While all ascidians (Urochordata) are hermaphroditic, some, including Halocynthia roretzi (Fuke, 1983) and Ciona intestinalis (Rosati & De Santis, 1978) are strictly self-sterile because of a self–nonself recognition system in the interaction between the sperm and the vitelline coat of the eggs. However, immature oocytes (Fuke & Numakunai, 1996) and acidic-seawater-treated mature oocytes (Morgan, 1939; Kawamura et al., 1991) are self-fertile. It is thought that a putative self–nonself recognition molecule, which is detached or modified by treatment with acidic seawater, may be attached to the vitelline coat during oocyte maturation. Although the existence of a self–nonself recognition system in the fertilisation process is well known, the molecular entity has yet to be conclusively identified. However, there have been several attempts to identify such a molecule in Ciona (Marino et al., 1999). In the present study, we have isolated and analysed a molecule which appears to be responsible for allorecognition in the interaction between sperm and eggs of the ascidian Halocynthia roretzi.

Biologicals. A solitary ascidian Halocynthia roretzi Type C was used in this study. The fertilisation experiment was carried out as described previously (Sawada et al., 1982).

Isolation and N-terminal Sequencing of Hr VC70. Vitelline coats were isolated from immature and mature oocytes of the ascidian by homogenisation and repeated washing with 5× diluted artificial seawater. The isolated vitelline coats were subjected to SDS-PAGE, followed by blotting to a PVDF membrane. The N-terminal amino acid sequence of the 70 kDa main component (HrVC70) was determined by a protein sequencer.