Behaviour of centrosomes in early Tubifex embryos: asymmetric segregation and mitotic cycle-dependent duplication.

An antibody raised against a highly conserved peptide of γ-tubulin (Joshi et al. 1992) recognized a 50 kDa polypeptide in centrosomes in Tubifex embryos. Centrosomes labelled with this antibody are found at both poles of the first meiotic spindle and at the inner pole of the second meiotic spindle. At the transition to the second meiosis, there is no change in morphology of the centrosomes which are retained in the egg proper. In contrast, as the second meiosis proceeds from anaphase to telophase, centrosomes labelled with the antibody gradually become smaller, but are still recognized as tiny dots; each egg exhibits no more than one tiny dot. The first cleavage spindles exhibit a centrosome at one pole but not at the other. The spindle pole with a centrosome forms an aster which is inherited by the larger cell, CD, of the two-cell embryo; the centrosome-free spindle pole then becomes anastral and is segregated to a smaller cell AB. Centrosomes are present in the C and D cell lineages but not in the A and B lineages, at least up to the eighth cleavage cycle. During cleavage stages, centrosomes duplicate early in telophase of each mitosis, and their size changes in a cell cycle-specific fashion. Centrosomes which otherwise duplicate asynchronously in separate cells do so synchronously in a common cytoplasm. Centrosome duplication is inhibited by nocodazole but not by cytochalasin D. An examination of embryos treated with cycloheximide or aphidicolin also suggests that centrosome duplication during cleavages requires protein synthesis but no DNA replication per se. These results suggest that the centrosome cycle in Tubifex blastomeres is linked to the mitotic cycle more closely than is that in other animals.