The cytosine oxidation cycle. The chemical structures of cytosine, 5-methylcytosine and each oxi-mC within DNA. Their synthesis route is depicted by arrows. The enzyme catalyzing each reaction and/or the physical process taking place, is labeled next to the arrow.

The 5mC and 5hmC content of the mouse (A) and zebrafish (B) during development. 5mC and 5hmC content are expressed as % of genomic CpG sites. Solid circles represent the overall genomic CpG 5mC status (Jiang et al., 2013; Amouroux et al., 2016; Okamoto et al., 2016; Guo et al., 2017), with crosses representing the overall genomic CpG 5hmC status (Pastor et al., 2013; Wu and Zhang, 2014; Kamstra et al., 2015; Amouroux et al., 2016). Sex symbols indicate the 5mC content of sperm or oocytes. Blue panels represent the general expression level of TET mRNA at particular stages (Yamaguchi et al., 2012; Ge et al., 2014; Amouroux et al., 2016; Bogdanović et al., 2016). X-axes labels indicate the developmental stage of the embryo, with shared positions being analogous between species.

The 5mC and 5hmC content of the mouse germline during development. 5mC content is expressed as % of genomic CpG sites, while 5hmC is expressed as relative change to genomic 5hmC status. Solid circles represent the overall genomic CpG 5mC status (Seisenberger et al., 2012; Guo et al., 2017; Iurlaro et al., 2017), with crosses representing the relative genomic CpG 5hmC status (Hackett et al., 2013; Yamaguchi et al., 2013a). Sex symbols indicate the 5mC content of sperm or oocytes. Blue panels represent the general activities of TET and DNMT enzymes at particular stages (Kagiwada et al., 2012; Hackett et al., 2013). Above each colored panel, the coinciding methylation phase is labeled. X-axes labels indicate the developmental stage of the embryo.

Acknowledgments
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