Compensatory replacement of the BigH1 variant histone by canonical H1 supports normal embryonic development in Drosophila

Histone variants carry specific functions in addition to those fulfilled by their canonical counterparts. Variants of the linker Histone H1 are prevalent in vertebrates and based on the pattern of their expression, many are presumed to function during germline and the earliest zygotic stages of development. While the existence of multiple H1 variants has hampered their study in vertebrates, a single variant, BigH1, was identified in Drosophila, promising to accelerate our understanding of the biological functions of H1 and H1 variants. Here we uncovered evidence for a compensatory activity that loads maternal H1 onto BigH1-devoid chromatin. Remarkably, this H1-based chromatin state is fully functional in supporting normal embryonic development, suggesting that H1 carries the essential function of the BigH1 molecule under the same developmental context. In addition, we discovered that this compensatory replacement of BigH1 with H1 might be limited to rapidly cycling cells in early embryos.