a, Segmented EfPiwi-piRNA cryo-EM reconstruction, colored by domain. b, Cartoon representation of EfPiwi model. Disordered piRNA nucleotides, dashed red line. Linear domain schematic shown below. c, Density maps (grey mesh) in space occupied by seed regions of hAgo2 (PDB 4OLA) and EfPiwi. Modeled guide RNAs as red sticks. d, Superposition of miRNA-class AGO structures (left, PDB: 4KXT, 4OLA, 5VM9, 6OON) and known PIWI structures (right, PDB: 7KX7, 5GUH, 6KR6). AGO central-gate and PIWI seed-gate structures indicated.