FIGURE 1.

PSR-induced chromatin modifications in Nasonia vitripennis and other genome silencing or elimination events. (A) Schematic representation of histone post-translational modifications associated with PSR-induced genome elimination. In the center row, PSR is carried into the egg by the sperm. The two pronuclei come together and undergo one round of replication. As the two sets enter into the first mitosis, the maternal chromatin resolves normally into individual chromosomes while the paternal chromatin fails to resolve, becoming the paternal chromatin mass (PCM). The maternal chromosomes and the PSR chromosome, which is not blocked from resolving, segregate together, forming two haploid (1N) PSR+ nuclei. Top, a chromatin fiber from a region of euchromatin, which becomes phosphorylated at serine residue 10 of histone H3. This mark is associated with mitotic chromosome condensation (box in top right). Bottom, a chromatin fiber in euchromatin of the PCM. After fertilization, the paternal chromatin obtains abnormally high levels of di- and tri-methylated histone H3K9, mono-methylated histone H3K27, and mono-methylated histone H4K20. The H3S10p mark is also placed, but the combination of all four marks blocks normal chromosome resolution (box in bottom right). Chromatin marks that have been characterized on the Nasonia PCM are summarized in (B) along with what is known about these marks on the mammalian inactivated X chromosome (Barr body) (Hall and Lawrence, 2010) and the eliminated paternal chromosomes (EC) of Sciara (Goday and Ruiz, 2002; Greciano and Goday, 2006; Escribá et al., 2011). Symbols indicate that the specified modification is enriched (+), depleted (–), or unchanged (UC) on the eliminated or silenced chromosome(s) while “ND” indicates modifications for which there is no available data.