PIWI clade Argonaute proteins silence transposon expression in animal gonads. 
Their target specificity is defined by bound ∼23- to 30-nucleotide (nt) 
PIWI-interacting RNAs (piRNAs) that are processed from single-stranded precursor 
transcripts via two distinct pathways. Primary piRNAs are defined by the 
endonuclease Zucchini, while biogenesis of secondary piRNAs depends on 
piRNA-guided transcript cleavage and results in piRNA amplification. Here, we 
analyze the interdependencies between these piRNA biogenesis pathways in 
developing Drosophila ovaries. We show that secondary piRNA-guided target 
slicing is the predominant mechanism that specifies transcripts—including those 
from piRNA clusters—as primary piRNA precursors and defines the spectrum of 
Piwi-bound piRNAs in germline cells. Post-transcriptional silencing in the 
cytoplasm therefore enforces nuclear transcriptional target silencing, which 
ensures the tight suppression of transposons during oogenesis. As target slicing 
also defines the nuclear piRNA pool during mouse spermatogenesis, our findings 
uncover an unexpected conceptual similarity between the mouse and fly piRNA 
pathways.