piRNAs silence foreign genes, such as transposons, to preserve genome integrity, 
but they also target endogenous mRNAs by mechanisms that are poorly understood. 
Caenorhabditis elegans piRNAs interact with both transposon and nontransposon 
mRNAs to initiate sustained silencing via the RNAi pathway. To assess the 
dysregulation of gene silencing caused by lack of piRNAs, we restored RNA 
silencing in RNAi-defective animals in the presence or absence of piRNAs. In the 
absence of piRNAs and a cellular memory of piRNA activity, essential and 
conserved genes are misrouted into the RNAi pathway to produce siRNAs that bind 
the nuclear Argonaute HRDE-1, resulting in dramatic defects in germ cell 
proliferation and function such that the animals are sterile. Inactivation of 
RNAi suppresses sterility, indicating that aberrant siRNAs produced in the 
absence of piRNAs target essential genes for silencing. Thus, by reanimating 
RNAi, we uncovered a role for piRNAs in protecting essential genes from RNA 
silencing.