The ascidian as a model organism in developmental and evolutionary biology

Application of Minos, one of the Tc1/mariner superfamily transposable elements, to ascidian embryos as a tool for insertional mutagenesis

As it has a simple genome structure, Ciona intestinalis is a good chordate species for studying the function of genes. To this end, it is a key requirement to introduce insertional mutagenesis using a transposable element to the ascidian system. The present study focuses on Minos, one of the Tc1/mariner superfamily transposons that is already used in a human cell line. By extrachromosomal excision and transposition assays, we found that Minos activity is very high in C. intestinalis. We also demonstrated the nuclear localization activity of Minos transposase in Ciona embryos. From these tests, we concluded that Minos transposase has complete activity when it is expressed in C. intestinalis, suggesting that Minos has the potential to be used for genome-wide insertional mutagenesis of C. intestinalis.

Direct evidence for the role of pigment cells in the brain of ascidian larvae by laser ablation

The anterior sensory vesicle of ascidian larvae contains a single large vesicle in which lie two distinct types of pigment cells, anterior and posterior. The ultrastructure of these pigment cells suggests that the anterior pigment cell is an otolith, presumably used for gravity detection, and the posterior pigment cell is an ocellus, used for photoreception. However, there is no direct experimental evidence for this assignment of function. Upward swimming behaviour occurring during the initial period of larval life was examined before and after laser ablation of the anterior pigment and posterior pigment cells. Posterior pigment cell-ablated larvae retained the upward swimming behaviour, but anterior pigment cell-ablated larvae lost it. These results suggest that the anterior pigment acts as a gravity sensor. The negatively phototactic swimming during the latter part of larval life was also examined before and after laser ablation of the anterior pigment or posterior pigment cells. Anterior pigment cell-ablated larvae retained the phototactic response, but posterior pigment cell-ablated larvae lost it. These results suggest that the posterior pigment of the sensory vesicle is involved in the negatively phototactic, downward swimming behavior. The effect of pressure on swimming behaviour was studied, and a putative pressure-detection organ was found not to be involved in the larval swimming behaviour. These are the first published experimental results that permit a functional role in ascidian larval behavior to be assigned to the sensory organs.

The ascidian tadpole larva: comparative molecular development and genomics

Evolution is of interest not only to developmental biology but also to genetics and genomics. We are witnessing a new era in which evolution, development, genetics and genomics are merging to form a new discipline, a good example of which is the study of the origin and evolution of the chordates. Recent studies on the formation of the notochord and the dorsal neural tube in the increasingly famous Ciona intestinalis tadpole larva, and the availability of its draft genome, show how the combination of comparative molecular development and evolutionary genomics might help us to better understand our chordate ancestor.

Generation of Ci-Brachyury-GFP stable transgenic lines in the ascidian Ciona savignyi

We report generation of stable transgenic lines of the ascidian Ciona savignyi carrying a Ciona intestinalis-Brachyury-promoter/Green Fluorescent Protein-reporter (Ci-Bra-GFP) construct. The transgenic lines were made using a technique in which the endonuclease I-SceI was coinjected into fertilized eggs with a transgene construct containing flanking recognition sites for I-SceI. Two founder animals, out of 12 F(0) adults tested, were found to transmit the transgene to their offspring (F(1)s) at frequencies of 42% and 23%. The transgene was further inherited by the F(2) in a Mendelian fashion and displayed nonmosaic expression, indicating integration into the genome. The Mendelian inheritance and the absence of mosaicism persisted through the F(3) and F(4) generations. Southern blot analyses showed that the transgene was organized in tandem arrays of no more than 10 copies. Using these Ci-Bra-GFP transgenics, we describe cellular movements and shape changes involved in notochord morphogenesis in both wildtype and mutant embryos.

A novel G protein alpha subunit in embryo of the ascidian, Halocynthia roretzi

A cDNA clone encoding a novel G protein alpha subunit, HrGalpha(n) was isolated from the larvae of ascidian, Halocynthia roretzi. In contrast with overall amino acid identity (63%) with G protein alpha subunit of G(i) or G(o) subclass, HrGalpha(n) has a unique amino acid sequence, which lacks a residue for pertussis toxin substrate, but retains for cholera toxin substrate for ADP-ribosylation. The sequence characteristics and molecular phylogenetic analysis suggest that HrGalpha(n) defines a novel subclass within G(i) class of G protein alpha subunits. The zygotic expression of HrGalpha(n) was first detected at the 64-cell stage and observed in all blastomeres except for B7.4, B7.5 and B7.6 cells till the 110-cell stage. As progress of the developmental stages, the expression of HrGalpha(n) became restricted and was observed in the muscle, mesenchyme and a part of trunk lateral cells in tailbud embryos. With HrGalpha(n)-GFP fusion-gene construct it was showed that the genomic fragment containing 2674 bp upstream of the putative translation start site of HrGalpha(n) contained the regulatory sequence responsible for the expression in the muscle and mesenchyme cells, and that the regulatory sequence functioned also in Ciona intestinalis. Our results suggest a possible involvement of HrGalpha(n) in the signaling system regulates the cell fate during the embryogenesis of the ascidian.

Urochordates and the origin of natural killer cells: identification of a CD94/NKR-P1-related receptor in blood cells of Botryllus

Transplantation immunity based on the recognition of MHC molecules is well described in vertebrates. Vertebrates, however, do not undergo transplantation reaction naturally. The phylogenetically closest group in which transplantation reactions can occur is the Urochordata. Therefore, these animals occupy a key position for understanding the evolution of the vertebrate immune system. When screening for genes differentially expressed during allorecognition in Botryllus schlosseri, we isolated a gene coding for a type II transmembrane protein with a C-type lectin-binding domain and close similarity to vertebrates CD94 and NKR-P1. Here we show that the gene, BsCD94-1, is differentially regulated during allorecognition and that a subpopulation of blood cells carries the corresponding receptor on its cell surface. Southern blot analysis with DNA from individual colonies and intronless BsCD94-1 probe reveal variation between individuals at the genomic level. CD94 in vertebrates is one of the markers for natural killer cells and binds to MHC class I molecules. Natural killer cells play a major role in recognition and elimination of allogeneic cells. Their evolutionary origin, however, remained unknown. The results presented here indicate that the elaboration of the vertebrate immune system may have its roots in an ancestral population of cells in the urochordate blood.

Early steps in the formation of neural tissue in ascidian embryos

Ascidians are simple invertebrate chordates whose lineage diverged from that of vertebrates at the base of the chordate tree. Their larvae display a typical chordate body plan, but are composed of a remarkably small number of cells. Ascidians develop with an invariant cell lineage, and their embryos can be easily experimentally manipulated during the cleavage stages. Their larval nervous system is organised in a similar way as in vertebrates but is composed of less than 130 neurones and around 230 glial cells. This remarkable simplicity offers an opportunity to understand, at the cellular and molecular levels, the ontogeny and function of each neural cell. Here, we first review the organisation of the ascidian nervous system and its lineage. We then focus on the current understanding of the processes of neural specification and patterning before and during gastrulation. We discuss these advances in the context of what is currently known in vertebrates.

The draft genome of Ciona intestinalis: insights into chordate and vertebrate origins

The first chordates appear in the fossil record at the time of the Cambrian explosion, nearly 550 million years ago. The modern ascidian tadpole represents a plausible approximation to these ancestral chordates. To illuminate the origins of chordate and vertebrates, we generated a draft of the protein-coding portion of the genome of the most studied ascidian, Ciona intestinalis. The Ciona genome contains approximately 16,000 protein-coding genes, similar to the number in other invertebrates, but only half that found in vertebrates. Vertebrate gene families are typically found in simplified form in Ciona, suggesting that ascidians contain the basic ancestral complement of genes involved in cell signaling and development. The ascidian genome has also acquired a number of lineage-specific innovations, including a group of genes engaged in cellulose metabolism that are related to those in bacteria and fungi.

Programmed cell death in the ascidian embryo: modulation by FoxA5 and Manx and roles in the evolution of larval development

Programmed cell death (PCD) has been discounted in the ascidian embryo because the descendants of every embryonic cell appear to be present in the tadpole larva. Here we show that apoptotic PCD is initiated in the epidermis and central nervous system (CNS) but not in the endoderm, mesenchyme, muscle, and notochord cells during embryogenesis in molgulid ascidians. However, the affected cells do not actually die until the beginning of metamorphosis. Although specific patterns of PCD were different in distantly related ascidian species, the results suggest that removal of CNS cells by apoptosis is a urchordate feature predating the origin of the vertebrates. Certain molgulid ascidian species have evolved an anural (tailless) larva in which notochord cells fail to undergo the morphogenetic movements culminating in tail development. These anural species include Molgula occulta, the sister species of the urodele (tailed) species Molgula oculata. We show that PCD in the notochord cell lineage precedes the arrest of tail development in M. occulta and other independently evolved anural species. The notochord cells are rescued from PCD and a tail develops in hybrid embryos produced by fertilizing M. occulta eggs with M. oculata sperm, implying that apoptosis is controlled zygotically. Antisense inhibition experiments show that zygotic expression of the FoxA5 and Manx genes is required to prevent notochord PCD in urodele species and hybrids with restored tails. The results provide the first indication of PCD in the ascidian embryo and suggest that apoptosis modulated by FoxA5 and Manx is involved in notochord and tail regression during anural development. Differences in PCD that occur between ascidian species suggest that diversity in programming apoptosis may explain differences in larval form.

Central nervous system-specific expression of G protein alpha subunits in the ascidian Ciona intestinalis

Heterotrimeric G proteins play crucial roles as mediators of signaling by many extracellular stimuli. The receptors that activate G proteins constitute the largest and most diverse family of cell surface molecules involved in signal transmission of metazoan cells. To investigate G protein signaling in the central nervous system (CNS) of chordates, we isolated cDNA fragments encoding five different G protein alpha subunits (CiGalpha(x), CiGalpha(q), CiGalpha(i1a), CiGalpha(i1b), and CiGalpha(i2)) from larvae of the ascidian, a simple chordate, Ciona intestinalis. In situ hybridization analysis revealed that each isoform had distinct patterns of spatial distribution in embryos. Among them, CiGalpha(i1a) and CiG alpha(i1b) mRNAs were specifically expressed in the CNS of the larva, whereas CiGalpha(q) transcripts were expressed in small parts of the trunk epidermis and the tip of the tail, but not in the CNS. The CiGalpha(x) expression was widely observed throughout the trunk and tail of the embryos, and the signals were stronger in the epidermis, mesenchyme, and tail muscle cells. Comparison of cDNA sequences and the exon-intron organization indicate that CiGalpha(i1a) and CiGalpha(i1b) are produced by alternative splicing of transcripts from a single gene, CiGalpha(i1). In the cleavage and gastrula stages, transcripts of CiGalpha(i1) were widely distributed in embryos, and the expression then became restricted to the CNS of tailbud embryos and larvae. An exhaustive search has failed to find transducin-type alpha subunits in C. intestinalis. Since CiGalpha(i1) is expressed in the ocellus, CiGalpha(i1) may mediate signals from Ci-opsin1, a visual pigment of the ocellus photoreceptor cells.

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.

Construction of BAC libraries derived from the ascidian Ciona intestinalis

Large insert genomic bacterial artificial chromosome (BAC) libraries were constructed from a basal chordate, the ascidian Ciona intestinalis. Insert analyses of randomly selected clones indicated that in the first library the mean insert size was 135 kb and predicted a 15-fold coverage of the Ciona genome, and in the second library the mean insert size was 165 kb and predicted a 5-fold coverage of the genome. These first large insert genomic libraries of the ascidian should increase the speed of genomic analyses of basal chordates.

Patterning the marginal zone of early ascidian embryos: localized maternal mRNA and inductive interactions

Early animal embryos are patterned by localized egg cytoplasmic factors and cell interactions. In invertebrate chordate ascidians, larval tail muscle originates from the posterior marginal zone of the early embryo. It has recently been demonstrated that maternal macho-1 mRNA encoding transcription factor acts as a localized muscle determinant. Other mesodermal tissues such as notochord and mesenchyme are also derived from the vegetal marginal zone. In contrast, formation of these tissues requires induction from endoderm precursors at the 32-cell stage. FGF-Ras-MAPK signaling is involved in the induction of both tissues. The responsiveness for induction to notochord or mesenchyme depends on the inheritance of localized egg cytoplasmic factors. Previous studies also point to critical roles of directed signaling in polarization of induced cells and in subsequent asymmetric divisions resulting in the formation of two daughter cells with distinct fates. One cell adopts an induced fate, while the other assumes a default fate. A simple model of mesoderm patterning in ascidian embryos is proposed in comparison with that of vertebrates.

Tail regression in Ciona intestinalis (Prochordate) involves a Caspase-dependent apoptosis event associated with ERK activation

Two apoptotic events take place during embryonic development of Ciona intestinalis. The first concerns extra-embryonic cells and precedes hatching. The second controls tail regression at metamorphosis, occurs through a polarized wave originating from tail extremity, and is caspase dependent. This was shown by: (1) in vivo incorporation of a fluorescent marker of caspase activation in different cell types of the tail; (2) detection of an activated form of caspase 3-like protein by western blotting; and (3) failure of 30% of larvae to undergo metamorphosis after treatment of fertilized eggs with a pan-caspase inhibitor. In addition, Ciona embryos express a single ERK protein, specifically phosphorylated at metamorphosis. ERK activation was shown to be located in cells of the tail. Addition of MEK inhibitor in the culture medium prevented ERK activation and metamorphosis. In silico analysis of Ciona genome pointed to 15 caspases with high homology with humans, and a single ERK gene with high homology to both mammalian ERK1 and ERK2. It is concluded that the sequence of events leading to metamorphosis includes ERK phosphorylation followed by caspase-dependent apoptosis and tail regression.

Movies available on-line

Control of intercalation is cell-autonomous in the notochord of Ciona intestinalis

Dishevelled signaling plays a critical role in the control of cell intercalation during convergent extension in vertebrates. This study presents evidence that Dishevelled serves a similar function in the Ciona notochord. Embryos transgenic for mutant Dishevelled fail to elongate their tails, and notochord cells fail to intercalate, though notochord cell fates are unaffected. Analysis of mosaic transgenics revealed that the effects of mutant Dishevelled on notochord intercalation are cell-autonomous in Ciona, though such defects have nonautonomous effects in Xenopus. Furthermore, our data indicate that notochord cell intercalation in Ciona does not require the progressive signals which coordinate cell intercalation in the Xenopus notochord, highlighting an important difference in how mediolateral cell intercalation is controlled in the two animals. Finally, this study establishes the Ciona embryo as an effective in vivo system for the study of the molecular control of morphogenetic cell movements in chordates.

Multiple functions of a Zic-like gene in the differentiation of notochord, central nervous system and muscle inCiona savignyiembryos

Multiple functions of a Zic-like zinc finger transcription factor gene (Cs-ZicL) were identified in Ciona savignyi embryos. cDNA clones for Cs-ZicL, a β-catenin downstream genes, were isolated and the gene was transiently expressed in the A-line notochord/nerve cord lineage and in B-line muscle lineage from the 32-cell stage and later in a-line CNS lineage from the 110-cell stage. Suppression of Cs-ZicL function with specific morpholino oligonucleotide indicated that Cs-ZicL is essential for the formation of A-line notochord cells but not of B-line notochord cells, essential for the CNS formation and essential for the maintenance of muscle differentiation. The expression of Cs-ZicL in the A-line cells is downstream of β-catenin and a β-catenin-target gene, Cs-FoxD, which is expressed in the endoderm cells from the 16-cell stage and is essential for the differentiation of notochord. In spite of its pivotal role in muscle specification, the expression of Cs-ZicL in the muscle precursors is independent of Cs-macho1, which is another Zic-like gene encoding a Ciona maternal muscle determinant, suggesting another genetic cascade for muscle specification independent of Cs-macho1. Cs-ZicL may provide a future experimental system to explore how the gene expression in multiple embryonic regions is controlled and how the single gene can perform different functions in multiple types of embryonic cells.

Genome-wide identification of tissue-specific enhancers in the Ciona tadpole

Less than 100 cis-regulatory DNAs have been characterized in the context of transgenic metazoan embryos. Here we investigate the feasibility of conducting a genome-wide search for tissue-specific enhancers in the ascidian Ciona intestinalis. A total of 138 random genomic DNA fragments with an average size of 1.7 kb were separately placed 5' of a lacZ reporter gene. Eleven of the lacZ fusion genes displayed localized patterns of expression in tadpole-stage Ciona embryos. At least five of these transgenes appear to contain bona fide tissue-specific enhancers that direct expression in the cerebral vesicle, neural tube, primordial adhesive organ, notochord, and tail epidermis. One of the enhancers maps near Distalless (Ci-Dll-A) and recapitulates most aspects of the endogenous expression pattern, including localized expression in the anterior-most regions of the neurogenic ectoderm. We discuss the prospects of creating a regulatory atlas of the Ciona genome, whereby every enhancer is identified for every gene.

Cytoplasmic intermediate filament protein expression in tunicate development: a specific marker for the test cells

The urochordate Ciona intestinalis is a well established system for embryological studies, and large scale EST sequences begin to emerge. We cloned five cytoplasmic intennediate filament (IF) cDNAs and made specific antibodies to the recombinant proteins. Self-assembly studies and immunofluorescence microscopy were used to study these proteins and their distribution. Confirming and extending previous studies in Styela, we found that Ciona protein IF-A is expressed in muscle and forms homopolymeric filaments while proteins IF-C and IF-D, which form only obligatory heteropolymeric filaments, resemble a keratin pair exclusively found in the entire epidermis. Protein IF-B and the new protein IF-F potentially reflect tunicate-specific IF proteins. They are found in the entire internal epithelia including the neural gland. We also extended the analysis to earlier developmental stages of Ciona. Protein IF-A is expressed in muscle from larval stages, whereas proteins IF-C and IF-D are found only in the tail epidermis. Protein IF-F is detected abundantly in the test cells of eggs, embryos and premetamorphic larvae. Our studies show that IF proteins could prove very useful markers in the study of cell fate determination in Ciona. They also support previous findings on the evolutionary relationships of different IF proteins. Non-vertebrate chordates have IF proteins which represent orthologs of vertebrate type I to III proteins, but also IF proteins that do not seem to fit into these classes. However, the intron positions of all tunicate IF genes are conserved with vertebrate type I to III genes, pointing to a common evolutionary origin.

Early embryonic expression ofFGF4/6/9gene and its role in the induction of mesenchyme and notochord inCiona savignyiembryos

In early Ciona savignyi embryos, nuclear localization of β-catenin is the first step of endodermal cell specification, and triggers the activation of various target genes. A cDNA for Cs-FGF4/6/9, a gene activated downstream of β-catenin signaling, was isolated and shown to encode an FGF protein with features of both FGF4/6 and FGF9/20. The early embryonic expression of Cs-FGF4/6/9 was transient and the transcript was seen in endodermal cells at the 16- and 32-cell stages, in notochord and muscle cells at the 64-cell stage, and in nerve cord and muscle cells at the 110-cell stage; the gene was then expressed again in cells of the nervous system after neurulation. When the gene function was suppressed with a specific antisense morpholino oligo, the differentiation of mesenchyme cells was completely blocked, and the fate of presumptive mesenchyme cells appeared to change into that of muscle cells. The inhibition of mesenchyme differentiation was abrogated by coinjection of the morpholino oligo and synthetic Cs-FGF4/6/9 mRNA. Downregulation of β-catenin nuclear localization resulted in the absence of mesenchyme cell differentiation due to failure of the formation of signal-producing endodermal cells. Injection of synthetic Cs-FGF4/6/9 mRNA in β-catenin-downregulated embryos evoked mesenchyme cell differentiation. These results strongly suggest that Cs-FGF4/6/9 produced by endodermal cells acts an inductive signal for the differentiation of mesenchyme cells. On the other hand, the role of Cs-FGF4/6/9 in the induction of notochord cells is partial; the initial process of the induction was inhibited by Cs-FGF4/6/9 morpholino oligo, but notochord-specific genes were expressed later to form a partial notochord.