Genetic mosaics and time-lapse imaging identify functions of histone H3.3 residues in mouse oocytes and embryos

During development from oocyte to embryo, genetic programs in mouse germ cells are reshaped by chromatin remodeling to orchestrate the onset of development. Epigenetic modifications of specific amino acid residues of core histones and their isoforms can dramatically alter activation and suppression of gene expression. H3.3 is a histone H3 variant that plays essential roles in mouse oocytes and early embryos, but the functional role of individual amino acid residues has been unclear because of technical hurdles. Here, we describe two strategies that successfully investigated the functions of three individual H3.3 residues in oogenesis, cleavage-stage embryogenesis and early development. We first generated genetic mosaic ovaries and blastocysts with stochastic expression of wild-type or mutant H3.3 alleles and showed dominant negative effects of H3.3R26 and H3.3K27 in modulating oogenesis and partitioning cells to the inner cell mass of the early embryo. Time-lapse imaging assays also revealed the essential roles of H3.3K56 in efficient H2B incorporation and paternal pronuclei formation. Application of these strategies can be extended to investigate roles of additional H3.3 residues and has implications for use in other developmental systems.