Plays a central role during gametogenesis by repressing transposable elements and preventing their mobilization, which is essential for the germline integrity (PubMed:14516689, PubMed:17418787, PubMed:18833190). Acts via the piRNA metabolic process, which mediates the repression of transposable elements during meiosis by forming complexes composed of piRNAs and Piwi proteins and governs the methylation and subsequent repression of transposons (PubMed:14516689, PubMed:17418787, PubMed:18833190). Directly binds methylated piRNAs, a class of 24 to 30 nucleotide RNAs that are generated by a Dicer-independent mechanism and are primarily derived from transposons and other repeated sequence elements (PubMed:14516689, PubMed:17418787, PubMed:18833190). Has a strong preference for piRNAs with a uridine nucleotide at their 5'-end (g1U preference, also named 1U-bias) and binds piRNAs in an opposite direction compared to piwil2/zili (PubMed:14516689, PubMed:17418787, PubMed:18833190). Participates in a piRNA amplification loop with piwil2/zili (PubMed:14516689, PubMed:17418787, PubMed:18833190). Not involved in the piRNA amplification loop, also named ping-pong amplification cycle. Acts as an endoribonuclease that cleaves transposon messenger RNAs (By similarity). Component of the meiotic nuage, also named P granule, a germ-cell-specific organelle required to repress transposon activity during meiosis. Expressed exclusively in the adult gonads; expression in the ovary weaker than in the testis (at protein level). During neurogenesis and organogenesis, expression is detected in CNS (midbrain and eye) and fin buds. Starting from 24 hours post-fertilization, expression is found in the genital ridge. Initially detected during embryonic segmentation which persists for at least 4 weeks post hatching. The PAZ domain specifically recognizes binds the 2'-O-methylated 3'-end of piRNAs. The MID region is required for recognition of uridine in the first position of piRNAs (g1U preference, also named 1U-bias). Methylated on arginine residues; required for the interaction with Tudor domain-containing protein and subsequent localization to the meiotic nuage, also named P granule. Progressive loss of germ cells due to apoptosis during larval development. Belongs to the argonaute family. Piwi subfamily.