Single-cell RNA-seq identifies protracted mouse germline X chromosome reactivation dynamics directed by a PRC2-dependent mechanism.

Female primordial germ cells (PGCs) undergo X chromosome reactivation (XCR) during genome-wide reprogramming. XCR kinetics and dynamics are poorly understood at a molecular level. Here, we apply single-cell RNA sequencing and chromatin profiling on germ cells from F1 mouse embryos, performing a precise appraisal of XCR spanning from migratory-stage PGCs to gonadal germ cells. Establishment of germ cell sexual dimorphism and X chromosome dosage compensation states in vivo are temporally linked to XCR. Allele-specific analysis evidence that the reactivating X chromosome is minimally active in embryonic day (E)9.5 female PGCs, reactivates gradually, and reaches parity to the active X chromosome in E16.5 oogonia. While Xist is repressed from E10.5 onward, epigenetic memory of X inactivation persists from self-sustained polycomb repressive complex 2 (PRC2) activity. The reactivating X is asymmetrically enriched for histone 3-lysine-27-trimethylation (H3K27me3) at E13.5, which is later reversed, permitting germline gene expression. Our findings relate XCR with PRC2 function in promoting female meiosis.