Global changes in genomic methylation levels during early development of the zebrafish embryo.

We have examined the methylation status of the zebrafish genome during early embryogenesis and we find evidence that methylation fluxes do occur in that organism. The parental genetic contributions to the zygote are, initially, differently methylated with the genome of the sperm being hypermethylated relative to the genome of the oocyte. Post-fertilization there is an immediate decrease in methylation of the embryonic genome but the methylation begins to increase rapidly and is re-established by the gastrulation stage. These results are consistent with the results of Santos et al. (Dev Biol 241:172-182, 2002), who examined the methylation of early mouse embryos, and this conservation argues that demethylation/re-methylation is an important part of vertebrate development.