Abstract: The ability of eukaryotic cells to fit nearly 2 meters of DNA into a single micron-scale nucleus, as well as selectively access the right genes at the right time, is a physical challenge that remains a key biological question. How DNA is folded in 3D can influence which genes are easy to turn on or off, particularly during stress. Emerging research in mammalian cells has shown that chromatin, the DNA and its associated histone proteins, are not arranged as a fixed, rigid hierarchy. Instead, chromatin has been found to form, dynamic, heterogenous "neighborhoods": nanoscale "packing domains" characterized by dense interior cores and more open outer regions, held in place by different factors, such as post-translational modifications of the histone proteins. These local 3D environments help shape gene activity and could contribute to cellular

"memory" of past conditions. This paradigm shift in chromatin 3D organization may be critically important for our understanding in the resilience of ecologically important species, as environments change due to anthropogenic activities. Here, we ask whether similar 3D packing principles occur in reef-building corals, sessile organisms which must respond quickly to environmental change, such as marine heat stress. Although the field of coral epigenetics is rapidly growing, the potential role of 3D genome organization in regulating gene expression remains largely unexplored. We adapt ChromSTEM, a high-resolution electron microscopy approach, to visualize and quantify DNA packing in coral cells at nanometer resolution and compare these patterns to those reported in human cells. This work is a first step toward connecting 3D genome structure with gene regulation and stress responses in a non-model, ecologically important organism.

Bio: Shanna Davidson is a postdoctoral scholar in the Marcelino lab in Civil and Environmental Engineering at Northwestern University. Prior to joining Northwestern, she earned her PhD in Chemical and Petroleum Engineering from the University of Pittsburgh. Her current research bridges physical genomics and environmental science by investigating how the three-dimensional organization of chromatin regulates cellular responses to environmental change. Utilizing a suite of nanoscale imaging techniques and molecular approaches, she studies genome architecture in corals to understand how these ecologically critical organisms respond to ocean warming.