Cell type- and chromosome-specific chromatin landscapes and DNA replication programs of Drosophila testis tumor stem-cell like cells

Stem cells have the unique ability to self-renew and differentiate into specialized cell types. Epigenetic mechanisms, including histones and their post-translational modifications, play a crucial role in regulating programs integral to a cells identity, like gene expression and DNA replication. However, the transcriptional, chromatin, and replication timing profiles of adult stem cells in vivo remain poorly understood. Containing germline stem cells (GSCs) and somatic cyst stem cells (CySCs), the Drosophila testis provides an excellent in vivo model for studying adult stem cells. However, the small number of stem cells and cellular heterogeneity of this tissue have limited comprehensive genomic studies. In this study, we developed cell type-specific genomic techniques to analyze the transcriptome, histone modification patterns, and replication timing of GSC-like and CySC-like cells. Single cell RNA sequencing validated previous findings on GSC-CySC intercellular communication and revealed high expression of chromatin regulators in GSC-like cells. To characterize chromatin landscapes, we developed a cell-type-specific chromatin profiling assay to map H3K4me3-, H3K27me3-, and H3K9me3- enriched regions, corresponding to euchromatic, facultative heterochromatic, and constitutive heterochromatic domains, respectively. Finally, we determined cell type-specific replication timing profiles, integrating our in vivo datasets with published data using cultured cell lines. Our results reveal that GSC-like cells display a distinct replication program compared to somatic lineages, that aligns with chromatin state differences. Collectively, our integrated transcriptomic, chromatin, and replication datasets provide a comprehensive framework for understanding genome regulation differences between these in vivo stem cell populations, demonstrating the power of multi-omics in uncovering cell type- specific regulatory features.