在过去的1000年间，很多逆转录酶病毒（像HIV和Rous肉瘤病毒那样的含有单链RNA和一个用来制造DNA的逆转录酶的病毒）已经成为了哺乳动物基因组的构成部分。大多数已经因为突变和删除而失去活性，但有些，像考拉逆转录酶病毒（KoRV），相对完好，被认为是较晚进入基因组的。对野生考拉流行的KoRV病毒所做的一项调查证实，它具有一种插入的、而且仍可传播的内生逆转录酶病毒的特点。令人吃惊的是，它在某些个体中完全没有，而在澳大利亚南部沿海附近孤立的Kangeroo岛上的所有种群中都没有。KoRV因此似乎正处在从外生向内生的过渡当中。这为研究明显的演化事件提供了一个模型。但也有重要的保护问题，因为KoRV已知能在这一独特而脆弱的物种中引发肿瘤。
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.
