Bivalency in Drosophila embryos is associated with strong inducibility of Polycomb target genes.

Polycomb group (PcG) and Trithorax group (TrxG) proteins orchestrate development of a multicellular organism by faithfully maintaining cell fate decisions made early in embryogenesis. An important chromatin mark connected to PcG/TrxG regulation are bivalent domains, the simultaneous presence of H3K27me3 and H3K4me3 on a given locus, originally identified in mammalian embryonic stem cells but considered to be absent in invertebrates. Here, we provide evidence for existence of bivalency in fly embryos. Using a recently described PcG reporter fly line, we observed a strong reporter inducibility in embryo and its sharp decrease in larval and adult stages. Analysis of the chromatin landscape of the reporter revealed a strong signal for the repressive PcG mark, H3K27me3, in all three developmental stages and, surprisingly, a strong signal for a transcriptionally activating H3K4me3 mark in embryo. Using re-ChIP experiments, bivalent domains were also uncovered at endogenous PcG targets like the Hox genes.