EVIDENCE FOR INTRODUCTION OF A VARIABLE G1 PHASE AT THE MIDBLASTULA TRANSITION DURING EARLY DEVELOPMENT IN AXOLOTL

After fertilization in axolotl, the synchronous cell cleavages are tri phasic (S, G2 and M phases). Midblastula transition (MBT) begins at th e ninth cleavage and is the consequence of lengthening of cell cycles. By spectrofluorometry and incorporation of H-3 thymidine into the nuc lear DNA followed by autoradiography on individual cells, the time at which a G1 phase appears during early development was investigated. Th e present results show that the G1 phase was introduced for the first time at MBT ana its duration was variable from one blastomere to anoth er. This variability could account for lengthening of cell cycles and be required for zygotic transcriptions necessary for DNA replication. From this point of view, axolotl represents an interesting alternative amphibian model to identify regulators involved in the G1-S transitio n at MBT during early development.