Conservation of chromatin conformation in carnivores

The Ruminant Telomere-to-Telomere (RT2T) Consortium

Telomere-to-telomere (T2T) assemblies reveal new insights into the structure and function of the previously 'invisible' parts of the genome and allow comparative analyses of complete genomes across entire clades. We present here an open collaborative effort, termed the 'Ruminant T2T Consortium' (RT2T), that aims to generate complete diploid assemblies for numerous species of the Artiodactyla suborder Ruminantia to examine chromosomal evolution in the context of natural selection and domestication of species used as livestock.

Strong association between genomic 3D structure and CRISPR cleavage efficiency

CRISPR is a gene editing technology which enables precise in-vivo genome editing; but its potential is hampered by its relatively low specificity and sensitivity. Improving CRISPR's on-target and off-target effects requires a better understanding of its mechanism and determinants. Here we demonstrate, for the first time, the chromosomal 3D spatial structure's association with CRISPR's cleavage efficiency, and its predictive capabilities. We used high-resolution Hi-C data to estimate the 3D distance between different regions in the human genome and utilized these spatial properties to generate 3D-based features, characterizing each region's density. We evaluated these features based on empirical, in-vivo CRISPR efficiency data and compared them to 425 features used in state-of-the-art models. The 3D features ranked in the top 13% of the features, and significantly improved the predictive power of LASSO and xgboost models trained with these features. The features indicated that sites with lower spatial density demonstrated higher efficiency. Understanding how CRISPR is affected by the 3D DNA structure provides insight into CRISPR's mechanism in general and improves our ability to correctly predict CRISPR's cleavage as well as design sgRNAs for therapeutic and scientific use.

Hi-C sequencing unravels dynamic three-dimensional chromatin interactions in muntjac lineage: insights from chromosome fusions in Fea's muntjac genome

Eukaryotes have varying numbers and structures of characteristic chromosomes across lineages or species. The evolutionary trajectory of species may have been affected by spontaneous genome rearrangements. Chromosome fusion drastically alters karyotypes. However, the mechanisms and consequences of chromosome fusions, particularly in muntjac species, are poorly understood. Recent research-based advancements in three-dimensional (3D) genomics, particularly high-throughput chromatin conformation capture (Hi-C) sequencing, have allowed for the identification of chromosome fusions and provided mechanistic insights into three muntjac species: Muntiacus muntjak, M. reevesi, and M. crinifrons. This study aimed to uncover potential genome rearrangement patterns in the threatened species Fea's muntjac (Muntiacus feae), which have not been previously examined for such characteristics. Deep Hi-C sequencing (31.42 × coverage) was performed to reveal the 3D chromatin architecture of the Fea's muntjac genome. Patterns of repeated chromosome fusions that were potentially mediated by high-abundance transposable elements were identified. Comparative Hi-C maps demonstrated linkage homology between the sex chromosomes in Fea's muntjac and autosomes in M. reevesi, indicating that fusions may have played a crucial role in the evolution of the sex chromosomes of the lineage. The species-level dynamics of topologically associated domains (TADs) suggest that TAD organization could be altered by differential chromosome interactions owing to repeated chromosome fusions. However, research on the effect of TADs on muntjac genome evolution is insufficient. This study generated Hi-C data for the Fea's muntjac, providing a genomic resource for future investigations of the evolutionary patterns of chromatin conformation at the chromosomal level.

Patterns of chromosome evolution in ruminants

Studying when and where gross genomic rearrangements occurred during evolution is key to understanding changes in genome structure with functional consequences that might eventually lead to speciation. Here we identified chromosome rearrangements in ruminants, a clade characterized by large chromosome differences. Using 26 genome assemblies, we reconstructed five ancestral karyotypes and classified the rearrangement events occurring in each lineage. With these reconstructions, we then identified evolutionary breakpoints regions (EBRs) and synteny fragments. Ruminant karyotype evolution is characterized by inversions, while interchromosomal rearrangements occurred preferentially in the oldest ancestor of ruminants. We found that EBRs are depleted of protein coding genes, including housekeeping genes. Similarly, EBRs are not enriched in high GC regions, suggesting that meiotic double strand breaks might not be their origin. Overall, our results characterize at fine detail the location of chromosome rearrangements in ruminant evolution and provide new insights into the formation of EBRs.

Predicting chromosomal compartments directly from the nucleotide sequence with DNA-DDA

Three-dimensional (3D) genome architecture is characterized by multi-scale patterns and plays an essential role in gene regulation. Chromatin conformation capturing experiments have revealed many properties underlying 3D genome architecture, such as the compartmentalization of chromatin based on transcriptional states. However, they are complex, costly and time consuming, and therefore only a limited number of cell types have been examined using these techniques. Increasing effort is being directed towards deriving computational methods that can predict chromatin conformation and associated structures. Here we present DNA-delay differential analysis (DDA), a purely sequence-based method based on chaos theory to predict genome-wide A and B compartments. We show that DNA-DDA models derived from a 20 Mb sequence are sufficient to predict genome wide compartmentalization at the scale of 100 kb in four different cell types. Although this is a proof-of-concept study, our method shows promise in elucidating the mechanisms responsible for genome folding as well as modeling the impact of genetic variation on 3D genome architecture and the processes regulated thereby.

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.

Principles of 3D chromosome folding and evolutionary genome reshuffling in mammals

Studying the similarities and differences in genomic interactions between species provides fertile grounds for determining the evolutionary dynamics underpinning genome function and speciation. Here, we describe the principles of 3D genome folding in vertebrates and show how lineage-specific patterns of genome reshuffling can result in different chromatin configurations. We (1) identified different patterns of chromosome folding in across vertebrate species (centromere clustering versus chromosomal territories); (2) reconstructed ancestral marsupial and afrotherian genomes analyzing whole-genome sequences of species representative of the major therian phylogroups; (3) detected lineage-specific chromosome rearrangements; and (4) identified the dynamics of the structural properties of genome reshuffling through therian evolution. We present evidence of chromatin configurational changes that result from ancestral inversions and fusions/fissions. We catalog the close interplay between chromatin higher-order organization and therian genome evolution and introduce an interpretative hypothesis that explains how chromatin folding influences evolutionary patterns of genome reshuffling.

Genome report: chromosome-level draft assemblies of the snow leopard, African leopard, and tiger (Panthera uncia, Panthera pardus pardus, and Panthera tigris)

The big cats (genus Panthera) represent some of the most popular and charismatic species on the planet. Although some reference genomes are available for this clade, few are at the chromosome level, inhibiting high-resolution genomic studies. We assembled genomes from 3 members of the genus, the tiger (Panthera tigris), the snow leopard (Panthera uncia), and the African leopard (Panthera pardus pardus), at chromosome or near-chromosome level. We used a combination of short- and long-read technologies, as well as proximity ligation data from Hi-C technology, to achieve high continuity and contiguity for each individual. We hope that these genomes will aid in further evolutionary and conservation research of this iconic group of mammals.

Evolution of the ancestral mammalian karyotype and syntenic regions

Computational reconstruction of ancestral mammalian karyotypes revealed a comprehensive picture of the chromosome rearrangements that occurred over the evolutionary history of mammals. Ancient gene order, in some cases extending to full chromosomes, was found conserved for more than 300 My, demonstrating strong evolutionary constraint against rearrangements in some regions. Conserved segments of chromosomes are enriched for genes that control developmental processes. Therefore, Darwinian selection likely maintains ancient gene combinations while allowing for genomic innovations within or near chromosomal sites that break and rearrange over evolutionary time. The revealed relationship between the three-dimensional structure of chromosomes and the evolutionary stability of chromosome segments provides additional insights into the mechanisms of chromosome evolution and diseases associated with genome rearrangements.

Decrypting the rearrangements that drive mammalian chromosome evolution is critical to understanding the molecular bases of speciation, adaptation, and disease susceptibility. Using 8 scaffolded and 26 chromosome-scale genome assemblies representing 23/26 mammal orders, we computationally reconstructed ancestral karyotypes and syntenic relationships at 16 nodes along the mammalian phylogeny. Three different reference genomes (human, sloth, and cattle) representing phylogenetically distinct mammalian superorders were used to assess reference bias in the reconstructed ancestral karyotypes and to expand the number of clades with reconstructed genomes. The mammalian ancestor likely had 19 pairs of autosomes, with nine of the smallest chromosomes shared with the common ancestor of all amniotes (three still conserved in extant mammals), demonstrating a striking conservation of synteny for ∼320 My of vertebrate evolution. The numbers and types of chromosome rearrangements were classified for transitions between the ancestral mammalian karyotype, descendent ancestors, and extant species. For example, 94 inversions, 16 fissions, and 14 fusions that occurred over 53 My differentiated the therian from the descendent eutherian ancestor. The highest breakpoint rate was observed between the mammalian and therian ancestors (3.9 breakpoints/My). Reconstructed mammalian ancestor chromosomes were found to have distinct evolutionary histories reflected in their rates and types of rearrangements. The distributions of genes, repetitive elements, topologically associating domains, and actively transcribed regions in multispecies homologous synteny blocks and evolutionary breakpoint regions indicate that purifying selection acted over millions of years of vertebrate evolution to maintain syntenic relationships of developmentally important genes and regulatory landscapes of gene-dense chromosomes.