Wasps that tamed viruses

If you pierce the ovary of a wasp called Microplite demolisher, viruses spurt out in large quantities, shimmering like iridescent blue toothpaste. “It’s very beautiful and just amazing that there’s so much virus in there,” says Gaelen Burke, an entomologist at the University of Georgia.

Mr. Demolitionistis a parasite that lays its eggs in caterpillars, and the particles in its ovaries are “domesticated” viruses that have been tuned to persist harmlessly in the wasps and serve their purposes. The viral particles are injected into the caterpillar by the wasp’s stinger, along with the wasp’s eggs. The viruses then dump their contents into the caterpillar’s cells, delivering different genes than a normal virus. These genes suppress the caterpillar’s immune system and control its development, turning it into a harmless nursery for the wasp’s young.

The insect world is full of species of parasitic wasps that spend their childhood eating other living insects. And for reasons that scientists don’t fully understand, they have repeatedly adopted and tamed disease-causing wild viruses and turned them into biological weapons. Half a dozen examples have already been described, and new research suggests many more.

By studying viruses at different stages of domestication, researchers are now trying to understand how the process takes place.

Partners in diversification

The quintessential example of a virus domesticated by a wasp involves a group called bracoviruses, which are thought to be descended from a virus that infected a wasp, or its caterpillar host, around 100 million years ago . This ancient virus inserted its DNA into the wasp’s genome. From then on, it becomes part of the wasp, transmitted to each new generation.

Over time, wasps diversified into new species, and their viruses diversified with them. Bracoviruses are now present in some 50,000 species of wasps, including Mr. Demolitionist. Other domesticated viruses are descended from different wild viruses that entered the wasp genome at different times.

Researchers debate whether domesticated viruses should be called viruses. “Some people say it’s still a virus; others say it is embedded and therefore part of the wasp,” says Marcel Dicke, an ecologist at Wageningen University in the Netherlands, who has described how domesticated viruses indirectly affect plants and other organisms. in an article published in 2020 in the Annual Review of Entomology.

As the wasp-virus composite evolves, the virus genome disperses throughout the wasp’s DNA. Some genes disintegrate, but a central set is preserved: those essential for manufacturing the infectious particles of the original virus. “The parts are all in different places in the wasp genome. But they can still talk to each other. And they still make products that cooperate with each other to produce virus particles,” says Michael Strand, an entomologist at the University of Georgia. But instead of containing a complete viral genome, as a wild virus would, the domesticated virus particles serve as vectors for the wasp’s weapons.

These weapons vary greatly. Some are proteins, while others are genes located on short segments of DNA. Most bear little resemblance to what is found in wasps or viruses, so it is not clear where they come from. And they are constantly evolving, engaged in evolving arms races with the defenses of caterpillars or other hosts.

In many cases, researchers have yet to discover what genes and proteins do in wasp hosts or prove that they function as weapons. But they unraveled some details.

For example, Mr. Demolitionistwasps use bracoviruses to deliver a gene called glc1.8in the immune cells of butterfly caterpillars. THE glc1.8This gene causes infected immune cells to produce mucus that prevents them from sticking to the wasp’s eggs. Other genes in Mr. DemolitionistBracoviruses force immune cells to commit suicide, while still others prevent caterpillars from smothering parasites in melanin sheaths.