Stronger Chromatin Boundaries Stand the Test of Evolutionary Time

For more than a decade, genome biologists have argued about how much of the three-dimensional architecture of our DNA actually matters. Topologically associating domains, or TADs, are stretches of the genome whose DNA sequences physically contact one another far more often than they contact neighboring regions, forming loop-like neighborhoods inside the cell nucleus. Some researchers view these domains as fundamental functional units of the genome, while others have pointed to their apparent shuffling across species as evidence that they are largely incidental byproducts of chromosome folding. A new study published in Genome Biology by Daniela C. Soto, Jonathan Flint and colleagues at the University of California, Los Angeles, adds a powerful quantitative twist to that debate, showing that the strength with which a domain boundary insulates its genomic neighborhood predicts how faithfully that boundary has been preserved across tens of millions of years of mammalian evolution.

The team attacked the problem with an unusually rich experimental resource: bulk Hi-C contact maps generated from the ventral hippocampus, a brain region central to memory and emotion, of eight inbred mouse strains. Five of these were classical laboratory strains and three were wild-derived, capturing a meaningful slice of the genetic diversity within a single species. The resulting dataset comprises 14.55 billion valid contact pairs, an enormous catalog of who-touches-whom information across the mouse genome. By comparing chromatin architecture across strains that differ at millions of sequence positions, the researchers could ask which features of genome folding are stable enough to survive both genetic drift and deliberate breeding, and which are labile.

Before any evolutionary claims could be made, however, the authors confronted a stubborn methodological problem. The most popular analytical tools treat TADs as discrete objects: an algorithm scans a Hi-C contact map and draws sharp lines where it detects a boundary. When the researchers applied such discrete calling across replicate samples, they found poor concordance between replicates, meaning the same biological sample analyzed twice could yield noticeably different sets of called domains. That instability makes discrete TAD calls a shaky foundation for cross-strain or cross-species comparison, because apparent differences might reflect algorithmic noise rather than genuine biology.

The solution was to abandon the binary view. Instead of asking whether a boundary exists at a given position, the team measured separation scores, a continuous metric that quantifies how strongly a genomic location insulates the regions on either side. Rather than a yes-or-no verdict, every point along the genome receives a graded insulation value, and weakly insulated boundaries can be distinguished from strongly insulated ones. This shift from categorical to continuous measurement substantially improved reproducibility between replicates, giving the researchers a reliable yardstick for comparing chromatin architecture across strains and, ultimately, across species. The lesson is one the field is likely to absorb broadly: when a biological property varies in degree, measuring it as a degree rather than a category can dissolve apparent irreproducibility.

Armed with this framework, the researchers identified 2,857 highly insulated genomic regions whose insulation is conserved across all eight mouse strains. These are places where the folding of the chromosome creates a particularly impermeable wall, and where that wall appears in every strain examined, from the most domesticated laboratory line to the wildest-derived strain. The consistency across such genetically diverse backgrounds suggests that these boundaries are not accidents of recent breeding history but features actively maintained by the cell.

What kind of genomic real estate hosts these conserved, strongly insulated boundaries? The team found they are markedly enriched for multiple binding sites of CTCF, the architectural protein best known for stitching DNA loops together and anchoring the boundaries of chromatin domains. They also sit in accessible, open chromatin, lie near super-enhancers, the dense clusters of regulatory elements that drive expression of genes defining cell identity, and are found close to housekeeping genes, the essential maintenance machinery every cell must keep running. In other words, the most insulated boundaries are not scattered randomly through the genome; they cluster around the regulatory and structural elements that biologists already consider among the most functionally consequential.

The evolutionary signal extends below the level of chromatin folding into the DNA sequence itself. The conserved highly insulated regions show greater DNA sequence conservation across placental mammals than would be expected by chance, indicating that the nucleotide letters underlying these boundaries have been constrained by purifying selection over deep evolutionary time. They also display more stable DNA methylation patterns, suggesting that their epigenetic state is buffered against the genetic perturbations that distinguish one mouse strain from another. Together, these converging lines of evidence, from protein binding, chromatin accessibility, sequence conservation and epigenetic stability, paint the strongly insulated boundary as a multiply protected feature of the mammalian genome.

The most striking result emerges when the comparison is pushed beyond mice. In a cross-species analysis, the researchers measured how often insulated regions occupy the same genomic positions in human and mouse, two lineages separated by roughly 90 million years of evolution. They found that the positional overlap between the two species increases steadily with insulation strength, climbing to 80.7 percent for the most highly insulated regions. That figure is remarkable: it means that for the strongest boundaries, the vast majority of locations are shared between human and mouse, implying that the folding architecture at these sites has been maintained essentially intact since the common ancestor of the two species. Weakly insulated boundaries, by contrast, overlap far less, consistent with the idea that only the most robust architectural walls are under strong selective constraint.

Not every difference in chromatin conformation between strains carries deep meaning, however. The study also examined structural genomic rearrangements, deletions, duplications and inversions of DNA segments that differ among mouse strains, and found that these are the primary driver of chromatin conformation differences between strains. Yet when the team searched for functional consequences of those conformational differences, they found only limited evidence that they matter for gene regulation or phenotype. This is a subtle but important nuance: much of the strain-to-strain variation in genome folding may be passive, a mechanical consequence of DNA sequence rearrangement rather than a cause of biological divergence. The functional story, the data suggest, lives in the conserved, strongly insulated boundaries rather than in the variable ones.

The practical payoff of the study is a prioritization tool. Because insulation strength is continuous, reproducible and predictive of conservation, researchers screening a disease-associated genomic variant can now consult the insulation landscape to judge whether a nearby TAD boundary is likely to be an ancient, functionally critical architectural element or a labile, probably inconsequential one. Given that disruptions of TAD boundaries have been implicated in developmental disorders and cancers, for example when structural variants allow an enhancer to reach a gene it was never meant to touch, a quantitative framework for ranking boundary importance could sharpen the interpretation of human genomic data. The work also reframes the long-running TAD debate: rather than asking whether TADs as a class are functionally meaningful, the field can now ask which specific boundaries carry the strongest evidence of constraint, with insulation strength serving as the guide. Funded in part by the National Institutes of Health and the Howard Hughes Medical Institute’s Hanna H. Gray Fellows Program, the study demonstrates that a careful rethinking of measurement, from discrete calls to continuous scores, can turn a contested question in genome biology into a tractable, quantitative one.

Subject of Research: Evolutionary conservation of topologically associating domain boundaries measured by Hi-C derived insulation strength in mouse and human genomes

Article Title: Hi-C derived insulation strength predicts evolutionary conservation of topologically associating domains

Article References: Soto, D. C., Chen, P. B., Huang, H., Galasso, J., Chen, Z. J., Luo, C., Ernst, J., & Flint, J. (2026). Hi-C derived insulation strength predicts evolutionary conservation of topologically associating domains. Genome Biology. https://doi.org/10.1186/s13059-026-04298-2

Image Credits: AI Generated

DOI: 10.1186/s13059-026-04298-2

Keywords: TADs, Hi-C, chromatin architecture, insulation strength, CTCF, genome evolution, mouse strains, separation scores, super-enhancers, DNA methylation, comparative genomics, Genome Biology