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Single-molecule chromosome tracing reveals a diversity of megabase heterochromatin domains

Pulido Barrera, D. C.; You, J. E.; Xu, F.; Verheijen, T.; Sawh, A. N.; Kos, P.; Giorgetti, L.; Molina, N.; Bruckner, D.; Mango, S. E.

2025-09-09 genomics
10.1101/2025.09.03.673923 bioRxiv
Show abstract

Chromosomes fold into distinct domains that regulate transcription, replication, and repair. Beyond well-characterized TADs and compartments, the diversity of heterochromatin domains remains poorly defined at the sequence level. Using single-molecule tracing of nascent heterochromatin in C. elegans, we identify three classes of megabase-scale domains: (i) sharp-boundary, Condensin I-dependent Topological Associating Domain-like domains (TADLs); (ii) similarly sized, but Condensin-independent, elegans Condensin-Independent Domains (elCIDs); and (iii) weaker, diffuse structures that are abundant in the population. TADLs arise early in development, preceding elCIDs, and both become progressively compacted through H3K9 methylation, which promotes intra- and inter-domain proximity. Condensin mutations disrupt TADLs, yet single molecules can still form domain-like structures, as recapitulated by free polymer simulations. However, these differ markedly in boundary positioning and biophysical properties. Our results uncover previously unrecognized heterochromatin architectures and demonstrate that single-molecule analysis and mutational dissection provide valuable approaches for distinguishing between domain types.

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