[Show abstract][Hide abstract] ABSTRACT: RNA interference-related silencing mechanisms concern very diverse and distinct biological processes, from gene regulation (via the microRNA pathway) to defense against molecular parasites (through the small interfering RNA and the Piwi-interacting RNA pathways). Small non-coding RNAs serve as specificity factors that guide effector proteins to ribonucleic acid targets via base-pairing interactions, to achieve transcriptional or post-transcriptional regulation. Because of the small sequence complementarity required for microRNA-dependent post-transcriptional regulation, thousands of microRNA (miRNA) putative targets have been annotated in Drosophila. In Drosophila somatic ovarian cells, genomic parasites, such as transposable elements (TEs), are transcriptionally repressed by chromatin changes induced by Piwi-interacting RNAs (piRNAs) that prevent them from invading the germinal genome. Here we show, for the first time, that a functional miRNA pathway is required for the piRNA-mediated transcriptional silencing of TEs in this tissue. Global miRNA depletion, caused by tissue- and stage-specific knock down of drosha (involved in miRNA biogenesis), AGO1 or gawky (both responsible for miRNA activity), resulted in loss of TE-derived piRNAs and chromatin-mediated transcriptional de-silencing of TEs. This specific TE de-repression was also observed upon individual titration (by expression of the complementary miRNA sponge) of two miRNAs (miR-14 and miR-34) as well as in a miR-14 loss-of-function mutant background. Interestingly, the miRNA defects differentially affected TE- and 3' UTR-derived piRNAs. To our knowledge, this is the first indication of possible differences in the biogenesis or stability of TE- and 3' UTR-derived piRNAs. This work is one of the examples of detectable phenotypes caused by loss of individual miRNAs in Drosophila and the first genetic evidence that miRNAs have a role in the maintenance of genome stability via piRNA-mediated TE repression.
[Show abstract][Hide abstract] ABSTRACT: piRNAs (piwi-interacting RNAs) are a class of small interfering RNAs that play a major role in the regulation of transposable elements (TEs) in Drosophila and are considered of fundamental importance in gonadal development. Genes encoding the effectors of the piRNA machinery are thus often though to be highly constrained. On the contrary, as actors of genetic immunity, these genes have also been shown to evolve rapidly and display a high level of sequence variability. In order to assess the support for these competing models, we analyzed seven genes of the piRNA pathway using a collection of wild-type strains of Drosophila simulans, which are known to display significant variability in their TE content between strains. We showed that these genes exhibited wide variation in transcript levels, and we discuss some evolutionary considerations regarding the observed variability in TE copy numbers.
[Show abstract][Hide abstract] ABSTRACT: Transposable elements (TEs), whose propagation can result in severe damage to the host genome, are silenced in the animal gonad by Piwi-interacting RNAs (piRNAs). piRNAs produced in the ovaries are deposited in the embryonic germline and initiate TE repression in the germline progeny. Whether the maternally transmitted piRNAs play a role in the silencing of somatic TEs is however unknown. Here we show that maternally transmitted piRNAs from the tirant retrotransposon in Drosophila are required for the somatic silencing of the TE and correlate with an increase in histone H3K9 trimethylation an active tirant copy.
[Show abstract][Hide abstract] ABSTRACT: Genes are important in defining genetic variability, but they do not constitute the largest component of genomes, which in most organisms contain large amounts of various repeated sequences including transposable elements (TEs), which have been shown to account for most of the genome size. TEs contribute to genetic diversity by their mutational potential as a result of their ability to insert into genes or gene regulator regions, to promote chromosomal rearrangements, and to interfere with gene networks. Also, TEs may be activated by environmental stresses (such as temperature or radiation) that interfere with epigenetic regulation systems, and makes them powerful mutation agents in nature. To understand the relationship between genotype and phenotype, we need to analyze the portions of the genome corresponding to TEs in great detail, and to decipher their relationships with the genes. For this purpose, we carried out comparative analyses of various natural populations of the closely-related species Drosophila melanogaster and Drosophila simulans, which differ with regard to their TE amounts as well as their ecology and population size.
Full-text · Article · May 2012 · Journal of Environmental Radioactivity
[Show abstract][Hide abstract] ABSTRACT: Endogenous retroviruses have the ability to become permanently integrated into the genomes of their host, and they are generally
transmitted vertically from parent to progeny. With the exception of gypsy, few endogenous retroviruses have been identified in insects. In this study, we describe the tirant endogenous retrovirus in a subset of Drosophila simulans natural populations. By focusing on the envelope gene, we show that the entire retroviral cycle (transcription, translation,
and retrotransposition) can be completed for tirant within one population of this species.
Full-text · Article · Jan 2012 · Journal of Virology