Casting histone variants during mammalian reproduction

Abstract

During mammalian reproduction, germ cell chromatin packaging is key to prepare parental genomes for fertilization and to initiate embryonic development. While chromatin modifications such as DNA methylation and histone post-translational modifications are well known to carry regulatory information, histone variants have received less attention in this context. Histone variants alter the stability, structure and function of nucleosomes and, as such, contribute to chromatin organization in germ cells. Here, we review histone variants expression dynamics during the production of male and female germ cells, and what is currently known about their parent-of-origin effects during reproduction. Finally, we discuss the apparent conundrum behind these important functions and their recent evolutionary diversification.

Keywords: Evolution; Histone; Mammals; Parental-effect; Reproduction; Variants.

Fig. 1

Expression timing of mouse histone variants during gametogenesis. The window of expression (RNA) or chromatin association (protein) is shown for histone variants discussed in this review. Stages of male (blue) or female (pink) germ cell differentiation are indicated above. H3mms and H2B.W expression has not been staged during gametogenesis. H3.3 and cenH3 are also included although their expression is not restricted to germ cells.

Fig. 2

Germline histone variants orthologs identified in mammals. Histone variants with germline specific functions or expression are classified by type (H3, H2A, H2B, H4 and H1). Filled boxes indicate identified orthologous genes in the mouse, human, and cow genomes, with chicken used an outgroup. Empty boxes indicate pseudogenization or other secondary loss event. “?” denotes unresolved orthology. In the following paragraphs, we briefly discuss these evolutionary scenarios for germline histone variants.