Oocytes of the surf clam, Spisula solidissima, underwent germinal vesicle breakdown and two meiotic divisions to give off polar bodies when they were fertilized or parthenogenetically activated with KCl. Fertilized eggs further proceeded to mitosis and cleaved, while parthenogenetically activated eggs remained uncleaved. We examined changes in microtubule-containing structures during meiotic divisions and subsequent mitotic processes by immunofluorescence. A monoclonal anti-tubulin antibody was applied to alcohol-fixed eggs from which the vitelline membrane had been removed by protease digestion. Up to the stage of second polar body formation, the pattern of microtubule organization in the first and second meiotic spindles was identical in both fertilized and parthenogenetically activated eggs. However, while fertilized eggs formed a sperm aster and mitotic spindles later, activated eggs formed only monaster- or ring-shaped microtubule-containing structures which underwent cycles of alternating formation and breakdown. Lacto-orcein staining of parthenogenetically activated eggs revealed that the chromosome cycle could occur in these eggs, in phase with this microtubule cycle.
Bibliographical noteFunding Information:
We thank Dr. J. V. Kilmartin of the MRC Laboratory of Molecular Biology, Cambridge, England, for providing monoclonal antitubulin antibody; Mr. H. Thorsen of the University of Wisconsin for printing photographs; Dr. R. P. Elinson, Ms. E. K. Shibuya, and Mr. H. J. Clarke of the University of Toronto for reading the manuscript. One of us (R.K.) expresses her thanks to Dr. J. L.‘Rosenbaum of the Yale Uni- versity for his kind support and advice. The authors are thankful to Mrs. Rossana Soo for her assistance in typing the manuscript. This work was supported by Step Toward Independence Fellowship from Marine Biological Laboratory at Woods Hole, Minnesota Medical Foundation CRF-69-85, ACS Institutional Grant IN-13-X-8 (R.K.), NIH Grant GM 30385 (G.G.B.), NIH Training Grant 5-T35HD07098 awarded to the Embryology Course, Marine Biological Laboratory, Woods Hole, and NSERC Canada Grant A5855 (Y.M.).