Asexual Development of Ascidians: Its Biological Significance, Diversity, and Morphogenesis

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Abstract
SYNOPSIS. Budding of colonial ascidians was reviewed from three different viewpoints. First, budding was classified into two functional types, propagative and survival. The former type is supposed to occur in all colonial ascidians; it is active and functions during colony growth. The latter type is passive and occurs in some ascidians under adverse environmental conditions to ensure the survival of the animal. Second, a comparative study was made on the development and budding of ascidians of the subfamily Polyclininae. Third, some morphological problems concerning the asexual development of ascidians were discussed. Special attention was paid to 1) morphological differences between oozoids and blastozooids, 2) establishment of bud polarity, and 3) transmission of bilateral asymmetry to the next asexual generation. The evidences cited in these discussions show that colonial ascidians have a great promise for the study of morphogenesis, especially pattern formation.

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