Imprinting of mammalian male gametes is gene specific and does not occur at a single stage of differentiation.

Epigenetic modifications such as DNA methylation and alterations to chromatin structure have been proposed as hallmarks of imprinting in somatic cells after fertilization. In the germ cell line, gene imprinting needs to be reset in order to transmit the correct sex-specific imprinting pattern to the next generation. The precise timing of imprint erasure and re-establishment for many genes remains to be determined and precise molecular mechanisms of genomic imprinting have not yet been fully characterized. Here, we have analysed the methylation state and DNase-I sensitivity of two genes with reciprocal genomic imprinting (U2af1-rs1 and H19 genes) in a male mouse primordial germ cell (PGC) derived cell line (EG-1), isolated post-natal spermatogonia and mature sperm cells. Our results show that establishment of imprinting of the U2af1-rs1 and H19 genes during male germ cell differentiation occurs at different stages of differentiation. Furthermore, the presence of DNase-I hypersensitive sites may constitute a molecular marker to identify alleles and subsequently acquire the appropriate methylation imprint. We propose that this molecular identifier may be present or absent for a specific gene according to the sex of the gamete.