In 1950, Australia used a virus as a biological weapon against millions of rabbits; scientists just decoded how they fought back, using DNA from a rabbit that once belonged to Charles Darwin

In 1950, Australia turned a virus into a weapon. The myxoma virus was deliberately introduced to control the out-of-control rabbit population in the country, and it worked right away. According to Alves et al. in the Science study ‘Parallel Adaptation of Rabbit Populations to Myxoma Virus,’ the disease spread over 2,000 kilometers in less than three months, killing 99% of infected animals. Then Alves et al. note that the virus was illegally introduced in France in 1952 and reached the UK by 1953, again with devastating results in both countries. Then, quietly, evolution started pushing back.

The backstory: a pest of biblical proportions
European rabbits were thought to have been introduced to Australia by an English settler, Thomas Austin, in the 1850s, according to Alves and colleagues. Within a century, there were hundreds of millions of them, wreaking havoc on native plants and animals in Australia. Myxomatosis seemed like the answer. But nature had other ideas.

Scientists read a rabbit's DNA across 150 years
Now, the landmark study in Science by scientists at the University of Cambridge and the CIBIO Institute in Porto has revealed, in fine genetic detail, how the rabbits rewired themselves to survive. An international team extracted DNA from nearly 200 rabbits dating from 1865 to 2013, including one that once belonged to Charles Darwin and is now in London’s Natural History Museum, said Alves et al. The samples were taken from 11 natural history museums in the UK, France, Australia, and the US. The team sequenced almost 20,000 genes to identify mutations that occurred after the pandemics of the 1950s.

The same fix, in three different countries
What the scientists found was striking. The study showed that modern rabbits in Australia, the UK, and France have all developed resistance through the same genetic changes, even though the populations evolved independently thousands of miles apart. Such parallel evolution is one of the clearest signs that natural selection is operating. According to the study, this resistance was not due to a single dramatic mutation in a single gene, but rather to the cumulative effect of many small changes spread throughout the genome, especially in genes involved in immunity.

Three of the most important mutations were found in the IFN-alpha 21A gene. Alves and team describe this gene as a gene that encodes a protein that acts as a biological alarm system and, when a virus infects the rabbit’s cells, it triggers an alarm to activate the immune response. The team made both the 1950s version of this protein and the modern version and then tested them against the virus. Senior author Professor Francis Jiggins from Cambridge's Department of Genetics said the team saw, at the molecular level, exactly how rabbits had been fighting back over the years.

The virus fought back, and it got meaner
The rabbits did not win fairly. According to the study, ‘Next Step in the Ongoing Arms Race Between Myxoma Virus and Wild Rabbits in Australia Is a Novel Disease Phenotype,’ published in Proceedings of the National Academy of Sciences, as rabbits developed resistance, the virus evolved a worrying counter-strategy. Later strains evolved a greater ability to suppress the rabbit immune system, leading to what researchers call an immune collapse syndrome. Kerr et al. state that this is a new escalation in which the virus is no longer just surviving but is actively undermining the host’s defenses. The arms race goes on.

A side effect that could protect more rabbits
There could be a surprise on the upside. According to Alves et al. , the protein conferring resistance to myxoma in modern rabbits has antiviral effects against an unrelated virus, vesicular stomatitis virus. Miguel Carneiro of the CIBIO Institute suggested that while fighting myxoma, rabbits might have developed greater resistance to other viruses, possibly even rabbit hemorrhagic disease, which is currently decimating rabbit populations across Europe.

Why this matters beyond the rabbit warren
To Americans, it might sound like a wildlife story from overseas. But it’s really about what happens when a new pathogen meets a population that has never seen it and how life reacts. The myxomatosis pandemics are among the most intense and well-documented episodes of natural selection in populations of wild animal hosts and have become a textbook case of host–parasite coevolution, the Science study notes.

Darwin’s rabbit could never have known that one day its bones would help to answer one of the biggest questions in evolution. But the arms race of which it is a part is far from over. The virus keeps changing. The rabbits continue to mutate. And scientists, watching closely, are learning lessons that extend far beyond the rabbit.