The role of parental positional information in the determination of antero-posterior polarity during palleal budding in ascidians.

In the polysteelid ascidian,Polyandrocarpa misakiensis, buds can arise at any antero-posterior positional level around the basal margin of a functional zooid. In this study, each bud was cut into anterior and posterior halves along its proximo-distal axis. When an anterior half was combined with a posterior half from a higher (more posterior) level, the future zooid developed the same anteroposterior polarity as a normal bud. When the same operation was performed with a posterior half from a lower (more anterior) level, the antero-posterior polarity was inverted, resulting in a reversed body asymmetry (situs inversus viscerum). Bud polarity depended not on the size or the growth stage of the fragmented buds, but on the difference in parental positional levels from which respective buds to be fused had originated. Surgically constructed doublehalf buds developed a single polarity instead of a pattern reduplication. Insertion experiments with bud pieces showed that the gap between the positional levels was recognized at the proximal region of operated buds. The results of this study are inconsistent with the predictions of the polar coordinate model for pattern formation. It is concluded that a parent zooid possesses antero-posterior position-specific potential for determining bud polarity.