If you’re a caterpillar, you do not want to meet a parasitic wasp. The winged insect will inject you full of eggs, which will grow inside your body, develop into larvae, and hatch from your corpse. But a new study reveals that wasps have given caterpillars something beneficial during these attacks as well: pieces of viral DNA that become part of the caterpillar genome, protecting them against an entirely different lethal virus. In essence, the wasps have turned caterpillars into genetically modified organisms.
“The key strength of the study is it clearly demonstrates that [viruses] have been a source of horizontal gene transfer for some insects,” says parasitic wasp expert Michael Strand of the University of Georgia, Athens, who was not involved in the study.
Study author and biologist Jean-Michel Drezen of François Rabelais University in Tours, France, has been studying the parasitic relationship between the wasps and their lepidopteran (butterflies and moths) victims for decades. He specializes in bracoviruses, which are injected by the wasps along with their eggs. Once inside the caterpillar, the bracovirus prevents a normal immune response by disrupting the cell’s cytoskeleton—a network of filamentlike proteins responsible for moving components and machinery around the cell. Without an immune response, the wasp eggs are free to grow unchecked.
But the discovery that the caterpillars have taken up and repurposed bits of the wasp virus for their own means came as a surprise. "I couldn’t believe it,” Drezen says. “We did not expect this at all.”
To understand the interplay between wasp, virus, and caterpillar, the scientists analyzed a computer database containing DNA from multiple species of moths and butterflies, including the iconic monarch, for sequences similar to a known segment of bracovirus DNA. The initial search showed that, indeed, certain segments of the lepidopteran genome closely resembled bracovirus sequences. At some points in evolutionary history—those rare cases where wasp parasitization did not prove fatal for the caterpillars—it seems that the injected virus genes found their way into germline cells and have been passed down in several lepidopteran lineages.
The researchers then confirmed the computer search by grinding up real insect specimens, extracting the DNA, and searching for the presence of bracovirus integrations. Not only did they find the evidence they were looking for, they also found the virus sometimes “picks up” genes from its original wasp host and brings those along, too. In simpler terms, this means wasp genes riding on a virus have been found within the caterpillar genome.
More surprising still was the finding that both the wasp and virus genes are active inside the caterpillars and have been passed down over many generations. This led the scientists to wonder whether the caterpillars had found their own use for the parasitic genes.
Further tests, both in cell cultures and in live specimens, revealed that the genes acquired from the wasp virus appear to protect the caterpillars from a different (but similarly named) pathogen called baculovirus. The baculoviridae family infects many different insects, and, in the case of moths and caterpillars, is usually acquired by eating leaves contaminated with the infectious particles. In the course of a normal infection, baculoviruses make heavy use of the cell’s cytoskeletal system in order to reproduce inside the caterpillar. The team’s results—reported online today in PLOS Genetics—suggest that the same bracovirus genes that disrupt the cytoskeleton and allow the wasp eggs to grow also cause enough disruption to inhibit the growth of baculovirus. In effect, the genes from the wasp virus impair the lepidopteran cells’ normal functions, but the disruption may be enough to keep out baculovirus—a silver lining on an otherwise nasty history of parasitization.
The authors point out that the addition of wasp genes to the butterfly and moth DNA means many lepidopteran species are naturally occurring genetically modified organisms. This type of gene transfer has been observed between other species in the wild before, mostly between bacteria (which have been known to share genes for antibiotic resistance). The scientists say that the finding should remind us of two things: that genetically modified organisms occur naturally and that any gene introduced into an ecosystem has the potential to spread between species. “We have very simple principles that, in practical situations, can be very, very complex,” Drezen says.