A novel organelle, the piNG-body, in the nuage of Drosophila male germ cells is associated with piRNA-mediated gene silencing

Proteins of the PIWI subfamily Aub and AGO3 associated with the germline-specific perinuclear granules (nuage) are involved in the silencing of retrotransposons and other selfish repetitive elements in the Drosophila genome. PIWI proteins and their 25- to 30-nt PIWI-interacting RNA (piRNAs) are considered as key participants of the piRNA pathway. Using immunostaining, we found a large, nuage-associated organelle in the testes, the piNG-body (piRNA nuage giant body), which was significantly more massive than an ordinary nuage granule. This body contains known ovarian nuage proteins, including Vasa, Aub, AGO3, Tud, Spn-E, Bel, Squ, and Cuff, as well as AGO1, the key component of the microRNA pathway. piNG-bodies emerge at the primary spermatocyte stage of spermatogenesis during the period of active transcription. Aub, Vasa, and Tud are located at the periphery of the piNG-body, whereas AGO3 is found in its core. Mutational analysis revealed that Vasa, Aub, and AGO3 were crucial for both the maintenance of the piNG-body structure and the silencing of selfish Stellate repeats. The piNG-body destruction caused by csul mutations that abolish specific posttranslational symmetrical arginine methylation of PIWI proteins is accompanied by strong derepression of Stellate genes known to be silenced via the piRNA pathway.