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 t adpole larva that provide new insights into chordate origins and evolu tion. The notochord of ascidian larvae and vertebrates appear to be ho mologous structures based on their induction by endoderm and expressio n 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 specifie d by cytoplasmic determinants localized in the egg as well as embryoni c induction. Studies of the tailless larvae of anural ascidians have r esulted in the identification of Manx, a gene that may control tail de velopment and evolution. These and other results support the ascidian tadpole prototype for the ancestral chordate.