Acts via the piwi-interacting RNA (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. 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. In ovary, associates predominantly with antisense piRNAs that contain uridine at their 5' end. In testis, associates with Su(Ste) antisense piRNAs (most abundant class of piRNAs found in complex with aub in testes) and negatively regulates Ste expression, most likely by cleaving its transcripts. Also in testis, may repress translation of vas when associated with a piRNA derived from chromosome X, termed AT-chX-1, whose sequence shows strong complementarity to vas mRNA. Aub-piRNA complexes from ovary and testis possess RNA cleavage activity. Involved in telomere regulation by repressing specialized telomeric retroelements HeT-A and TART; Drosophila telomeres being maintained by transposition of specialized telomeric retroelements. Also involved in telomeric trans-silencing, a repression mechanism by which a transposon or a transgene inserted in subtelomeric heterochromatin has the capacity to repress in trans, in the female germline, a homologous transposon, or transgene located in euchromatin. Involved in the suppression of meiotic drive of sex chromosomes and autosomes. Involved in transposon silencing in the adult brain. Required for dorsal-ventral as well as anterior-posterior patterning of the egg. Required during oogenesis for primordial germ cell formation and activation of RNA interference. During early oogenesis, required for osk mRNA silencing and polarization of the microtubule cytoskeleton. During mid-oogenesis, required for osk mRNA localization to the posterior pole and efficient translation of osk and grk. During embryogenesis, required for posterior localization of nos mRNA, independently of osk, and pole cell formation. Essential for the formation and/or structural integrity of perinuclear nuage particles. Required for the localization of Mael to the meiotic nuage. Forms a complex with smg, twin, AGO3 and specific piRNAs that targets nos mRNA (and probably other maternal mRNAS) for deadenylation promoting its decay during early embryogenesis.