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

Morphogenetic tissue interactions during posterior commitment in palleal buds of the polystyelid ascidian, Polyandrocarpa misakiensis

The development of chimeric double-anterior and double-posterior buds in the polystyelid ascidian, Polyandrocarpa misakiensis, was studied. Bud pieces used were allowed to develop for various time periods before the fusion operation. One-day double-half buds developed into complete zooids with normal anteroposterior polarity. In these cases, the bud half whose site of origin on the parent was higher (more posterior) than the partner formed the posterior organ, the digestive tract, of a resultant zooid. Two-day double-posterior buds developed into biposterior zooids that had two digestive tracts, whereas 2-day double-anterior buds still underwent pattern regulation in the same manner as 1-day buds (23-24 degrees C). Bianterior zooids could only be induced from 3-day double-anterior buds, suggesting that the anterior determination occurs later than the posterior determination. Next, a 3-day posterior half was combined with a 1-day posterior half from a higher site. Though "high-level" tissue has a high potential for posterior differentiation, the 1-day bud half failed to form the digestive tract. This result suggests that the 3-day bud half has acquired the ability to inhibit the formation of additional posterior structures. The kinetics of posterior-forming activity of developing bud pieces was measured using a 12-hr bud piece of the highest level as a probe. The activity increased with increasing developmental time, and the increase was accompanied by posterior inhibition activity. Buds of different positional levels showed different time courses of these activities. The results of this study show that the posterior end is the dominant region of developing ascidian buds.