Enigmatic Pachytene PIWI-Interacting RNAs

PIWI-interacting RNAs (piRNAs), a class of small RNAs, are renowned for their roles in sequencing-dependent targeting and suppressing transposable elements (TEs). Nevertheless, a majority of mammalian piRNAs, expressing at pachytene stage of meiosis, known as pachytene piRNAs, are devoid of discernible targets, casting a veil of enigma over their functional significance. Overturning the notion that this unusual class of piRNAs functions beyond TE silencing, we recently demonstrated that pachytene piRNAs play an essential and conserved role in silencing young and actively transposing TEs across amniotes. However, only 1% of pachytene piRNAs target active TEs. The biological significance of the abundant non-TE piRNAs, coproduced from the same precursors as TE piRNAs, remains unclear. Here, we provide a comprehensive summary of the potential roles of non-TE piRNAs, and thus propose that these non-TE piRNAs either bolster the action of TE piRNAs or provide the host genome a preexisting mechanism to suppress the potential invasion of novel TEs in the future.