FIGURE 3.

Chromatin dynamics during male gametogenesis. This scheme summarizes cytogenetic and molecular profiling data suggesting large-scale chromatin dynamics events during male gametophyte development. Although disparate in the level of investigation and plant species analyzed it provides a conceptual framework, yet to be completed, for apprehending the extent and potential significance of chromatin dynamics during this developmental stage. In Arabidopsis, the microspore harbors low levels of CHH methylation at retrotransposon loci, but retains CG methylation. After the first mitosis, the vegetative nucleus restores CHH methylation, but undergoes CG demethylation at a subset of TE loci (Calarco et al., 2012). The chromatin of the vegetative cell is highly decondensed, mostly deprived of linker H1 (Wenjing She and Célia Baroux, unpublished) and H3K9me2 (Schoft et al., 2009). Additionally, the somatic patterns of histone H3 variants are erased, and only a few H3 variants are retained including HTR5, HTR8, and HTR14 (Ingouff et al., 2010). Compared to that in somatic nuclei, the chromatin of vegetative cell in rye lost H3K4me2, H3K9ac and H3K9me2, but retains H3K27me3, which can be traced back to the bicellular stage (Houben et al., 2011). In contrast, the sperm chromatin inherits the pattern of DNA methylation from the microspore nucleus, with low levels of CHH methylation, and enrichment of methylated CG (Calarco et al., 2012). It accumulates linker histone H1.1 (Wenjing She and Célia Baroux, unpublished) and H3K9me2 (Schoft et al., 2009). Dynamic changes in the histone H3 repertoire are also observed, with erasure of the somatic variants, but enrichment in HTR5, HTR10 in the sperm nucleus (Ingouff et al., 2010). In rye, it was shown that the sperm chromatin is enriched in H3K4me2, H3K9ac and H3K9me2 modifications, but depleted of H3K27me3, a state that can be traced back to the generative cell at the bicellular stage (Houben et al., 2011).