Announcements of the latest animal to have its genome sequenced are coming so thick and fast non-biologists may wonder at the point. However, the recent sequencing of the koala genome is demonstrating how much other species' genomes can teach us. In fact, koala genetics could be key to a large portion of the human genome, and explain retroviruses in distant species.
"Retroviruses insert their genome into their host's chromosome, from where they make more copies of themselves," explained Professor Paul Young of the University of Queensland, co-author of a new study published in Proceedings of the National Academy of Sciences. "Some can also infect what are known as germline cells, which alters the host genetic code and that of all their descendants."
Today humanity is plagued by retroviruses in the form of HIV and the human T-cell lymphotropic virus. On the other hand, some 5 million years ago a retrovirus found its way into our genetic inheritance. The virus may have had initially devastating health effects, but eventually it and humanity formed a bargain – it stopped harming its hosts, and they passed its genetic sequence on in the form of “junk DNA”, the part of our genome that does not code for proteins. This arrangement has stuck through the evolution of multiple species to survive as approximately 8 percent of Homo sapiens' genome.
Young noted that, while we can see the legacy of this retrovirus taming in our genetic code, we don't know how it happened. However, retrovirus infection of koalas is happening right now, suppressing their immune system and making them vulnerable to certain cancers, but providing a live demonstration of retrovirus incorporation.
Young says, at this point, the retrovirus is not fixed in its position in the koala genome, allowing us to watch the relationship evolve.
At the same university, Dr Gregory Simmons is exploring the koala retrovirus' background, noting its similarity to one infecting the native Australian grassland mosaic-tailed rat (Melomys burtoni).
“The melomys virus is closely related to koala retrovirus, but it is also closely related to the gibbon ape leukemia retrovirus and may be another strain of this virus,” Simmons said. “It was then apparent that the two viruses almost certainly share a common ancestor and are the result of cross-species transmission in the past.”
In Virus Genes, Simmons previously proposed the virus reached gibbons from melomys relatives, via humans.
Koalas and gibbons are too geographically distant to have infected each other. Instead, Simmons proposes the virus somehow transferred from melomys relatives in Papua New Guinea to humans, making the jump to gibbons from biological waste samples. If true, the theory emphasizes the danger of zoonotic diseases – transmitted from animal to human – goes both ways.