Mammalian-Specific Traits Generated by LTR Retrotransposon-Derived SIRH Genes

What is the mechanism by which mammalian-specific genes derived from long terminal repeat (LTR) retrotransposons played a role in generating mammalian-specific traits, such as a unique viviparous reproductive system and a highly developed central nervous system? A series of knockout mouse studies has clearly demonstrated that at least some sushi-ichi-related retrotransposon homologues (SIRH genes) play essential roles in placental development and brain function. Some SIRH genes are conserved in all eutherians, whereas other SIRH genes became pseudogenes in a species- or lineage-specific manner, implying that LTR retrotransposons served as a critical driving force in mammalian evolution and diversification by generating mammalian-specific and species- or lineage-specific genes, respectively. Interestingly, most SIRH genes are located on the X chromosome. We discuss whether there is a specific reason for or advantage of having an X-linked chromosomal location, and we also discuss the role of X chromosome inactivation during this process.