In Drosophila melanogaster the COM Locus Directs the Somatic Silencing of Two Retrotransposons through both Piwi-Dependent and -Independent Pathways

Background In the Drosophila germ line, repeat-associated small interfering RNAs (rasiRNAs) ensure genomic stability by silencing endogenous transposable elements. This RNA silencing involves small RNAs of 26-30 nucleotides that are mainly produced from the antisense strand and function through the Piwi protein. Piwi belongs to the subclass of the Argonaute family of RNA interference effector proteins, which are expressed in the germline and in surrounding somatic tissues of the reproductive apparatus. In addition to this germ-line expression, Piwi has also been implicated in diverse functions in somatic cells. Principal Findings Here, we show that two LTR retrotransposons from Drosophila melanogaster, ZAM and Idefix, are silenced by an RNA silencing pathway that has characteristics of the rasiRNA pathway and that specifically recognizes and destroys the sense-strand RNAs of the retrotransposons. This silencing depends on Piwi in the follicle cells surrounding the oocyte. Interestingly, this silencing is active in all the somatic tissues examined from embryos to adult flies. In these somatic cells, while the silencing still involves the strict recognition of sense-strand transcripts, it displays the marked difference of being independent of the Piwi protein. Finally, we present evidence that in all the tissues examined, the repression is controlled by the heterochromatic COM locus. Conclusion Our data shed further light on the silencing mechanism that acts to target Drosophila LTR retrotransposons in somatic cells throughout fly development. They demonstrate that different RNA silencing pathways are involved in ovarian versus other somatic tissues, since Piwi is necessary for silencing in the former tissues but is dispensable in the latter. They further demonstrate that these pathways are controlled by the heterochromatic COM locus which ensures the overall protection of Drosophila against the detrimental effects of random retrotransposon mobilization.

[1]  Haifan Lin,et al.  An epigenetic activation role of Piwi and a Piwi-associated piRNA in Drosophila melanogaster , 2007, Nature.

[2]  Manolis Kellis,et al.  Discrete Small RNA-Generating Loci as Master Regulators of Transposon Activity in Drosophila , 2007, Cell.

[3]  Kuniaki Saito,et al.  A Slicer-Mediated Mechanism for Repeat-Associated siRNA 5' End Formation in Drosophila , 2007, Science.

[4]  Alain Bucheton,et al.  A Novel Repeat-Associated Small Interfering RNA-Mediated Silencing Pathway Downregulates Complementary Sense gypsy Transcripts in Somatic Cells of the Drosophila Ovary , 2006, Journal of Virology.

[5]  Christian Biémont,et al.  Genetics: Junk DNA as an evolutionary force , 2006, Nature.

[6]  Kuniaki Saito,et al.  Specific association of Piwi with rasiRNAs derived from retrotransposon and heterochromatic regions in the Drosophila genome. , 2006, Genes & development.

[7]  Vladimir Gvozdev,et al.  A Distinct Small RNA Pathway Silences Selfish Genetic Elements in the Germline , 2006, Science.

[8]  C. Sander,et al.  A novel class of small RNAs bind to MILI protein in mouse testes , 2006, Nature.

[9]  Ravi Sachidanandam,et al.  A germline-specific class of small RNAs binds mammalian Piwi proteins , 2006, Nature.

[10]  Giacomo Cavalli,et al.  RNAi Components Are Required for Nuclear Clustering of Polycomb Group Response Elements , 2006, Cell.

[11]  Phillip D Zamore,et al.  microPrimer: the biogenesis and function of microRNA , 2005, Development.

[12]  V. Gvozdev,et al.  Argonaute protein PIWI controls mobilization of retrotransposons in the Drosophila male germline , 2005, Nucleic acids research.

[13]  V. Ambros.  The functions of animal microRNAs , 2004, Nature.

[14]  Giacomo Cavalli,et al.  Dissection of a Natural RNA Silencing Process in the Drosophila melanogaster Germ Line , 2004, Molecular and Cellular Biology.

[15]  E. Sontheimer,et al.  Distinct Roles for Drosophila Dicer-1 and Dicer-2 in the siRNA/miRNA Silencing Pathways , 2004, Cell.

[16]  A. Pélisson,et al.  Evidence for a piwi-Dependent RNA Silencing of the gypsy Endogenous Retrovirus by the Drosophila melanogaster flamenco Gene , 2004, Genetics.

[17]  James A. Birchler,et al.  Heterochromatic Silencing and HP1 Localization in Drosophila Are Dependent on the RNAi Machinery , 2004, Science.

[18]  B. Dastugue,et al.  Intercellular communication between germ line and somatic line is utilized to control the transcription of ZAM, an endogenous retrovirus from Drosophila melanogaster. , 2004, Nucleic acids research.

[19]  J. Birchler,et al.  Interrelationship of RNA interference and transcriptional gene silencing in Drosophila. , 2004, Cold Spring Harbor symposia on quantitative biology.

[20]  B. Dastugue,et al.  COM, a heterochromatic locus governing the control of independent endogenous retroviruses from Drosophila melanogaster. , 2003, Genetics.

[21]  Michael Rowe,et al.  The rest is silence. , 2002, Health affairs.

[22]  G. Rubin,et al.  ARGONAUTE1 is required for efficient RNA interference in Drosophila embryos , 2002, Proceedings of the National Academy of Sciences of the United States of America.

[23]  J. Birchler,et al.  RNAi related mechanisms affect both transcriptional and posttranscriptional transgene silencing in Drosophila. , 2002, Molecular cell.

[24]  G. Hannon,et al.  C . elegans involved in developmental timing in Dicer functions in RNA interference and in synthesis of small RNA , 2001 .

[25]  A. Aravin,et al.  Double-stranded RNA-mediated silencing of genomic tandem repeats and transposable elements in the D. melanogaster germline , 2001, Current Biology.

[26]  V. Robert,et al.  Characterization of the flamenco region of the Drosophila melanogaster genome. , 2001, Genetics.

[27]  B. Dastugue,et al.  Life Cycle of an Endogenous Retrovirus,ZAM, in Drosophila melanogaster , 2000, Journal of Virology.

[28]  R. Plasterk,et al.  mut-7 of C. elegans, Required for Transposon Silencing and RNA Interference, Is a Homolog of Werner Syndrome Helicase and RNaseD , 1999, Cell.

[29]  B. Dastugue,et al.  Mobilization of two retroelements, ZAM and Idefix, in a novel unstable line of Drosophila melanogaster. , 1999, Molecular biology and evolution.

[30]  B. Dastugue,et al.  Invertebrate retroviruses: ZAM a new candidate in D.melanogaster , 1997, The EMBO journal.

[31]  K. Golic,et al.  Somatic reversion of chromosomal position effects in Drosophila melanogaster. , 1996, Genetics.

[32]  N. Perrimon,et al.  Targeted gene expression as a means of altering cell fates and generating dominant phenotypes. , 1993, Development.