High-quality genome (re)assembly using chromosomal contact data

A deep learning-based method enables the automatic and accurate assembly of chromosome-level genomes

The application of high-throughput chromosome conformation capture (Hi-C) technology enables the construction of chromosome-level assemblies. However, the correction of errors and the anchoring of sequences to chromosomes in the assembly remain significant challenges. In this study, we developed a deep learning-based method, AutoHiC, to address the challenges in chromosome-level genome assembly by enhancing contiguity and accuracy. Conventional Hi-C-aided scaffolding often requires manual refinement, but AutoHiC instead utilizes Hi-C data for automated workflows and iterative error correction. When trained on data from 300+ species, AutoHiC demonstrated a robust average error detection accuracy exceeding 90%. The benchmarking results confirmed its significant impact on genome contiguity and error correction. The innovative approach and comprehensive results of AutoHiC constitute a breakthrough in automated error detection, promising more accurate genome assemblies for advancing genomics research.

Hi-C techniques: from genome assemblies to transcription regulation

The invention of chromosome conformation capture (3C) techniques, in particular the key method Hi-C providing genome-wide information about chromatin contacts, revolutionized the way we study the three-dimensional organization of the nuclear genome and how it affects transcription, replication, and DNA repair. Because the frequency of chromatin contacts between pairs of genomic segments predictably relates to the distance in the linear genome, the information obtained by Hi-C has also proved useful for scaffolding genomic sequences. Here, we review recent improvements in experimental procedures of Hi-C and its various derivatives, such as Micro-C, HiChIP, and Capture Hi-C. We assess the advantages and limitations of the techniques, and present examples of their use in recent plant studies. We also report on progress in the development of computational tools used in assembling genome sequences.

Unique territorial and compartmental organization of chromosomes in the holocentric silkmoth

The hallmarks of chromosome organization in multicellular eukaryotes are chromosome territories (CT), chromatin compartments, and insulated domains, including topologically associated domains (TADs). Yet, most of these elements of chromosome organization are derived from analyses of a limited set of model organisms, while large eukaryotic groups, including insects, remain mostly unexplored. Here we combine Hi-C, biophysical modeling, and microscopy to characterize the 3D genome architecture of the silkmoth, Bombyx mori. In contrast to other eukaryotes, B. mori chromosomes form highly separated territories. Similar to other eukaryotes, B. mori chromosomes segregate into active A and inactive B compartments, yet unlike in vertebrate systems, contacts between euchromatic A regions appear to be a strong driver of compartmentalization. Remarkably, we also identify a third compartment, called secluded "S," with a unique contact pattern. Each S region shows prominent short-range self-contacts and is remarkably devoid of contacts with the rest of the chromosome, including other S regions. Compartment S hosts a unique combination of genetic and epigenetic features, localizes towards the periphery of CTs, and shows developmental plasticity. Biophysical modeling reveals that the formation of such secluded domains requires highly localized loop extrusion within them, along with a low level of extrusion in A and B. Our Hi-C data supports predicted genome-wide and localized extrusion. Such a broad, non-uniform distribution of extruders has not been seen in other organisms. Overall, our analyses support loop extrusion in insects and highlight the evolutionary plasticity of 3D genome organization, driven by a new combination of known processes.

Unique territorial and compartmental organization of chromosomes in the holocentric silkmoth

The hallmarks of chromosome organization in multicellular eukaryotes are chromosome territories (CT), chromatin compartments, and insulated domains, including topologically associated domains (TADs). Yet, most of these elements of chromosome organization are derived from analyses of a limited set of model organisms, while large eukaryotic groups, including insects, remain mostly unexplored. Here we combine Hi-C, biophysical modeling, and microscopy to characterize the 3D genome architecture of the silkworm, Bombyx mori. In contrast to other eukaryotes, B. mori chromosomes form highly separated territories. Similar to other eukaryotes, B. mori chromosomes segregate into active A and inactive B compartments, yet unlike in vertebrate systems, contacts between euchromatic A regions appear to be a strong driver of compartmentalization. Remarkably, we also identify a third compartment, called secluded S, with a unique contact pattern. Each S region shows prominent short-range self-contacts and is remarkably devoid of contacts with the rest of the chromosome, including other S regions. Compartment S hosts a unique combination of genetic and epigenetic features, localizes towards the periphery of CTs, and shows developmental plasticity. Biophysical modeling reveals that the formation of such secluded domains requires highly localized loop extrusion within them, along with a low level of extrusion in A and B. Our Hi-C data supports predicted genome-wide and localized extrusion. Such a broad, non-uniform distribution of extruders has not been seen in other organisms. Overall, our analyses support loop extrusion in insects and highlight the evolutionary plasticity of 3D genome organization, driven by a new combination of known processes.

Chromosome-scale assembly of the streamlined picoeukaryote Picochlorum sp. SENEW3 genome reveals Rabl-like chromatin structure and potential for C4 photosynthesis

Genome sequencing and assembly of the photosynthetic picoeukaryotic Picochlorum sp. SENEW3 revealed a compact genome with a reduced gene set, few repetitive sequences, and an organized Rabl-like chromatin structure. Hi-C chromosome conformation capture revealed evidence of possible chromosomal translocations, as well as putative centromere locations. Maintenance of a relatively few selenoproteins, as compared to similarly sized marine picoprasinophytes Mamiellales, and broad halotolerance compared to others in Trebouxiophyceae, suggests evolutionary adaptation to variable salinity environments. Such adaptation may have driven size and genome minimization and have been enabled by the retention of a high number of membrane transporters. Identification of required pathway genes for both CAM and C4 photosynthetic carbon fixation, known to exist in the marine mamiellale pico-prasinophytes and seaweed Ulva, but few other chlorophyte species, further highlights the unique adaptations of this robust alga. This high-quality assembly provides a significant advance in the resources available for genomic investigations of this and other photosynthetic picoeukaryotes.

Subtelomeric plasticity contributes to gene family expansion in the human parasitic flatworm Schistosoma mansoni

BACKGROUND

The genomic region that lies between the telomere and chromosome body, termed the subtelomere, is heterochromatic, repeat-rich, and frequently undergoes rearrangement. Within this region, large-scale structural changes enable gene diversification, and, as such, large multicopy gene families are often found at the subtelomere. In some parasites, genes associated with proliferation, invasion, and survival are often found in these regions, where they benefit from the subtelomere's highly plastic, rapidly changing nature. The increasing availability of complete (or near complete) parasite genomes provides an opportunity to investigate these typically poorly defined and overlooked genomic regions and potentially reveal relevant gene families necessary for the parasite's lifestyle.

RESULTS

Using the latest chromosome-scale genome assembly and hallmark repeat richness observed at chromosome termini, we have identified and characterised the subtelomeres of Schistosoma mansoni, a metazoan parasitic flatworm that infects over 250 million people worldwide. Approximately 12% of the S. mansoni genome is classified as subtelomeric, and, in line with other organisms, we find these regions to be gene-poor but rich in transposable elements. We find that S. mansoni subtelomeres have undergone extensive interchromosomal recombination and that these sites disproportionately contribute to the 2.3% of the genome derived from segmental duplications. This recombination has led to the expansion of subtelomeric gene clusters containing 103 genes, including the immunomodulatory annexins and other gene families with unknown roles. The largest of these is a 49-copy plexin domain-containing protein cluster, exclusively expressed in the tegument-the tissue located at the host-parasite physical interface-of intramolluscan life stages.

CONCLUSIONS

We propose that subtelomeric regions act as a genomic playground for trial-and-error of gene duplication and subsequent divergence. Owing to the importance of subtelomeric genes in other parasites, gene families implicated in this subtelomeric expansion within S. mansoni warrant further characterisation for a potential role in parasitism.

Orchestrating chromosome conformation capture analysis with Bioconductor

Genome-wide chromatin conformation capture assays provide formidable insights into the spatial organization of genomes. However, due to the complexity of the data structure, their integration in multi-omics workflows remains challenging. We present data structures, computational methods and visualization tools available in Bioconductor to investigate Hi-C, micro-C and other 3C-related data, in R. An online book ( https://bioconductor.org/books/OHCA/ ) further provides prospective end users with a number of workflows to process, import, analyze and visualize any type of chromosome conformation capture data.

Repeatability of adaptation in sunflowers reveals that genomic regions harbouring inversions also drive adaptation in species lacking an inversion

Local adaptation commonly involves alleles of large effect, which experience fitness advantages when in positive linkage disequilibrium (LD). Because segregating inversions suppress recombination and facilitate the maintenance of LD between locally adapted loci, they are also commonly found to be associated with adaptive divergence. However, it is unclear what fraction of an adaptive response can be attributed to inversions and alleles of large effect, and whether the loci within an inversion could still drive adaptation in the absence of its recombination-suppressing effect. Here, we use genome-wide association studies to explore patterns of local adaptation in three species of sunflower: Helianthus annuus, Helianthus argophyllus, and Helianthus petiolaris, which each harbour a large number of species-specific inversions. We find evidence of significant genome-wide repeatability in signatures of association to phenotypes and environments, which are particularly enriched within regions of the genome harbouring an inversion in one species. This shows that while inversions may facilitate local adaptation, at least some of the loci can still harbour mutations that make substantial contributions without the benefit of recombination suppression in species lacking a segregating inversion. While a large number of genomic regions show evidence of repeated adaptation, most of the strongest signatures of association still tend to be species-specific, indicating substantial genotypic redundancy for local adaptation in these species.

Design, construction, and functional characterization of a tRNA neochromosome in yeast

Here, we report the design, construction, and characterization of a tRNA neochromosome, a designer chromosome that functions as an additional, de novo counterpart to the native complement of Saccharomyces cerevisiae. Intending to address one of the central design principles of the Sc2.0 project, the ∼190-kb tRNA neochromosome houses all 275 relocated nuclear tRNA genes. To maximize stability, the design incorporates orthogonal genetic elements from non-S. cerevisiae yeast species. Furthermore, the presence of 283 rox recombination sites enables an orthogonal tRNA SCRaMbLE system. Following construction in yeast, we obtained evidence of a potent selective force, manifesting as a spontaneous doubling in cell ploidy. Furthermore, tRNA sequencing, transcriptomics, proteomics, nucleosome mapping, replication profiling, FISH, and Hi-C were undertaken to investigate questions of tRNA neochromosome behavior and function. Its construction demonstrates the remarkable tractability of the yeast model and opens up opportunities to directly test hypotheses surrounding these essential non-coding RNAs.

Genomics in the long-read sequencing era

Long-read sequencing (LRS) technologies have provided extremely powerful tools to explore genomes. While in the early years these methods suffered technical limitations, they have recently made significant progress in terms of read length, throughput, and accuracy and bioinformatics tools have strongly improved. Here, we aim to review the current status of LRS technologies, the development of novel methods, and the impact on genomics research. We will explore the most impactful recent findings made possible by these technologies focusing on high-resolution sequencing of genomes and transcriptomes and the direct detection of DNA and RNA modifications. We will also discuss how LRS methods promise a more comprehensive understanding of human genetic variation, transcriptomics, and epigenetics for the coming years.

Telomere-to-telomere assemblies of 142 strains characterize the genome structural landscape in Saccharomyces cerevisiae

Pangenomes provide access to an accurate representation of the genetic diversity of species, both in terms of sequence polymorphisms and structural variants (SVs). Here we generated the Saccharomyces cerevisiae Reference Assembly Panel (ScRAP) comprising reference-quality genomes for 142 strains representing the species' phylogenetic and ecological diversity. The ScRAP includes phased haplotype assemblies for several heterozygous diploid and polyploid isolates. We identified circa (ca.) 4,800 nonredundant SVs that provide a broad view of the genomic diversity, including the dynamics of telomere length and transposable elements. We uncovered frequent cases of complex aneuploidies where large chromosomes underwent large deletions and translocations. We found that SVs can impact gene expression near the breakpoints and substantially contribute to gene repertoire evolution. We also discovered that horizontally acquired regions insert at chromosome ends and can generate new telomeres. Overall, the ScRAP demonstrates the benefit of a pangenome in understanding genome evolution at population scale.

Bismuth-nickel bimetal nanosheets with a porous structure for efficient hydrogen production in neutral and alkaline media

Active and durable electrocatalysts are very important for efficient and economically sustainable hydrogen generation via electrocatalytic water splitting. A bismuth-nickel (Bi-Ni) bimetal nanosheet with a mesoporous structure was prepared via a self-template electrochemical in situ process. The Bi-Ni catalyst required overpotentials of 56 mV and 183 mV at 10 mA cm^-2 for the hydrogen evolution reaction (HER), which were close to that of commercial Pt/C in 1.0 M KOH and 1.0 M PBS (pH 7.0), respectively. The electrocatalyst maintained a steady current density during 20 h electrolysis in 1.0 M KOH and 1.0 M PBS (pH 7.0). Density functional theory (DFT) indicated that the alloying effect could induce charge transfer from the Bi atom to Ni atom and thus modulate the d-band centre of Bi-Ni nanosheets, which could efficiently accelerate H* conversion and H₂ desorption at the Ni active site. This promotes the HER kinetics. By adopting the Bi_84.8Ni_15.2 alloy as the cathode to establish a full-cell (IrO₂∥Bi_84.8Ni_15.2) for water splitting in 1.0 M KOH, the required cell voltage was 1.53 V to drive 10 mA cm^-2, which was lower than that of the IrO₂∥Pt/C electrolyzer (1.64 V@10 mA cm^-2).

Integration of Hi-C with short and long-read genome sequencing reveals the structure of germline rearranged genomes

Structural variants are a common cause of disease and contribute to a large extent to inter-individual variability, but their detection and interpretation remain a challenge. Here, we investigate 11 individuals with complex genomic rearrangements including germline chromothripsis by combining short- and long-read genome sequencing (GS) with Hi-C. Large-scale genomic rearrangements are identified in Hi-C interaction maps, allowing for an independent assessment of breakpoint calls derived from the GS methods, resulting in >300 genomic junctions. Based on a comprehensive breakpoint detection and Hi-C, we achieve a reconstruction of whole rearranged chromosomes. Integrating information on the three-dimensional organization of chromatin, we observe that breakpoints occur more frequently than expected in lamina-associated domains (LADs) and that a majority reshuffle topologically associating domains (TADs). By applying phased RNA-seq, we observe an enrichment of genes showing allelic imbalanced expression (AIG) within 100 kb around the breakpoints. Interestingly, the AIGs hit by a breakpoint (19/22) display both up- and downregulation, thereby suggesting different mechanisms at play, such as gene disruption and rearrangements of regulatory information. However, the majority of interpretable genes located 200 kb around a breakpoint do not show significant expression changes. Thus, there is an overall robustness in the genome towards large-scale chromosome rearrangements.

Chromosome-scale assemblies of Acanthamoeba castellanii genomes provide insights into Legionella pneumophila infection-related chromatin reorganization

The unicellular amoeba Acanthamoeba castellanii is ubiquitous in aquatic environments, where it preys on bacteria. The organism also hosts bacterial endosymbionts, some of which are parasitic, including human pathogens such as Chlamydia and Legionella spp. Here we report complete, high-quality genome sequences for two extensively studied A. castellanii strains, Neff and C3. Combining long- and short-read data with Hi-C, we generated near chromosome-level assemblies for both strains with 90% of the genome contained in 29 scaffolds for the Neff strain and 31 for the C3 strain. Comparative genomics revealed strain-specific functional enrichment, most notably genes related to signal transduction in the C3 strain and to viral replication in Neff. Furthermore, we characterized the spatial organization of the A. castellanii genome and showed that it is reorganized during infection by Legionella pneumophila Infection-dependent chromatin loops were found to be enriched in genes for signal transduction and phosphorylation processes. In genomic regions where chromatin organization changed during Legionella infection, we found functional enrichment for genes associated with metabolism, organelle assembly, and cytoskeleton organization. Given Legionella infection is known to alter its host's cell cycle, to exploit the host's organelles, and to modulate the host's metabolism in its favor, these changes in chromatin organization may partly be related to mechanisms of host control during Legionella infection.

Trichoderma - genomes and genomics as treasure troves for research towards biology, biotechnology and agriculture

The genus Trichoderma is among the best studied groups of filamentous fungi, largely because of its high relevance in applications from agriculture to enzyme biosynthesis to biofuel production. However, the physiological competences of these fungi, that led to these beneficial applications are intriguing also from a scientific and ecological point of view. This review therefore summarizes recent developments in studies of fungal genomes, updates on previously started genome annotation efforts and novel discoveries as well as efforts towards bioprospecting for enzymes and bioactive compounds such as cellulases, enzymes degrading xenobiotics and metabolites with potential pharmaceutical value. Thereby insights are provided into genomes, mitochondrial genomes and genomes of mycoviruses of Trichoderma strains relevant for enzyme production, biocontrol and mycoremediation. In several cases, production of bioactive compounds could be associated with responsible genes or clusters and bioremediation capabilities could be supported or predicted using genome information. Insights into evolution of the genus Trichoderma revealed large scale horizontal gene transfer, predominantly of CAZyme genes, but also secondary metabolite clusters. Investigation of sexual development showed that Trichoderma species are competent of repeat induced point mutation (RIP) and in some cases, segmental aneuploidy was observed. Some random mutants finally gave away their crucial mutations like T. reesei QM9978 and QM9136 and the fertility defect of QM6a was traced back to its gene defect. The Trichoderma core genome was narrowed down to 7000 genes and gene clustering was investigated in the genomes of multiple species. Finally, recent developments in application of CRISPR/Cas9 in Trichoderma, cloning and expression strategies for the workhorse T. reesei as well as the use genome mining tools for bioprospecting Trichoderma are highlighted. The intriguing new findings on evolution, genomics and physiology highlight emerging trends and illustrate worthwhile perspectives in diverse fields of research with Trichoderma.

The Spruce Budworm Genome: Reconstructing the Evolutionary History of Antifreeze Proteins

Insects have developed various adaptations to survive harsh winter conditions. Among freeze-intolerant species, some produce "antifreeze proteins" (AFPs) that bind to nascent ice crystals and inhibit further ice growth. Such is the case of the spruce budworm, Choristoneura fumiferana (Lepidoptera: Tortricidae), a destructive North American conifer pest that can withstand temperatures below -30°C. Despite the potential importance of AFPs in the adaptive diversification of Choristoneura, genomic tools to explore their origins have until now been limited. Here we present a chromosome-scale genome assembly for C. fumiferana, which we used to conduct comparative genomic analyses aimed at reconstructing the evolutionary history of tortricid AFPs. The budworm genome features 16 genes homologous to previously reported C. fumiferana AFPs (CfAFPs), 15 of which map to a single region on chromosome 18. Fourteen of these were also detected in five congeneric species, indicating Choristoneura AFP diversification occurred before the speciation event that led to C. fumiferana. Although budworm AFPs were previously considered unique to the genus Choristoneura, a search for homologs targeting recently sequenced tortricid genomes identified seven CfAFP-like genes in the distantly related Notocelia uddmanniana. High structural similarity between Notocelia and Choristoneura AFPs suggests a common origin, despite the absence of homologs in three related tortricids. Interestingly, one Notocelia AFP formed the C-terminus of a "zonadhesin-like" protein, possibly representing the ancestral condition from which tortricid AFPs evolved. Future work should clarify the evolutionary path of AFPs between Notocelia and Choristoneura and assess the role of the "zonadhesin-like" protein as precursor of tortricid AFPs.

Mutation load in sunflower inversions is negatively correlated with inversion heterozygosity

Recombination is critical both for accelerating adaptation and purging deleterious mutations. Chromosomal inversions can act as recombination modifiers that suppress local recombination in heterozygotes and thus, under some conditions, are predicted to accumulate such mutations. In this study, we investigated patterns of recombination, transposable element abundance and coding sequence evolution across the genomes of 1,445 individuals from three sunflower species, as well as within nine inversions segregating within species. We also analyzed the effects of inversion genotypes on 87 phenotypic traits to test for overdominance. We found significant negative correlations of long terminal repeat retrotransposon abundance and deleterious mutations with recombination rates across the genome in all three species. However, we failed to detect an increase in these features in the inversions, except for a modest increase in the proportion of stop codon mutations in several very large or rare inversions. Consistent with this finding, there was little evidence of overdominance of inversions in phenotypes that may relate to fitness. On the other hand, significantly greater load was observed for inversions in populations polymorphic for a given inversion compared to populations monomorphic for one of the arrangements, suggesting that the local state of inversion polymorphism affects deleterious load. These seemingly contradictory results can be explained by the low frequency of inversion heterozygotes in wild sunflower populations, apparently due to divergent selection and associated geographic structure. Inversions contributing to local adaptation represent ideal recombination modifiers, acting to facilitate adaptive divergence with gene flow, while largely escaping the accumulation of deleterious mutations.

Metagenomes Binning Using Proximity-Ligation Data

Microbial communities are key components of all ecosystems, but characterization of their complete genomic structure remains challenging. Typical analysis tends to elude the complexity of the mixes in terms of species, strains, as well as extrachromosomal DNA molecules. Recently, approaches have been developed that bins DNA contigs into individual genomes and episomes according to their 3D contact frequencies. Those contacts are quantified by chromosome conformation capture experiments (3C, Hi-C), also known as proximity-ligation approaches, applied to metagenomics samples. Here, we present a simple computational pipeline that allows to recover high-quality Metagenomics Assemble Genomes (MAGs) starting from metagenomic 3C or Hi-C datasets and a metagenome assembly.

Normalization of Chromosome Contact Maps: Matrix Balancing and Visualization

Over the last decade, genomic proximity ligation approaches have reshaped our vision of chromosomes 3D organizations, from bacteria nucleoids to larger eukaryotic genomes. The different protocols (3Cseq, Hi-C, TCC, MicroC [XL], Hi-CO, etc.) rely on common steps (chemical fixation digestion, ligation…) to detect pairs of genomic positions in close proximity. The most common way to represent these data is a matrix, or contact map, which allows visualizing the different chromatin structures (compartments, loops, etc.) that can be associated to other signals such as transcription, protein occupancy, etc. as well as, in some instances, to biological functions.In this chapter we present and discuss the filtering of the events recovered in proximity ligation experiments as well as the application of the balancing normalization procedure on the resulting contact map. We also describe a computational tool for visualizing normalized contact data dubbed Scalogram.The different processes described here are illustrated and supported by the laboratory custom-made scripts pooled into "hicstuff," an open-access python package accessible on github ( https://github.com/koszullab/hicstuff ).

Saccharomyces yeast hybrids on the rise

Saccharomyces hybrid yeasts are receiving increasing attention as a powerful model system to understand adaptation to environmental stress and speciation mechanisms, using experimental evolution and omics techniques. We compiled all genomic resources available from public repositories of the eight recognized Saccharomyces species and their interspecific hybrids. We present the newest numbers on genomes sequenced, assemblies, annotations, and sequencing runs, and an updated species phylogeny using orthogroup inference. While genomic resources are highly skewed towards Saccharomyces cerevisiae, there is a noticeable movement to use wild, recently discovered yeast species in recent years. To illustrate the degree and potential causes of reproductive isolation, we reanalyzed published data on hybrid spore viabilities across the entire genus and tested for the role of genetic, geographic, and ecological divergence within and between species (28 cross types and 371 independent crosses). Hybrid viability generally decreased with parental genetic distance likely due to antirecombination and negative epistasis, but notable exceptions emphasize the importance of strain-specific structural variation and ploidy differences. Surprisingly, the viability of crosses within species varied widely, from near reproductive isolation to near-perfect viability. Geographic and ecological origins of the parents predicted cross viability to an extent, but with certain caveats. Finally, we highlight publication trends in the field and point out areas of special interest, where hybrid yeasts are particularly promising for innovation through research and development, and experimental evolution and fermentation.

Technical considerations in Hi-C scaffolding and evaluation of chromosome-scale genome assemblies

The recent development of ecological studies has been fueled by the introduction of massive information based on chromosome-scale genome sequences, even for species for which genetic linkage is not accessible. This was enabled mainly by the application of Hi-C, a method for genome-wide chromosome conformation capture that was originally developed for investigating the long-range interaction of chromatins. Performing genomic scaffolding using Hi-C data is highly resource-demanding and employs elaborate laboratory steps for sample preparation. It starts with building a primary genome sequence assembly as an input, which is followed by computation for genome scaffolding using Hi-C data, requiring careful validation. This article presents technical considerations for obtaining optimal Hi-C scaffolding results and provides a test case of its application to a reptile species, the Madagascar ground gecko (Paroedura picta). Among the metrics that are frequently used for evaluating scaffolding results, we investigate the validity of the completeness assessment of chromosome-scale genome assemblies using single-copy reference orthologues.

Aborting meiosis allows recombination in sterile diploid yeast hybrids

Hybrids between diverged lineages contain novel genetic combinations but an impaired meiosis often makes them evolutionary dead ends. Here, we explore to what extent an aborted meiosis followed by a return-to-growth (RTG) promotes recombination across a panel of 20 Saccharomyces cerevisiae and S. paradoxus diploid hybrids with different genomic structures and levels of sterility. Genome analyses of 275 clones reveal that RTG promotes recombination and generates extensive regions of loss-of-heterozygosity in sterile hybrids with either a defective meiosis or a heavily rearranged karyotype, whereas RTG recombination is reduced by high sequence divergence between parental subgenomes. The RTG recombination preferentially arises in regions with low local heterozygosity and near meiotic recombination hotspots. The loss-of-heterozygosity has a profound impact on sexual and asexual fitness, and enables genetic mapping of phenotypic differences in sterile lineages where linkage analysis would fail. We propose that RTG gives sterile yeast hybrids access to a natural route for genome recombination and adaptation.

Filling gaps of genome scaffolds via probabilistic searching optical maps against assembly graph

BACKGROUND

Optical maps record locations of specific enzyme recognition sites within long genome fragments. This long-distance information enables aligning genome assembly contigs onto optical maps and ordering contigs into scaffolds. The generated scaffolds, however, often contain a large amount of gaps. To fill these gaps, a feasible way is to search genome assembly graph for the best-matching contig paths that connect boundary contigs of gaps. The combination of searching and evaluation procedures might be "searching followed by evaluation", which is infeasible for long gaps, or "searching by evaluation", which heavily relies on heuristics and thus usually yields unreliable contig paths.

RESULTS

We here report an accurate and efficient approach to filling gaps of genome scaffolds with aids of optical maps. Using simulated data from 12 species and real data from 3 species, we demonstrate the successful application of our approach in gap filling with improved accuracy and completeness of genome scaffolds.

CONCLUSION

Our approach applies a sequential Bayesian updating technique to measure the similarity between optical maps and candidate contig paths. Using this similarity to guide path searching, our approach achieves higher accuracy than the existing "searching by evaluation" strategy that relies on heuristics. Furthermore, unlike the "searching followed by evaluation" strategy enumerating all possible paths, our approach prunes the unlikely sub-paths and extends the highly-probable ones only, thus significantly increasing searching efficiency.

Hi-C scaffolded short- and long-read genome assemblies of the California sea lion are broadly consistent for syntenic inference across 45 million years of evolution

With the advent of chromatin-interaction maps, chromosome-level genome assemblies have become a reality for a wide range of organisms. Scaffolding quality is, however, difficult to judge. To explore this gap, we generated multiple chromosome-scale genome assemblies of an emerging wild animal model for carcinogenesis, the California sea lion (Zalophus californianus). Short-read assemblies were scaffolded with two independent chromatin interaction mapping data sets (Hi-C and Chicago), and long-read assemblies with three data types (Hi-C, optical maps and 10X linked reads) following the "Vertebrate Genomes Project (VGP)" pipeline. In both approaches, 18 major scaffolds recovered the karyotype (2n = 36), with scaffold N50s of 138 and 147 Mb, respectively. Synteny relationships at the chromosome level with other pinniped genomes (2n = 32-36), ferret (2n = 34), red panda (2n = 36) and domestic dog (2n = 78) were consistent across approaches and recovered known fissions and fusions. Comparative chromosome painting and multicolour chromosome tiling with a panel of 264 genome-integrated single-locus canine bacterial artificial chromosome probes provided independent evaluation of genome organization. Broad-scale discrepancies between the approaches were observed within chromosomes, most commonly in translocations centred around centromeres and telomeres, which were better resolved in the VGP assembly. Genomic and cytological approaches agreed on near-perfect synteny of the X chromosome, and in combination allowed detailed investigation of autosomal rearrangements between dog and sea lion. This study presents high-quality genomes of an emerging cancer model and highlights that even highly fragmented short-read assemblies scaffolded with Hi-C can yield reliable chromosome-level scaffolds suitable for comparative genomic analyses.

Genome reconstruction and haplotype phasing using chromosome conformation capture methodologies

Genomic analysis of individuals or organisms is predicated on the availability of high-quality reference and genotype information. With the rapidly dropping costs of high-throughput DNA sequencing, this is becoming readily available for diverse organisms and for increasingly large populations of individuals. Despite these advances, there are still aspects of genome sequencing that remain challenging for existing sequencing methods. This includes the generation of long-range contiguity during genome assembly, identification of structural variants in both germline and somatic tissues, the phasing of haplotypes in diploid organisms and the resolution of genome sequence for organisms derived from complex samples. These types of information are valuable for understanding the role of genome sequence and genetic variation on genome function, and numerous approaches have been developed to address them. Recently, chromosome conformation capture (3C) experiments, such as the Hi-C assay, have emerged as powerful tools to aid in these challenges for genome reconstruction. We will review the current use of Hi-C as a tool for aiding in genome sequencing, addressing the applications, strengths, limitations and potential future directions for the use of 3C data in genome analysis. We argue that unique features of Hi-C experiments make this data type a powerful tool to address challenges in genome sequencing, and that future integration of Hi-C data with alternative sequencing assays will facilitate the continuing revolution in genomic analysis and genome sequencing.

Closed and High-Quality Bacterial Genome Sequences of the Oligo-Mouse-Microbiota Community

The Oligo-Mouse-Microbiota (OMM¹²) gnotobiotic murine model is an increasingly popular model in microbiota studies. However, following Illumina and PacBio sequencing, the genomes of the 12 strains could not be closed. Here, we used genomic chromosome conformation capture (Hi-C) data to reorganize, close, and improve the quality of these 12 genomes.

The Oligo-Mouse-Microbiota (OMM¹²) gnotobiotic murine model is an increasingly popular model in microbiota studies. However, following Illumina and PacBio sequencing, the genomes of the 12 strains could not be closed. Here, we used genomic chromosome conformation capture (Hi-C) data to reorganize, close, and improve the quality of these 12 genomes.

Opportunities and challenges for high-quality biodiversity tissue archives in the age of long-read sequencing

The technological ability to characterize genetic variation at a genome-wide scale provides an unprecedented opportunity to study the genetic underpinnings and evolutionary mechanisms that promote and sustain biodiversity. The transition from short- to long-read sequencing is particularly promising and allows a more holistic view on any changes in genetic diversity across time and space. Long-read sequencing has tremendous potential but sequencing success strongly depends on the long-range integrity of DNA molecules and therefore on the availability of high-quality tissue samples. With the scope of genomic experiments expanding and wild populations simultaneously disappearing at an unprecedented rate, access to high-quality samples may soon be a major concern for many projects. The need for high-quality biodiversity tissue archives is therefore urgent but sampling and preserving high-quality samples is not a trivial exercise. In this review, I will briefly outline how long-read sequencing can benefit the study of molecular ecology, how this will substantially increase the demand for high-quality tissues and why it is challenging to preserve DNA integrity. I will then provide an overview of preservation approaches and end with a call for support to acknowledge the efforts needed to assemble high-quality tissue archives. In doing so, I hope to simultaneously motivate field biologists to expand sampling practices and molecular biologists to develop (cost) efficient guidelines for the sampling and long-term storage of tissues. A concerted, interdisciplinary, effort is needed to catalogue the genetic variation underlying contemporary biodiversity and will eventually provide a critical resource for future studies.

Long insert clone experimental evidence for assembly improvement and chimeric chromosomes detection in an allopentaploid beer yeast

Lager beer is made with the hybrid Saccharomyces pastorianus. Many publicly available S. pastorianus genome assemblies are highly fragmented due to the difficulties of assembling hybrid genomes, such as the presence of homeologous chromosomes from both parental types, and translocations between them. To improve the assembly of a previously sequenced lager yeast hybrid Saccharomyces sp. 790 and elucidate its genome structure, we proposed the use of alternative experimental evidence. We determined the phylogenetic position of Saccharomyces sp. 790 and established it as S. pastorianus 790. Then, we obtained from this yeast a bacterial artificial chromosome (BAC) genomic library with its BAC-end sequences (BESs). To analyze these data, we developed a pipeline (applicable to other assemblies) that classifies BES pairs alignments according to their orientation. For the case of S. pastorianus 790, paired-end BESs alignments validated parts of the assembly and unpaired-end ones suggested contig joins or misassemblies. Importantly, the BACs library was preserved and used for verification experiments. Unpaired-end alignments were used to upgrade the previous assembly and provided an improved detection of translocations. With this, we proposed a genome structure of S. pastorianus 790, which was similar to that of other lager yeasts; however, when we estimated chromosome copy number and experimentally measured its genome size, we discovered that one key difference is the outstanding S. pastorianus 790 ploidy level (allopentaploid). Altogether, our results show the value of combining bioinformatic analyses with experimental data such as long-insert clone information to improve a short-read assembly of a hybrid genome.

A cookbook for DNase Hi-C

Background

The Hi-C technique is widely employed to study the 3-dimensional chromatin architecture and to assemble genomes. The conventional in situ Hi-C protocol employs restriction enzymes to digest chromatin, which results in nonuniform genomic coverage. Using sequence-agnostic restriction enzymes, such as DNAse I, could help to overcome this limitation.

Results

In this study, we compare different DNAse Hi-C protocols and identify the critical steps that significantly affect the efficiency of the protocol. In particular, we show that the SDS quenching strategy strongly affects subsequent chromatin digestion. The presence of biotinylated oligonucleotide adapters may lead to ligase reaction by-products, which can be avoided by rational design of the adapter sequences. Moreover, the use of nucleotide-exchange enzymes for biotin fill-in enables simultaneous labelling and repair of DNA ends, similar to the conventional Hi-C protocol. These improvements simplify the protocol, making it less expensive and time-consuming.

Conclusions

We propose a new robust protocol for the preparation of DNAse Hi-C libraries from cultured human cells and blood samples supplemented with experimental controls and computational tools for the evaluation of library quality.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13072-021-00389-5.

Chromosome-level genome assemblies of the malaria vectors Anopheles coluzzii and Anopheles arabiensis

BACKGROUND

Anopheles coluzzii and Anopheles arabiensis belong to the Anopheles gambiae complex and are among the major malaria vectors in sub-Saharan Africa. However, chromosome-level reference genome assemblies are still lacking for these medically important mosquito species.

FINDINGS

In this study, we produced de novo chromosome-level genome assemblies for A. coluzzii and A. arabiensis using the long-read Oxford Nanopore sequencing technology and the Hi-C scaffolding approach. We obtained 273.4 and 256.8 Mb of the total assemblies for A. coluzzii and A. arabiensis, respectively. Each assembly consists of 3 chromosome-scale scaffolds (X, 2, 3), complete mitochondrion, and unordered contigs identified as autosomal pericentromeric DNA, X pericentromeric DNA, and Y sequences. Comparison of these assemblies with the existing assemblies for these species demonstrated that we obtained improved reference-quality genomes. The new assemblies allowed us to identify genomic coordinates for the breakpoint regions of fixed and polymorphic chromosomal inversions in A. coluzzii and A. arabiensis.

CONCLUSION

The new chromosome-level assemblies will facilitate functional and population genomic studies in A. coluzzii and A. arabiensis. The presented assembly pipeline will accelerate progress toward creating high-quality genome references for other disease vectors.

A comprehensive review of scaffolding methods in genome assembly

In the field of genome assembly, scaffolding methods make it possible to obtain a more complete and contiguous reference genome, which is the cornerstone of genomic research. Scaffolding methods typically utilize the alignments between contigs and sequencing data (reads) to determine the orientation and order among contigs and to produce longer scaffolds, which are helpful for genomic downstream analysis. With the rapid development of high-throughput sequencing technologies, diverse types of reads have emerged over the past decade, especially in long-range sequencing, which have greatly enhanced the assembly quality of scaffolding methods. As the number of scaffolding methods increases, biology and bioinformatics researchers need to perform in-depth analyses of state-of-the-art scaffolding methods. In this article, we focus on the difficulties in scaffolding, the differences in characteristics among various kinds of reads, the methods by which current scaffolding methods address these difficulties, and future research opportunities. We hope this work will benefit the design of new scaffolding methods and the selection of appropriate scaffolding methods for specific biological studies.

Trichoderma reesei Isolated From Austrian Soil With High Potential for Biotechnological Application

Fungi of the genus Trichoderma are of high importance for biotechnological applications, in biocontrol and for production of homologous and heterologous proteins. However, sexual crossing under laboratory conditions has so far only been achieved with the species Trichoderma reesei, which was so far only isolated from tropical regions. Our isolation efforts aimed at the collection of Trichoderma strains from Austrian soils surprisingly also yielded 12 strains of the species T. reesei, which was previously not known to occur in Europe. Their identity was confirmed with tef1- and rpb2-sequencing and phylogenetic analysis. They could clearly be distinguished from tropical strains including the common laboratory wildtypes by UP-PCR and genetic variations adjacent to the mating type locus. The strains readily mated with reference strains derived from CBS999.97. Secreted cellulase and xylanase levels of these isolates were up to six-fold higher than those of QM6a indicating a high potential for strain improvement. The strains showed different responses to injury in terms of induction of sporulation, but a correlation to alterations in the nox1-gene sequence was not detected. Several synonymous SNPs were found in the sequence of the regulator gene noxR of the soil isolates compared to QM6a. Only in one strain, non-synonymous SNPs were found which impact a PEST sequence of NoxR, suggesting altered protein stability. The availability of sexually fertile strains from middle Europe naturally producing decent amounts of plant cell wall degrading enzymes opens up novel perspectives for non-GMO strain improvement and biological pretreatment of plant biomass for bioethanol production. Moreover, the varied response of these strains to injury in terms of sporulation, which is independent of Nox1 and NoxR suggests that additional regulators impact this phenomenon in T. reesei.

Genetic Transformation of Trichoderma spp

The production of biofuels from plant biomass is dependent on the availability of enzymes that can hydrolyze the plant cell wall polysaccharides to their monosaccharides. These enzyme mixtures are formed by microorganisms but their native compositions and properties are often not ideal for application. Genetic engineering of these microorganisms is therefore necessary, in which introduction of DNA is an essential precondition. The filamentous fungus Trichoderma reesei-the main producer of plant-cell-wall-degrading enzymes for biofuels and other industries-has been subjected to intensive genetic engineering toward this goal and has become one of the iconic examples of the successful genetic improvement of fungi. However, the genetic manipulation of other enzyme-producing Trichoderma species is frequently less efficient and, therefore, rarely managed. In this chapter, we therefore describe the two potent methods of Trichoderma transformation mediated by either (a) polyethylene glycol (PEG) or (b) Agrobacterium. The methods are optimized for T. reesei but can also be applied for such transformation-resilient species as T. harzianum and T. guizhouense, which are putative upcoming alternatives for T. reesei in this field. The protocols are simple, do not require extensive training or special equipment, and can be further adjusted for T. reesei mutants with particular properties.

Ecological Genomics and Evolution of Trichoderma reesei

The filamentous fungus Trichoderma reesei (Hypocreales, Ascomycota) is an efficient industrial cell factory for the production of cellulolytic enzymes used for biofuel and other applications. Therefore, researches addressing T. reesei are relatively advanced compared to other Trichoderma spp. because of the significant bulk of available knowledge, multiple genomic data, and gene manipulation techniques. However, the established role of T. reesei in industry has resulted in a frequently biased understanding of the biology of this fungus. Thus, the recent studies unexpectedly show that the superior cellulolytic activity of T. reesei and other Trichoderma species evolved due to multiple lateral gene transfer events, while the innate ability to parasitize other fungi (mycoparasitism) was maintained in the genus, including T. reesei. In this chapter, we will follow the concept of ecological genomics and describe the ecology, distribution, and evolution of T. reesei, as well as critically discuss several common misconceptions that originate from the success of this species in applied sciences and industry.

PacBio Long-Read Sequencing, Assembly, and Funannotate Reannotation of the Complete Genome of Trichoderma reesei QM6a

Single-molecule real-time (SMRT) sequencing developed by Pacific BioSciences (PacBio) offers three major advantages compared to second-generation sequencing: long read length and high consensus accuracy, and a low degree of bias. Together with high sequencing coverage, these advantages overcome the difficulty of sequencing genomic regions such as long AT-rich islands and repeated regions (e.g., ribosomal DNA) in the genome of Trichoderma reesei QM6a. Herein, we describe a protocol for preparing high-quality, high molecular weight genomic DNA for PacBio long-read sequencing, de novo assembly and streamlined annotation of the QM6a genome.

The G-protein Coupled Receptor GPR8 Regulates Secondary Metabolism in Trichoderma reesei

Changing environmental conditions are of utmost importance for regulation of secondary metabolism in fungi. Different environmental cues including the carbon source, light and the presence of a mating partner can lead to altered production of compounds. Thereby, the heterotrimeric G-protein pathway is of major importance for sensing and adjustment of gene regulation. Regulation of secondary metabolism is crucial in the biotechnological workhorse Trichoderma reesei for knowledge-based adjustment in industrial fermentations, but also with respect to the potential use as a host for heterologous compound production. We investigated the function of the class VII G-protein coupled receptor (GPCR) gene gpr8 that is localized in the vicinity of the SOR cluster, which is responsible for biosynthesis of sorbicillinoids. GPR8 positively impacts regulation of the genes in this cluster in darkness. Accordingly, abundance of trichodimerol and dihydrotrichotetronine as well as other secondary metabolites is decreased in the deletion mutant. Transcriptome analysis moreover showed the major role of GPR8 being exerted in darkness with a considerable influence on regulation of secondary metabolism. Genes regulated in Δgpr8 overlap with those regulated directly or indirectly by the transcription factor YPR2, especially concerning genes related to secondary metabolism. The predicted FAD/FMN containing dehydrogenase gene sor7, one of the positive targets of the cascade triggered by GPR8, has a positive effect on secondary metabolite production, but also cellulase gene expression. Hence SOR7 has some overlapping, but also additional functions compared to GPR8. The G-protein coupled receptor GPR8 exerts a light dependent impact on secondary metabolism, which is in part mediated by the transcription factor YPR2 and the function of SOR7. Hence, T. reesei may apply GPR8 to adjust production of secondary metabolites and hence chemical communication to signals from the environment.

Multifaceted Hi-C benchmarking: what makes a difference in chromosome-scale genome scaffolding?

Background

Hi-C is derived from chromosome conformation capture (3C) and targets chromatin contacts on a genomic scale. This method has also been used frequently in scaffolding nucleotide sequences obtained by de novo genome sequencing and assembly, in which the number of resultant sequences rarely converges to the chromosome number. Despite its prevalent use, the sample preparation methods for Hi-C have not been intensively discussed, especially from the standpoint of genome scaffolding.

Results

To gain insight into the best practice of Hi-C scaffolding, we performed a multifaceted methodological comparison using vertebrate samples and optimized various factors during sample preparation, sequencing, and computation. As a result, we identified several key factors that helped improve Hi-C scaffolding, including the choice and preparation of tissues, library preparation conditions, the choice of restriction enzyme(s), and the choice of scaffolding program and its usage.

Conclusions

This study provides the first comparison of multiple sample preparation kits/protocols and computational programs for Hi-C scaffolding by an academic third party. We introduce a customized protocol designated “inexpensive and controllable Hi-C (iconHi-C) protocol,” which incorporates the optimal conditions identified in this study, and demonstrate this technique on chromosome-scale genome sequences of the Chinese softshell turtle Pelodiscus sinensis.

A chromosome-scale assembly of the black gram (Vigna mungo) genome

Black gram (Vigna mungo) is an important short duration grain legume crop. Black gram seeds provide an inexpensive source of dietary protein. Here, we applied the 10X Genomics linked-read technology to obtain a de novo whole genome assembly of V. mungo cultivated variety Chai Nat 80 (CN80). The preliminary assembly contained 12,228 contigs and had an N50 length of 5.2 Mb. Subsequent scaffolding using the long-range Chicago and HiC techniques yielded the first high-quality, chromosome-level assembly of 499 Mb comprising 11 pseudomolecules. Comparative genomics analyses based on sequence information from single-copy orthologous genes revealed that black gram and mungbean (Vigna radiata) diverged about 2.7 million years ago . The transversion rate (4DTv) analysis in V. mungo revealed no evidence supporting a recent genome-wide duplication event observed in the tetraploid créole bean (Vigna reflexo-pilosa). The proportion of repetitive elements in the black gram genome is slightly lower than the numbers reported for related Vigna species. The majority of long terminal repeat retrotransposons appeared to integrate into the genome within the last five million years. We also examined alternative splicing events in V. mungo using full-length transcript sequences. While intron retention was the most prevalent mode of alternative splicing in several plant species, alternative 3' acceptor site selection represented the majority of events in black gram. Our high-quality genome assembly along with the genomic variation information from the germplasm provides valuable resources for accelerating the development of elite varieties through marker-assisted breeding and for future comparative genomics and phylogenetic studies in legume species.

Repetitive Elements Contribute to the Diversity and Evolution of Centromeres in the Fungal Genus Verticillium

Centromeres are chromosomal regions that are crucial for chromosome segregation during mitosis and meiosis, and failed centromere formation can contribute to chromosomal anomalies. Despite this conserved function, centromeres differ significantly between and even within species. Thus far, systematic studies into the organization and evolution of fungal centromeres remain scarce. In this study, we identified the centromeres in each of the 10 species of the fungal genus Verticillium and characterized their organization and evolution. Chromatin immunoprecipitation of the centromere-specific histone CenH3 (ChIP-seq) and chromatin conformation capture (Hi-C) followed by high-throughput sequencing identified eight conserved, large (∼150-kb), AT-, and repeat-rich regional centromeres that are embedded in heterochromatin in the plant pathogen Verticillium dahliae Using Hi-C, we similarly identified repeat-rich centromeres in the other Verticillium species. Strikingly, a single degenerated long terminal repeat (LTR) retrotransposon is strongly associated with centromeric regions in some but not all Verticillium species. Extensive chromosomal rearrangements occurred during Verticillium evolution, of which some could be linked to centromeres, suggesting that centromeres contributed to chromosomal evolution. The size and organization of centromeres differ considerably between species, and centromere size was found to correlate with the genome-wide repeat content. Overall, our study highlights the contribution of repetitive elements to the diversity and rapid evolution of centromeres within the fungal genus VerticilliumIMPORTANCE The genus Verticillium contains 10 species of plant-associated fungi, some of which are notorious pathogens. Verticillium species evolved by frequent chromosomal rearrangements that contribute to genome plasticity. Centromeres are instrumental for separation of chromosomes during mitosis and meiosis, and failed centromere functionality can lead to chromosomal anomalies. Here, we used a combination of experimental techniques to identify and characterize centromeres in each of the Verticillium species. Intriguingly, we could strongly associate a single repetitive element to the centromeres of some of the Verticillium species. The presence of this element in the centromeres coincides with increased centromere sizes and genome-wide repeat expansions. Collectively, our findings signify a role of repetitive elements in the function, organization, and rapid evolution of centromeres in a set of closely related fungal species.

Massive haplotypes underlie ecotypic differentiation in sunflowers

Species often include multiple ecotypes that are adapted to different environments¹. However, it is unclear how ecotypes arise and how their distinctive combinations of adaptive alleles are maintained despite hybridization with non-adapted populations^2-4. Here, by resequencing 1,506 wild sunflowers from 3 species (Helianthus annuus, Helianthus petiolaris and Helianthus argophyllus), we identify 37 large (1-100 Mbp in size), non-recombining haplotype blocks that are associated with numerous ecologically relevant traits, as well as soil and climate characteristics. Limited recombination in these haplotype blocks keeps adaptive alleles together, and these regions differentiate sunflower ecotypes. For example, haplotype blocks control a 77-day difference in flowering between ecotypes of the silverleaf sunflower H. argophyllus (probably through deletion of a homologue of FLOWERING LOCUS T (FT)), and are associated with seed size, flowering time and soil fertility in dune-adapted sunflowers. These haplotypes are highly divergent, frequently associated with structural variants and often appear to represent introgressions from other-possibly now-extinct-congeners. These results highlight a pervasive role of structural variation in ecotypic adaptation.

instaGRAAL: chromosome-level quality scaffolding of genomes using a proximity ligation-based scaffolder

Hi-C exploits contact frequencies between pairs of loci to bridge and order contigs during genome assembly, resulting in chromosome-level assemblies. Because few robust programs are available for this type of data, we developed instaGRAAL, a complete overhaul of the GRAAL program, which has adapted the latter to allow efficient assembly of large genomes. instaGRAAL features a number of improvements over GRAAL, including a modular correction approach that optionally integrates independent data. We validate the program using data for two brown algae, and human, to generate near-complete assemblies with minimal human intervention.

Subtelomeres as Specialized Chromatin Domains

Specificities associated with chromosomal linearity are not restricted to telomeres. Here, recent results obtained on fission and budding yeast are summarized and an attempt is made to define subtelomeres using chromatin features extending beyond the heterochromatin emanating from telomeres. Subtelomeres, the chromosome domains adjacent to telomeres, differ from the rest of the genome by their gene content, rapid evolution, and chromatin features that together contribute to organism adaptation. However, current definitions of subtelomeres are generally based on synteny and are largely gene-centered. Taking into consideration both the peculiar gene content and dynamics as well as the chromatin properties of those domains, it is discussed how chromatin features can contribute to subtelomeric properties and functions, and play a pivotal role in the emergence of subtelomeres.

Seq'ing identity and function in a repeat-derived noncoding RNA world

Innovations in high-throughout sequencing approaches are being marshaled to both reveal the composition of the abundant and heterogeneous noncoding RNAs that populate cell nuclei and lend insight to the mechanisms by which noncoding RNAs influence chromosome biology and gene expression. This review focuses on some of the recent technological developments that have enabled the isolation of nascent transcripts and chromatin-associated and DNA-interacting RNAs. Coupled with emerging genome assembly and analytical approaches, the field is poised to achieve a comprehensive catalog of nuclear noncoding RNAs, including those derived from repetitive regions within eukaryotic genomes. Herein, particular attention is paid to the challenges and advances in the sequence analyses of repeat and transposable element-derived noncoding RNAs and in ascribing specific function(s) to such RNAs.

Chitin and chitosan remodeling defines vegetative development and Trichoderma biocontrol

Fungal parasitism depends on the ability to invade host organisms and mandates adaptive cell wall remodeling to avoid detection and defense reactions by the host. All plant and human pathogens share invasive strategies, which aid to escape the chitin-triggered and chitin-targeted host immune system. Here we describe the full spectrum of the chitin/chitosan-modifying enzymes in the mycoparasite Trichoderma atroviride with a central role in cell wall remodeling. Rapid adaption to a variety of growth conditions, environmental stresses and host defense mechanisms such as oxidative stress depend on the concerted interplay of these enzymes and, ultimately, are necessary for the success of the mycoparasitic attack. To our knowledge, we provide the first in class description of chitin and associated glycopolymer synthesis in a mycoparasite and demonstrate that they are essential for biocontrol. Eight chitin synthases, six chitin deacetylases, additional chitinolytic enzymes, including six chitosanases, transglycosylases as well as accessory proteins are involved in this intricately regulated process. Systematic and biochemical classification, phenotypic characterization and mycoparasitic confrontation assays emphasize the importance of chitin and chitosan assembly in vegetative development and biocontrol in T. atroviride. Our findings critically contribute to understanding the molecular mechanism of chitin synthesis in filamentous fungi and mycoparasites with the overarching goal to selectively exploit the discovered biocontrol strategies.

The Coix Genome Provides Insights into Panicoideae Evolution and Papery Hull Domestication

Coix is a grass crop domesticated as early as the Neolithic era. It is still widely cultivated for both highly nutritional food and medicinal use. However, the genetic study and breeding of this crop are hindered by the lack of a sequenced genome. Here, we report de novo sequencing and assembly of the 1619-Mb genome of Coix, and annotation of 75.39% repeats and 39 629 protein-coding genes. Comparative genomics analysis showed that Coix is more closely related to sorghum than maize, but intriguingly only Coix and maize had a recent genome duplication event, which was not detected in sorghum. We further constructed a genetic map and mapped several important traits, especially the strength of hull. Selection of papery hull (thin: easy dehulling) from the stony hull (thick: difficult dehulling) in wild progenitors was a key step in Coix domestication. The papery hull makes seed easier to process and germinate. Anatomic and global transcriptome analysis revealed that the papery hull is a result of inhibition of cell division and wall biogenesis. We also successfully demonstrated that seed hull pressure resistance is controlled by two major quantitative trait loci (QTLs), which are associated with hull thickness and color, respectively. The two QTLs were further fine mapped within intervals of 250 kb and 146 kb, respectively. These resources provide a platform for evolutionary studies and will facilitate molecular breeding of this important crop.

Tracking microbial evolution in the human gut using Hi-C reveals extensive horizontal gene transfer, persistence and adaptation

Despite the importance of horizontal gene transfer for rapid bacterial evolution, reliable assignment of mobile genetic elements to their microbial hosts in natural communities such as the human gut microbiota is lacking. We used high-throughput chromosomal conformation capture coupled with probabilistic modelling of experimental noise to resolve 88 strain-level metagenome-assembled genomes of distal gut bacteria from two participants, including 12,251 accessory elements. Comparisons of two samples collected 10 years apart for each of the participants revealed extensive in situ exchange of accessory elements as well as evidence of adaptive evolution in core genomes. Accessory elements were predominantly promiscuous and prevalent in the distal gut metagenomes of 218 adult individuals. This research provides a foundation and approach for studying microbial evolution in natural environments.

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

In this study, Renschler et al. set out to analyze the impact of genomic rearrangements on genome topology using the Drosophila genus and X chromosome dosage compensation as a model. The authors developed a scaffolding algorithm and generated chromosome-length assemblies from Hi-C data for studying genome topology in three distantly related Drosophila species. Their data provides unique insights into genome topology evolution. RA

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.

The role of PKAc1 in gene regulation and trichodimerol production in Trichoderma reesei

Background

Trichoderma reesei represents a model system for investigation of plant cell wall degradation and its connection to light response. The cyclic adenosine monophosphate pathway (cAMP pathway) plays an important role in both physiological outputs, being crucial for regulation of photoreceptor function as well as for cellulase regulation on different carbon sources. Phosphorylation of photoreceptors and of the carbon catabolite repressor CRE1 was shown in ascomycetes, indicating a relevance of protein kinase A in regulation of the target genes of these transcription factors as well as an impact on regulation of induction specific genes. Moreover, the cAMP pathway impacts growth and development.

Results

Here, we investigated gene regulation by the catalytic subunit of protein kinase A (PKAc1) upon growth on cellulose. We found distinct gene sets for regulation upon growth in light and darkness with an overlap of only 13 genes. PKAc1 regulates metabolic genes as well as transport and defense functions. The overlap of gene regulation by PKAc1 with the genes representing the cAMP dependent regulatory output of the photoreceptor ENV1 indicates an involvement of PKA in this pathway, which counteracts its effects by contrasting regulation. Moreover, we found considerable overlap with the gene sets regulated under cellulase inducing conditions and by the carbon catabolite repressor CRE1. Our analysis also showed that PKAc1 regulates the genes of the SOR cluster associated with the biosynthesis of sorbicillinoids. The homologue of gin4, encoding a CAMK type kinase, which is regulated by PKAc1, CRE1 and YPR2 showed a moderate impact on trichodimerol production. We isolated trichodimerol as representative sorbicillin compound and established a method for its quantification in large sample sets using high performance thin layer chromatography (HPTLC), which can be broadly applied for secondary metabolite screening of mutants or different growth conditions. Due to the high expression levels of the SOR cluster under conditions of sexual development we crosschecked the relevance of PKAc1 under these conditions. We could show that PKAc1 impacts biosynthesis of trichodimerol in axenic growth and upon mating.

Conclusions

We conclude that PKAc1 is involved in light dependent regulation of plant cell wall degradation, including carbon catabolite repression as well as secondary metabolism and development in T. reesei.

Improved inference of chromosome conformation from images of labeled loci

We previously published a method that infers chromosome conformation from images of fluorescently-tagged genomic loci, for the case when there are many loci labeled with each distinguishable color.  Here we build on our previous work and improve the reconstruction algorithm to address previous limitations.  We show that these improvements 1) increase the reconstruction accuracy and 2) allow the method to be used on large-scale problems involving several hundred labeled loci.  Simulations indicate that full-chromosome reconstructions at 1/2 Mb resolution are possible using existing labeling and imaging technologies.  The updated reconstruction code and the script files used for this paper are available at:  https://github.com/heltilda/align3d.