PRECOCIOUS BREAKDOWN OF THE GERMINAL VESICLE INDUCES PARTHENOGENETIC DEVELOPMENT IN SEA CUCUMBERS

¹ Department of Zoology, Faculty of Science, Kyoto University, Kitashirakawa-Oiwakecho, Sakyo-Ku, Kyoto, 606 Japan

Meiotic divisions of sea cucumber oocytes can be induced and completed by dithiothreitol (DTT) treatment, but DTT-matured, unfertilized eggs are arrested at the pronucleus stage and do not develop. By pipetting the oocyte suspension, the membrane of the germinal vesicle of the full-grown oocyte ruptured and then disappeared. In immature oocytes the rupture of the germinal vesicle eventually resulted in fragmentation of the oocyte proper without formation of the polar body and the fertilization membrane. In DTT-matured, unfertilized oocytes, artificial rupture of the germinal vesicle before its normal breakdown caused parthenogenetic development following polar body formation. Pipetting after normal germinal vesicle breakdown had no such effects. Pipetting did not cause immediate elevation of the fertilization membrane and pipetted oocytes remained fertilizable. These results indicate that the direct outcome of rupturing the germinal vesicle is not the activation of the oocyte in the usual sense of the term, but a provision of some conditions for the oocyte to initiate cleavage division cycles after the meiotic divisions. It is suggested that the germinal vesicle materials contain some factors that induce the "activation" of the oocyte.