Unique territorial and compartmental organization of chromosomes in the holocentric silkworm.

Hallmarks of multicellular eukaryotic genome organization are chromosome territories, compartments, and loop-extrusion-mediated structures, including TADs. However, these are mainly observed in model organisms, and most eukaryotes remain unexplored. Using Hi-C in the silkworm Bombyx mori we discover a novel chromatin folding structure, compartment S, which is "secluded" from the rest of the chromosome. This compartment exhibits loop extrusion features and a unique genetic and epigenetic landscape, and it localizes towards the periphery of chromosome territories. While euchromatin and heterochromatin display preferential compartmental contacts, S domains are remarkably devoid of contacts with other regions, including with other S domains. Polymer simulations show that this contact pattern can only be explained by high loop-extrusion activity within compartment S, combined with low extrusion elsewhere through the genome. This unique, targeted extrusion represents a novel phenomenon and underscores how evolutionarily conserved mechanisms-compartmentalization and loop extrusion-can be repurposed to create new 3D genome architectures.