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MIT 7 72 - Study Guide

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REVIEWS–A PEER REVIEWED FORUMCiona intestinalis: Chordate DevelopmentMade SimpleYale J. Passamaneck and Anna Di Gregorio*Thanks to their transparent and rapidly developing mosaic embryos, ascidians (or sea squirts) have been amodel system for embryological studies for over a century. Recently, ascidians have entered thepostgenomic era, with the sequencing of the Ciona intestinalis genome and the accumulation of molecularresources that rival those available for fruit flies and mice. One strength of ascidians as a model system istheir close similarity to vertebrates. Literature reporting molecular homologies between vertebrate andascidian tissues has flourished over the past 15 years, since the first ascidian genes were cloned. However,it should not be forgotten that ascidians diverged from the lineage leading to vertebrates over 500 millionyears ago. Here, we review the main similarities and differences so far identified, at the molecular level,between ascidian and vertebrate tissues and discuss the evolution of the compact ascidian genome.Developmental Dynamics 233:1–19, 2005.© 2005 Wiley-Liss, Inc.Key words: Ciona; ascidian; chordate; development; evolution; genomeReceived 10 June 2004; Revised 2 November 2004; Accepted 3 November 2004INTRODUCTIONThe ascidian Ciona intestinalis hasemerged recently as a model systemfor understanding the evolution ofchordate development and genome or-ganization. Ascidians are membersof the chordate clade Urochordata,which diverged from the last commonancestor of all chordates at least 520million years ago (Chen et al., 2003).This divergence creates over one bil-lion years of independent evolutionbetween extant ascidians and modernvertebrates, such as human, mouse,chick, frog, and zebrafish. Despite thisevolutionary distance, the basic fea-tures of the chordate body plan re-main recognizable in ascidian larvae.Because of their relative simplicityand their position as an outgroup tothe vertebrates, ascidians haveunique potential to illuminate the mo-lecular mechanisms underlying theprimitive body plan from which mod-ern chordates diversified.The close relationship between ascid-ians and vertebrates was first recog-nized in the mid-19th century by theembryologist Alexander Kowalevsky,who noted the striking similarities be-tween ascidian larvae and vertebrateembryos (Kowalevsky, 1866). Kowa-levsky’s observation of a notochord anda dorsal neural tube in the ascidianlarva provided clear evidence that as-cidians are, along with vertebrates andthe cephalochordate amphioxus, mem-bers of the phylum Chordata. The adultascidian also possesses recognizablechordate features, even though it is con-sidered more divergent than the larvaand is a sedentary filter-feeder (al-though some deep-sea ascidians areadapted to capture larger food particles;Fig. 1). The feeding basket of the adultABBREVIATIONS CNS central nervous system kb kilobase(s) Mb megabase(s) bHLH basic helix-loop-helix FGF fibroblast growth factorTVC trunk ventral cell DCEN dorsocaudal epidermal neuron VCEN ventrocaudal epidermal neuron GFP green fluorescent proteinDepartment of Cell and Developmental Biology, Weill Medical College of Cornell University, New York, New YorkGrant sponsor: March of Dimes Birth Defects Foundation; Grant number: #5-FY03-153.*Correspondence to: Anna Di Gregorio, Ph.D., Department of Cell and Developmental Biology, Weill Medical College ofCornell University, 1300 York Avenue, Box 60, Whitney Pavilion, W-505, New York, NY 10021.E-mail: [email protected] 10.1002/dvdy.20300Published online 11 March 2005 in Wiley InterScience (www.interscience.wiley.com).DEVELOPMENTAL DYNAMICS 233:1–19, 2005© 2005 Wiley-Liss, Inc.contains gill slits that appear to share acommon origin with the gill slits ofother chordates (Aros and Viragh,1969). Likewise, the endostyle of theadult ascidian is a homolog of the ver-tebrate thyroid gland, sequestering io-dine and producing thyroid hormone(Eales, 1997).As a model system for understandingchordate development, ascidians offerseveral advantages. First, the ascidianlarva is relatively simple, being com-posed of only ⬃2,500 cells when fullydeveloped (Satoh, 1994). Furthermore,the cleavage program of the embryo isinvariant, and accurate fate maps havebeen drawn to trace the embryonic de-velopment of different ascidian species(Conklin, 1905; Ortolani, 1971; Nishida,1987). When fully developed, the ascid-ian larva contains only six tissue types:nervous system, notochord, muscle, ec-toderm, endoderm, and mesenchyme(Fig. 2). The notochord is composed ofonly 40 cells; the muscle of 36 cells (40cells in the Japanese ascidian Halocyn-thia roretzi); and the central nervoussystem (CNS) of ⬃350 cells, of which⬃100 are neurons (Nicol and Mein-ertzhagen, 1991; Satoh, 1994). Cionaembryos and larvae are transparent, al-lowing for direct observation of tissueswithout need for sectioning.The recent sequencing and assemblyof the C. intestinalis genome by theJoint Genome Institute (JGI; Dehal etal., 2002), greatly aids molecular stud-ies, allowing rapid identification ofgenes of interest and of the noncodingregions flanking them. The ongoing se-quencing of the closely related speciesCiona savignyi (http://www.broad.mit.edu/annotation/ciona/) should providean invaluable resource for comparativeapproaches, such as phylogenetic foot-printing, to identify conserved cis-regu-latory regions (e.g., Zhang and Ger-stein, 2003).Ciona embryos are readily amenableto experimentation in the laboratory.Adults are hermaphrodites, and in vitrofertilization can produce thousands ofsynchronously dividing embryos. In ad-dition, the C. intestinalis embryo devel-ops rapidly, with the tadpole larva com-pleting development in 18 hr whenreared at 18°C (Whittaker, 1977).Fig. 1. Morphological diversity of adult ascidians. A,B,C,E: Red arrowheads point to the oral siphon; yellow arrowheads point to the atrial siphon. A:The solitary ascidian Halocynthia roretzi, which is used as food crop in Japan. B: The colonial ascidian Botryllus schlosseri. Colonies may be composedof up to 100 systems of 7–10 zooids produced asexually through budding. Each zooid has its own oral siphon, whereas all zooids in a system sharea common atrial siphon. The colony shown is in “Stage D,” or the takeover stage of colony development. Older zooids are regressing throughapoptosis, whereas younger buds are preparing to open their oral siphons (photograph courtesy of Dr. Diana


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