Structure, regulation, and function of micro1 in the sea urchin Hemicentrotus pulcherrimus

Architecture and evolution of the cis-regulatory system of the echinoderm kirrelL gene

The gene regulatory network (GRN) that underlies echinoderm skeletogenesis is a prominent model of GRN architecture and evolution. KirrelL is an essential downstream effector gene in this network and encodes an Ig-superfamily protein required for the fusion of skeletogenic cells and the formation of the skeleton. In this study, we dissected the transcriptional control region of the kirrelL gene of the purple sea urchin, Strongylocentrotus purpuratus. Using plasmid- and bacterial artificial chromosome-based transgenic reporter assays, we identified key cis-regulatory elements (CREs) and transcription factor inputs that regulate Sp-kirrelL, including direct, positive inputs from two key transcription factors in the skeletogenic GRN, Alx1 and Ets1. We next identified kirrelL cis-regulatory regions from seven other echinoderm species that together represent all classes within the phylum. By introducing these heterologous regulatory regions into developing sea urchin embryos we provide evidence of their remarkable conservation across ~500 million years of evolution. We dissected in detail the kirrelL regulatory region of the sea star, Patiria miniata, and demonstrated that it also receives direct inputs from Alx1 and Ets1. Our findings identify kirrelL as a component of the ancestral echinoderm skeletogenic GRN. They support the view that GRN subcircuits, including specific transcription factor–CRE interactions, can remain stable over vast periods of evolutionary history. Lastly, our analysis of kirrelL establishes direct linkages between a developmental GRN and an effector gene that controls a key morphogenetic cell behavior, cell–cell fusion, providing a paradigm for extending the explanatory power of GRNs.

Lessons from a transcription factor: Alx1 provides insights into gene regulatory networks, cellular reprogramming, and cell type evolution

The skeleton-forming cells of sea urchins and other echinoderms have been studied by developmental biologists as models of cell specification and morphogenesis for many decades. The gene regulatory network (GRN) deployed in the embryonic skeletogenic cells of euechinoid sea urchins is one of the best understood in any developing animal. Recent comparative studies have leveraged the information contained in this GRN, bringing renewed attention to the diverse patterns of skeletogenesis within the phylum and the evolutionary basis for this diversity. The homeodomain-containing transcription factor, Alx1, was originally shown to be a core component of the skeletogenic GRN of the sea urchin embryo. Alx1 has since been found to be key regulator of skeletal cell identity throughout the phylum. As such, Alx1 is currently serving as a lens through which multiple developmental processes are being investigated. These include not only GRN organization and evolution, but also cell reprogramming, cell type evolution, and the gene regulatory control of morphogenesis. This review summarizes our current state of knowledge concerning Alx1 and highlights the insights it is yielding into these important developmental and evolutionary processes.

The gene regulatory control of sea urchin gastrulation

The cell behaviors associated with gastrulation in sea urchins have been well described. More recently, considerable progress has been made in elucidating gene regulatory networks (GRNs) that underlie the specification of early embryonic territories in this experimental model. This review integrates information from these two avenues of work. I discuss the principal cell movements that take place during sea urchin gastrulation, with an emphasis on molecular effectors of the movements, and summarize our current understanding of the gene regulatory circuitry upstream of those effectors. A case is made that GRN biology can provide a causal explanation of gastrulation, although additional analysis is needed at several levels of biological organization in order to provide a deeper understanding of this complex morphogenetic process.

pmar1/phb homeobox genes and the evolution of the double-negative gate for endomesoderm specification in echinoderms

In several model animals, the earliest phases of embryogenesis are regulated by lineage-specific genes, such as Drosophila bicoid Sea urchin (echinoid) embryogenesis is initiated by zygotic expression of pmar1, a paired-class homeobox gene that has been considered to be present only in the lineage of modern urchins (euechinoids). In euechinoids, Pmar1 promotes endomesoderm specification by repressing the hairy and enhancer of split C (hesC) gene. Here, we have identified the basal echinoid (cidaroid) pmar1 gene, which also promotes endomesoderm specification but not by repressing hesC A further search for related genes demonstrated that other echinoderms have pmar1-related genes named phb Functional analyses of starfish Phb proteins indicated that, similar to cidaroid Pmar1, they promote activation of endomesoderm regulatory gene orthologs via an unknown repressor that is not HesC. Based on these results, we propose that Pmar1 may have recapitulated the regulatory function of Phb during the early diversification of echinoids and that the additional repressor HesC was placed under the control of Pmar1 in the euechinoid lineage. This case provides an exceptional model for understanding how early developmental processes diverge.

Expression of exogenous mRNAs to study gene function in echinoderm embryos

With the completion of the genome sequencing projects, a new challenge for developmental biologists is to assign a function to the thousands of genes identified. Expression of exogenous mRNAs is a powerful, versatile and rapid technique that can be used to study gene function during development of the sea urchin. This chapter describes how this technique can be used to analyze gene function in echinoderm embryos, how it can be combined with cell transplantation to perform mosaic analysis and how it can be applied to identify downstream targets genes of transcription factors and signaling pathways. We describe specific examples of the use of overexpression of mRNA to analyze gene function, mention the benefits and current limitations of the technique and emphasize the importance of using different controls to assess the specificity of the effects observed. Finally, this chapter details the different steps, vectors and protocols for in vitro production of mRNA and phenotypic analysis.

From genome to anatomy: The architecture and evolution of the skeletogenic gene regulatory network of sea urchins and other echinoderms

The skeletogenic gene regulatory network (GRN) of sea urchins and other echinoderms is one of the most intensively studied transcriptional networks in any developing organism. As such, it serves as a preeminent model of GRN architecture and evolution. This review summarizes our current understanding of this developmental network. We describe in detail the most comprehensive model of the skeletogenic GRN, one developed for the euechinoid sea urchin Strongylocentrotus purpuratus, including its initial deployment by maternal inputs, its elaboration and stabilization through regulatory gene interactions, and its control of downstream effector genes that directly drive skeletal morphogenesis. We highlight recent comparative studies that have leveraged the euechinoid GRN model to examine the evolution of skeletogenic programs in diverse echinoderms, studies that have revealed both conserved and divergent features of skeletogenesis within the phylum. Last, we summarize the major insights that have emerged from analysis of the structure and evolution of the echinoderm skeletogenic GRN and identify key, unresolved questions as a guide for future work.

p38 MAPK as an essential regulator of dorsal-ventral axis specification and skeletogenesis during sea urchin development: a re-evaluation

Dorsal-ventral axis formation in the sea urchin embryo relies on the asymmetrical expression of the TGFβ Nodal. The p38-MAPK pathway has been proposed to be essential for dorsal-ventral axis formation by acting upstream of nodal expression. Here, we report that, in contrast to previous studies that used pharmacological inhibitors of p38, manipulating the activity of p38 by genetic means has no obvious impact on morphogenesis. Instead, we discovered that p38 inhibitors strongly disrupt specification of all germ layers by blocking signalling from the Nodal receptor and by interfering with the ERK pathway. Strikingly, while expression of a mutant p38 that is resistant to SB203580 did not rescue dorsal-ventral axis formation or skeletogenesis in embryos treated with this inhibitor, expression of mutant Nodal receptors that are resistant to SB203580 fully restored nodal expression in SB203580-treated embryos. Taken together, these results establish that p38 activity is not required for dorsal-ventral axis formation through nodal expression nor for skeletogenesis. Our results prompt a re-evaluation of the conclusions of several recent studies that linked p38 activity to dorsal-ventral axis formation and to patterning of the skeleton.

Diversification of spatiotemporal expression and copy number variation of the echinoid hbox12/pmar1/micro1 multigene family

Changes occurring during evolution in the cis-regulatory landscapes of individual members of multigene families might impart diversification in their spatiotemporal expression and function. The archetypal member of the echinoid hbox12/pmar1/micro1 family is hbox12-a, a homeobox-containing gene expressed exclusively by dorsal blastomeres, where it governs the dorsal/ventral gene regulatory network during embryogenesis of the sea urchin Paracentrotus lividus. Here we describe the inventory of the hbox12/pmar1/micro1 genes in P. lividus, highlighting that gene copy number variation occurs across individual sea urchins of the same species. We show that the various hbox12/pmar1/micro1 genes group into three subfamilies according to their spatiotemporal expression, which ranges from broad transcription throughout development to transient expression in either the animal hemisphere or micromeres of the early embryo. Interestingly, the promoter regions of those genes showing comparable expression patterns are highly similar, while differing from those of the other subfamilies. Strikingly, phylogenetic analysis suggests that the hbox12/pmar1/micro1 genes are species-specific, exhibiting extensive divergence in their noncoding, but not in their coding, sequences across three distinct sea urchin species. In spite of this, two micromere-specific genes of P. lividus possess a TCF/LEF-binding motif in a similar position, and their transcription relies on Wnt/β-catenin signaling, similar to the pmar1 and micro1 genes, which in other sea urchin species are involved in micromere specification. Altogether, our findings suggest that the hbox12/pmar1/micro1 gene family evolved rather rapidly, generating paralogs whose cis-regulatory sequences diverged following multiple rounds of duplication from a common ancestor.

Roles of hesC and gcm in echinoid larval mesenchyme cell development

To understand the roles of hesC and gcm during larval mesenchyme specification and differentiation in echinoids, we performed perturbation experiments for these genes in two distantly related euechinoids, Hemicentrotus pulcherrimus and Scaphechinus mirabilis. The number of larval mesenchyme cells increased when the translation of hesC was inhibited, thereby suggesting that hesC has a general role in larval mesenchyme development. We confirmed previous results by demonstrating that gcm is involved in pigment cell differentiation. Simultaneous inhibition of the translation of hesC and gcm induced a significant increase in the number of skeletogenic cells, which suggests that gcm functions in skeletogenic fate repression. Based on these observations, we suggest that: (i) hesC participates in some general aspects of mesenchymal cell development; and (ii) gcm is involved in the mechanism responsible for the binary specification of skeletogenic and pigment cell fates.

The Maternal Maverick/GDF15-like TGF-β Ligand Panda Directs Dorsal-Ventral Axis Formation by Restricting Nodal Expression in the Sea Urchin Embryo

Specification of the dorsal-ventral axis in the highly regulative sea urchin embryo critically relies on the zygotic expression of nodal, but whether maternal factors provide the initial spatial cue to orient this axis is not known. Although redox gradients have been proposed to entrain the dorsal-ventral axis by acting upstream of nodal, manipulating the activity of redox gradients only has modest consequences, suggesting that other factors are responsible for orienting nodal expression and defining the dorsal-ventral axis. Here we uncover the function of Panda, a maternally provided transforming growth factor beta (TGF-β) ligand that requires the activin receptor-like kinases (Alk) Alk3/6 and Alk1/2 receptors to break the radial symmetry of the embryo and orient the dorsal-ventral axis by restricting nodal expression. We found that the double inhibition of the bone morphogenetic protein (BMP) type I receptors Alk3/6 and Alk1/2 causes a phenotype dramatically more severe than the BMP2/4 loss-of-function phenotype, leading to extreme ventralization of the embryo through massive ectopic expression of nodal, suggesting that an unidentified signal acting through BMP type I receptors cooperates with BMP2/4 to restrict nodal expression. We identified this ligand as the product of maternal Panda mRNA. Double inactivation of panda and bmp2/4 led to extreme ventralization, mimicking the phenotype caused by inactivation of the two BMP receptors. Inhibition of maternal panda mRNA translation disrupted the early spatial restriction of nodal, leading to persistent massive ectopic expression of nodal on the dorsal side despite the presence of Lefty. Phylogenetic analysis indicates that Panda is not a prototypical BMP ligand but a member of a subfamily of TGF-β distantly related to Inhibins, Lefty, and TGF-β that includes Maverick from Drosophila and GDF15 from vertebrates. Indeed, overexpression of Panda does not appear to directly or strongly activate phosphoSmad1/5/8 signaling, suggesting that although this TGF-β may require Alk1/2 and/or Alk3/6 to antagonize nodal expression, it may do so by sequestering a factor essential for Nodal signaling, by activating a non-Smad pathway downstream of the type I receptors, or by activating extremely low levels of pSmad1/5/8. We provide evidence that, although panda mRNA is broadly distributed in the early embryo, local expression of panda mRNA efficiently orients the dorsal-ventral axis and that Panda activity is required locally in the early embryo to specify this axis. Taken together, these findings demonstrate that maternal panda mRNA is both necessary and sufficient to orient the dorsal-ventral axis. These results therefore provide evidence that in the highly regulative sea urchin embryo, the activity of spatially restricted maternal factors regulates patterning along the dorsal-ventral axis.

Developmental gene regulatory network evolution: insights from comparative studies in echinoderms

One of the central concerns of Evolutionary Developmental biology is to understand how the specification of cell types can change during evolution. In the last decade, developmental biology has progressed toward a systems level understanding of cell specification processes. In particular, the focus has been on determining the regulatory interactions of the repertoire of genes that make up gene regulatory networks (GRNs). Echinoderms provide an extraordinary model system for determining how GRNs evolve. This review highlights the comparative GRN analyses arising from the echinoderm system. This work shows that certain types of GRN subcircuits or motifs, i.e., those involving positive feedback, tend to be conserved and may provide a constraint on development. This conservation may be due to a required arrangement of transcription factor binding sites in cis regulatory modules. The review will also discuss ways in which novelty may arise, in particular through the co-option of regulatory genes and subcircuits. The development of the sea urchin larval skeleton, a novel feature that arose in echinoderms, has provided a model for study of co-option mechanisms. Finally, the types of GRNs that can permit the great diversity in the patterns of ciliary bands and their associated neurons found among these taxa are discussed. The availability of genomic resources is rapidly expanding for echinoderms, including genome sequences not only for multiple species of sea urchins but also a species of sea star, sea cucumber, and brittle star. This will enable echinoderms to become a particularly powerful system for understanding how developmental GRNs evolve.

Par6 regulates skeletogenesis and gut differentiation in sea urchin larvae

Partitioning-defective (par) genes were originally identified as genes that are essential for the asymmetric division of the Caenorhabditis elegans zygote. Studies have since revealed that the gene products are part of an evolutionarily conserved PAR-atypical protein kinase C system involved in cell polarity in various biological contexts. In this study, we analyzed the function of par6 during sea urchin morphogenesis by morpholino-mediated knockdown and by manipulation swapping of the primary mesenchyme cells (PMCs). Loss of Par6 resulted in defects in skeletogenesis and gut differentiation in larvae. Phenotypic analyses of chimeras constructed by PMC swapping showed that Par6 in non-PMCs is required for differentiation of archenteron into functional gut. In contrast, Par6 in both PMCs and ectodermal cells cooperatively regulates skeletogenesis. We suggest that Par6 in PMCs plays an immediate role in the deposition of biomineral in the syncytial cable, whereas Par6 in ectoderm may stabilize skeletal rods via an unknown signal(s).

Implication of HpEts in gene regulatory networks responsible for specification of sea urchin skeletogenic primary mesenchyme cells

The large micromeres of the 32-cell stage of sea urchin embryos are autonomously specified and differentiate into primary mesenchyme cells (PMCs), giving rise to the skeletogenic cells. We previously demonstrated that HpEts, an ets-related transcription factor, plays an essential role in the specification of PMCs in sea urchin embryos. In order to clarify the function of HpEts in the gene regulatory network involved in PMC specification, we analyzed the zygotic expression pattern and the cis-regulatory region of HpEts, and examined the activity of the HpEts protein as a transcription factor. Intron-based PCR reveals that zygotic expression of HpEts starts at the cleavage stage, and that the rate of transcription reaches maximum at the unhatched blastula stage. A series of progressive deletions of the fragments from -4.2 kbp to +1206 bp of the HpEts, which directs PMC-specific expression, caused a gradual decrease in the specificity, implying that coordination of several cis-regulatory elements regulates the expression in PMCs. A minimum cis-element required for the temporal expression is located within a 10 bp from -243 bp to -234 bp. The HpEts protein remains in the cytoplasm of entire embryonic cells in the cleavage stage. At the unhatched blastula stage, the HpEts protein translocates into the nucleus in presumptive PMCs. Transactivation assays demonstrate that the HpEts protein activates a promoter of Spicule Matrix Protein 50 (SM50), which is a target of HpEts, which binds to the regulatory region of SM50.

Activation of the skeletogenic gene regulatory network in the early sea urchin embryo

The gene regulatory network (GRN) that underlies the development of the embryonic skeleton in sea urchins is an important model for understanding the architecture and evolution of developmental GRNs. The initial deployment of the network is thought to be regulated by a derepression mechanism, which is mediated by the products of the pmar1 and hesC genes. Here, we show that the activation of the skeletogenic network occurs by a mechanism that is distinct from the transcriptional repression of hesC. By means of quantitative, fluorescent whole-mount in situ hybridization, we find that two pivotal early genes in the network, alx1 and delta, are activated in prospective skeletogenic cells prior to the downregulation of hesC expression. An analysis of the upstream regulation of alx1 shows that this gene is regulated by MAPK signaling and by the transcription factor Ets1; however, these inputs influence only the maintenance of alx1 expression and not its activation, which occurs by a distinct mechanism. By altering normal cleavage patterns, we show that the zygotic activation of alx1 and delta, but not that of pmar1, is dependent upon the unequal division of vegetal blastomeres. Based on these findings, we conclude that the widely accepted double-repression model is insufficient to account for the localized activation of the skeletogenic GRN. We postulate the existence of additional, unidentified repressors that are controlled by pmar1, and propose that the ability of pmar1 to derepress alx1 and delta is regulated by the unequal division of vegetal blastomeres.

Evolutionary modification of T-brain (tbr) expression patterns in sand dollar

The sand dollars are a group of irregular echinoids that diverged from other regular sea urchins approximately 200 million years ago. We isolated two orthologs of T-brain (tbr), Smtbr and Pjtbr, from the indirect developing sand dollar Scaphechinus mirabilis and the direct developing sand dollar Peronella japonica, respectively. The expression patterns of Smtbr and Pjtbr during early development were examined by whole mount in situ hybridization. The expression of Smtbr was first detected in micromere descendants in early blastula stage, similar to tbr expression in regular sea urchins. However, unlike in regular sea urchin, Smtbr expression in middle blastula stage was detected in micromere-descendent cells and a subset of macromere-descendant cells. At gastrula stage, expression of Smtbr was detected in part of the archenteron as well as primary mesenchyme cells. A similar pattern of tbr expression was observed in early Peronella embryos. A comparison of tbr expression patterns between sand dollars and other echinoderm species suggested that broader expression in the endomesoderm is an ancestral character of echinoderms. In addition to the endomesoderm, Pjtbr expression was detected in the apical organ, the animal-most part of the ectoderm.

Lessons from a gene regulatory network: echinoderm skeletogenesis provides insights into evolution, plasticity and morphogenesis

Significant new insights have emerged from the analysis of a gene regulatory network (GRN) that underlies the development of the endoskeleton of the sea urchin embryo. Comparative studies have revealed ways in which this GRN has been modified (and conserved) during echinoderm evolution, and point to mechanisms associated with the evolution of a new cell lineage. The skeletogenic GRN has also recently been used to study the long-standing problem of developmental plasticity. Other recent findings have linked this transcriptional GRN to morphoregulatory proteins that control skeletal anatomy. These new studies highlight powerful new ways in which GRNs can be used to dissect development and the evolution of morphogenesis.

cis-Regulatory sequences driving the expression of the Hbox12 homeobox-containing gene in the presumptive aboral ectoderm territory of the Paracentrotus lividus sea urchin embryo

Embryonic development is coordinated by networks of evolutionary conserved regulatory genes encoding transcription factors and components of cell signalling pathways. In the sea urchin embryo, a number of genes encoding transcription factors display territorial restricted expression. Among these, the zygotic Hbox12 homeobox gene is transiently transcribed in a limited number of cells of the animal-lateral half of the early Paracentrotus lividus embryo, whose descendants will constitute part of the ectoderm territory. To obtain insights on the regulation of Hbox12 expression, we have explored the cis-regulatory apparatus of the gene. In this paper, we show that the intergenic region of the tandem Hbox12 repeats drives GFP expression in the presumptive aboral ectoderm and that a 234 bp fragment, defined aboral ectoderm (AE) module, accounts for the restricted expression of the transgene. Within this module, a consensus sequence for a Sox factor and the binding of the Otx activator are both required for correct Hbox12 gene expression. Spatial restriction to the aboral ectoderm is achieved by a combination of different repressive sequence elements. Negative sequence elements necessary for repression in the endomesoderm map within the most upstream 60 bp region and nearby the Sox binding site. Strikingly, a Myb-like consensus is necessary for repression in the oral ectoderm, while down-regulation at the gastrula stage depends on a GA-rich region. These results suggest a role for Hbox12 in aboral ectoderm specification and represent our first attempt in the identification of the gene regulatory circuits involved in this process.

Krüppel-like is required for nonskeletogenic mesoderm specification in the sea urchin embryo

The canonical Wnt pathway plays a central role in specifying vegetal cell fate in sea urchin embryos. SpKrl has been cloned as a direct target of nuclear beta-catenin. Using Hemicentrotus pulcherrimus embryos, here we show that HpKrl controls the specification of secondary mesenchyme cells (SMCs) through both cell-autonomous and non-autonomous means. Like SpKrl, HpKrl was activated in both micromere and macromere progenies. To examine the functions of HpKrl in each blastomere, we constructed chimeric embryos composed of blastomeres from control and morpholino-mediated HpKrl-knockdown embryos and analyzed the phenotypes of the chimeras. Micromere-swapping experiments showed that HpKrl is not involved in micromere specification, while micromere-deprivation assays indicated that macromeres require HpKrl for cell-autonomous specification. Transplantation of normal micromeres into a micromere-less host with morpholino revealed that macromeres are able to receive at least some micromere signals regardless of HpKrl function. From these observations, we propose that two distinct pathways of endomesoderm formation exist in macromeres, a Krl-dependent pathway and a Krl-independent pathway. The Krl-independent pathway may correspond to the Delta/Notch signaling pathway via GataE and Gcm. We suggest that Krl may be a downstream component of nuclear beta-catenin required by macromeres for formation of more vegetal tissues, not as a member of the Delta/Notch pathway, but as a parallel effector of the signaling (Krl-dependent pathway).

Gene regulatory networks and developmental plasticity in the early sea urchin embryo: alternative deployment of the skeletogenic gene regulatory network

Cell fates in the sea urchin embryo are remarkably labile, despite the fact that maternal polarity and zygotic programs of differential gene expression pattern the embryo from the earliest stages. Recent work has focused on transcriptional gene regulatory networks (GRNs) deployed in specific embryonic territories during early development. The micromere-primary mesenchyme cell(PMC) GRN drives the development of the embryonic skeleton. Although normally deployed only by presumptive PMCs, every lineage of the early embryo has the potential to activate this pathway. Here, we focus on one striking example of regulative activation of the skeletogenic GRN; the transfating of non-skeletogenic mesoderm (NSM) cells to a PMC fate during gastrulation. We show that transfating is accompanied by the de novo expression of terminal,biomineralization-related genes in the PMC GRN, as well as genes encoding two upstream transcription factors, Lvalx1 and Lvtbr. We report that Lvalx1, a key component of the skeletogenic GRN in the PMC lineage, plays an essential role in the regulative pathway both in NSM cells and in animal blastomeres. MAPK signaling is required for the expression of Lvalx1 and downstream skeletogenic genes in NSM cells, mirroring its role in the PMC lineage. We also demonstrate that Lvalx1 regulates the signal from PMCs that normally suppresses NSM transfating. Significantly,misexpression of Lvalx1 in macromeres (the progenitors of NSM cells)is sufficient to activate the skeletogenic GRN. We suggest that NSM cells normally deploy a basal mesodermal pathway and require only an Lvalx1-mediated sub-program to express a PMC fate. Finally, we provide evidence that, in contrast to the normal pathway, activation of the skeletogenic GRN in NSM cells is independent of Lvpmar1. Our studies reveal that, although most features of the micromere-PMC GRN are recapitulated in transfating NSM cells, different inputs activate this GRN during normal and regulative development.

The micro1 gene is necessary and sufficient for micromere differentiation and mid/hindgut-inducing activity in the sea urchin embryo

In the sea urchin embryo, micromeres have two distinct functions: they differentiate cell autonomously into the skeletogenic mesenchyme cells and act as an organizing center that induces endomesoderm formation. We demonstrated that micro1 controls micromere specification as a transcriptional repressor. Because micro1 is a multicopy gene with at least six polymorphic loci, it has been difficult to consistently block micro1 function by morpholino-mediated knockdown. Here, to block micro1 function, we used an active activator of micro1 consisting of a fusion protein of the VP16 activation domain and the micro1 homeodomain. Embryos injected with mRNA encoding the fusion protein exhibited a phenotype similar to that of micromere-less embryos. To evaluate micro1 function in the micromere, we constructed chimeric embryos composed of animal cap mesomeres and a micromere quartet from embryos injected with the fusion protein mRNA. The chimeras developed into dauerblastulae with no vegetal structures, in which the micromere progeny constituted the blastula wall. We also analyzed the phenotype of chimeras composed of an animal cap and a mesomere expressing micro1. These chimeras developed into pluteus larvae, in which the mesomere descendants ingressed as primary mesenchyme cells and formed a complete set of skeletal rods. The hindgut and a part of the midgut were also generated from host mesomeres. However, the foregut and nonskeletogenic mesoderm were not formed in the larvae. From these observations, we conclude that micro1 is necessary and sufficient for both micromere differentiation and mid/hindgut-inducing activity, and we also suggest that micro1 may not fulfill all micromere functions.