Evolution of brain functions in mammals and LTR retrotransposon-derived genes

In the human genome, there are approximately 30 LTR retrotransposon-derived genes, such as the sushi-ichi retrotransposon homologues (SIRH) and the paraneoplastic Ma antigen (PNMA) family genes. They are derivatives from the original LTR retrotransposons and each gene seems to have its own unique function. PEG10/SIRH1 as well as PEG11/RTL1/SIRH2 and SIRH7/LDOC1 play essential roles in placenta formation, maintenance of fetal capillaries and the differentiation/maturation of a variety of placental cells, respectively. All of this evidence provides strong support for their contribution to the evolution of viviparity in mammals via their eutherian-specific functions. SIRH11/ZCCHC16 is an X-linked gene that encodes a CCHC type of zinc-finger protein that exhibits high sequence identity to the LTR retrotransposon Gag protein and its deletion causes abnormal behavior related to cognition, including attention, impulsivity and working memory, possibly via the locus coeruleus noradrenaergic system in mice. Therefore, we have suggested that the acquisition of SIRH11/ZCCHC16 was involved in eutherian brain evolution. Interestingly, SIRH11/ZCCHC16 displays lineage-specific structural and putative species-specific functional variations in eutherians, suggesting that it contributed to the diversification of eutherians via increasing evolutionary fitness by these changes.