Chromatin architecture in the fly: Living without CTCF/cohesin loop extrusion?

The organization of the genome into topological domains (TADs) appears to
be a fundamental process occurring across a wide range of eukaryote
organisms, and it likely plays an important role in providing an architectural
foundation for gene regulation. Initial studies emphasized the remarkable
parallels between TAD organization in organisms as diverse as Drosophila
and mammals. However, whereas CTCF/cohesin loop extrusion is emerging
as a key mechanism for the formation of mammalian topological domains, the
genome organization in Drosophila appears to depend primarily on the
partitioning of chromatin state domains. We discuss recent work suggesting a
fundamental conserved role of chromatin state in building domain
architecture, and we consider insights into genome organization from recent
studies in Drosophila.