Differential condensation of sister chromatids acts with Cdc6 to ensure asynchronous S-phase entry in Drosophila male germline stem cell lineage

During Drosophila melanogaster male GSC (germline stem cell) asymmetric division, preexisting old versus newly synthesized histones H3 and H4 are asymmetrically inherited. However, the biological outcomes of this phenomenon remained unclear. Here, we tracked old and new histones throughout the GSC cell cycle using high spatial and temporal resolution microscopy. We found unique features that differ between old and new histone-enriched sister chromatids, including differences in nucleosome density, chromosomal condensation, and H3 Ser10 phosphorylation. These distinct chromosomal features lead to their differential association with Cdc6, a pre-replication complex component, and subsequent asynchronous DNA replication initiation in the resulting daughter cells. Disruption of asymmetric histone inheritance abolishes differential Cdc6 association and asynchronous S-phase entry, demonstrating that histone asymmetry acts upstream of these critical cell cycle progression events. Furthermore, disruption of these GSC-specific chromatin features leads to GSC defects, indicating a connection between histone inheritance, cell cycle progression, and cell fate determination.