C. Heimann1, A.E. van Loon2, H.J. Goedemans2, A.W.C. Dorresteijn1
1 Institut für Zoologie, Universität Mainz, Mainz,
Germany
2 Dep. of Exp. Zoology, State University at Utrecht, Utrecht,
The Netherlands
In embryos of Platynereis the blastomeres differ in size, cytoplasmic composition, and in the duration of cell cycles from the two cell stage on. The rate of cell cycle progression in the blastomeres correlates with their amount of yolk free cytoplasm. In normal embryos the yolk free cytoplasm is shunted preferentially into the D-cell line (Dorresteijn AWC 1990 Roux’s Arch Dev Biol 199: 14). Experimental change of this cytoplasmic distribution prior to first cleavage leads to embryos in which two instead of one cell line contain large amounts of yolk free cytoplasm. Both these cell lines proliferate rapidly and the resulting larvae show a duplication of D-cell line specific structures (Dorresteijn AWC et al 1987 Roux’s Arch Dev Biol 196: 51; Dorresteijn AWC, Eich P 1991 Roux’s Arch Dev Biol 200: 342). This data indicate that factors within the yolk free cytoplasm specify both the cell cycle duration of the blastomeres as well as the fate of at least some D-quadrant cells. Therefore, we focus on two questions: (1) Which molecular mechanisms control the embryonic cell cycles and make them asynchronous? (2) Are the differences in cell cycle duration a nessesary prerequisite for the determination of D-quadrant cells? Using the BrdU/anti-BrdU-technique we could demonstrate that early blastomeres enter S--phase shortly after the end of mitisis without or with only a very short G1-phase. That means that the G1- to S-transition is not crucial for the differences in the cell cycle duration of the blastomeres (Heimann C 1994 Diploma thesis, Mainz). We are presently interested in molecules which are involved in the regulation of the second major checkpoint, the G2-M-transition. With an antibody against Bufo-japonicus-cyclin B1 we could demonstrate the cycling activity of a G2/M-specific cyclin in Platynereis as well. Thus, the rate of cyclin-synthesis during interphase could be a limiting factor for entering mitosis. In a first step to isolate complete cell cycle regulating genes from cDNA-libraries, we have now amplified and sequenced PCR-fragments of a Platynereis-cyclin (embl-AJ224983) and of a Platynereis wee1-homolog (embl-AJ224984). Sense- or antisense-RNA of these genes will be injected to alter the normal sequence of cleavages and to study possible effects on the determination of the blastomeres.
Acknowledgements: We thank T. Kishimoto (Tokyo, Japan) and R. Streeck (Mainz, Germany) for their support. The B. japonicus-cyclin-B1 antibody was a kind gift of M. Yamashita (Sapporo, Japan). C. H. was supported by a grant of the Heinrich J. Klein Förderstiftung, Mainz (Germany) and by a grant of the University of Mainz (LGFG).
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