Model for delayed MZT in the PGCs relative to the soma. (a-c) Wild type, (d-f) smaug mutant. (a, d) The model. (b, c, e, f) The dynamics of maternal decay and zygotic genome activation with the model diagrammed above the curves. (a, b) In wild-type soma, maternal mRNAs are targeted for decay by a maternally encoded decay machinery ('M') that includes Smaug. Among the targeted transcripts are ones encoding transcriptional repressors that keep the zygotic genome silent. As these are eliminated so zygotic genome activation (ZGA) initiates. Among the zygotic transcripts are components of the zygotically encoded decay machinery ('Z') that feed back to further destabilize maternal mRNAs. ZGA also produces transcriptional activators that feed back to upregulate transcription. (a, c) In wild-type PGCs additional layers of regulation occur (black boxes): protection of a subset of maternal mRNAs from decay factors such as Smaug; and PGC-specific transcriptional repressors that keep the zygotic genome silent (for example, Polar granule component). Only after these are eliminated does the delayed MZT commence in the PGCs. (d, e) In smaug-mutant soma, a subset of maternal mRNAs is stabilized (60%) while a subset of ZGA fails (40%). (d, f) In the smaug-mutant PGCs, there is a similar effect but on a different scale: 34% of unstable maternal mRNAs fail to be cleared and 36% of ZGA fails.