Genes important for early development display promoter hypomethylation in zebrafish sperm

Abstract

Fertilization involves the joining of sperm and egg to form a pluripotent zygote. One key question for development is whether the promoters of genes important for pluripotency, or for guiding early development (transcription and signaling factors), bear chromatin marks or composition that facilitate their expression at the onset of zygotic transcription. As mature sperm are reported to be transcriptionally inactive and have much higher levels of DNA methylation than somatic cells (or eggs), it has been assumed that the sperm genome is essentially inert, is largely reprogrammed by the egg chromatin, and makes only a few unique contributions to development: the sex chromosome, paternal imprints and certain RNA molecules. To test the status of sperm chromatin, we performed genome‐wide DNA methylation profiling on zebrafish sperm and adult tissues. Surprisingly, although zebrafish sperm is globally hypermethylated, the promoters of many genes encoding important developmental transcription factors and signaling proteins are distinctly hypomethylated. This raises the interesting possibility that certain genes are poised in sperm by hypomethylation to assist in their expression in the developing embryo. Our current efforts to understand how DNA methylation levels are correlated with, and related to, histone modifications will also be presented.