There's no consent for these surrogate parents. Tens of thousands of wasp species lay their eggs inside caterpillars, injecting toxins that paralyze the hosts and allow their young to feast on the innards with impunity. Researchers have long wondered what exactly these toxins are and where they came from. The answers, a new genetic analysis reveals, have to do with a virus that infected wasps millions of years ago.
The first clue to the nature of the wasp's toxins came in the 1970s. Using electron microscopy, researchers found that they consisted of protein-encased, double-stranded DNA particles that were produced in the wasps' ovaries. Because they resemble viruses, the toxins were christened polydnaviruses.
But the virus classification immediately provoked debate. Genetic analysis revealed that the particles harbored mostly wasp DNA and didn't seem to contain any of the proteins most viruses use to replicate. These two observations led some researchers to argue that the particles were actually "genetic secretions" of the wasp itself rather than an independent virus.
Determined to resolve the debate, entomologist Jean-Michel Drezen of the University of Tours in France and colleagues decoded the first complete sequence of a polydnavirus in 2004. But those results also showed mostly wasp DNA (Science, 8 October 2004, p. 286). So in the new study, Drezen's team looked at DNA from wasp ovaries, in which the polydnaviruses are made. They analyzed DNA from three different wasp species and checked the sequences against those of known insect viruses. In one group of wasps, 22 genes matched those of an ancient family of viruses called nudiviruses, the researchers report tomorrow in Science. Further experiments showed that these genes code for key structural proteins in the wasps' polydnavirus toxins.
What this means, explains Drezen, is that nudiviruses infected wasps a few million years ago and that, over time, the viral DNA fully integrated into the wasp genome. As it currently stands, the wasps need the virus to survive, because the virus helps the insects lay eggs in caterpillars. The virus also needs the wasp to survive, because the virus can only replicate in the wasp's ovaries. The virus cannot replicate inside the caterpillar, because all of its replication machinery is inside the wasp.
The findings provide the most convincing evidence to date that some polydnaviruses descended from a type of nudivirus that infected wasps millions of years ago, says Nancy Beckage, an entomologist at the University of California, Riverside. The researchers could not explain how the second group of polydnaviruses--which didn't contain nudivirus genes--developed. But another kind of insect virus, as yet undiscovered or even extinct, might have infiltrated wasps to create those groups, says James Webb, an entomologist at the University of Kentucky, Lexington.
Learning more about how these viruses work could have clinical applications, says Drezen. The polydnavirus acts as a gene vector, carrying much larger chunks of DNA to the caterpillar than any synthetic gene therapy agent can transport. Thus, studying these viruses could enhance gene-therapy techniques, Drezen says.