ABSTRACT The formation of a diploid zygote is a highly complex cellular process that is entirely controlled by maternal gene products stored in the egg cytoplasm. This highly specialized transcriptional program is tightly controlled at the chromatin level in the female germline. As an extreme case in point, the massive and specific ovarian expression of the essential thioredoxin Deadhead (DHD) is critically regulated in Drosophila by the histone demethylase Lid and its partner, the histone deacetylase complex scaffold Sin3A, via yet unknown mechanisms. Here, we identified the Brahma chromatin remodeler sub-unit Snr1 and the insulator component Mod(mdg4) as essential for dhd expression and investigated how these epigenomic effectors act with Lid and Sin3A to hyperactivate dhd . Using Cut&Run chromatin profiling with a dedicated data analysis procedure, we found that dhd is intriguingly embedded in an H3K27me3/H3K9me3-enriched mini-domain flanked by DNA regulatory elements, including a dhd promoter-proximal element essential for its expression. Surprisingly, Lid, Sin3A, Snr1 and Mod(mdg4) impact H3K27me3 and this regulatory element in distinct manners. However, we show that these effectors activate dhd independently of H3K27me3/H3K9me3, and that these marks are not required to repress dhd . Together, our study demonstrates an atypical and critical role for chromatin regulators Lid, Sin3A, Snr1 and Mod(mdg4) to trigger tissue-specific hyperactivation within a unique heterochromatin mini-domain. AUTHOR SUMMARY Gene expression is tightly regulated by conserved protein complexes that act at the chromatin level to allow or restrict transcription. Such epigenetic control of gene activity defines the identity of different cell types during development, as well as their response to environmental cues. Yet, how multiple chromatin factors converge to achieve precise gene regulation remains difficult to address, partly due to the lack of biological situations where these intricate relationships can be studied. In this paper, we have addressed this issue by dissecting the regulation of deadhead , an essential gene specifically and massively expressed in the Drosophila germline. Unexpectedly, we found that its hyperactivation occurs despite deadhead being embedded in an apparently unfavorable chromatin mini-domain, notably featuring repressive histone modifications. We further demonstrate that four chromatin effectors, Lid, Sin3A, Snr1 and Mod(mdg4), have distinct, atypical and essential roles to ensure deadhead expression within this chromatin environment. Together, our findings put into perspective our understanding on these regulatory factors by illustrating how they can exert a biologically essential function via non-canonical mechanisms.
In Drosophila melanogaster, as in many animal and plant species, centromere identity is specified epigenetically. In proliferating cells, a centromere-specific histone H3 variant (CenH3), named Cid in Drosophila and Cenp-A in humans, is a crucial component of the epigenetic centromere mark. Hence, maintenance of the amount and chromosomal location of CenH3 during mitotic proliferation is important. Interestingly, CenH3 may have different roles during meiosis and the onset of embryogenesis. In gametes of Caenorhabditis elegans, and possibly in plants, centromere marking is independent of CenH3. Moreover, male gamete differentiation in animals often includes global nucleosome for protamine exchange that potentially could remove CenH3 nucleosomes. Here we demonstrate that the control of Cid loading during male meiosis is distinct from the regulation observed during the mitotic cycles of early embryogenesis. But Cid is present in mature sperm. After strong Cid depletion in sperm, paternal centromeres fail to integrate into the gonomeric spindle of the first mitosis, resulting in gynogenetic haploid embryos. Furthermore, after moderate depletion, paternal centromeres are unable to re-acquire normal Cid levels in the next generation. We conclude that Cid in sperm is an essential component of the epigenetic centromere mark on paternal chromosomes and it exerts quantitative control over centromeric Cid levels throughout development. Hence, the amount of Cid that is loaded during each cell cycle appears to be determined primarily by the preexisting centromeric Cid, with little flexibility for compensation of accidental losses.
> L’architecture du noyau eucaryote est extremement variable selon le type cellulaire. La structure intime de la chromatine est en revanche remarquablement conservee a l’echelle de son unite fonctionnelle, le nucleosome, particule resultant de l’assemblage des quatre types d’histones (H2A, H2B, H3 et H4) et de la molecule d’ADN. Chez de nombreuses especes, le noyau du spermatozoide deroge a cette regle fondamentale en adoptant une organisation de sa chromatine radicalement differente. Selon les especes, les histones sont totalement ou partiellement remplacees dans le noyau du gamete mâle par des petites proteines chromosomiques tres basiques appelees protamines. Les protamines permettent a l’ADN d’atteindre un niveau de compaction tres eleve. A la fecondation, lorsque le noyau du spermatozoide est libere dans le cytoplasme de l’œuf, la chromatine paternelle doit rapidement eliminer ses protamines pour reacquerir une organisation en nucleosomes essentielle pour le developpement de l’embryon. Nous avons identifie une mutation chez la drosophile, appelee sesame, qui affecte tres specifiquement ce processus [1]. Dans les œufs pondus par des femelles mutantes, le noyau du spermatozoide provenant d’un mâle sauvage est incapable de decondenser sa chromatine. Le pronucleus mâle, anormalement condense, est alors exclu de la premiere mitose zygotique et les embryons ne se developpent qu’avec le seul jeu de chromosomes maternels. Nous avons recemment decouvert que la mutation sesame affecte le gene Hira qui code un facteur d’assemblage de la chromatine present chez tous les eucaryotes [2]. La mutation induit le remplacement d’un seul residu tres conserve, dans un domaine de HIRA predit pour interagir avec d’autres proteines. Les proprietes d’assemblage de la chromatine du facteur HIRA ont ete recemment caracterisees in vitro par le groupe de Genevieve Almouzni et ses collaborateurs [3]. HIRA fait ainsi partie d’un complexe proteique capable d’assembler des nucleosomes independamment de la synthese d’ADN, par opposition au complexe CAF-1, dont la fonction est d’assembler les nucleosomes sur l’ADN en cours de replication ou de reparation. Dans un œuf sauvage de drosophile, le noyau du spermatozoide doit assembler sa chromatine en remplacant ses protamines par des histones qui sont fournies par l’œuf. Ce processus, qui s’accompagne de la decondensation de la chromatine paternelle, intervient bien avant que le pronucleus mâle ne replique son ADN, ce qu’il fait au moment ou il rejoint son partenaire femelle. La formation du pronucleus mâle releve donc d’un mode d’assemblage independant de la HIRA, une molecule de l’œuf qui controle la formation du pronucleus mâle
The union of haploid gametes at fertilization initiates the formation of the diploid zygote in sexually reproducing animals. This founding event of embryogenesis includes several fascinating cellular and nuclear processes, such as sperm–egg cellular interactions, sperm chromatin remodelling, centrosome formation or pronuclear migration. In comparison with other aspects of development, the exploration of animal fertilization at the functional level has remained so far relatively limited, even in classical model organisms. Here, we have reviewed our current knowledge of fertilization in Drosophila melanogaster , with a special emphasis on the genes involved in the complex transformation of the fertilizing sperm nucleus into a replicated set of paternal chromosomes.
Abstract Segregation Distorter ( SD ) is a male meiotic drive system in Drosophila melanogaster. Males heterozygous for a selfish SD chromosome rarely transmit the homologous SD + chromosome. It is well established that distortion results from an interaction between Sd , the primary distorting locus on the SD chromosome and its target, a satellite DNA called Rsp, on the SD + chromosome. However, the molecular and cellular mechanisms leading to post-meiotic SD + sperm elimination remain unclear. Here we show that SD/SD + males of different genotypes but with similarly strong degrees of distortion have distinct spermiogenic phenotypes. In some genotypes, SD + spermatids fail to fully incorporate protamines after the removal of histones, and degenerate during the individualization stage of spermiogenesis. In contrast, in other SD/SD + genotypes, protamine incorporation appears less disturbed, yet spermatid nuclei are abnormally compacted, and mature sperm nuclei are eventually released in the seminal vesicle. Our analyses of different SD + chromosomes suggest that the severity of the spermiogenic defects associates with the copy number of the Rsp satellite. We propose that when Rsp copy number is very high (> 2000), spermatid nuclear compaction defects reach a threshold that triggers a checkpoint controlling sperm chromatin quality to eliminate abnormal spermatids during individualization.
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