RESUMO
A new class of small RNAs (endo-siRNAs) produced from endogenous double-stranded RNA (dsRNA) precursors was recently shown to mediate transposable element (TE) silencing in the Drosophila soma. These endo-siRNAs might play a role in heterochromatin formation, as has been shown in S. pombe for siRNAs derived from repetitive sequences in chromosome pericentromeres. To address this possibility, we used the viral suppressors of RNA silencing B2 and P19. These proteins normally counteract the RNAi host defense by blocking the biogenesis or activity of virus-derived siRNAs. We hypothesized that both proteins would similarly block endo-siRNA processing or function, thereby revealing the contribution of endo-siRNA to heterochromatin formation. Accordingly, P19 as well as a nuclear form of P19 expressed in Drosophila somatic cells were found to sequester TE-derived siRNAs whereas B2 predominantly bound their longer precursors. Strikingly, B2 or the nuclear form of P19, but not P19, suppressed silencing of heterochromatin gene markers in adult flies, and altered histone H3-K9 methylation as well as chromosomal distribution of histone methyl transferase Su(var)3-9 and Heterochromatin Protein 1 in larvae. Similar effects were observed in dcr2, r2d2, and ago2 mutants. Our findings provide evidence that a nuclear pool of TE-derived endo-siRNAs is involved in heterochromatin formation in somatic tissues in Drosophila.
Assuntos
Heterocromatina/metabolismo , RNA Interferente Pequeno/fisiologia , Animais , Animais Geneticamente Modificados , Cromossomos , Elementos de DNA Transponíveis/genética , Drosophila , Inativação Gênica , Marcadores Genéticos , Histonas/metabolismo , Metilação , RNA Interferente Pequeno/antagonistas & inibidoresAssuntos
Sequência Conservada , Epigênese Genética/genética , Evolução Molecular , Sequências Reguladoras de Ácido Nucleico/genética , Rombencéfalo/embriologia , Animais , Animais Geneticamente Modificados , Galinhas/genética , Proteínas de Ligação a DNA/genética , Proteínas de Ligação a DNA/fisiologia , Proteínas de Drosophila/genética , Proteínas de Drosophila/fisiologia , Drosophila melanogaster/genética , Desenvolvimento Embrionário/genética , Elementos Facilitadores Genéticos/genética , Humanos , Fator de Transcrição MafB/genética , Fator de Transcrição MafB/fisiologia , Mamíferos/genética , Proteínas de Membrana/genética , Proteínas de Membrana/fisiologia , Camundongos , Morfogênese/genética , Proteínas do Tecido Nervoso/genética , Proteínas do Tecido Nervoso/fisiologia , Proteínas Nucleares/genética , Proteínas Nucleares/fisiologia , Proteínas do Grupo Polycomb , Proteínas Repressoras/genética , Proteínas Repressoras/fisiologia , Inversão de Sequência , Fatores de Transcrição/genética , Fatores de Transcrição/fisiologiaRESUMO
The potential to produce new cells during adult life depends on the number of stem cell niches and the capacity of stem cells to divide, and is therefore under the control of programs ensuring developmental homeostasis. However, it remains generally unknown how the number of stem cell niches is controlled. In the insect ovary, each germline stem cell (GSC) niche is embedded in a functional unit called an ovariole. The number of ovarioles, and thus the number of GSC niches, varies widely among species. In Drosophila, morphogenesis of ovarioles starts in larvae with the formation of terminal filaments (TFs), each made of 8-10 cells that pile up and sort in stacks. TFs constitute organizers of individual germline stem cell niches during larval and early pupal development. In the Drosophila melanogaster subgroup, the number of ovarioles varies interspecifically from 8 to 20. Here we show that pipsqueak, Trithorax-like, batman and the bric-à-brac (bab) locus, all encoding nuclear BTB/POZ factors of the Tramtrack Group, are involved in limiting the number of ovarioles in D. melanogaster. At least two different processes are differentially perturbed by reducing the function of these genes. We found that when the bab dose is reduced, sorting of TF cells into TFs was affected such that each TF contains fewer cells and more TFs are formed. In contrast, psq mutants exhibited a greater number of TF cells per ovary, with a normal number of cells per TF, thereby leading to formation of more TFs per ovary than in the wild type. Our results indicate that two parallel genetic pathways under the control of a network of nuclear BTB factors are combined in order to negatively control the number of germline stem cell niches.