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Viruses can kill wasp larvae that grow inside infected caterpillars

When parasitic wasps call, some caterpillars have an amazing ally: a viral infection.

Insects called parasitoid wasps lay their eggs inside larvae of young moths, turning the caterpillars into involuntary incubators destined to die for possibly hundreds of wasp offspring. That’s bad news for viruses trying to use caterpillars as replication factories. For caterpillars, viral infections can be lethal, but their chances of survival are probably higher than if wasps chose them as a living nursery.

Now, a study shows how certain viruses can help caterpillars to parasitoid wasps. A group of proteins called parasitoid killing factor (PKF), which are found in some insect viruses, is incredibly toxic to young parasitoid wasps, researchers reported on July 30th.

The new discovery shows that viruses and caterpillars can come together to fight a common enemy of wasp, says study co-author Madoka Nakai, an insect virologist at Tokyo University of Agriculture and Technology. A parasitoid wasp would kill a host that the virus needs to survive, so the virus fights for its home. “He’s very smart,” Nakai says.

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What’s more, some moth caterpillars make the proteins that kill the wasp on their own, the team found. It is possible that in the distant past, some moths survived a viral infection and "got some gifts" in the form of genetic instructions on how to make proteins, says study co-author Salvador Herrero, an insect pathologist and geneticist at the University of Valencia in Spain. . Those insects could pass the ability offspring. In this case, “what doesn’t kill you makes you stronger,” Herrero says.

Previous studies have shown that viruses and insects, including moths, can exchange genes with each other. The new finding is one of the latest examples of this activity, says Michael Strand, an entomologist at the University of Georgia in Athens who did not participate in the work.

“Parasite-host relationships are very specialized,” he says. "Factors like (PKF) are probably important in defining which hosts can be used with which parasites." But it’s still unclear if the caterpillars stole the genetic instructions from the virus proteins or if the viruses originally stole the instructions from another host, Strand says.

Researchers discovered in the 1970s that virus-infected caterpillars could kill parasitoid wasp larvae using an unknown viral protein. In the new study, Herrero and colleagues identified PKF as wasp-killing proteins. The team infected moth caterpillars with one of three insect viruses that carry the genetic blueprints to make the proteins. The researchers then allowed the wasps to lay their eggs in the caterpillars or expose the wasp’s larvae to the hemolymph (the blood-equivalent insect) of the infected caterpillars.

Virus-infected caterpillars were poor hosts of the parasitoid wasp Cotesia kariyai; most young wasps died before they had a chance to get out of the caterpillars into the world. The hemolymph of infected caterpillars was also an effective killer of wasp larvae, normally destroying more than 90 percent of the young.

C. kariyai wasp larvae also did not survive in the caterpillars, including the beet worm (Spodoptera exigua), which make their own PKF. When the researchers blocked the genes in the proteins of these caterpillars, the wasps were alive, a sign that proteins are critical to the caterpillars ’defenses.

Some parasitoid wasps, including Meteorus pulchricornis, were not affected by PKF from viruses and also from beet army worms, which allowed the wasp's offspring to thrive within the caterpillars. That finding suggests that the ability to fight the wasp is species-specific, says Elisabeth Herniou, an insect virologist at the CNRS and the University of Tours in France who did not participate in the work. Identifying why some wasps are not susceptible could reveal the details of a long evolutionary battle between the three types of organisms.

The study highlights that “individual genes can interfere with the outcome of (these) interactions,” Herniou says. “One virus can have this gene and the other doesn’t have it,” and that can change what happens when the virus, the caterpillar, and the parasitoid collide.

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