How Endogenous Retroviruses Predict Evolution by Common Descent

I highly recommend the Facts of Evolution Series on Youtube. In this latest installment, the author talks about retroviruses. These are viruses that reverse transcribe RNA into DNA (which is the opposite of what  most organisms do). Examples of such retroviruses are the HTLV-1 virus which causes leukemia and the HIV virus which causes AIDS. However, some retroviruses can insert their DNA into host germ cells where they are inherited by descendants of the host. These are called endogenous retroviruses (ERV), and they persist in the genome of the species that becomes infected.

Under the idea of common descent, we can predict the evolutionary relationships of different species by comparing their genomes and identifying ERVs. If the ancestor of humans have acquired ERV1 after the split from other apes, we can expect to find ERV1 in the genome of all humans but not in the genome of any other ape. With the advance of genetics, scientists are comparing ERVs between species and so far everything has added up, thus providing another strong evidence for evolution.

Interestingly, where mammals except monotremes are concerned, ERVs are known for role in allowing pregnancy to occur. During pregnancy, some ERVs are turned on and they cause an immunosuppressive effect to protect the embryo from the maternal immune system. It is thought that at some point a retrovirus infected an early mammalian ancestor, and this enabled the evolution of vivaparity as the fetus became more resistant to the mother’s immune system. They also play a role in the formation of the placental syncytium, via viral fusion proteins. An example of a viral fusion protein in retroviruses we see today is gp41 seen in HIV.

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