Abstract
The DNA methylome of spermatozoa results from a unique epigenetic reprogramming crucial for chromatin compaction and the protection of the paternal genetic heritage. Although bull semen is widely used for artificial insemination (AI), little is known about the sperm epigenome in cattle. The purpose of this review is to synthetize recent work on the bull sperm methylome in light of the knowledge accumulated in humans and model species. We will address sperm-specific DNA methylation features and their potential evolutionary impacts, with particular emphasis on hypomethylated regions and repetitive elements. We will review recent examples of interindividual variability and intra-individual plasticity of the bull sperm methylome as related to fertility and age, respectively. Finally, we will address paternal methylome reprogramming after fertilization, as well as the mechanisms potentially involved in epigenetic inheritance, and provide some examples of disturbances that alter the dynamics of reprogramming in cattle. Because the selection of AI bulls is closely based on their genotypes, we will also discuss the complex interplay between sequence polymorphism and DNA methylation, which represents both a difficulty in addressing the role of DNA methylation in shaping phenotypes and an opportunity to better understand genome plasticity.
Highlights
Epigenetics refers to the molecular mechanisms which alter gene regulation in a DNA sequence-independent fashion and are transmitted to the daughter cells through cell divisions
Even though transcription is barely detectable in mature sperm cells, the male germline differentiation programme is orchestrated by a dynamic sequence of transcriptional regulations that are directly reliant on epigenetic reprogramming (Seisenberger et al 2012; Hammoud et al 2015; Stewart et al 2016; Hill et al 2018)
We turn to the reprogramming dynamics of the repetitive elements, which we found to be hypomethylated in bull sperm
Summary
Epigenetics refers to the molecular mechanisms which alter gene regulation in a DNA sequence-independent fashion and are transmitted to the daughter cells through cell divisions. This issue has not been addressed in the male offspring, epigenetic changes in spermatozoa in response to maternal metabolism have been reported in mice and humans; suggesting that a similar phenomenon may occur in the gametes of bulls conceived from lactating elite cows (Chavatte-Palmer et al 2016). Retrotransposons display variable methylation attributes in sperm according to the subfamily to which they belong and the density of CpGs and the species, and hypomethylated promoters are found in larger numbers in sperm than in ES cells (Molaro et al 2011) This is consistent with the findings of another study which reports a lower methylation percentage among promoters in sperm compared with ES cells (Popp et al 2010).
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