DrosophilaMales Use 5′-to-3′ Phased Biogenesis to MakeStellate-silencing piRNAs that Lack Homology to Maternally Deposited piRNA Guides

Abstract

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.

Genetic control of a sex-specific piRNA program

“…In fact, the effect of the Y is independent of its parental origin and mothers' sex chromosome contents. The ability of Y-linked piRNA loci to act in both male and female cellular environments independently of its inheritance path implies unique regulatory mechanisms 36 employed by the Y and distinctive evolutionary forces acting on the Y.…”

Autonomous Shaping of the piRNA Sequence Repertoire by Competition between Adjacent Ping-Pong Sites

“…For TEs, sequence mutation is the simplest strategy to evade piRNA-mediated silencing. On the other hand, the piRNA system has more relaxed rules for target recognition and cleavage than the siRNA system, and thus it is thought to be better equipped to silence rapidly diverging TEs 20 .…”

“…The ping-pong pathway plays a dual role: it silences TEs by cleavage while amplifying new piRNAs using the cleavage fragments 5,12,18 . Amplified piRNAs can then be maternally inherited by the germline of the next generation, serving as a robust trans-generational repository of TE sequence information for target silencing 19,20 . Indeed, if a TE, whose sequence is absent from the maternal pool of piRNAs, is inherited only paternally, the TE escapes silencing and causes adult sterility syndrome 6,21 .…”

“…A classic example is P-M hybrid dysgenesis in Drosophila, where crosses between a male fly with P elements (P strain) and a female without P elements (M strain) cause sterility in the offspring, while reciprocal crosses between a P strain female and an M strain male produce fertile progeny because of maternally inherited piRNAs that target P elements 6,22 . In addition, piRNAs can tolerate more mismatches than small interfering RNAs (siRNAs), another class of small RNAs that guide AGO proteins, for target recognition and cleavage, making them suitable for combating rapidly diverging or newly invading TEs 20,23 . Still, efficient cleavage by piRNAs requires more than 15 contiguous base pairs 20 , which is expected to be particularly important in the ping-pong pathway for reciprocal cleavage cycles.…”

“…In addition, piRNAs can tolerate more mismatches than small interfering RNAs (siRNAs), another class of small RNAs that guide AGO proteins, for target recognition and cleavage, making them suitable for combating rapidly diverging or newly invading TEs 20,23 . Still, efficient cleavage by piRNAs requires more than 15 contiguous base pairs 20 , which is expected to be particularly important in the ping-pong pathway for reciprocal cleavage cycles. Therefore, it must be critical for the piRNA system to choose the appropriate sites for ping-pong-mediated piRNA production and to have the flexibility to adjust them according to diverging TEs for achieving long-term TE silencing over generations.…”