cells and mesoderm cells lose normal gene expression (Sox17 and Wnt8c, respectively) when transplanted outside of their normal position of origin. Moreover, when rostral or caudal primitive streak segments are transplanted into rostral blastoderm isolates (RBIs), both types of transplants express Sox17 4–6 hours later–consistent with their new position, regardless of their presumptive germ layer origin–and prospective mesoderm transplants, which normally express Wnt8c, turn off expression, suggesting that signals within the rostral blastoderm induce endoderm gene expression, and repress mesoderm gene expression, during gastrulation.ConclusionOur results demonstrate that germ layer identity is fixed at the time populations of endoderm and mesoderm cells ingress through the primitive streak, whereas their gene expression patterns remain labile. In addition, our results show that inductive and repressive signals are present, and that these signals regulate gene expression of both ingressed endoderm and mesoderm cells. Thus, gastrula cells display elements of both pre-patterning and plasticity, with endoderm the first germ layer becoming committed to its fate during early gastrulation stages.

Early Development of Zooxanthella-containing Eggs of the Corals Porites cylindrica and Montipora digitata: The Endodermal Localization of Zooxanthellae

We studied the early development of zooxanthellae-containing eggs of the scleractinian corals Porites cylindrica and Montipora digitata to elucidate how zooxanthellae become localized to the endoderm of planulae during the course of development. In both species, zooxanthellae were distributed evenly in the oocytes and delivered almost equally to the blastomeres during cleavage. In P. cylindrica, gastrulation occurred via delamination or ingression, and blastomeres containing zooxanthellae dropped into the blastocoel during gastrulation. Thus, zooxanthellae were restricted to the endodermal cells at the gastrula or early planula stage in P. cylindrica. In M. digitata, gastrulation occurred by a combination of invagination and epiboly to form a somewhat concave gastrula. Zooxanthellae were present in both endodermal and ectodermal cells of early planulae, but they disappeared from the ectoderm as the planulae matured. In our previous study on two species of Pocillopora, we found that zooxanthellae were localized in eggs as well as in embryos, and that blastomeres containing zooxanthellae later dropped into the blastocoel to become restricted to the endoderm (Hirose et al., 2000). The timing and mechanism of zooxanthella localization and types of gastrulation differed among species belonging to the three genera. These results suggest that zooxanthella localization in the embryos reflects the timing of the determination of presumptive endoderm cells and/or specificity of zooxanthellae toward presumptive endoderm cells.

Effects of U0126 and fibroblast growth factor on gene expression profile in Ciona intestinalis embryos as revealed by microarray analysis

Fibroblast growth factor (FGF) induces the notochord and mesenchyme in ascidian embryos, via extracellular signal-regulated kinase (ERK) that belongs to the mitogen-activated protein kinase (MAPK) family. A cDNA microarray analysis was carried out to identify genes affected by an inhibitor of MAPK/ERK kinase (MEK), U0126, in embryos of the ascidian Ciona intestinalis. Data obtained from the microarray and in situ hybridization suggest that the majority of genes are downregulated by U0126 treatment. Genes that were downregulated in U0126-treated embryos included Ci-Bra and Ci-Twist-like1 that are master regulatory genes of notochord and mesenchyme differentiation, respectively. The plasminogen mRNA was downregulated by U0126 in presumptive endoderm cells. This suggests that a MEK-mediated extracellular signal is necessary for gene expression in tissues whose specification does not depend on cell-to-cell interaction. Among 85 cDNA clusters that were not affected by U0126, 30 showed mitochondria-like mRNA localization in the nerve cord/muscle lineage blastomeres in the equatorial region. The expression level and asymmetric distribution of these mRNA were independent of MEK signaling.

Mosaic and regulative development: two faces of one coin

“There seems to be no more a completely mosaic egg than a completely regulation one” Waddington, Principles of Embryology 1956, Allen and Unwin, p63.

Expression pattern of three putative RNA-binding proteins during early development of the sea urchin Paracentrotus lividus

We report the expression patterns of three transcripts encoding RNA-binding proteins during early development of the Mediterranean sea urchin Paracentrotus lividus. Two of these genes encode KH-domains RNA-binding proteins closely related to the vertebrate neuro-oncological ventral antigen 1 (Nova) and RING Finger and KH-domain (RKHD). The third encodes the sea urchin ortholog of the polypyrimidine tract binding protein (PTB). Zygotic expression of nova and rkhd starts at mesenchyme blastula stage and is restricted to the presumptive endoderm territory. During gastrulation, expression of nova is restricted to the midgut and hindgut, while expression of rkhd become more complex and includes the foregut and hindgut territories as well as previously unknown territories within the ectoderm. PTB is first expressed ubiquitously but starting at the late gastrula stage, then PTB transcripts become highly enriched in the foregut and oral ectoderm. We further report that expression of nova and rkhd in the endomesoderm is under the control of the Wnt/β-catenin pathway and occurs in a cell-autonomous manner while expression of rkhd and PTB in the oral ectoderm is regulated by Nodal signaling.

Dorso/Ventral Genes Are Asymmetrically Expressed and Involved in Germ-Layer Demarcation during Cnidarian Gastrulation

Cnidarians (corals, sea anemones, hydroids, and jellyfish) are a basal taxon closely related to bilaterally symmetrical animals and have been characterized as diploblastic and as radially symmetrical around their longitudinal axis. We show that some orthologs of key bilaterian dorso/ventral (D/V) patterning genes, including the TGFbeta signaling molecules NvDpp and NvBMP5-8 and their antagonist NvChordin, are initially expressed asymmetrically at the onset of gastrulation in the anthozoan sea anemone Nematostella vectensis. Surprisingly, unlike flies and vertebrates, the TGFbeta ligands and their antagonist are colocalized at the onset of gastrulation but then segregate by germ layer as gastrulation proceeds. TGFbeta ligands, their extracellular enhancer, NvTolloid, and components of their downstream signaling pathway (NvSmad1/5 and NvSmad4) are all coexpressed in presumptive endoderm, indicating that only planar TGFbeta signaling operates at these stages. NvChordin expression forms a boundary between TGFbeta-expressing endodermal cells and aboral ectoderm. Manipulation of nuclear beta-catenin localization affects TGFbeta ligand and antagonist expression, suggesting that the ancestral role of the dpp/chordin antagonism during gastrulation may have been in germ-layer segregation and/or epithelial patterning rather than dorsal/ventral patterning.