Rhino breaks the deadlock in Drosophila testis

Overview & Synopsis

In the early 2000s, Aravin and colleagues discovered, in the Drosophila melanogaster testis, a new class of small regulatory RNAs initially named repeat-associated small interfering RNA (rasiRNAs). rasiRNAs were first described as regulating a protein-coding gene. They are derived from the Suppressor of Stellate [Su(Ste)] locus located on the Y chromosome and were shown to target the X-linked Stellate repeated genes by sequence complementarity. Stellate genes encode proteins with homology to the regulatory β subunit of the protein kinase CK2. Stellate repression occurs during male gametogenesis and is essential for male fertility. These small RNAs were renamed as PIWI-interacting RNAs (pAU : Pleasenotethat}piRNAs}hasbeendefineda iRNAs); they are 23 to 29 nucleotides (nAU : Pleasenotethat}nt}hasbeendefinedas}nucleotides}inthesentence}ThesesmallRNAswererenamed t) long and bind to proteins of the PIWI family. Accordingly, piRNA pathway mutants are sterile, and they contain crystalline aggregates of Stellate-coded protein [1]. piRNAs were then discovered in mammals and in most other animal germ cells [2]. Nowadays, we know that piRNAs are mainly devoted to protecting the genome from active mobile genetic elements in the metazoan germline. In D. melanogaster, both sexes depend on a functional piRNA pathway for their fertility. Our understanding of piRNA biogenesis and function comes predominantly from studies of the female Drosophila germline, but the male's piRNA pathway remains poorly understood. Previously, studies reported that many proteins involved in the female piRNA pathway are also required for male fertility and Stellate silencing in the testis, supporting the conservation of piRNA pathway machinery in both sexes [3]. While Aub is expressed broadly from germline stem cells (GSCs) to primary spermatocytes, Ago3 and PIWI were detected only in mitotically dividing germline cells (GSCs and spermatogonia), indicating stage-specific modulations of the piRNA pathway [4-6] (Fig 1A). The 2 most active piRNA clusters in the testis are dual strand: Su(Ste) genes and AT-chX [5,7,8]. Interestingly, Aravin's team have recently defined novel piRNA clusters in D. melanogaster spermatogenesis and show piRNAs adaptation, dependent on sex-specific expression of transposons [8]. Dualstrand piRNA clusters generally lack promoters, and their expression depends on the Rhino-Deadlock-Cutoff (RDC) complex that licenses noncanonical transcription. The RDC is anchored to H3K9me3-marked chromatin via Rhino's chromodomain. This process involves 5 0-end protection of nascent RNAs and suppression of transcription termination [9]. Importantly, since fertility in male Rhino mutants was not compromised and stellate crystals were absent, RDC function in spermatogenesis was neglected [10]. In this issue of PLOS Genetics, Chen and colleagues explored whether piRNA loci in the D. melanogaster testis use the same RDC noncanonical transcription mechanism as in the female germline [11]. Unexpectedly, a detailed examination of male fertility by sperm exhaustion assays revealed germline defects and subfertility in RDC mutant males. A careful spatiotemporal analysis showed that the RDC complex is not ovary specific. RDC assembled also in early spermatogenesis to regulate piRNA cluster expression required for an efficient transposable element (TAU : Pleasenotethat}TE}hasbeendefinedas}transposableelement}inthesentence}RDCassem E) silencing in the testis. Like in ovaries, they observed that RDC marks all dual