Animals use 18–33-nucleotide PIWI-interacting RNAs (piRNAs) to silence transposons in germ cells1–3. In addition to transposon-silencing piRNAs, placental mammals make pachytene piRNAs4,5, an abundant class of testis-specific small RNAs derived from long noncoding RNA precursors. Although the sites of pachytene piRNA precursor transcription are often conserved among placental mammals, the sequences of the piRNAs themselves are rapidly diverging, even in the human population6. Consequently, the biological function and mechanism of action of pachytene piRNAs remain debated. Here we report that most mouse pachytene piRNAs have no biological function but instead ‘selfishly’ promote their own production. Our data suggest that pachytene piRNAs direct endonucleolytic cleavage of partially complementary targets and neither activate nor repress mRNA translation. Although many pachytene piRNAs guide cleavage of specific mRNAs, few alter the steady-state abundance of their targets. The minority of pachytene piRNAs that reduce target mRNA abundance enhance sperm fitness, thereby ensuring production of the entire pachytene piRNA repertoire. Together, our findings explain the lack of conservation of most pachytene piRNA sequences and suggest that these ‘selfish’ small RNAs persist in mammalian evolution because target cleavage by a tiny minority of piRNAs supports male fertility.
