Ciona intestinalis, sea squirt model tunicate

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Taxonomy

cellular organisms - Eukaryota - Fungi/Metazoa group - Metazoa - Eumetazoa - Bilateria - Coelomata - Deuterostomia - Chordata - Urochordata - Ascidiacea - Enterogona - Phlebobranchia - Cionidae - Ciona - Ciona intestinalis

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Brief facts

• Ciona intestinalis is a large solitary sea squirt which grows up to 15 cm in length.
• The body is soft, retractile and a pale translucent greenish/yellow, through which the internal organs are visible.
• Ciona intestinalis prefers habitats with low wave exposure and some water flow. It grows not only on bedrock and boulders but also on artificial surfaces such as metal and concrete. Other organisms, such as algae, are also used as substrata. C. intestinalis is the only species of a cosmopolitan ascidian.

Importance as a model organism

• Tunicates have some of the smallest bilaterian genomes: the haploid content is only about 0.196 pg DNA.
• Many characteristics make Ciona a highly tractable model: Ciona has only 2,500 cells in the late larva; cell fates are determined very early in embryogenesis compared to those of other animals, and developmental genes have concomitantly altered functions in patterning the embryo.
• Ciona intestinalis belongs to the subphylum Urochordata. Urochordates split off earliest from other chordates, and the characteristics we share with them may help to shape an image of the eochordate, the ancestor of the modern chordates. Chordates, like echinodems (for example, sea urchins) and hemichordates (for example, acorn worms), are deuterostomes, a division of animals that are distinguished from other animals (protostomes) by the development of the embryonic blastopore into the anus rather than the mouth.
• Two contrasting modes of early embryogenesis were proposed: (1) mosaic (cell-autonomous) mode where the egg is organized by way of localized and inherited maternal factors that are responsible for the development of parts of the embryo and adult and where each cell fate is predetermined and restricted; (2) inducer or "organizer" (non-cell-autonomous) mode where some isolated parts of the embryo can compensate and make a full larva in a process of self-regulation, also, some parts of the embryo, when transplanted, are capable of inducing development in the surrounding cells. Although ascidian embryogenesis was long considered as "text-book" example of mosaic development, recent studies show that it also involves inductive events and that both modes interact with each other.
• There are striking parallels between the tunicate, Ciona intestinalis and the nematode, Caenorhabditis elegans, in terms of reduction of the nuclear genome, rapid evolution of the mitochondrial genome, the acquisition of determinate development and simplification of the body plan.

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Organs

• siphon a tubular organ by which water is taken in or expelled
  • buccal siphon inhalant or incurrent siphon; it is situated at the end of the oval outline of the animal; water and food particles enter the buccal siphon and move to the pharynx lumen
  • atrial siphon exhalant or excurrent siphon; it is situated on the side of the body; digestive wastes, gametes, and the feeding current carried out of the atrium by the atrial siphon
• body wall
  • tunic the outermost layer of the body wall; tunic serves as exoskeleton that grows as the animal grows and does not require molting; it consists of a matrix cellulose (tunicin), protein fibers, cells, and proteoglycan ground substance
  • mantle the inner layer of the body wall consists of the epidermis, connective tissue, circular and longitudinal muscles
• gonad gonads are hermaphroditic and consist of an ovary and testis on each side
  • ovary
  • oviduct
  • testis
• pharynx respiratory organ; large and thin, it underlines most of the mantle extending from the buccal siphon; pharyngeal wall is perforated by minute gill slits (stigmata)
• heart
• digestive
  • pyloric gland digestive gland
  • gastrointestinal tract
    • esophagus
    • stomach
    • intestine
• endostyle a special organ in the pharynx of Urochordata, Cephalochordata, and Cyclostomata; it composed of ciliated and glandular cells; the glandular cells secrete an iodine-containing mucus net used for food capture by the branchial basket; the endostyle has a functional homology to the vertebrate thyroid gland
• nervous
  • cerebral ganglion serves as a primitive brain
  • nerve nerve cords that exit from each end of the ganglion and branch out
• neural gland contains no neurons and has no nervous role; homology with pituitary gland of vertebrates has been proposed

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Life cycle

C. intestinalis can spawn year around. Generation time is about 2-3 months.

Life Cycle Stages

• fertilized
  egg MeSH C. intestinalis is cross-fertile and very rarely self-fertile; fertilized eggs maintained at 20°C usually hatch in 14-16 hours after fertilization, or in 16 - 18 hours at 18°C
• embryo
  • cleavage MeSH dividing egg; the first cleavage occurs in 1 h after ferticlization at 18°C, after two more synchronous and four asynchronous cleavages, the embryo reaches the 110-cell stage, after which the gastrulation starts
  • gastrula MeSH starts at ~ 5 h after fertilization; a single-cell layers of endoderm and mesoderm cells in the vegetal pole invaginate into the interior toward animal pole while the ectoderm migrates toward the vegetal pole to form a layer surrounding the embryo
  • neurula neurulation begins soon after gastrulation is complete (~7 h after fertilization); during this stage the neural plate is folded up dorsally to form the neural tube; once it is closed, the tail becomes noticable; at the same time the notochord cells converge and extend along anterior-posterior axis
  • tailbud ~9 h after fertilization; during this stage the tail continues to elongate until the embryo is ready to hatch
• larval motile tadpole larva; tadpole larva consists of trunk ("tadpole's head") and tail, which has a notochord and a dorsal nervous chord (typical chordate features); the anterior end of the trunk has a preoral lobe with stalk and adhesive papillae, which are primary organs used for settlement of the ascidian; the larva also has a sensory vesicle with both a statocyst and a light-sensitive organ; the free-swimming period may last from 6 to 36 hours, usually more than 12 hours; during this period the larva is constantly evolving and passes many stages of its development until the attachment
• metamorphosis
  MeSH metamorphosis begins with attachment of the larva to the substrate; metamorphic events include loss of the tail and most of the nervous system, transformation of heart and digestive system; this stage takes about 1-2 days; at day 3 after hatching, the buccal siphon begins to contract in response to stimuli, and ascidians become able to feed by filtration and atrial siphon also becomes functional (for excretion)
• juvenile
  • 1^st ascidian stage attached (sessile) zooid with underdeveloped gill slits; starts from early stage with only 2 protostigmata on each side in the pharynx and lasts until the two pre-atrial siphons fuse
  • 2^nd ascidian stage starts from 6 rows protostigmatas and fused pre-atrial siphons and continues until all adult organs including gonads are fully developed; late stage is essentially undersized sexually immature zooid
• adult sexually mature and ready to spawn zooid; under ideal laboratory conditions sexual maturity is attained at about 2 months of age at size of 50-60 mm; life span is about 6 months

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Appendix I: C. intestinalis adult and larva

Cañestro C, Bassham S, Postlethwait JH. Seeing chordate evolution through the Ciona genome sequence. Genome Biol. 2003;4(3):208.

a) The Ciona adult develops from a tadpole larva by a dramatic metamorphosis, during which it resorbs its tail, modifies its central nervous system, and transforms its digestive system into incurrent and excurrent siphons that filter plankton through perforations in the pharynx (gill slits). Image courtesy of Andrew Martinez.
(b) An early tadpole hybridized with a probe for the alcohol dehydrogenase gene CiAdh3 to show the anterior endoderm. Chordate features such as the notochord and the muscular tail are visible, and the positions of the sensory vesicle (brain) and dorsal nerve cord are indicated.
(c) Electroporation is an efficient procedure by which to introduce DNA into Ciona for functional experiments, as is shown in this larva expressing a reporter construct, a lacZ gene driven by the brachyury promoter in the notochord.

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Appendix II: tail morphogenesis in C. intestinalis

Passamaneck YJ, Hadjantonakis AK, Di Gregorio A. Dynamic and polarized muscle cell behaviors accompany tail morphogenesis in the ascidian Ciona intestinalis. PLoS One. 2007 Aug 8;2(1):e714. PMID: 17684560

Schematic illustrations depicting muscle cell development in Ciona and high-resolution imaging of muscle development by dual-tagging muscle cells with spectrally distinct, subcellularly-localized fluorescent proteins. (A) Schematics of the muscle lineage of the Ciona intestinalis embryo at the one-cell, 8-cell, 32-cell, 110-cell, neurula, early tailbud, mid tailbud and late tailbud stages, with the cell lineages marked by conventional nomenclature. Only one side of the embryo is labeled. Tail muscle precursors are labeled in orange, neural tissue in light blue, trunk mesenchyme in light purple and trunk ventral cells (heart progenitors) in dark purple. Blastomeres that give rise to more than one tissue are stippled with the colors corresponding to their fates. (B–I) Time series of embryos co-electroporated with sna>GPI-GFP and sna>H2B-RFP. (B,F) neurula, (C,G) early tailbud, (D,H) mid tailbud, and (E,I) late tailbud stages are shown. (B–E) A single slice in the z-axis is shown in the xy-plane of view. Cross-sectional slices in the xz and yz-planes are shown above and to the left of the xy-axis view, respectively, and the positions of the slices are represented in the insets. (F–I) A maximum intensity projection of all slices along the z-axis is shown in the xy-plane of view. Scale bars, 40 µm.

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References

PubMed articles

• Kumano G, Nishida H. Ascidian embryonic development: an emerging model system for the study of cell fate specification in chordates. Dev Dyn. 2007 Jul;236(7):1732-47. PMID: 17366575
• Joly JS et al. Culture of Ciona intestinalis in closed systems. Dev Dyn. 2007 Jul;236(7):1832-40. PMID: 17394236
• Liu LP et al. Ciona intestinalis as an emerging model organism: its regeneration under controlled conditions and methodology for egg dechorionation. J Zhejiang Univ Sci B. 2006 Jun;7(6):467-74. PMID: 16691641
• Kawashima T et al. Dynamic changes in developmental gene expression in the basal chordate Ciona intestinalis. Dev Growth Differ. 2005 Apr;47(3):187-99. PMID: 15840003
• Passamaneck YJ, Di Gregorio A. Ciona intestinalis: chordate development made simple. Dev Dyn. 2005 May;233(1):1-19. PMID: 15765512
• Chiba S et al. Development of Ciona intestinalis juveniles (through 2nd ascidian stage). Zoolog Sci. 2004 Mar PMID: 15056923
• MICHAEL J. KATZ. COMPARATIVE ANATOMY OF THE TUNICATE TADPOLE, CIONA INTESTINALIS MICHAEL J. KATZ Free full text
• Free full text articles: major topic Ciona intestinalis

Websites and other references

• Wikipedia: Ciona intestinalis