Dynamic distribution of the replacement histone variant H3.3 in the mouse oocyte and preimplantation embryos.

Upon fertilization, the gametes undergo a drastic reprogramming that includes changes in DNA methylation and histone modifications. Currently, it is not known whether replacement of the major histones by histone variants is also involved in these processes. Here we have examined the expression and localization of the histone variant H3.3 in early mouse embryogenesis. We show that H3.3 is present in the oocyte as a maternal factor. It is then incorporated preferentially into the male pronucleus before genome activation, pointing towards an asymmetry in histone composition between the two pronuclei. This is in line with the male pronucleus bearing transcriptional activation first. The same distribution was observed when we followed the localisation of a tagged version of H3.3. We detected H3.3 in the nuclei of mouse embryos in all of the stages analysed, from the zygote to the blastocyst stage, suggesting that the epigenetic mechanisms in the early embryo not only involve changes in histone modifications but may also include histone replacement.