Timing the Drosophila Mid-Blastula Transition: A Cell Cycle-Centered View

At the mid-blastula transition (MBT), externally developing embryos refocus from increasing cell number to elaboration of the body plan. Studies in Drosophila reveal a sequence of changes in regulators of Cyclin:Cdk1 that increasingly restricts the activity of this cell cycle kinase to slow cell cycles during early embryogenesis. By reviewing these events, we provide an outline of the mechanisms slowing the cell cycle at and around the time of MBT. The perspectives developed should provide a guiding paradigm for the study of other MBT changes as the embryo transits from maternal control to a regulatory program centered on the expression of zygotic genes.

TrendsDevelopmental constraints force the early egg to adopt an unusual biology.Normal biological controls are introduced in a rapid but progressive sequence, which we exemplify by changes in key cell cycle regulators that slow and regularize the cell cycle.This succession of regulatory changes contrasts to the usual depiction of the rapid transformation prior to gastrulation as a monolithic switch called the MBT.While there are some particularly decisive steps, the entire progression is temporally stereotyped.Thus the MBT, while rapid and transformative, should be considered as a developmental process that engages much of the biology of embryo.Progressive changes in Cdk1, an influential cell cycle kinase, coordinate much of this developmental program.