Friday, March 27, 2015

Major Evolutionary Event: Retrovirus Invading Koala DNA!

Koalas throughout Australia are being infected with a circulating endogenous retrovirus. [Source: Quanta Magazine]

In the 1990s, a large number of koalas in Dreamworld, an Australian theme park, started dying of cancer. When it comes to animals, cancer clusters like these are often caused by retroviruses. These viruses insert their genetic material into the host genome, sometimes leading to oncogenesis. However, as veterinarians tested koalas for a retrovirus, they were shocked to discover that almost every koala was infected with koala retrovirus (KoRV).

KoRV is still spreading throughout Australia. Nearly all of the koalas in Queensland have been infected and there are moderate levels of KoRV between Queensland and the Southern islands, where the virus has yet to be detected. However, it may only be a matter of time.

The pervasiveness of KoRV indicates that it is likely infected the koala germ-line. As an endogenous retrovirus, it would be passed on from parent koalas to their offspring along with the rest of the koalas's genetic information.

All vertebrates have retroviral DNA embedded in their genomes. In fact, about 8% of the human genome is made up of endogenous retrovirus. However, these retroviral jumps into the germ-line genome occurred way back in our evolutionary history (and sometimes even drove evolution  -- we have endogenous retroviruses to thank for our placentas!)

Regular vs. Endogenous retroviruses, an infographic. [Source: Quanta Magazine]

This discovery is so exciting not only because endogenous retroviruses are awesome (certainly reason enough), but also because this is the first time scientists can observe a recent retroviral jump and its affects on a species.

Currently, KoRV is found circulating in the blood of koalas as well as in their sperm and eggs. It is also quite deadly. Usually endogenous retroviruses have been silenced in the host genome and do not cause disease. The infection was likely introduced tens of thousands of years ago (not so long on an evolutionary timescale), and KoRV already has two mutations that lowered its virulence, so this may be a general trend.

Researchers hope that KoRV will help explain one of the great endogenous retrovirus mysteries: "How does a deadly virus become one with the host without killing off the host population entirely?" Studying KoRV and its coevolution with koalas may also provide critical insights into our own evolutionary history.



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