Oral-aboral Axis Research Articles

Secondary axis specification in sea urchin embryos

AbstractSecondary axis specification is a process that relies on asymmetric nuclearization of transcription factors in flies and vertebrates, such that the crucial factor is nuclear and therefore functional only in cells along one side of the embryo. In vertebrates, this transcription factor is β‐catenin, which is canonically activated downstream from Wnt signals. However, the sea urchin uses asymmetric β‐catenin nuclearization during specification of the primary animal‐vegetal axis, rather than the secondary oral‐aboral (OA) axis. OA specification relies instead on the asymmetric localization of p38 MAPK, a signaling kinase that directly modulates transcription factor activity. A number of genes are expressed in the oral territory downstream from p38, including Nodal and Goosecoid, both of which are associated with secondary axis specification in vertebrate embryos. Because the p38 asymmetry is the earliest known event in the specification of the OA axis, an outstanding question concerns identifying the apparatus upstream from p38 that regulates its asymmetrical activity. Intriguingly, this may be controlled by reactive oxygen species released from the mitochondria, which are asymmetrically distributed about the OA axis.

The Wnt code: cnidarians signal the way

Cnidarians are the simplest metazoans with a nervous system. They are well known for their regeneration capacity, which is based on the restoration of a signalling centre (organizer). Recent work has identified the canonical Wnt pathway in the freshwater polyp Hydra, where it acts in organizer formation and regeneration. Wnt signalling is also essential for cnidarian embryogenesis. In the sea anemone Nematostella vectensis 11 of the 12 known wnt gene subfamilies were identified. Different wnt genes exhibit serial and overlapping expression domains along the oral-aboral axis of the embryo (the 'wnt code'). This is reminiscent of the hox code (cluster) in bilaterian embryogenesis that is, however, absent in cnidarians. It is proposed that the common ancestor of cnidarians and bilaterians invented a set of wnt genes that patterned the ancient main body axis. Major antagonists of Wnt ligands (e.g. Dkk 1/2/4) that were previously known only from chordates, are also present in cnidarians and exhibit a similar conserved function. The unexpectedly high level of genetic complexity of wnt genes evolved in early multi-cellular animals about 650 Myr ago and suggests a radical expansion of the genetic repertoire, concurrent with the evolution of multi-cellularity and the diversification of eumetazoan body plans.

RTK and TGF-β signaling pathways genes in the sea urchin genome

The Receptor Tyrosine kinase (RTK) and TGF-β signaling pathways play essential roles during development in many organisms and regulate a plethora of cellular responses. From the genome sequence of Strongylocentrotus purpuratus, we have made an inventory of the genes encoding receptor tyrosine kinases and their ligands, and of the genes encoding cytokines of the TGF-β superfamily and their downstream components. The sea urchin genome contains at least 20 genes coding for canonical receptor tyrosine kinases. Seventeen of the nineteen vertebrate RTK families are represented in the sea urchin. Fourteen of these RTK among which ALK, CCK4/PTK7, DDR, EGFR, EPH, LMR, MET/RON, MUSK, RET, ROR, ROS, RYK, TIE and TRK are present as single copy genes while pairs of related genes are present for VEGFR, FGFR and INSR. Similarly, nearly all the subfamilies of TGF-β ligands identified in vertebrates are present in the sea urchin genome including the BMP, ADMP, GDF, Activin, Myostatin, Nodal and Lefty, as well as the TGF-β sensu stricto that had not been characterized in invertebrates so far. Expression analysis indicates that the early expression of nodal, BMP2/4 and lefty is restricted to the oral ectoderm reflecting their role in providing positional information along the oral–aboral axis of the embryo. The coincidence between the emergence of TGF-β-related factors such as Nodal and Lefty and the emergence of the deuterostome lineage strongly suggests that the ancestral function of Nodal could have been related to the secondary opening of the mouth which characterizes this clade, a hypothesis supported by functional data in the extant species. The sea urchin genome contains 6 genes encoding TGF-β receptors and 4 genes encoding prototypical Smad proteins. Furthermore, most of the transcriptional activators and repressors shown to interact with Smads in vertebrates have orthologues in echinoderms. Finally, the sea urchin genome contains an almost complete repertoire of genes encoding extracellular modulators of BMP signaling including Chordin, Noggin, Sclerotin, SFRP, Gremlin, DAN and Twisted gastrulation. Taken together, these findings indicate that the sea urchin complement of genes of the RTK and TGF-β signaling pathways is qualitatively very similar to the repertoire present in vertebrates, and that these genes are part of the common genetool kit for intercellular signaling of deuterostomes.

Molecular evidence for deep evolutionary roots of bilaterality in animal development.

Nearly all metazoans show signs of bilaterality, yet it is believed the bilaterians arose from radially symmetric forms hundreds of millions of years ago. Cnidarians (corals, sea anemones, and "jellyfish") diverged from other animals before the radiation of the Bilateria. They are diploblastic and are often characterized as being radially symmetrical around their longitudinal (oral-aboral) axis. We have studied the deployment of orthologs of a number of family members of developmental regulatory genes that are expressed asymmetrically during bilaterian embryogenesis from the sea anemone, Nematostella vectensis. The secreted TGF-beta genes Nv-dpp, Nv-BMP5-8, six TGF-beta antagonists (NvChordin, NvNoggin1, NvNoggin2, NvGremlin, NvFollistatin, and NvFollistatin-like), the homeodomain proteins NvGoosecoid (NvGsc) and NvGbx, and the secreted guidance factor, NvNetrin, were studied. NvDpp, NvChordin, NvNoggin1, NvGsc, and NvNetrin are expressed asymmetrically along the axis perpendicular to the oral-aboral axis, the directive axis. Furthermore, NvGbx, and NvChordin are expressed in restricted domains on the left and right sides of the body, suggesting that the directive axis is homologous with the bilaterian dorsal-ventral axis. The asymmetric expression of NvNoggin1 and NvGsc appear to be maintained by the canonical Wnt signaling pathway. The asymmetric expression of NvNoggin1, NvNetrin, and Hox orthologs NvAnthox7, NvAnthox8, NvAnthox1a, and NvAnthox6, in conjunction with the observation that NvNoggin1 is able to induce a secondary axis in Xenopus embryos argues that N. vectensis could possess antecedents of the organization of the bilaterian central nervous system.

Asymmetric expression of the BMP antagonists chordin and gremlin in the sea anemone Nematostella vectensis: Implications for the evolution of axial patterning

The evolutionary origin of the anterior–posterior and the dorsoventral body axes of Bilateria is a long-standing question. It is unclear how the main body axis of Cnidaria, the sister group to the Bilateria, is related to the two body axes of Bilateria. The conserved antagonism between two secreted factors, BMP2/4 (Dpp in Drosophila) and its antagonist Chordin (Short gastrulation in Drosophila) is a crucial component in the establishment of the dorsoventral body axis of Bilateria and could therefore provide important insight into the evolutionary origin of bilaterian axes. Here, we cloned and characterized two BMP ligands, dpp and GDF5-like as well as two secreted antagonists, chordin and gremlin, from the basal cnidarian Nematostella vectensis. Injection experiments in zebrafish show that the ventralizing activity of NvDpp mRNA is counteracted by NvGremlin and NvChordin, suggesting that Gremlin and Chordin proteins can function as endogenous antagonists of NvDpp. Expression analysis during embryonic and larval development of Nematostella reveals asymmetric expression of all four genes along both the oral–aboral body axis and along an axis perpendicular to this one, the directive axis. Unexpectedly, NvDpp and NvChordin show complex and overlapping expression on the same side of the embryo, whereas NvGDF5-like and NvGremlin are both expressed on the opposite side. Yet, the two pairs of ligands and antagonists only partially overlap, suggesting complex gradients of BMP activity along the directive axis but also along the oral–aboral axis. We conclude that a molecular interaction between BMP-like molecules and their secreted antagonists was already employed in the common ancestor of Cnidaria and Bilateria to create axial asymmetries, but that there is no simple relationship between the oral–aboral body axis of Nematostella and one particular body axis of Bilateria.

Animal pole determinants define oral–aboral axis polarity and endodermal cell-fate in hydrozoan jellyfish Podocoryne carnea

Cnidarians, in contrast with bilaterians, are generally considered to exhibit radial symmetry around a single body axis (oral–aboral) throughout their life-cycles. We have investigated how the oral–aboral axis is established in the hydrozoan jellyfish Podocoryne carnea. Vital labeling experiments showed that the oral end of the blastula derives from the animal pole region of the egg as has been demonstrated for other cnidarian species. Gastrulation is restricted to the oral pole such that the oral 20% of blastula cells give rise to endoderm. Unexpectedly, bisection experiments at the 8-cell stage showed that animal regions are able to develop into normally polarized larvae, but that vegetal (aboral) blastomeres completely fail to develop endoderm or to elongate. These vegetal-derived larvae also failed to polarize, as indicated by a lack of oral-specific RFamide-positive nerve cells and a disorganized tyrosinated tubulin-positive nerve net. A different result was obtained following bisection of the late blastula stage: aboral halves still lacked the capacity to develop endoderm but retained features of axial polarity including elongation of the larva and directional swimming. These results demonstrate for the first time in a cnidarian the presence of localized determinants responsible for axis determination and endoderm formation at the animal pole of the egg. They also show that axial polarity and endoderm formation are controlled by separable pathways after the blastula stage.

P38 MAPK is essential for secondary axis specification and patterning in sea urchin embryos

Most eggs in the animal kingdom establish a primary, animal-vegetal axis maternally, and specify the remaining two axes during development. In sea urchin embryos, the expression of Nodal on the oral (ventral) side of the embryo is the first known molecular determinant of the oral-aboral axis (the embryonic dorsoventral axis), and is crucial for specification of the oral territory. We show that p38 MAPK acts upstream of Nodal and is required for Nodal expression in the oral territory. p38 is uniformly activated early in development, but, for a short interval at late blastula stage, is asymmetrically inactivated in future aboral nuclei. Experiments show that this transient asymmetry of p38 activation corresponds temporally to both oral specification and the onset of oral Nodal expression. Uniform inhibition of p38 prevents Nodal expression and axis specification, resulting in aboralized embryos. Nodal and its target Gsc each rescue oral-aboral specification and patterning when expressed asymmetrically in p38-inhibited embryos. Thus, our results indicate that p38 is required for oral specification through its promotion of Nodal expression in the oral territory.

Developmental Potential of Small Micromeres in Sea Urchin Embryos

The large micromeres (lMics) of echinoid embryos are reported to have distinct potentials with regard to inducing endo-mesoderm and autonomous differentiation into skeletogenic cells. However, the developmental potential of small micromeres (sMics), the sibling of lMics, has not been clearly demonstrated. In this study we produced chimeric embryos from an animal cap recombined with various numbers of sMics, in order to investigate the developmental potential of sMics in the sea urchin Hemicentrotus pulcherrimus and the sand dollar Scaphechinus mirabilis. We found that sMics of H. pulcherrimus had weak potential for inducing presumptive ectoderm cells to form endo-mesoderm structures. The inducing potential of ten sMics was almost equivalent to that of one lMic. The sMics also had the potential to differentiate autonomously into skeletogenic cells. Conversely, the sMics of S. mirabilis did not show either inductive or skeletogenic differentiation potential. The sMics of both species had the potential to induce oral-aboral axis establishment. These results suggest that the potential for sMics to differentiate into skeletogenic cells and for inducing the presumptive ectoderm to differentiate into endomesoderm differs across species, while the potential of sMics to induce the oral-aboral axis is conserved among species.

From larval bodies to adult body plans: patterning the development of the presumptive adult ectoderm in the sea urchin larva

Echinoderms are unique among bilaterians for their derived, nonbilateral adult body plan. Their radial symmetry emerges from the bilateral larval body plan by the establishment of a new axis, the adult oral-aboral axis, involving local mesoderm-ectoderm interactions. We examine the mechanisms underlying this transition in the direct-developing sea urchin Heliocidaris erythrogramma. Adult ectoderm arises from vestibular ectoderm in the left vegetal quadrant. Inductive signals from the left coelom are required for adult ectodermal development but not for initial vestibule formation. We surgically removed gastrula archenteron, making whole-ectoderm explants, left-, right-, and animal-half ectoderm explants, and recombinants of these explants with left coelom. Vestibule formation was analyzed morphologically and with radioactive in situ hybridization with HeET-1, an ectodermal marker. Whole ectodermal explants in the absence of coelom developed vestibules on the left side or ventrally but not on the right side, indicating that left-right polarity is ectoderm autonomous by the gastrula stage. However, right-half ectodermal explants robustly formed vestibules that went on to form adult structures when recombined with the left coelom, indicating that the right side retains vestibule-forming potential that is normally suppressed by signals from the left-side ectoderm. Animal-half explants formed vestibules only about half the time, demonstrating that animal-vegetal axis determination occurs earlier. However, when combined with the left coelom, animal-half ectoderm always formed a vestibule, indicating that the left coelom can induce vestibule formation. This suggests that although coelomic signals are not required for vestibule formation, they may play a role in coordinating the coelom-vestibule interaction that establishes the adult oral-aboral axis.

Sexual reproduction of the solitary sunset cup coral Leptopsammia pruvoti (Scleractinia: Dendrophylliidae) in the Mediterranean. 1. Morphological aspects of gametogenesis and ontogenesis

Information on the reproduction in scleractinian solitary corals and in those living in temperate zones is notably scant. Leptopsammia pruvoti is a solitary coral living in the Mediterranean Sea and along Atlantic coasts from Portugal to southern England. This coral lives in shaded habitats, from the surface to 70 m in depth, reaching population densities of >17,000 individuals m−2. In this paper, we discuss the morphological aspects of sexual reproduction in this species. In a separate paper, we report the quantitative data on the annual reproductive cycle and make an interspecific comparison of reproductive traits among Dendrophylliidae aimed at defining different reproductive strategies. The present study on L. pruvoti is the first in-depth investigation of the reproductive biology of a species of this genus. As expected for a member of the family Dendrophylliidae, L. pruvoti is a gonochoric and brooding coral. The gastrodermal tissue of the gametogenetic mesenteries we examined was swollen and granular, which led us to hypothesize that interstitial cells could have a trophic function favoring gametogenesis. Undifferentiated germ cells arose in the gastrodermis and subsequently migrated to the mesoglea, where they completed gametogenesis. During spermary development, spermary diameter increased from a minimum of 14 μm during the immature stages to a maximum of 410 μm during the mature stages. As oogenesis progressed, we observed a gradual reduction in the nucleus to cytoplasm ratio due to the steady synthesis of yolk. During the final stages of oogenesis, after having migrated to the extreme periphery of the oocyte and having firmly adhered to the oolemma, the nucleus became indented, assuming a sickle or dome shape. We can hypothesize that the nucleus’ migration and change of shape may have to do with facilitating fertilization and determining the future embryonic axis. During oogenesis, oocyte diameter increased from a minimum of 20 μm during the immature stage to a maximum of 680 μm when mature. Embryogenesis took place in the coelenteron. We did not see any evidence that even hinted at the formation of a blastocoel; embryonic development proceeded via stereoblastulae with superficial cleavage. Gastrulation took place by delamination. Early and late embryos had diameters of 204–724 μm and 290–736 μm, respectively. When released, the larvae had completed ontogenesis and swam by a ciliary movement with the aboral pole at the anterior, their shape varied from spherical to cylindrical (in the latter the oral–aboral axis measured 695–1,595 μm and the transversal one measured 267–633 μm).

Oral–aboral axis specification in the sea urchin embryo: II. Mitochondrial distribution and redox state contribute to establishing polarity in Strongylocentrotus purpuratus

The initial asymmetry that specifies the oral–aboral (OA) axis of the sea urchin embryo has long been a mystery. It was shown previously that OA polarity can be entrained in embryos by imposing a respiratory asymmetry, with the most oxidizing side of the embryo tending to develop as the oral pole. This suggests that one of the earliest observable asymmetries along the incipient OA axis, a redox gradient established by a higher density and/or activity of mitochondria on the prospective oral side of the embryo, might play a causal role in establishing the axis. Here, we examine the origin and functional significance of this early redox gradient. Using MitoTracker Green, we show that mitochondria are asymmetrically distributed in the unfertilized egg of Strongylocentrotus purpuratus, and that the polarity of the maternal asymmetry is maintained in the zygote. Vital staining indicates that the side of the embryo that inherits the highest density of mitochondria tends to develop into the oral pole. This correlation holds when mitochondria are redistributed by centrifugation of eggs or by transfer of purified mitochondria into zygotes, indicating that an asymmetric mitochondrial distribution can entrain OA polarity, possibly through effects on intracellular redox state. In support of this possibility, we find that specification of oral ectoderm is suppressed when embryos are cultured under hypoxic conditions that enforce a relatively reducing redox state. This effect is reversed by overexpression of nodal, an early zygotic marker of oral specification whose localized expression suffices to organize the entire OA axis, indicating that redox state is upstream of nodal expression. We therefore propose that a threshold level of intracellular oxidation is required to effectively activate nodal, and that precocious attainment of this threshold within the blastomeres containing the highest density of mitochondria results in asymmetric nodal activity and consequent specification of the OA axis.

Nodal and BMP2/4 Signaling Organizes the Oral-Aboral Axis of the Sea Urchin Embryo

In the sea urchin embryo, the oral-aboral axis is specified after fertilization by mechanisms that are largely unknown. We report that early sea urchin embryos express Nodal and Antivin in the presumptive oral ectoderm and demonstrate that these genes control formation of the oral-aboral axis. Overexpression of nodal converted the whole ectoderm into oral ectoderm and induced ectopic expression of the orally expressed genes goosecoid, brachyury, BMP2/4, and antivin. Conversely, when the function of Nodal was blocked, by injection of an antisense Morpholino oligonucleotide or by injection of antivin mRNA, neither the oral nor the aboral ectoderm were specified. Injection of nodal mRNA into Nodal-deficient embryos induced an oral-aboral axis in a largely non-cell-autonomous manner. These observations suggest that the mechanisms responsible for patterning the oral-aboral axis of the sea urchin embryo may share similarities with mechanisms that pattern the dorsoventral axis of other deuterostomes.

The origins of axial patterning in the metazoa: how old is bilateral symmetry?

Bilateral symmetry is a hallmark of the Bilateria. It is achieved by the intersection of two orthogonal axes of polarity: the anterior-posterior (A-P) axis and the dorsal-ventral (D-V) axis. It is widely thought that bilateral symmetry evolved in the common ancestor of the Bilateria. However, it has long been known that members of the phylum Cnidaria, an outgroup to the Bilateria, also exhibit bilateral symmetry. Recent studies have examined the developmental expression of axial patterning genes in members of the phylum Cnidaria. Hox genes play a conserved role in patterning the A-P axis of bilaterians. Hox genes are expressed in staggered axial domains along the oral-aboral axis of cnidarians, suggesting that Hox patterning of the primary body axis was already present in the cnidarian-bilaterian ancestor. Dpp plays a conserved role patterning the D-V axis of bilaterians. Asymmetric expression of dpp about the directive axis of cnidarians implies that this patterning system is similarly ancient. Taken together, these result imply that bilateral symmetry had already evolved before the Cnidaria diverged from the Bilateria.

An ancient role for nuclear beta-catenin in the evolution of axial polarity and germ layer segregation.

The human oncogene beta-catenin is a bifunctional protein with critical roles in both cell adhesion and transcriptional regulation in the Wnt pathway. Wnt/beta-catenin signalling has been implicated in developmental processes as diverse as elaboration of embryonic polarity, formation of germ layers, neural patterning, spindle orientation and gap junction communication, but the ancestral function of beta-catenin remains unclear. In many animal embryos, activation of beta-catenin signalling occurs in blastomeres that mark the site of gastrulation and endomesoderm formation, raising the possibility that asymmetric activation of beta-catenin signalling specified embryonic polarity and segregated germ layers in the common ancestor of bilaterally symmetrical animals. To test whether nuclear translocation of beta-catenin is involved in axial identity and/or germ layer formation in 'pre-bilaterians', we examined the in vivo distribution, stability and function of beta-catenin protein in embryos of the sea anemone Nematostella vectensis (Cnidaria, Anthozoa). Here we show that N. vectensis beta-catenin is differentially stabilized along the oral-aboral axis, translocated into nuclei in cells at the site of gastrulation and used to specify entoderm, indicating an evolutionarily ancient role for this protein in early pattern formation.

Coquillette, a sea urchin T-box gene of the Tbx2 subfamily, is expressed asymmetrically along the oral–aboral axis of the embryo and is involved in skeletogenesis

Transcription factors of the T-domain family regulate many developmental processes. We have isolated from the sea urchin a new member of the Tbx2 subfamily: coquillette. Coquillette has a late zygotic expression whose localization is dynamic: at the blastula stage it is restricted to the aboral side of most of the presumptive ectoderm and endoderm territories and from gastrulation on, to the aboral-most primary mesenchyme cells. Perturbation of coquillette function delays gastrulation and strongly disorganizes the skeleton of the larva. Coquillette is sensitive to alteration of the oral–aboral (OA) axis and we identify goosecoid, which controls oral and aboral fates in the ectoderm, as a probable upstream regulator. Coquillette appears to be an integral part of the patterning system along the OA axis.

Head regeneration in Hydra.

Hydra, a primitive metazoan, has a simple structure consisting of a head, body column, and foot aligned along a single oral-aboral axis. The body column has a high capacity for regeneration of both the head and foot. Because of the tissue dynamics that take place in adult Hydra, the processes governing axial patterning are continuously active to maintain the form of the animal. Regeneration in hydra is morphallactic and closely related to these axial patterning processes. As might be expected, analysis at the molecular level indicates that the same set of genes are involved in head regeneration and the maintenance of the head in the context of the tissue dynamics of the adult. The genes analyzed so far play roles in axial patterning processes in bilaterians.

Expression of the ctenophore Brain Factor 1 forkhead gene ortholog (ctenoBF-1) mRNA is restricted to the presumptive mouth and feeding apparatus: implications for axial organization in the Metazoa.

Ctenophores are thoroughly modern animals whose ancestors are derived from a separate evolutionary branch than that of other eumetazoans. Their major longitudinal body axis is the oral-aboral axis. An apical sense organ, called the apical organ, is located at the aboral pole and contains a highly innervated statocyst and photodetecting cells. The apical organ integrates sensory information and controls the locomotory apparatus of ctenophores, the eight longitudinal rows of ctene/comb plates. In an effort to understand the developmental and evolutionary organization of axial properties of ctenophores we have isolated a forkhead gene from the Brain Factor 1 (BF-1) family. This gene, ctenoBF-1, is the first full-length nuclear gene reported from ctenophores. This makes ctenophores the most basal metazoan (to date) known to express definitive forkhead class transcription factors. Orthologs of BF-1 in vertebrates, Drosophila, and Caenorhabditis elegans are expressed in anterior neural structures. Surprisingly, in situ hybridizations with ctenoBF-1 antisense riboprobes show that this gene is not expressed in the apical organ of ctenophores. CtenoBF-1 is expressed prior to first cleavage. Transcripts become localized to the aboral pole by the 8-cell stage and are inherited by ectodermal micromeres generated from this region at the 16- and 32-cell stages. Expression in subsets of these cells persists and is seen around the edge of the blastopore (presumptive mouth) and in distinct ectodermal regions along the tentacular poles. Following gastrulation, stomodeal expression begins to fade and intense staining becomes restricted to two distinct domains in each tentacular feeding apparatus. We suggest that the apical organ is not homologous to the brain of bilaterians but that the oral pole of ctenophores corresponds to the anterior pole of bilaterian animals.

Sexual reproduction in the Mediterranean solitary coral Balanophyllia europaea (Scleractinia, Dendrophylliidae)

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 229:83-94 (2002) - doi:10.3354/meps229083 Sexual reproduction in the Mediterranean solitary coral Balanophyllia europaea (Scleractinia, Dendrophylliidae) Stefano Goffredo*, Silvia Arnone, Francesco Zaccanti Dipartimento di Biologia Evoluzionistica Sperimentale, Università degli Studi di Bologna, via F. Selmi 3, 40126 Bologna, Italy *E-mail: sgoff@tin.it ABSTRACT: Balanophyllia europaea (Risso, 1826) is a common scleractinian coral living on subtidal rocky substrates in the Mediterranean Sea. The annual sexual reproductive cycle of this species is studied in specimens from an area near Leghorn (Tuscany, Italy). This study represents the first report of a reproductive cycle in a Mediterranean scleractinian coral. B. europaea is a simultaneous hermaphrodite and brooder. Polyp size at sexual maturity occurs at approximately 3 yr of age (6 to 10 mm maximum oral disc diameter). Observation of hermaphroditic polyps shows several significant differences in the distribution of spermaries and oocytes along the oral-aboral axis. Mean distance of spermaries from the oral pole is less than that of oocytes. Also, large mature spermaries tend to line up towards the mouth, while large mature oocytes tend to line up towards the polyp base. This differential gamete distribution could result in a barrier to self-fertilization, as it reduces encounters between gametes of the opposite sex produced by the same individual. Testes require 1 yr to reach maturity with a maximum spermary diameter of 500 µm. Oogenesis lasts 2 yr with a maximum oocyte diameter of 1400 µm. Fecundity varies significantly with polyp size. The rate of gonadal development increases significantly during January and February, fertilization takes place in March to June and planulation in August and September. Released larvae have a maximum diameter of 2000 µm. The annual cycle of photoperiod and water temperature appear to coincide with the reproductive cycle in B. europaea. KEY WORDS: Coral · Balanophyllia europaea · Reproduction · Gametogenesis · Fecundity · Mediterranean Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 229. Online publication date: March 20, 2002 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2002 Inter-Research.

Molecular patterning along the sea urchin animal-vegetal axis

The molecular regulatory mechanisms underlying primary axis formation during sea urchin development have recently been identified. Two opposing maternally inherited systems, one animalizing and one vegetalizing, set up the animal-vegetal (A-V) axis. The vegetal system relies in part on the Wnt-beta-catenin-Tcf/Lef signaling pathway and the animal system is based on a cohort of animalizing transcription factors that includes members of the Ets and Sox classes. The two systems autonomously define three zones of cell-type specification along the A-V axis. The vegetalmost zone gives rise to the skeletogenic mesenchyme lineage; the animalmost zone gives rise to ectoderm; and the zone in which the two systems overlap generates endoderm, secondary mesenchyme, and ectoderm. Patterning along the A-V also depends on cellular interactions involving Wnt, Notch, and BMP signaling. We discuss how these systems impact the formation of the second axis, the oral-aboral axis; how they connect to later developmental events; and how they lead to cell-type-specific gene expression via cis-regulatory networks associated with transcriptional control regions. We also discuss how these systems may confer on the embryo its spectacular regulatory capacity to replace missing parts.

Sea urchin goosecoid function links fate specification along the animal-vegetal and oral-aboral embryonic axes.

We have identified a single homolog of goosecoid, SpGsc, that regulates cell fates along both the animal-vegetal and oral-aboral axes of sea urchin embryos. SpGsc mRNA is expressed briefly in presumptive mesenchyme cells of the approximately 200-cell blastula and, beginning at about the same time, accumulates in the presumptive oral ectoderm through pluteus stage. Loss-of-function assays with morpholine-substituted antisense oligonucleotides show that SpGsc is required for endoderm and pigment cell differentiation and for gastrulation. These experiments and gain-of-function tests by mRNA injection show that SpGsc is a repressor that antagonizes aboral ectoderm fate specification and promotes oral ectoderm differentiation. We show that SpGsc competes for binding to specific cis elements with SpOtx, a ubiquitous transcription activator that promotes aboral ectoderm differentiation. Moreover, SpGsc represses transcription in vivo from an artificial promoter driven by SpOtx. As SpOtx appears long before SpGsc transcription is activated, we propose that SpGsc diverts ectoderm towards oral fate by repressing SpOtx target genes. Based on the SpGsc-SpOtx example and other available data, we propose that ectoderm is first specified as aboral by broadly expressed activators, including SpOtx, and that the oral region is subsequently respecified by the action of negative regulators, including SpGsc. Accumulation of SpGsc in oral ectoderm depends on cell-cell interactions initiated by nuclear beta-catenin function, which is known to be required for specification of vegetal tissues, because transcripts are undetectable in dissociated or in cadherin mRNA-injected embryos. This is the first identified molecular mechanism underlying the known dependence of oral-aboral ectoderm polarity on intercellular signaling.