Koalas aren't exactly closely related to humans, but these furry animals are helping to shed light on the human genome, according to a new study.
Eight percent of our genome derives from retroviruses, or RNA viruses, that inserted themselves into human sex cells millions of years ago. And right now the koala retrovirus (KoRV) is invading koala genomes, a process that can help us understand our own viral lineage.
There are two types of retroviruses: exogenous, in which the virus infects a host and uses its DNA to replicate, and endogenous - meaning the retrovirus infects an egg or sperm and passes its genes down to the host's offspring.
According to the study, 39 different KoRVs in a koala's genome were all endogenous, proving it as the first organism known to transition from exogenous to endogenous retrovirus (ERV). It's possible that thousands or even tens of thousands of KoRVs are in the population, but koalas have evolved ways of combating these viruses, which can lead to several conditions like leukemia, lymphoma and immune suppression.
"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 that ERV will disappear," researcher Alfred Roca explained in a statement.
Humans too have developed evolutionary defenses against endogenization. Learning how Australia's iconic marsupial came to live with retroviruses could help scientists understand the same process occurred in humans.
Home Sweet Home
It takes retroviruses, like KoRV, many thousands of years to become a fixed part of the koala genome, like the eight percent of retroviral DNA that all humans share. This process can often be devastating for whatever species finds itself the victim. However, the silver lining is that over time, as retroviruses get comfortable in the host, "the detrimental effects go down and the beneficial effects go up," Roca said.
ERVs that are successfully passed down are protected by the koala's DNA repair mechanisms, leading to very low mutation rates. And based on the large number of mutations researchers saw in this study, Roca's team estimates that the KoRVs became part of the koala's host genome less than 50,000 years ago.
"This is quite recent compared with other ERVs that are millions of years old and have accumulated mutations," added first author Yasuko Ishida.
The Lesser of Two Evils
While koalas may have learned to live in harmony with KoRVs, they still pose a threat to certain populations, like almost all koalas living in northern Australia.
Koalas are most recognized by their soft, wool-like gray fur. And while this fuzzy coat makes these "bears" - which are actually marsupials - extra cuddly, it also made them targets of extensive hunting. These nocturnal mammals, once numbering in the millions, suffered major declines in population during the 1920s, according to Defenders of Wildlife.
The now vulnerable population estimates at fewer than 100,000 animals.
In addition to hunting, habitat loss, traffic deaths, and attacks by dogs kill an estimated 4,000 of these pouched mammals per year.
So in an effort to preserve koalas, a few were moved to an island off the coast of Australia, researchers say, until years later when they were reintroduced to southern Australia. Their years on the island led to horrible inbreeding, but fortunately, they avoided the retrovirus.
Now scientists are struggling to figure out which is the lesser of two evils - whether they should protect the northern or southern koala population.
"Do you try to conserve genetic diversity, which is present in the northern populations along with the retrovirus," Roca said, "or do you conserve southern populations that don't have the retrovirus but are horribly inbred?"
Today koalas can be found in Queensland, New South Whales, Victoria and South Australia, munching on eucalyptus trees and sleeping for up to 16 hours a day. Australian national laws protect these plump, fuzzy mammals, but it's up to each individual state to actively conserve the species.
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