Post-fertilization transcription initiation in an ancestral LTR retrotransposon drives lineage-specific genomic imprinting of .

The imprinted gene is regulated through a unique mechanism involving a transient paternal transcript in early embryos, rather than persistent gametic DNA methylation. In humans and mice, this transcript- (also known as ) or the long isoform of ()-arises from the unmethylated paternal allele and initiates secondary epigenetic marks that maintain expression. Here, we investigate the evolutionary origin of this mechanism, and show that the first exon of human overlaps with a MER21C long terminal repeat (LTR), a retrotransposon subfamily specific to Boreoeutherian mammals. Comparative analyses revealed that this MER21C insertion occurred in the common ancestor of Euarchontoglires, including primates, rodents, and rabbits. Although not annotated, the first exon of mouse displays conserved features with the MER21C-overlapping exon in humans. In rabbit and nonhuman primate placentas, orthologs with LTR-embedded first exons were also identified. In contrast, in non-Euarchontoglire mammals such as cow and tammar wallaby, is biallelically expressed, suggesting absence of imprinting. These findings suggest that imprinting emerged in Euarchontoglires via MER21C insertion. Together with our prior work on LTR-driven imprinting in oocytes, our findings demonstrate that post-fertilization activation of retrotransposons can also drive lineage-specific acquisition of imprinting.