PIWI-interacting RNAs (piRNAs) direct PIWI proteins to silence complementary targets such as transposons. In animals with a maternally specified germline, e.g. Drosophila melanogaster, maternally deposited piRNAs initiate piRNA biogenesis in the progeny. Normal fertility in D. melanogaster males requires repression of tandemly repeated Stellate genes by piRNAs from Suppressor of Stellate [Su(Ste)]. Because the Su(Ste) loci are on the Y chromosome, Su(Ste) piRNAs are not deposited in oocytes. How the male germline produces Su(Ste) piRNAs in the absence of maternally deposited Su(Ste) piRNAs is unknown. Here, we show that Su(Ste) piRNAs are made in the early male germline via 5′-to-3′ phased piRNA biogenesis triggered by maternally deposited 1360/Hoppel transposon piRNAs. Strikingly, deposition of Su(Ste) piRNAs from XXY mothers obviates the need for phased piRNA biogenesis in sons. Together, our study uncovers the developmentally programmed mechanism that allows fly mothers to protect their sons using a Y-linked piRNA locus.

Piwi-interacting RNAs (piRNAs) play essential roles in silencing repetitive elements to promote fertility in metazoans. Studies in worms, flies, and mammals reveal that piRNAs are expressed in a sex-specific manner. However, the mechanisms underlying this sex-specific regulation are unknown. Here we identify SNPC-1.3, a variant of a conserved subunit of the snRNA activating protein complex, as a male-specific piRNA transcription factor in C. elegans. Binding of SNPC-1.3 at male piRNA loci drives spermatogenic piRNA transcription and requires the core piRNA transcription factor SNPC-4. Loss of snpc-1.3 leads to depletion of male piRNAs and defects in male-dependent fertility. Furthermore, TRA-1, a master regulator of sex determination, binds to the snpc-1.3 promoter and represses its expression during oogenesis. Loss of TRA-1 targeting causes ectopic expression of snpc-1.3 and male piRNAs during oogenesis. Thus, sexual dimorphic regulation of snpc-1.3 coordinates male and female piRNA expression during germline development.

The piRNA pathway protects germline genomes from selfish genetic elements (e.g. transposons) through their transcript cleavage in the cytoplasm and/or their transcriptional silencing in the nucleus. Here, we describe a mechanism by which the nuclear and cytoplasmic arms of the piRNA pathway are linked. We find that during mitosis of Drosophila spermatogonia, nuclear Piwi interacts with nuage, the compartment that mediates the cytoplasmic arm of the piRNA pathway. At the end of mitosis, Piwi leaves nuage to return to the nucleus. Dissociation of Piwi from nuage occurs at the depolymerizing microtubules of the central spindle, mediated by a microtubule-depolymerizing kinesin, Klp10A. Depletion of klp10A delays the return of Piwi to the nucleus and affects piRNA production, suggesting the role of nuclearcytoplasmic communication in piRNA biogenesis. We propose that cell cycle-dependent communication between the nuclear and cytoplasmic arms of the piRNA pathway may play a previously unappreciated role in piRNA regulation.

35The piRNA pathway protects germline genomes through transcript cleavage of selfish 36 genetic elements, such as transposons, in the cytoplasm and their transcriptional silencing 37 in the nucleus. Here, we describe a mechanism by which the nuclear and cytoplasmic 38 arms of the silencing mechanism are linked. During mitosis of Drosophila 39 spermatogonia, nuclear Piwi interacts with nuage, the compartment that mediates the 40 cytoplasmic arm of piRNA-mediated silencing. At the end of mitosis, Piwi leaves nuage 41 to return to the nucleus. We found that dissociation of Piwi from nuage occurs at the 42 depolymerizing microtubules of the central spindle, mediated by a microtubule-43 depolymerizing kinesin Klp10A. Depletion of klp10A delays Piwi's return to the nucleus 44 and affects piRNA production, suggesting the importance of nuclear-cytoplasmic 45 communication in piRNA biogenesis. We propose that cell cycle-dependent 46 communication between the nuclear and cytoplasmic arms of the piRNA pathway plays 47 important roles in coordinated piRNA production. 48

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