Figure: Tiny wasp eggs (white) surrounding their host, the caterpillar (green).
On July 29, researchers at the University of Valencia published a study about parasitoid-killing factors with many other researchers from Japan, Canada, and South Korea. Parasitoid-killing factors (PKF) are proteins whose genes are in several double-stranded DNA viruses that infect lepidopteran insects, such as moths, butterflies, and skippers. These proteins—carried by either caterpillars or viruses—are highly poisonous to wasp eggs laid inside their hosts (caterpillars).
When the parasitic wasp’s eggs steal the “home” of the virus (“home” = caterpillar), the virus and its host form a mutualistic relationship. In this relationship, the virus protects the caterpillar by destroying the parasitic wasp's eggs and preventing them from consuming the host's flesh. Consumption of the host's body would result in the eventual death of the host. Although the virus alone is harmful to its host, the virus gives the host a higher probability for survival than the parasitoid wasps that use the caterpillar or host as a “living nursery.”
Not only did researchers discover PKF in the entomopathogenic DNA viruses, but they also found the parasitoid-killing factors inside the insects themselves. For instance, some moth caterpillars could form the factor proteins themselves to kill the eggs laid by the parasitoid wasps. The study’s Co-author Salvador Herrero, a pathologist and geneticist at the University of Valencia, presumed that in the distant past, several moth caterpillars that survived viral infections could have learned how to create PKF as survival techniques. Those advantages were likely to have been passed down to the offspring.
The research of the relationships between the three organisms—caterpillars (hosts), parasitoid wasps, and viruses—dates back to the 1970s. Entomologist Harry Kaya at the University of California, Davis, et al. had already observed that virus-infected caterpillars were protected from the parasitoids with the benefit of a viral protein. However, the viral protein still remained unknown, and the researchers couldn’t understand the mechanism of protection. Only in a recent study did researchers Herrero and his collaborator Madoka Nakai et al. from Tokyo University of Agriculture and Technology identify the protein as PKF and understand that mechanism.
Madoka Nakai and her team were the first to discover the parasitoid-killing factors in northern armyworm caterpillars infected with entomopoxvirus—a subfamily of the Poxviridae family in which humans and other insects (besides caterpillars) can serve as hosts. From their discovery, they concluded that the infected hosts or caterpillars were protected from certain parasitoid species. To test their hypothesis, Salvador Herrero and his team exposed the young parasitic wasps to plasma from virus-infected caterpillars taken of any virus particles. As a result, the wasp larvae died. The team then compared the plasma proteins in the healthy larvae to virus-infected larvae and soon identified a 28-kDa protein: PKF. The protein was present exclusively in the infected larvae.
Afterward, the researchers allowed the wasps to lay their eggs inside their victims (caterpillars) or have them (the wasp larvae) exposed to hemolymph, a fluid similar to blood in most invertebrates. The results were that both groups—the virus-infected caterpillars and hemolymph—were efficient killers of wasp larvae. However, further findings suggested that some parasitoid wasps resist the parasitoid-killing factors because the ability of the wasps to fight back is species-specific.
Q&A:
Sally: Where can this discovery be applied? What are some industries or agriculture that might benefit from this? In other words, is there a way our society can benefit from this?
This discovery can allow scientists to comprehend how living organisms interact within an environment. Scientists examine their behaviors and the organisms’ needs to survive and reproduce. Researchers can further unearth new data on evolution regarding the three organisms: parasitic wasps, caterpillars, and viruses.
Hannah: You mentioned that the virus alone is harmful to the caterpillar. If so, can it really be considered a mutualistic relationship? In the absence of parasitic wasps, the relationship between the virus and the caterpillar cannot be mutualistic: the virus will definitely harm its host. However, when the third party chimes in, the virus will fight for its territory by destroying the parasitic wasps’ eggs and inadvertently saving the caterpillar. Indeed, the virus works its way into the host for its own benefit. But unlike the parasitic wasps, the virus prevents the caterpillar from immediate death or death itself.
Xavier: What is the significance of this study? Is it an urgent issue that should continue to be researched?
The study helps researchers make hypothetical statements that evolution—natural selection—could have occurred over thousands of years to these caterpillars to survive viral infections. Scientists can presume that genes that create PKF could have been passed down to caterpillars’ offspring and be advantageous. The study further helps scientists understand what relationships living organisms, other than humans, may have influenced us and the ecosystem. The issue may not seem urgent, but considering the relationship between parasitic wasps, caterpillars, and viruses is likely to allow scientists to apply that attained knowledge to other fields in science that may develop our society and understanding of living organisms.
Jennah: Is this study only applicable to certain lepidopteran insects? For instance, the article specifically mentions the finding of PFK in a relationship between a parasitic wasp and the caterpillar. Are there other insects (or pairs of insects) that may benefit from this discovery?
So far, parasitoid-killing factors have been found in some entomopathogenic (microorganisms that cause diseases upon tiny insects) DNA virus families and lepidopteran genomes. Living organisms that share parasitic relationships with other organisms may benefit from PKF to prevent harm from being inflicted on the victim organisms.
Wooseok: Are phenomena like this common in the natural world? If so, is there an aspect that makes this concept unique?
Although parasitic relationships like the wasps and the caterpillars and the snails and the parasitic worms are relatively common in the natural world, the phenomenon of viruses protecting caterpillars that are attacked by parasitic wasps is new. Viruses have been sought to exclusively harm its hosts, not protect.
John: How do the parasitic wasps lay their eggs in the caterpillars?
A female wasp attacks a designated caterpillar, piercing into the host’s flesh with an ovipositor, a tube-like organ. As the wasp attacks, it also lays approximately 80 eggs into the caterpillar’s body cavity. Subsequently, the eggs hatch inside the caterpillar and feed off their host’s body.
Fabian: The article mentions that some parasitoid wasps are resistant to the PFK. What variants of wasps carry this trait, and what efforts are done by the scientists to overcome the wasps’ resistance?
Meteorus pulchricornis wasps, for instance, were not affected by the parasitoid-killing factors. Further details are yet to be found by scientists to answer why these wasps were not affected. However, discovering why some wasps are not affected will allow researchers to find new details on evolutionary relationships between the three organisms: parasitoid wasps, caterpillars, and viruses.
Works Cited
Jesús, Erin Garcia de. “Viruses Can Kill Wasp Larvae That Grow inside Infected Caterpillars.” Science News, 29 July 2021, www.sciencenews.org/article/viruses-wasp-larvae-caterpillars.
Melchor, Annie. “Genes Shared With Viruses Protect Caterpillars from Parasitic Wasps.” The Scientist Magazine®, www.the-scientist.com/news-opinion/viral-proteins-protect-caterpillars-from-parasitic-wasp-larvae-69047.
“Viruses Can Kill Managed Wasp Larvae in Infected Caterpillars.” Pledge Times, 2 Aug. 2021, pledgetimes.com/viruses-can-kill-managed-wasp-larvae-in-infected-caterpillars/.
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