Chasing tails in ascidians: developmental insights into the origin and evolution of chordates.

The ascidian tadpole larva is regarded as a prototype of the ancestral chordate. Here we consider recent studies on the development of the tadpole larva that provide new insights into chordate origins and evolution. The notochord of ascidian larvae and vertebrates appear to be homologous structures based on their induction by endoderm and expression of the Brachyury (T) gene. The muscle cells of ascidian larvae also appear homologous to those of vertebrates based on their expression of bHLH myogenic and muscle-type actin genes, although they are specified by cytoplasmic determinants localized in the egg as well as embryonic induction. Studies of the tailless larvae of anural ascidians have resulted in the identification of Manx, a gene that may control tail development and evolution. These and other results support the ascidian tadpole prototype for the ancestral chordate.