"Who knows not where a wasp does wear his sting?" Shakespeare asks in Taming of the Shrew. "In his tail."
Most biologists would agree with the Bard, but a genetic peek inside some parasitic wasps suggests the story is a bit more complicated. The wasps' stings pack a punch partly due to paralyzing particles stripped out of an ancient virus, suggests a team led by Annie Bezier of France's Université François Rabelais.
"Many species of parasitoid wasps inject polydnavirus (virus-like) particles in order to manipulate host defenses and development," begins the study in the current Science magazine. What does that mean? Remember the exploding chest scene in the movie, Alien?
Yep, that's what the researchers are talking about — some 17,500 wasp species that hunt down caterpillars and inject their eggs into them. The wasps also inject polydnavirus particles whose genes express proteins that paralyze the hapless caterpillars, disabling their immune system and growth, which allows the wasp embryos to feast from within until they kill their host and explode from the carcass to begin the cycle anew.
Pretty grisly stuff, but gardeners and farmers mostly see it as just desserts for the caterpillars. And since 1967, the paralysis particles have puzzled researchers as to their origin. Unlike real viruses, the particles don't reproduce after invading their host, and lack any genetic machinery to spawn. So, how did they get into wasps? Could something so nifty, which acted and looked exactly like toxins expressed by viruses, have developed independently through the evolution of ancestral wasps, or was there another explanation?
To find out, Bezier and colleagues cracked open wasp ovaries (the calyx) of distantly-related parasitic wasps, focusing on cotesia congregata wasps, which prey particularly on young tobacco hookworms. The calyx is the only place the wasp makes the viral particles, prior to injecting them into their soft-skinned egg incubators. There, within the genes of the wasps, they found the machinery for reproducing the particles. And then they compared those genes to known viruses, looking for a match, which they found within a family of viruses known to afflict moths, beetles, crickets and shrimp.
Specifically, the researchers found that 22 calyx genes matched those from the moth virus family, with small differences suggesting they are remnants of an infection of an ancestral wasp about 100 million years ago. The researchers have "convincingly demonstrated" that this ancient virus inserted its genes into the DNA of the wasps, finds immunologist Donald Stoltz of Canada's Dalhousie University and entomologist James Whitfield of the University of Illinois, in a commentary accompanying the study.
Essentially, the ancestral wasp beat up a robber (the virus) invading its home, took his gun, and her descendants now make a living shooting up caterpillars.
"The authors solve a long-standing mystery, and at the same time establish a new paradigm in virology," add Stoltz and Whitfield, in a more congratulatory vein. Episodes of "horizontal" transfer of genes, wholesale borrowing of useful ones, are common among microbes, and genome researchers have long shown that animal genomes, including ours, are riddled with remnants of viral DNA that blundered their way into our genes and sometimes play useful roles, for example in the development of the placenta in pregnant women. But the wasp-virus incorporations "represent the only example, so far," say the study authors, of a creature lifting gene machinery that allows it to inject and fire off genes into another creature. The finding, they suggest, opens a new avenue for gene therapy.
More broadly, the virus finding reveals a new way for viruses and the hosts to make a living off each other, suggest Stoltz and Whitfield. The original virus doesn't seem to be around any more, but its genes live on in the belly of the wasp. "Were the (virus-like) polydnaviruses really viruses?" they ask? It all depends on how virologists define viruses, they conclude, something neglected in the most recent 1,259-page Virus Taxonomy: VIIIth Report, the bible of virology.
"The more interesting lesson here for virologists and for evolutionary biologists may be that there is now reason to start thinking about virus-host relationships in much broader terms," adds the commentary. The wasp polydnavirus-style mutual back-scratching among viruses and their victims may be more common than previously supposed. "How did this kind of relationship arise?" they ask.
In the Taming of The Shrew, the play's shrew, Katharina, instead locates the wasp's sting, "In his tongue." The sarcastic rejoinder to Shakespeare's tail suggestion may contain a hint of the answer to viruses' origin. Most likely, concludes the commentary, larval wasps long ago munching on caterpillars infected with the original virus imbibed the same infection, which made its way, eventually, into their sting.