Repression of zygotic gene expression in the putative germline cells in ascidian embryos

H3K27me3 suppresses sister-lineage somatic gene expression in late embryonic germline cells of the ascidian, Halocynthia roretzi

Protection of the germline from somatic differentiation programs is crucial for germ cell development. In many animals, whose germline development relies on the maternally inherited germ plasm, such protection in particular at early stages of embryogenesis is achieved by maternally localized global transcriptional repressors, such as PIE-1 of Caenorhabditis elegans, Pgc of Drosophila melanogaster and Pem of ascidians. However, zygotic gene expression starts in later germline cells eventually and mechanisms by which somatic gene expression is selectively kept under repression in the transcriptionally active cells are poorly understood. By using the ascidian species Halocynthia roretzi, we found that H3K27me3, a repressive transcription-related chromatin mark, became enriched in germline cells starting at the 64-cell stage when Pem protein level and its contribution to transcriptional repression decrease. Interestingly, inhibition of H3K27me3 together with Pem knockdown resulted in ectopic expression in germline cells of muscle developmental genes Muscle actin (MA4) and Snail, and of Clone 22 (which is expressed in all somatic but not germline cells), but not of other tissue-specific genes such as the notochord gene Brachyury, the nerve cord marker ETR-1 and a heart precursor gene Mesp, at the 110-cell stage. Importantly, these ectopically expressed genes are normally expressed in the germline sister cells (B7.5), the last somatic lineage separated from the germline. Also, the ectopic expression of MA4 was dependent on a maternally localized muscle determinant Macho-1. Taken together, we propose that H3K27me3 may be responsible for selective transcriptional repression for somatic genes in later germline cells in Halocynthia embryos and that the preferential repression of germline sister-lineage genes may be related to the mechanism of germline segregation in ascidian embryos, where the germline is segregated progressively by successive asymmetric cell divisions during cell cleavage stages. Together with findings from C. elegans and D. melanogaster, our data for this urochordate animal support the proposal for a mechanism, conserved widely throughout the animal kingdom, where germline transcriptional repression is mediated initially by maternally localized factors and subsequently by a chromatin-based mechanism.

Maternal Nanos inhibits Importin-α2/Pendulin-dependent nuclear import to prevent somatic gene expression in the Drosophila germline

Repression of somatic gene expression in germline progenitors is one of the critical mechanisms involved in establishing the germ/soma dichotomy. In Drosophila, the maternal Nanos (Nos) and Polar granule component (Pgc) proteins are required for repression of somatic gene expression in the primordial germ cells, or pole cells. Pgc suppresses RNA polymerase II-dependent global transcription in pole cells, but it remains unclear how Nos represses somatic gene expression. Here, we show that Nos represses somatic gene expression by inhibiting translation of maternal importin-α2 (impα2) mRNA. Mis-expression of Impα2 caused aberrant nuclear import of a transcriptional activator, Ftz-F1, which in turn activated a somatic gene, fushi tarazu (ftz), in pole cells when Pgc-dependent transcriptional repression was impaired. Because ftz expression was not fully activated in pole cells in the absence of either Nos or Pgc, we propose that Nos-dependent repression of nuclear import of transcriptional activator(s) and Pgc-dependent suppression of global transcription act as a 'double-lock' mechanism to inhibit somatic gene expression in germline progenitors.

Control of Pem protein level by localized maternal factors for transcriptional regulation in the germline of the ascidian, Halocynthia roretzi

Localized maternal mRNAs play important roles in embryogenesis, e.g. the establishment of embryonic axes and the developmental cell fate specification, in various animal species. In ascidians, a group of maternal mRNAs, called postplasmic/PEM RNAs, is localized to a subcellular structure, called the Centrosome-Attracting Body (CAB), which contains the ascidian germ plasm, and is inherited by the germline cells during embryogenesis. Posterior end mark (Pem), a postplasmic/PEM RNAs member, represses somatic gene expression in the germline during cleavage stages by inhibition of RNA polymerase II activity. However, the functions of other postplasmic/ PEM RNAs members in germline formation are largely unknown. In this study, we analyzed the functions of two postplasmic/PEM RNAs, Popk-1 and Zf-1, in transcriptional regulation in the germline cells. We show that Popk-1 contributes to transcriptional quiescence by controlling the size of the CAB and amount of Pem protein translated at the CAB. Our studies also indicated that zygotic expression of a germline gene starts around the onset of gastrulation and that the decrease of Pem protein is necessary and sufficient for the zygotic germline gene expression. Finally, further studies showed that the decrease of the Pem protein level is facilitated by Zf-1. Taken together, we propose that postplasmic/PEM RNAs such as Popk-1 and Zf-1 control the protein level of the transcriptional repressor Pem and regulate its transcriptional state in the ascidian germline.

Cytoplasmic localization and reorganization in ascidian eggs: role of postplasmic/PEM RNAs in axis formation and fate determination

Localization of maternal molecules in eggs and embryos and cytoplasmic movements to relocalize them are fundamental for the orderly cellular and genetic processes during early embryogenesis. Ascidian embryos have been known as 'mosaic eggs' because of their autonomous differentiation abilities based on localized cell fate determinants. This review gives a historical overview of the concept of cytoplasmic localization, and then explains the key features such as ooplasmic movements and cell lineages that are essential to grasp the process of ascidian development mediated by localized determinant activities. These activities are partly executed by localized molecules named postplasmic/PEM RNAs, originating from approximately 50 genes, of which the muscle determinant, macho-1, is an example. The cortical domain containing these RNAs is relocalized to the posterior-vegetal region of the egg by cytoskeletal movements after fertilization, and plays crucial roles in axis formation and cell fate determination. The cortical domain contains endoplasmic reticulum and characteristic granules, and gives rise to a subcellular structure called the centrosome-attracting body (CAB), in which postplasmic/PEM RNAs are highly concentrated. The CAB is responsible for a series of unequal partitionings of the posterior-vegetal cytoplasmic domain and the postplasmic/PEM RNAs at the posterior pole during cleavage. Some components of this domain, which is rich in granules, are eventually inherited by prospective germline cells with particular postplasmic/PEM RNAs such as vasa. The postplasmic/PEM RNAs are classified into two groups according to their final cellular destinations and localization pathways. Localization of these RNAs is regulated by specific nucleotide sequences in the 3' untranslated regions (3'UTRs).

Ci-Pem-1 localizes to the nucleus and represses somatic gene transcription in the germline of Ciona intestinalis embryos

In many animal embryos, germ-cell formation depends on maternal factors located in the germ plasm. To ensure the development of germ cells, germline progenitors must be prevented from differentiating inappropriately into somatic cells. A common mechanism for this appears to be the active repression of somatic gene transcription. Species-specific germ-plasm components, such as Pgc in Drosophila and PIE-1 in C. elegans, establish germline transcriptional quiescence by inhibiting general transcriptional machineries. In the ascidian Ciona intestinalis, although transcriptional repression in the germline has been proposed, the factors and mechanisms involved have been unknown. We found that the protein products of Ci-pem-1 RNA, which is an ascidian-specific component of the postplasm (the germ plasm equivalent in ascidians), localized to the nucleus of germline blastomeres, as well as to the postplasm. Morpholino oligonucleotide-mediated Ci-pem-1 knockdown resulted in the ectopic expression of several somatic genes that are usually silent in the germline. In the Ci-pem-1 knockdown embryos, the expression of both β-catenin- and GATAa-dependent genes was derepressed in the germline blastomeres, suggesting that Ci-Pem-1 broadly represses germline mRNA transcription. Immunoprecipitation assays showed that Ci-Pem-1 could interact with two C. intestinalis homologs of Groucho, which is a general co-repressor of mRNA transcription. These results suggest that Ci-pem-1 is the C. intestinalis version of a germ-plasm RNA whose protein product represses the transcription of somatic genes during specification of the germ-cell fate, and that this repression may be operated through interactions between Ci-Pem-1 and Groucho co-repressors.

A maternal factor unique to ascidians silences the germline via binding to P-TEFb and RNAP II regulation

Suppression of zygotic transcription in early embryonic germline cells is tightly linked to their separation from the somatic lineage. Many invertebrate embryos utilize localized maternal factors that are successively inherited by the germline cells for silencing the germline. Germline quiescence has also been associated with the underphosphorylation of Ser2 of the C-terminal domain (CTD-Ser2) of RNA polymerase II [1-3]. Here, using the ascidian Halocynthia roretzi, we identified a first deuterostome example of a maternally localized factor, posterior end mark (PEM), which globally represses germline transcription. PEM knockdown resulted in ectopic transcription and ectopic phosphorylation of CTD-Ser2 in the germline. Overexpression of PEM abolished all transcription and led to the underphosphorylation of CTD-Ser2 in the somatic cells. PEM protein was reiteratively detected in the nucleus of the germline cells and coimmunoprecipitated with CDK9, a component of posterior transcription elongation factor b (P-TEFb). These results suggest that nonhomologous proteins, PEM and Pgc of Drosophila [3-5] and PIE-1 of C. elegans [1, 6, 7], repress germline gene expression through analogous functions: by keeping CTD-Ser2 underphosphorylated through binding to the P-TEFb complex. The present study is an interesting example of evolutionary constraint on how a mechanism of germline silencing can evolve in diverse animals.

Repression of early zygotic transcription in the germline

Germ cells, the progenitors of gametes, are often specified and segregated from somatic lineages early in embryogenesis. As germ cells are essential to create the next generation in sexually reproducing organisms, they must be prevented from differentiating inappropriately into somatic cells. In Drosophila and Caenorhabditis elegans embryos, this is governed by the transient and global repression of mRNA transcription. Furthermore, the inhibition of somatic transcriptional programs is also crucial for germ cell specification in the mouse. Therefore, the active repression of somatic transcriptional programs appears to be a common mechanism for launching the germline. In this review, we will discuss the mechanisms of transcriptional repression during germ cell specification and their interspecies similarities and differences.

Germline cell formation and gonad regeneration in solitary and colonial ascidians

The morphology of ascidian gonad is very similar among species. The testis consists of variable number of testicular follicles; the ovary consists of ovarian tubes that are thickened forming the germinal epithelium with stem cells for female germ cells with the exception of botryllid ascidians. Peculiar accessory cells that would be germline in origin accompany the oocytes. Using vasa homologues as a molecular marker, germline precursor cells can be traced back to the embryonic posterior-most blastomeres and are found in the tail of tailbud embryo in some solitary and colonial ascidians. In Ciona, they are subsequently located in the larval tail, while in colonial botryllid ascidians vasa-expressing cells become obscure in the tail. Recent evidence suggests that ascidian germ cells can regenerate from cells other than embryonic germline. An ensemble of the embryonic stringency of germ cell lineage and the postembryonic flexibility of gonad formation is discussed.

Repression of zygotic gene expression in the Xenopus germline

Primordial germ cells (PGCs) in Xenopus are specified through the inheritance of germ plasm. During gastrulation, PGCs remain totipotent while surrounding cells in the vegetal mass become committed to endoderm through the action of the vegetal localized maternal transcription factor VegT. We find that although PGCs contain maternal VegT RNA, they do not express its downstream targets at the mid-blastula transition (MBT). Transcriptional repression in PGCs correlates with the failure to phosphorylate serine 2 in the carboxy-terminal domain (CTD) of the large subunit of RNA polymerase II (RNAPII). As serine 5 is phosphorylated, these results are consistent with a block after the initiation step but before the elongation step of RNAPII-based transcription. Repression of PGC gene expression occurs despite an apparently permissive chromatin environment. Phosphorylation of CTD-serine 2 and expression of zygotic mRNAs in PGCs are first detected at neurula, some 10 hours after MBT, indicating that transcription is significantly delayed in the germ cell lineage. Significantly, Oct-91, a POU subclass V transcription factor related to mammalian Oct3/4, is among the earliest zygotic transcripts detected in PGCs and is a likely mediator of pluripotency. Our findings suggest that PGCs are unable to respond to maternally inherited endoderm determinants because RNAPII activity is transiently blocked while these determinants are present. Our results in a vertebrate system further support the concept that one strategy used repeatedly during evolution for preserving the germline is RNAPII repression.

Inhibition of transcription by the Caenorhabditis elegans germline protein PIE-1: genetic evidence for distinct mechanisms targeting initiation and elongation

In Caenorhabditis elegans embryos, specification of the germ lineage depends on PIE-1, a maternal protein that blocks mRNA transcription in germline blastomeres. Studies in mammalian cell culture have suggested that PIE-1 inhibits P-TEFb, a kinase that phosphorylates serine 2 in the carboxyl-terminal domain (CTD) repeats of RNA polymerase II during transcriptional elongation. We have tested this hypothesis using an in vivo complementation assay for PIE-1 function. Our results support the view that PIE-1 inhibits P-TEFb using the CTD-like motif YAPMAPT. This activity is required to block serine 2 phosphorylation in germline blastomeres, but unexpectedly is not essential for transcriptional repression or specification of the germline. We find that sequences outside of the YAPMAPT are required to inhibit serine 5 phosphorylation, and that this second inhibitory mechanism is essential for transcriptional repression and specification of the germ lineage. Our results suggest that PIE-1 uses partially redundant mechanisms to block transcription by targeting both the initiation and elongation phases of the transcription cycle.

Cleavage pattern, gastrulation, and neurulation in the appendicularian, Oikopleura dioica

The appendicularian, Oikopleura dioica is a chordate. Its life cycle is extremely short--approximately 5 days--and its tadpole shape with a beating tail is retained throughout entire life. The tadpole hatches after 3 h of development at 20 degrees C. Here, we describe the cleavage pattern and morphogenetic cell movements during gastrulation and neurulation. Cleavage showed an invariant pattern. It is basically bilateral but also shows various minor left-right asymmetries starting from the four-cell stage. We observed two rounds of unequal cleavage of the posterior-vegetal B-line cells at the posterior pole. The nature of the unequal cleavages is reminiscent of those in ascidian embryos and suggests the presence of a centrosome-attracting body, a special subcellular structure at the posterior pole. The representation of the cell division pattern in this report will aid the identification of each cell, a prerequisite for clarifying the gene expression patterns in early embryos. Gastrulation started as early as the 32-cell stage and progressed in three phases. By the end of the second phase at the 64-cell stage, every vegetal cell had ingressed into the embryo, and animal cells had covered the entire embryo by epiboly. There was no archenteron formation. In the anterior region, eight A-line cells were aligned as a 2x4 array along the anterior-posterior axis and become internalized during the 64-cell stage. This process was considered to correspond to neurulation. The simple and accelerated development of Oikopleura, nevertheless giving rise to a conserved chordate body plan, is advantageous for studying developmental mechanisms using molecular and genetic approaches and makes this animal the simplest model organism in the phylum Chordata.

Postplasmic/PEM RNAs: a class of localized maternal mRNAs with multiple roles in cell polarity and development in ascidian embryos

Ascidian is a good model to understand the cellular and molecular mechanisms responsible for mRNA localization with the discovery of a large family of localized maternal mRNAs, called postplasmic/PEM RNAs, which includes more than 40 members in three different ascidian species (Halocynthia roretzi, Ciona intestinalis, and C. savignyi). Among these mRNAs, two types (Type I and Type II) have been identified and show two different localization patterns from fertilization to the eight-cell stage. At the eight-cell stage, both types concentrate to a macromolecular cortical structure called CAB (for Centrosome Attracting Body) in the posterior-vegetal B4.1 blastomeres. The CAB is responsible for unequal cleavages and the partitioning of postplasmic/PEM RNAs at the posterior pole of embryos during cleavage stages. It has also been suggested that the CAB region could contain putative germ granules. In this review, we discuss recent data obtained on the distribution of Type I postplasmic/PEM RNAs from oogenesis to late development, in relation to their localization and translational control. We have first regrouped localization patterns for Type I and Type II into a comparative diagram and included all important definitions in the field. We also have made an exhaustive classification of their embryonic expression profiles (Type I or Type II), and analyzed their functions after knockdown and/or overexpression experiments and the role of the 3'-untranslated region (3'UTR) controlling both their localization and translation. Finally, we propose a speculative model integrating recent data, and we also discuss the relationship between postplasmic/PEM RNAs, posterior specification, and germ cell formation in ascidians.

Brain induction in ascidian embryos is dependent on juxtaposition of FGF9/16/20-producing and -receiving cells

Coordinated regulation of inductive events, both spatially and temporally, during animal development ensures that tissues are induced at their specific positions within the embryo. The ascidian brain is induced in cells at the anterior edge of the animal hemisphere by fibroblast growth factor (FGF) signals secreted from vegetal cells. To clarify how this process is spatially regulated, we first identified the sources of the FGF signal by examining the expression of brain markers Hr-Otx and Hr-ETR-1 in embryos in which FGF signaling is locally inhibited by injecting individual blastomeres with morpholino oligonucleotide against Hr-FGF9/16/20, which encodes an endogenous brain inducer. The blastomeres identified as the inducing sources are A5.1 and A5.2 at the 16-cell stage and A6.2 and A6.4 at the 24-cell stage, which are juxtaposed with brain precursors at the anterior periphery of the embryo at the respective stages. We also showed that all the cells of the animal hemisphere are capable of expressing Hr-Otx in response to the FGF signal. These results suggest that the position of inducers, rather than competence, plays an important role in determining which animal cells are induced to become brain tissues during ascidian embryogenesis. This situation in brain induction contrasts with that in mesoderm induction, where the positions at which the notochord and mesenchyme are induced are determined mainly by intrinsic competence factors that are inherited by signal-receiving cells.

From oocyte to 16-cell stage: Cytoplasmic and cortical reorganizations that pattern the ascidian embryo

The dorsoventral and anteroposterior axes of the ascidian embryo are defined before first cleavage by means of a series of reorganizations that reposition cytoplasmic and cortical domains established during oogenesis. These domains situated in the periphery of the oocyte contain developmental determinants and a population of maternal postplasmic/PEM RNAs. One of these RNAs (macho-1) is a determinant for the muscle cells of the tadpole embryo. Oocytes acquire a primary animal-vegetal (a-v) axis during meiotic maturation, when a subcortical mitochondria-rich domain (myoplasm) and a domain rich in cortical endoplasmic reticulum (cER) and maternal postplasmic/PEM RNAs (cER-mRNA domain) become polarized and asymmetrically enriched in the vegetal hemisphere. Fertilization at metaphase of meiosis I initiates a series of dramatic cytoplasmic and cortical reorganizations of the zygote, which occur in two major phases. The first major phase depends on sperm entry which triggers a calcium wave leading in turn to an actomyosin-driven contraction wave. The contraction concentrates the cER-mRNA domain and myoplasm in and around a vegetal/contraction pole. The precise localization of the vegetal/contraction pole depends on both the a-v axis and the location of sperm entry and prefigures the future site of gastrulation and dorsal side of the embryo. The second major phase of reorganization occurs between meiosis completion and first cleavage. Sperm aster microtubules and then cortical microfilaments cause the cER-mRNA domain and myoplasm to reposition toward the posterior of the zygote. The location of the posterior pole depends on the localization of the sperm centrosome/aster attained during the first major phase of reorganization. Both cER-mRNA and myoplasm domains localized in the posterior region are partitioned equally between the first two blastomeres and then asymmetrically over the next two cleavages. At the eight-cell stage the cER-mRNA domain compacts and gives rise to a macroscopic cortical structure called the Centrosome Attracting Body (CAB). The CAB is responsible for a series of unequal divisions in posterior-vegetal blastomeres, and the postplasmic/PEM RNAs it contains are involved in patterning the posterior region of the embryo. In this review, we discuss these multiple events and phases of reorganizations in detail and their relationship to physiological, cell cycle, and cytoskeletal events. We also examine the role of the reorganizations in localizing determinants, postplasmic/PEM RNAs, and PAR polarity proteins in the cortex. Finally, we summarize some of the remaining questions concerning polarization of the ascidian embryo and provide comparisons to a few other species. A large collection of films illustrating the reorganizations can be consulted by clicking on "Film archive: ascidian eggs and embryos" at http://biodev.obs-vlfr.fr/recherche/biomarcell/.

Ciona intestinalis as a model for cardiac development

The primitive chordate Ciona intestinalis has emerged as a significant model system for the study of heart development. The Ciona embryo employs a conserved heart gene network in the context of extremely low cell numbers and reduced genetic redundancy. Here, I review recent studies on the molecular genetics of Ciona cardiogenesis as well as classic work on heart anatomy and physiology. I also discuss the potential of employing Ciona to decipher a comprehensive chordate gene network and to determine how this network controls heart morphogenesis.

Dynamic redistribution of vasa homolog and exclusion of somatic cell determinants during germ cell specification in Ciona intestinalis

Ascidian embryos sequester a specific cytoplasm, called the postplasm, at the posterior pole, where many maternal RNAs and proteins accumulate. Although the postplasm is thought to act as the germ plasm, it is also highly enriched in several factors essential for somatic cell development, and how the postplasm components regulate both germ and somatic cell differentiation remains elusive. Using a vasa homolog, CiVH, and other postplasmic components as markers, we found that the postplasm-containing blastomeres, the B7.6 cells, undergo an asymmetric cell division during gastrulation to produce two distinct daughter cells: B8.11 and B8.12. Most of the postplasmic components segregate only into the B8.11 cells, which never coalesce into the gonad. By contrast, the maternal CiVH RNA and protein are specifically distributed into the B8.12 cells, which divide further and are incorporated into the gonad in juveniles. In the B8.12 cells, CiVH production is upregulated from the maternal RNA source, resulting in the formation of perinuclear CiVH granules, which may be the nuage, a hallmark of germ cells in many animal species. We propose that the redistribution of specific maternal molecules into the B8.12 cells is essential for germ-cell specification in ascidians.

The aPKC-PAR-6-PAR-3 cell polarity complex localizes to the centrosome attracting body, a macroscopic cortical structure responsible for asymmetric divisions in the early ascidian embryo

Posterior blastomeres of 8-cell stage ascidian embryos undergo a series of asymmetric divisions that generate cells of unequal sizes and segregate muscle from germ cell fates. These divisions are orchestrated by a macroscopic cortical structure, the ;centrosome attracting body' (CAB) which controls spindle positioning and distribution of mRNA determinants. The CAB is composed of a mass of cortical endoplasmic reticulum containing mRNAs (the cER-mRNA domain) and an electron dense matrix, but little is known about its precise structure and functions. We have examined the ascidian homologues of PAR proteins, known to regulate polarity in many cell types. We found that aPKC, PAR-6 and PAR-3 proteins, but not their mRNAs, localize to the CAB during the series of asymmetric divisions. Surface particles rich in aPKC concentrate in the CAB at the level of cortical actin microfilaments and form a localized patch sandwiched between the plasma membrane and the cER-mRNA domain. Localization of aPKC to the CAB is dependent on actin but not microtubules. Both the aPKC layer and cER-mRNA domain adhere to cortical fragments prepared from 8-cell stage embryos. Astral microtubules emanating from the proximal centrosome contact the aPKC-rich cortical domain. Our observations indicate that asymmetric division involves the accumulation of the aPKC-PAR-6-PAR-3 complex at the cortical position beneath the pre-existing cER-mRNA domain.

Specification of embryonic axis and mosaic development in ascidians

Setting up future body axes is the first important event before and at the beginning of embryogenesis. The ascidian embryo is a classic model that has been used to gain insight into developmental processes for over a century. This review summarizes advances made in this decade in our understanding of the developmental processes involved in the specification of the embryonic axes and cell fates during early ascidian embryogenesis. Maternal factors, including mRNAs, are translocated to specific regions of the egg by cytoplasmic and cortical reorganization, so-called ooplasmic segregation, and specify the animal-vegetal axis and the one perpendicular to it, which is defined as the anteroposterior axis in ascidians. Some postplasmic/PEM RNAs that are anchored to cortical endoplasmic reticulum are brought to the future posterior pole of fertilized eggs, and play crucial roles in posterior development. Following specification of the animal-vegetal axis, nuclear localization of beta-catenin takes place in the vegetal blastomeres; this occurrence is important for the acquisition of the vegetal character of the blastomeres in later development. Positioning of these maternal factors lead to subsequent cell interactions and zygotic gene expression responsible for axis establishment and for cell fate specification. We describe how endoderm blastomeres in the vegetal pole region emanate inductive signals mainly attributable to fibroblast growth factor. Marginal blastomeres next to endoderm blastomeres respond differently in ways that are determined by intrinsic competence factors. Expression patterns of developmentally important genes, including key transcription factors of each tissue type, are also summarized.

Maternal determinants and mRNAs in the cortex of ascidian oocytes, zygotes and embryos

The peripheral region of ascidian oocytes and zygotes contains five determinants for morphogenesis and differentiation of the embryo. The determinant for the 24 primary muscle cells of the tadpole, macho1, is one of several cortical mRNAs localized in a gradient along the animal-vegetal axis in the oocyte. After fertilization these mRNAs, together with cortical endoplasmic reticulum (cER) and a subcortical mitochondria-rich domain (myoplasm), relocate in two major reorganization phases forming the posterior plasm (postplasm) of the zygote. At the 8-cell stage cortical mRNAs concentrate in a macroscopic cortical structure called the centrosome-attracting body (CAB), forming a characteristic posterior end mark (PEM) in the two posterior vegetal blastomeres. We propose to call the numerous mRNAs showing this particular cortical localization in the posterior region of the embryo postplasmic/PEM RNAs and suggest a nomemclature. We do not know how postplasmic/PEM RNAs reach their polarized distribution in the oocyte cortex but at least PEM1 and macho1 (and probably others) bind to the network of cER retained in isolated cortical fragments. We propose that after fertilization, these postplasmic/PEM mRNAs move in the zygote cortex together with the cER network (cER/mRNA domain) via microfilament- and microtubule-driven translocations. The cER/mRNA domain is localized posteriorly at the time of first cleavage and distributed equally between the first two blastomeres. After the third cleavage, the cER/mRNA domain and dense particles compact to form the CAB in posterior vegetal blastomeres of the 8-cell stage. We discuss the identity of postplasmic/PEM RNAs, how they localize, anchor, relocate and may be translated. We also examine their roles in unequal cleavage and as a source of posterior morphogenetic and differentiation factors.

Germ cells: finding programs of mass repression

Transcription is globally silenced in the germline of animals. Recent studies have shown that, in Caenorhabditis elegans, this silencing is initially mediated through direct repression, but in Drosophila, the factors involved include pgc, a non-coding cytoplasmic RNA. Why are these mechanisms so diverse and complex?

A conserved chromatin architecture marks and maintains the restricted germ cell lineage in worms and flies

In C. elegans, mRNA production is initially repressed in the embryonic germline by a protein unique to C. elegans germ cells, PIE-1. PIE-1 is degraded upon the birth of the germ cell precursors, Z2 and Z3. We have identified a chromatin-based mechanism that succeeds PIE-1 repression in these cells. A subset of nucleosomal histone modifications, methylated lysine 4 on histone H3 (H3meK4) and acetylated lysine 8 on histone H4 (H4acetylK8), are globally lost and the DNA appears more condensed. This coincides with PIE-1 degradation and requires that germline identity is not disrupted. Drosophila pole cell chromatin also lacks H3meK4, indicating that a unique chromatin architecture is a conserved feature of embryonic germ cells. Regulation of the germline-specific chromatin architecture requires functional nanos activity in both organisms. These results indicate that genome-wide repression via a nanos-regulated, germ cell-specific chromatin organization is a conserved feature of germline maintenance during embryogenesis.

Mechanisms of germ cell specification across the metazoans: epigenesis and preformation

Germ cells play a unique role in gamete production, heredity and evolution. Therefore, to understand the mechanisms that specify germ cells is a central challenge in developmental and evolutionary biology. Data from model organisms show that germ cells can be specified either by maternally inherited determinants (preformation) or by inductive signals (epigenesis). Here we review existing data on 28 metazoan phyla, which indicate that although preformation is seen in most model organisms, it is actually the less prevalent mode of germ cell specification, and that epigenetic germ cell specification may be ancestral to the Metazoa.

Evolutionary origins of the vertebrate heart: Specification of the cardiac lineage in Ciona intestinalis

Here we exploit the extensive cell lineage information and streamlined genome of the ascidian, Ciona intestinalis, to investigate heart development in a basal chordate. Several cardiac genes were analyzed, including the sole Ciona ortholog of the Drosophila tinman gene, and tissue-specific enhancers were isolated for some of the genes. Conserved sequence motifs within these enhancers facilitated the isolation of a heart enhancer for the Ciona Hand-like gene. Altogether, these studies provide a regulatory framework for the differentiation of the cardiac mesoderm, beginning at the 110-cell stage, and extending through the fusion of cardiac progenitors during tail elongation. The cardiac lineage shares a common origin with the germ line, and zygotic transcription is first detected in the heart progenitors only after its separation from the germ line at the 64-cell stage. We propose that germ-line determinants influence the specification of the cardiac mesoderm, both by inhibiting inductive signals required for the development of noncardiac mesoderm lineages, and by providing a localized source of Wnt-5 and other signals required for heart development. We discuss the possibility that the germ line also influences the specification of the vertebrate heart.

Suppression of macho-1-directed muscle fate by FGF and BMP is required for formation of posterior endoderm in ascidian embryos

Specification of germ layers is a crucial event in early embryogenesis. In embryos of the ascidian, Halocynthia roretzi, endoderm cells originate from two distinct lineages in the vegetal hemisphere. Cell dissociation experiments suggest that cell interactions are required for posterior endoderm formation, which has hitherto been thought to be solely regulated by localized egg cytoplasmic factors. Without cell interaction, every descendant of posterior-vegetal blastomeres, including endoderm precursors, assumed muscle fate. Cell interactions are required for suppression of muscle fate and thereby promote endoderm differentiation in the posterior endoderm precursors. The cell interactions take place at the 16- to 32-cell stage. Inhibition of cell signaling by FGF receptor and MEK inhibitor also supported the requirement of cell interactions. Consistently, FGF was a potent signaling molecule, whose signaling is transduced by MEK-MAPK. By contrast, such cell interactions are not required for formation of the anterior endoderm. Our results suggest that another redundant signaling molecule is also involved in the posterior endoderm formation, which is likely to be mediated by BMP. Suppression of the function of macho-1, a muscle determinant in ascidian eggs, by antisense oligonucleotide was enough to allow autonomous endoderm specification. Therefore, the cell interactions induce endoderm formation by suppressing the function of macho-1, which is to promote muscle fate. These findings suggest the presence of novel mechanisms that suppress functions of inappropriately distributed maternal determinants via cell interactions after embryogenesis starts. Such cell interactions would restrict the regions where maternal determinants work, and play a key role in marking precise boundaries between precursor cells of different tissue types.