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T he origin and evolution of the chordates has been the subject of extensive investigation and speculation for more than a centuryl-5. The phylum Chordata con- sists of the subphyla Urochordata (tunicat&, Cephalo- chordata (amphioxus) and Vertebrata. These groups exhibit a notochord, a dorsal nerve cord and pharyrr- g& gill slits, which are hallmarks of the chordate body plan. &hough many extant invertebrate groups and several hypothetical forms have been considered as potential chordate ancestors, a consensus has settled on emergence from a deuterostome ancestor1-6. Phy- logeties inferred by molecular7s and cladistic analysis9 suggest that: (I) the deuterostomes are a monophyletic group; (2) the cephalochordates and vertebrates are sister groups; and (3) the urochordates (larvaceans, salps and ascidians) are the sister group of the cephalo- chordate/vertebrate line (Fig. 1). The urochordates occupy a central position in most theories of chordate evolution, largely because of their body planl? Adult (larvaceans) or larval (ascidians) urochordates have a motile tail containing a notochord, muscle cells and a dorsal nervous system. Therefore, the development of the ascidian tadpole larva may provide clues concem- ing the origin and evolution of chordates. Development and orga&ation of the ascidian tadpole larva Ascidians (sea squirts) are ubiquitous marine animals, which have served as model systems in developmental biology for more than a centurylOJt. Ascidian embryos exhibit bilateral cleavage and a highly determinate mode of development. The complete cell lineage has been described up to the gastrula stagelz. Gastrulation begins at the IlO-cell stage, followed by neurulation and tail formation. A swimming tadpole larva usually hatches within a day after fertilization. The tadpole larva con- sists of about 2600 cells and a small number of tissues: endoderm, epidermis, nervous system (brain and spinal cord), notochord, muscle and mesenchyme (Fig, 2). The tadpole is organized into a trunk and tail (Fig. 2a). The trunk contains endoderm, mesenchyme and a dorsal brain with sensory organs. The tail contains a noto- chord flanked by the spinal cord, endodermal strand and three rows of striated muscle cells (Fig, 2b). The entire surface of the larva is covered by an epidermis. The non-feeding larva is a dispersal phase. During metamorphosis, the larval tail is destroyed, and the adult tissues and organs differentiate from precursors in the trunk. No&chord development The notochord consists of 40 cells aligned in single file in the larval tail (Figs 2 and 3ej. As shown in Fig. 3a-e, the 40 notochord cells consist of 32 A-line (derivatives of the Ad.1 cell-pair of the &cell embryo) and 8 B-line (derivatives of the B4.1 cell-pair) blastomeres. Blastomere isolation and recombination experiments show that A-line notochord cells (~6.2 or A6.4 cells) are induced during the 32-cell stage by a signal emanating from the adjacent presumptive endo- derm blastomeres (~6.1 or ~6.3 cells)l3. The presump- tive notochord blastomeres are able to induce mutually to form notochordl? The prospective notochord cells Chasing tails in ascidians: developmental insights into the origin and evolution of chordates NORMJKI SATOH AND WILLIAM R. JEFFERY me ascidkan tadpole larva is regard&d as aprototype of tbe ancestral cbordhte. Here we considkr recent stud&s on tbe devehpment of tbe tadpole hawa tbatprovidk new insights into cbordhte origins a& evokrtion a*be notocbord of ascidia~ larvae and vertebrates appear to be bomohgous structures based on their induction by endodhrm and expression of tbe Bracbyury (13 gene. Ebe muscle cells of ascidian larvae also appear bomobgous to those of vertebrates based on their expression of bIELH myogenic and muscle-type actin genes, altbougb they are spec@ed by cytopksmk determinants localked in tbe egg as well as embryonic itulucth Studies of tbe taiks larvae of anural ascidhas bave resutied in tbe idknt@ation of Manx, agene tbat may control tail deve@ment and evo&iot~ These and other resu&s * support tbe ascidian tadpole prototype fw tbe ancestral cbordhte. involute over the anterior lip of the blastopore during gastrulation and subsequently undergo differentiation, a change in cell shape and morphogenetic movements . leading to the extension of the tail. As the notochord also arises by induction in vertebratesl*, the mecha- nisms of notochord development may be conserved among the chordates. The development of the central nervous system and sensory organs is also achieved by in- duction in ascidian embryoSJ6. Understanding the molecular basis of notochord development in vertebrates has been advanced by cloning the mouse BEK~~UPJI (T) genen. Homologs of the mouse T gene have been isolated in X~opus and zebrafish. In these vertebrates, the T gene is expressed in the developing notochord and other mesodermal derivatives. Mutations in the mouse T gene lead to de- fects in the primitive streak, lack df notochord and pos- terior mesoderm differentiation, and failure of tail elon- gation. We have identified cDNA clones encoding an ascidian homolog (As-T) of the vertebrate T genel*J9. The N-terminal of the As-T protein has 78% an-r&o acid identity to the mouse T protein. The As-Ttranscripts are frost detected at the 64-cell stage, peak during the gas- trula stage, and decrease during the neurula and tailbud stages. The initiation of the expression of the As-Tgene is correlated with fate restriction vh the notochord lin- eage1*J9. As shown in Fig. 3b, the 32-cell embryo con- tains three pairs of presumptive notochord cells, ~6.2, A6.4 and B6.2. At this stage, these cells are also fated to produce spinal cord, muscle and mesenchyme cells. The As-Tgene is not expressed in the 32-cell embryo (Fig. 3f). The 64-ceil embryo also has three pairs of notochord-hqeage cells, A7.3, A7.7 and B7.3 (Fig. 3~). At this stage, the A7.3 and A7.7 pairs are destined to TIG SEWER 1995 VOL. 11 No. 9 0 1995 Ekvier Science Ud 0168 - 9525/95/$09.50 354