Three-dimensional genome reorganization foreshadows zygotic genome activation in Drosophila

How chromatin conformation relates to chromatin state remains a central challenge in genome regulation. Here we present Pico-C, a low-input Micro-C approach that enables high-resolution, temporally resolved three-dimensional genome mapping during early Drosophila embryogenesis. Contrary to a prevailing view of a disorganized genome before zygotic genome activation (ZGA), we uncover a dynamic and ordered emergence of chromatin loops during pre-ZGA nuclear cycles. Spatial autocorrelation analysis points to context-dependent regulatory influences on chromatin. Notably, inhibition of transcriptional elongation has site-specific effects, retaining some early loops while weakening insulation at active promoters, suggesting distinct regulatory dependencies. Machine learning models trained on sequence features identify orthogonal, motif-specific contributions to architecture. Co-depletion of the pioneer factors Zelda and GAF leads to factor-specific perturbations in chromatin architecture, further highlighting a modular regulatory logic in genome establishment. Together, our findings reveal that early genome organization is orchestrated by an interplay of overlapping yet separable regulatory inputs.