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Many animals, including humans, have DNA left over from ancient viral infections. In koalas, researchers are studying the process in real time. Koalas have been running into hard times. They have suffered for years from habitat destruction, dog attacks, automobile accidents. But that’s only the beginning. They are also plagued by chlamydia and cancers like leukemia and lymphoma, and in researching those problems, scientists have found a natural laboratory in which to study one of the hottest topics in biology: how viruses can insert themselves into an animal’s DNA and sometimes change the course of evolution.
The target of this research is Koala retrovirus, or KoRV, a bit of protein and genetic material in the same family as H.I.V. that began inserting itself into the koala genome about 40,000 years ago and is now passed on from generation to generation, like genes. It is also still passed from animal, as a typical viral infection. In recent years, scientists have found that the insertion of viruses into the genomes of animals has occurred over and over again. An estimated 8 percent of the human genome is made up of viruses left over from ancient infections, ancient as in millions of years ago, many of them in primate ancestors before human beings existed. The koala retrovirus is unusual because 40,000 years is the blink of an eye in evolutionary time, and because the process appears to be continuing. A group of scientists reported in Cell on Thursday that they observed a genome immune system fighting to render the virus inactive now that it has established itself in the koala DNA. They also reported that koala retrovirus may have activated other ancient viral DNA. All of this activity stirs the pot of mutation and variation that is the raw material for natural selection.
Koala genetics are a gold mine, said William Theurkauf, a professor in molecular medicine at the University of Massachusetts Medical School and one of the authors of the report. “What they are going through is the process of what’s driven the evolution of every animal on the planet.” Past viral infections have led to major evolutionary changes, he said. For example: “A gene that is absolutely essential for the placenta was derived from the shell of a virus millions of years ago.” Humans would not exist without that ancient retroviral infection. Retroviruses are made of RNA, a single strand of genetic information. When they infect a cell, they translate themselves into DNA, the two-stranded molecule that carries all the information for making humans, koalas and other animals. The retroviruses take over the DNA machinery to make more of themselves, which keeps the process going. That process makes us and other animals sick. AIDS is probably the best known retroviral disease. But when the insertion of a retrovirus occurs in a sperm or an egg cell, the change can become permanent, passed on forever. When retroviruses become part of an animal’s inherited DNA, they are called endogenous and eventually they no longer cause the kind of original infection they once did. But they can still be used by the animal's genetic machinery for other purposes, like making a placenta...
Additional information published in Cell on October 10, 2019:
