During mammalian fertilization, two sets of genomes from male and female gametes join together and then undergo large-scale reprogramming to restore the totipotency. However, although reside in the same zygotic cytoplasm, the paternal and maternal genomes are reprogrammed in different ways. It is well known that several epigenetic modifications are involved in the reprogramming events [1
The first described epigenetic modification is DNA methylation. DNA methylation at CpG dinucleotides is associated with the repression of gene transcription and is essential for mammalian development [3
]. In mouse zygotes, the paternal genome undergoes active DNA demethylation shortly after fertilization, while the maternal genomic DNA remains methylated throughout the first mitosis [4
]. Although the active demethylation of paternal genome has been observed in several mammalian species [6
], with the same immunostaining approach no paternal DNA demethylation can be detected in sheep, rabbit and goat zygotes [7
]. Therefore, the paternal demethylation event appears to be variable among species, but the mechanism underlying it is still unclear.
In addition to DNA methylation, covalent modifications of nucleosomal histone, including acetylation, methylation, phosphorylation and ubiquitination, also play critical roles in regulation of gene expression and are involved in the processes of epigenetic reprogramming [1
]. Modifications can occur at several amino acid residuals, of which the lysine residue 9 of histone H3 (H3K9) can be either acetylated or methylated. In general, acetylated H3K9 represents gene transcription permissive status while methylated H3K9 mediates gene silencing [10
]. In particular, H3K9 methylation has been suggested to be mechanistically linked to DNA methylation [11
]. In mouse zygotes, methylated H3K9 is distributed asymmetrically between the maternal and paternal pronucleus [14
], which is coincident with the distribution pattern of DNA methylation [4
]. The absence of methylated H3K9 from the paternal pronucleus has been thought to attribute to the paternal DNA demethylation [17
As sheep zygotes differ from mouse zygotes in the aspect of DNA methylation [7
], it would be interesting to ask the question of whether the epigenetic differences are also reflected in histone modifications. To address this issue, this study detected the methylation and acetylation patterns of H3K9 in sheep zygotes and compared with that of the mouse zygotes. Furthermore, the possible relationship between H3K9 modification and DNA methylation was examined in sheep zygotes. Our results indicate that sheep zygotes display similar H3K9 modification patterns to the mouse and DNA methylation is not closely correlated with H3K9 modification.