Evolution of the sperm methylome of primates is associated with retrotransposon insertions and genome instability

Changes in gene expression resulting from epigenetic and/or genetic changes play an important role in the evolutionary divergence of phenotypes. To explore how epigenetic and genetic changes are linked during primate evolution, we have compared the genome-wide DNA methylation profiles (methylomes) of humans and chimpanzees, which have a 1.2% DNA sequence divergence, of sperm, the frontal cortices, B cells, and neutrophils. We revealed that species-specific differentially methylated regions (S-DMRs), ranging from several hundred base pairs (bp) to several kilo base pairs (kb), were frequently associated with sequence changes in transcription factor-binding sites and insertions of Alu and SVA retrotransposons. We then generated a reference macaque sperm methylome map and revealed, in sperm, that both human and chimpanzee S-DMRs arose more frequently owing to methylation loss rather than gain. Moreover, we observed that the sperm methylomes contained many more hypomethylated domains (HMDs), ranging from 20 to 500 kb, than did the somatic methylomes. Interestingly, the sperm HMDs changed rapidly during primate evolution; hundreds of sperm HMDs were specific to humans, whereas most somatic HMDs were highly conserved between humans and chimpanzees. Notably, these human-specific sperm HMDs frequently occurred in regions exhibiting copy number variations. Our findings indicate that primate evolution, particularly in the germline, is significantly impacted by reciprocal changes in the genome and epigenome.