Convergence of topological domain boundaries, insulators, and polytene interbands revealed by high-resolution mapping of chromatin contacts in the early Drosophila melanogaster embryo

Stem-loop and circle-loop TADs generated by directional pairing of boundary elements have distinct physical and regulatory properties

The chromosomes in multicellular eukaryotes are organized into a series of topologically independent loops called TADs. In flies, TADs are formed by physical interactions between neighboring boundaries. Fly boundaries exhibit distinct partner preferences, and pairing interactions between boundaries are typically orientation-dependent. Pairing can be head-to-tail or head-to-head. The former generates a stem-loop TAD, while the latter gives a circle-loop TAD. The TAD that encompasses the Drosophila even skipped (eve) gene is formed by the head-to-tail pairing of the nhomie and homie boundaries. To explore the relationship between loop topology and the physical and regulatory landscape, we flanked the nhomie boundary region with two attP sites. The attP sites were then used to generate four boundary replacements: λ DNA, nhomie forward (WT orientation), nhomie reverse (opposite of WT orientation), and homie forward (same orientation as WT homie). The nhomie forward replacement restores the WT physical and regulatory landscape: in MicroC experiments, the eve TAD is a 'volcano' triangle topped by a plume, and the eve gene and its regulatory elements are sequestered from interactions with neighbors. The λ DNA replacement lacks boundary function: the endpoint of the 'new' eve TAD on the nhomie side is ill-defined, and eve stripe enhancers activate a nearby gene, eIF3j. While nhomie reverse and homie forward restore the eve TAD, the topology is a circle-loop, and this changes the local physical and regulatory landscape. In MicroC experiments, the eve TAD interacts with its neighbors, and the plume at the top of the eve triangle peak is converted to a pair of 'clouds' of contacts with the next-door TADs. Consistent with the loss of isolation afforded by the stem-loop topology, the eve enhancers weakly activate genes in the neighboring TADs. Conversely, eve function is partially disrupted.

Genome organization regulates nuclear pore complex formation and promotes differentiation during Drosophila oogenesis

Genome organization can regulate gene expression and promote cell fate transitions. The differentiation of germline stem cells (GSCs) to oocytes in Drosophila involves changes in genome organization mediated by heterochromatin and the nuclear pore complex (NPC). Heterochromatin represses germ cell genes during differentiation, and NPCs anchor these silenced genes to the nuclear periphery, maintaining silencing to allow for oocyte development. Surprisingly, we found that genome organization also contributes to NPC formation, mediated by the transcription factor Stonewall (Stwl). As GSCs differentiate, Stwl accumulates at boundaries between silenced and active gene compartments. Stwl at these boundaries plays a pivotal role in transitioning germ cell genes into a silenced state and activating a group of oocyte genes and nucleoporins (Nups). The upregulation of these Nups during differentiation is crucial for NPC formation and further genome organization. Thus, cross-talk between genome architecture and NPCs is essential for successful cell fate transitions.

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.

Developmental and Housekeeping Genes: Two Types of Genetic Organization in the Drosophila Genome

We developed a procedure for locating genes on Drosophila melanogaster polytene chromosomes and described three types of chromosome structures (gray bands, black bands, and interbands), which differed markedly in morphological and genetic properties. This was reached through the use of our original methods of molecular and genetic analysis, electron microscopy, and bioinformatics data processing. Analysis of the genome-wide distribution of these properties led us to a bioinformatics model of the Drosophila genome organization, in which the genome was divided into two groups of genes. One was constituted by 65, in which the genome was divided into two groups, 62 genes that are expressed in most cell types during life cycle and perform basic cellular functions (the so-called "housekeeping genes"). The other one was made up of 3162 genes that are expressed only at particular stages of development ("developmental genes"). These two groups of genes are so different that we may state that the genome has two types of genetic organization. Different are the timings of their expression, chromatin packaging levels, the composition of activating and deactivating proteins, the sizes of these genes, the lengths of their introns, the organization of the promoter regions of the genes, the locations of origin recognition complexes (ORCs), and DNA replication timings.

Cis-regulatory modes of Ultrabithorax inactivation in butterfly forewings

Hox gene clusters encode transcription factors that drive regional specialization during animal development: for example the Hox factor Ubx is expressed in the insect metathoracic (T3) wing appendages and differentiates them from T2 mesothoracic identities. Hox transcriptional regulation requires silencing activities that prevent spurious activation and regulatory crosstalks in the wrong tissues, but this has seldom been studied in insects other than Drosophila, which shows a derived Hox dislocation into two genomic clusters that disjoined Antennapedia (Antp) and Ultrabithorax (Ubx). Here, we investigated how Ubx is restricted to the hindwing in butterflies, amidst a contiguous Hox cluster. By analysing Hi-C and ATAC-seq data in the butterfly Junonia coenia, we show that a Topologically Associated Domain (TAD) maintains a hindwing-enriched profile of chromatin opening around Ubx. This TAD is bordered by a Boundary Element (BE) that separates it from a region of joined wing activity around the Antp locus. CRISPR mutational perturbation of this BE releases ectopic Ubx expression in forewings, inducing homeotic clones with hindwing identities. Further mutational interrogation of two non-coding RNA encoding regions and one putative cis-regulatory module within the Ubx TAD cause rare homeotic transformations in both directions, indicating the presence of both activating and repressing chromatin features. We also describe a series of spontaneous forewing homeotic phenotypes obtained in Heliconius butterflies, and discuss their possible mutational basis. By leveraging the extensive wing specialization found in butterflies, our initial exploration of Ubx regulation demonstrates the existence of silencing and insulating sequences that prevent its spurious expression in forewings.

Genome organization regulates nuclear pore complex formation and promotes differentiation during Drosophila oogenesis

Genome organization can regulate gene expression and promote cell fate transitions. The differentiation of germline stem cells (GSCs) to oocytes in Drosophila involves changes in genome organization mediated by heterochromatin and the nuclear pore complex (NPC). Heterochromatin represses germ-cell genes during differentiation and NPCs anchor these silenced genes to the nuclear periphery, maintaining silencing to allow for oocyte development. Surprisingly, we find that genome organization also contributes to NPC formation, mediated by the transcription factor Stonewall (Stwl). As GSCs differentiate, Stwl accumulates at boundaries between silenced and active gene compartments. Stwl at these boundaries plays a pivotal role in transitioning germ-cell genes into a silenced state and activating a group of oocyte genes and Nucleoporins (Nups). The upregulation of these Nups during differentiation is crucial for NPC formation and further genome organization. Thus, crosstalk between genome architecture and NPCs is essential for successful cell fate transitions.

Setting the stage for development: the maternal-to-zygotic transition in Drosophila

The zygote has a daunting task ahead of itself; it must develop from a single cell (fertilized egg) into a fully functioning adult with a multitude of different cell types. In the beginning, the zygote has help from its mother, in the form of gene products deposited into the egg, but eventually, it must rely on its own resources to proceed through development. The transfer of developmental control from the mother to the embryo is called the maternal-to-zygotic transition (MZT). All animals undergo this transition, which is defined by two main processes-the degradation of maternal RNAs and the synthesis of new RNAs from the zygote's own genome. Here, we review the regulation of the MZT in Drosophila, but given the broad conservation of this essential process, much of the regulation is shared among metazoans.

Localization of the Drosophila pioneer factor GAF to subnuclear foci is driven by DNA binding and required to silence satellite repeat expression

The eukaryotic genome is organized to enable the precise regulation of gene expression. This organization is established as the embryo transitions from a fertilized gamete to a totipotent zygote. To understand the factors and processes that drive genomic organization, we focused on the pioneer factor GAGA factor (GAF) that is required for early development in Drosophila. GAF transcriptionally activates the zygotic genome and is localized to subnuclear foci. This non-uniform distribution is driven by binding to highly abundant GA repeats. At GA repeats, GAF is necessary to form heterochromatin and silence transcription. Thus, GAF is required to establish both active and silent regions. We propose that foci formation enables GAF to have opposing transcriptional roles within a single nucleus. Our data support a model in which the subnuclear concentration of transcription factors acts to organize the nucleus into functionally distinct domains essential for the robust regulation of gene expression.

Chromosome-level organization of the regulatory genome in the Drosophila nervous system

Previous studies have identified topologically associating domains (TADs) as basic units of genome organization. We present evidence of a previously unreported level of genome folding, where distant TAD pairs, megabases apart, interact to form meta-domains. Within meta-domains, gene promoters and structural intergenic elements present in distant TADs are specifically paired. The associated genes encode neuronal determinants, including those engaged in axonal guidance and adhesion. These long-range associations occur in a large fraction of neurons but support transcription in only a subset of neurons. Meta-domains are formed by diverse transcription factors that are able to pair over long and flexible distances. We present evidence that two such factors, GAF and CTCF, play direct roles in this process. The relative simplicity of higher-order meta-domain interactions in Drosophila, compared with those previously described in mammals, allowed the demonstration that genomes can fold into highly specialized cell-type-specific scaffolds that enable megabase-scale regulatory associations.

Precisely timed regulation of enhancer activity defines the binary expression pattern of Fushi tarazu in the Drosophila embryo

The genes that drive development each typically have many different enhancers. Properly coordinating the action of these different enhancers is crucial to correctly specifying cell-fate decisions, yet it remains poorly understood how their activity is choregraphed in time. To shed light on this question, we used recently developed single-cell live imaging tools to dissect the regulation of Fushi tarazu (Ftz) in Drosophila melanogaster embryos. Ftz is a transcription factor that is expressed in asymmetric stripes by two distinct enhancers: autoregulatory and zebra. The anterior edge of each stripe needs to be sharply defined to specify essential lineage boundaries. Here, we tracked how boundary cells commit to either a high-Ftz or low-Ftz fate by measuring Ftz protein traces in real time and simultaneously quantifying transcription from the endogenous locus and individual enhancers. This revealed that the autoregulatory enhancer does not establish this fate choice. Instead, it perpetuates the decision defined by zebra. This is contrary to the prevailing view that autoregulation drives the fate decision by causing bi-stable Ftz expression. Furthermore, we showed that the autoregulatory enhancer is not activated based on a Ftz-concentration threshold but through a timing-based mechanism. We hypothesize that this is regulated by several ubiquitously expressed pioneer-like transcription factors, which have recently been shown to act as timers in the embryo. Our work provides new insight into how precisely timed enhancer activity can directly regulate the dynamics of gene regulatory networks, which may be a general mechanism for ensuring that embryogenesis runs like clockwork.

Stress-sensitive dynamics of miRNAs and Elba1 in Drosophila embryogenesis

Early-life stress can result in life-long effects that impact adult health and disease risk, but little is known about how such programming is established and maintained. Here, we show that such epigenetic memories can be initiated in the Drosophila embryo before the major wave of zygotic transcription, and higher-order chromatin structures are established. An early short heat shock results in elevated levels of maternal miRNA and reduced levels of a subgroup of zygotic genes in stage 5 embryos. Using a Dicer-1 mutant, we show that the stress-induced decrease in one of these genes, the insulator-binding factor Elba1, is dependent on functional miRNA biogenesis. Reduction in Elba1 correlates with the upregulation of early developmental genes and promotes a sustained weakening of heterochromatin in the adult fly as indicated by an increased expression of the PEV w^m4h reporter. We propose that maternal miRNAs, retained in response to an early embryonic heat shock, shape the subsequent de novo heterochromatin establishment that occurs during early development via direct or indirect regulation of some of the earliest expressed genes, including Elba1.

Stochastic motion and transcriptional dynamics of pairs of distal DNA loci on a compacted chromosome

Chromosomes in the eukaryotic nucleus are highly compacted. However, for many functional processes, including transcription initiation, the 3D pair-wise motion of distal chromosomal elements, such as enhancers and promoters, is essential and necessitates dynamic fluidity. Therefore, the interplay of chromosome organization and dynamics is crucial for gene regulation. Here, we use a live imaging assay to simultaneously measure the positions of pairs of enhancers and promoters and their transcriptional output in the developing fly embryo while systematically varying the genomic separation between these two DNA loci. Our analysis reveals a combination of a compact globular organization and fast subdiffusive dynamics. These combined features cause an anomalous scaling of polymer relaxation times with genomic separation and lead to long-ranged correlations compared to existing polymer models. This scaling implies that encounter times of DNA loci are much less dependent on genomic separation than predicted by existing polymer models, with potentially significant consequences for eukaryotic gene expression.

The 3D genome landscape: Diverse chromosomal interactions and their functional implications

Genome organization includes contacts both within a single chromosome and between distinct chromosomes. Thus, regulatory organization in the nucleus may include interplay of these two types of chromosomal interactions with genome activity. Emerging advances in omics and single-cell imaging technologies have allowed new insights into chromosomal contacts, including those of homologs and sister chromatids, and their significance to genome function. In this review, we highlight recent studies in this field and discuss their impact on understanding the principles of chromosome organization and associated functional implications in diverse cellular processes. Specifically, we describe the contributions of intra-chromosomal, inter-homolog, and inter-sister chromatid contacts to genome organization and gene expression.

Application of the 3C Method to Study the Developmental Genes in Drosophila Larvae

A transition from one developmental stage to another is accompanied by activation of developmental programs and corresponding gene ensembles. Changes in the spatial conformation of the corresponding loci are associated with this activation and can be investigated with the help of the Chromosome Conformation Capture (3C) methodology. Application of 3C to specific developmental stages is a sophisticated task. Here, we describe the use of the 3C method to study the spatial organization of developmental loci in Drosophila larvae. We critically analyzed the existing protocols and offered our own solutions and the optimized protocol to overcome limitations. To demonstrate the efficiency of our procedure, we studied the spatial organization of the developmental locus Dad in 3rd instar Drosophila larvae. Differences in locus conformation were found between embryonic cells and living wild-type larvae. We also observed the establishment of novel regulatory interactions in the presence of an adjacent transgene upon activation of its expression in larvae. Our work fills the gap in the application of the 3C method to Drosophila larvae and provides a useful guide for establishing 3C on an animal model.

Single-nucleus RNA-sequencing in pre-cellularization Drosophila melanogaster embryos

Our current understanding of the regulation of gene expression in the early Drosophila melanogaster embryo comes from observations of a few genes at a time, as with in situ hybridizations, or observation of gene expression levels without regards to patterning, as with RNA-sequencing. Single-nucleus RNA-sequencing however, has the potential to provide new insights into the regulation of gene expression for many genes at once while simultaneously retaining information regarding the position of each nucleus prior to dissociation based on patterned gene expression. In order to establish the use of single-nucleus RNA sequencing in Drosophila embryos prior to cellularization, here we look at gene expression in control and insulator protein, dCTCF, maternal null embryos during zygotic genome activation at nuclear cycle 14. We find that early embryonic nuclei can be grouped into distinct clusters according to gene expression. From both virtual and published in situ hybridizations, we also find that these clusters correspond to spatial regions of the embryo. Lastly, we provide a resource of candidate differentially expressed genes that might show local changes in gene expression between control and maternal dCTCF null nuclei with no detectable differential expression in bulk. These results highlight the potential for single-nucleus RNA-sequencing to reveal new insights into the regulation of gene expression in the early Drosophila melanogaster embryo.

Chromosome organization in 4D: insights from C. elegans development

Eukaryotic genome organization is ordered and multilayered, from the nucleosome to chromosomal scales. These layers are not static during development, but are remodeled over time and between tissues. Thus, animal model studies with high spatiotemporal resolution are necessary to understand the various forms and functions of genome organization in vivo. In C. elegans, sequencing- and imaging-based advances have provided insight on how histone modifications, regulatory elements, and large-scale chromosome conformations are established and changed. Recent observations include unexpected physiological roles for topologically associating domains, different roles for the nuclear lamina at different chromatin scales, cell-type-specific enhancer and promoter regulatory grammars, and prevalent compartment variability in early development. Here, we summarize these and other recent findings in C. elegans, and suggest future avenues of research to enrich our in vivo knowledge of the forms and functions of nuclear organization.

The role of insulators and transcription in 3D chromatin organization of flies

The DNA in many organisms, including humans, is shown to be organized in topologically associating domains (TADs). In Drosophila, several architectural proteins are enriched at TAD borders, but it is still unclear whether these proteins play a functional role in the formation and maintenance of TADs. Here, we show that depletion of BEAF-32, Cp190, Chro, and Dref leads to changes in TAD organization and chromatin loops. Their depletion predominantly affects TAD borders located in regions moderately enriched in repressive modifications and depleted in active ones, whereas TAD borders located in euchromatin are resilient to these knockdowns. Furthermore, transcriptomic data has revealed hundreds of genes displaying differential expression in these knockdowns and showed that the majority of differentially expressed genes are located within reorganized TADs. Our work identifies a novel and functional role for architectural proteins at TAD borders in Drosophila and a link between TAD reorganization and subsequent changes in gene expression.

OUP accepted manuscript

Eukaryotic chromosomes are spatially segregated into topologically associating domains (TADs). Some TADs are attached to the nuclear lamina (NL) through lamina-associated domains (LADs). Here, we identified LADs and TADs at two stages of Drosophila spermatogenesis – in bam^Δ86 mutant testes which is the commonly used model of spermatogonia (SpG) and in larval testes mainly filled with spermatocytes (SpCs). We found that initiation of SpC-specific transcription correlates with promoters’ detachment from the NL and with local spatial insulation of adjacent regions. However, this insulation does not result in the partitioning of inactive TADs into sub-TADs. We also revealed an increased contact frequency between SpC-specific genes in SpCs implying their de novo gathering into transcription factories. In addition, we uncovered the specific X chromosome organization in the male germline. In SpG and SpCs, a single X chromosome is stronger associated with the NL than autosomes. Nevertheless, active chromatin regions in the X chromosome interact with each other more frequently than in autosomes. Moreover, despite the absence of dosage compensation complex in the male germline, randomly inserted SpG-specific reporter is expressed higher in the X chromosome than in autosomes, thus evidencing that non-canonical dosage compensation operates in SpG.

Super-resolution microscopy reveals stochastic initiation of replication in Drosophila polytene chromosomes

Studying the probability distribution of replication initiation along a chromosome is a huge challenge. Drosophila polytene chromosomes in combination with super-resolution microscopy provide a unique opportunity for analyzing the probabilistic nature of replication initiation at the ultrastructural level. Here, we developed a method for synchronizing S-phase induction among salivary gland cells. An analysis of the replication label distribution in the first minutes of S phase and in the following hours after the induction revealed the dynamics of replication initiation. Spatial super-resolution structured illumination microscopy allowed identifying multiple discrete replication signals and to investigate the behavior of replication signals in the first minutes of the S phase at the ultrastructural level. We identified replication initiation zones where initiation occurs stochastically. These zones differ significantly in the probability of replication initiation per time unit. There are zones in which initiation occurs on most strands of the polytene chromosome in a few minutes. In other zones, the initiation on all strands takes several hours. Compact bands are free of replication initiation events, and the replication runs from outer edges to the middle, where band shapes may alter.

Using the CRISPR/Cas9 System for Dissection of Functional Sites of the Notch Gene in Drosophila melanogaster

The Notch gene is a key factor in the signaling cascade that allows communication between neighboring cells in many organisms, from worms and insects to humans. The relative simplicity of the Notch pathway in Drosophila, combined with a powerful set of molecular and cytogenetic methods, makes this model attractive for studying the fundamental principles of Notch regulation and functioning. Here, using the CRISPR/Cas9 system in combination with homologous recombination, for the first time at the level of the whole organism, we obtained a directed deletion of the 5'-regulatory region and the first exon of the Notch gene, which were replaced by the attP integration site of the ΦC31 phage. Based on this approach, we obtained and characterized new Notch mutations. Thus, a new powerful tool is provided for studying the genetic regulation of the Notch gene and the organization of chromatin at this locus.

Spatial organization of transcribing loci during early genome activation in Drosophila

The early Drosophila embryo provides unique experimental advantages for addressing fundamental questions of gene regulation at multiple levels of organization, from individual gene loci to the entire genome. Using 1.5-h-old Drosophila embryos undergoing the first wave of genome activation,¹ we detected ∼110 discrete "speckles" of RNA polymerase II (RNA Pol II) per nucleus, two of which were larger and localized to the histone locus bodies (HLBs).^2,3 In the absence of the primary driver of Drosophila genome activation, the pioneer factor Zelda (Zld),^1,4,5 70% fewer speckles were present; however, the HLBs tended to be larger than wild-type (WT) HLBs, indicating that RNA Pol II accumulates at the HLBs in the absence of robust early-gene transcription. We observed a uniform distribution of distances between active genes in the nuclei of both WT and zld mutant embryos, indicating that early co-regulated genes do not cluster into nuclear sub-domains. However, in instances whereby transcribing genes did come into close 3D proximity (within 400 nm), they were found to have distinct RNA Pol II speckles. In contrast to the emerging model whereby active genes are clustered to facilitate co-regulation and sharing of transcriptional resources, our data support an "individualist" model of gene control at early genome activation in Drosophila. This model is in contrast to a "collectivist" model, where active genes are spatially clustered and share transcriptional resources, motivating rigorous tests of both models in other experimental systems.

FISH Going Meso-Scale: A Microscopic Search for Chromatin Domains

The intimate relationships between genome structure and function direct efforts toward deciphering three-dimensional chromatin organization within the interphase nuclei at different genomic length scales. For decades, major insights into chromatin structure at the level of large-scale euchromatin and heterochromatin compartments, chromosome territories, and subchromosomal regions resulted from the evolution of light microscopy and fluorescence in situ hybridization. Studies of nanoscale nucleosomal chromatin organization benefited from a variety of electron microscopy techniques. Recent breakthroughs in the investigation of mesoscale chromatin structures have emerged from chromatin conformation capture methods (C-methods). Chromatin has been found to form hierarchical domains with high frequency of local interactions from loop domains to topologically associating domains and compartments. During the last decade, advances in super-resolution light microscopy made these levels of chromatin folding amenable for microscopic examination. Here we are reviewing recent developments in FISH-based approaches for detection, quantitative measurements, and validation of contact chromatin domains deduced from C-based data. We specifically focus on the design and application of Oligopaint probes, which marked the latest progress in the imaging of chromatin domains. Vivid examples of chromatin domain FISH-visualization by means of conventional, super-resolution light and electron microscopy in different model organisms are provided.

Investigation of the Basic Steps in the Chromosome Conformation Capture Procedure

A constellation of chromosome conformation capture methods (С-methods) are an important tool for biochemical analysis of the spatial interactions between DNA regions that are separated in the primary sequence. All these methods are based on the long sequence of basic steps of treating cells, nuclei, chromatin, and finally DNA, thus representing a significant technical challenge. Here, we present an in-depth study of the basic steps in the chromatin conformation capture procedure (3С), which was performed using Drosophila Schneider 2 cells as a model. We investigated the steps of cell lysis, nuclei washing, nucleoplasm extraction, chromatin treatment with SDS/Triton X-100, restriction enzyme digestion, chromatin ligation, reversion of cross-links, DNA extraction, treatment of a 3C library with RNases, and purification of the 3C library. Several options were studied, and optimal conditions were found. Our work contributes to the understanding of the 3C basic steps and provides a useful guide to the 3C procedure.

Insulators in Plants: Progress and Open Questions

The genomes of higher eukaryotes are partitioned into topologically associated domains or TADs, and insulators (also known as boundary elements) are the key elements responsible for their formation and maintenance. Insulators were first identified and extensively studied in Drosophila as well as mammalian genomes, and have also been described in yeast and plants. In addition, many insulator proteins are known in Drosophila, and some have been investigated in mammals. However, much less is known about this important class of non-coding DNA elements in plant genomes. In this review, we take a detailed look at known plant insulators across different species and provide an overview of potential determinants of plant insulator functions, including cis-elements and boundary proteins. We also discuss methods previously used in attempts to identify plant insulators, provide a perspective on their importance for research and biotechnology, and discuss areas of potential future research.

3D genome organisation in Drosophila

Ever since Thomas Hunt Morgan's discovery of the chromosomal basis of inheritance by using Drosophila melanogaster as a model organism, the fruit fly has remained an essential model system in studies of genome biology, including chromatin organisation. Very much as in vertebrates, in Drosophila, the genome is organised in territories, compartments and topologically associating domains (TADs). However, these domains might be formed through a slightly different mechanism than in vertebrates due to the presence of a large and potentially redundant set of insulator proteins and the minor role of dCTCF in TAD boundary formation. Here, we review the different levels of chromatin organisation in Drosophila and discuss mechanisms and factors that might be involved in TAD formation. The dynamics of TADs and enhancer-promoter interactions in the context of transcription are covered in the light of currently conflicting results. Finally, we illustrate the value of polymer modelling approaches to infer the principles governing the three-dimensional organisation of the Drosophila genome.

DNA transposons mediate duplications via transposition-independent and -dependent mechanisms in metazoans

Despite long being considered as "junk", transposable elements (TEs) are now accepted as catalysts of evolution. One example is Mutator-like elements (MULEs, one type of terminal inverted repeat DNA TEs, or TIR TEs) capturing sequences as Pack-MULEs in plants. However, their origination mechanism remains perplexing, and whether TIR TEs mediate duplication in animals is almost unexplored. Here we identify 370 Pack-TIRs in 100 animal reference genomes and one Pack-TIR (Ssk-FB4) family in fly populations. We find that single-copy Pack-TIRs are mostly generated via transposition-independent gap filling, and multicopy Pack-TIRs are likely generated by transposition after replication fork switching. We show that a proportion of Pack-TIRs are transcribed and often form chimeras with hosts. We also find that Ssk-FB4s represent a young protein family, as supported by proteomics and signatures of positive selection. Thus, TIR TEs catalyze new gene structures and new genes in animals via both transposition-independent and -dependent mechanisms.

Multi-Scale Organization of the Drosophila melanogaster Genome

Interphase chromatin, despite its appearance, is a highly organized framework of loops and bends. Chromosomes are folded into topologically associating domains, or TADs, and each chromosome and its homolog occupy a distinct territory within the nucleus. In Drosophila, genome organization is exceptional because homologous chromosome pairing is in both germline and somatic tissues, which promote interhomolog interactions such as transvection that can affect gene expression in trans. In this review, we focus on what is known about genome organization in Drosophila and discuss it from TADs to territory. We start by examining intrachromosomal organization at the sub-chromosome level into TADs, followed by a comprehensive analysis of the known proteins that play a key role in TAD formation and boundary establishment. We then zoom out to examine interhomolog interactions such as pairing and transvection that are abundant in Drosophila but rare in other model systems. Finally, we discuss chromosome territories that form within the nucleus, resulting in a complete picture of the multi-scale organization of the Drosophila genome.

Mechanism and functional role of the interaction between CP190 and the architectural protein Pita in Drosophila melanogaster

Background

Pita is required for Drosophila development and binds specifically to a long motif in active promoters and insulators. Pita belongs to the Drosophila family of zinc-finger architectural proteins, which also includes Su(Hw) and the conserved among higher eukaryotes CTCF. The architectural proteins maintain the active state of regulatory elements and the long-distance interactions between them. In particular, Pita is involved in the formation of several boundaries between regulatory domains that controlled the expression of three hox genes in the Bithorax complex (BX-C). The CP190 protein is recruited to chromatin through interaction with the architectural proteins.

Results

Using in vitro pull-down analysis, we precisely mapped two unstructured regions of Pita that interact with the BTB domain of CP190. Then we constructed transgenic lines expressing the Pita protein of the wild-type and mutant variants lacking CP190-interacting regions. We have demonstrated that CP190-interacting region of the Pita can maintain nucleosome-free open chromatin and is critical for Pita-mediated enhancer blocking activity in BX-C. At the same time, interaction with CP190 is not required for the in vivo function of the mutant Pita protein, which binds to the same regions of the genome as the wild-type protein. Unexpectedly, we found that CP190 was still associated with the most of genome regions bound by the mutant Pita protein, which suggested that other architectural proteins were continuing to recruit CP190 to these regions.

Conclusions

The results directly demonstrate role of CP190 in insulation and support a model in which the regulatory elements are composed of combinations of binding sites that interact with several architectural proteins with similar functions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13072-021-00391-x.

SetDB1 and Su(var)3-9 play non-overlapping roles in somatic cell chromosomes of Drosophila melanogaster

We explored functional roles of two H3K9-specific histone methyltransferases of Drosophila melanogaster, SetDB1 (also known as Eggless) and Su(var)3-9. Using the DamID approach, we generated the binding profile for SetDB1 in Drosophila salivary gland chromosomes, and matched it to the profile of Su(var)3-9. Unlike Su(var)3-9, SetDB1 turned out to be an euchromatic protein that is absent from repeated DNA compartments, and is largely restricted to transcription start sites (TSSs) and 5' untranslated regions (5'UTRs) of ubiquitously expressed genes. Significant SetDB1 association is also observed at binding sites for the insulator protein CP190. SetDB1 and H3K9 di- and tri-methylated (me2 and me3)-enriched sites tend to display poor overlap. At the same time, SetDB1 has a clear connection with the distribution of H3K27me3 mark. SetDB1 binds outside the domains possessing this modification, and about half of the borders of H3K27me3 domains are decorated by SetDB1 together with actively transcribed genes. On the basis of poor correlation between the distribution of SetDB1 and H3K9 methylation marks, we speculate that, in somatic cells, SetDB1 may contribute to the methylation of a broader set of chromosomal proteins than just H3K9. In addition, SetDB1 can be expected to play a role in the establishment of chromatin functional domains.

Independence of chromatin conformation and gene regulation during Drosophila dorsoventral patterning

The relationship between chromatin organization and gene regulation remains unclear. While disruption of chromatin domains and domain boundaries can lead to misexpression of developmental genes, acute depletion of regulators of genome organization has a relatively small effect on gene expression. It is therefore uncertain whether gene expression and chromatin state drive chromatin organization or whether changes in chromatin organization facilitate cell-type-specific activation of gene expression. Here, using the dorsoventral patterning of the Drosophila melanogaster embryo as a model system, we provide evidence for the independence of chromatin organization and dorsoventral gene expression. We define tissue-specific enhancers and link them to expression patterns using single-cell RNA-seq. Surprisingly, despite tissue-specific chromatin states and gene expression, chromatin organization is largely maintained across tissues. Our results indicate that tissue-specific chromatin conformation is not necessary for tissue-specific gene expression but rather acts as a scaffold facilitating gene expression when enhancers become active.

3D genome evolution and reorganization in the Drosophila melanogaster species group

Topologically associating domains, or TADs, are functional units that organize chromosomes into 3D structures of interacting chromatin. TADs play an important role in regulating gene expression by constraining enhancer-promoter contacts and there is evidence that deletion of TAD boundaries leads to aberrant expression of neighboring genes. While the mechanisms of TAD formation have been well-studied, current knowledge on the patterns of TAD evolution across species is limited. Due to the integral role TADs play in gene regulation, their structure and organization is expected to be conserved during evolution. However, more recent research suggests that TAD structures diverge relatively rapidly. We use Hi-C chromosome conformation capture to measure evolutionary conservation of whole TADs and TAD boundary elements between D. melanogaster and D. triauraria, two early-branching species from the melanogaster species group which diverged ∼15 million years ago. We find that the majority of TADs have been reorganized since the common ancestor of D. melanogaster and D. triauraria, via a combination of chromosomal rearrangements and gain/loss of TAD boundaries. TAD reorganization between these two species is associated with a localized effect on gene expression, near the site of disruption. By separating TADs into subtypes based on their chromatin state, we find that different subtypes are evolving under different evolutionary forces. TADs enriched for broadly expressed, transcriptionally active genes are evolving rapidly, potentially due to positive selection, whereas TADs enriched for developmentally-regulated genes remain conserved, presumably due to their importance in restricting gene-regulatory element interactions. These results provide novel insight into the evolutionary dynamics of TADs and help to reconcile contradictory reports related to the evolutionary conservation of TADs and whether changes in TAD structure affect gene expression.

Defining the Boundaries of Polycomb Domains in Drosophila

Polycomb group (PcG) proteins are an important group of transcriptional repressors that act by modifying chromatin. PcG target genes are covered by the repressive chromatin mark H3K27me3. Polycomb repressive complex 2 (PRC2) is a multiprotein complex that is responsible for generating H3K27me3. In Drosophila, PRC2 is recruited by Polycomb Response Elements (PREs) and then trimethylates flanking nucleosomes, spreading the H3K27me3 mark over large regions of the genome, the "Polycomb domains." What defines the boundary of a Polycomb domain? There is experimental evidence that insulators, PolII, and active transcription can all form the boundaries of Polycomb domains. Here we divide the boundaries of larval Polycomb domains into six different categories. In one category, genes are transcribed toward the Polycomb domain, where active transcription is thought to stop the spreading of H3K27me3. In agreement with this, we show that introducing a transcriptional terminator into such a transcription unit causes an extension of the Polycomb domain. Additional data suggest that active transcription of a boundary gene may restrict the range of enhancer activity of a Polycomb-regulated gene.

The Functions and Mechanisms of Action of Insulators in the Genomes of Higher Eukaryotes

The mechanisms underlying long-range interactions between chromatin regions and the principles of chromosomal architecture formation are currently under extensive scrutiny. A special class of regulatory elements known as insulators is believed to be involved in the regulation of specific long-range interactions between enhancers and promoters. This review focuses on the insulators of Drosophila and mammals, and it also briefly characterizes the proteins responsible for their functional activity. It was initially believed that the main properties of insulators are blocking of enhancers and the formation of independent transcription domains. We present experimental data proving that the chromatin loops formed by insulators play only an auxiliary role in enhancer blocking. The review also discusses the mechanisms involved in the formation of topologically associating domains and their role in the formation of the chromosomal architecture and regulation of gene transcription.

Evolution of Regulated Transcription

The genomes of all organisms abound with various cis-regulatory elements, which control gene activity. Transcriptional enhancers are a key group of such elements in eukaryotes and are DNA regions that form physical contacts with gene promoters and precisely orchestrate gene expression programs. Here, we follow gradual evolution of this regulatory system and discuss its features in different organisms. In eubacteria, an enhancer-like element is often a single regulatory element, is usually proximal to the core promoter, and is occupied by one or a few activators. Activation of gene expression in archaea is accompanied by the recruitment of an activator to several enhancer-like sites in the upstream promoter region. In eukaryotes, activation of expression is accompanied by the recruitment of activators to multiple enhancers, which may be distant from the core promoter, and the activators act through coactivators. The role of the general DNA architecture in transcription control increases in evolution. As a whole, it can be seen that enhancers of multicellular eukaryotes evolved from the corresponding prototypic enhancer-like regulatory elements with the gradually increasing genome size of organisms.

Disparate expression specificities coded by a shared Hox-C enhancer

Can a single regulatory sequence be shared by two genes undergoing functional divergence? Here we describe a single promiscuous enhancer within the Drosophila Antennapedia Complex, EO053, that directs aspects of the expression of two adjacent genes, pb (a Hox2 ortholog) and zen2 (a divergent Hox3 paralog), with disparate spatial and temporal expression patterns. We were unable to separate the pb-like and zen2-like specificities within EO053, and we identify sequences affecting both expression patterns. Importantly, genomic deletion experiments demonstrate that EO053 cooperates with additional pb- and zen2-specific enhancers to regulate the mRNA expression of both genes. We examine sequence conservation of EO053 within the Schizophora, and show that patterns of synteny between the Hox2 and Hox3 orthologs in Arthropods are consistent with a shared regulatory relationship extending prior to the Hox3/zen divergence. Thus, EO053 represents an example of two genes having evolved disparate outputs while utilizing this shared regulatory region.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

Chromatin dynamics at the maternal to zygotic transition: recent advances from the zebrafish model

Early animal development is characterized by intense reorganization of the embryonic genome, including large-scale changes in chromatin structure and in the DNA and histone modifications that help shape this structure. Particularly profound shifts in the chromatin landscape are associated with the maternal-to-zygotic transition, when the zygotic genome is first transcribed and maternally loaded transcripts are degraded. The accessibility of the early zebrafish embryo facilitates the interrogation of chromatin during this critical window of development, making it an important model for early chromatin regulation. Here, we review our current understanding of chromatin dynamics during early zebrafish development, highlighting new advances as well as similarities and differences between early chromatin regulation in zebrafish and other species.

Demarcation of Topologically Associating Domains Is Uncoupled from Enriched CTCF Binding in Developing Zebrafish

CCCTC-binding factor (CTCF) is a conserved architectural protein that plays crucial roles in gene regulation and three-dimensional (3D) chromatin organization. To better understand mechanisms and evolution of vertebrate genome organization, we analyzed genome occupancy of CTCF in zebrafish utilizing an endogenously epitope-tagged CTCF knock-in allele. Zebrafish CTCF shares similar facets with its mammalian counterparts, including binding to enhancers, active promoters and repeat elements, and bipartite sequence motifs of its binding sites. However, we found that in vivo CTCF binding is not enriched at boundaries of topologically associating domains (TADs) in developing zebrafish, whereas TAD demarcation by chromatin marks did not differ from mammals. Our data suggest that general mechanisms underlying 3D chromatin organization, and in particular the involvement of CTCF in this process, differ between distant vertebrate species.

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Identification of CTCF occupancy in zebrafish embryos using a tagged ctcf allele

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CTCF binding at promoters correlates with gene expression levels

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No general CTCF enrichment at topological domain boundaries in zebrafish embryos

Advances in Chromatin Imaging at Kilobase-Scale Resolution

It is now widely appreciated that the spatial organization of the genome is nonrandom, and its complex 3D folding has important consequences for many genome processes. Recent developments in multiplexed, super-resolution microscopy have enabled an unprecedented view of the polymeric structure of chromatin - from the loose folds of whole chromosomes to the detailed loops of cis-regulatory elements that regulate gene expression. Facilitated by the use of robotics, microfluidics, and improved approaches to super-resolution, thousands to hundreds of thousands of individual cells can now be analyzed in an individual experiment. This has led to new insights into the nature of genomic structural features identified by sequencing, such as topologically associated domains (TADs), and the nature of enhancer-promoter interactions underlying transcriptional regulation. We review these recent improvements.

Pericentromeric heterochromatin is hierarchically organized and spatially contacts H3K9me2 islands in euchromatin

Membraneless pericentromeric heterochromatin (PCH) domains play vital roles in chromosome dynamics and genome stability. However, our current understanding of 3D genome organization does not include PCH domains because of technical challenges associated with repetitive sequences enriched in PCH genomic regions. We investigated the 3D architecture of Drosophila melanogaster PCH domains and their spatial associations with the euchromatic genome by developing a novel analysis method that incorporates genome-wide Hi-C reads originating from PCH DNA. Combined with cytogenetic analysis, we reveal a hierarchical organization of the PCH domains into distinct “territories.” Strikingly, H3K9me2-enriched regions embedded in the euchromatic genome show prevalent 3D interactions with the PCH domain. These spatial contacts require H3K9me2 enrichment, are likely mediated by liquid-liquid phase separation, and may influence organismal fitness. Our findings have important implications for how PCH architecture influences the function and evolution of both repetitive heterochromatin and the gene-rich euchromatin.

The three dimensional (3D) organization of genomes in cell nuclei can influence a wide variety of genome functions. However, most of our understanding of this critical architecture has been limited to the gene-rich euchromatin, and largely ignores the gene-poor and repeat-rich pericentromeric heterochromatin, or PCH. PCH comprises a large part of most eukaryotic genomes, forms 3D membraneless PCH domains in nuclei, and plays a vital role in chromosome dynamics and genome stability. In this study, we developed a new method that overcomes the technical challenges imposed by the highly repetitive PCH DNA, and generated a comprehensive picture of its 3D organization. Combined with image analyses, we reveal a hierarchical organization of the PCH domains. Surprisingly, we showed that distant euchromatic regions enriched for repressive epigenetic marks also dynamically interact with the main PCH domains. These 3D interactions are likely mediated by liquid-liquid phase separation (similar to how oil and vinegar separate in salad dressing) and the resulting liquid-like fusion events, and can influence the fitness of individuals. Our discoveries have strong implications for how seemingly “junk” DNA could impact functions in the gene-rich euchromatin.

Lamina-Dependent Stretching and Unconventional Chromosome Compartments in Early C. elegans Embryos

Current models suggest that chromosome domains segregate into either an active (A) or inactive (B) compartment. B-compartment chromatin is physically separated from the A compartment and compacted by the nuclear lamina. To examine these models in the developmental context of C. elegans embryogenesis, we undertook chromosome tracing to map the trajectories of entire autosomes. Early embryonic chromosomes organized into an unconventional barbell-like configuration, with two densely folded B compartments separated by a central A compartment. Upon gastrulation, this conformation matured into conventional A/B compartments. We used unsupervised clustering to uncover subpopulations with differing folding properties and variable positioning of compartment boundaries. These conformations relied on tethering to the lamina to stretch the chromosome; detachment from the lamina compacted, and allowed intermingling between, A/B compartments. These findings reveal the diverse conformations of early embryonic chromosomes and uncover a previously unappreciated role for the lamina in systemic chromosome stretching.

Ordered patterning of the sensory system is susceptible to stochastic features of gene expression

Sensory neuron numbers and positions are precisely organized to accurately map environmental signals in the brain. This precision emerges from biochemical processes within and between cells that are inherently stochastic. We investigated impact of stochastic gene expression on pattern formation, focusing on senseless (sens), a key determinant of sensory fate in Drosophila. Perturbing microRNA regulation or genomic location of sens produced distinct noise signatures. Noise was greatly enhanced when both sens alleles were present in homologous loci such that each allele was regulated in trans by the other allele. This led to disordered patterning. In contrast, loss of microRNA repression of sens increased protein abundance but not sensory pattern disorder. This suggests that gene expression stochasticity is a critical feature that must be constrained during development to allow rapid yet accurate cell fate resolution.

In situ dissection of domain boundaries affect genome topology and gene transcription in Drosophila

Chromosomes are organized into high-frequency chromatin interaction domains called topologically associating domains (TADs), which are separated from each other by domain boundaries. The molecular mechanisms responsible for TAD formation are not yet fully understood. In Drosophila, it has been proposed that transcription is fundamental for TAD organization while the participation of genetic sequences bound by architectural proteins (APs) remains controversial. Here, we investigate the contribution of domain boundaries to TAD organization and the regulation of gene expression at the Notch gene locus in Drosophila. We find that deletion of domain boundaries results in TAD fusion and long-range topological defects that are accompanied by loss of APs and RNA Pol II chromatin binding as well as defects in transcription. Together, our results provide compelling evidence of the contribution of discrete genetic sequences bound by APs and RNA Pol II in the partition of the genome into TADs and in the regulation of gene expression in Drosophila.

In Drosophila, transcription is thought to be required for TAD formation, while the role of architectural proteins remains controversial. Here, the authors find that deletion of domain boundaries at the fly Notch locus results in TAD fusion and long-range topological defects, loss of architectural protein and RNA Pol II chromatin binding, and transcription defects.

Patterns of Genomic Differentiation in the Drosophila nasuta Species Complex

The Drosophila nasuta species complex contains over a dozen recently diverged species that are distributed widely across South-East Asia, and which shows varying degrees of pre- and postzygotic isolation. Here, we assemble a high-quality genome for D. albomicans using single-molecule sequencing and chromatin conformation capture, and draft genomes for 11 additional species and 67 individuals across the clade, to infer the species phylogeny and patterns of genetic diversity in this group. Our assembly recovers entire chromosomes, and we date the origin of this radiation ∼2 Ma. Despite low levels of overall differentiation, most species or subspecies show clear clustering into their designated taxonomic groups using population genetics and phylogenetic methods. Local evolutionary history is heterogeneous across the genome, and differs between the autosomes and the X chromosome for species in the sulfurigaster subgroup, likely due to autosomal introgression. Our study establishes the nasuta species complex as a promising model system to further characterize the evolution of pre- and postzygotic isolation in this clade.

Brd4 and P300 Confer Transcriptional Competency during Zygotic Genome Activation

The awakening of the genome after fertilization is a cornerstone of animal development. However, the mechanisms that activate the silent genome after fertilization are poorly understood. Here, we show that transcriptional competency is regulated by Brd4- and P300-dependent histone acetylation in zebrafish. Live imaging of transcription revealed that genome activation, beginning at the miR-430 locus, is gradual and stochastic. We show that genome activation does not require slowdown of the cell cycle and is regulated through the translation of maternally inherited mRNAs. Among these, the enhancer regulators P300 and Brd4 can prematurely activate transcription and restore transcriptional competency when maternal mRNA translation is blocked, whereas inhibition of histone acetylation blocks genome activation. We conclude that P300 and Brd4 are sufficient to trigger genome-wide transcriptional competency by regulating histone acetylation on the first zygotic genes in zebrafish. This mechanism is critical for initiating zygotic development and developmental reprogramming.

Structural and Functional Dissection of the 5' Region of the Notch Gene in Drosophila melanogaster

Notch is a key factor of a signaling cascade which regulates cell differentiation in all multicellular organisms. Numerous investigations have been directed mainly at studying the mechanism of Notch protein action; however, very little is known about the regulation of activity of the gene itself. Here, we provide the results of targeted 5'-end editing of the Drosophila Notch gene in its native environment and genetic and cytological effects of these changes. Using the Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR associated protein 9 (CRISPR/Cas9) system in combination with homologous recombination, we obtained a founder fly stock in which a 4-kb fragment, including the 5' nontranscribed region, the first exon, and a part of the first intron of Notch, was replaced by an attachment Phage (attP) site. Then, fly lines carrying a set of six deletions within the 5'untranscribed region of the gene were obtained by ΦC31-mediated integration of transgenic constructs. Part of these deletions does not affect gene activity, but their combinations with transgenic construct in the first intron of the gene cause defects in the Notch target tissues. At the polytene chromosome level we defined a DNA segment (~250 bp) in the Notch5'-nontranscribed region which when deleted leads to disappearance of the 3C6/C7 interband and elimination of CTC-Factor (CTCF) and Chromator (CHRIZ) insulator proteins in this region.

Distinct Elements Confer the Blocking and Bypass Functions of the Bithorax Fab-8 Boundary

Boundaries in the Drosophila bithorax complex (BX-C) enable the regulatory domains that drive parasegment-specific expression of the three Hox genes to function autonomously. The four regulatory domains (iab-5, iab-6, iab-7, and iab-8) that control the expression of the Abdominal-B (Abd-B) gene are located downstream of the transcription unit, and are delimited by the Mcp, Fab-6, Fab-7, and Fab-8 boundaries. These boundaries function to block cross talk between neighboring regulatory domains. In addition, three of the boundaries (Fab-6, Fab-7, and Fab-8) must also have bypass activity so that regulatory domains distal to the boundaries can contact the Abd-B promoter. In the studies reported here, we have undertaken a functional dissection of the Fab-8 boundary using a boundary-replacement strategy. Our studies indicate that the Fab-8 boundary has two separable subelements. The distal subelement blocks cross talk, but cannot support bypass. The proximal subelement has only minimal blocking activity but is able to mediate bypass. A large multiprotein complex, the LBC (large boundary complex), binds to sequences in the proximal subelement and contributes to its bypass activity. The same LBC complex has been implicated in the bypass activity of the Fab-7 boundary.

Highly structured homolog pairing reflects functional organization of the Drosophila genome

Trans-homolog interactions have been studied extensively in Drosophila, where homologs are paired in somatic cells and transvection is prevalent. Nevertheless, the detailed structure of pairing and its functional impact have not been thoroughly investigated. Accordingly, we generated a diploid cell line from divergent parents and applied haplotype-resolved Hi-C, showing that homologs pair with varying precision genome-wide, in addition to establishing trans-homolog domains and compartments. We also elucidate the structure of pairing with unprecedented detail, observing significant variation across the genome and revealing at least two forms of pairing: tight pairing, spanning contiguous small domains, and loose pairing, consisting of single larger domains. Strikingly, active genomic regions (A-type compartments, active chromatin, expressed genes) correlated with tight pairing, suggesting that pairing has a functional implication genome-wide. Finally, using RNAi and haplotype-resolved Hi-C, we show that disruption of pairing-promoting factors results in global changes in pairing, including the disruption of some interaction peaks.

The genome-wide multi-layered architecture of chromosome pairing in early Drosophila embryos

Genome organization involves cis and trans chromosomal interactions, both implicated in gene regulation, development, and disease. Here, we focus on trans interactions in Drosophila, where homologous chromosomes are paired in somatic cells from embryogenesis through adulthood. We first address long-standing questions regarding the structure of embryonic homolog pairing and, to this end, develop a haplotype-resolved Hi-C approach to minimize homolog misassignment and thus robustly distinguish trans-homolog from cis contacts. This computational approach, which we call Ohm, reveals pairing to be surprisingly structured genome-wide, with trans-homolog domains, compartments, and interaction peaks, many coinciding with analogous cis features. We also find a significant genome-wide correlation between pairing, transcription during zygotic genome activation, and binding of the pioneer factor Zelda. Our findings reveal a complex, highly structured organization underlying homolog pairing, first discovered a century ago in Drosophila. Finally, we demonstrate the versatility of our haplotype-resolved approach by applying it to mammalian embryos.