The Ascidian as a Model Organism in Developmental and Evolutionary Biology

What Is an Ascidian? Ascidians, or sea squirts, are invertebrate chordates that belong to the earliest branch in the chordate phylum, the subphylum Urochordata or Tunicata (Figure 1A). Ascidian larvae possess a prototypical chordate body plan Joseph C. Corbo,1,3 Anna Di Gregorio,2 and Michael Levine2,3 Department of Pathology Brigham and Women’s Hospital and Harvard Medical School Boston, Massachusetts 02115 that includes a dorsal neural tube, an axial notochord 2 Department of Molecular and Cellular Biology flanked by muscles, and a ventral endodermal strand University of California at Berkeley (Satoh, 1994; Figure 1B). Gastrulation and neurulation Berkeley, California 94720 involve cellular rearrangements that are comparable to those seen in vertebrates, except that ascidian embryos ...at an extremely remote period a group of animals are composed of just a few hundred cells whereas comexisted resembling in many respects the larvae of our parable vertebrate embryos contain many thousands of present Ascidians, which diverged into two great cells. The resultant motile larva ( 1 mm long) is analobranches—the one retrograding in development and gous to the amphibian tadpole. This larva ultimately producing the present class of Ascidians, the other rising undergoes metamorphosis into a sessile, filter-feeding to the crown and summit of the animal kingdom by adult (Figure 1C), hence Darwin’s reference to ascidians giving birth to the vertebrata. “retrograding in development.” —Charles Darwin, The Descent of Man (1871) The larva of Ciona intestinalis, the most cosmopolitan ascidian species, has about 2,600 cells including only 36 Several years after the publication of On the Origin of muscle cells and 40 notochord cells. It has a remarkably Species in 1859, Darwin recognized the importance of simple central nervous system (CNS) with only 330 cells the ascidian larva as a prototype of modern chordates. of which fewer than 100 are neurons, the remainder Despite their key phylogenetic position, ascidians were being glial cells (Meinertzhagen and Okamura, 2001). largely neglected by experimental biologists through These figures place the ascidian larva on a level of cellumost of the twentieth century. This fact is surprising lar complexity comparable to that of C. elegans which since ascidian embryos provided the very first classical has 959 somatic cells including 95 muscle cells and 302 evidence that localized determinants control cell fate neurons. As with C. elegans, ascidian cell lineages are specification (Conklin, 1905). Furthermore, the ascidian invariant and have been completely defined for several larva represents an ideal model of early chordate develtissues. The cell lineage of the C. intestinalis CNS has opment: it possesses the basic developmental and been traced through the penultimate cell division, and the morphologic features of vertebrates but has the cellular connectivity of all neurons is being mapped (Meinertzand genomic simplicity of invertebrates. The purpose hagen and Okamura, 2001 and references therein). Such of this review is to describe the recent revitalization of an undertaking is simply not feasible in other chordates. interest in ascidians and their potential as a model The cellular simplicity of the ascidian larva is mirrored system for developmental and evolutionary biology. by its small, compact genome. The haploid genome size of C. intestinalis is 160 Mb and contains 15,500 genes Correspondence: [email protected], mlevine@uclink4. berkeley.edu (Simmen et al., 1998). This low figure, along with other