Cell cycle timing in colchicine‐treated sea urchin eggs: Persistent coordination between the nuclear cycles and the rhythm of cortical stiffness

Cyclic changes in cortical stiffness, condensation‐decondensation of chromosomes, and breakdown‐reformation of the nuclear envelope were found to continue in eggs of Dendraster excentricus treated with colchicine even though mitosis and cleavage were suppressed. Furthermore, a consistent and precise synchronization persisted between these cyclic phenomena; each rise and fall of cortical stiffness coincided with the breakdown and reformation, respectively, of the nuclear envelope. The overall cycle length was prolonged relative to controls because of an extended intermitotic phase, indicating that assembled mitotic microtubules do not influence cell cycle timing in this species. In colchicine‐treated eggs of Strongylocentrotus purpuratus , however, the cortex and the nucleus became arrested in conditions equivalent to metaphase, as if the mitotic phase was indefinitely prolonged. The cortex was stabilized in a state of high stiffness, the chromosomes remained condensed, and the nuclear envelopes remained broken down. In this species, even in the absence of rhythmic change, a strict correlation between the cortical status and the nuclear condition was still consistently observed. The persistence of coordination, in spite of diverse species‐specific responses to colchicine, suggests the existence of a common control mechanism between the cytoplasm and nucleus in normal eggs that is important in cell cycle timing. The insensitivity to colchicine indicates that the mechanism of coordination is independent of microtubules.