Developmental regulation of heterochromatin establishment at the maternal to zygotic transition

Heterochromatin has important functions in the organization of nuclear structure, the maintenance of genome integrity, and the regulation of gene expression programs1-3. Here, we demonstrate that the genome of the early zebrafish embryo is packaged in an atypical chromatin state that lacks features characteristic of heterochromatin. The heterochromatic histone modification, histone H3 lysine 9 trimethyl (H3K9me3), is undetectable in embryonic chromatin and embryonic nuclei are devoid of condensed chromatin prior to activation of the zygotic genome. The BAF complex catalytic subunit Smarca2 is a guardian of this unique chromatin state, and miR-430 mediated degradation of maternal smarca2 RNA is required for heterochromatin establishment at the maternal to zygotic transition. Our results reveal a period of profound chromatin reorganization in the early zebrafish embryo and identify Smarca2 as an essential regulator of heterochromatin establishment during early development. Zebrafish embryo samples collected at 2.5, 4.5, and 6 hours post fertilization. Cells lysed and subjected to chromatin immunoprecipitation. 20 total samples, two replicates, Scramble Morpholino control