Timing of early morphogenetic events in tetraploid starfish embryos

Abstract

When starfish eggs were treated with caffeine, they developed parthenogenetically as tetraploid embryos. The timing of cleavages and gastrulation was compared between these tetraploid embryos and control diploid embryos. The following results were obtained: 1) The number of synchronous cleavages was ten in controls but nine in tetraploid embryos; 2) tetraploid embryos began to gastrulate at almost the same time after first cleavage as did control embryos; 3) the average number of constituent cells of tetraploid gastrulae was about 2,300, whereas that of control gastrulae was about 4,000. These results indicate that 1) the initiation of asynchronous cleavages is closely associated with the nucleocytoplasmic ratio; and 2) the timing of gastrulation is not determined by the number of cell divisions. These conclusions are in good accord with the results of a previous study on timing of early morphogenetic events in half eggs whose cytoplasm was decreased to half that of normal eggs (Mita, '83).

Heterozygosity and developmental stability in gynogenetic diploid and triploid rainbow trout

“…The similarity between the developmental rate of diploid and triploid fishes implies that developmental rate is not controlled by cell number or the number of cell divisions. This is similar to the developmental programme of starfish (Mita and Obata, 1984), but different from that of the mouse where the timing of certain events is known to depend on total cell number (Lewis and Rossant, 1982). A possible reason for the inability to produce viable polyploid mammals is due to the dependence of the timing of developmental events on cell number.…”

Distinct molecular mechanism regulate cell cycle timing at successive stages of Drosophila embryogenesis.

“…This makes the mechanism inherently self-limiting, since increases in Cyclin B substrates would cause ever greater depletion of the Cyclins at mitosis, and this would cause the cycle to slow. Titration and depletion of Cyclins by the proliferating mitotic apparati provides an attractive explanation of results showing that slowing of the maternally controlled divisions is caused by increases in the nucleo: cytoplasmic ratio not only in Drosophila, but in embryos of diverse species (Koboyakawa and Kubota 1981;Newport and Kirschner 1982;Mita and Obata 1984;Yasuda and Schubiger 1992). The transition to a Cyclin-limited cell cycle, followed by destruction of maternal String and transition to a String-limited cycle, are major developmental events that are coupled with the onset of transcription, and initiation of zygotically programmed morphogenesis, respectively.…”

Zygotic degradation of two maternal Cdc25 mRNAs terminates Drosophila's early cell cycle program.

“…Experimental manipulations that increase the nucleo/cytoplasmic ratio slow the maternal cell cycle oscillator prematurely and advance the MZT by one or even two cell cycles in a variety of different embryo types. Decreasing the nucleo/cytoplasmic ratio has the opposite effect: Rapid cell cycles continue for longer, and the MZT occurs after more cycles than normal (Okada et al 1980;Kobayakawa and Kubota 1981;Newport and Kirschner 1982a, b;Mita 1983;Mita and Obata 1984;Dasso and Newport 1990;Yasuda et al 1991).…”