Koala Retroviruses Reveal New Clues About the Origins of the Human Genome

Koalas may be revealing new clues about the origins of the human genome. Scientists have recently discovered that 39 different koala retroviruses in a koala's genome were all endogenous, which means they've been passed down to the koala from one parent or the other.

Eight percent of the human genome derives from retroviruses that inserted themselves into human sex cells millions of years ago. Right now, the koala retrovirus (KoRV) is invading koala genomes, which is a process that can help us understand our own viral lineage.

Koalas are actually the only known organisms where a retrovirus is transitioning from exogenous to endogenous. An exogenous retrovirus infects a host, inserts its genetic information into the cell's DNA, and then uses the host cell's machinery to manufacture more viruses. But when an exogenous retrovirus infects an egg or sperm cell and the viral genetic information is then passed down to the host's offspring, the virus becomes an endogenous retrovirus (ERV).

"During the early stages of endogenization, there are huge numbers of retroviruses," said Alfred Roca, one of the researchers, in a news release. "KoRVs are present all across koalas' genomes, with many thousands or tens of thousands of KoRVs in the population. Over time most of them will disappear because these copies of the virus may be present in as few as one individual chromosome. If that one individual happens to not reproduce, or if it reproduces and the other chromosome is passed down, then the ERV will disappear."

It actually takes retroviruses, like KoRV, many thousands of years to become a fixed part of a species' genome, like the eight percent of retroviral DNA that all humans share.

In this case, the researchers found that the KoRVs integrated into the host germ line in koalas less than 50,000 years ago. This is surprisingly recent compared with other ERVs.

"It seems likely that for thousands of years since this virus integrated, the koala host has suffered fitness effects," said Roca. "But once retroviruses become part of the host, they begin to help the host because that is how they survive. They will be better off if they evolve to protect the host. Over time, the detrimental effects go down and the beneficial effects go up."

The findings reveal a bit more about retroviruses which, in turn, can tell us a bit about the evolution not only of the koala genome, but of the human genome as well.

The findings are published in the journal Molecular Biology and Evolution.