TADs pair homologous chromosomes to promote interchromosomal gene regulation
Homologous chromosomes colocalize to regulate gene expression in processes including genomic imprinting and X-inactivation, but the mechanisms driving these interactions are poorly understood. In Drosophila, homologous chromosomes pair throughout development, promoting an interchromosomal gene regulatory mechanism called transvection. Despite over a century of study, the molecular features that facilitate chromosome-wide pairing are unknown. The \"button\" model of pairing proposes that specific regions along chromosomes pair with a higher affinity than their surrounding regions, but only a handful of DNA elements that drive homologous pairing between chromosomes have been described. Here, we identify button loci interspersed across the fly genome that have the ability to pair with their homologous sequences. Buttons are characterized by topologically associated domains (TADs), which drive pairing with their endogenous loci from multiple locations in the genome. Fragments of TADs do not pair, suggesting a model in which combinations of elements interspersed along the full length of a TAD are required for pairing. Though DNA-binding insulator proteins are not associated with pairing, buttons are enriched for insulator cofactors, suggesting that these proteins may mediate higher order interactions between homologous TADs. Using a TAD spanning the spinelessd gene as a paradigm, we find that pairing is necessary but not sufficient for transvection. spineless pairing and transvection are cell-type-specific, suggesting that local buttoning and unbuttoning regulates transvection efficiency between cell types. Together, our data support a model in which specialized TADs button homologous chromosomes together to facilitate cell-type-specific interchromosomal gene regulation.