Ascidians (tunicates) are primitive chordates. In spite of their elevated phylogenetic position in the animal kingdom, ascidians have evolved a varied reproductive repertoire; some of them live as individuals (solitary ascidians), while others form colonies (colonial ascidians). Colonial ascidians propagate asexually by budding and strobilation, and they have an extensive capacity for regeneration. However, the orthodox taxonomic classification of ascidians categorizes them into two major groups (the orders Enterogona and Pleurogona), irrespective of their solitary or colonial life style. To examine whether the orthodox classification of ascidians is substantiated by molecular phylogeny, the complete nucleotide sequence of a region of about 1000 base pairs in the central part of their respective 18S rDNAs was determined, and the sequences were compared among five solitary and three colonial ascidians. The phylogenetic tree deduced from these results suggests that the three species of Enterogona and the five species of Pleurogona examined form discrete and separate groups irrespective of their potential to form colonies. Therefore, a solitary or colonial life style is likely to have developed independently after the divergence of the two major groups of ascidians.

Eggs of the hermaphrodite, self-sterile ascidian, Ciona intestinalis, were washed with acid seawater (pH 3.2), and the washing solution was then adjusted to pH 8.2. This solution was found to inhibit only the binding of non-autologous sperm to the vitelline coat (VC) of eggs, indicating that it contained self-nonself recognition activity. This activity was heat-stable and insensitive to trypsin, but was destroyed by V-8 protease and a-glucosidase. Both the hydrophobic and hydrophilic components of a lyophilized powder of the extract showed allo-recognizing activity. On TLC, the hydrophobic components gave a major spot of glucose (Glc) and a peptide spot(s) containing mainly glutarnic acid and/or glutamine (Glx). The glucosyl conjugate was purified by HPLC and shown to block sperm-egg binding to various extents. Individual peptide subfractions had no inhibitory activity, but in combination they showed inhibitory activity. These findings suggest that the acid extract of Ciona eggs contains a Glc-enriched nonspecific inhibitor of sperm-egg binding, which could be the primary effector of self-incompatibility, and Glx-enriched modulators, which serve as acceptors of allo-sperm. The cooperative interactions of these components may be responsible for the diversity of allo-recognition in Ciona gametes.

No abstract

The continuous observation of living animals and the reconstruction of serial sections of fixed specimens have revealed 15 developmental stages and their timetable in palleal buds of the compound ascidian, Symplegma reptans. The bud primordium (stage 1) appeared on the right anterior region of the atrial epithelium of its parental bud (stage 9). It evaginated to form a vesicle along with the epidermis (stage 3) about two days later (at 18°C). Stage 4 was characterized by the formation of a test vessel through which the bud received active bloodflow. Body axes and bilateral asymmetry first became visible at this stage. Rudiments of the neural complex, pharynx, gut, and endostyle were formed directly from the inner vesicle of the bud. The neural complex placode was the first organ rudiment observed, in 3.5-day-old buds (stage 5). It changed into a tube sac whose anterior half had cilia on the luminal surface (stage 9, eight days old); the ciliated duct thus formed. In 6-day buds (stage 8), a large cell mass could be observed histologically beneath the neural complex. The cell bodies were soon arranged at the periphery of the cell mass, thus forming the dorsal ganglion. Rudiments of the pericardium, gonad, and pyloric duct appeared first as small aggregates of mesenchymal cells at stages 7, 8, and 11 (12 days old), respectively. Muscle precursors appeared in the mesenchymal space in association with the epidermis at stage 11. They differentiated into the musculus sphincter and longitudinal body musculature during stage 12, at which time, too, the stigmata perforated the pharyngeal wall. Zooids began to feed and thus attained functional maturity after about two weeks of development from buds.

No abstract