Cell lineage and timing of fate restriction, determination and gene expression in ascidian embryos.

Tadpole larvae of ascidians show the basic body plan of chordates. An ascidian larva consists of only few types of cells and has a relatively small number of cells. Cell lineages are simple and invariant among individuals and have been described in detail. The clonal restriction of developmental fate takes place considerably early in development. I review here the temporal relationship between fate restriction, determination and initiation of lineage-specific gene expression during ascidian embryogenesis. In several cases, determination and initiation of gene expression precede fate restriction and occur during the last cell cycle before fate restriction. Such a phenomenon contradicts the traditional view of fate specification and has several important implications for the understanding of the way in which cells execute the developmental pathway.