PEG10 (paternally expressed gene 10) is a paternally expressed gene (meaning only the paternal allele is expressed) that has an essential role in the formation of the placenta of the mouse. In the mouse genome, the PEG10 gene is flanked by the SGCE and PPP1R9A genes. To study the origin of PEG10, you examine syntenic regions spanning the SGCE and PPP1R9A loci in the genomes of several vertebrates, and you note that the PEG10 gene is present in the genomes of placental and marsupial mammals but not in the platypus, chicken, or fugu genomes.

The green bars in the figure indicate the exons of each gene. The gray bars represent LINEs and SINEs, and the blue bars represent long terminal repeat (LTR) elements of retrotransposons. Solid black diagonal lines link introns, and dashed black lines connect orthologous exons. Arrowheads indicate direction of transcription.
Using the predicted protein sequence of PEG10, you perform a tblastn search for homologous genes and find that the most similar sequences are in a class of retrotransposons (the sushi-ichi retrotransposons). Propose an evolutionary scenario for the origin of the PEG10 gene, and relate its origin to its biological function. <>