Numerous histone variants exist in eukaryotes, and the replacement of canonical histones with such variants probably contributes to chromatin remodelling. Chromatin remodelling occurs during fertilisation, as germ cells become totipotent zygotes, but the role of histone variants during this process is unknown. On , Fugaku Aoki and colleagues assess the dynamics of histone H2A and its variants, H2A.X, H2A.Z and macroH2A, during mouse oogenesis and pre-implantation development. They report that all variants are present in oocytes; by contrast, only H2A.X is abundant in one-cell embryos. The authors confirmed this postfertilisation reduction in H2A, H2A.Z and macroH2A using transgenic mice that express tagged H2 variants, and by microinjecting embryos with mRNA for these variants. Domain-swapping experiments showed that the C-terminal 23 amino acids of H2A.X enable its incorporation into chromatin after fertilisation, and that the concomitant reduction of H2A.Z and macroH2A is required for normal development. The authors suggest that altered histone composition might therefore contribute to the genome remodelling, and hence reprogramming, that occurs postfertilisation.
