Unique and dynamic regulation of Pol II in the embryonic germline. (A) Transcription status and histone modification dynamics in C. elegans early embryonic blastomeres. The asymmetric divisions of P-lineage cells (P0 to P3) produce both germline and somatic blastomeres. The last P cell division (P4) is symmetric and produces two germline-committed primordial germ cells (PGCs), Z2 and Z3 and these cells arrest at G2 phase through the rest of embryogenesis . Two maternally expressed CCCH zinc finger proteins, OMA-1/2 and PIE-1, act sequentially in P0 to P4 to maintain transcriptional quiescence independently of the chromatin environment [14, 69, 70] (orange line). At the P4 division, PIE-1 is degraded quickly and 'active Pol II' (H5 staining; magenta line) appears in the PGCs (circled by blue dotted line). H3K36me3 (black) and H3K4me2 (green) are initially present in both the transcriptionally quiescent P-lineage and their somatic sisters, where transcription is activated. MES-4, which is essential for germ cell viability, produces H3K36me in the germline precursors independently of transcription . H3K4me2 is also maintained through the P-lineage by an unidentified mechanism, but the level of H3K4me2 becomes almost undetectable in the PGCs  (indicated by dotted green lines). (B-L) H5 staining of Pol II Ser2P in the PGCs of (B-E) wild type, (F-I) mes-4 and (J-L) mes-2during embryogenesis; (B,F)~150 minutes post-fertilization at 22°C, ~26-cell stage; (C,G,J) ~200 minutes, ~90-cell stage; (D,H,K) ~450 minutes, ~1.5-fold stage; (E,I,L) >500 minutes, 2- to 3-fold stages. PGCs are boxed and shown by arrows. DAPI staining, red; H5 antibody staining, green; PGL-1 (germline marker) staining, blue. In the lower panels, PGCs are enlarged, and the separated channels for DAPI and H5 staining are shown in grayscale. Scale bars: 10 μm.