Single-molecule chromatin tracing reveals a diversity of megabase heterochromatin domains.

Single-molecule tracing in Caenorhabditis elegans identifies three classes of megabase-scale heterochromatin domains, including condensin I-dependent sharp-boundary TAD-like domains.

• Why it matters: Chromosome folding is known to involve TADs and compartments, but heterochromatin domain diversity is poorly defined at the sequence level, especially in early embryos.
• What they did: The study used single-molecule tracing of nascent heterochromatin in Caenorhabditis elegans to classify megabase domains by boundary sharpness and condensin dependence, including condensin-independent elCIDs and diffuse decompacted structures.
• The result: Condensin I localizes to TADL borders, while DNA-binding proteins, active histone marks, and maternally active genes flank elCIDs, implying distinct boundary mechanisms and maternal transcriptional activity shaping early-embryo domain boundaries.

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