A little while back (late November) I started a series of posts discussing some of my favourite facts, or tidbits, that I’ve learned about evolutionary biology, with my first post written about the reproductive cycle of the liver lancet fluke. This post is thus my second, of a probable total of four, and is about endogenous retroviruses (how fucking cool it is that they exist; how their discovery has led to better dating and understanding of evolutionary origins or similarities/differences between species; how they may act as a force for evolutionary change; and how understanding them could help cure diseases like HIV/AIDS.)
Most of what I know about endogenous retroviruses comes from a New Yorker article I read a couple years ago. The article, titled ‘Darwin’s Surprise’, can be found here. ***
Viruses, broadly defined, are vectors that carry genetic information (DNA or RNA) and replicate by placing this information into another host. The host then reads this information and creates more of the viruses. A retrovirus is a special bread of virus that uses its RNA to convince the host to duplicate the RNA into its own DNA (using the enzyme reverse transcriptase). The DNA is then incorporated into the host’s genome by an integrase enzyme.
All of this is neat and dandy, but what’s really interesting are endogenous retroviruses. Most viruses attack cells in the somatic line: everything except the germline. Germline cells are the ones that contain genetic material that will be passed to an offspring (sperm or egg). Endogenous retroviruses are viruses that manage to place their DNA into the cell line of the offspring: thus securing its place in all the cells of that individual (provided the embryo survives). When this individual then has offspring, the virus may continue to be passed down throughout the generations, and may secure its place in the blueprint of the species.
Well, it turns out that just such a thing has happened many times in the past. Enough times in fact that eight percent of our DNA is composed of the remnants of broken and disabled retroviruses, dating back millions of years. (!!!)
The article goes on to discuss how the researcher Thierry Heidman managed to bring one of the retroviruses ‘back to life’. That is, managed to reconstruct the genetic material of the virus and then injected it into a host cell, where it promptly inserted itself into the host’s DNA. (Cool, right?! Right!?)
Anyways, the article further talks about how the entire evolution of placental animals (read: mammals) may have occurred due to a mammalian ancestor (common to the spiny anteater and the duck-billed platypus) becoming infected with such an endogenous retrovirus:
The placenta is essentially a modified egg. In the early nineteen-seventies, biologists who were scanning baboon placentas with an electron microscope were surprised to see retroviruses on a layer of tissue known as the syncytium, which forms the principal barrier between mother and fetus. They were even more surprised to see that all the animals were healthy. The same phenomenon was soon observed in mice, cats, guinea pigs, and humans. For many years, however, embryologists were not quite sure what to make of these placental discoveries. Most remained focussed on the potential harm a retrovirus could cause, rather than on any possible benefit. Cell fusion is a fundamental characteristic of the mammalian placenta but also, it turns out, of endogenous retroviruses. In fact, the protein syncytin, which causes placental cells to fuse together, employs the exact mechanism that enables retroviruses to latch on to the cells they infect.
Finally, the rest of the article also talks about how that eight percent of virus DNA in our DNA is among the really strong evidence in support of evolution (because we share a large part of it with our closest ancestors, the great apes, and less so with the ancestors further away), and how these viruses are helping researchers study H.I.V.
***(aside: I thought I’d just quickly mention a fact that I came across that reminded me of something else: 50 million people died from the Spanish flu. This, 1) is astonishing, and 2) immediately made me think of Jonathan Safran Foer’s book Eating Animals. In the book, he mentions the Spanish flu, and discusses how living in close quarters with animals, especially animals who are kept in less than optimal conditions for their health, increases our risks of catching diseases from them (Mad Cow, H1N1, SARS, etc. etc.). That then reminded me of Jared Diamond’s Guns, Germs and Steel, for the idea that what aided European settlers in decimating the native populations of America was the fact that they carried diseases their immune systems had become accustomed to which they caught from living with domesticated animals. Besides this being interesting to me as someone who thinks that, at the very least, our consumption of animals should be reduced, it also hits home because, as anyone who knows me or has read enough of these blog posts knows: my parents don’t believe that vaccines do anything. Now, although the spanish flu was not combatted with vaccines, smallpox was, and smallpox killed about 10 times as many people (half a billion) in the twentieth century. That disease was eradicated in 1979 according to the WHO (thanks to vaccines). Next on their list is Polio.)