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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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ZAM and Idefix are regulated by a PIWI-independent pathway outside of the reproductive apparatus.The pGFP-ZU sensor transgene driven by 24B-Gal4 is not expressed in larvae, pupae, or adult stages in a [S/S; piwi+/+] genetic background (right panel). Only a very faint level of fluorescence, corresponding to the background, is detected. A clear GFP expression is observed in these stages of development in a [U/U; piwi+/+] line (middle panel). In piwi mutant backgrounds, in homozygous [S/S; piwi3/3] lines, the silencing of the sensor transgene is not released. A very faint fluorescence level similar to that observed in homozygous [S/S; piwi+/+] lines is observed (left panel).
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pone-0001526-g007: ZAM and Idefix are regulated by a PIWI-independent pathway outside of the reproductive apparatus.The pGFP-ZU sensor transgene driven by 24B-Gal4 is not expressed in larvae, pupae, or adult stages in a [S/S; piwi+/+] genetic background (right panel). Only a very faint level of fluorescence, corresponding to the background, is detected. A clear GFP expression is observed in these stages of development in a [U/U; piwi+/+] line (middle panel). In piwi mutant backgrounds, in homozygous [S/S; piwi3/3] lines, the silencing of the sensor transgene is not released. A very faint fluorescence level similar to that observed in homozygous [S/S; piwi+/+] lines is observed (left panel).

Mentions: Since the control exerted by COM is active in the somatic cells outside of the reproductive apparatus, the question arose then to know whether Piwi is also a component of this somatic silencing pathway. Indeed, in addition to its function in the reproductive apparatus of flies, diverse functions have been attributed to Piwi in somatic tissues at different stages of fly development [23]–[26]. Thus, GFP expression of the sensor transgenes pGFP-ZAM or pGFP-Id, driven by 24B-Gal4, was analyzed in larvae, pupae, and adult S flies mutated or not for the piwi gene. Piwi2/2, piwi3/3, and transheterozygous piwi3/2 mutations were tested in these experiments. The results obtained indicated that the silencing of the sensor transgenes in the somatic tissues of the fly outside of the ovaries does not depend on the presence of the Piwi protein. Indeed, in contrast to what was observed in the gonads, GFP expression was not recovered in the somatic tissues of S lines when the Piwi gene was mutated (see Figure 7 and data not shown).


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)

ZAM and Idefix are regulated by a PIWI-independent pathway outside of the reproductive apparatus.The pGFP-ZU sensor transgene driven by 24B-Gal4 is not expressed in larvae, pupae, or adult stages in a [S/S; piwi+/+] genetic background (right panel). Only a very faint level of fluorescence, corresponding to the background, is detected. A clear GFP expression is observed in these stages of development in a [U/U; piwi+/+] line (middle panel). In piwi mutant backgrounds, in homozygous [S/S; piwi3/3] lines, the silencing of the sensor transgene is not released. A very faint fluorescence level similar to that observed in homozygous [S/S; piwi+/+] lines is observed (left panel).
© Copyright Policy
Related In: Results  -  Collection

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

pone-0001526-g007: ZAM and Idefix are regulated by a PIWI-independent pathway outside of the reproductive apparatus.The pGFP-ZU sensor transgene driven by 24B-Gal4 is not expressed in larvae, pupae, or adult stages in a [S/S; piwi+/+] genetic background (right panel). Only a very faint level of fluorescence, corresponding to the background, is detected. A clear GFP expression is observed in these stages of development in a [U/U; piwi+/+] line (middle panel). In piwi mutant backgrounds, in homozygous [S/S; piwi3/3] lines, the silencing of the sensor transgene is not released. A very faint fluorescence level similar to that observed in homozygous [S/S; piwi+/+] lines is observed (left panel).
Mentions: Since the control exerted by COM is active in the somatic cells outside of the reproductive apparatus, the question arose then to know whether Piwi is also a component of this somatic silencing pathway. Indeed, in addition to its function in the reproductive apparatus of flies, diverse functions have been attributed to Piwi in somatic tissues at different stages of fly development [23]–[26]. Thus, GFP expression of the sensor transgenes pGFP-ZAM or pGFP-Id, driven by 24B-Gal4, was analyzed in larvae, pupae, and adult S flies mutated or not for the piwi gene. Piwi2/2, piwi3/3, and transheterozygous piwi3/2 mutations were tested in these experiments. The results obtained indicated that the silencing of the sensor transgenes in the somatic tissues of the fly outside of the ovaries does not depend on the presence of the Piwi protein. Indeed, in contrast to what was observed in the gonads, GFP expression was not recovered in the somatic tissues of S lines when the Piwi gene was mutated (see Figure 7 and data not shown).

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