Cleavage asynchrony in the Tubifex embryo: involvement of cytoplasmic and nucleus-associated factors.

The smaller AB-cell of the 2-cell embryo of Tubifex divides 40 min later than the larger CD-cell. The present study was undertaken to elucidate those factors that are responsible for this cleavage asynchrony. An examination of chromosome cycles showed that the AB-cell entered M-phase 5 min later and spent about 35 min longer in the prometaphase portion of M-phase than did the CD-cell. Even if cultured in isolation, the length of M-phase of AB isolates continued to be comparable to that of intact AB-cells. When AB and CD nuclei were brought into a common cytoplasm, they entered M-phase simultaneously. The durations of M-phase portions from prometaphase through telophase were not altered even in a common cytoplasm, as long as the mitotic figures of AB and CD nuclei were separated from each other. Only when the mitotic spindles of both nuclei were united at at least one pole was the length of prometaphase of the AB nucleus significantly shortened. Whole-mount immunocytochemistry of microtubules revealed that asters were involved in mitotic spindle assembly in the CD-cell but not in the AB-cell. The fully formed spindle of the AB-cell was anastral and barrel-shaped, while that of the CD-cell possessed asters at its poles. The morphology of the spindle was not altered by changes in the cytoplasmic environments; that is, the spindle of AB nucleus was found to be anastral even in the presence of a CD-cell cytoplasm. These results suggest that unlike the case of other animals so far studied, cleavage asynchrony in the Tubifex embryo is brought about not only by cytoplasmic components but also by nucleus-associated factors.