The prevention of smaller blastomeres of early Tubifex embryos from entering mitosis by unreplicated DNA.

The zygote of Tubifex divides unequally into a smaller AB-cell and a larger CD-cell; the latter produces a smaller C-cell and a larger D-cell at its next division. The present study was undertaken to examine mitotic cycles in early Tubifex embryos whose DNA replication was blocked by aphidicolin. Although zygotes and larger blastomeres (i.e., CD- and D-cells) which contained unreplicated DNA entered mitosis on schedule, their smaller sister blastomeres (i.e., AB- and C-cells) were prevented from entering mitosis for 8 hr or more, viz. through three or more rounds of mitotic cycles of control cells. Whole-mount immunocytochemistry of microtubules confirmed that these smaller blastomeres were blocked at interphase. Entry into mitosis of smaller blastomeres which contained unreplicated DNA was significantly accelerated by caffeine. Among descendants of the D-quadrant containing unreplicated DNA, macromere 1D was the only cell that entered mitosis on schedule; its daughter cells 2d and 2D were no longer immune to aphidicolin-induced inhibition of mitosis. When CD-cells were induced to divide equally in the presence of aphidicolin, two daughter cells of individual CD-cells entered mitosis at the same time in about one-half of the cases or with a 20- to 60-min lag in the timing of the remaining cases, although their entry into mitosis lagged about 60 min behind that of the control cells. An examination of the distribution of pole plasms in these cells suggested that susceptibility to unreplicated DNA of blastomeres inversely correlates to the amount of pole plasm they inherit. These results suggest that Tubifex embryos contain a feedback pathway which couples the completion of DNA replication to the initiation of mitosis and that the replication feedback system operates in smaller blastomeres, but not in larger blastomeres of early Tubifex embryos.