Dynamics of embryonic cleavage divisions.

At the start of animal embryogenesis, cleavage divisions partition a unicellular zygote into progressively smaller compartments. I will present two results from our work on embryonic cleavages in Drosophila, where 13 synchronous cleavage divisions generate a blastoderm with close to 6000 nuclei within just two hours after egg fertilization. First, we discovered that maternal stores of dNTPs in the egg are sufficient for making only half of the genomes formed by 13 cleavages. The remainder of monomers required for DNA polymerization is synthesized by the embryo, and this strategy is essential for proper onset of zygotic transcription. Second, we found that synchrony of embryonic cleavages relies on a pacemaker-type mechanism, whereby cell cycles at the embryonic poles are sped up by the ERK signaling pathway. While previous work on ERK signaling at the poles focused on its transcriptional effects, the ERK-dependent synchronization of cleavage divisions is independent of transcription.