Maternal-to-zygotic transition (MZT) in the soma and in primordial germ cells (PGCs). Profiles of unstable maternal mRNAs (blue) and of zygotic mRNAs (orange) are shown. In whole embryos, 14% of maternal mRNAs are degraded by maternal activities (Maternal decay), 22% by zygotic activities (Zygotic decay), and 25% by both (Mixed decay) . Smaug is part of the maternal mRNA decay machinery and the miR-309 cluster is part of the zygotic machinery. Oskar might prevent Smg-dependent mRNA decay in the PGCs by binding to Smg. The dashed vertical lines indicate the time at which major zygotic transcription begins and the time of PGC formation. Time is indicated in hours after egg deposition (at 25°C).