This is a story about viruses that became domesticated by parasitic wasps, which use them as biological weapons for corrupting the bodies of caterpillars, which in turn can steal the viral genes and incorporate them into their own genomes, where they protect the caterpillars from yet more viruses. Evolution, you have outdone yourself with this one.

The wasps in question are called braconids. There are more than 17,000 known species, and they're all parasites. The females lay their eggs in the bodies of still-living caterpillars, which their grubs then devour alive.

As early as 1967, scientists realised that the wasps were also injecting the caterpillars with some kind of small particle, alongside their eggs. It took almost a decade to realise that those particles were viruses, which have since become known as bracoviruses. Each species of braconid wasp has its own specific bracovirus, but they all do the same thing: They suppress the caterpillar’s immune system and tweak its metabolism to favour the growing wasp. Without these viral allies, the wasp grubs would be killed by their host bodies.

So, the viruses are essential for the wasps—but the reverse is also true. Unlike most other kinds of virus, these bracoviruses cannot make copies of themselves. They are only manufactured in the ovaries of the wasps, and once they get into the caterpillars, their life cycle ends. Some might say they’re not true viruses are all. They're almost like secretions of the wasp’s body.

The bracoviruses can't independently reproduce because they lack genes for making the protein coats that give them form and structure. Those coat genes didn't vanish. In 2009, Anne Bezier and Jean-Michel Drezen from Francois Rabelais University showed that they exist within the wasp genomes. The bracoviruses aren’t just allies for the wasps: They are part of the wasps.

Based on the diversity of these viral genes within different species of braconid wasps, Bezier and Drezen estimated that they must have entered the wasp genome around 100 million years ago, before the braconid dynasty expanded into its current lush state. Back then, an ancient virus infected an ancient wasp, inserted its genes among those of its host, and created a partnership that has been dooming caterpillars ever since.

More recently, Gaelen Burke and Michael Strand from University of Georgia showed that the wasp genomes contain two separate clusters of viral genes. The first is a replication set, which the wasps use to turn their ovaries into virus-making factories. The second is a virulence set, which attacks the caterpillars. But when the wasps build the viruses, they fill them only with the virulence genes, not the replication ones. That’s why the resulting particles can attack caterpillars, but can’t reproduce or spread to new hosts. They are fully domesticated.

The caterpillars aren't just helpless victims in this drama. Sometimes, they successfully fight off the wasp-and-virus tag team. Other times, the wasps screw up, by attacking caterpillars of the wrong species, against whom their particular viruses are useless. Either way, caterpillars occasionally survive their encounters with braconids, but still end up with swarms of bracoviruses in their bodies. What happens then? Since those viruses were originally part of one insect genome (the wasp’s), could they find their way into another (the caterpillar’s)?

The answer is yes. Last year, Sean Schneider and James Thomas from the University of Washington found evidence of bracovirus genes in the genomes of the silk moth and the monarch butterfly. The duo described the wasps as “accidental genetic engineers,” implanting the genomes of their caterpillar victims with their own (viral) DNA. In other words, one insect was genetically modifying another with viral genes, via a sting.

“What’s kind of funny is that such a species as iconic as the monarch has been genetically modified by the parasitic wasp virus and can thus be considered as a natural GMO,” says Drezen, in an email. He, together with Salvador Herrero from the University of Valencia, has now found similar genes in a wider range of butterfly and moth species, including important pests like the beet armyworm and fall armyworm. And they’ve found that these sequences may not just be passive hitchhikers.

Herrero is an expert on baculoviruses, a group of viruses that infect and kill insects, and are often used in biological control. While looking for moth genes that resist these infections, he found a few that had no counterparts in related species. Instead, their closest matches were the bracovirus genes in braconid wasps. It looked as if these viral genes, which had hopped from wasp to moth, were now protecting their new hosts from baculoviruses.  

By coincidence, Drezen had discovered the same thing on his own, and found himself sitting next to Herrero at a conference dinner. They talked, realized that they were working on the same problem, and teamed up. Together, they showed that one of the viral genes in the beet armyworm prevents baculoviruses from reproducing in insect cells. Another stops baculoviruses from entering the cells in the first place, blocking infections entirely.

Herrero speculates that the wasps might have originally used these genes to stop baculoviruses from prematurely killing their hosts before their grubs could develop. When the genes made their way into the caterpillars, they played exactly the same role, but on behalf of a different owner. Where they once preserved the caterpillar’s life so the wasp could later kill it, now they just preserve its life, full-stop.

Michael Strand says that the team haven’t conclusively shown that the viral genes play an active role in the moths; the data, he says, are “suggestive” but not conclusive. Herrero acknowledges this, and is trying to get more unambiguous proof. He plans to disable the transferred viral genes by editing them, to see if their caterpillar owners more readily suffer from baculovirus infections. In other words, he plans to genetically modify the moths to show that the wasps have been doing so all along.