Ascidian tail formation requires caudal function

Although the tail is one of the major characteristics of animals of the phy lum Chordata, evolutionary aspects of the molecular mechanisms involved in its formation are not clear. To obtain insights into these issues, we have isolated and investigated the caudal gene of an ascidian, one of the lower animal groups among chordates. Ascidian caudal is expressed from the midgas trula stage onward in the lateral walt of the posterior neural tube cell li neage and also in the posterior epidermal cells from the neurula stage. Thu s, ascidian caudal expression is restricted to the ectoderm of a tail-formi ng region throughout embryogenesis. Suppression of caudal function by an an tisense oligonucleotide or a dominant negative construct caused inhibition of the cell movement required for tail formation. Overexpression of wild-ty pe caudal mRNA in an ascidian animal cap, an animal half explant prepared a t the eight-cell stage, caused elongation of the cap. Furthermore, Xenopus embryos injected with dominant negative ascidian caudal exhibited defects i n elongation, suggesting a conserved caudal function among chordates. These results indicate that caudal function is required for chordate tail format ion and may play a key role in its evolution. (C) 1999 Academic Press.