Molecular Mechanisms Underlying Self-Sterility In Ascidian Gametes

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The ascidians are hermaphrodite and many species are self-sterile. In the self-sterile solitary ascidian Ciona intestinalis, only heterologous, but no autologous. spermatozoa can bind to the vitelline coat (VC). It is postulated that fucosyl proteins of the VC and a- L- fucosidase of sperm mediate the binding, yet the molecules controlling allorecognition between the gametes remain unknown. In another self-sterile solitary ascidian Halocynthia roretzi. both heterologous and autologous spermatozoa bind to the VC by a similar mechanism us ing fucosy I proteins and a - L - f ucos i dase. Because the oocytes deprived of the VC are self-fertile. the autologous spermatozoa seem incapable of penetrating the VC in this species.By using in vitro fertilization assay of oocytes from Qi-Qna ovaries, we have found that self-sterility is contributed or controlled by products. presumably a glycoprotein. of the overlying follicle cells. When fully-grown ovarian oocytes with a germinal vesicle were inc … More ubated 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 cellfree oocytes were incubated with the free follicle cells. they became selfsterile 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 layer was similarly observed in follicle cell-free oocytes only when they had been incubated with the free follicle cells. We have purified a chymoteypsin-like protease as a VC-lysin in Ciona spermatozoa. Interestingly, the enzyme purified from Ciona spermatozoa digested the electron-dense layer.These observations suggest that, during oogenesis, follicle-cells synthesize a self-sterility controlling factor (probably a glycoprotein) which. following germinal vesicle breakdown, is secreted to coat the VC in a way that confers self-sterility to the oocytes. It is an interesting question to ask whether the allorecognition molecule (s) is contained in the outermost electrondense layer of the VC or not. Less