Induction of malformed zooids and determination of polarity in palleal buds of the polystyelid ascidian, <i>Polyandrocarpa misakiensis</i>

Abstract

Abstract The development of malformations was studied in palleal buds of the polystyelid ascidian, Polyandrocarpa misakiensis . Various malformations were induced by transection of protruding buds. Three types could be distinguished: zooids with reversed bilateral asymmetry, zooids with double malformations, and zooids with three apertures. During the early developmental stages of transected proximal pieces, when depigmentation occurred in the distal area, the pieces produced zooids with reversed bilateral asymmetry. When depigmentation occurred in the distal and proximal areas, the pieces induced either zooids with double malformations or zooids with three apertures. In the former, the two depigmented areas were about the same size, whereas in the latter the two areas were very different in size. The individuals occurring from the depigmented proximal area had the same polarity as the parent. Individuals derived from the depigmented distal area displayed either reversal of the left‐right (L‐R) axis (reversed laterality) or reversal of the L‐R and anteroposterior (A‐P) axes (normal laterality). The malformed zooid with reversed L‐R axis produced buds with reversed laterality. We examined the frequency of induction of these malformed zooids in relation to the following four points: 1) the position of the parent body at which buds are produced, 2) the different developmental stages of blastogenesis, 3) the different levels along the maximum protruding bud, and 4) the different transecting times of the maximum protruding bud. The results are discussed in relation to the mechanism of the expression of polarity in the normal development of the palleal bud.