Viral Endosymbionts and Endogenous Retrovirus
During pregnancy in viviparous mammals, endogenous retroviruses (ERVs) are activated and produced in high quantities during the implantation of the embryo. On one hand they act as immunodepressors, and protect the embryo from the immune system of the mother and on the other hand viral fusion proteins cause the formation of the placental syncytium in order to limit the exchange of migratory cells between the developing embryo and the body of the mother, where an epithelium won't do because certain blood cells are specialized to be able to insert themselves between adjacent epithelial cells. The ERV is a virus similar to HIV (the virus causing AIDS in humans). The immunodepressive action was the initial normal behavior of the virus, similar to HIV. The fusion proteins was a way to spread the infection to other cells by simply merging them with the infected one (similar to HIV). It is believed that the ancestors of modern vivipary mammals evolved after an accidental infection of an ancestor with this virus, that permitted the fetus to survive the immune system of the mother.
The human genome project found several thousand ERVs, which are organized into 24 families.
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