Keep your friends close; keep your enemies’ DNA

By Anna Tiley

This July I spent a day volunteering at Soapbox Science in London, an exciting event aimed at promoting women in Science. I was paired with Dr. Ravinder Kanda who gave a fascinating talk entitled “Genome invaders: friend or foe?” Dr Kanda works in an area known as Paleovirology (the study of ancient viruses and their evolution) and her talk was the inspiration for this article.

Viruses are tiny infectious particles which, despite their small size, can cause a lot of havoc. Not only are they the causal agents of major human diseases such as flu, herpes and AIDS but plant viruses are also responsible for millions of pounds worth of crop loss each year.

It may therefore come as a surprise to hear that these troublesome particles are also responsible for the origin of many beneficial adaptations in humans. In fact it has been estimated that at least 8% of our genome has viral ancestry.

So how does this happen?

In order to survive, viruses must infect and reproduce inside a host. Occasionally during this process bits of the viral DNA are integrated into the host’s genome by accident. If this occurs in germline cells, the cells that make a new organism, then the viral DNA gets passed down from parent to offspring.

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Some types of viral DNA provide the host with an evolutionary advantage and are “captured” to carry out a function different to what they had been adapted for in the virus. This mechanism by which a gene is used for something different to what it was evolved for is known as co-option.

One example of co-option of viral genes occurred during the evolution of mammals. The mammalian gene important for the formation of the placenta, called syncitin has been shown to have viral origin. Research shows that it came from an internalised retroviral gene responsible for the attachment to and subsequent invasion of cells. This gene was then integrated and co-opted into the mammalian ancestor’s genome.

Further examples include viral genes which have become part of the immune system in mice and ironically help to defend from viral attack. These genes make non-reactive proteins similar to active viral proteins. The non-lethal proteins block receptors on the cell surface which are normally used by viral pathogens to gain entry into the cell. As a result, the invading viruses cannot infect the cell.

It may be odd to think that some of our genome is composed of regions originating from organisms that usually do us harm. However, these viral genes were integrated a long time ago so are no longer a threat.

The resulting co-opted genes are therefore fascinating examples of how the process of evolution can result in the formation of novel adaptations from pre-existing characteristics.

General review for further reading:

Aswad A, Katzourakis A. (2012) Paleovirology and virally derived immunity

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