In mammals, although size differences exist, most of organs consists of the same cells and exhibits the same structures. However, placentas are quite diverse in cell components, structures and the association between fetal membranes and maternal uteri. These differences have not been well characterized. Recently, endogenous retroviruses (ERVs) have been thought to have caused such diversity, which require both PEG type genes and syncytins.
It has long been thought that viruses/transposons components exist in organism’ genomes. In 2000, Mi et al., found that endogenous retrovirus (ERV, Syncytin-1) exists in the human placenta. Since then, syncytin-like structure and their functions have been reported in many animal species, but none of them contain the same nucleotide structures, strongly suggesting that these ERVs are independently captured and integrated into mammalian genomes. These observations revealed that structural diversity could be explained by these syncytin-like ERVs.
In this review, we presented sufficient evidence that the evolution of placenta or placental structures requires PEG10 and PRG11/RTL1, paternally expressed imprinted genes, and structural diversity comes from the syncytin-like ERVs. We also presented recent observations on ERVs and how these ERVs control gene expression of both functional genes as well as ERV themselves. Based on the recent information, we presented the baton-pass hypothesis, successive integration of ERVs and new models explaining placental diversity.
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