Establishment of self-sterility of eggs in the ovary of the solitary ascidian, Halocynthia roretzi

Abstract

When unfertilized eggs (UFE) of the solitary ascidian, Halocynthia roretzi, are released naturally they are strictly self-sterile, whereas almost all ovarian eggs isolated after spawning are self-fertile. Self-sterile eggs are prepared within a relatively short period of several hours before the spawning. The morphological changes in ovarian eggs during late oogenesis were studied with special reference to the establishment of self-sterility. Four types of eggs at serial developmental stages were classified according to the morphology of their external envelopes. Self-sterility was established in the last stage, from the ovarian egg type 3 (OVE3) to UFE stages. Ovarian eggs which had become committed to UFE were denoted as full-grown ovarian eggs (FOE). FOE were able to differentiate into self-sterile UFE in vitro, whereas OVE3 could not. Several morphological differences between OVE3 and UFE were found. OVE3 had a germinal vesicle (GV), a type of vitelline coat (VC-OVE3) and no expanded perivitelline space, whereas UFE had completed germinal vesicle break down (GVBD), had another type of coat (VC-UFE) and showed an expanded perivitelline space. There were also some differences in the mode of fertilization between OVE3 and UFE. In UFE, sperm became bound firmly to the vitelline coat and passed through the coat with the help of follicle cells, whereas in OVE3, sperm did not bind so strongly and entered the perivitelline space without the aid of follicle cells. The relationships between the establishment of self-sterility and these morphological and functional changes in ovarian eggs are discussed.

Common Cell‐Surface Antigens Functioning in Self‐Recognition Reactions by Both Somatic Cells and Gametes in the Solitary Ascidian Halocynthia roretzi

“…Naturally spawned unfertilized eggs and sperm from the testis were isolated as described by Fuke and Numakunai (9,12). Unfertilized eggs (100-150 eggs) were incubated with (50 ug/ml) or without mAbs in a final volume of I m1 filtered SW for 1 hr at 10 C. After washing with SW, eggs (50-100 eggs) were inseminated with 1 X 10 6 of allogeneic sperm in 1 ml of filtered SW. After 2 to 3 hr, eggs were fixed with 1.0% glutaraldehyde in Sw.…”

Self-fertilization as an alternative mode of reproduction in the solitary tunicate Pyura chilensis

“…We suggest that fertilization failure in Pyura chilensis can be attributed to fertilization blocking when gametes of the same individual come into contact, although the mechanisms involved in blocking self-fertilization are still unknown. In ascidians, the site that discriminates between self-and non-self has been found to reside in the vitelline coat of the egg (Morgan 1923, Rosati & De Santis 1978, Fuke & Numakunai 1996. In laboratory experiments, it has been observed that the solitary ascidian Halocynthia roretzi induces self-sterility just before spawning by a trypsin-like protease in the ovary.…”

“…In laboratory experiments, it has been observed that the solitary ascidian Halocynthia roretzi induces self-sterility just before spawning by a trypsin-like protease in the ovary. However, in nature the establishment of self-sterility begins about 20 h after spawning (Fuke & Numakunai 1996). Moreover, in Ciona intestinalis self-fertilization can be induced artificially (Kawamura et al 1987).…”

Re-Examination of Sibling Cross-Sterility in the Ascidian, Ciona intestinalis: Genetic Background of the Self-Sterility

“…Besides these facts, it is now known that the eggs acquire self-fertility first and, during a very last phase of egg maturation, they establish self-sterility. The acquisition of self-sterility in eggs is reconstructed in vitro in C. intestinalis (De Santis and Pinto, 1991) and in H. roretzi (Fuke and Numakunai, 1996). It is also known that the acquisition is exactly coincident with the translocation of a low molecular weight substance, most likely a peptide, from the follicle cells to the vitelline coat with the aid of a heat-shock protein and proteasome.…”