Figure 7. Chromatin remodeling events in spermiogenesis affect sperm histone-dependent regulation of gene expression during embryonic genome activation (working model).

Chromatin remodeling during spermiogenesis (left panel) leads to the exchange of nucleosomes (blue) with their specific paternal tail modifications by protamines that normally leads to regulated retention of histones in certain domains of a given gene locus (A). Modulation of this remodeling process, for instance by altering PAR metabolism, results in either insufficient exchange of histones (B) or excessive remodeling causing more intense depletion of histones from that locus (C). As a result, histone association of this locus can be variable in sperm (middle panel). After fertilization, the paternal chromatin becomes rapidly remodeled, again with the exchange of protamines, but presumably not paternal histones, for maternally provided histones (pink) with maternal tail modifications that are mostly activating or nondescript in nature (right hand panel). As a result, the ratio of maternal and paternal histones can vary at the time point of genome activation, leading to continued differential epigenetic remodeling of the locus and ensuing differential expression (DE) in the early embryo.