By integrating themselves into the germ line of their host, retroviruses change the genetic code of their host. The only known case where this process can be currently observed is in Koalas. As an international team of scientists from Australia, Europe and North America just found out, this process may take longer than expected, with the virus continuing to have a serious pathological impact on the host which may go on for centuries.
These findings have been just published online in Molecular Biology and Evolution.
"The process by which a retrovirus invades the host germ line appears to be quite drawn out in this case, so that the koala population has suffered the strongly pathological effects of the virus for many generations during the process of retroviral endogenisation," said Alfred Roca of the University of Illinois at Urbana-Champaign, one of the authors of the study. Koala retrovirus is associated with Chlamydia infection and leukemia in modern koala populations. Given the similarity between ancient and modern koala retrovirus (KoRV) sequences, ancient KoRV would have caused similar pathologies.
When the scientists examined KoRV in koala museum skins from the late 19th and 20th centuries using ancient DNA analysing techniques, they got a number of surprises. Contrary to their expectations, the oldest sample showed that KoRV was already widespread 120 years ago in koalas in northern Australia, and the virus sequence has changed little for more than a century. This suggested that KoRV initially infected koalas much earlier than previously thought but had spread very slowly across koala populations. Alex Greenwood of the Leibniz Institute for Zoo and Wildlife Research (IZW) in Berlin, leader of the research consortium, notes that "this actually makes sense as koalas are quite sedentary so it is hard to imagine how KoRV would have spread through the entire population in the short time of 200 years. The koala retrovirus might be more than a thousand or even ten thousand years old, unfortunately we have no older specimens."
In order to shed light on how retroviruses invade the germ lines of their hosts, the scientists examined a seemingly unlikely model species, the Australian koala. This is because the koala retrovirus is unique as the only known retrovirus in animals currently in the midst of invading its host germ line. Whereas almost all koalas in northern Australia already possess endogenous KoRV, the percentage of koalas infected drops drastically when going south, suggesting that KoRV may be quite a young virus. Greenwood noticed that "given a potential 200 year time frame from first infection to the current distribution of KoRV, we thought that using ancient DNA methods on museum skins would reveal the changes that occurred in the virus over time. Going into the study, we expected that KoRV would be less widespread the farther back in time we went and that we would detect many changes in KoRV as it adapted from being an infectious virus to being part of the koala germ line. But the results of the study showed that the KoRV must be ancient and the sequences of KoRV in old and new samples were nearly the same."
Retroviruses comprise a large and complex group of viruses that includes the human immunodeficiency viruses (HIV) which causes AIDS. Unlike other viruses, retroviruses must actually copy their genetic material into the host genome as part of their life cycle. On occasion, a retrovirus may integrate into the reproductive cells of the host that give rise to future generations, thus becoming a permanent part of the host genome. In humans, about 8% of the entire genome derives from these endogenous retrovirus (ERV) copies, which now are passed on from parents to offspring like any other gene. In some cases, ERVs have been associated with human diseases. Yet it has been difficult to study the progression of events by which retroviruses invade a host germ line, because all human ERVs resulted from retroviral invasions that occurred millions of years ago in the distant past.
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