Phylogenetic conservation of cytostatic factor related genes in the ascidian Ciona intestinalis

In all vertebrates, mature oocytes arrest at the metaphase of the II meiotic division, while some invertebrates arrest at metaphase-I, others at prophase-I. Fertilization induces completion of meiosis and entry into the first mitotic division. Several experimental models have been considered from both vertebrates and invertebrates in order to shed light on the peculiar aspects of meiotic division, such as the regulation of the cytostatic factor (CSF) and the maturation promoting factor (MPF) in metaphase I or II. Recently, we proposed the oocytes of ascidian Ciona intestinalis as a new model to study the meiotic division. Here, taking advantage of the recent publication of the C. intestinalis genome, we presented a phylogenetic analysis of key molecular components of the CSF-related machinery. We showed that the Mos/MAP kinase pathway is perfectly conserved in ascidians. We demonstrated the presence of a CSF-like activity in metaphase-I arrested C. intestinalis oocytes able to block cell division in two-cell embryos. We further investigated the regulation of CSF by demonstrating that both CSF and MPF inactivation, at the exit of metaphase-I, are independent from protein synthesis, indicating the absence of short-lived factors that regulate metaphase stability, as in other invertebrate species. The results obtained suggest that meiotic regulation in C. intestinalis resembles that of vertebrates, such as Xenopus accordingly to the position of this organism in the evolutionary tree.

Making very similar embryos with divergent genomes: conservation of regulatory mechanisms of Otx between the ascidians Halocynthia roretzi and Ciona intestinalis

Ascidian embryos develop with a fixed cell lineage into simple tadpoles. Their lineage is almost perfectly conserved, even between the evolutionarily distant species Halocynthia roretzi and Ciona intestinalis, which show no detectable sequence conservation in the non-coding regions of studied orthologous genes. To address how a common developmental program can be maintained without detectable cis-regulatory sequence conservation, we compared in both species the regulation of Otx, a gene with a shared complex expression pattern. We found that in Halocynthia, the regulatory logic is based on the use of very simple cell line-specific regulatory modules, the activities of which are conserved, in most cases, in the Ciona embryo. The activity of each of these enhancer modules relies on the conservation of a few repeated crucial binding sites for transcriptional activators, without obvious constraints on their precise number, order or orientation, or on the surrounding sequences. We propose that a combination of simplicity and degeneracy allows the conservation of the regulatory logic, despite drastic sequence divergence. The regulation of Otx in the anterior endoderm by Lhx and Fox factors may even be conserved with vertebrates.

The central nervous system of the ascidian larva: mitotic history of cells forming the neural tube in late embryonic Ciona intestinalis

Ascidian larvae develop after an invariant pattern of embryonic cleavage. Fewer than 400 cells constitute the larval central nervous system (CNS), which forms without either extensive migration or cell death. We catalogue the mitotic history of these cells in Ciona intestinalis, using confocal microscopy of whole-mount embryos at stages from neurulation until hatching. The positions of cells contributing to the CNS were reconstructed from confocal image stacks of embryonic nuclei, and maps of successive stages were used to chart the mitotic descent, thereby creating a cell lineage for each cell. The entire CNS is formed from 10th- to 14th-generation cells. Although minor differences exist in cell position, lineage is invariant in cells derived from A-line blastomeres, which form the caudal nerve cord and visceral ganglion. We document the lineage of five pairs of presumed motor neurons within the visceral ganglion: one pair arises from A/A 10.57, and four from progeny of A/A 9.30. The remaining cells of the visceral ganglion are in their 13th and 14th generations at hatching, with most mitotic activity ceasing around 85% of embryonic development. Of the approximately 330 larval cells previously reported in the CNS of Ciona, we document the lineage of 226 that derive predominantly from A-line blastomeres.

Specification of developmental fates in ascidian embryos: molecular approach to maternal determinants and signaling molecules

Tadpole larvae of ascidians represent the basic body plan of chordates with a relatively small number and few types of cells. Because of their simplicity, ascidians have been intensively studied. More than a century of research on ascidian embryogenesis has uncovered many cellular and molecular mechanisms responsible for cell fate specification in the early embryo. This review describes recent advances in our understanding of the molecular mechanisms of fate specification mainly uncovered in model ascidian species--Halocynthia roretzi, Ciona intestinalis, and Ciona savignyi. One category of developmentally important molecules represents maternal localized mRNAs that are involved in cell-autonomous processes. In the second category, signaling molecules and downstream transcription factors are involved in inductive cell interactions. Together with genome-wide information, there is a renewed interest in studying ascidian embryos as a fascinating model system for understanding how single-celled eggs develop a highly organized chordate body plan.

Ascidian otx gene Hroth activates transcription of the brain-specific gene HrTRP

The brain (sensory vesicle) of the ascidian larvae is thought to be homologous to the vertebrate forebrain and midbrain and, thus, is proposed as a simplified model to investigate mechanisms of brain formation in vertebrates. However, the genetic circuitry that governs formation of the sensory vesicle is largely unknown. To address this issue, we investigated the transcriptional regulation of the sensory vesicle-specific gene HrTRP by Hroth, the otx gene of the ascidian Halocynthia roretzi. A 133-bp 5'-flanking region of HrTRP, identified as a promoter that can drive expression of the reporter gene in the sensory vesicle, contains two otx binding consensus sites. When the two otx sites were deleted or mutated, the promoter activity of this region was decreased. Hroth overexpression can transactivate this promoter in an otx site-dependent manner. Transactivation of HrTRP promoter by Hroth overexpression was mimicked by overexpression of Hroth/VP16, which encodes a fusion protein of Hroth and the activator domain of VP16, and is suppressed by coexpression with Hroth/En, which encodes a fusion protein of Hroth and the Engrailed repressor domain. Finally, translational interference of Hroth by a morpholino oligonucleotide resulted in the reduction of HrTRP expression in the ascidian embryos. These results suggest that Hroth acts as a direct activator of HrTRP transcription during sensory vesicle development.

Ciona intestinalis cDNA projects: expressed sequence tag analyses and gene expression profiles during embryogenesis

Ascidians are primitive chordates. Their fertilized egg develops quickly into a tadpole-type larva, which consists of a small number but distinct types of cells, including those of epidermis, central nervous system with two sensory organs, endoderm and mesenchyme in the trunk, and notochord and muscle in the tail. This configuration of the ascidian tadpole is thought to represent the most simplified and primitive chordate body plan. In addition, the free-swimming and non-feeding larvae metamorphose into sessile and filter-feeding adults. The genome size of Ciona intestinalis is estimated to be about 160 Mb, and the number of genes approximately 15,500. The present Ciona cDNA projects focused on gene expression profiles of fertilized eggs, 32-110-cell stage embryos, tailbud embryos, larvae, and young adults. Expressed sequence tags (ESTs) of the 5'-most end and 3'-most end of more than 3000 clones were determined at each developmental stage, and the clones were categorized into independent clusters using the 3'-end sequences. Nearly 1000 clusters of them were then analyzed in detail of their sequences against a BLASTX search. This analysis demonstrates that, on average, half of the clusters showed proteins with sequence similarities to known proteins and the other half did not show sequence similarities to known proteins. Genes with sequence similarities were further categorized into three major subclasses, depending on their functions. Furthermore, the expression profiles of all of the clusters were analyzed by whole-mount in situ hybridization. This analysis highlights gene expression patterns characteristic to each developmental stage. As a result, the present study provides many new molecular markers for each of the tissues and/or organs that constitutes the Ciona tailbud embryo. This sequence information will be used for further comparative genome studies to explore molecular mechanisms involved in the formation of one of the most primitive chordate body plans. All of the data fully characterized may be viewed at the web site http://ghost.zool.kyoto-u.ac.jp.

Large-scale cDNA analysis of the maternal genetic information in the egg of Halocynthia roretzi for a gene expression catalog of ascidian development

The ascidian egg is a well-known mosaic egg. In order to investigate the molecular nature of the maternal genetic information stored in the egg, we have prepared cDNAs from the mRNAs in the fertilized eggs of the ascidian, Halocynthia roretzi. The cDNAs of the ascidian embryo were sequenced, and the localization of individual mRNA was examined in staged embryos by whole-mount in situ hybridization. The data obtained were stored in the database MAGEST (http://www.genome.ad.jp/magest) and further analyzed. A total of 4240 cDNA clones were found to represent 2221 gene transcripts, including at least 934 different protein-coding sequences. The mRNA population of the egg consisted of a low prevalence, high complexity sequence set. The majority of the clones were of the rare sequence class, and of these, 42% of the clones showed significant matches with known peptides, mainly consisting of proteins with housekeeping functions such as metabolism and cell division. In addition, we found cDNAs encoding components involved in different signal transduction pathways and cDNAs encoding nucleotide-binding proteins. Large-scale analyses of the distribution of the RNA corresponding to each cDNA in the eight-cell, 110-cell and early tailbud embryos were simultaneously carried out. These analyses revealed that a small fraction of the maternal RNAs were localized in the eight-cell embryo, and that 7.9% of the clones were exclusively maternal, while 40.6% of the maternal clones showed expression in the later stages. This study provides global insights about the genes expressed during early development.

Involvement of Rel/NF-kappaB in regulation of ascidian notochord formation

The Rel/NF-kappaB family is known to be involved in a wide variety of biological processes, including morphogenesis. In the present study, two protochordate cDNA clones encoding Rel/NF-kappaB proteins, named As-rel1 and As-rel2, were isolated from a fertilized egg cDNA library of the ascidian Halocynthia roretzi. The As-rel1 protein is a typical Rel/NF-kappaB family member, containing a Rel homology domain, a nuclear localization sequence and a C-terminal putative transcription activation domain, while the As-rel2 protein is a novel Rel/NF-kappaB family member that lacks a nuclear localization sequence and the C-terminal domain. Northern blot analyses showed that both transcripts were maternally expressed and that their expression changed during development of H. roretzi embryos. Although injection of the As-rel2 mRNA into H. roretzi fertilized eggs had little effect on embryonic development, injection of the As-rel1 mRNA interfered greatly with notochord formation, resulting in a shortened tail with a reduced number of notochord cells. In contrast, embryos co-injected with As-rel1 and As-rel2 mRNA developed normally, indicating that the As-rel2 protein rescued the defect in notochord formation induced by the injection of As-rel1 mRNA alone. These results strongly suggest that the As-rel1 protein functions as a suppressor in ascidian notochord formation, while the As-rel2 protein has an antagonistic effect on the action of the As-rel1 protein.

Determinants of cell and positional fate in ascidian embryos

Ascidians have played a major role in studies to understand the function of cytoplasmic determinants in animal development. Special qualities, including eggs with colored cytoplasmic regions, an invariant cleavage pattern and cell lineage, embryos with low cell numbers, larvae with typical chordate features and only six different tissues, rapid development, and a small genome, combine to make these animals a unique system for studying cytoplasmic determinants. There is evidence for determinants that specify the cleavage pattern; the differentiation of epidermal, endodermal, and muscle cells; and cell movements associated with gastrulation. The muscle determinants appear to be modified in concert with tail and muscle regression in species that have evolved an anural, or tailless, larva. Several lines of evidence suggest that determinants may be localized maternal mRNAs, which encode transcription factors or signal transduction components responsible for initiating differential gene activity. Different approaches and strategies are being used to isolate and characterize the function of these localized maternal mRNAs.

Expression and functional analysis of Cititf1, an ascidian NK-2 class gene, suggest its role in endoderm development

In solitary ascidians the fate of endoderm is determined at a very early stage of development and depends on cytoplasmic factors whose nature has not been determined. We have isolated a member of the NK-2 gene family, Cititf1, from the ascidian Ciona intestinalis, showing high sequence homology to mammalian TITF1. The Cititf1 gene was expressed in all endodermal precursors at the pregastrula and gastrula stages, and is thus the first specific regulatory endodermal marker to be isolated from an ascidian. Cititf1 expression was downregulated at the end of gastrulation to reappear at middle tailbud and larval stages in the most anterior and ventral parts of head endoderm, regions which give rise, after metamorphosis, to the adult endostyle, where Cititf1 mRNA was still present. Microinjection of Cititf1 mRNA into fertilized eggs resulted in tadpole larvae with abnormalities in head-trunk development consequent to the formation of excess endoderm, perhaps due to recruitment of notochord precursors to an endodermal fate. These data suggest that Cititf1 plays an important role in normal endoderm differentiation during ascidian embryogenesis.

Ascidian tail formation requires caudal function

Although the tail is one of the major characteristics of animals of the phylum Chordata, evolutionary aspects of the molecular mechanisms involved in its formation are not clear. To obtain insights into these issues, we have isolated and investigated the caudal gene of an ascidian, one of the lower animal groups among chordates. Ascidian caudal is expressed from the midgastrula stage onward in the lateral walls of the posterior neural tube cell lineage and also in the posterior epidermal cells from the neurula stage. Thus, ascidian caudal expression is restricted to the ectoderm of a tail-forming region throughout embryogenesis. Suppression of caudal function by an antisense oligonucleotide or a dominant negative construct caused inhibition of the cell movement required for tail formation. Overexpression of wild-type caudal mRNA in an ascidian animal cap, an animal half explant prepared at the eight-cell stage, caused elongation of the cap. Furthermore, Xenopus embryos injected with dominant negative ascidian caudal exhibited defects in elongation, suggesting a conserved caudal function among chordates. These results indicate that caudal function is required for chordate tail formation and may play a key role in its evolution.

posterior end mark 3 (pem-3), an ascidian maternally expressed gene with localized mRNA encodes a protein with Caenorhabditis elegans MEX-3-like KH domains

Maternal factors localized in the posterior-vegetal cytoplasm of an ascidian egg are essential for cell specification and pattern formation of the embryo. The molecular identification of the localized factors and the elucidation of the machinery associated with the localization are therefore key research subjects. I report here the isolation and characterization of a novel maternally expressed gene, posterior end mark 3 (pem-3). The pem-3 cDNA was obtained from a cDNA library of fertilized egg mRNAs subtracted with gastrula mRNAs of Ciona savignyi. As in the case of pem (Yoshida et al., 1996, Development 122, 2005-2012), the pem-3 maternal transcript was gradually concentrated after fertilization in the posterior-vegetal cytoplasm of the egg, and it later marked the posterior end of developing embryos. The PEM-3 protein was also detected in the posterior end of early embryos. The nucleotide sequence predicted that pem-3 encodes a probable RNA-binding protein with two KH domains that have an extensive similarity with those of Caenorhabditis elegans MEX-3. MEX-3 is also localized in nematode embryos (Draper et al., 1996, Cell 87, 205-216), suggesting that PEM-3 is a candidate homologue of MEX-3. In addition to maternal expression, a zygotic transcript of pem-3 and its gene product were detected in cells of the neural plate, mesenchyme, and epidermis of embryos after the neural-plate stage. Inhibition of zygotic expression using an antisense oligonucleotide resulted in the development of abnormal larvae without sensory pigment cells, suggesting that the zygotic PEM-3 plays a role in the differentiation of the brain of the ascidian larva.

Phases of cytoplasmic and cortical reorganizations of the ascidian zygote between fertilization and first division

Many eggs undergo reorganizations that localize determinants specifying the developmental axes and the differentiation of various cell types. In ascidians, fertilization triggers spectacular reorganizations that result in the formation and localization of distinct cytoplasmic domains that are inherited by early blastomeres that develop autonomously. By applying various imaging techniques to the transparent eggs of Phallusia mammillata, we now define 9 events and phases in the reorganization of the surface, cortex and the cytoplasm between fertilization and first cleavage. We show that two of the domains that preexist in the egg (the ER-rich cortical domain and the mitochondria-rich subcortical myoplasm) are localized successively by a microfilament-driven cortical contraction, a microtubule-driven migration and rotation of the sperm aster with respect to the cortex, and finally, a novel microfilament-dependant relaxation of the vegetal cortex. The phases of reorganization we have observed can best be explained in terms of cell cycle-regulated phases of coupling, uncoupling and recoupling of the motions of cortical and subcortical layers (ER-rich cortical domain and mitochondria-rich domain) with respect to the surface of the zygote. At the end of the meiotic cell cycle we can distinguish up to 5 cortical and cytoplasmic domains (including two novel ones; the vegetal body and a yolk-rich domain) layered against the vegetal cortex. We have also analyzed how the myoplasm is partitioned into distinct blastomeres at the 32-cell stage and the effects on development of the ablation of precisely located small fragments. On the basis of our observations and of the ablation/ transplantation experiments done in the zygotes of Phallusia and several other ascidians, we suggest that the determinants for unequal cleavage, gastrulation and for the differentiation of muscle and endoderm cells may reside in 4 distinct cortical and cytoplasmic domains localized in the egg between fertilization and cleavage.

Cross-coupling between voltage-dependent Ca2+ channels and ryanodine receptors in developing ascidian muscle blastomeres

1. Ascidian blastomeres of muscle lineage express voltage-dependent calcium channels (VDCCs) despite isolation and cleavage arrest. Taking advantage of these large developing cells, developmental changes in functional relations between VDCC currents and intracellular Ca2+ stores were studied. 2. Inactivation of ascidian VDCCs is Ca2+ dependent, as demonstrated by two pieces of evidence: (1) a bell-shaped relationship between prepulse voltage and amplitude during the test pulse in Ca2+, but not in Ba2+, and (2) the decay kinetics of Ca2+ currents (ICa) obtained as the size of tail currents. 3. During replacement in the external solution of Ca2+ with Ba2+, the inward current appeared biphasic: it showed rapid decay followed by recovery and slow decay. This current profile was most evident in the mixed bath solution (2 % Ca2+ and 98 % Ba2+, abbreviated to '2Ca/98Ba'). 4. The biphasic profile of I2Ca/98Ba was significantly attenuated in caffeine and in ryanodine, indicating that Ca2+ release is involved in shaping the current kinetics of VDCCs. After washing out the caffeine, the biphasic pattern was reproducibly restored by depolarizing the membrane in calcium-rich solution, which is expected to refill the internal Ca2+ stores. 5. The inhibitors of endoplasmic reticulum (ER) Ca2+-ATPase (SERCAs) cyclopiazonic acid (CPA) and thapsigargin facilitated elimination of the biphasic profile with repetitive depolarization. 6. At a stage earlier than 36 h after fertilization, the biphasic profile of I2Ca/98Ba was not observed. However, caffeine induced a remarkable decrease in the amplitude of I2Ca/98Ba and this suppression was blocked by microinjection of the Ca2+ chelator BAPTA, showing the presence of caffeine-sensitive Ca2+ stores at this stage. 7. Electron microscopic observation shows that sarcoplasmic membranes (SR) arrange closer to the sarcolemma with maturation, suggesting that the formation of the ultrastructural machinery underlies development of the cross-coupling between VDCCs and Ca2+ stores.

The developing dorsal ganglion of the salp Thalia democratica, and the nature of the ancestral chordate brain

The development of the dorsal ganglion of the salp, Thalia democratica , is described from electron microscope reconstructions up to the stage of central neuropile formation. The central nervous system (CNS) rudiment is initially tubular with an open central canal. Early developmental events include: (i) the formation of a thick dorsal mantle of neuroblasts from which paired dorsal paraxial neuropiles arise; (ii) the differentiation of clusters of primary motor neurons along the ventral margin of the mantle; and (iii) the development from the latter of a series of peripheral nerves. The dorsal paraxial neuropiles ultimately connect to the large central neuropile, which develops later. Direct contact between neuroblasts and muscle appears to be involved in the development of some anterior nerves. The caudal nerves responsible for innervating more distant targets in the posterior part of the body develop without such contacts, which suggests that a different patterning mechanism may be employed in this part of the neuromuscular system. The results are compared with patterns of brain organization in other chordates. Because the salp CNS is symmetrical and generally less reduced than that of ascidian larvae, it is more easily compared with the CNS of amphioxus and vertebrates. The dorsal paraxial centres in the salp resemble the dorsolateral tectal centres in amphioxus in both position and organization; the central neuropile in salps likewise resembles the translumenal system in amphioxus. The neurons themselves are similar in that many of their neurites appear to be derived from the apical surface instead of the basal surface of the cell. Such neurons, with extensively developed apical neurites, may represent a new cell type that evolved in the earliest chordates in conjunction with the formation of translumenal or intralumenal integrative centres. In comparing the salp ganglion with vertebrates, we suggest that the main core of the ganglion is most like the mes-metencephalic region of the vertebrate brain, i.e. the zone occupied by the midbrain, isthmus, and anterior hindbrain. Counterparts of more anterior regions (forebrain) and posterior ones (segmented hindbrain) appear to be absent in salps, but are found in other tunicates, suggesting that evolution has acted quite differently on the main subdivisions of the CNS in different types of tunicates.

Functional analysis of an ascidian homologue of vertebrate Bmp-2/Bmp-4 suggests its role in the inhibition of neural fate specification

The ascidian tadpole larva is thought to be close to a prototype of the ancestral chordate. The vertebrate body plan is established by a series of inductive cellular interactions, whereas ascidians show a highly determinate mode of development. Recent studies however, suggest some roles of cell-cell interaction during ascidian embryogenesis. To elucidate the signaling molecules responsible for the cellular interaction, we isolated HrBMPb, an ascidian homologue of the vertebrate bone morphogenetic protein (BMP) gene, from Halocynthia roretzi. The amino acid sequence of HrBMPb closely resembled those of vertebrate BMP-2 and BMP-4 and of Drosophila Decapentaplegic (DPP). In addition to the sequence similarity, HrBMPb overexpression induced the ventralization of Xenopus embryos, suggesting functional conservation. The zygotic expression of HrBMPb was first detected around gastrulation. HrBMPb expression was maintained in some cells at the lateral edges of the neural plate through gastrulation to neurulation, although that in the presumptive muscle cells was downregulated. HrBMPb was not expressed in the presumptive epidermis during gastrulation. When HrBMPb mRNA was injected into fertilized Halocynthia eggs, cells that normally give rise to the neural tissue differentiated into epidermis, causing a loss of anterior neural tissue in the larva. In addition, HrBMPb might function synergistically with HrBMPa, an ascidian homologue of BMPs-5 to 8. However, HrBMPb overexpression did not affect differentiation of the notochord and muscle cells. These results suggest that HrBMPb functions as a neural inhibitor and as an epidermal inducer but not as a ventralizing agent in ascidian development.

Posterior end mark 2 (pem-2), pem-4, pem-5, and pem-6: maternal genes with localized mRNA in the ascidian embryo

The posterior-vegetal cytoplasm of an ascidian egg contains maternal factors required for pattern formation and cell specification of the embryo. We report here the isolation and characterization of cDNA clones for novel maternal genes, posterior end mark 2 (pem-2), pem-4, pem-5, and pem-6. We obtained these clones from a cDNA library of Ciona savignyi fertilized egg mRNAs subtracted with gastrula mRNAs by examining the localization of the corresponding mRNAs of randomly selected clones by whole-mount in situ hybridization. As in the case of pem, all of these mRNAs were localized in the posterior-vegetal cytoplasm of the egg, and they later marked the posterior end of early embryos. The predicted amino acid sequence suggested that PEM-2 contains a signal for nuclear localization, an src homology 3 (SH3) domain, and a consensus sequence of the CDC24 family guanine nucleotide dissociation stimulators (GDSs). PEM-4 has a signal for nuclear localization and three C2H2-type zinc finger motifs, while PEM-5 and PEM-6 show no similarity to known proteins. These results provide further evidence that the ascidian egg contains maternal messages that are localized in the posterior-vegetal cytoplasm.

Autonomy of ascidian fork head/HNF-3 gene expression

We have characterized the expression pattern of a class I fork head/HNF-3 gene (HrHNF3-1) of the ascidian Halocynthia roretzi. Zygotic HrHNF3-1 expression was detectable as early as the 16-cell stage, and the transcript was evident in blastomeres of the endoderm, notochord and mesenchyme lineages of the early embryos. After the late gastrula stage, HrHNF3-1 was also expressed in the presumptive spinal cord cells and some brain cells. The spinal cord of the ascidian tadpole consists of four layers of cells; the dorsal layer, two lateral layers and the ventral layer, the latter of which simply lies on the notochord. Cross-sections of in situ hybridized specimens showed that HrHNF3-1 was expressed in cells of the ventral layer, reminiscent of the floor plate of vertebrate embryos. In addition, we found autonomy in the initiation of early HrHNF3-1 expression, because the gene was expressed in blastomeres continuously dissociated from the first cleavage until the 16-cell stage.

Distinct neuronal lineages of the ascidian embryo revealed by expression of a sodium channel gene

The ascidian larva contains tubular neural tissue, one of the prominent anatomical features of the chordates. The cell-cleavage pattern and cell maps of the nervous system have been described in the ascidian larva in great detail. Cell types in the neural tube, however, have not yet been defined due to the lack of a suitable molecular marker. In the present work, we identified neuronal cells in the caudal neural tube of the Halocynthia embryo by utilizing a voltage-gated Na+ channel gene, TuNa I, as a molecular marker. Microinjection of a lineage tracer revealed that TuNa I-positive neurons in the brain and in the trunk epidermis are derived from the a-line of the eight-cell embryo, which includes cell fates to epidermal and neural tissue. On the other hand, TuNa I-positive cells in the more caudal part of the neural tissue were not stained by microinjection into the a-line. These neurons are derived from the A-line, which contains fates of notochord and muscle, but not of epidermis. Electron microscopic observation confirmed that A-line-derived neurons consist of motor neurons innervating the dorsal and ventral muscle cells. Isolated A-line blastomeres have active membrane excitability distinct from those of the a-line-derived neuronal cells after culture under cleavage arrest, suggesting that the A-line gives rise to a neuronal cell distinct from that of the a-lineage. TuNa I expression in the a-line requires signals from another cell lineage, whereas that in the A-line occurs without tight cell contact. Thus, there are at least two distinct neuronal lineages with distinct cellular behaviors in the ascidian larva: the a-line gives rise to numerous neuronal cells, including sensory cells, controlled by a mechanism similar to vertebrate neural induction, whereas A-line cells give rise to motor neurons and ependymal cells in the caudal neural tube that develop in close association with the notochord or muscle lineage, but not with the epidermal lineage.

Specific cellular localization of tyrosinase mRNA during Ciona intestinalis larval development

A Ciona intestinalis cDNA clone that encodes a protein highly homologous to other tyrosinases was isolated. Northern blot analysis showed that expression of Ciona tyrosinase starts at the early neurula stage and continues throughout the tail-bud and tadpole larval stages. The earliest tyrosinase expression was detected, by in situ hybridization, at the neural plate stage, in pigment precursor cells located along the two neural folds, in the animal region of the embryo. In the course of embryonic development the strong hybridization signal was always localized, within the rostral part of the developing brain, in the pigment precursor cells and was later detected in the otolith and ocellus. These results are discussed in relation to tyrosinase as an early marker of neural induction.