THE ZEBRAFISH MIDBLASTULA TRANSITION

The zebrafish midblastula transition (MBT) begins at cycle 10. It is c haracterized by cell cycle lengthening, loss of cell synchrony, activa tion of transcription and appearance of cell motility. Superceding a 1 5 minute oscillator that controls the first nine cycles, the nucleocyt oplasmic ratio appears to govern the MBT. This timing mechanism operat es cell autonomously: clones of labeled cells initiate cell cycle leng thening independently of neighbors but dependent on immediate lineal a ncestors. Unequal divisions, when they occur, produce asymmetric cell cycle lengthening based on the volume of each daughter. During the sev eral cycles after the MBT begins, cycle length is correlated with the reciprocal of the blastomere volume, suggesting a continuation of cell cycle regulation by the nucleocytoplasmic ratio during an interval th at we term the 'MBT period'.