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In Drosophila melanogaster the COM locus directs the somatic silencing of two retrotransposons through both Piwi-dependent and -independent pathways.

Desset S, Buchon N, Meignin C, Coiffet M, Vaury C - PLoS ONE (2008)

Bottom Line: In the Drosophila germ line, repeat-associated small interfering RNAs (rasiRNAs) ensure genomic stability by silencing endogenous transposable elements.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.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.

View Article: PubMed Central - PubMed

Affiliation: Centre National de la Recherche Scientifique (CNRS), UMR6247-GReD, Clermont Université; INSERM, Faculté de Médecine, BP38, Clermont-Ferrand, France.

ABSTRACT

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.

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The silencing mechanism targeting ZAM and Idefix is active in somatic tissues throughout fly development.A) In an S/S genetic background, the pGFP-IdU sensor transgene driven by 24B-Gal4 is not expressed in embryos, larvae, or adults (top, middle and bottom panels, respectively). Only a very faint level of fluorescence, corresponding to the background expression of GFP, is detected. B) In a U/U genetic background, the GFP-IdU transgene silencing is released and GFP fluorescence is clearly observed in the three stages analyzed. The fluorescence pattern recapitulates the expression of the HOW gene in muscle and tendon cells, as expected for the 24B-Gal4 driver [22]. C) In an S/S genetic background, the pGFP-IdUAS sensor transgene carrying the 5′UTR of Idefix in the opposite orientation is not subjected to the silencing exerted on the Idefix sequences. pGFP-IdUAS is correctly expressed and GFP is detected in embryos, larvae, and adult flies.
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pone-0001526-g006: The silencing mechanism targeting ZAM and Idefix is active in somatic tissues throughout fly development.A) In an S/S genetic background, the pGFP-IdU sensor transgene driven by 24B-Gal4 is not expressed in embryos, larvae, or adults (top, middle and bottom panels, respectively). Only a very faint level of fluorescence, corresponding to the background expression of GFP, is detected. B) In a U/U genetic background, the GFP-IdU transgene silencing is released and GFP fluorescence is clearly observed in the three stages analyzed. The fluorescence pattern recapitulates the expression of the HOW gene in muscle and tendon cells, as expected for the 24B-Gal4 driver [22]. C) In an S/S genetic background, the pGFP-IdUAS sensor transgene carrying the 5′UTR of Idefix in the opposite orientation is not subjected to the silencing exerted on the Idefix sequences. pGFP-IdUAS is correctly expressed and GFP is detected in embryos, larvae, and adult flies.

Mentions: In the S/S genetic background, no fluorescence was detected with any of the transgenes (pGFP-ZU, pGFP-Zenv, pGFP-IdU or pGFP-IdGag), regardless of the driver used (actin-Gal4 or 24B-Gal4). It should be noted that, if the microscope settings are optimized, a very faint level of fluorescence can be detected at each stage of development. This transgene silencing was observed in all the cells examined and throughout fly development, including in embryos, larvae, and adult flies. As an example, results obtained with pGFP-IdU driven by 24B-Gal4 are presented Fig. 6, column A. In contrast, when the X-chromosome in S-transgenic lines was replaced by one from a U line, clear fluorescence resulting from the expression of the GFP reporter gene driven by 24B-Gal4 was detected in embryos, larvae, and adult flies (Fig. 6 column B).


In Drosophila melanogaster the COM locus directs the somatic silencing of two retrotransposons through both Piwi-dependent and -independent pathways.

Desset S, Buchon N, Meignin C, Coiffet M, Vaury C - PLoS ONE (2008)

The silencing mechanism targeting ZAM and Idefix is active in somatic tissues throughout fly development.A) In an S/S genetic background, the pGFP-IdU sensor transgene driven by 24B-Gal4 is not expressed in embryos, larvae, or adults (top, middle and bottom panels, respectively). Only a very faint level of fluorescence, corresponding to the background expression of GFP, is detected. B) In a U/U genetic background, the GFP-IdU transgene silencing is released and GFP fluorescence is clearly observed in the three stages analyzed. The fluorescence pattern recapitulates the expression of the HOW gene in muscle and tendon cells, as expected for the 24B-Gal4 driver [22]. C) In an S/S genetic background, the pGFP-IdUAS sensor transgene carrying the 5′UTR of Idefix in the opposite orientation is not subjected to the silencing exerted on the Idefix sequences. pGFP-IdUAS is correctly expressed and GFP is detected in embryos, larvae, and adult flies.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2211404&req=5

pone-0001526-g006: The silencing mechanism targeting ZAM and Idefix is active in somatic tissues throughout fly development.A) In an S/S genetic background, the pGFP-IdU sensor transgene driven by 24B-Gal4 is not expressed in embryos, larvae, or adults (top, middle and bottom panels, respectively). Only a very faint level of fluorescence, corresponding to the background expression of GFP, is detected. B) In a U/U genetic background, the GFP-IdU transgene silencing is released and GFP fluorescence is clearly observed in the three stages analyzed. The fluorescence pattern recapitulates the expression of the HOW gene in muscle and tendon cells, as expected for the 24B-Gal4 driver [22]. C) In an S/S genetic background, the pGFP-IdUAS sensor transgene carrying the 5′UTR of Idefix in the opposite orientation is not subjected to the silencing exerted on the Idefix sequences. pGFP-IdUAS is correctly expressed and GFP is detected in embryos, larvae, and adult flies.
Mentions: In the S/S genetic background, no fluorescence was detected with any of the transgenes (pGFP-ZU, pGFP-Zenv, pGFP-IdU or pGFP-IdGag), regardless of the driver used (actin-Gal4 or 24B-Gal4). It should be noted that, if the microscope settings are optimized, a very faint level of fluorescence can be detected at each stage of development. This transgene silencing was observed in all the cells examined and throughout fly development, including in embryos, larvae, and adult flies. As an example, results obtained with pGFP-IdU driven by 24B-Gal4 are presented Fig. 6, column A. In contrast, when the X-chromosome in S-transgenic lines was replaced by one from a U line, clear fluorescence resulting from the expression of the GFP reporter gene driven by 24B-Gal4 was detected in embryos, larvae, and adult flies (Fig. 6 column B).

Bottom Line: In the Drosophila germ line, repeat-associated small interfering RNAs (rasiRNAs) ensure genomic stability by silencing endogenous transposable elements.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.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.

View Article: PubMed Central - PubMed

Affiliation: Centre National de la Recherche Scientifique (CNRS), UMR6247-GReD, Clermont Université; INSERM, Faculté de Médecine, BP38, Clermont-Ferrand, France.

ABSTRACT

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.

Show MeSH
Related in: MedlinePlus