The researchers wrote another chapter in the classic case of the arms race between a host and its pathogen. The main characters in this 70-year-old drama are the voracious European rabbit (Oryctolagus cuniculus) and a virus deliberately released in France and Australia to kill rabbits and protect fields and pastures. Working with museum specimens collected several decades ago, a team discovered that rabbits from two continents have undergone the same genetic modifications to fight the virus – before the virus itself transforms and regains its upper hand. .
Discovery is a striking example of how evolution sometimes repeats itself, and it can help to understand how the human immune system reacts to pathogens. The rabbit work, published online today in Science, "provides key new information on one of the biggest stories of evolution," said Edward Holmes, an evolution biologist at the University of Sydney in Australia, who is studying the biocontrol virus.
In Australia, a few dozen European rabbits introduced in the mid-1800s for hunters did what the animals did in a famous way. They have multiplied until hundreds of millions of people consume crops. Thus, in 1950, after a smallpox-like virus found in South American rabbits killed the European parent, the Australian authorities released him in the wild, reducing the number of rabbits by 99%. A few years later, the virus, called myxoma, was released in France and eventually spread to the UK.
The result was "an opportunity to trace the arms race between pathogens and pathogens under the gaze of our hosts," says Jia Liu, a biologist at the University of Arkansas for Medical Sciences in Little Rock. . In the space of a decade, the number of rabbits was again on the rise as some resisted this deadly infection and the virus itself became less deadly.
To understand the adaptations of the rabbit, Joel Alves, today an evolutionary biologist at the University of Oxford in the UK, the evolutionary geneticist Francis Jiggins of the University of Cambridge in the UK, and his colleagues spotted specimens from UK, Australia and France. O. cuniculus collected by museums before the introduction of the virus. They sequenced all the genes and other DNA that could influence the body's immune defenses and compared the results to those of modern rabbit sequences living in the same places. Comparisons revealed changes in many genes, usually a change in the frequency of some versions, or alleles, of a gene. It is striking to note that half of the changes were shared by rabbits from all three countries – evidence of a parallel evolution.
An allele change has affected the interferon of rabbits, a protein released by immune cells that fires the alarm bell when attacked by a virus and helps trigger an immune response. Compared to pre-infected rabbits, modern rabbits produce an interferon that responds better to the biocontrol virus, researchers have found by adding different versions of the protein to rabbit cell lines.
The virus did not stay still. In 2017, Holmes and colleagues reported that in the 1970s, the virus had developed a greater ability to suppress immune responses in rabbits. This change, along with the natural emergence of another virus that kills rabbits, has caused a further decline in populations. But unlike parallel evolution in the rabbit, myxoma viruses at various locations have borrowed different genetic pathways to regain their potency.
Andrew Read, an evolution microbiologist at Pennsylvania State University at State College, suggests that viral counterattack "is a cautionary tale" for researchers seeking to take charge of the arms race of evolution introducing biocontrol agents or making crops or livestock more resistant to disease. "You have to be careful about evolution and the counter-revolution," he says. "The rabbit did not win."