Equalization of unequal first cleavage in the Tubifex egg by introduction of an additional centrosome: implications for the absence of cortical mechanisms for mitotic spindle asymmetry.

The first cleavage in the Tubifex egg is unequal and involves a monastral mitotic apparatus (MA), which remains at the egg's center from metaphase through telophase. The monastral form of the MA is thought to arise due to the involvement of a single maternal centrosome in the MA assembly (R. Ishii and T. Shimizu, Dev. Growth Differ. 37, 687-701, 1995). To investigate the mechanisms that generate asymmetry in this division, we have examined the cleavage patterns in eggs that are manipulated to inherit two centrosomes during the first mitosis. When eggs are prevented from extruding polar bodies, centrosomes located at both poles of the first meiotic spindle persist into the first mitosis without showing any sign of duplication and generate astral spindle poles. Eggs that inherit these centrosomes exhibit two types of MA configurations, viz., an amphiastral MA and twin monastral MAs. We found that eggs with amphiastral MAs undergo bipolar equal divisions, while those with twin MAs divide into three cells simultaneously (tripolar divisions). The amphiastral MAs are located at the egg's center and their astral poles are organized symmetrically, suggesting that Tubifex eggs are unable to generate asymmetry in the amphiastral MAs during the first mitosis. These results suggest that inheritance of a single centrosome during the first mitosis is critical for Tubifex eggs to undergo unequal cleavage. We propose that the cortical mechanisms for MA asymmetry are lacking in the Tubifex egg during the first mitosis.