Cartoons comparing ESCs and undifferentiated cells of bam testis. (a) During ESC differentiation, bivalent genes resolve into monovalent genes according to cell type specificity in somatic lineages. In the Drosophila male germline lineage, monovalent genes in undifferentiated cells may either retain their chromatin signature or switch to another pattern. Differentiation genes that are required for spermatogenesis require additional activation mechanisms to turn on their expression robustly, in addition to the removal of the repressive H3K27me3 mark. (b) A potential molecular mechanism that renders the bivalency and poised status dispensable in the Drosophila germline stem cell lineage, due to the lack of endogenous DNA methylase activity. DNMT, DNA methyltransferase.
Gan et al. Genome Biology 2010 11:R42 doi:10.1186/gb-2010-11-4-r42