Possible Role of Atrial Epithelium as a Carrier of the Positional Information in the Compound Ascidian, Polyandrocarpa misakiensis

There are two phenotypically different strains in the compound ascidian, Polyandrocarpa misakiensis. One is white spot strain with large circular white spot on the dorsal side between branchial and atrial siphons. The other is spotless strain with fine white band instead of the circular spot at the same area. Anatomical study has been revealed that the difference between two strains based mainly on the distribution of red pigments on epidermal layers. If two anterior fragments are fused together at their cut surfaces, double-zooid monsters are produced, each regenerating the posterior part at the fusion area. Induction of posterior organization is responsible for the gap of positional values. Therefore, the boundary of cells carrying the positional information in the monster may be regarded as the line joining two posterior ends. If such a monster is produced between two strains, the boundary of cells concerning color pattern of two strains can be drawn by the distribution of two strains of blastogenic zooids arising from the margin of the monsters. The boundary of cells carrying the positional information was not always consistent with that of cells concerning color pattern. Therefore, it is concluded that epidermal layer is not responsible for the positional information. Possible role of atrial epithelium as a carrier of the positional information is suggested in the ascidian