Published online 12 February 2009 | Nature | doi:10.1038/news.2009.97

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Origins of virus-like particles confirmed

A Cotesia wasp in parasitic action Alex Wild/myrmecos.net

Parasitic wasps exploit the genetic remnants of an ancient virus to stun their hosts into submission, a study has found.

The research sheds light on the origins of particles that some parasitic wasps inject, along with their eggs, into caterpillars. These particles, called polydnaviruses, carry genes that halt the caterpillar's immune response when expressed, allowing the wasp larvae to grow. But polydnaviruses do not replicate inside their host, nor do they contain any genes encoding their own protein coat - making them an oddity of the viral world.

Researchers have now confirmed that viral genes which integrated into wasp DNA 100 million years ago are used to make the coats of the polydnaviruses. Wasp-derived genes make up the particles' innards. The research "pretty much nails down the question" of where the particles came from, says James Whitfield, an entomologist at the University of Illinois at Urbana-Champaign, who provided some of the wasp DNA for the analysis.

Virologist Jean-Michel Drezen from France's national research centre (CNRS), based in Tours, and his colleagues, found genetic clues in the ovaries of female wasps, where the particles are produced. In the wasp species Chelonus inanitus and Cotesia congregata, the ovaries expressed proteins from 22 genes similar to those found in a virus group called nudiviruses. Two of these genes were found in four related wasp subfamilies, suggesting that an ancient nudivirus had inserted its DNA into the wasp genome before these species diverged. The work appears in Science1.

Not a virus?

Scientists already suspected that polydnaviruses had a viral origin, but the study "provides an important bit of information indicating who the ancient ancestor probably was", says Michael Strand, an entomologist at the University of Georgia in Athens who has worked with one of the study's authors. It makes sense that polydnaviruses come from nudiviruses, he says, because nudiviruses commonly infect the reproductive tracts of insects.

The team also studied part of the genome of one wasp species, C. congregata, and found five nudivirus-like genes clustered together. This clustering could mean that a complete nudivirus genome, not just individual genes, was integrated into the wasp genome, Drezen says. Based on similarities to genes from well-studied viruses, the researchers concluded that most of the viral genes in the wasp genome code for parts of the protein shell, a hypothesis verified by matching the DNA sequences to protein sequences from polydnavirus particles.

The findings suggest that polydnaviruses are sufficiently integrated to be considered parts of the wasp, rather than independent viruses, says Brian Federici, an entomologist at the University of California, Riverside. "It's more evidence that these are organelles, and they're not viruses," he says. A similar situation arose when bacteria took up residence into the cells of eukaryotes, he says, and became so integrated that they are now called mitochondria.

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Polydnaviruses are currently classified as viruses, but "whether we want to call them viruses now is an issue for the virological community", says Whitfield.

Scientists might also be able to create better gene therapy vectors by mimicking the polydnavirus, Drezen says. The particles are able to deliver 560 kb of DNA, an amount not possible with today's vectors.

The next step is to find the viral ancestor in other groups of parasitic wasp, says Drezen. The study found nudivirus-like genes in a subset of parasitic wasps called braconid wasps, but not in a second group called ichneumonid wasps. The particles produced by ichneumonid wasps may have originated from a virus that has not yet been studied, he says.