Development of the appendicularian Oikopleura dioica: culture, genome, and cell lineages.

Appendicularians are planktonic tunicates (urochordates), and retain a swimming tadpole shape throughout their life. Together with ascidians, they are the closest relatives of the vertebrates. Oikopleura dioica is characterized by its simplified life habit and anatomical organization. It has a tiny genome, the smallest ever found in a chordate. Its life cycle is extremely short - about 5 days - and it can be maintained in the laboratory over many generations. Embryos and adults are transparent and consist of a small number of cells. The anatomy of juveniles and adults has been described in detail. Cleavage pattern, cell lineages, and morphogenetic movements during embryogenesis have also been comprehensively documented. A draft genome sequence is now available. These features make this organism a suitable experimental model animal in which genetic manipulations would be feasible, as in Drosophila and Caenorhabditis elegans. In this review, I summarize a hundred years' knowledge on the development throughout the life cycle of this organism. Oikopleura is an attractive organism for developmental and evolutionary studies of chordates. It offers considerable promise for future genetic approaches.