Telomeric Trans-Silencing in Drosophila melanogaster: Tissue Specificity, Development and Functional Interactions between Non-Homologous Telomeres

BACKGROUND: The study of P element repression in Drosophila melanogaster led to the discovery of the telomeric Trans-Silencing Effect (TSE), a homology-dependent repression mechanism by which a P-transgene inserted in subtelomeric heterochromatin (Telomeric Associated Sequences, "TAS") has the capacity to repress in trans, in the female germline, a homologous P-lacZ transgene located in euchromatin. TSE can show variegation in ovaries, displays a maternal effect as well as an epigenetic transmission through meiosis and involves heterochromatin and RNA silencing pathways. PRINCIPAL FINDINGS: Here, we analyze phenotypic and genetic properties of TSE. We report that TSE does not occur in the soma at the adult stage, but appears restricted to the female germline. It is detectable during development at the third instar larvae where it presents the same tissue specificity and maternal effect as in adults. Transgenes located in TAS at the telomeres of the main chromosomes can be silencers which in each case show the maternal effect. Silencers located at non-homologous telomeres functionally interact since they stimulate each other via the maternally-transmitted component. All germinally-expressed euchromatic transgenes tested, located on all major chromosomes, were found to be repressed by a telomeric silencer: thus we detected no TSE escaper. The presence of the euchromatic target transgene is not necessary to establish the maternal inheritance of TSE, responsible for its epigenetic behavior. A single telomeric silencer locus can simultaneously repress two P-lacZ targets located on different chromosomal arms. CONCLUSIONS AND SIGNIFICANCE: Therefore TSE appears to be a widespread phenomenon which can involve different telomeres and work across the genome. It can explain the P cytotype establishment by telomeric P elements in natural Drosophila populations

Paramutation in Drosophila Requires Both Nuclear and Cytoplasmic Actors of the piRNA Pathway and Induces Cis-spreading of piRNA Production

International audienceTransposable element activity is repressed in the germline in animals by PIWI-interacting RNAs (piRNAs), a class of small RNAs produced by genomic loci mostly composed of TE sequences. The mechanism of induction of piRNA production by these loci is still enigmatic. We have shown that, in Drosophila melanogaster, a cluster of tandemly repeated P-lacZ-white transgenes can be activated for piRNA production by maternal inheritance of a cytoplasm containing homologous piRNAs. This activated state is stably transmitted over generations and allows trans-silencing of a homologous transgenic target in the female germline. Such an epigenetic conversion displays the functional characteristics of a paramutation, i.e., a heritable epigenetic modification of one allele by the other. We report here that piRNA production and trans-silencing capacities of the paramutated cluster depend on the function of the rhino, cutoff, and zucchini genes involved in primary piRNA biogenesis in the germline, as well as on that of the aubergine gene implicated in the ping-pong piRNA amplification step. The 21-nt RNAs, which are produced by the paramutated cluster, in addition to 23- to 28-nt piRNAs are not necessary for paramutation to occur. Production of these 21-nt RNAs requires Dicer-2 but also all the piRNA genes tested. Moreover, cytoplasmic transmission of piRNAs homologous to only a subregion of the transgenic locus can generate a strong paramutated locus that produces piRNAs along the whole length of the transgenes. Finally, we observed that maternally inherited transgenic small RNAs can also impact transgene expression in the soma. In conclusion, paramutation involves both nuclear (Rhino, Cutoff) and cytoplasmic (Aubergine, Zucchini) actors of the piRNA pathway. In addition, since it is observed between nonfully homologous loci located on different chromosomes, paramutation may play a crucial role in epigenome shaping in Drosophila natural populations

Inheritance of paramutation in plants and Drosophila

International audienceno abstrac

piRNAs and epigenetic conversion in Drosophila

International audienceTransposable element (TE) activity is repressed in the Drosophila germline by Piwi-Interacting RNAs (piRNAs), a class of small non-coding RNAs. These piRNAs are produced by discrete genomic loci containing TE fragments. In a recent publication, we tested for the existence of a strict epigenetic induction of piRNA production capacity by a locus in the D. melanogaster genome. We used 2 lines carrying a transgenic 7-copy tandem cluster (P-lacZ-white) at the same genomic site. This cluster generates in both lines a local heterochromatic sector. One line (T-1) produces high levels of ovarian piRNAs homologous to the P-lacZ-white transgenes and shows a strong capacity to repress homologous sequences in trans, whereas the other line (BX2) is devoid of both of these capacities. The properties of these 2 lines are perfectly stable over generations. We have shown that the maternal transmission of a cytoplasm carrying piRNAs from the first line can confer to the inert transgenic locus of the second, a totally de novo capacity to produce high levels of piRNAs as well as the ability to induce homology-dependent silencing in trans. These new properties are stably inherited over generations (n > 50). Furthermore, the converted locus has itself become able to convert an inert transgenic locus via cytoplasmic maternal inheritance. This results in a stable epigenetic conversion process, which can be performed recurrently-a phenomenon termed paramutation and discovered in Maize 60 y ago. Paramutation in Drosophila corresponds to the first stable paramutation in animals and provides a model system to investigate the epigenetically induced emergence of a piRNA-producing locus, a crucial step in epigenome shaping. In this Extra View, we discuss some additional functional aspects and the possible molecular mechanism of this piRNA-linked paramutation