Eight percent of your 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, a process that can help us understand our own viral lineage and make decisions about managing this vulnerable species.
In a recent study, published in Molecular Biology and Evolution, scientists from the University of Illinois discovered that 39 different KoRVs in a koala's genome were all endogenous, which means passed down to the koala from one parent or the other; one of the KoRVs was found in both parents.
Koalas are 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 uses the host cell's machinery to manufacture more viruses. 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).