Molecular Mechanisms of Sperm-Egg Interactions in the Ascidians

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The ascidians are hermaphrodite and many species are seif-sterile. Using two species of self-sterile solitary ascidiansas model systems, Ciona intestinalis (Enterogona) and Halocynthia roretzi (Pleurogona), molecular mechanisms underlying the sperm-egg interactions were studied.We postulated previously that a sperm glycosidase (e.g., alpha-L-fucosidase in Ciona) and corresponding sugar chains of the vitelling coat (VC) mediate the binding of ascidian sperm to the homologous VC. Several lines of experimental evidence has further supported this hypothesis in both Ciona and Halocynthia. In H. roretzi, we have purified the major VC glycoprotein of about 70 kDa. It has terminal fucose residues and presumably serves as a sperm-receptor. Its amino acid sequence was partially revealed.A chymotrypsin-like protease was purified from Ciona sperm as a VC-lysin and proved to digest the outermost thin layer of the VC. It was also shown that a Zn^<++>-dependent metalloprotease is involved in sperm-eg … More g fusion in Ciona.By using in vitro fertilization assay of oocytes from Ciona ovaries, we have found that self-sterility is controlled by products, presumably a glycoprotein, of the overlying follicle cells. When fully-grown ovarian oocytes with a germinal vesicle were incubated in sea water at room temperature, they underwent germinal vesicle breakdown within 20 minutes and became fertile, and eventually acquired self-sterility within a maximum of 3 hours of incubation. When they were deprived of follicle cells by gentle shaking before incubation, most of them remained self-fertile even after 3 hours. If follicle cell-free oocytes were incubated with the free follicle cells, they became self-sterile as well as follicle cell-intact oocytes.The acquisition of self-sterility was accompanied by an ultrastructural change in the VC fixed with tannic acid : an electron-dense, thin layer appeared on the outer surface of the VC after 3 hours of incubation. This was the layer digested by the chymotryptic lysin in the sperm. Therefore it is suggested that the induction of acrosome reaction is the site modulated by allo-recognition between sperm and VC. Less