If there weren’t viruses, our cats and dogs would not develop a placenta. A group of researchers from France discovered that in cats and dogs, the gene syncytin, responsible for creating a placenta, is stolen from retroviruses. Syncytin is highly conserved within species in the carnivora. In other words, there is a little DNA variation in the syncytin sequence. By comparing syncytin in species, the gene is stolen from viruses about 70-80 million years ago. Interestingly, syncytin is identified as an envelope gene that belongs to a provirus (provirus means a virus genome integrated into cat’s or dog’s genome). An envelope gene codes for an envelope protein. Go to the figure below: envelope protein is important in binding to cells for the virus to enter the cell. Stealing env gene from viruses is, therefore, an important mechanism for mammals to evolve. Evolution is a slow process. It depends on mutations that give rise to characteristics that may convey fitness to a particular species. By stealing an entire gene from viruses, mammals essentially skip the slow process of accumulating mutations. The researcher wrote,
“Therefore, it seems that, on several occasions in the course of mammalian evolution, env genes from endogenous retroviruses have been co-opted by their host to participate in the formation of the placenta”
The researchers also asked an interesting question after correlating the syncytin gene with the envelope gene of retroviruses. Can viruses use syncytin gene from cats and dogs as their envelope (see figure to see where the envelope protein is) to enter the cell? This group of researchers made virus mutants with cats’ or dogs’ syncytin as their envelope protein (envelope protein is responsible for binding to specific receptor to enter the cells). Then, they measured the number of viruses produced to see if syncytin is functional. In Figure C, you can see that viruses with either cat’s and dog’s syncytin can replicate in cat and dog’s cell. In sum, syncytin gene is still a functional envelope gene for virus replication.
Humans also have syncytin.
What if there are no viruses on Earth? Life would probably be developed with a difference mechanism. Given the slow rate of evolution, forming a placenta is quite unlikely.