Specification of cell fate in the sea urchin embryo: summary and some proposed mechanisms

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.

PI3K inhibitors block skeletogenesis but not patterning in sea urchin embryos

Skeletogenesis in the sea urchin embryo is a simple model of biomineralization, pattern formation, and cell-cell communication during embryonic development. The calcium carbonate skeletal spicules are secreted by primary mesenchyme cells (PMCs), but the skeletal pattern is dictated by the embryonic ectoderm. Although the process of skeletogenesis is well characterized, there is little molecular understanding of the basis of patterning within this system. In this study, we examined the contribution of phosphatidylinositide 3-kinase (PI3K)-mediated signaling to the skeletogenic process in sea urchin embryos by using the well-established PI3K inhibitors LY294002 and wortmannin. Our results show that PI3K inhibitors specifically and reversibly block skeletogenesis, and that this blockade occurs within the PMCs rather than in the ectoderm, because the inhibitors block spiculogenesis in cultured micromeres. Our results are consistent with a model in which PI3K signaling is required, not for pattern sensing or interpretation but rather for the biomineralization process itself in the sea urchin embryo.

Expression of tryptophan 5-hydroxylase gene during sea urchin neurogenesis and role of serotonergic nervous system in larval behavior

Tryptophan 5-hydroxylase (TPH) is the rate-limiting enzyme in the biosynthesis of serotonin. cDNA cloning of TPH was carried out, and the occurrence of spatiotemporal transcription of TPH message was examined in larvae of the sea urchin, Hemicentrotus pulcherrimus (HpTPH), with in situ hybridization by using the tyramide signal amplification (TSA) technique and Northern hybridization. Based on deduced amino acids sequence of HpTPH, phylogenetically sea urchin locates at the closest position to vertebrates among invertebrates, and HpTPH had common conserved sequences in a catalytic domain. Initiation of HpTPH transcription occurred at the late gastrula stage exclusively in serotonin cells of apical ganglion (SAG) that was composed of a cluster of HpTPH-positive cells and the negative cells in between. In situ hybridization showed that the mRNA expression pattern was similar to the immunohistochemical localization of serotonin cells reported before (Bisgrove and Burke [1986] Dev. Growth Differ. 28:557-569; Yaguchi et al. [2000] Dev. Growth Differ. 42:479-488). p-Chlorophenylalanine (CPA), an irreversible inhibitor of TPH activity, considerably decreased serotonin content in the serotonin cells, whereas the HpTPH expression pattern and timing, and the extension of neurofibers from SAG cells were apparently unaffected, suggesting CPA exclusively perturbed synthesis of serotonin but not nervous system organization. CPA-treated larvae did not swim, despite the occurrence of ciliary beating in culture chamber, suggesting that proper serotonin synthesis is necessary for normal swimming of the larvae.

Tight regulation of SpSoxB factors is required for patterning and morphogenesis in sea urchin embryos

Previous studies in sea urchin embryos have demonstrated that nuclearization of beta-catenin is essential for initial steps in the specification of endoderm and mesenchyme, which are derived from vegetal blastomeres. This process begins at the 4th and extends through the 9th cleavage stage, an interval in which the SpSoxB1 transcription regulator is downregulated by beta-catenin-dependent gene products that include the transcription repressor SpKrl. These observations raise the possibility that SpSoxB1 removal is required to allow vegetal development to proceed. Here we show that elevated and ectopic expression of this factor suppresses differentiation of all vegetal cell types, a phenotype that is very similar to that caused by the suppression of beta-catenin nuclear function by cadherin overexpression. Suppression of vegetal fates involves interference at the protein-protein level because a mutation of SpSoxB1 that prevents its binding to DNA does not significantly reduce this activity. Reduction in SpSoxB1 level results in elevated TCF/Lef-beta-catenin-dependent expression of a luciferase reporter gene in vivo, indicating that in the normal embryo this protein suppresses the primary vegetal signaling mechanism that is required for specification of mesenchyme and endoderm. Surprisingly, normal expression of SpSoxB1 is required for gastrulation and endoderm differentiation, as shown by both morpholino-mediated translational interference and expression of a dominant negative protein. Similar gain-of-function and loss-of-function assays of a closely related factor, SpSoxB2, demonstrate that it, too, is required for gastrulation and that its overexpression can suppress vegetal development. However, significant phenotypic differences are apparent in the two perturbations, indicating that SpSoxB1 and SpSoxB2 have at least some distinct developmental functions. The results of all these studies support a model in which the concentration of SpSoxB factors must be tightly regulated along the animal-vegetal axis of the early sea urchin embryo to allow beta-catenin-dependent specification of endoderm and mesenchyme cell fates as well as to activate target genes required for gastrulation.

Reproduction: widespread cloning in echinoderm larvae

Asexual reproduction by free-living invertebrate larvae is a rare and enigmatic phenomenon and, although it is known to occur in sea stars and brittle stars, it has not been detected in other echinoderms despite more than a century of intensive study. Here we describe spontaneous larval cloning in three species from two more echinoderm classes: a sea cucumber (Holothuroidea), a sand dollar and a sea urchin (Echinoidea). Larval cloning may therefore be an ancient ability of echinoderms and possibly of deutero-stomes - the group that includes echinoderms, acorn worms, sea squirts and vertebrates.

Spdeadringer, a sea urchin embryo gene required separately in skeletogenic and oral ectoderm gene regulatory networks

The Spdeadringer (Spdri) gene encodes an ARID-class transcription factor not previously known in sea urchin embryos. We show that Spdri is a key player in two separate developmental gene regulatory networks (GRNs). Spdri is expressed in a biphasic manner, first, after 12 h and until ingression in the skeletogenic descendants of the large micromeres; second, after about 20 h in the oral ectoderm, where its transcripts remain present at 30-50 mRNA molecules/cell far into development. In both territories, the periods of Spdri expression follow prior territorial specification events. The functional significance of each phase of expression was assessed by determining the effect of an alphaSpdri morpholino antisense oligonucleotide (MASO) on expression of 17 different mesodermal genes, 8 different oral ectoderm genes, and 18 other genes expressed specifically during endomesoderm specification. These effects were measured by quantitative PCR, supplemented by whole-mount in situ hybridization and morphological observations. Spdri is shown to act in the micromere descendants in the pathways that result in the expression of batteries of terminal skeletogenic genes. But, in the oral ectoderm, the same gene participates in the central GRN controlling oral ectoderm identity. Spdri is linked in the oral ectoderm GRN with several other genes encoding transcriptional regulators that are expressed specifically in various regions of the oral ectoderm. If its expression is blocked by treatment with alphaSpdri MASO, oral-specific features disappear and expression of the aboral ectoderm marker spec1 encompasses the whole of the ectoderm. In addition to disappearance of the oral ectoderm, morphological consequences of alphaSpdri MASO treatment include failure of spiculogenesis and of correct primary mesenchyme cell (pmc) patterning in the postgastrular embryo, and also failure of gastrulation. To further analyze these phenotypes, chimeric embryos were constructed consisting of two labeled micromeres combined with micromereless 4th cleavage host embryos; either the micromeres or the hosts contained alphaSpdri MASO. These experiments showed that, while Spdri expression is required autonomously for expression of skeletogenic genes prior to ingression, complete skeletogenesis also requires the expression of oral ectoderm patterning information. Presentation of this information on the oral side of the blastocoel in turn depends on Spdri expression in the oral ectoderm. Failure of gastrulation is not due to indirect interference with endomesodermal specification per se, since all endomesodermal genes tested function normally in alphaSpdri MASO embryos. Part of its cause is interference by alphaSpdri MASO with a late signaling function on the part of the micromere descendants that is needed to complete clearance of the Soxb1 repressor of gastrulation from the prospective endoderm, but in addition there is a nonautonomous oral ectoderm effect.

Regulatory punctuated equilibrium and convergence in the evolution of developmental pathways in direct-developing sea urchins

We made hybrid crosses between closely and distantly related sea urchin species to test two hypotheses about the evolution of gene regulatory systems in the evolution of ontogenetic pathways and larval form. The first hypothesis is that gene regulatory systems governing development evolve in a punctuational manner during periods of rapid morphological evolution but are relatively stable over long periods of slow morphological evolution. We compared hybrids between direct and indirect developers from closely and distantly related families. Hybrids between eggs of the direct developer Heliocidaris erythrogramma and sperm of the 4-million year distant species H. tuberculata, an indirect developer, restored feeding larval structures and paternal gene expression that were lost in the evolution of the direct-developing maternal parent. Hybrids resulting from the cross between eggs of H. erythrogramma and sperm of the 40-million year distant indirect-developer Pseudoboletia maculata are strikingly similar to hybrids between the congeneric hybrids. The marked similarities in ontogenetic trajectory and morphological outcome in crosses of involving either closely or distantly related indirect developing species indicates that their regulatory mechanisms interact with those of H. erythrogramma in the same way, supporting remarkable conservation of molecular control pathways among indirect developers. Second, we tested the hypothesis that convergent developmental pathways in independently evolved direct developers reflect convergence of the underlying regulatory systems. Crosses between two independently evolved direct-developing species from two 70-million year distant families, H. erythrogramma and Holopneustes purpurescens, produced harmoniously developing hybrid larvae that maintained the direct mode of development and did not exhibit any obvious restoration of indirect-developing features. These results are consistent with parallel evolution of direct-developing features in these two lineages.

Mechanical induction of Twist in the Drosophila foregut/stomodeal primordium

BACKGROUND

Morphogenetic movements are closely regulated by the expression of developmental genes. Here I examine whether developmental gene expression can in turn be mechanically regulated by morphogenetic movements. I have analyzed the effects of mechanical stress on the expression of Twist, which is normally expressed only in the most ventral cells of the cellular blastoderm embryo under the control of the Dorsal morphogen gradient. At embryogenesis gastrulation (stage 7), Twist is also expressed in the anterior foregut and stomodeal primordia.

RESULTS

Submitting the early Drosophila embryo to a transient 10% uniaxial lateral deformation induces the ectopic expression of Twist around the entire dorsal-ventral axis and results in the ventralization of the embryo. This induction is independent of the Dorsal gradient and is triggered by mechanically induced Armadillo nuclear translocation. I also show that Twist is not expressed in the anterior foregut and stomodeal primordia at stage 7 in mutants that block the morphogenetic movement of germ-band extension. Because I can rescue the mutants with gentle compression of these cells, my interpretation is that the stomodeal-cell compression normally caused by the germ-band extension induces the expression of Twist. Correspondingly, laser ablation of dorsal cells in wild-type embryos relaxes stomodeal cell compression and reduces Twist expression in the stomodeal primordium. I also demonstrate that the induction of Twist in these cells depends on the nuclear translocation of Armadillo.

CONCLUSIONS

I propose that anterior-gut formation is mechanically induced by the movement of germ-band extension through the induction of Twist expression in stomodeal cells.

Expression of AmKrox, a starfish ortholog of a sea urchin transcription factor essential for endomesodermal specification

The sea urchin zinc finger transcription factor SpKrox is a regulatory gene that functions early in endomesodermal specification. We report here the cloning and expression of an ortholog of this gene, AmKrox, from the starfish Asterina miniata. The echinoderm Krox proteins belong to a class of transcription factors that includes the vertebrate Blimp-1 proteins and two putative insect proteins. AmKrox is expressed in a ring around the vegetal pole of the blastula. During gastrulation, expression is detected surrounding the blastopore and in the posterior archenteron. In the early bipinnaria larva transcripts are detected in the midgut and hindgut. Despite differences in early development of sea urchins and starfish, the expression of these Krox transcription factors is highly conserved.

Expression of a gene encoding a Gata transcription factor during embryogenesis of the starfish Asterina miniata

The sea urchin zinc finger transcription factor SpGatae is a major activator of endomesoderm-specific regulatory genes. We have cloned the ortholog of this gene, AmGatae, from a distantly related echinoderm, the starfish Asterina miniata. Expression of AmGatae is first detected in a ring around the vegetal pole of the blastula. During gastrulation, transcripts are detected surrounding the blastopore, in the posterior archenteron and more faintly in the anterior mesoderm of the archenteron. In early bipinnaria larva, expression is localized to the midgut and hindgut and to the developing coelomic pouches. These observations show that despite differences in the early specification processes of the endomesoderm in starfish and sea urchins, gatae factors are expressed very similarly in these two taxa.

Patterning the sea urchin embryo: gene regulatory networks, signaling pathways, and cellular interactions

We discuss steps in the specification of major tissue territories of the sea urchin embryo that occur between fertilization and hatching blastula stage and the cellular interactions required to coordinate morphogenetic processes that begin after hatching. We review evidence that has led to new ideas about how this embryo is initially patterned: (1) Specification of most of the tissue territories is not direct, but proceeds gradually by progressive subdivision of broad, maternally specified domains that depend on opposing gradients in the ratios of animalizing transcription factors (ATFs) and vegetalizing (beta-catenin) transcription factors; (2) the range of maternal nuclear beta-catenin extends further than previously proposed, that is, into the animal hemisphere, where it programs many cells to adopt early aboral ectoderm characteristics; (3) cells at the extreme animal pole constitute a unique ectoderm region, lacking nuclear beta-catenin; (4) the pluripotential mesendoderm is created by the combined outputs of ATFs and nuclear beta-catenin, which initially overlap in the macromeres, and by an undefined early micromere signal; (5) later micromere signals, which activate Notch and Wnt pathways, subdivide mesendoderm into secondary mesenchyme and endoderm; and (6) oral ectoderm specification requires reprogramming early aboral ectoderm at about the hatching blastula stage. Morphogenetic processes that follow initial fate specification depend critically on continued interactions among cells in different territories. As illustrations, we discuss the regulation of (1) the ectoderm/endoderm boundary, (2) mesenchyme positioning and skeletal growth, (3) ciliated band formation, and (4) several suppressive interactions operating late in embryogenesis to limit the fates of multipotent cells.

The gene encoding the sea urchin complement protein, SpC3, is expressed in embryos and can be upregulated by bacteria

Sea urchins have an innate immune response that functions in the absence of adaptive capabilities. It is mediated, in part, by components of the complement system, an important subsystem of the innate response in mammals. A homologue of complement C3, SpC3, has been identified in adult Strongylocentrotus purpuratus and is expressed in coelomocytes. In this study, transcript levels from the gene, Sp064, which encodes SpC3, were examined in developing embryos and found to be present in unfertilized eggs and throughout embryogenesis with a peak in transcript levels just prior to and during gastrulation. In addition, continuous exposure of embryos, beginning with the hatched blastula stage, to heat killed Vibrio diazatrophicus, a marine pathogen of sea urchins, significantly increased Sp064 message content in plutei compared to unexposed controls. These results suggest that sea urchin embryos may use a complement-based immune system for defense against pathogens in their aquatic environment.

Activation of pmar1 controls specification of micromeres in the sea urchin embryo

pmar1 is a transcription factor in the paired class homeodomain family that was identified and found to be transcribed in micromeres beginning at the fourth cleavage of sea urchin development [Dev. Biol. 246 (2002), 209]. Based on in situ data, molecular perturbation studies, and QPCR data, the recently published gene regulatory network (GRN) model for endomesoderm specification [Science 295 (2002) 1669; Dev. Biol. 246 (2002), 162] places pmar1 early in the micromere specification pathway, and upstream of two important micromere induction signals. The goal of this study was to test these three predictions of the network model. A series of embryo chimeras were produced in which pmar1 activity was perturbed in one cell that was transplanted to control hosts. At the fourth cleavage, micromeres bearing altered pmar1 activity were combined with a normal micromereless host embryo. If beta-catenin signaling is blocked, the micromeres remain unspecified and are unable to signal to the host cells. When such beta-catenin-blocked micromeres also express Pmar1, all observed micromere functions are rescued. The rescue includes expression of the primary mesenchyme cell (PMC) differentiation program, expression and execution of the Delta signal to induce secondary mesoderm cell (SMC) specification in macromere progeny, and expression of the early endomesoderm induction signal necessary for full specification of the endoderm. Additionally, Pmar1 expressed mosaically from inserted DNA constructs causes induction of ectopic Endo 16 in adjacent cells, demonstrating further that Pmar1 controls expression of the early endomesoderm induction signal. Based on these experiments, Pmar1 is an important transcription factor necessary for initiating the micromere specification program and for the expression of two inductive signals produced by micromeres. Each of the tests we describe supports the placement and function of Pmar1 in the endomesoderm GRN model.

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.

Conservative segregation of maternally inherited CS histone variants in larval stages of sea urchin development

Three sets of histone variants are coexisting in the embryo at larval stages of sea urchin's development: the maternally inherited cleavage stage variants (CS) expressed during the two initial cleavage divisions, the early histone variants, which are recruited into embryonic chromatin from middle cleavage stages until hatching and the late variants, that are fundamentally expressed from blastula stage onward. Since the expression of the CS histones is confined to the initial cleavage stages, these variants represent a very minor proportion of the histones present in the plutei larvae, whereas the late histone variants are predominant. To determine the position of these CS in the embryonic territories, we have immunolocalized the CS histone variants in plutei larvas harvested 72 h post-fertilization. In parallel, we have pulse labeled the DNA replicated during the initial cleavage cycle with bromodeoxyuridine (BrdU) and its position was further determined in the plutei larvas by immunofluorescence. We have found that the CS histone variants were segregated to specific territories in the plutei. The position in which the CS histone variants were found to be segregated was consistent with the position in which the DNA molecules that were replicated during the initial cleavage divisions were localized. These results strongly suggest that a specification of embryonic nuclei occurs at the initial cleavage divisions which is determined by a chromatin organized by CS histone variants.

Regulatory gene networks and the properties of the developmental process

Genomic instructions for development are encoded in arrays of regulatory DNA. These specify large networks of interactions among genes producing transcription factors and signaling components. The architecture of such networks both explains and predicts developmental phenomenology. Although network analysis is yet in its early stages, some fundamental commonalities are already emerging. Two such are the use of multigenic feedback loops to ensure the progressivity of developmental regulatory states and the prevalence of repressive regulatory interactions in spatial control processes. Gene regulatory networks make it possible to explain the process of development in causal terms and eventually will enable the redesign of developmental regulatory circuitry to achieve different outcomes.

Conserved expression pattern of BMP-2/4 in hemichordate acorn worm and echinoderm sea cucumber embryos

The auricularia larva of sea cucumbers and tornaria larva of acorn worms share striking developmental and morphological similarities. They are regarded as not only an archetype of the nonchordate deuterostome larva, but also an archetype of the origin of chordates. Here we report the characterization and spatial expression patterns of the BMP-2/4 genes of a hemichordate acorn worm (Pf-bmp2/4) and an echinoderm sea cucumber (Sj-bmp2/4). Both the Pf-bmp2/4 and Sj-bmp2/4 genes exhibited apparently conserved expression in the region of the coelomopore complex. This is in agreement with the homology between their basic larval body plans with respect to coelomogenesis and allows us to discuss the evolutionary counterparts of the coelomopore complex in chordates.

Defining the molecular and cellular basis of toxicity using comparative models

A critical element of any experimental design is the selection of the model that will be used to test the hypothesis. As Claude Bernard proposed over 100 years ago "the solution of a physiological or pathological problem often depends solely on the appropriate choice of the animal for the experiment so as to make the result clear and searching." Likewise, the Danish physiologist August Krogh in 1929 wrote that "For a large number of problems there will be some animal of choice, or a few such animals, on which it can be most conveniently studied." This scientific principle has been validated repeatedly in the intervening years as investigators have described unique models that exploit natural differences in chemical and molecular structure, biochemical function, or physiological response between different cells, tissues, and organisms to address specific hypotheses. Despite the power of this comparative approach, investigators have generally been reluctant to utilize nonmammalian or nonclassical experimental models to address questions of human biology. The perception has been that studies in relatively simple or evolutionarily ancient organisms would provide little insight into "complex" human biology. This perception, although always somewhat misguided, is now even less tenable given the results of the genome sequencing projects, which demonstrate that the human genome is remarkably similar to that of evolutionarily ancient organisms. Thus, the various life forms on Earth share much more in common then anyone had previously envisioned. This realization provides additional rationale for the use of nonclassical experimental models and provides perhaps the strongest validation of Bernard's and Krogh's assertions. This overview emphasizes some of the special attributes of alternative animal models that may be exploited to define the molecular and cellular basis of toxicity. For each attribute, selected examples of animal models and experimental approaches are presented. It focuses on the areas of neurotoxicology, reproductive and developmental toxicology, organ systems toxicology, carcinogenesis, and functional genomics/toxicogenomics and highlights the use of fish, avian, Drosophila, Caenorhabditis elegans, and yeast models in such studies.

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.

New early zygotic regulators expressed in endomesoderm of sea urchin embryos discovered by differential array hybridization

Genes that are upregulated by LiCl treatment of sea urchin embryos and/or downregulated by injection into the egg of mRNA encoding an internal fragment of cadherin (Cad) were detected in a differential macroarray screen. The method was that recently described by J. P. Rast et al. (2000, Dev. Biol. 228, 270-296). Almost 10(5) clones from a 12-h cDNA library were screened. Measurements on internal standards showed that the screening procedure was sufficiently sensitive to afford detection of differentially expressed mRNAs of the most rare class, those present in only a few copies per average cell. The injection of Cad mRNA, which specifically blocks nuclearization of beta-catenin, resulted in many-fold decreases in the levels of transcripts of a suite of marker genes expressed zygotically during endomesoderm specification. These measurements substantiated the use of Cad mRNA as the basis for a differential screen for discovery of new endomesodermal genes. By use of the newly developed BioArray software for analysis of macroarray screens, 1106 clones representing differentially expressed genes and yielding useful sequence were recovered. The 367 clones that gave significant BLASTX matches to known cellular proteins fell into 264 nonredundant sequence classes. Those of particular interest for this work were clones encoding DNA-binding transcription factors, signal transduction pathway components, proteases, kinases, and phosphatases. Quantitative PCR analysis of 66 such selected clones revealed that the large majority of these clones had been selected because they are upregulated by LiCl treatment, which affects the expression of a much greater diversity and number of genes than are involved in endomesoderm specification. Seven transcript species were identified that responded sharply to injection of Cad mRNA, and that are not represented in maternal mRNA. Six of those encode transcription factors. We focused on three transcription factor genes of this set that were previously unknown in sea urchin embryos. By whole-mount in situ hybridization, these genes are expressed in specific domains of the endomesodermal territory. They are: (1) Speve, an evenskipped orthologue expressed very early in all vegetal blastomeres and then gradually shifting to veg(1) derivatives by the mesenchyme blastula stage; (2) Spgcm, an orthologue of the fruit fly gene glial cells missing, which is first expressed specifically and exclusively in part of the prospective secondary mesenchyme (mesodermal) domain at late-cleavage blastula stage; and (3) Spfoxc, which is first expressed in the early blastula only in the four small micromeres, and later only expressed in that coelomic pouch which gives rise to the mesoderm of the ventral surface of the adult rudiment.

A provisional regulatory gene network for specification of endomesoderm in the sea urchin embryo

We present the current form of a provisional DNA sequence-based regulatory gene network that explains in outline how endomesodermal specification in the sea urchin embryo is controlled. The model of the network is in a continuous process of revision and growth as new genes are added and new experimental results become available; see http://www.its.caltech.edu/~mirsky/endomeso.htm (End-mes Gene Network Update) for the latest version. The network contains over 40 genes at present, many newly uncovered in the course of this work, and most encoding DNA-binding transcriptional regulatory factors. The architecture of the network was approached initially by construction of a logic model that integrated the extensive experimental evidence now available on endomesoderm specification. The internal linkages between genes in the network have been determined functionally, by measurement of the effects of regulatory perturbations on the expression of all relevant genes in the network. Five kinds of perturbation have been applied: (1) use of morpholino antisense oligonucleotides targeted to many of the key regulatory genes in the network; (2) transformation of other regulatory factors into dominant repressors by construction of Engrailed repressor domain fusions; (3) ectopic expression of given regulatory factors, from genetic expression constructs and from injected mRNAs; (4) blockade of the beta-catenin/Tcf pathway by introduction of mRNA encoding the intracellular domain of cadherin; and (5) blockade of the Notch signaling pathway by introduction of mRNA encoding the extracellular domain of the Notch receptor. The network model predicts the cis-regulatory inputs that link each gene into the network. Therefore, its architecture is testable by cis-regulatory analysis. Strongylocentrotus purpuratus and Lytechinus variegatus genomic BAC recombinants that include a large number of the genes in the network have been sequenced and annotated. Tests of the cis-regulatory predictions of the model are greatly facilitated by interspecific computational sequence comparison, which affords a rapid identification of likely cis-regulatory elements in advance of experimental analysis. The network specifies genomically encoded regulatory processes between early cleavage and gastrula stages. These control the specification of the micromere lineage and of the initial veg(2) endomesodermal domain; the blastula-stage separation of the central veg(2) mesodermal domain (i.e., the secondary mesenchyme progenitor field) from the peripheral veg(2) endodermal domain; the stabilization of specification state within these domains; and activation of some downstream differentiation genes. Each of the temporal-spatial phases of specification is represented in a subelement of the network model, that treats regulatory events within the relevant embryonic nuclei at particular stages.

A regulatory gene network that directs micromere specification in the sea urchin embryo

Micromeres and their immediate descendants have three known developmental functions in regularly developing sea urchins: immediately after their initial segregation, they are the source of an unidentified signal to the adjacent veg(2) cells that is required for normal endomesodermal specification; a few cleavages later, they express Delta, a Notch ligand which triggers the conditional specification of the central mesodermal domain of the vegetal plate; and they exclusively give rise to the skeletogenic mesenchyme of the postgastrular embryo. We demonstrate the key components of the zygotic regulatory gene network that accounts for micromere specificity. This network is a subelement of the overall endomesoderm specification network of the Strongylocentrotus purpuratus embryo. A central role is played by a newly discovered gene encoding a paired class homeodomain transcription factor which in micromeres acts as a repressor of a repressor: the gene is named pmar1 (paired-class micromere anti-repressor). pmar1 is expressed only during cleavage and early blastula stages, and exclusively in micromeres. It is initially activated as soon as the micromeres are formed, in response to Otx and beta-Catenin/Tcf inputs. The repressive nature of the interactions mediated by the pmar1 gene product was shown by the identical effect of introducing mRNA encoding the Pmar1 factor, and mRNA encoding an Engrailed-Pmar1 (En-Pmar1) repressor domain fusion. In both cases, the effects are derepression: of the delta gene; and of skeletogenic genes, including several transcription factors normally expressed only in micromere descendants, and also a set of downstream skeletogenic differentiation genes. The spatial phenotype of embryos bearing exogenous mRNA encoding Pmar1 factor or En-Pmar1 is expansion of the domains of expression of the downstream genes over most or all of the embryo. This results in transformation of much of the embryo into skeletogenic mesenchyme cells that express skeletogenic markers. The normal role of pmarl is to prevent, exclusively in the micromeres, the expression of a repressor that is otherwise operative throughout the embryo. This function accounts for the localization of delta transcription in micromeres, and thereby for the conditional specification of the vegetal plate mesoderm. It also explains why skeletogenic differentiation gene batteries normally function only in micromere descendants. More generally, the regulatory network subelement emerging from this work shows how the specificity of micromere function depends on continuing global regulatory interactions, as well as on early localized inputs.

Functional characterization of Ets-binding sites in the sea urchin embryo: three base pair conversions redirect expression from mesoderm to ectoderm and endoderm

Because of the limited knowledge of target genes for the ets family of transcription factors, it is yet unclear how specificity of biological function among different members is achieved in this class of proteins. In the present study, we compared two Ets-binding sites in two differentially expressed genes of the sea urchin embryo. The first gene examined is the cytoskeletal actin CyIIa, which is transiently expressed in skeletogenic and secondary mesenchyme and in its terminal and permanent phase in the gut. The second one encodes the hatching enzyme gene of Strongylocentrotus purpuratus, and is regulated cell-autonomously and asymmetrically along the maternally determined animal-vegetal axis. The Ets sites within the regulatory regions of these two genes interact and form different binding complexes with proteins present in the nuclei of mesenchyme blastula embryos. We also demonstrated that the DNA binding specificity of the CyIIa Ets-binding site can be converted to the other type of Ets site, as in the hatching enzyme promoter, by changing only three base pairs near the Ets core sequence. Switching of these three base pairs near the central GGA trinucleotide motif characteristic of all Ets-binding targets was also sufficient to redirect expression of a reporter gene construct containing a heterologous basal promoter from mesenchyme to non-mesenchyme cell type in transgenic sea urchin embryos. These observations suggest that binding affinity of ets transcription factors plays an important role in determining cell type-specific gene expression.

A genomic regulatory network for development

Development of the body plan is controlled by large networks of regulatory genes. A gene regulatory network that controls the specification of endoderm and mesoderm in the sea urchin embryo is summarized here. The network was derived from large-scale perturbation analyses, in combination with computational methodologies, genomic data, cis-regulatory analysis, and molecular embryology. The network contains over 40 genes at present, and each node can be directly verified at the DNA sequence level by cis-regulatory analysis. Its architecture reveals specific and general aspects of development, such as how given cells generate their ordained fates in the embryo and why the process moves inexorably forward in developmental time.

HOX genes in the sepiolid squid Euprymna scolopes: implications for the evolution of complex body plans

Molluscs display a rich diversity of body plans ranging from the wormlike appearance of aplacophorans to the complex body plan of the cephalopods with highly developed sensory organs, a complex central nervous system, and cognitive abilities unrivaled among the invertebrates. The aim of the current study is to define molecular parameters relevant to the developmental evolution of cephalopods by using the sepiolid squid Euprymna scolopes as a model system. Using PCR-based approaches, we identified one anterior, one paralog group 3, five central, and two posterior group Hox genes. The deduced homeodomain sequences of the E. scolopes Hox cluster genes are most similar to known annelid, brachiopod, and nemertean Hox gene homeodomain sequences. Our results are consistent with the presence of a single Hox gene cluster in cephalopods. Our data also corroborate the proposed existence of a differentiated Hox gene cluster in the last common ancestor of Bilaterians. Furthermore, our phylogenetic analysis and in particular the identification of Post-1 and Post-2 homologs support the Lophotrochozoan clade.

Potential of veg2 blastomeres to induce endoderm differentiation in sea urchin embryos

Two different modes of gastrulation in sea urchin embryos have been reported. The first mode, reported in Hemicentrotus pulcherrimus and some other species, consists of two phases: a primary and a secondary invagination. The second mode involves gastrulation with a continuous convolution of cells near the blastopore; this mode has been reported to occur in the embryos of the sand dollar, Scaphechinus mirabilis. The rudimentary gut is comprised of fewer cells in the embryos of the former species than in the latter. We assumed that the differences in gastrulation modes could be related to the different potentials of the veg2 layer to induce endoderm differentiation in the upper layer. In the present study, we produced chimeric embryos consisting of an animal cap recombined with veg2 layer blastomere(s) to compare the inductive effect of the veg2 layer and/or the blastomere(s) in H. pulcherrimus and S. mirabilis embryos. Our results showed that the inductive effect of the veg2 layer is stronger in S. mirabilis embryos than in H. pulcherrimus embryos. Moreover, it was suggested that the difference in the strength of inductive effects of veg2 layers is related to the difference in gastrulation modes.

Behavior of pigment cells in gastrula-stage embryos of Hemicentrotus pulcherrimus and Scaphechinus mirabilis

The behavior of pigment cells in sea urchin embryos, especially at the gastrula stage, is not well understood, due to the lack of an appropriate method to detect pigment cells. We found that pigment cells emanated autofluorescence when they were fixed with formalin and irradiated with ultraviolet or green light. In Hemicentrotus pulcherrimus, fluorescent pigment cells became visible at the archenteron tip at the mid-gastrula stage. The cells detached from the archenteron slightly before the initiation of secondary invagination and migrated toward the apical plate. Most pigment cells entered the apical plate. This entry site seemed to be restricted, because pigment cells could not enter the ectoderm and remained in the blastocoele at the vegetal pole side when elongation of archenteron was blocked. Pigment cells that had entered the apical plate soon began to migrate in the aboral ectoderm toward the vegetal pole. In contrast, pigment cells of Scaphechinus mirabilis embryos were first detected in the vegetal plate before the onset of gastrulation. Without entering the blastocoele, these cells began to migrate preferentially in the aboral ectoderm toward the animal pole. When the archenteron tip reached the apical plate, pigment cells had already distributed throughout the aboral ectoderm. Thus, the behavior of pigment cells was quite different between H. pulcherrimus and S. mirabilis.

Deuterostome evolution: early development in the enteropneust hemichordate, Ptychodera flava

Molecular and morphological comparisons indicate that the Echinodermata and Hemichordata represent closely related sister-phyla within the Deuterostomia. Much less is known about the development of the hemichordates compared to other deuterostomes. For the first time, cell lineage analyses have been carried out for an indirect-developing representative of the enteropneust hemichordates, Ptychodera flava. Single blastomeres were iontophoretically labeled with Dil at the 2- through 16-cell stages, and their fates followed through development to the tornaria larval stage. The early cleavage pattern of P. flava is similar to that of the direct-developing hemichordate, Saccoglossus kowalevskii, as well as that displayed by indirect-developing echinoids. The 16-celled embryo contains eight animal "mesomeres," four slightly larger "macromeres," and four somewhat smaller vegetal "micromeres." The first cleavage plane was not found to bear one specific relationship relative to the larval dorsoventral axis. Although individual blastomeres generate discrete clones of cells, the appearance and exact locations of these clones are variable with respect to the embryonic dorsoventral and bilateral axes. The eight animal mesomeres generate anterior (animal) ectoderm of the larva, which includes the apical organ; however, contributions to the apical organ were found to be variable as only a subset of the animal blastomeres end up contributing to its formation and this varies from embryo to embryo. The macromeres generate posterior larval ectoderm, and the vegetal micromeres form all the internal, endomesodermal tissues. These blastomere contributions are similar to those found during development of the only other hemichordate studied, the direct-developing enteropneust, S. kowalevskii. Finally, isolated blastomeres prepared at either the two- or the four-cell stage are capable of forming normal-appearing, miniature tornaria larvae. These findings indicate that the fates of these cells and embryonic dorsoventral axial properties are not committed at these early stages of development. Comparisons with the developmental programs of other deuterostome phyla allow one to speculate on the conservation of some key developmental events/mechanisms and propose basal character states shared by the ancestor of echinoderms and hemichordates.

The role of Brachyury (T) during gastrulation movements in the sea urchin Lytechinus variegatus

The studies described here sought to identify and characterize genes involved in the gastrulation and morphogenetic movements that occur during sea urchin embryogenesis. An orthologue of the T-box family transcription factor, Brachyury, was cloned through a candidate gene approach. Brachyury (T) is the founding member of this T-box transcription factor family and has been implicated in gastrulation movements in Xenopus, zebrafish, and mouse embryogenesis. Polyclonal serum was generated to LvBrac in order to characterize protein expression. LvBrac initially appears at mesenchyme blastula stage in two distinct regions with embryonic expression perduring until pluteus stage. Vegetally, LvBrac expression is in endoderm and lies circumferentially around the blastopore. This torus-shaped area of LvBrac expression remains constant in size as endoderm cells express LvBrac upon moving into that circumference and cease LvBrac expression as they leave the circumference. Vegetal expression remains around the anus through pluteus stage. The second domain of LvBrac expression first appears broadly in the oral ectoderm at mesenchyme blastula stage and at later embryonic stages is refined to just the stomodael opening. Vegetal LvBrac expression depends on autonomous beta-catenin signaling in macromeres and does not require micromere or veg2-inductive signals. It was then determined that LvBrac is necessary for the morphogenetic movements occurring in both expression regions. A dominant-interfering construct was generated by fusing the DNA binding domain of LvBrac to the transcriptional repression module of the Drosophila Engrailed gene in order to perturb gene function. Microinjection of mRNA encoding this LvBrac-EN construct resulted in a block in gastrulation movements but not expression of endoderm and mesoderm marker genes. Furthermore, injection of LvBrac-EN into one of two blastomeres resulted in normal gastrulation movements of tissues derived from the injected blastomere, indicating that LvBrac downstream function may be nonautonomous during sea urchin gastrulation.

Evidence for a mesodermal embryonic regulator of the sea urchin CyIIa gene

The CyIIa gene of the sea urchin embryo is a model for study of cis-regulation downstream of cell-type specification, as CyIIa transcription follows the specification and initial differentiation of the embryonic domains in which it is expressed. These are the skeletogenic and secondary mesenchyme and gut. We carried out a detailed structural and functional analysis of a cis-regulatory region of this gene, extending 780 bp upstream and 125 bp downstream of the transcription start site, that had been shown earlier to reproduce faithfully the complex and dynamic CyIIa pattern of expression. This analysis revealed that the overall pattern of expression of the CyIIa gene appears to be governed mainly by two independent sets of DNA elements, which are target sites for specific proteins present in blastula-stage nuclear extract. One type of element, which controls a dynamic program of expression in both skeletogenic and secondary mesenchyme cells, contains the consensus-binding site for a member of the ets transcription factor family. The other, which is responsible for the terminal or permanent phase of CyIIa expression in the gut, shares homologies with the late module of the endoderm-specific Endo16 gene (endo16 Module B). Oligonucleotides containing replicas of these two target sites fused upstream of a sea urchin basal promoter are sufficient to confer accurate mesenchyme and late gut expression of an injected GFP construct. The finding of a single protein target site that recapitulates CyIIa expression in both primary and secondary mesenchyme cells suggests the existence of a pan-mesodermal gene expression program in the sea urchin embryo.

Regeneration neurohormones and growth factors in echinoderms

There has been much recent interest in the presence and biological functions of growth regulators in invertebrates. In spite of the different distribution patterns of these molecules in different phyla (from molluscs, insects, and annelids to echinoderms and tunicates), they seem always to be extensively involved in developmental processes, both embryonic and regenerative. Echinoderms are well known for their striking regenerative potential and many can completely regenerate arms that, for example, are lost following self-induced or traumatic amputation. Thus, they provide a valuable experimental model for the study of regenerative processes from the macroscopic to the molecular level. In crinoids as well as probably all ophiuroids, regeneration is rapid and occurs by means of a mechanism that involves blastema formation, known as epimorphosis, where the new tissues arise from undifferentiated cells. In asteroids, morphallaxis is the mechanism employed, replacement cells being derived from existing tissues following differentiation and (or) transdifferentiation. This paper focuses on the possible contribution of neurohormones and growth factors during both repair and regenerative processes. Three different classes of regulatory molecules are proposed as plausible candidates for growth-promoting factors in regeneration: neurotransmitters (monoamines), neuropeptides (substance P, SALMFamides 1 and 2), and growth-factor-like molecules (TGF-β (transforming growth factor β), NGF (nerve growth factor), RGF-2 (basic fibroblast growth factor)).

Correct Expression of spec2a in the sea urchin embryo requires both Otx and other cis-regulatory elements

Strongylocentrotus purpuratus Otx (SpOtx) is required simultaneously in sea urchin development for the activation of endo16 in the vegetal plate and for the activation of spec2a in the aboral ectoderm. Because Otx binding sites alone do not appear to be responsible for the spatially restricted expression of spec2a, additional DNA elements were sought. We show here that consensus Otx binding sites fused to basal promoters are sufficient to activate CAT reporter gene expression in all cell types, although expression in endomesoderm progenitors is enhanced. On the other hand, three non-Otx elements derived from the spec2a enhancer are needed together with Otx sites for specifically aboral ectoderm expression. A DNA element termed Y/CBF, lying just downstream from an Otx site within the spec2a enhancer, mediates general activation in the ectoderm. A second element lying between the Otx and Y/CBF sites, called OER, functions to prevent expression in the oral ectoderm. A third site, called ENR, overlapping another Otx site, is required to repress endoderm expression. Three distinct DNA binding proteins interact sequence specifically at the Y/CBF, OER, and ENR elements. The spec2a enhancer thus consists of closely linked activator and repressor elements that function collectively to cause expression of the spec2a gene in the aboral ectoderm.

The neurobiology and evolution of cannabinoid signalling

The plant Cannabis sativa has been used by humans for thousands of years because of its psychoactivity. The major psychoactive ingredient of cannabis is Delta(9)-tetrahydrocannabinol, which exerts effects in the brain by binding to a G-protein-coupled receptor known as the CB1 cannabinoid receptor. The discovery of this receptor indicated that endogenous cannabinoids may occur in the brain, which act as physiological ligands for CB1. Two putative endocannabinoid ligands, arachidonylethanolamide ('anandamide') and 2-arachidonylglycerol, have been identified, giving rise to the concept of a cannabinoid signalling system. Little is known about how or where these compounds are synthesized in the brain and how this relates to CB1 expression. However, detailed neuroanatomical and electrophysiological analysis of mammalian nervous systems has revealed that the CB1 receptor is targeted to the presynaptic terminals of neurons where it acts to inhibit release of 'classical' neurotransmitters. Moreover, an enzyme that inactivates endocannabinoids, fatty acid amide hydrolase, appears to be preferentially targeted to the somatodendritic compartment of neurons that are postsynaptic to CB1-expressing axon terminals. Based on these findings, we present here a model of cannabinoid signalling in which anandamide is synthesized by postsynaptic cells and acts as a retrograde messenger molecule to modulate neurotransmitter release from presynaptic terminals. Using this model as a framework, we discuss the role of cannabinoid signalling in different regions of the nervous system in relation to the characteristic physiological actions of cannabinoids in mammals, which include effects on movement, memory, pain and smooth muscle contractility. The discovery of the cannabinoid signalling system in mammals has prompted investigation of the occurrence of this pathway in non-mammalian animals. Here we review the evidence for the existence of cannabinoid receptors in non-mammalian vertebrates and invertebrates and discuss the evolution of the cannabinoid signalling system. Genes encoding orthologues of the mammalian CB1 receptor have been identified in a fish, an amphibian and a bird, indicating that CB1 receptors may occur throughout the vertebrates. Pharmacological actions of cannabinoids and specific binding sites for cannabinoids have been reported in several invertebrate species, but the molecular basis for these effects is not known. Importantly, however, the genomes of the protostomian invertebrates Drosophila melanogaster and Caenorhabditis elegans do not contain CB1 orthologues, indicating that CB1-like cannabinoid receptors may have evolved after the divergence of deuterostomes (e.g. vertebrates and echinoderms) and protostomes. Phylogenetic analysis of the relationship of vertebrate CB1 receptors with other G-protein-coupled receptors reveals that the paralogues that appear to share the most recent common evolutionary origin with CB1 are lysophospholipid receptors, melanocortin receptors and adenosine receptors. Interestingly, as with CB1, each of these receptor types does not appear to have Drosophila orthologues, indicating that this group of receptors may not occur in protostomian invertebrates. We conclude that the cannabinoid signalling system may be quite restricted in its phylogenetic distribution, probably occurring only in the deuterostomian clade of the animal kingdom and possibly only in vertebrates.

Cis-regulatory logic in the endo16 gene: switching from a specification to a differentiation mode of control

The endo16 gene of Strongylocentrotus purpuratus encodes a secreted protein of the embryonic and larval midgut. The overall functional organization of the spatial and temporal control system of this gene are relatively well known from a series of earlier cis-regulatory studies. Our recent computational model for the logic operations of the proximal region of the endo16 control system (Module A) specifies the function of interactions at each transcription factor target site of Module A. Here, we extend sequence level functional analysis to the adjacent cis-regulatory region, Module B. The computational logic model is broadened to include B/A interactions as well as other Module B functions. Module B drives expression later in development and its major activator is responsible for a sharp, gut-specific increase in transcription after gastrulation. As shown earlier, Module B output undergoes a synergistic amplification that requires interactions within Module A. The interactions within Module B that are required to generate and transmit its output to Module A are identified. Logic considerations predicted an internal cis-regulatory switch by which spatial control of endo16 expression is shifted from Module A (early) to Module B (later). This prediction was confirmed experimentally and a distinct set of interactions in Module B that mediate the switch function was demonstrated. The endo16 computational model now provides a detailed explanation of the information processing functions executed by the cis-regulatory system of this gene throughout embryogenesis. Early in development the gene participates in the specification events that define the endomesoderm; later it functions as a gut-specific differentiation gene. The cis-regulatory switch mediates this functional change.

Oral-aboral axis specification in the sea urchin embryo. I. Axis entrainment by respiratory asymmetry

In embryos of indirectly developing echinoids, the secondary (oral-aboral) larval axis is established after fertilization by an as yet undiscovered process. One of the earliest manifestations of this axis is an asymmetry in mitochondrial respiration, with the prospective oral side of the embryo exhibiting a higher rate of respiration than the prospective aboral side. We show here that respiratory asymmetry can be experimentally induced within embryos by immobilizing them in tight clusters of four ("rosettes"). Within such clusters a redox gradient is established from the inside to the outside of the rosette. Vital staining of clustered embryos demonstrates that the side of the embryo facing the outside of the rosette (i.e., the most oxidizing) tends to become the oral side, while the side facing the inside tends to become the aboral side. Effective entrainment of the oral-aboral axis requires that the embryos remain immobilized in rosettes until the hatching blastula stage. To begin to investigate the molecular mechanisms underlying this effect we made use of P3A2, a transcriptional regulatory protein whose activity is spatially modulated along the oral-aboral axis. When synthetic mRNA encoding P3A2 fused to the VP16 activation domain is injected into eggs, it activates embryonic expression of a green fluorescent protein reporter gene containing a basal promoter and a single strong P3A2 target site. In embryo rosettes, such activation occurs predominantly on the outside of the rosette, suggesting that the activity of the P3A2 protein is spatially regulated by the respiratory asymmetry established by clustering the embryos. These findings are discussed with reference to earlier work on both oral-aboral axis specification and P3A2 and used to develop a testable model of the mechanism of oral-aboral axis specification in the sea urchin embryo.

Spatial expression of Hox cluster genes in the ontogeny of a sea urchin

The Hox cluster of the sea urchin Strongylocentrous purpuratus contains ten genes in a 500 kb span of the genome. Only two of these genes are expressed during embryogenesis, while all of eight genes tested are expressed during development of the adult body plan in the larval stage. We report the spatial expression during larval development of the five ‘posterior’ genes of the cluster: SpHox7, SpHox8, SpHox9/10, SpHox11/13a and SpHox11/13b. The five genes exhibit a dynamic, largely mesodermal program of expression. Only SpHox7 displays extensive expression within the pentameral rudiment itself. A spatially sequential and colinear arrangement of expression domains is found in the somatocoels, the paired posterior mesodermal structures that will become the adult perivisceral coeloms. No such sequential expression pattern is observed in endodermal, epidermal or neural tissues of either the larva or the presumptive juvenile sea urchin. The spatial expression patterns of the Hox genes illuminate the evolutionary process by which the pentameral echinoderm body plan emerged from a bilateral ancestor.

Initial analysis of immunochemical cell surface properties, location and formation of the serotonergic apical ganglion in sea urchin embryos

In the present study it was found that serotonergic apical ganglion (SAG)-forming cells in plutei of the sea urchin, Hemicentrotus pulcherrimus, possessed a characteristic pear shape with broad apical sides and a pointed basal side in the acron epithelium. The basal side extended axons through the space between the epithelium and the basal lamina toward the midline of the embryo that aligned parallel to the embryonic anteroposterior axis. Serotonergic apical ganglion-forming cells had epithelial cell surface-specific proteins on their entire surface. The SAG in 4-arm plutei was composed of a 4-cell trunk region that aligned at right angles to the embryonic anteroposterior axis, and forked into two branches of one to two cells at both ends. Two branches extended toward the oral and the other two toward the aboral region, respectively. Double-stained immunohistochemistry using antiserotonin antibodies and oral ectoderm-specific anti-Ecto V monoclonal antibody or aboral ectoderm-specific anti-Ars antibodies indicated that SAG was in the aboral ectoderm region. Serotonergic apical ganglion cells were first detected in late gastrulae and increased in number rapidly between 36 and 48 h after fertilization, and then slowly afterwards. A 5-bromo-2-deoxyuridine incorporation study indicated that none of the increased SAG cells were in the S phase during the aforementioned period, suggesting that SAG cells do not proliferate by cell division, but acquire the property in particular cells by transdifferentiation using a mechanism that has yet to be elucidated.

A sea urchin genome project: sequence scan, virtual map, and additional resources

Results of a first-stage Sea Urchin Genome Project are summarized here. The species chosen was Strongylocentrotus purpuratus, a research model of major importance in developmental and molecular biology. A virtual map of the genome was constructed by sequencing the ends of 76,020 bacterial artificial chromosome (BAC) recombinants (average length, 125 kb). The BAC-end sequence tag connectors (STCs) occur an average of 10 kb apart, and, together with restriction digest patterns recorded for the same BAC clones, they provide immediate access to contigs of several hundred kilobases surrounding any gene of interest. The STCs survey >5% of the genome and provide the estimate that this genome contains approximately 27,350 protein-coding genes. The frequency distribution and canonical sequences of all middle and highly repetitive sequence families in the genome were obtained from the STCs as well. The 500-kb Hox gene complex of this species is being sequenced in its entirety. In addition, arrayed cDNA libraries of >10(5) clones each were constructed from every major stage of embryogenesis, several individual cell types, and adult tissues and are available to the community. The accumulated STC data and an expanding expressed sequence tag database (at present including >12, 000 sequences) have been reported to GenBank and are accessible on public web sites.

Sox regulates transcription of the sea urchin arylsulfatase gene

A 50 bp region from -194 bp to -144 bp of the arylsulfatase gene (HpArs) of the sea urchin, Hemicentrotus pulcherrimus, is related to the temporally regulated expression of this gene. This region contains a Sox (Sry-related HMG box)-binding site, and the introduction of sequence mutations to this site significantly reduced the activity of the HpArs promoter, even in the presence of the C15 enhancer, which consists of HpOtx and CAAT motifs. A protein that binds to the Sox-binding site in the 50 bp region of the HpArs gene was detected in nuclear extracts of mesenchyme blastulae and a protein synthesized in vitro using SoxB1 cDNA of another sea urchin, Strongylocentrotus purpuratus, also bound to this Sox site. These results suggest that HpSox, which is maternally expressed and remains abundant by the pluteus stage, is clearly implicated in regulation of the HpArs gene. The presence of a negatively acting cis element in this 50 bp region has also been detected.

Novel gene expression patterns in hybrid embryos between species with different modes of development

Cross-species hybrids between eggs of the direct-developing sea urchin, Heliocidaris erythrogramma, and sperm from its congeneric indirect-developing species, Heliocidaris tuberculata, show restoration of features of the paternal feeding pluteus larva, including the gut, and pluteus spicular skeleton. Unlike other reported sea urchin cross-species hybrids, Heliocidaris hybrids express genes derived from both maternal and paternal species at high levels. Ectodermal cell types, which differ radically between the two parental species, are of intermediate form in the hybrids. Gene expression patterns in hybrid embryo tissues represent a number of combinations of parental gene expression patterns: genes that are not expressed in one paternal species, but are expressed in hybrids as in the expressing parent; genes that show additive expression patterns plus novel sites of expression; a gene that is misexpressed in the hybrids; and genes expressed identically in both parents and in hybrids. The results indicate that both conserved and novel gene regulatory interactions are present. Only one gene, CyIII actin, has lost cell-type-specific regulation in the hybrids. Hybrids thus reveal that disparate parental genomes, each with its own genic regulatory system, can produce in combination a novel gene expression entity with a unique ontogeny. This outcome may derive from conserved gene regulatory regions in downstream genes of both parental species responding in conserved ways to higher-level regulators that determine modular gene expression territories.

A BMP pathway regulates cell fate allocation along the sea urchin animal-vegetal embryonic axis

To examine whether a BMP signaling pathway functions in specification of cell fates in sea urchin embryos, we have cloned sea urchin BMP2/4, analyzed its expression in time and space in developing embryos and assayed the developmental consequences of changing its concentration through mRNA injection experiments. These studies show that BMP4 mRNAs accumulate transiently during blastula stages, beginning around the 200-cell stage, 14 hours postfertilization. Soon after the hatching blastula stage, BMP2/4 transcripts can be detected in presumptive ectoderm, where they are enriched on the oral side. Injection of BMP2/4 mRNA at the one-cell stage causes a dose-dependent suppression of commitment of cells to vegetal fates and ectoderm differentiates almost exclusively as a squamous epithelial tissue. In contrast, NOGGIN, an antagonist of BMP2/4, enhances differentiation of endoderm, a vegetal tissue, and promotes differentiation of cells characteristic of the ciliated band, which contains neurogenic ectoderm. These findings support a model in which the balance of BMP2/4 signals produced by animal cell progeny and opposing vegetalizing signals sent during cleavage stages regulate the position of the ectoderm/ endoderm boundary. In addition, BMP2/4 levels influence the decision within ectoderm between epidermal and nonepidermal differentiation.

Modularity and dissociation in the evolution of gene expression territories in development

Modularity is a salient feature of development and crucial to its evolution. This paper extends modularity to include the concept of gene expression territory, as established for sea urchin embryos. Territories provide a mechanism for partitioning of the cells of a rapidly developing embryo into functional units of a feeding larva. Territories exhibit the characteristics of modules. The paper asks if the embryo and the nonfeeding larva of the direct-developing sea urchin Heliocidaris erythrogramma are organized into gene expression territories, and if its territories correspond to the canonical territories of the pluteus. An analysis of cell lineage and gene expression data for H. erythrogramma shows that skeletogenic cell, coelomic, and vegetal plate gene expression territories are conserved, although they arise from cell lineages distinct from those of the pluteus, and the overall morphology of the larva differs from that of a pluteus. The ectoderm, as in indirect developers, is divided into territories. However, the oral ectodermal territory characteristic of the pluteus is absent in H. erythrogramma. Oral ectoderm is restored in hybrids of H. erythrogramma eggs fertilized by Heliocidaris tuberculata sperm. This indicates that embryonic modules evolve by changes in expression of dominant regulatory genes within territories and that entire modules can be eliminated in evolution of embryos.

Bilaterian origins: significance of new experimental observations

Several recent laboratory observations that bear on the origin of the Bilateria are reviewed and interpreted in light of our set-aside cell theory for bilaterian origins. We first discuss new data concerning the phylogeny of bilaterian phyla. Next, we use systematic, molecular, and paleontological lines of evidence to argue that the latest common ancestor of echinoderms plus hemichordates used a maximal indirect mode of development. Furthermore, the latest common ancestor of molluscs and annelids was also indirectly developing. Finally, we discuss new data on Hox gene expression patterns which suggest that both sea urchins and polychaete annelids use Hox genes in a very similar fashion. Neither utilizes the complete Hox complex in the development of the larva per se, while the Hox complex is expressed in the set-aside cells from which the adult body plan is formed. Our current views on the ancestry of the bilaterians are summarized in phylogenetic terms, incorporating the characters discussed in this paper.

Animal-vegetal axis patterning mechanisms in the early sea urchin embryo

We discuss recent progress in understanding how cell fates are specified along the animal-vegetal axis of the sea urchin embryo. This process is initiated by cell-autonomous, maternally directed, mechanisms that establish three unique gene-regulatory domains. These domains are defined by distinct sets of vegetalizing (beta-catenin) and animalizing transcription factor (ATF) activities and their region of overlap in the macromeres, which specifies these cells as early mesendoderm. Subsequent signaling among cleavage-stage blastomeres further subdivides fates of macromere progeny to yield major embryonic tissues. Zygotically produced Wnt8 reinforces maternally regulated levels of nuclear beta-catenin in vegetal derivatives to down regulate ATF activity and further promote mesendoderm fates. Signaling through the Notch receptor from the vegetal micromere lineages diverts adjacent mesendoderm to secondary mesenchyme fates. Continued Wnt signaling expands the vegetal domain of beta-catenin's transcriptional regulatory activity and competes with animal signaling factors, including BMP2/4, to specify the endoderm-ectoderm border within veg(1) progeny. This model places new emphasis on the importance of the ratio of maternally regulated vegetal and animal transcription factor activities in initial specification events along the animal-vegetal axis.

TCF is the nuclear effector of the beta-catenin signal that patterns the sea urchin animal-vegetal axis

The mechanism of animal-vegetal (AV) axis formation in the sea urchin embryo is incompletely understood. Specification of the axis is thought to involve a combination of cell-cell signals and as yet unidentified maternal determinants. In Xenopus the Wnt pathway plays a crucial role in defining the embryonic axes. Recent experiments in sea urchins have shown that at least two components of the Wnt signaling pathway, GSK3beta and beta-catenin, are involved in embryonic AV axis patterning. These results support the notion that the developmental network that regulates axial patterning in deuterostomes is evolutionarily conserved. To further test this hypothesis, we have examined the role of beta-catenin nuclear binding partners, members of the TCF family of transcriptional regulators, in sea urchin AV axis patterning. To test the role of TCFs in mediating beta-catenin signals in sea urchin AV axis development we examined the consequences of microinjecting RNAs encoding altered forms of TCF on sea urchin development. We show that expression of a dominant negative TCF results in a classic "animalized" embryo. In contrast, microinjected RNA encoding an activated TCF produces a highly "vegetalized" embryo. We show that the transactivational activity of endogenous sea urchin TCF is potentiated by LiCl treatment, which vegetalizes embryos by inhibiting GSK3, consistent with an in vivo interaction between endogenous beta-catenin and TCF. We also provide evidence indicating that all of beta-catenin's activity in patterning the sea urchin AV axis is mediated by TCF. Using a glucocorticoid-responsive TCF, we show that TCF transcriptional activity affects specification along the AV axis between fertilization and the 60-cell stage.

Regulation of cell-fate determination in Dictyostelium

A key step in the development of all multicellular organisms is the differentiation of specialized cell types. The eukaryotic microorganism Dictyostelium discoideum provides a unique experimental system for studying cell-type determination and spatial patterning in a developing multicellular organism. Unlike metazoans, which become multicellular by undergoing many rounds of cell division after fertilization of an egg, the social amoeba Dictyostelium achieves multicellularity by the aggregation of approximately 10(5) cells in response to nutrient depletion. Following aggregation, cell-type differentiation and morphogenesis result in a multicellular organism with only a few cell types that exhibit a defined patterning along the anterior-posterior axis of the organism. Analysis of the mechanisms that control these processes is facilitated by the relative simplicity of Dictyostelium development and the availability of molecular, genetic, and cell biological tools. Interestingly, analysis has shown that many molecules that play integral roles in the development of higher eukaryotes, such as PKA, STATs, and GSK-3, are also essential for cell-type differentiation and patterning in Dictyostelium. The role of these and other signaling pathways in the induction, maintenance, and patterning of cell types during Dictyostelium development is discussed.