If you prick the ovary of a wasp called Microplitis demolitorviruses burst forth in huge numbers, glistening like iridescent blue toothpaste. “It’s really pretty and amazing that there are so many viruses in there,” says Gaelen Burke, an entomologist at the University of Georgia.
M. demolition is a parasite that lays its eggs in caterpillars, and the particles in its ovaries are “domesticated” viruses that have been fine-tuned to persist harmlessly in wasps and serve their purposes. The virus particles are injected into the caterpillar through the wasp’s stinger, along with its own eggs. The viruses then pour their contents into the caterpillar’s cells, delivering genes which are different from those of 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 offspring.
The insect world is full of parasitic wasp species that spend their childhood eating other insects alive. And for reasons scientists don’t fully understand, they have repeatedly adopted and domesticated wild disease-causing viruses and turned them into biological weapons. Half a dozen examples have already been described, and new research points to many more.
By studying viruses at different stages of domestication, researchers are unraveling how the process unfolds.
Partners in diversification
The quintessential example of a virus domesticated by a wasp is the group of bracoviruses, which are believed to descend from a virus that infected a wasp, or its host caterpillar, about 100 million years agoThat ancient virus spliced its DNA into the wasp’s genome. From then on, it became part of the wasp, being passed on to each new generation.
Over time, wasps diversified into new species, and their viruses diversified with them. Bracoviruses are now found in about 50,000 species of wasps, including M. demolitionOther domesticated viruses are descended from different wild viruses that entered the genomes of wasps at different times.
Researchers debate whether domesticated viruses should be called viruses. “Some say it is still a virus; others say it is integrated and part of the wasp,” explains Marcel Dicke, an ecologist at Wageningen University in the Netherlands who 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 is dispersed throughout the DNA from the wasp. Some genes are broken down, but a core set remains — those essential for making the original infectious virus particles. “The parts are in different places in the wasp genome. But they can still communicate with each other. And they’re still making products that cooperate with each other to create virus particles,” explains Michael Strand, an entomologist at the University of Georgia. But rather than containing a complete viral genome, as a wild virus would, the domesticated virus particles serve as delivery vehicles for the wasp’s weapons.
![These are the steps in the life of a parasitic wasp that hosts a bracovirus.](https://imagenes.elpais.com/resizer/v2/ELAMVEE5XZBV7EJ7ENRNPCHZJA.png?auth=b200c84b51e5fad271d3066ca5e351efe85e68e02442de621b99378130047605&width=414)
These weapons vary widely. Some are proteins, while others are genes on short stretches of DNA. Most bear little resemblance to those of wasps or viruses, so their origin is unclear. And they change constantly, locked in evolutionary arms races with the defenses of caterpillars or other hosts.
In many cases, researchers have yet to discover what the genes and proteins inside the wasp hosts do or prove that they function as weapons. But they have unraveled some details.
For example, wasps M. demolition They use bracovirus to introduce a gene called glc1.8 in the immune cells of moth caterpillars. The gene glc1.8 causes infected immune cells to produce a mucus that prevents them from attaching to the wasp’s eggs. Other genes in the bracoviruses M. demolition force immune cells to commit suicide, while others prevent caterpillars from suffocating parasites in melanin sheaths.
Wasps keep control
Domesticating viruses is arguably a dangerous undertaking. After all, wild relatives of domesticated viruses can be deadly, as they force cells to produce viral particles and then burst, releasing their contents. Some of them cause insects’ innards to dissolve. Indeed, even in the domesticated situation, sometimes specialized cells in wasps’ ovaries must burst to release viral particles.
“The wasp has to find a way to control that virus so it doesn’t infect and kill the wasp itself,” says Kelsey Coffman, an entomologist at the University of Tennessee.
How have wasps evolved to control their pet viruses? Most importantly, they have castrated them. The virus particles cannot reproduce because they do not contain the crucial genes for creating new virus particles. These remain in the wasp’s genome.
Wasps also control where and when the domesticated virus particles are produced, presumably to reduce the risk of the virus going rogue. Bracovirus particles are only produced in one cavity of the female’s reproductive tract, and only for a limited time.
And key viral genes have been completely lost, so that domesticated viruses cannot replicate their own DNA. This loss is seen even in recently domesticated viruses, suggesting that this is an important first step.
In fact, any viral gene that does not help the wasp will accumulate mutations. In bracoviruses, so much time has passed that unused genes are unrecognizable. In more recently domesticated viruses, remnants can still be identified.
A “missing link” uncovered
There’s nothing special about having a genome full of dead viruses. Viruses jump into animal genomes all the time; even our own DNA is littered with their remnants. But only parasitic wasps are known to maintain entire sets of genes that still work together to build viral particles.
Researchers are eager to understand how these relationships begin. For clues, some turn to a tiny orange wasp called Diachasmimorpha longicaudatawhich may be in the early stages of domesticating a poxvirus. The poxvirus is not a true domesticated virus because its DNA has not entered the wasp’s genome. Instead, it replicates on its own in the wasp’s venom glands.
Like other virus-taming wasps, the D. longicaudata injects viral particles into its host, which in this case is a fruit fly maggot. And Coffman and Burke, with researcher Taylor Harrell, have shown that without the poxvirus, Most wasp larvae dieBut unlike fully domesticated viruses, the poxvirus also replicates outside the wasp, producing new viral particles in the worm’s cells. The wasp benefits from the poxvirus, but does not fully control it.
This weak control may reflect the type of virus the wasps started out with, Coffman says. Most domesticated viruses are descended from types called nudiviruses, which can integrate into wasp genomes more easily than poxviruses.
But it’s also possible that the wasps just haven’t had enough time yet. In fact, the wasp-poxvirus association is so new that it only seems to be present in one species of wasp. It’s even absent from another species so closely related that Coffman didn’t realize at first that he had both wasps in his lab.
However, the virus is isolated in certain tissues and only replicates when the eggs are developing, which could mean that D. longicaudata has already set up some defenses. The viruses also appear to be losing their ability to be transmitted without the wasp’s help. “I’ve tried feeding the flies a lot of virus and they don’t seem to get infected that way,” Coffman says.
The poxvirus system is exciting, Coffman adds, because so little is known about how virus domestication begins. “We can’t go back in time and figure out how it started. But this system is new. We have this snapshot of, you might say, the missing link.” Although no one knows for sure why viruses keep getting domesticated in parasitic wasps, researchers suspect it has to do with their lifestyle. Internal parasites live in the guts of their hosts, dangerous environments that actively try to kill them. From the wasps’ perspective, viruses are like toolkits loaded with tools to solve this very serious problem.
The wasp has to find a way to control that virus so that it doesn’t infect and kill it.”
Kelsey Coffman, an entomologist at the University of Tennessee.
This idea is supported by research conducted in 2023 that analyzed the genomes of more than 120 species of wasps, ants, and bees. The researchers searched these genomes for clues to the types of viruses that tend to become domesticated. They inferred the presence of domesticated viruses by detecting genes from viruses that have been maintained in a functional state over evolutionary time. This conservation would not be expected unless the genes helped wasps survive or reproduce.
As expected, non-parasitic insects showed little evidence of having these domesticated viruses. The same was true for parasites that develop outside their hosts’ bodies, where the immune system can’t attack them. But in parasites that develop inside other insects—so-called endoparasitoids—the domesticated viruses seemed to be much more common.
“There is a special connection between viruses and these endoparasitoids,” says Julien Varaldi, an evolutionary biologist at Claude Bernard Lyon 1 University in France and one of the authors of the study. “It suggests that these viruses play an important role in the evolution of this way of life.”
And with hundreds of thousands of wasp species and countless strains of viruses, there’s every chance the two entities will team up. It is, Strand says, “an evolutionary sandbox full of opportunity.”
Article translated by Debbie Ponchner. This article Originally appeared in Knowable in Spanisha non-profit publication dedicated to making scientific knowledge accessible to everyone.
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