Fig. 1. The cell lineages and gene expression domains of the ascidian CNS. (A) Cell lineages of the CNS. Embryonic stages are indicated below the drawings. Blastomere names are indicated on the left half of the drawings. At the neural plate stage (neural plate shown only), blastomere names should be prefixed with an a9. for a-line and A9. for A-line, as indicated. Bars connecting blastomeres on the right half of the drawings indicate a sister cell relationship. A-line lineages are shown in yellow for medial precursors and tan for lateral precursors, with the secondary muscle precursor coloured in blue. a-line lineages are indicated in red (sensory vesicle) and pink (anterior epidermis and pharynx; because of the cell arrangement, these cells are considered part of the neural plate although they do not contribute to the CNS). In addition, cells from the b-line, positioned lateral to the A- and a-line neural plate, contribute to the dorsal part of the CNS (data not shown). The same colour code is used in all subsequent figures. At the neural plate stages, cells are organised into four columns (1-4) and six rows (I-VI), as indicated. (B) Schematic representation of the gene expression profiles in A-line neural lineages at early gastrula and neural plate stages. The right-hand-side A-line neural blastomeres of the bilaterally symmetrical embryo are represented; the position of the medial axis is indicated on the left. Bars above and below the schematics indicate gene expression for the markers used in this study (row II above, row I below). Position of the columns and rows are indicated (top and right). Sna, Snail; 07j15, cicl07j15; FGF8, FGF8/17/18; Chd, Chordin; FGF9, FGF9/16/20.