Widespread 3D genome reorganization precedes programmed DNA rearrangement in .

Genome organization recapitulates function, yet ciliates like possess highly-specialized germline genomes, which are largely transcriptionally silent. During post-zygotic development, 's germline undergoes large-scale genome editing, rearranging precursor genome elements into a transcriptionally-active genome with thousands of gene-sized nanochromosomes. Transgenerationally-inherited RNAs, derived from the parental somatic genome, program the retention and reordering of germline fragments. Retained and eliminated DNA must be distinguished and processed separately, but the role of chromatin organization in this process is unknown. We developed tools for studying nuclei and apply them to map the 3D organization of precursor and developmental states using Hi-C. We find that the precursor conformation primes the germline for development, while a massive spatial reorganization during development differentiates retained from eliminated regions before DNA rearrangement. Further experiments suggest a role for RNA-DNA interactions and chromatin remodeling in this process, implying a critical role for 3D architecture in programmed genome rearrangement.