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, PubMed:27693359). Plays an essential role in meiotic differentiation of spermatocytes, germ cell differentiation and in self-renewal of spermatogonial stem cells (PubMed:19460866, PubMed:25558067, PubMed:27693359). 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, PubMed:27693359). 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, PubMed:27693359). Recognizes piRNAs containing a phosphate at the 5'-end and a 2'-O-methylation modification at the 3'-end (PubMed:27693359). Strongly prefers a uridine in the first position of their guide (g1U preference, also named 1U-bias) and a complementary adenosine in the target (t1A bias) (PubMed:24757166, PubMed:27693359). 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. In this process Siwi acts as a 'slicer-competent' piRNA endoribonuclease that cleaves primary piRNAs, which are then loaded onto Ago3 (PubMed:27693359). In this process, Siwi requires the RNA unwinding activity of the RNA helicase Vasa for the release of the cleavage products (PubMed:25558067). Interacts (when symmetrically methylated) with Papi/TDRKH (PubMed:23970546, PubMed:26919431). Interacts with Vasa (PubMed:25558067). Component of the meiotic nuage, also named P granule, a germ-cell-specific organelle required to repress transposon activity during meiosis (PubMed:25558067). Highly expressed in the larval testis, pupal ovary and adult eggs. Shows a strong structural resemblance to the argonaute/Ago subfamily. The domains (N, PAZ, MID and PIWI) and linkers (L0, L1, and L2) assemble into a typical bi-lobed (N-PAZ lobe and MID-PIWI lobe) architecture, in which the 5' and 3' ends of the bound piRNA are anchored by the MID-PIWI and PAZ domains, respectively. However, the relative orientation of these two lobes differs significantly between Siwi and the Ago-clade proteins: the N-PAZ lobe in Siwi rotates significantly downward. Additional hinge motions between N and PAZ domains exist, due to a distinct set of contacts at the interface. Three out of the four residues in the DEDH catalytic tetrad are well-preserved. The fourth, Glu-708, is located in a disordered loop and likely joins the active site upon the guide-target RNA duplex formation. This 'unplugged' active site scheme is found in Agos of some bacteria. Arginine methylation is required for the interaction with Tudor domain-containing protein Papi/TDRKH (PubMed:23970546, PubMed:26919431). Belongs to the argonaute family. Piwi subfamily.