Intergenerationally Maintained Histone H4 Lysine 16 Acetylation Is Instructive for Future Gene Activation.

Before zygotic genome activation (ZGA), the quiescent genome undergoes reprogramming to transition into the transcriptionally active state. However, the mechanisms underlying euchromatin establishment during early embryogenesis remain poorly understood. Here, we show that histone H4 lysine 16 acetylation (H4K16ac) is maintained from oocytes to fertilized embryos in Drosophila and mammals. H4K16ac forms large domains that control nucleosome accessibility of promoters prior to ZGA in flies. Maternal depletion of MOF acetyltransferase leading to H4K16ac loss causes aberrant RNA Pol II recruitment, compromises the 3D organization of the active genomic compartments during ZGA, and causes downregulation of post-zygotically expressed genes. Germline depletion of histone deacetylases revealed that other acetyl marks cannot compensate for H4K16ac loss in the oocyte. Moreover, zygotic re-expression of MOF was neither able to restore embryonic viability nor onset of X chromosome dosage compensation. Thus, maternal H4K16ac provides an instructive function to the offspring, priming future gene activation.

Alternating between even and odd ploidy levels switches on and off the recombination control, even near the centromeres.

Meiotic recombination is a key biological process in plant evolution and breeding, as it generates genetic diversity in each generation through the formation of crossovers (COs). However, due to their importance in genome stability, COs are highly regulated in frequency and distribution. We previously demonstrated that this strict regulation of COs can be modified, both in terms of CO frequency and distribution, in allotriploid Brassica hybrids (2n = 3x = 29; AAC) resulting from a cross between Brassica napus (2n = 4x = 38; AACC) and Brassica rapa (2n = 2x = 20; AA). Using the recently updated B. napus genome now including pericentromeres, we demonstrated that COs occur in these cold regions in allotriploids, as close as 375 kb from the centromere. Reverse transcription quantitative PCR (RT-qPCR) of various meiotic genes indicated that Class I COs are likely involved in the increased recombination frequency observed in allotriploids. We also demonstrated that this modified recombination landscape can be maintained via successive generations of allotriploidy (odd ploidy level). This deregulated meiotic behavior reverts to strict regulation in allotetraploid (even ploidy level) progeny in the second generation. Overall, we provide an easy way to manipulate tight recombination control in a polyploid crop.

Hi-C guided assemblies reveal conserved regulatory topologies on X and autosomes despite extensive genome shuffling.

Genome rearrangements that occur during evolution impose major challenges on regulatory mechanisms that rely on three-dimensional genome architecture. Here, we developed a scaffolding algorithm and generated chromosome-length assemblies from Hi-C data for studying genome topology in three distantly related Drosophila species. We observe extensive genome shuffling between these species with one synteny breakpoint after approximately every six genes. A/B compartments, a set of large gene-dense topologically associating domains (TADs), and spatial contacts between high-affinity sites (HAS) located on the X chromosome are maintained over 40 million years, indicating architectural conservation at various hierarchies. Evolutionary conserved genes cluster in the vicinity of HAS, while HAS locations appear evolutionarily flexible, thus uncoupling functional requirement of dosage compensation from individual positions on the linear X chromosome. Therefore, 3D architecture is preserved even in scenarios of thousands of rearrangements highlighting its relevance for essential processes such as dosage compensation of the X chromosome.

Characterization of Influenza D Virus Reassortant Strain in Swine from Mixed Pig and Beef Farm, France.

Influenza D virus was isolated from pigs on a mixed pig and beef farm in France. Investigation suggested bull-to-pig transmission and spread among pigs. The swine influenza D virus recovered was a reassortant of D/660 and D/OK lineages. Reported mutations in the receptor binding site might be related to swine host adaptation.

Pathogenicity and escape to pre-existing immunity of a new genotype of swine influenza H1N2 virus that emerged in France in 2020.

In 2020, a new genotype of swine H1N2 influenza virus (H1avN2-HA 1C.2.4) was identified in France. It rapidly spread within the pig population and supplanted the previously predominant H1avN1-HA 1C.2.1 virus. To characterize this new genotype which is genetically and antigenically distant from the other H1avNx viruses detected in France, an experimental study was conducted to compare the outcomes of H1avN2 and H1avN1 infections in pigs and evaluate the protection conferred by the only inactivated vaccine currently licensed in Europe containing an HA 1C (clade 1C.2.2) antigen. Infection with H1avN2 induced stronger clinical signs and earlier shedding than H1avN1. The neutralizing antibodies produced following H1avN2 infection were unable to neutralize H1avN1, and vice versa, whereas the cellular-mediated immunity cross-reacted. Vaccination slightly altered the impact of H1avN2 infection at the clinical level, but did not prevent shedding of infectious virus particles. It induced a cellular-mediated immune response towards H1avN2, but did not produce neutralizing antibodies against this virus. As in vaccinated animals, animals previously infected by H1avN1 developed a cross-reacting cellular immune response but no neutralizing antibodies against H1avN2. However, H1avN1 pre-infection induced a better protection against the H1avN2 infection than vaccination, probably due to higher levels of non-neutralizing antibodies and a mucosal immunity. Altogether, these results showed that the new H1avN2 genotype induced a severe respiratory infection and that the actual vaccine was less effective against this H1avN2-HA 1C.2.4 than against H1avN1-HA 1C.2.1, which may have contributed to the H1avN2 epizootic and dissemination in pig farms in France.

Epigenomic and structural events preclude recombination in Brassica napus.

Meiotic recombination is a major evolutionary process generating genetic diversity at each generation in sexual organisms. However, this process is highly regulated, with the majority of crossovers lying in the distal chromosomal regions that harbor low DNA methylation levels. Even in these regions, some islands without recombination remain, for which we investigated the underlying causes. Genetic maps were established in two Brassica napus hybrids to detect the presence of such large nonrecombinant islands. The role played by DNA methylation and structural variations in this local absence of recombination was determined by performing bisulfite sequencing and whole genome comparisons. Inferred structural variations were validated using either optical mapping or oligo fluorescence in situ hybridization. Hypermethylated or inverted regions between Brassica genomes were associated with the absence of recombination. Pairwise comparisons of nine B. napus genome assemblies revealed that such inversions occur frequently and may contain key agronomic genes such as resistance to biotic stresses. We conclude that such islands without recombination can have different origins, such as DNA methylation or structural variations in B. napus. It is thus essential to take into account these features in breeding programs as they may hamper the efficient combination of favorable alleles in elite varieties.

Galaxy HiCExplorer 3: a web server for reproducible Hi-C, capture Hi-C and single-cell Hi-C data analysis, quality control and visualization.

The Galaxy HiCExplorer provides a web service at https://hicexplorer.usegalaxy.eu. It enables the integrative analysis of chromosome conformation by providing tools and computational resources to pre-process, analyse and visualize Hi-C, Capture Hi-C (cHi-C) and single-cell Hi-C (scHi-C) data. Since the last publication, Galaxy HiCExplorer has been expanded considerably with new tools to facilitate the analysis of cHi-C and to provide an in-depth analysis of Hi-C data. Moreover, it supports the analysis of scHi-C data by offering a broad range of tools. With the help of the standard graphical user interface of Galaxy, presented workflows, extensive documentation and tutorials, novices as well as Hi-C experts are supported in their Hi-C data analysis with Galaxy HiCExplorer.

The transposable element-rich genome of the cereal pest Sitophilus oryzae.

BACKGROUND: The rice weevil Sitophilus oryzae is one of the most important agricultural pests, causing extensive damage to cereal in fields and to stored grains. S. oryzae has an intracellular symbiotic relationship (endosymbiosis) with the Gram-negative bacterium Sodalis pierantonius and is a valuable model to decipher host-symbiont molecular interactions. RESULTS: We sequenced the Sitophilus oryzae genome using a combination of short and long reads to produce the best assembly for a Curculionidae species to date. We show that S. oryzae has undergone successive bursts of transposable element (TE) amplification, representing 72% of the genome. In addition, we show that many TE families are transcriptionally active, and changes in their expression are associated with insect endosymbiotic state. S. oryzae has undergone a high gene expansion rate, when compared to other beetles. Reconstruction of host-symbiont metabolic networks revealed that, despite its recent association with cereal weevils (30 kyear), S. pierantonius relies on the host for several amino acids and nucleotides to survive and to produce vitamins and essential amino acids required for insect development and cuticle biosynthesis. CONCLUSIONS: Here we present the genome of an agricultural pest beetle, which may act as a foundation for pest control. In addition, S. oryzae may be a useful model for endosymbiosis, and studying TE evolution and regulation, along with the impact of TEs on eukaryotic genomes.

Selection following Gene Duplication Shapes Recent Genome Evolution in the Pea Aphid Acyrthosiphon pisum.

Ecology of insects is as wide as their diversity, which reflects their high capacity of adaptation in most of the environments of our planet. Aphids, with over 4,000 species, have developed a series of adaptations including a high phenotypic plasticity and the ability to feed on the phloem sap of plants, which is enriched in sugars derived from photosynthesis. Recent analyses of aphid genomes have indicated a high level of shared ancestral gene duplications that might represent a basis for genetic innovation and broad adaptations. In addition, there are a large number of recent, species-specific gene duplications whose role in adaptation remains poorly understood. Here, we tested whether duplicates specific to the pea aphid Acyrthosiphon pisum are related to genomic innovation by combining comparative genomics, transcriptomics, and chromatin accessibility analyses. Consistent with large levels of neofunctionalization, we found that most of the recent pairs of gene duplicates evolved asymmetrically, showing divergent patterns of positive selection and gene expression. Genes under selection involved a plethora of biological functions, suggesting that neofunctionalization and tissue specificity, among other evolutionary mechanisms, have orchestrated the evolution of recent paralogs in the pea aphid and may have facilitated host-symbiont cooperation. Our comprehensive phylogenomics analysis allowed us to tackle the history of duplicated genes to pave the road toward understanding the role of gene duplication in ecological adaptation.

Galaxy HiCExplorer: a web server for reproducible Hi-C data analysis, quality control and visualization.

Galaxy HiCExplorer is a web server that facilitates the study of the 3D conformation of chromatin by allowing Hi-C data processing, analysis and visualization. With the Galaxy HiCExplorer web server, users with little bioinformatic background can perform every step of the analysis in one workflow: mapping of the raw sequence data, creation of Hi-C contact matrices, quality assessment, correction of contact matrices and identification of topological associated domains (TADs) and A/B compartments. Users can create publication ready plots of the contact matrix, A/B compartments, and TADs on a selected genomic locus, along with additional information like gene tracks or ChIP-seq signals. Galaxy HiCExplorer is freely usable at: https://hicexplorer.usegalaxy.eu and is available as a Docker container: https://github.com/deeptools/docker-galaxy-hicexplorer.

Hypoxia response in Arabidopsis roots infected by Plasmodiophora brassicae supports the development of clubroot.

BACKGROUND: The induction of alcohol fermentation in roots is a plant adaptive response to flooding stress and oxygen deprivation. Available transcriptomic data suggest that fermentation-related genes are also frequently induced in roots infected with gall forming pathogens, but the biological significance of this induction is unclear. In this study, we addressed the role of hypoxia responses in Arabidopsis roots during infection by the clubroot agent Plasmodiophora brassicae. RESULTS: The hypoxia-related gene markers PYRUVATE DECARBOXYLASE 1 (PDC1), PYRUVATE DECARBOXYLASE 2 (PDC2) and ALCOHOL DEHYDROGENASE 1 (ADH1) were induced during secondary infection by two isolates of P. brassicae, eH and e2. PDC2 was highly induced as soon as 7 days post inoculation (dpi), i.e., before the development of gall symptoms, and GUS staining revealed that ADH1 induction was localised in infected cortical cells of root galls at 21 dpi. Clubroot symptoms were significantly milder in the pdc1 and pdc2 mutants compared with Col-0, but a null T-DNA insertional mutation of ADH1 did not affect clubroot susceptibility. The Arg/N-end rule pathway of ubiquitin-mediated proteolysis controls oxygen sensing in plants. Mutants of components of this pathway, ate1 ate2 and prt6, that both exhibit constitutive hypoxia responses, showed enhanced clubroot symptoms. In contrast, gall development was reduced in quintuple and sextuple mutants where the activity of all oxygen-sensing Group VII Ethylene Response Factor transcription factors (ERFVIIs) is absent (erfVII and prt6 erfVII). CONCLUSIONS: Our data demonstrate that the induction of PDC1 and PDC2 during the secondary infection of roots by P. brassicae contributes positively to clubroot development, and that this is controlled by oxygen-sensing through ERFVIIs. The absence of any major role of ADH1 in symptom development may also suggest that PDC activity could contribute to the formation of galls through the activation of a PDH bypass.

Contribution of Epigenetic Mechanisms in the Regulation of Environmentally-Induced Polyphenism in Insects.

Many insect species display a remarkable ability to produce discrete phenotypes in response to changes in environmental conditions. Such phenotypic plasticity is referred to as polyphenism. Seasonal, dispersal and caste polyphenisms correspond to the most-studied examples that are environmentally-induced in insects. Cues that induce such dramatic phenotypic changes are very diverse, ranging from seasonal cues, habitat quality changes or differential larval nutrition. Once these signals are perceived, they are transduced by the neuroendocrine system towards their target tissues where gene expression reprogramming underlying phenotypic changes occur. Epigenetic mechanisms are key regulators that allow for genome expression plasticity associated with such developmental switches. These mechanisms include DNA methylation, chromatin remodelling and histone post-transcriptional modifications (PTMs) as well as non-coding RNAs and have been studied to various extents in insect polyphenism. Differential patterns of DNA methylation between phenotypes are usually correlated with changes in gene expression and alternative splicing events, especially in the cases of dispersal and caste polyphenism. Combinatorial patterns of histone PTMs provide phenotype-specific epigenomic landscape associated with the expression of specific transcriptional programs, as revealed during caste determination in honeybees and ants. Alternative phenotypes are also usually associated with specific non-coding RNA profiles. This review will provide a summary of the current knowledge of the epigenetic changes associated with polyphenism in insects and highlights the potential for these mechanisms to be key regulators of developmental transitions triggered by environmental cues.

Long-read assembly of the Brassica napus reference genome Darmor-bzh.

BACKGROUND: The combination of long reads and long-range information to produce genome assemblies is now accepted as a common standard. This strategy not only allows access to the gene catalogue of a given species but also reveals the architecture and organization of chromosomes, including complex regions such as telomeres and centromeres. The Brassica genus is not exempt, and many assemblies based on long reads are now available. The reference genome for Brassica napus, Darmor-bzh, which was published in 2014, was produced using short reads and its contiguity was extremely low compared with current assemblies of the Brassica genus. FINDINGS: Herein, we report the new long-read assembly of Darmor-bzh genome (Brassica napus) generated by combining long-read sequencing data and optical and genetic maps. Using the PromethION device and 6 flowcells, we generated ∼16 million long reads representing 93× coverage and, more importantly, 6× with reads longer than 100 kb. This ultralong-read dataset allows us to generate one of the most contiguous and complete assemblies of a Brassica genome to date (contig N50 > 10 Mb). In addition, we exploited all the advantages of the nanopore technology to detect modified bases and sequence transcriptomic data using direct RNA to annotate the genome and focus on resistance genes. CONCLUSION: Using these cutting-edge technologies, and in particular by relying on all the advantages of the nanopore technology, we provide the most contiguous Brassica napus assembly, a resource that will be valuable to the Brassica community for crop improvement and will facilitate the rapid selection of agronomically important traits.

Epigenetics and insect polyphenism: mechanisms and climate change impacts.

Phenotypic plasticity is a ubiquitous process found in all living organisms. Polyphenism is an extreme case of phenotypic plasticity which shares a common scheme in insects such as honeybees, locusts or aphids: an initial perception of environmental stimuli, a neuroendocrine transmission of these signals to the target tissues, the activation of epigenetic mechanisms allowing the setup of alternative transcriptional programs responsible for the establishment of discrete phenotypes. Climate change can modulate the environmental stimuli triggering polyphenisms, and/or some epigenetics marks, thus modifying on the short and long terms the discrete phenotype proportions within populations. This might result in critical ecosystem changes.

Dosage compensation and sex-specific epigenetic landscape of the X chromosome in the pea aphid.

BACKGROUND: Heterogametic species display a differential number of sex chromosomes resulting in imbalanced transcription levels for these chromosomes between males and females. To correct this disequilibrium, dosage compensation mechanisms involving gene expression and chromatin accessibility regulations have emerged throughout evolution. In insects, these mechanisms have been extensively characterized only in Drosophila but not in insects of agronomical importance. Aphids are indeed major pests of a wide range of crops. Their remarkable ability to switch from asexual to sexual reproduction during their life cycle largely explains the economic losses they can cause. As heterogametic insects, male aphids are X0, while females (asexual and sexual) are XX. RESULTS: Here, we analyzed transcriptomic and open chromatin data obtained from whole male and female individuals to evaluate the putative existence of a dosage compensation mechanism involving differential chromatin accessibility of the pea aphid's X chromosome. Transcriptomic analyses first showed X/AA and XX/AA expression ratios for expressed genes close to 1 in males and females, respectively, suggesting dosage compensation in the pea aphid. Analyses of open chromatin data obtained by Formaldehyde-Assisted Isolation of Regulatory Elements (FAIRE-seq) revealed a X chromosome chromatin accessibility globally and significantly higher in males than in females, while autosomes' chromatin accessibility is similar between sexes. Moreover, chromatin environment of X-linked genes displaying similar expression levels in males and females-and thus likely to be compensated-is significantly more accessible in males. CONCLUSIONS: Our results suggest the existence of an underlying epigenetic mechanism enhancing the X chromosome chromatin accessibility in males to allow X-linked gene dose correction between sexes in the pea aphid, similar to Drosophila. Our study gives new evidence into the comprehension of dosage compensation in link with chromatin biology in insects and newly in a major crop pest, taking benefits from both transcriptomic and open chromatin data.