Transgenerational continuity of histone modifications in the germline. Continuity in germline chromatin is illustrated for three chromatin modifications: H3K27 methylation (orange), H3K36 methylation (blue), and H3K4methylation (green). Vertical dotted lines demark separate generations; the arrows at the bottom delineate embryonic and postembryonic germ cell stages within each generation. H3K27 methylation levels are maintained by the PRC2 complex at all stages, with an increase in H3K27me3 levels in Z2/Z3 in the embryo (not shown). H3K36 methylation maintenance by MES-4 occurs at all stages, but its maintenance is critical in the embryo (light blue). In postembryonic germ cell stages, co-transcriptional H3K36 methylation by MET-1 occurs (dark blue). H3K4 methylation (green) is maintained in the early embryo by the C. elegans MLL complex, but extensive loss/reprogramming of this mark is observed at the birth of the PGCs, Z2/Z3. H3K4 methylation in the postembryonic germ line is then re-established and maintained by a combination of MLL-dependent and MLL-independent/transcription-dependent mechanisms.