When these parasitoid wasps lay their eggs in caterpillar hosts, they also leave behind a cache of viral DNA that creeps into the caterpillar’s genes and changes their physiology.
The natural world is rife with horrific cases of parasitism, from microbes that turn shrimp into cannibals to protozoans that brainwash mice into seeking out their feline predators. But things don’t always turn out poorly for the host. To spice up the eternal battle between predator and prey, evolution can even turn a deadly parasite into a blessing in disguise.
That’s what has happened between parasitic braconid wasps and their caterpillar hosts. Parasitoid wasps typically lay their eggs in other insects so that the larvae hatch right on top of an all-you-can-eat buffet. The class of braconid wasps spice up their flavor of parasitism by dispatching a virus along with their eggs, which incapacitates the caterpillar’s normal immune response and allows the eggs to flourish. This mutually beneficial relationship between braconid wasps and bracoviruses dates back at least 100 million years, and as a result their genetic material are intimately associated.
While studying the DNA of various lepidopteran (butterfly and moth) species, researchers from the University of Valencia in Spain and the University of Tours in France found traces of genes that belong to a bracovirus linked with the wasp Cotesia congregate. This wasp usually lays its eggs in caterpillars of the tobacco hornworm moth, Manduca sexta, so those bracovirus genes had no business invading silkworms, beet armyworm moths, fall armyworm moths, and Monarch butterflies.
Researchers Jean-Michel Drezen and Salvador Herrero initially thought the experiment had somehow become contaminated. But they confirmed the findings with new specimens from the field, where they found bracoviral DNA integrated into the lepidopteran genomes. This evidence clearly testified that viruses provide an avenue for horizontal gene transfer among different species of insects (as opposed to vertical gene transfer between parents and offspring through sexual reproduction).
When the researchers dug deeper, they discovered that one of the lepidopteran species, an armyworm, carried genes for a certain protein that actually looked more similar to the wasp version than to lepidopteran versions. The gene transfer had not only embedded foreign DNA into the host genomes, but also altered the host’s copies of its own DNA. This gene flow is an ancient, completely natural example of genetically modified insects.
And here’s where the caterpillars outsmarted the parasites and wrangled an advantage out of a seemingly toxic relationship: at least one bracovirus gene was found to provide the armyworm caterpillars with protection from baculovirus, a common insect pathogen. The armyworm had domesticated these viral intruders into useful genes that improve its own fitness.
But the burning question is how did these viral genes wind up in non-host species in the first place? Braconid larvae usually devour their hapless hosts long before the tobacco hornworm caterpillars get the chance to reproduce and pass down their infected genomes. So Drezen and Herrero hypothesize that braconid wasps just lay their eggs willy-nilly on whatever caterpillars they happen upon. Other lepidopterans aren’t quite as vulnerable to the bracovirus, so they can overthrow the braconid larval infection and survive with a battle trophy of bracoviral DNA.
This instance of genetic modification has obvious implications for our attempts to control agricultural pests. Some scientists want to take advantage of the natural parasitic behavior of these wasps to keep crop-munching caterpillars in check. But before we can weaponize parasitoid wasps, we’d have to make them resistant to the insecticides routinely sprayed in crop fields. If these wasps can transfer their genes to other species so easily, there’d be no way to control the spread of insecticide-resistance genes. Just as antibiotic resistance has run rampant throughout multiple populations of deadly bacteria, we’d have a crisis of super-pests on our hands.
The researchers plan to test their theory that the viral genes have propagated due to their advantageous effect, perhaps by suppressing the bracovirus genes in a new strain of armyworms and exposing them to baculoviruses. They also hope to find other instances of horizontal gene transfer that will further illuminate how these bracoviral genes have become intrinsic elements of so many lepidopteran genomes.