Cleavage of mRNAs by a minority of pachytene piRNAs improves sperm fitness

Abstract Animals use 18–33-nucleotide PIWI-interacting RNAs (piRNAs) to silence transposons in germ cells 1–3 . In addition to transposon-silencing piRNAs, placental mammals make pachytene piRNAs 4,5 , 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 population 6 . 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…