Genome-invading retroviruses are a nasty surprise: the case of the koalas

Koalas at the Tiergarten Schönbrunn in Vienna, Austria
Koalas at the Tiergarten Schönbrunn in Vienna, Austria; Photo: Barbara Feldmann

Retroviruses are able to change the genetic code of germ lines in Koalas. In Koalas the process by which a host species adapts to a retrovirus invading its germ line may be long and drawn out, and an endogenising retrovirus may have a negative impact on the host species that lasts for centuries, at least in some cases. These findings have been just published online in Molecular Biology and Evolution by an international team of scientists from.

“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.




Caption Photo:

Koalas at the Tiergarten Schönbrunn in Vienna, Austria; Photo: Barbara Feldmann


Contact information:

Leibniz Institute for Zoo and Wildlife Research (IZW)

in the Forschungsverbund Berlin e.V.

Alfred-Kowalke-Str. 17

10315 Berlin


Prof. Alex Greenwood, +49 30 5168 255,


Steven Seet, +49 30 5168 108,



Background information:

The Leibniz Institute for Zoo and Wildlife Research (IZW) investigates the vitality and adaptability of wildlife populations in mammalian and avian species of outstanding ecological interest that face anthropogenic challenges. It studies the adaptive value of traits in the life cycle of wildlife, wildlife diseases and clarifies the biological basis and development of methods for the protection of threatened species. Such knowledge is a precondition for a scientifically based approach to conservation and for the development of concepts for the ecologically sustainable use of natural resources.


The Leibniz Association is made up of 86 independent research and scientific institutes, as well as two associated members. Their public and research functions are of national importance and comprise a major component of Germany’s publicly-funded research potential. Leibniz Institutes maintain more than 2,300 contracted cooperations with international partners in academia and industry, and some 2,200 foreign scientists contribute to Leibniz Institutes’ output on a temporary basis each year. Formal cooperative partnerships have been or are currently being developed with scientific institutions in France, Japan, Korea, Canada, Poland, Taiwan, and India. Third-party funds of about € 330 million per year indicate high competitiveness and excellence. Leibniz Institutes currently coordinate 75 projects funded by the European Union. They were also awarded grants by the European Union (with a value of € 42 million) and the German Research Foundation (DFG, € 55 million) in 2010, while € 51 million are a result of cooperations with industry partners. Leibniz Institutes contribute to clusters of excellence in fields such as mathematics, optical technologies, materials research, bio-medical research, environmental research, bio- and nanotechnology, as well as biodiversity, economic policy, and educational research. Altogether, ca. 17,200 people are employed at Leibniz Institutes, among them 8,200 researchers, including 3,300 junior scientists.