Sequential action of the nuclear Argonautes HRDE-1 and NRDE-3 promotes global transcriptional silencing during starvation in the nascent C. elegans germline

In C. elegans, if embryos hatch into an environment lacking nutrients the primordial germ cells (PGCs) will arrest mRNA transcription and enter quiescence until food becomes available. Our previous work had shown that the global transcriptional silencing (GTS) that occurs in PGCs during L1 starvation requires a "hyperdeposition" of the repressive histone mark H3K9me3 in germline chromatin. Hyperdeposition is defined as an increase in the amount of H3K9me3 embedded in germline chromatin, relative to neighboring somatic nuclei. Hyperdeposition initiates in embryonic PGCs and is maintained in starved L1s. Here, we show that the nuclear RNAi pathway contributes to both the initiation and maintenance of H3K9me3 hyperdeposition. Interestingly, both known nuclear Argonautes, HRDE-1 and NRDE-3, play a role in hyperdeposition and they do so in a sequential manner. We show that HRDE-1 acts in embryonic PGCs to initiate hyperdeposition and then, at the embryo-to-L1 transition, HRDE-1 becomes dispensable and NRDE-3 is required to maintain H3K9me3 levels, and for GTS. We also examine the timing of H3K9me3 hyperdeposition and find that it initiates soon after germline zygotic genome activation (ZGA) occurs. Our data suggest a model where ZGA promotes both gene expression and H3K9me3 deposition at active loci, and under starvation conditions these H3K9me3 marks are then employed to silence the germline genome.