Maternal piRNAs are essential for germline development following de-novo establishment of endo-siRNAs in Caenorhabditis elegans. Caenorhabditis elegans

The Piwi-piRNA pathway represents a germline specific transposon-defense system. C. elegans Piwi, prg-1, is a non-essential gene and triggers a secondary RNAi response that depends on so-called mutator genes, endo-siRNAs (22G-RNAs) and at least one 22G-RNA-binding Argonaute protein, HRDE-1. Interestingly, through a poorly understood mechanism, silencing of PRG-1 targets can become PRG-1 independent. This state, also known as RNAe, is heritable and depends on mutator genes and HRDE-1. We studied how the transgenerational memory of RNAe and the piRNA pathway interact. We find that maternally provided PRG-1 is required for the de-novo establishment of 22G-RNA populations, especially those targeting transposons. Strikingly, attempts to re-establish 22G-RNAs in absence of both PRG-1 and RNAe memory result in severe germline proliferation defects. This is accompanied by a disturbed balance between gene-activating and -repressing 22G-RNA pathways. We propose a model in which CSR-1 prevents the loading of HRDE-1 and that both PRG-1 and HRDE-1 help to keep mutator activity focused on the proper targets. Overall design: Small RNA sequencing of C. elegans L1 and L2 larvae from wild-type and mutants with impaired endo-siRNA pathway. There are 29 samples that include wild-type controls and mutants with at least 2 biological replicates for each genetic background. Small RNA sequencing of input and HRDE-1 immunoprecipitates from wild-type and cde-1(tm1021) mutants.