Endoribonuclease that plays a central role during spermatogenesis by repressing transposable elements and preventing their mobilization, which is essential for the germline integrity (PubMed:19460866). Plays an essential role in meiotic differentiation of spermatocytes, germ cell differentiation and in self-renewal of spermatogonial stem cells (PubMed:19460866, PubMed:25558067). Its presence in oocytes suggests that it may participate in similar functions during oogenesis in females (PubMed:18191035). 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 govern the methylation and subsequent repression of transposons (PubMed:19460866, PubMed:25558067). Directly binds 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:19460866, PubMed:25558067). Strongly prefers a have adenine at position 10 of their guide (g10A preference) (PubMed:24757166, PubMed:25558067). Plays a key role in the piRNA amplification loop, also named ping-pong amplification cycle: antisense piRNA-bound Siwi and sense piRNA-bound Ago3 reciprocally cleave complementary transcripts, to couple the amplification of piRNAs with the repression of transposable elements (PubMed:25558067). Interacts (when symmetrically methylated) with Papi/TDRKH (PubMed:23970546). Interacts with Vasa. Component of the meiotic nuage, also named P granule, a germ-cell-specific organelle required to repress transposon activity during meiosis. Highly expressed in the larval testis, pupal ovary and adult eggs. Arginine methylation is required for the interaction with Tudor domain-containing protein Papi/TDRKH. Belongs to the argonaute family. Piwi subfamily.