The Enigmatic Non-Transposon 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, 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 a specific and fundamental role in silencing young and actively-transposing TEs. However, only 1% of pachytene piRNAs target active TEs. The biological significance of the abundant non-TE piRNAs, co-produced from the same loci, remains unclear. Here, we make 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 host genome a pre-existing mechanism to suppress potential invasion of novel TEs in the future.