From Discover magazine blog
Original article:
Magiorkinis, Gifford, Katzourakis, de Ranter & Belshaw. 2012. Env-less endogenous retroviruses are genomic superspreaders. PNAS http://dx.doi.org/10.1073/pnas.1200913109
... By looking at IAPs across 18 species, Magiorkinis found that those that lost their env were also far more abundant. This loss doesnt just reflect the normal genetic wear-and-tear that happens with time. In many viruses, env broke down even though other viral genes were intact. And this process only goes in one direction. ERVs often lose env and spread like wildlife, but those without the gene never regain it.
This trend isnt restricted to IAPs. Magiorkinis searched for other ERVs among the genomes of 38 mammals, including mice, bats, elephants, and more. Of the many families he found, almost all the most abundant ones had lost or degraded their copies of env. The viruses had repeatedly invaded genomes, lost the ability to spread across them, and become many times better at spreading within them.
You can think of the rise of ERVs as an epidemic thats confined within a single genome, and plays out over millions of years. This isnt just a cute metaphor. Magiorkinis found that the biggest 20 per cent of the ERV families accounted for 80 per cent of the total sequences. This distribution is remarkably similar to the 20/80 rule in epidemics, where the most infectious 20 per cent of infected individuals account for 80 per cent of transmissions.
Why should the loss of a single gene make so much difference? Its possible that env might harm the health of the viruss host, either by allowing the virus to infect more cells, or by directly weakening the immune system. By losing the gene, the virus might get a longer-lived carrier, and more chances to be passed on to future generations.
Alternatively, spreading within a genome might just be a more efficient way for the virus to copy itself. It doesnt need adaptations that allow it to survive outside of cells, or avoid defences that target its coat proteins. By hanging up its coat, it might get an easier lifestyle.
Whatever the reason, its clear that this shift is a successful strategy. ERVs have been found in every mammal genome, and they have been our passengers for millions of years. By losing the ability to infect, they could truly go viral.
Original article:
Magiorkinis, Gifford, Katzourakis, de Ranter & Belshaw. 2012. Env-less endogenous retroviruses are genomic superspreaders. PNAS http://dx.doi.org/10.1073/pnas.1200913109
We thus borrow simple epidemiological and ecological models and show that retrotransposition and loss of env is the trait that leads endogenous retroviruses to becoming genomic superspreaders that take over a significant proportion of their host's genome.