Anopheles mosquitoes reveal new principles of 3D genome organization in insects

Functional characterization of transcriptional enhancers in an Anopheles genetic locus controlling natural resistance to the malaria parasite, Plasmodium falciparum

BACKGROUND

Anopheles mosquitoes and the malaria parasites they transmit remain a significant global health problem. Most genomic and functional genomic studies of mosquitoes have focused on the protein-coding genome, and comparatively little is known about the importance of noncoding transcriptional enhancers in controlling their gene expression and phenotypic variation. Here we evaluate nine enhancers previously identified in a STARR-seq screen and present in a genetic locus that was identified as a major influence on susceptibility to malaria infection in wild Anopheles coluzzii mosquitoes.

RESULT

We developed an analytical pipeline to filter nine enhancers in the malaria susceptibility locus on chromosome 2L. First, ATAC-seq revealed that only three of the nine enhancers were located in open chromatin and thus likely to be active in somatic cells. Next, we cloned these three enhancers from malaria-susceptible and resistant mosquitoes and measured their enhancer activity by luciferase reporter assays. Only two of the three open-chromatin enhancers displayed significantly different enhancer activity between resistant and susceptible alleles. Finally, alleles of just one of these enhancers, ENH_2L-03, contained nucleotide variants which also segregated in wild mosquitoes, and ENH_2L-03 was prioritized for further study. A noncoding RNA was detected within ENH_2L-03, consistent with an enhancer RNA (eRNA), which we depleted in mosquitoes using RNAi in order to silence the enhancer activity. Transcriptional profiling of ENH_2L-03-silenced mosquitoes revealed 15 differentially expressed genes, which share a transcription factor binding motif suggestive of coordinate regulation. However, silencing ENH_2L-03 did not influence infection levels of either human or rodent malaria parasites.

CONCLUSION

Despite the absence of an ENH_2L-03 effect on infection outcome, multiple enhancers can cooperate to influence a phenotype, and further examination of this enhancer is warranted. Overall, we provide a pipeline for the in vivo functional study of transcriptional enhancers in Anopheles, towards understanding how enhancer function may control important vector phenotypes.

Genome Mountaineering: Expanding Horizons of the 3D Genome for the Intrepid Evolutionary Adventurer

The physical positioning of DNA in 3D space within the nucleus can be important for understanding how genetic changes influence gene regulation and consequently phenotype. The costs of 3D genomic assays are falling, concomitant with the rapid innovation of newer, more customizable methods. Thus, evolutionary researchers are increasingly able to engage with these approaches as barriers diminish. As we apply these methods to a broader range of organisms, we learn more about principles governing genome structure and regulatory evolution in 3D space. Here, we use recent studies in primarily nonmodel organisms to illustrate how these approaches can provide novel insights into evolutionary processes. We focus on these cases as motivation for further research into evolutionary conservation and change in 3D organization; the relationship between 3D organization and structural changes in the genome; and the impact of 3D organization in the evolution of gene regulation and organismal traits. We argue that 3D genomic information can help resolve a wide range of outstanding questions in evolutionary biology, particularly as technologies improve and become more accessible in nonmodel systems.

Genome-wide cataloging and orthology analysis of long noncoding RNA expression in three species of Anopheles mosquito

BACKGROUND

Long noncoding RNAs (lncRNAs) are versatile regulatory molecules that affect cellular phenotypes through context-specific expression. While their role in controlling cellular pathways is well-established in insects, investigating lncRNA expression in Anopheles in a tissue- and species-specific manner could add to our understanding of malaria transmission by this important vector.

METHODOLOGY

We performed de novo transcriptome assembly of Anopheles minimus, Anopheles albimanus, and Anopheles arabiensis utilising publicly available RNA-Seq datasets of male reproductive tissues, male carcasses, female reproductive tissues, and female carcasses. Various bioinformatics tools were subsequently used for lncRNA identification, conservation analysis, and differential expression analysis across sexes and tissues.

RESULTS

We identified 9331, 5372, and 5256 lncRNA transcripts in An. albimanus, An. arabiensis, and An. minimus, respectively. Compared with An. albimanus lncRNAs, conservation analysis revealed that a total of 1964 and 1400 lncRNAs were conserved in An. arabiensis and An. minimus; however, only 283 and 253 lncRNAs presented sequence-level conservation. Differential expression (DE) analysis revealed that the carcasses presented the lowest difference in lncRNA expression, whereas in each comparison, the reproductive tissues (whether male or female) presented relatively high levels of differential expression. Additionally, 69 lncRNAs were found to be conserved at the sequence level in all 3 species. These lncRNAs were almost exclusively upregulated in males (reproductive tissue as well as carcasses) and exclusively downregulated in female carcasses.

CONCLUSIONS

The genes in the vicinity of differentially expressed, conserved lncRNAs were found to be involved in critical pathways such as nuclear structure, chromatin remodelling, protein and RNA metabolism, and cell cycle in each of these species. Future studies on these lncRNAs can provide useful insights into how these functions control sexually-dimorphic physiological phenomena such as host-seeking and biting behaviour of female mosquitoes, blood meal metabolism, reproductive behaviour, and disease-carrying capacity.

Divergent 3D genome organization in livers of cave and surface morphs of Astyanax mexicanus as a potential driver of unique metabolic adaptations in cave environment

The cave morphs of Astyanax mexicanus have evolved a suite of distinct adaptations to life in perpetual darkness, including the loss of eyes and pigmentation loss, as well as profound metabolic changes such as hyperphagia and starvation resilience, traits that sharply contrast with those of their river-dwelling surface counterparts. While changed gene expression is a primary driver of these adaptations, the underlying role of 3D genome organization - a key regulator of gene expression - remains unexplored. Here, we investigate the 3D genome architecture of the livers of surface fish and two cavefish morphs (Pachón and Tinaja) using Hi-C, performing the first comparative 3D genomic analysis in this species. We analyzed and identified cave-specific 3D genomic features, such as genomic compartments and loops, which were conserved in both the cave populations but absent in surface fish. Integrating the 3D genome data with transcriptomic and epigenetic datasets, linked these changes to differential expression of metabolically relevant genes, such as Arhgef19 and Endog . Additionally, our study also uncovered genomic inversions unique to cavefish, potentially tied to cave adaptation. Our findings suggest that 3D genome organization contributes to transcriptomic shifts underlying cavefish phenotypes, providing a novel intra-species and morph specific perspective on 3D chromatin evolution. This study establishes a foundation for exploring how genome architecture potentially facilitates adaptation to new environments. Comparison of morphs within the same species also establishes a foundation for better understanding of how 3D genome reorganization may drive speciation and phenotypic diversity.

Combining chromosome conformation capture and exome sequencing for simultaneous detection of structural and single-nucleotide variants

BACKGROUND

Effective molecular diagnosis of congenital diseases hinges on comprehensive genomic analysis, traditionally reliant on various methodologies specific to each variant type-whole exome or genome sequencing for single nucleotide variants (SNVs), array CGH for copy-number variants (CNVs), and microscopy for structural variants (SVs).

METHODS

We introduce a novel, integrative approach combining exome sequencing with chromosome conformation capture, termed Exo-C. This method enables the concurrent identification of SNVs in clinically relevant genes and SVs across the genome and allows analysis of heterozygous and mosaic carriers. Enhanced with targeted long-read sequencing, Exo-C evolves into a cost-efficient solution capable of resolving complex SVs at base-pair accuracy.

RESULTS

Applied to 66 human samples Exo-C achieved 100% recall and 73% precision in detecting chromosomal translocations and SNVs. We further benchmarked its performance for inversions and CNVs and demonstrated its utility in detecting mosaic SVs and resolving diagnostically challenging cases.

CONCLUSIONS

Through several case studies, we demonstrate how Exo-C's multifaceted application can effectively uncover diverse causative variants and elucidate disease mechanisms in patients with rare disorders.

Chromosome-scale scaffolds of the fungus gnat genome reveal multi-Mb-scale chromosome-folding interactions, centromeric enrichments of retrotransposons, and candidate telomere sequences

BACKGROUND

The lower Dipteran fungus gnat, Bradysia (aka Sciara) coprophila, has compelling chromosome biology. Paternal chromosomes are eliminated during male meiosis I and both maternal X sister chromatids are retained in male meiosis II. Embryos start with three copies of the X chromosome, but 1-2 copies are eliminated from somatic cells as part of sex determination, and one is eliminated in the germline to restore diploidy. In addition, there is gene amplification in larval polytene chromosomes, and the X polytene chromosome folds back on itself mediated by extremely long-range interactions between three loci. These developmentally normal events present opportunities to study chromosome behaviors that are unusual in other systems. Moreover, little is known about the centromeric and telomeric sequences of lower Dipterans in general, and there are recent claims of horizontally-transferred genes in fungus gnats. Overall, there is a pressing need to learn more about the fungus gnat chromosome sequences.

RESULTS

We produced the first chromosome-scale models of the X and autosomal chromosomes where each somatic chromosome is represented by a single scaffold. Extensive analysis supports the chromosome identity and structural accuracy of the scaffolds, demonstrating they are co-linear with historical polytene maps, consistent with evolutionary expectations, and have accurate centromere positions, chromosome lengths, and copy numbers. The positions of alleged horizontally-transferred genes in the nuclear chromosomes were broadly confirmed by genomic analyses of the chromosome scaffolds using Hi-C and single-molecule long-read datasets. The chromosomal context of repeats shows family-specific biases, such as retrotransposons correlated with the centromeres. Moreover, scaffold termini were enriched with arrays of retrotransposon-related sequence as well as nucleosome-length (~ 175 bp) satellite repeats. Finally, the Hi-C data captured Mb-scale physical interactions on the X chromosome that are seen in polytene spreads, and we characterize these interesting "fold-back regions" at the sequence level for the first time.

CONCLUSIONS

The chromosome scaffolds were shown to be of exceptional quality, including loci harboring horizontally-transferred genes. Repeat analyses demonstrate family-specific biases and telomere repeat candidates. Hi-C analyses revealed the sequences of ultra-long-range interactions on the X chromosome. The chromosome-scale scaffolds pave the way for further studies of the unusual chromosome movements in Bradysia coprophila.

Evolution of 3D Chromatin Folding

Studies examining the evolution of genomes have focused mainly on sequence conservation. However, the inner working of a cell implies tightly regulated crosstalk between complex gene networks controlled by small dispersed regulatory elements of physically contacting DNA regions. How these different levels of chromatin organization crosstalk in different species underpins the potential for genome evolutionary plasticity. We review the evolution of chromatin organization across the Animal Tree of Life. We introduce general aspects of the mode and tempo of genome evolution to later explore the multiple layers of genome organization. We argue that both genome and chromosome size modulate patterns of chromatin folding and that chromatin interactions facilitate the formation of lineage-specific chromosomal reorganizations, especially in germ cells. Overall, analyzing the mechanistic forces involved in the maintenance of chromatin structure and function of the germ line is critical for understanding genome evolution, maintenance, and inheritance.

From macro to micro: De novo genomes of Aedes mosquitoes enable comparative genomics among close and distant relatives

The yellow fever mosquito (Aedes aegypti) is an organism of high medical importance because it is the primary vector for diseases such as yellow fever, Zika, dengue, and chikungunya. Its medical importance has made it a subject of numerous efforts to understand their biology. One such effort, was the development of a high-quality reference genome (AaegL5). However, this reference genome was sourced from a highly inbred laboratory strain with unknown geographic origin. Thus, the reference is not representative of a wild mosquito, let alone one from its native range in sub-Saharan Africa. To better understand the genetic architecture of Ae. aegypti and their sister species, we developed two de novo chromosome-scale genomes with sequences sourced from single individuals: one of Ae. aegypti formosus (Aaf) from Burkina Faso and one of Ae. mascarensis (Am) from Mauritius. Both genomes exhibit high contiguity and gene completeness, comparable to AaegL5. While Aaf exhibits high degree of synteny to AaegL5, it also exhibits several large inversions. We further conducted comparative genomic analyses using our genomes and other publicly available culicid reference genomes to find extensive chromosomal rearrangements between major lineages. Overrepresentation analysis of expanded genes in Aaf, AaegL5, and Am revealed that while the overarching category of genes that have expanded are similar, the specific genes that have expanded differ. Our findings elucidate novel insights into chromosome evolution at both microevolutionary and macroevolutionary scales. The genomic resources we present are additions to the arsenal of biologists in understanding mosquito biology and genome evolution.

Evolution and function of chromatin domains across the tree of life

The genome of all organisms is spatially organized to function efficiently. The advent of genome-wide chromatin conformation capture (Hi-C) methods has revolutionized our ability to probe the three-dimensional (3D) organization of genomes across diverse species. In this Review, we compare 3D chromatin folding from bacteria and archaea to that in mammals and plants, focusing on topology at the level of gene regulatory domains. In doing so, we consider systematic similarities and differences that hint at the origin and evolution of spatial chromatin folding and its relation to gene activity. We discuss the universality of spatial chromatin domains in all kingdoms, each encompassing one to several genes. We also highlight differences between organisms and suggest that similar features in Hi-C matrices do not necessarily reflect the same biological process or function. Furthermore, we discuss the evolution of domain boundaries and boundary-forming proteins, which indicates that structural maintenance of chromosome (SMC) proteins and the transcription machinery are the ancestral sculptors of the genome. Architectural proteins such as CTCF serve as clade-specific determinants of genome organization. Finally, studies in many non-model organisms show that, despite the ancient origin of 3D chromatin folding and its intricate link to gene activity, evolution tolerates substantial changes in genome organization.

Chromosomal inversions and their impact on insect evolution

Insects can adapt quickly and effectively to rapid environmental change and maintain long-term adaptations, but the genetic mechanisms underlying this response are not fully understood. In this review, we summarize studies on the potential impact of chromosomal inversion polymorphisms on insect evolution at different spatial and temporal scales, ranging from long-term evolutionary stability to rapid emergence in response to emerging biotic and abiotic factors. The study of inversions has recently been advanced by comparative, population, and 3D genomics methods. The impact of inversions on insect genome evolution can be profound, including increased gene order rearrangements on sex chromosomes, accumulation of transposable elements, and facilitation of genome divergence. Understanding these processes provides critical insights into the evolutionary mechanisms shaping insect diversity.

Prediction of YY1 loop anchor based on multi-omics features

The three-dimensional structure of chromatin is crucial for the regulation of gene expression. YY1 promotes enhancer-promoter interactions in a manner analogous to CTCF-mediated chromatin interactions. However, little is known about which YY1 binding sites can form loop anchors. In this study, the LightGBM model was used to predict YY1-loop anchors by integrating multi-omics data. Due to the large imbalance in the number of positive and negative samples, we use AUPRC to reflect the quality of the classifier. The results show that the LightGBM model exhibits strong predictive performance (AUPRC≥0.93). To verify the robustness of the model, the dataset was divided into training and test sets at a 4:1 ratio. The results show that the model performs well for YY1-loop anchor prediction on both the training and independent test sets. Additionally, we ranked the importance of the features and found that the formation of YY1-loop anchors is primarily influenced by the co-binding of transcription factors CTCF, SMC3, and RAD21, as well as histone modifications and sequence context.

Sequence-Based Machine Learning Reveals 3D Genome Differences between Bonobos and Chimpanzees

The 3D structure of the genome is an important mediator of gene expression. As phenotypic divergence is largely driven by gene regulatory variation, comparing genome 3D contacts across species can further understanding of the molecular basis of species differences. However, while experimental data on genome 3D contacts in humans are increasingly abundant, only a handful of 3D genome contact maps exist for other species. Here, we demonstrate that human experimental data can be used to close this data gap. We apply a machine learning model that predicts 3D genome contacts from DNA sequence to the genomes from 56 bonobos and chimpanzees and identify species-specific patterns of genome folding. We estimated 3D divergence between individuals from the resulting contact maps in 4,420 1 Mb genomic windows, of which ∼17% were substantially divergent in predicted genome contacts. Bonobos and chimpanzees diverged at 89 windows, overlapping genes associated with multiple traits implicated in Pan phenotypic divergence. We discovered 51 bonobo-specific variants that individually produce the observed bonobo contact pattern in bonobo-chimpanzee divergent windows. Our results demonstrate that machine learning methods can leverage human data to fill in data gaps across species, offering the first look at population-level 3D genome variation in nonhuman primates. We also identify loci where changes in 3D folding may contribute to phenotypic differences in our closest living relatives.

Chromosome-scale genome of the polyphagous pest Anastrepha ludens (Diptera: Tephritidae) provides insights on sex chromosome evolution in Anastrepha

The Mexican fruit fly, Anastrepha ludens, is a polyphagous true fruit fly (Diptera: Tephritidae) considered one of the most serious insect pests in Central and North America to various economically relevant fruits. Despite its agricultural relevance, a high-quality genome assembly has not been reported. Here, we described the generation of a chromosome-level genome for the A. ludens using a combination of PacBio high fidelity long-reads and chromatin conformation capture sequencing data. The final assembly consisted of 140 scaffolds (821 Mb, N50 = 131 Mb), containing 99.27% complete conserved orthologs (BUSCO) for Diptera. We identified the sex chromosomes using three strategies: 1) visual inspection of Hi-C contact map and coverage analysis using the HiFi reads, 2) synteny with Drosophila melanogaster, and 3) the difference in the average read depth of autosomal versus sex chromosomal scaffolds. The X chromosome was found in one major scaffold (100 Mb) and eight smaller contigs (1.8 Mb), and the Y chromosome was recovered in one large scaffold (6.1 Mb) and 35 smaller contigs (4.3 Mb). Sex chromosomes and autosomes showed considerable differences of transposable elements and gene content. Moreover, evolutionary rates of orthologs of A. ludens and Anastrepha obliqua revealed a faster evolution of X-linked, compared to autosome-linked, genes, consistent with the faster-X effect, leading us to new insights on the evolution of sex chromosomes in this diverse group of flies. This genome assembly provides a valuable resource for future evolutionary, genetic, and genomic translational research supporting the management of this important agricultural pest.

Chromosomal reference genome sequences for the malaria mosquito, Anopheles coustani, Laveran, 1900

We present genome assembly from individual female An. coustani (African malaria mosquito; Arthropoda; Insecta; Diptera; Culicidae) from Lopé, Gabon. The genome sequence is 270 megabases in span. Most of the assembly is scaffolded into three chromosomal pseudomolecules with the X sex chromosome assembled for both species. The complete mitochondrial genome was also assembled and is 15.4 kilobases in length.

Hi-C-guided many-polymer model to decipher 3D genome organization

We propose a high-throughput chromosome conformation capture data-based many-polymer model that allows us to generate an ensemble of multi-scale genome structures. We demonstrate the efficacy of our model by validating the generated structures against experimental measurements and employ them to address key questions regarding genome organization. Our model first confirms a significant correlation between chromosome size and nuclear positioning. Specifically, smaller chromosomes are distributed at the core region, whereas larger chromosomes are at the periphery, interacting with the nuclear envelope. The spatial distribution of A- and B-type compartments, which is nontrivial to infer from the corresponding high-throughput chromosome conformation capture maps alone, can also be elucidated using our model, accounting for an issue such as the effect of chromatin-lamina interaction on the compartmentalization of conventional and inverted nuclei. In accordance with imaging data, the overall shape of the 3D genome structures generated from our model displays significant variation. As a case study, we apply our method to the yellow fever mosquito genome, finding that the predicted morphology displays, on average, a more globular shape than the previously suggested spindle-like organization and that our prediction better aligns with the fluorescence in situ hybridization data. Our model has great potential to be extended to investigate many outstanding issues concerning 3D genome organization.

Evolutionary patterns and functional effects of 3D chromatin structures in butterflies with extensive genome rearrangements

Chromosome rearrangements may distort 3D chromatin architectures and thus change gene regulation, yet how 3D chromatin structures evolve in insects is largely unknown. Here, we obtain chromosome-level genomes for four butterfly species, Graphium cloanthus, Graphium sarpedon, Graphium eurypylus with 2n = 30, 40, and 60, respectively, and Papilio bianor with 2n = 60. Together with large-scale Hi-C data, we find that inter-chromosome rearrangements very rarely disrupted the pre-existing 3D chromatin structure of ancestral chromosomes. However, some intra-chromosome rearrangements changed 3D chromatin structures compared to the ancestral configuration. We find that new TADs and subTADs have emerged across the rearrangement sites where their adjacent compartments exhibit uniform types. Two intra-chromosome rearrangements altered Rel and lft regulation, potentially contributing to wing patterning differentiation and host plant choice. Notably, butterflies exhibited chromatin loops between Hox gene cluster ANT-C and BX-C, unlike Drosophila. Our CRISPR-Cas9 experiments in butterflies confirm that knocking out the CTCF binding site of the loops in BX-C affected the phenotypes regulated by Antp in ANT-C, resulting in legless larva. Our results reveal evolutionary patterns of insect 3D chromatin structures and provide evidence that 3D chromatin structure changes can play important roles in the evolution of traits.

The probability of chromatin to be at the nuclear lamina has no systematic effect on its transcription level in fruit flies

BACKGROUND

Multiple studies have demonstrated a negative correlation between gene expression and positioning of genes at the nuclear envelope (NE) lined by nuclear lamina, but the exact relationship remains unclear, especially in light of the highly stochastic, transient nature of the gene association with the NE.

RESULTS

In this paper, we ask whether there is a causal, systematic, genome-wide relationship between the expression levels of the groups of genes in topologically associating domains (TADs) of Drosophila nuclei and the probabilities of TADs to be found at the NE. To investigate the nature of this possible relationship, we combine a coarse-grained dynamic model of the entire Drosophila nucleus with genome-wide gene expression data; we analyze the TAD averaged transcription levels of genes against the probabilities of individual TADs to be in contact with the NE in the control and lamins-depleted nuclei. Our findings demonstrate that, within the statistical error margin, the stochastic positioning of Drosophila melanogaster TADs at the NE does not, by itself, systematically affect the mean level of gene expression in these TADs, while the expected negative correlation is confirmed. The correlation is weak and disappears completely for TADs not containing lamina-associated domains (LADs) or TADs containing LADs, considered separately. Verifiable hypotheses regarding the underlying mechanism for the presence of the correlation without causality are discussed. These include the possibility that the epigenetic marks and affinity to the NE of a TAD are determined by various non-mutually exclusive mechanisms and remain relatively stable during interphase.

CONCLUSIONS

At the level of TADs, the probability of chromatin being in contact with the nuclear envelope has no systematic, causal effect on the transcription level in Drosophila. The conclusion is reached by combining model-derived time-evolution of TAD locations within the nucleus with their experimental gene expression levels.

Two Nested Inversions in the X Chromosome Differentiate the Dominant Malaria Vectors in Europe, Anopheles atroparvus and Anopheles messeae

The Maculipennis subgroup of malaria mosquitoes includes both dominant malaria vectors and non-vectors in Eurasia. Understanding the genetic factors, particularly chromosomal inversions, that differentiate Anopheles species can provide valuable insights for vector control strategies. Although autosomal inversions between the species in this subgroup have been characterized based on the chromosomal banding patterns, the number and positions of rearrangements in the X chromosome remain unclear due to the divergent banding patterns. Here, we identified two large X chromosomal inversions, approximately 13 Mb and 10 Mb in size, using fluorescence in situ hybridization. The inversion breakpoint regions were mapped by hybridizing 53 gene markers with polytene chromosomes of An. messeae. The DNA probes were designed based on gene sequences from the annotated An. atroparvus genome. The two nested inversions resulted in five syntenic blocks. Only two small syntenic blocks, which encompass 181 annotated genes in the An. atroparvus genome, changed their position and orientation in the X chromosome. The analysis of the An. atroparvus genome revealed an enrichment of gene ontology terms associated with immune system and mating behavior in the rearranged syntenic blocks. Additionally, the enrichment of DNA transposons was found in sequences homologous to three of the four breakpoint regions. This study demonstrates the successful application of the physical genome mapping approach to identify rearrangements that differentiate species in insects with polytene chromosomes.

Towards Development of the 4C-Based Method Detecting Interactions of Plasmid DNA with Host Genome

Chromosome conformation capture techniques have revolutionized our understanding of chromatin architecture and dynamics at the genome-wide scale. In recent years, these methods have been applied to a diverse array of species, revealing fundamental principles of chromosomal organization. However, structural organization of the extrachromosomal entities, like viral genomes or plasmids, and their interactions with the host genome, remain relatively underexplored. In this work, we introduce an enhanced 4C-protocol tailored for probing plasmid DNA interactions. We design specific plasmid vector and optimize protocol to allow high detection rate of contacts between the plasmid and host DNA.

The chromosome-scale genome assembly for the West Nile vector Culex quinquefasciatus uncovers patterns of genome evolution in mosquitoes

BACKGROUND

Understanding genome organization and evolution is important for species involved in transmission of human diseases, such as mosquitoes. Anophelinae and Culicinae subfamilies of mosquitoes show striking differences in genome sizes, sex chromosome arrangements, behavior, and ability to transmit pathogens. However, the genomic basis of these differences is not fully understood.

METHODS

In this study, we used a combination of advanced genome technologies such as Oxford Nanopore Technology sequencing, Hi-C scaffolding, Bionano, and cytogenetic mapping to develop an improved chromosome-scale genome assembly for the West Nile vector Culex quinquefasciatus.

RESULTS

We then used this assembly to annotate odorant receptors, odorant binding proteins, and transposable elements. A genomic region containing male-specific sequences on chromosome 1 and a polymorphic inversion on chromosome 3 were identified in the Cx. quinquefasciatus genome. In addition, the genome of Cx. quinquefasciatus was compared with the genomes of other mosquitoes such as malaria vectors An. coluzzi and An. albimanus, and the vector of arboviruses Ae. aegypti. Our work confirms significant expansion of the two chemosensory gene families in Cx. quinquefasciatus, as well as a significant increase and relocation of the transposable elements in both Cx. quinquefasciatus and Ae. aegypti relative to the Anophelines. Phylogenetic analysis clarifies the divergence time between the mosquito species. Our study provides new insights into chromosomal evolution in mosquitoes and finds that the X chromosome of Anophelinae and the sex-determining chromosome 1 of Culicinae have a significantly higher rate of evolution than autosomes.

CONCLUSION

The improved Cx. quinquefasciatus genome assembly uncovered new details of mosquito genome evolution and has the potential to speed up the development of novel vector control strategies.

Expanding the list of sequence-agnostic enzymes for chromatin conformation capture assays with S1 nuclease

This study presents a novel approach for mapping global chromatin interactions using S1 nuclease, a sequence-agnostic enzyme. We develop and outline a protocol that leverages S1 nuclease's ability to effectively introduce breaks into both open and closed chromatin regions, allowing for comprehensive profiling of chromatin properties. Our S1 Hi-C method enables the preparation of high-quality Hi-C libraries, marking a significant advancement over previously established DNase I Hi-C protocols. Moreover, S1 nuclease's capability to fragment chromatin to mono-nucleosomes suggests the potential for mapping the three-dimensional organization of the genome at high resolution. This methodology holds promise for an improved understanding of chromatin state-dependent activities and may facilitate the development of new genomic methods.

Functional genomics in Spiralia

Our understanding of the mechanisms that modulate gene expression in animals is strongly biased by studying a handful of model species that mainly belong to three groups: Insecta, Nematoda and Vertebrata. However, over half of the animal phyla belong to Spiralia, a morphologically and ecologically diverse animal clade with many species of economic and biomedical importance. Therefore, investigating genome regulation in this group is central to uncovering ancestral and derived features in genome functioning in animals, which can also be of significant societal impact. Here, we focus on five aspects of gene expression regulation to review our current knowledge of functional genomics in Spiralia. Although some fields, such as single-cell transcriptomics, are becoming more common, the study of chromatin accessibility, DNA methylation, histone post-translational modifications and genome architecture are still in their infancy. Recent efforts to generate chromosome-scale reference genome assemblies for greater species diversity and optimise state-of-the-art approaches for emerging spiralian research systems will address the existing knowledge gaps in functional genomics in this animal group.

Emerging questions on the mechanisms and dynamics of 3D genome evolution in spiralians

Information on how 3D genome topology emerged in animal evolution, how stable it is during development, its role in the evolution of phenotypic novelties and how exactly it affects gene expression is highly debated. So far, data to address these questions are lacking with the exception of a few key model species. Several gene regulatory mechanisms have been proposed, including scenarios where genome topology has little to no impact on gene expression, and vice versa. The ancient and diverse clade of spiralians may provide a crucial testing ground for such mechanisms. Sprialians have followed distinct evolutionary trajectories, with some clades experiencing genome expansions and/or large-scale genome rearrangements, and others undergoing genome contraction, substantially impacting their size and organisation. These changes have been associated with many phenotypic innovations in this clade. In this review, we describe how emerging genome topology data, along with functional tools, allow for testing these scenarios and discuss their predicted outcomes.

Sequence-based machine learning reveals 3D genome differences between bonobos and chimpanzees

Phenotypic divergence between closely related species, including bonobos and chimpanzees (genus Pan), is largely driven by variation in gene regulation. The 3D structure of the genome mediates gene expression; however, genome folding differences in Pan are not well understood. Here, we apply machine learning to predict genome-wide 3D genome contact maps from DNA sequence for 56 bonobos and chimpanzees, encompassing all five extant lineages. We use a pairwise approach to estimate 3D divergence between individuals from the resulting contact maps in 4,420 1 Mb genomic windows. While most pairs were similar, ∼17% were predicted to be substantially divergent in genome folding. The most dissimilar maps were largely driven by single individuals with rare variants that produce unique 3D genome folding in a region. We also identified 89 genomic windows where bonobo and chimpanzee contact maps substantially diverged, including several windows harboring genes associated with traits implicated in Pan phenotypic divergence. We used in silico mutagenesis to identify 51 3D-modifying variants in these bonobo-chimpanzee divergent windows, finding that 34 or 66.67% induce genome folding changes via CTCF binding motif disruption. Our results reveal 3D genome variation at the population-level and identify genomic regions where changes in 3D folding may contribute to phenotypic differences in our closest living relatives.

Strong interactions between highly dynamic lamina-associated domains and the nuclear envelope stabilize the 3D architecture of Drosophila interphase chromatin

BACKGROUND

Interactions among topologically associating domains (TADs), and between the nuclear envelope (NE) and lamina-associated domains (LADs) are expected to shape various aspects of three-dimensional (3D) chromatin structure and dynamics; however, relevant genome-wide experiments that may provide statistically significant conclusions remain difficult.

RESULTS

We have developed a coarse-grained dynamical model of D. melanogaster nuclei at TAD resolution that explicitly accounts for four distinct epigenetic classes of TADs and LAD-NE interactions. The model is parameterized to reproduce the experimental Hi-C map of the wild type (WT) nuclei; it describes time evolution of the chromatin over the G1 phase of the interphase. The simulations include an ensemble of nuclei, corresponding to the experimentally observed set of several possible mutual arrangements of chromosomal arms. The model is validated against multiple structural features of chromatin from several different experiments not used in model development. Predicted positioning of all LADs at the NE is highly dynamic-the same LAD can attach, detach and move far away from the NE multiple times during interphase. The probabilities of LADs to be in contact with the NE vary by an order of magnitude, despite all having the same affinity to the NE in the model. These probabilities are mostly determined by a highly variable local linear density of LADs along the genome, which also has the same strong effect on the predicted positioning of individual TADs -- higher probability of a TAD to be near NE is largely determined by a higher linear density of LADs surrounding this TAD. The distribution of LADs along the chromosome chains plays a notable role in maintaining a non-random average global structure of chromatin. Relatively high affinity of LADs to the NE in the WT nuclei substantially reduces sensitivity of the global radial chromatin distribution to variations in the strength of TAD-TAD interactions compared to the lamin depleted nuclei, where a small (0.5 kT) increase of cross-type TAD-TAD interactions doubles the chromatin density in the central nucleus region.

CONCLUSIONS

A dynamical model of the entire fruit fly genome makes multiple genome-wide predictions of biological interest. The distribution of LADs along the chromatin chains affects their probabilities to be in contact with the NE and radial positioning of highly mobile TADs, playing a notable role in creating a non-random average global structure of the chromatin. We conjecture that an important role of attractive LAD-NE interactions is to stabilize global chromatin structure against inevitable cell-to-cell variations in TAD-TAD interactions.

Genomic rearrangements and evolutionary changes in 3D chromatin topologies in the cotton tribe (Gossypieae)

BACKGROUND

Analysis of the relationship between chromosomal structural variation (synteny breaks) and 3D-chromatin architectural changes among closely related species has the potential to reveal causes and correlates between chromosomal change and chromatin remodeling. Of note, contrary to extensive studies in animal species, the pace and pattern of chromatin architectural changes following the speciation of plants remain unexplored; moreover, there is little exploration of the occurrence of synteny breaks in the context of multiple genome topological hierarchies within the same model species.

RESULTS

Here we used Hi-C and epigenomic analyses to characterize and compare the profiles of hierarchical chromatin architectural features in representative species of the cotton tribe (Gossypieae), including Gossypium arboreum, Gossypium raimondii, and Gossypioides kirkii, which differ with respect to chromosome rearrangements. We found that (i) overall chromatin architectural territories were preserved in Gossypioides and Gossypium, which was reflected in their similar intra-chromosomal contact patterns and spatial chromosomal distributions; (ii) the non-random preferential occurrence of synteny breaks in A compartment significantly associate with the B-to-A compartment switch in syntenic blocks flanking synteny breaks; (iii) synteny changes co-localize with open-chromatin boundaries of topologically associating domains, while TAD stabilization has a greater influence on regulating orthologous expression divergence than do rearrangements; and (iv) rearranged chromosome segments largely maintain ancestral in-cis interactions.

CONCLUSIONS

Our findings provide insights into the non-random occurrence of epigenomic remodeling relative to the genomic landscape and its evolutionary and functional connections to alterations of hierarchical chromatin architecture, on a known evolutionary timescale.

Function and Evolution of the Loop Extrusion Machinery in Animals

Structural maintenance of chromosomes (SMC) complexes are essential proteins found in genomes of all cellular organisms. Essential functions of these proteins, such as mitotic chromosome formation and sister chromatid cohesion, were discovered a long time ago. Recent advances in chromatin biology showed that SMC proteins are involved in many other genomic processes, acting as active motors extruding DNA, which leads to the formation of chromatin loops. Some loops formed by SMC proteins are highly cell type and developmental stage specific, such as SMC-mediated DNA loops required for VDJ recombination in B-cell progenitors, or dosage compensation in Caenorhabditis elegans and X-chromosome inactivation in mice. In this review, we focus on the extrusion-based mechanisms that are common for multiple cell types and species. We will first describe an anatomy of SMC complexes and their accessory proteins. Next, we provide biochemical details of the extrusion process. We follow this by the sections describing the role of SMC complexes in gene regulation, DNA repair, and chromatin topology.

A Cre-LoxP-based approach for combinatorial chromosome rearrangements in human HAP1 cells

Alterations of human karyotype caused by chromosomal rearrangements are often associated with considerable phenotypic effects. Studying molecular mechanisms underlying these effects requires an efficient and scalable experimental model. Here, we propose a Cre-LoxP-based approach for the generation of combinatorial diversity of chromosomal rearrangements. We demonstrate that using the developed system, both intra- and inter-chromosomal rearrangements can be induced in the human haploid HAP1 cells, although the latter is significantly less effective. The obtained genetically modified HAP1 cell line can be used to dissect genomic effects associated with intra-chromosomal structural variations.

Evolution of 3D chromatin organization at different scales

Most animal genomes fold in 3D chromatin domains called topologically associated domains (TADs) that facilitate interactions between cis-regulatory elements (CREs) and promoters. Owing to their critical role in the control of developmental gene expression, we explore how TADs have shaped animal evolution. In the light of recent studies that profile TADs in disparate animal lineages, we discuss their phylogenetic distribution and the mechanisms that underlie their formation. We present evidence indicating that TADs are plastic entities composed of genomic strata of different ages: ancient cores are combined with newer regions and brought into extant TADs through genomic rearrangements. We highlight that newly incorporated TAD strata enable the establishment of new CRE-promoter interactions and in turn new expression patterns that can drive phenotypical innovation. We further highlight how subtle changes in chromatin folding may fine-tune the expression levels of developmental genes and hold a potential for evolutionary significance.

Interphase chromosomes of the Aedes aegypti mosquito are liquid crystalline and can sense mechanical cues

We use data-driven physical simulations to study the three-dimensional architecture of the Aedes aegypti genome. Hi-C maps exhibit both a broad diagonal and compartmentalization with telomeres and centromeres clustering together. Physical modeling reveals that these observations correspond to an ensemble of 3D chromosomal structures that are folded over and partially condensed. Clustering of the centromeres and telomeres near the nuclear lamina appears to be a necessary condition for the formation of the observed structures. Further analysis of the mechanical properties of the genome reveals that the chromosomes of Aedes aegypti, by virtue of their atypical structural organization, are highly sensitive to the deformation of the nuclei. This last finding provides a possible physical mechanism linking mechanical cues to gene regulation.

Visualization of the Linear and Spatial Organization of Chromosomes in Mosquitoes

Mosquitoes are vectors of dangerous human diseases such as malaria, dengue, Zika, West Nile fever, and lymphatic filariasis. Visualization of the linear and spatial organization of mosquito chromosomes is important for understanding genome structure and function. Utilization of chromosomal inversions as markers for population genetics studies yields insights into mosquito adaptation and evolution. Cytogenetic approaches assist with the development of chromosome-scale genome assemblies that are useful tools for studying mosquito biology and for designing novel vector control strategies. Fluorescence in situ hybridization is a powerful technique for localizing specific DNA sequences within the linear chromosome structure and within the spatial organization of the cell nucleus. Here, we introduce protocols used in our laboratories for chromosome visualization and their application in mosquitoes.