Figure 2. Germline reprogramming of DNA methylation in mice and plants.

A. In mice there are at least two rounds of genome wide DNA methylation reprogramming. The first occurs just after fertilization, in the zygote and early cleavage stages, to erase gametic (sperm and oocyte) epigenomic marks. During this phase of reprogramming, genomic imprints are maintained. The other major reprogramming process occurs in the germ line where the paternal and maternal somatic programs are erased, together with imprints and the inactive X is reactivated. Subsequent to this, parent-specific imprints are laid down in the germ line. In each reprogramming window, a specific set of mechanisms regulates erasure and re-establishment of DNA methylation. Recent studies have uncovered roles for the TET3 hydroxylase and passive demethylation, together with base excision repair (BER) and the elongator complex, in methylation erasure from the zygote (Seisenberger et al., 2013). In the germ line, deamination by AID, BER and passive demethylation have been implicated in reprogramming, but the processes are still poorly understood.

B. In plants meiocytes differentiate from somatic cells, and the germline undergoes 2–3 sterotypical mitotic divisions after formation of the haploid microspore (pollen) and megaspore (ovule) (Gutierrez-Marcos and Dickinson, 2012). In pollen, symmetric CG and CHG methylation (H = A,C,T) is retained in the microspore and sperm cells, but CG methylation is lost from a few hundred imprinted and other genes in the companion vegetative cell nucleus. CHH methylation is sharply reduced in the microspore and sperm cells. 21nt epigenetically activated siRNA and a subset of 24nt siRNA arise in the vegetative nucleus but accumulate in sperm cells where they contribute to imprinting and epigenetic transposon control. Modified from Calarco et al. 2012 Cell. 2012 Sep 28;151(1):194–205.