Biology subjects

Loppin, B.

Publications and source records attributed to Loppin, B..

5 recordsLinked to original sources

bioRxiv2024

Overdrive is essential for targeted sperm elimination by Segregation Distorter

Intra-genomic conflict driven by selfish chromosomes is a powerful force that shapes the evolution of genomes and species. In the male germline, many selfish chromosomes bias transmission in their own favor by eliminating spermatids bearing the competing homologous chromosomes. However, the mechanisms of targeted gamete elimination remain mysterious. Here, we show that Overdrive (Ovd), a gene required for both segregation distortion and male sterility in Drosophila pseudoobscura hybrids, is broadly conserved in Dipteran insects but dispensable for viability and fertility. In D. melanogaster, Ovd is required for targeted Responder spermatid elimination after the histone-to-protamine transition in the classical Segregation Distorter system. We propose that Ovd functions as a general spermatid quality checkpoint that is hijacked by independent selfish chromosomes to eliminate competing gametes.

genetics↗

bioRxiv2022

Biophysical ordering transitions underlie genome 3D re-organization during cricket spermiogenesis.

Spermiogenesis is a radical process of differentiation whereby sperm cells acquire a compact and specialized morphology to cope with the constraints of sexual reproduction while preserving their main cargo, an intact copy of the paternal genome. In animals, this often involves the replacement of most histones by sperm-specific nuclear basic proteins (SNBPs). Yet, how the SNBP-structured genome achieves compaction and accommodates shaping remain largely unknown. Here, we exploited confocal, electron and super-resolution microscopy observations, coupled with polymer modeling simulations to identify the higher-order architecture of sperm chromatin in the needle-shaped nucleus of the emerging model cricket Gryllus bimaculatus. Accompanying spermatid differentiation and shaping, the SNBP-based genome was strikingly reorganized as ~25nm-thick fibers orderly coiled along the elongated nucleus axis. This chromatin spool was further found to achieve large-scale helical twisting in the final stages of spermiogenesis, favoring its ultracompaction. Through a combination of microscopy observations and polymer simulations, we revealed that these dramatic transitions may be recapitulated by a surprisingly simple biophysical principle based on a nucleated rigidification of chromatin linked to the histone-to-SNBP transition within a confined nuclear space. Our work highlights a unique, liquid crystal-like mode of higher-order genome organization in ultracompact cricket sperm completely distinct from nucleosomal chromatin, and establishes a multidisciplinary methodological framework to explore the diversity of non-canonical modes of DNA organization. SIGNIFICANCE STATEMENTAnimal sperm cells are highly compact and atypically shaped compared to other cell types. How DNA is packaged and organized in the 3D space of sperm cell nuclei to cope with these constraints is poorly understood. In this work, we identified an original and elegant solution to this problem in crickets, whereby DNA fibers orderly spool and twist to fit into ultracompact, needle-shaped sperm cells. To understand this reorganization, we modeled DNA fibers in the nucleus as polymers and found that a relatively simple mechanism through which fibers become more rigid bit by bit can largely recapitulate our observations. Our multidisciplinary work highlights a simple solution to compact DNA to extreme levels in specialized nuclei.

cell biology↗

bioRxiv2021

Three classes of epigenomic regulators converge to hyperactivate the essential maternal gene deadhead within a heterochromatin mini-domain.

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 SUMMARYGene 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.

genetics↗

bioRxiv2021

Distinct spermiogenic phenotypes underlie sperm elimination in the Segregation Distorter meiotic drive system

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.

genetics↗

bioRxiv2019

The Lid/KDM5 histone demethylase complex activates a critical effector of the oocyte-to-zygote transition

Following fertilization of a mature oocyte, the formation of a diploid zygote involves a series of coordinated cellular events that ends with the first embryonic mitosis. In animals, this complex developmental transition is almost entirely controlled by maternal gene products. How such a crucial transcriptional program is established during oogenesis remains poorly understood. Here, we have performed an shRNA-based genetic screen in Drosophila to identify genes required to form a diploid zygote. We found that the Lid/KDM5 histone demethylase and its partner, the Sin3A-HDAC1 deacetylase complex, are necessary for sperm nuclear decompaction and karyogamy. Surprisingly, transcriptomic analyses revealed that these histone modifiers are required for the massive transcriptional activation of deadhead (dhd), which encodes a maternal thioredoxin involved in sperm chromatin remodeling. Unexpectedly, while lid knock-down tends to slightly favor the accumulation of its target, H3K4me3, on the genome, this mark was lost at the dhd locus. We propose that Lid/KDM5 and Sin3A cooperate to establish a local chromatin environment facilitating the unusually high expression of dhd, a key effector of the oocyte-to-zygote transition.

genetics↗