Single-nucleus Hi-C reveals unique chromatin reorganization at oocyte-to-zygote transition

Abstract

Chromatin is reprogrammed after fertilization to produce a totipotent zygote with the potential to generate a new organism ¹ . The maternal genome inherited through the oocyte and the paternal genome provided by sperm coexist as separate haploid nuclei in the zygote. How these two epigenetically distinct genomes are spatially organized is poorly understood. Existing chromosome conformation capture-based methods ^{2– 5} are inapplicable to oocytes and zygotes due to a paucity of material. To study the 3D chromatin organization in rare cell types, we developed a single-nucleus Hi-C (snHi-C) protocol that provides >10-fold more contacts per cell than the previous method ² . Here we show that chromatin architecture is uniquely reorganized during the mouse oocyte-to-zygote transition and is distinct in paternal and maternal nuclei within single-cell zygotes. Features of genomic organization including compartments, topologically associating domains (TADs) and loops are present in individual oocytes when averaged over the genome; each feature at a locus is variable between cells. At the sub-megabase level, we observe stochastic clusters of contacts that violate TAD boundaries but average into TADs. Strikingly, we found that TADs and loops but not compartments are present in zygotic maternal chromatin, suggesting that these are generated by different mechanisms. Our results demonstrate that the global chromatin organization of zygote nuclei is fundamentally different from other interphase cells. An understanding of this zygotic chromatin “ground state” has the potential to provide insights into reprogramming to totipotency.