Global Chromatin Reprogramming in Fetal Germ Cells weakly for H3K9me2 compared to somatic cells at each developmental stage (Figure 4) except at 15.5 dpc, when both male and female germ cells reached the staining level of somatic cells. H3K9me3 levels were higher in gem cells of both sexes than in somatic cells throughout fetal development (Figure 5). H3K9me3 levels increased in male germ cells at 15.5 and 17.5 dpc. H3K27me3 was high in male and female germ cells compared to the respective somatic cells at 11.5 and 13.5 dpc (Figure 6), when DNA methylation is the lowest. This is in agreement with previous reports that showed increased H3K27me3 levels in PGCs between 9.5 and 11.5 dpc [8,9]. It is tempting to speculate that Polycomb-mediated repression may be the major repressive force in midgestation germ cells at the time when DNA methylation is globally erased. At 15.5 dpc there was a dramatic increase in H3K27me3 levels in male germ cells, but at the same time H3K27me3 levels dropped in female germ cells. Figure 4. Global changes of H3K9me2 in fetal germ cells. Gonad preparations were stained with anti-H3K9me2 antibody. DAPI was used to visualize nuclei (blue). Other details are as in legend to Figure 1. doi:10.1371/journal.pone.0023848.g004 PLoS ONE | www.plosone.org 5 August 2011 | Volume 6 | Issue 8 | e23848