Metazoan genomes are spatially organized at multiple scales, from packaging of DNA around individual nucleosomes to segregation of whole chromosomes into distinct territories1–5. At the intermediate scale of kilobases to megabases, which encompasses the sizes of genes, gene clusters and regulatory domains, the three-dimensional (3D) organization of DNA is implicated in multiple gene regulatory mechanisms2–4,6–8, but understanding this organization remains a challenge. At this scale, the genome is partitioned into domains of different epigenetic states that are essential for regulating gene expression9–11. Here, we investigate the 3D organization of chromatin in different epigenetic states using super-resolution imaging. We classified genomic domains in Drosophila cells into transcriptionally active, inactive, or Polycomb-repressed states and observed distinct chromatin organizations for each state. Remarkably, all three types of chromatin domains exhibit power-law scaling between their physical sizes in 3D and their domain lengths, but each type has a distinct scaling exponent. Polycomb-repressed chromatin shows the densest packing and most intriguing folding behaviour in which packing density increases with domain length. Distinct from the self-similar organization displayed by transcriptionally active and inactive chromatin, the Polycomb-repressed domains are characterized by a high degree of chromatin intermixing within the domain. Moreover, compared to inactive domains, Polycomb-repressed domains spatially exclude neighbouring active chromatin to a much stronger degree. Computational modelling and knockdown experiments suggest that reversible chromatin interactions mediated by Polycomb-group proteins plays an important role in these unique packaging properties of the repressed chromatin. Taken together, our super-resolution images reveal distinct chromatin packaging for different epigenetic states at the kilobase-to-megabase scale, a length scale that is directly relevant to genome regulation.

The spatial organization of chromatin critically impacts genome function. Recent chromosome-conformation-capture studies have revealed topologically-associating domains (TADs) as a conserved feature of chromatin organization, but how TADs are spatially organized in individual chromosomes remains unknown. Here we developed an imaging method for mapping the spatial positions of numerous genomic regions along individual chromosomes and traced the positions of TADs in human interphase autosomes and X chromosomes. We observed that chromosome folding deviates from the ideal fractal-globule model at large length scales and that TADs are largely organized into two compartments spatially arranged in a polarized fashion in individual chromosomes. Active and inactive X chromosomes adopt different folding and compartmentalization configurations. These results suggest that the spatial organization of chromatin domains can change in response to regulation.

A host of observations demonstrating the relationship between nuclear architecture and processes such as gene expression have led to a number of new technologies for interrogating chromosome positioning. Whereas some of these technologies reconstruct intermolecular interactions, others have enhanced our ability to visualize chromosomes in situ. Here, we describe an oligonucleotide-and PCR-based strategy for fluorescence in situ hybridization (FISH) and a bioinformatic platform that enables this technology to be extended to any organism whose genome has been sequenced. The oligonucleotide probes are renewable, highly efficient, and able to robustly label chromosomes in cell culture, fixed tissues, and metaphase spreads. Our method gives researchers precise control over the sequences they target and allows for single and multicolor imaging of regions ranging from tens of kilobases to megabases with the same basic protocol. We anticipate this technology will lead to an enhanced ability to visualize interphase and metaphase chromosomes.T he role of chromosome positioning in gene regulation and chromosome stability is fueling a growing interest in technologies that reveal the in situ organization of the genome. Among these technologies are chromosome conformation capture (3C) (1) and its several iterations, such as Hi-C (2), which are applied to populations of nuclei to identify chromosomal regions that are in close proximity to each other (3, 4). Another technology is fluorescence in situ hybridization (FISH), wherein nucleic acids are targeted by fluorescently labeled probes and then visualized via microscopy; this technology is an extension of methods that once used radioactive probes and autoradiography but have since been adapted to use nonradioactive labels (5-11). FISH is a single-cell assay, making it especially powerful for the detection of rare events that might otherwise be lost in mixed or asynchronous populations of cells. In addition, because FISH is applied to fixed cells, it can reveal the positioning of chromosomes relative to nuclear, cytoplasmic, and even tissue structures. FISH can also be used to visualize RNA, permitting the simultaneous assessment of gene expression, chromosome position, and protein localization.FISH probes are typically derived from cloned genomic regions or flow-sorted chromosomes, which are labeled directly via nick translation or PCR in the presence of fluorophore-conjugated nucleotides or labeled indirectly with nucleotide-conjugated haptens, such as biotin and digoxigenin, and then visualized with secondary detection reagents. Probe DNA is often fragmented into ∼150-to 250-bp pieces to facilitate its penetration into fixed cells (12) and, as many genomic clones contain repetitive sequences that occur abundantly in the genome, hybridization is typically performed in the presence of unlabeled repetitive DNA (13). Another limitation to clone-based probes is that the genomic regions that can be visualized with them are restricted by the availability of clones and the size of ...

Fluorescence in situ hybridization (FISH) is a powerful single-cell technique for studying nuclear structure and organization. Here, we report two advances in FISH-based imaging. We first describe the in situ visualization of single-copy regions of the genome using two single-molecule super-resolution methodologies. We then introduce a robust and reliable system that harnesses single nucleotide polymorphisms (SNPs) to visually distinguish the maternal and paternal homologous chromosomes in mammalian and insect systems. Both of these new technologies are enabled by renewable, bioinformatically-designed, oligonucleotide-based Oligopaint probes, which we augment with a strategy that uses secondary oligonucleotides (oligos) to produce and enhance fluorescent signals. These advances should substantially expand the capability to query parent-of-origin specific chromosome positioning and gene expression on a cell-by-cell basis.

Fluorescence in situ hybridization (FISH) reveals the abundance and positioning of nucleic acid sequences in fixed samples. Despite recent advances in multiplexed amplification of FISH signals, it remains challenging to achieve high levels of simultaneous amplification and sequential detection with high sampling efficiency and simple workflows. Here, we introduce signal amplification by exchange reaction (SABER), which endows oligo-based FISH probes with long, single-stranded DNA concatemers that aggregate a multitude of short complementary fluorescent imager strands. We show that SABER amplifies RNA and DNA FISH signals (5 to 450-fold) in fixed cells and tissues, apply 17 orthogonal amplifiers against chromosomal targets simultaneously, and detect mRNAs with high efficiency. We further apply 10-plexSABER-FISH to identify in vivo introduced enhancers with cell type-specific activity in the mouse retina. SABER represents a simple and versatile molecular toolkit for rapid and cost-effective multiplexed imaging of nucleic acid targets.