© Alex Wild/myrmecos.netA parasitic wasp lays its eggs into a caterpillar, at the same time delivering a hybrid virus.
A historical viral infection gave some insects genes that allow them to parasitise their caterpillar hosts, a new study finds.
Many species of wasps lay their eggs inside caterpillars. To make this possible, the wasps' have a secret weapon in the form of a dose of virus-like particles that are injected along with the eggs.
Not only do these disable the caterpillars' immune system to stop it attacking the eggs, they also cause paralysis and keep the host from pupating - turning the caterpillar into an eternally youthful larder and nursery for the wasp grubs.
A closer look at these particles reveals that, although they look like viruses, they contain genetic material from the wasp, which is transcribed into the caterpillars' DNA - causing production of the very toxins that bring about their downfall.
Essentially, the caterpillars produce their own poison, says Jean-Michel Drezen, a molecular biologist at the National Center for Scientific Research (CNRS) in Tours, France, who led the study.
Now Drezen's team says it has identified traces of a roughly 100-million-year-old viral infection that gave rise to the unique wasp-virus hybrids.
Scientists first identified the particles in the 1960s, yet didn't know what to make of them. They resemble other viruses under the gaze of an electron microscope, but their genomes don't match those of any other known virus.
Some scientists suggested that the wasps independently evolved the ability to produce the virus-mimicking particles, says James Whitfield, an entomologist at the University of Illinois in Urbana-Champagne, who had a hunch that true viruses were involved.Wasp hijackers
"The spectre arose that the wasps were doing really clever genetic engineering that looks just like a virus but is really a wasp invention," he says.
The new study casts doubt on this possibility, though. Drezen's team identified up to 22 viral genes expressed in wasp ovaries, yet not housed in the virus particles. The sequences matched those of an obscure family of insect pathogens, called nudiviruses.
These genes produce proteins that form the coat of nudiviruses, as well as package DNA into virus particles. Viruses usually stuff their own DNA into these capsid shells, along with the occasional host stowaway. The wasps, it seems, have hijacked the entire system to shove its genes into the virus particles.
Drezen is still piecing together this evolutionary puzzle, but he thinks parasitic wasps were infected with a nudivirus roughly 100 million years ago.'Lottery win'
Gradually, the wasp and virus coevolved to a mutually beneficial relationship: viruses save the wasps the trouble of producing their own toxins, while the viruses live on in the wasp genome.
Viruses "won the lottery big time if you're thinking about their genes," says Whitfield.
The sinister nature of this practice won't surprise anyone familiar with parasitic wasps. Some turn their victims into mindless zombies, left only barely alive until the wasp eggs hatch.
Yet another species keeps fly larvae alive long enough so it can survive the winter, thanks to an antifreeze compound made by the flies.
Charles Darwin even used one family of parasitic wasps as evidence for natural selection, writing to a colleague: "I cannot persuade myself that a beneficent and omnipotent God would have designedly created the Ichneumonidae with the express intention of their feeding within the living bodies of caterpillars."
Journal reference: Science