Jing Liu

The functional integrity of a mammalian chromosome is shaped by its long-term, co-evolution with species-specific nuclear environment. How chromosomes co-adapt with their native environment to define 3D architecture and transcriptional activity remain poorly understood, largely due to a lack of experimental models capable of systematically dissecting this co-evolution relationship. Here, we report a cross-species chromosome substitution (CROSS) method, a robust genomic engineering method that enables the stable, scarless replacement of host chromosomes with evolutionarily divergent orthologs. By integrating microcell-mediated chromosome transfer with CRISPR/Cas9, we imported the intact 158-Mb rat X chromosome into mouse embryonic stem cells, and subsequently achieved targeted substitution of its endogenous mouse counterpart, maintaining stability and integrity. Using this model, we found that rat-specific LINE1 and RatSatRep2 repeats failed to adequately recruit host SETDB1 in mouse cells, leading to localized erosion of H3K9me3 heterochromatin. This further triggered 3D structural remodeling, characterized by the de novo formation of topologically associating domain (TAD) boundaries that aberrantly activated adjacent genes—including Rhox5 , the master regulator of the Rhox cluster—impairing cellular differentiation. Our method provides a powerful chromosome engineering platform for dissecting how genomic sequences and epigenetic mechanisms cooperate in regulating chromosome architecture and function, and for evaluating the structural and functional fidelity of large-scale synthetic or heterologous DNA across species.

Chromosomes of mammals and reptiles have diverged extensively in sequence and three-dimensional (3D) nuclear organization since their last common amniote ancestor. The fine-scale evolutionary patterns and mechanisms underlying this divergence and the relationship between chromosome rearrangements and 3D genome architecture remain unclear. Here we reconstruct ancestral linkage groups (ALGs) of the amniote ancestor and its descendent nodes using chromosome-level assemblies from 36 species, including two newly generated genomes. We infer that most microchromosomes originated in the jawed vertebrate ancestor were retained as distinct ALGs across all ancestral amniote nodes. They were mainly involved in fusions with each other in reptiles, but underwent frequent translocations with the larger ALGs (macro-ALGs) during the mammalian evolution. Despite extensive rearrangements, sequences derived from micro-ALGs consistently occupy nuclear interior positions in both reptiles and mammals, likely enriched near nuclear speckles, supported by published lamina-positioning data and our lamin-B1 ChIP-seq data. In contrast, mammals independently evolved interchromosomal interactions associated with lineage specific transposable elements, reflecting local innovations in nuclear architecture. Together, these results reveal contrasting rearrangement regimes in mammals and reptiles, yet a conserved higher-order nuclear organization linked to ancient micro- versus macro-ALG chromatin segregation, suggesting deep constraints on chromosome evolution.

In eukaryotes, chromosomes can be arranged with centromeres and/or telomeres clustered at opposite poles of the nucleus (Rabl-like, RBL) or as discrete spatial domains (chromosome territory, CT). These patterns were thought to be species specific, possibly linked to the presence or absence of condensin II subunits. Time-series embryonic chromatin maps from three distantly related animals reveal that RBL and CT reflect cell state rather than species identity, and a conserved RBL-to-CT transition has occurred during development. This shift is not significantly correlated with condensin II gene expression, but may be influenced by the genomic distribution of repetitive sequences. In late developmental stages, long-range cis- and trans-interactions between active A1 subcompartments progressively intensify, ultimately establishing CT as the dominant architecture. These results uncover a conserved, developmentally regulated reorganization of 3D genome structure across animals.

Little is known about how the epigenomic states change during development and evolution in a 3D genome context. Here we use Drosophila pseudoobscura with complex turnover of sex chromosomes as a model to address this, by collecting massive epigenomic and Hi-C data from five developmental stages and three adult tissues. We reveal that over 60% of the genes and transposable elements (TE) exhibit at least one developmental transition of chromatin state. Transitions on specific but not housekeeping enhancers are associated with specific chromatin loops and topologically associated domain borders (TABs). While evolutionarily young TEs are generally silenced, old TEs more often have been domesticated as interacting TABs or specific enhancers. But on the recently evolved X chromosome, young TEs are instead often active and recruited as TABs, due to acquisition of dosage compensation. Overall we characterize how Drosophila epigenomic landscapes change during development and in response to chromosome evolution, and highlight the important roles of TEs in genome organization and regulation.

The slow-evolving invertebrate amphioxus has an irreplaceable role in advancing our understanding of the vertebrate origin and innovations. Here we resolve the nearly complete chromosomal genomes of three amphioxus species, one of which best recapitulates the 17 chordate ancestor linkage groups. We reconstruct the fusions, retention, or rearrangements between descendants of whole-genome duplications, which gave rise to the extant microchromosomes likely existed in the vertebrate ancestor. Similar to vertebrates, the amphioxus genome gradually establishes its three-dimensional chromatin architecture at the onset of zygotic activation and forms two topologically associated domains at the Hox gene cluster. We find that all three amphioxus species have ZW sex chromosomes with little sequence differentiation, and their putative sex-determining regions are nonhomologous to each other. Our results illuminate the unappreciated interspecific diversity and developmental dynamics of amphioxus genomes and provide high-quality references for understanding the mechanisms of chordate functional genome evolution.

Microchromosomes are prevalent in nonmammalian vertebrates [P. D. Waters et al. , Proc. Natl. Acad. Sci. U.S.A. 118 (2021)], but a few of them are missing in bird genome assemblies. Here, we present a new chicken reference genome containing all autosomes, a Z and a W chromosome, with all gaps closed except for the W. We identified ten small microchromosomes (termed dot chromosomes) with distinct sequence and epigenetic features, among which six were newly assembled. Those dot chromosomes exhibit extremely high GC content and a high level of DNA methylation and are enriched for housekeeping genes. The pericentromeric heterochromatin of dot chromosomes is disproportionately large and continues to expand with the proliferation of satellite DNA and testis-expressed genes. Our analyses revealed that the 41-bp CNM repeat frequently forms higher-order repeats (HORs) at the centromeres of acrocentric chromosomes. The centromere core regions where the kinetochore attaches often encompass telomeric sequence (TTAGGG)n, and in a one of the dot chromosomes, the centromere core recruits an endogenous retrovirus (ERV). We further demonstrate that the W chromosome shares some common features with dot chromosomes, having large arrays of hypermethylated tandem repeats. Finally, using the complete chicken chromosome models, we reconstructed a fine picture of chordate karyotype evolution, revealing frequent chromosomal fusions before and after vertebrate whole-genome duplications. Our sequence and epigenetic characterization of chicken chromosomes shed insights into the understanding of vertebrate genome evolution and chromosome biology.

Chromatin states of genes and transposable elements (TEs) dictated by combinations of various histone modifications comprise key information for understanding the mechanisms of genome organization and regulation. However, little is known about the principles of their dynamic changes during development and evolution in a three-dimensional genome context. To address this, we study Drosophila pseudoobscura , a Drosophila model species that diverged from D. melanogaster about 25 million years ago. We collected 71 epigenomic datasets targeting 11 histone modification marks and 4 Hi-C datasets, and projected 15 chromatin states across four different developmental stages and two adult tissues. We estimate that before zygotic genome activation, 41% of the genome has already been deposited with histone modifications, while 20% of the rest genome switches from a ‘null’ state to an active/inactive chromatin state after the zygotic genome activation. Over two thirds of the genomic region exhibit at least one transition between different chromatin states during development. And such transitions on cis -regulatory regions are associated with tissue- or stage-specific formation of chromatin loops or topologically associated domain borders (TABs), as well as specific activation of gene expression. We further demonstrate that while evolutionarily young TEs are preferentially targeted by silencing histone modifications, old TEs are more frequently domesticated as TABs or specific enhancers that further contribute to the genome organization or local gene regulation. Interestingly, this trend is reversed on the newly evolved X chromosome in D. pseudoobscura , due to the acquisition of dosage compensation mechanism. Overall we characterize the developmental and evolutionary dynamics of Drosophila epigenomic states, and highlight the roles of certain TEs of different evolutionary ages in genome organization and regulation.

African swine fever virus (ASFV) has been circulating in China for 5 years, and low virulent strains with changes in the genome have been reported. Nevertheless, there is still a lack of knowledge about the epidemic strains at the whole-genome level. This study reported a novel strain and further analyzed its genomic and biological characteristics. In addition, our study also suggests that whole-genome sequencing plays a key role in the epidemiology investigation of ASFV variations.