FIGURE 2.

Capture of retroviral genes for placental development. (A) Schematic of endogenization and capture of a syncytin gene. A retrovirus infecting a germline cell integrates into the host genome as a provirus with characteristic features, the 5′LTR, gag, pol and env genes, and the 3′LTR. Over time, in a process termed endogenization, most of the provirus acquires deleterious mutations. In this example, the env gene retains an intact open reading frame and is captured by the host to fulfill functions during placental development. (B) Evolution of placental species and capture of ERV-derived env genes. The phylogenetic tree is based on previously published data (Cornelis et al., 2017; Imakawa and Nakagawa, 2017). (C) Structural representation of the retroviral Env protein with the surface (SU), transmembrane (TM), and immunosuppressive domain (ISD) subunits, as well as the fusion peptide. Panel (C) has been modified from Lavialle et al. (2013).