Lineage specific trimethylation of H3 on lysine 4 during C. elegans early embryogenesis.

In many organisms early embryogenesis is characterised by a period refractory to transcription. In Caenorhabditis elegans, the one-cell embryo is transcriptionally inactive, but at around eight-cell stage transcription is activated in the somatic lineage. This model suggests that histone tail modifications associated with activation of transcription, such as di- or trimethylation of histone 3 on lysine 4 (H3K4me2/me3) should be enriched in the somatic lineage. Here, we have investigated the deposition of H3K4me3 during embryogenesis and found that it is more dynamic than anticipated. In the eight-cell stage embryo, H3K4me3 deposition is poor in the germline blastomere, as expected, but surprisingly three somatic blastomeres also remain poor in H3K4me3. All the other somatic blastomeres show robust deposition of H3K4me3. Interestingly, the three somatic blastomeres poor in H3K4me3 are descendants of the first germline blastomere, implying an activity that impedes on H3K4me3 deposition in these cells. In contrast, the deposition of H3K4me2 and H3K27me2/3 is not lineage restricted. Taken together, our data reveal that H3K4me3 deposition is highly regulated according to the cell lineage involved.