A Planet of Viruses - Carl Zimmer [18]
Microbiologists have been getting used to the viral roots of the microbes they study for decades now. And as long as microbes were the only organisms with much evidence of virus-imported genes, we could try to ignore this philosophical weirdness by thinking of it merely as a fluke of “lower” life forms. But now we can no longer find comfort this way. If we look inside our own genome, we now see viruses. Thousands of them.
We have the jackalope to thank for this realization. The myth of the jackalope was one of the clues that led virologists to discover that some viruses cause cancer. In the 1960s, one of the most intensely studied cancer-causing viruses was avian leukosis virus. At the time, the virus was sweeping across chicken farms and threatening the entire poultry industry. Scientists found that avian leukosis virus belonged to a group of species known as retroviruses. Retroviruses insert their genetic material into their host cell’s DNA. When the host cell divides, it copies the virus’s DNA along with its own. Under the certain conditions, the cell is forced to produce new viruses—complete with genes and a protein shell—which can then escape to infect a new cell. Retroviruses sometimes trigger cells to turn cancerous if their genetic material is accidentally inserted in the wrong place in their host’s genome. Retroviruses have genetic “on switches” that prompt their host cell to make proteins out of nearby genes. Sometimes their switches turn on host genes that ought to be kept shut off, and cancer can result.
Avian leukosis virus proved to be a very strange retrovirus. At the time, scientists tested for the presence of the virus by screening chicken blood for one of the virus’s proteins. Sometimes they would find the avian leukosis virus protein in the blood of chickens that were perfectly healthy and never developed cancer. Stranger still, healthy hens carrying the protein could produce chicks that were also healthy and also carried the protein.
Robin Weiss, a virologist then working at the University of Washington, wondered if the virus had become a permanent, harmless part of the chicken DNA. He and his colleagues treated cells from healthy chickens with mutation-triggering chemicals and radiation to see if they could flush the virus out from its hiding place. Just as they had suspected, the mutant cells started to churn out the avian leukosis virus. In other words, these healthy chickens were not simply infected with avian leukosis virus in some of their cells; the genetic instructions for making the virus were implanted in all of their cells, and they passed those instructions down to their descendants.
These hidden viruses were not limited to just one oddball breed of chickens. Weiss and other scientists found avian leukosis virus embedded in many breeds, raising the possibility that the virus was an ancient component of chicken DNA. To see just how long ago avian leukosis viruses infected the ancestors of today’s chickens, Weiss and his colleagues travelled to the jungles of Malaysia. There they trapped red jungle fowl, the closest wild relatives of chickens. The red jungle fowl carried the same avian leukosis virus, Weiss found. On later expeditions, he found that other species of jungle fowl lacked the virus.
Out of the research on avian leukosis virus emerged a hypothesis for how it had merged with chickens. Thousands of years ago, the virus plagued the common ancestor of domesticated chickens and red jungle fowl. It invaded cells, made new copies of itself, and infected new birds, leaving tumors in its wake. But in at least one bird, something else happened. Instead of giving the bird cancer, the virus was kept in check by the bird’s immune system. As it spread harmlessly through the bird’s body, it infected the chicken’s sexual organs. When an infected bird mated, its fertilized egg also contained the virus’s DNA in its own genes.
As the infected embryo grew and divided, all of its cells also inherited the virus DNA. When the chick emerged from