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

The hallmarks of chromosome organization in multicellular eukaryotes are chromosome territories (CT), chromatin compartments, and insulated domains, including topologically associated domains (TADs). Yet, most of these elements of chromosome organization are derived from analyses of a limited set of model organisms, while large eukaryotic groups, including insects, remain mostly unexplored. Here we combine Hi-C, biophysical modeling, and microscopy to characterize the 3D genome architecture of the silkmoth, Bombyx mori. In contrast to other eukaryotes, B. mori chromosomes form highly separated territories. Similar to other eukaryotes, B. mori chromosomes segregate into active A and inactive B compartments, yet unlike in vertebrate systems, contacts between euchromatic A regions appear to be a strong driver of compartmentalization. Remarkably, we also identify a third compartment, called secluded "S," with a unique contact pattern. Each S region shows prominent short-range self-contacts and is remarkably devoid of contacts with the rest of the chromosome, including other S regions. Compartment S hosts a unique combination of genetic and epigenetic features, localizes towards the periphery of CTs, and shows developmental plasticity. Biophysical modeling reveals that the formation of such secluded domains requires highly localized loop extrusion within them, along with a low level of extrusion in A and B. Our Hi-C data supports predicted genome-wide and localized extrusion. Such a broad, non-uniform distribution of extruders has not been seen in other organisms. Overall, our analyses support loop extrusion in insects and highlight the evolutionary plasticity of 3D genome organization, driven by a new combination of known processes.

Unique territorial and sub-chromosomal organization revealed in the holocentric moth Bombyx mori.

The hallmarks of chromosome organization in multicellular eukaryotes are chromosome territories (CT), chromatin compartments, and different types of domains, including topologically associated domains (TADs). Yet, most of these concepts derive from analyses of organisms with monocentric chromosomes. Here we describe the 3D genome architecture of an organism with holocentric chromosomes, the silkworm Bombyx mori . At the genome-wide scale, B. mori chromosomes form highly separated territories and lack substantial trans contacts. As described in other eukaryotes, B. mori chromosomes segregate into an active A and an inactive B compartment. Remarkably, we also identify a third compartment, Secluded "S", with a unique contact pattern. Compartment S shows strong enrichment of short-range contacts and depletion of long-range contacts. It hosts a unique combination of genetic and epigenetic features, localizes at the periphery of CTs and shows developmental plasticity. Biophysical modeling shows that formation of such secluded domains requires a new mechanism - a high density of extruded loops within them along with low level of extrusion and compartmentalization of A and B. Together with other evidence of loop extrusion in interphase, this suggests SMC-mediated loop extrusion in this insect. Overall, our analyses highlight the evolutionary plasticity of 3D genome organization driven by a new combination of known processes.