The mechanism of anteroposterior cell determination in ascidian palleal buds: A gap of positional values triggers posterior formation.

Cell-cell interactions during the development of bud polarity in the polystyelid ascidianPolyandrocarpa misakiensis were studied by juxtaposing normally nondjacent bud pieces from marked stocks. These chimeras have been used to determine the prospective fate of each bud piece with respect to positional disparity. The results showed that the posterior end of a future zooid was always established around the proximal boundary region of the two bud pieces. The anteroposterior axis was skewed from this posterior end toward the bud piece arising from a lower (more anterior) level of the parental zooid, suggesting that the lower level tissue may provide a cue which establishes the site of the future anterior end. The role of positional disparity between bud pieces was further examined by sandwiching a host bud between two grafts of various parental positions. The results showed that an additional positional gap resulted in the formation of an additional posterior structure such as the digestive tract. For example, when the grafts were taken from two or more positions posterior to the host bud, biposterior zooids were induced in 20-30% of the cases. This induction ratio increased to 72.4% if two host buds were used instead of a single host. It is, therefore, concluded that the gap of positional values in ascidian buds triggers the formation of the posterior-most positional value. This cellular response to a positional gap contrasts with the intercalation theory that is predicted by the polar coordinate model for pattern formation.