在過去的1000年間，很多逆轉(zhuǎn)錄酶病毒（像HIV和Rous肉瘤病毒那樣的含有單鏈RNA和一個(gè)用來制造DNA的逆轉(zhuǎn)錄酶的病毒）已經(jīng)成為了哺乳動(dòng)物基因組的構(gòu)成部分,。大多數(shù)已經(jīng)因?yàn)橥蛔兒蛣h除而失去活性,，但有些，像考拉逆轉(zhuǎn)錄酶病毒（KoRV）,，相對完好,，被認(rèn)為是較晚進(jìn)入基因組的。對野生考拉流行的KoRV病毒所做的一項(xiàng)調(diào)查證實(shí),，它具有一種插入的,、而且仍可傳播的內(nèi)生逆轉(zhuǎn)錄酶病毒的特點(diǎn)。令人吃驚的是,，它在某些個(gè)體中完全沒有,，而在澳大利亞南部沿海附近孤立的Kangeroo島上的所有種群中都沒有。KoRV因此似乎正處在從外生向內(nèi)生的過渡當(dāng)中,。這為研究明顯的演化事件提供了一個(gè)模型,。但也有重要的保護(hù)問題，因?yàn)镵oRV已知能在這一獨(dú)特而脆弱的物種中引發(fā)腫瘤,。
A koala retrovirus is integrated into the koala genome and its genetic material is passed from parent to offspring (Image: Reuters/David Gray)
A cancer-causing virus is gradually invading the genome of Australia's koalas, researchers say.
Rachael Tarlinton, a PhD student from the University of Queensland, and colleagues made the unexpected discovery while studying koala retrovirus (KoRV), which causes leukaemia and immune deficiencies.
Until now, scientists had thought KoRV was an endogenous virus, a virus that has become integrated into its host's genome and passed from parent to child like normal genes.
But when Tarlinton and her colleagues examined the virus in koalas throughout Australia they found that some populations were infected with the virus and others were free of it.
Their findings are published today in the journal Nature.
The researchers also found that KoRV is highly active and variable between individual koalas, suggesting it is in transition between infectious and endogenous forms.
"That was the real surprise," says co-author Associate Professor Paul Young. "That just totally changes what we think about endogenous retroviruses."
Endogenous viruses are found throughout the animal kingdom, including in humans, but have normally been integrated into their host genomes for thousands of years.
Island life
When the researchers studied koalas on Kangaroo Island, off the south coast of Australia, they found that the marsupials were completely free of the virus.
The population on that island was established around 1900 when koalas on the mainland were being killed by hunting.
Together with other evolutionary evidence, this suggests that KoRV probably began invading the koala genome between 100 and 200 years ago, Young says.
Early studies, he says, suggest the virus might have come from Asian rodents.
"We've had incursions from Asian rodents into northern Australia repeatedly over thousands of years," he says. "We think that's probably the route."
Causing cancer
Because the virus has only recently begun integrating into the koala genome, it still often causes cancers, the researchers note.
"The koala is having to live with the high levels of cancers ... and there's not a lot we can do about that in the wild," Young says.
For the scientists, the new findings offer a rare glimpse into what happens when an animal is faced with a viral challenge like this.
"Coming to grips with how the koala handles this initial viral onslaught may give us insights into the dynamic events that occurred millions of years ago when retroviruses first invaded the human genome," Young says.
