Reproductive phasiRNAs are the piRNAs of plants

Small RNAs are fundamental to gene expression regulation, with specialized classes playing critical roles in reproduction. This review compares animal PIWI-interacting RNAs (piRNAs) and plant reproductive phased small interfering RNAs (phasiRNAs), which show remarkable similarities. Both originate from Pol II-transcribed precursors but have distinct biogenesis pathways. piRNA processing in metazoans is Dicer-independent, involving PIWI-clade proteins for amplification via 'ping-pong' and phased cleavage. Reproductive phasiRNAs are Dicer-dependent and are initiated by miRNA-guided cleavage to generate phased sRNAs. A well-defined piRNA function is transposon silencing, but roles for nontransposon-targeting piRNAs and most reproductive phasiRNAs remain unresolved. Comparing these independently evolved systems reveals common strategies for reproductive success and highlights key unresolved questions regarding their molecular targets, functions, and evolutionary pressures that shaped them.