Along with their eggs, parasitic wasps are injecting DNA into silkworms and the caterpillars of monarch butterflies – making them naturally occurring genetically modified organisms, according to a new PLOS Genetics study published this week. 

Symbiotic viruses called bracoviruses are associated with tens of thousands parasitic wasp species that develop within the body of butterfly and moth hosts. Viral particles produced in wasp ovaries are injected into caterpillars along with the wasp eggs. Once there, the viral DNA integrates into the DNA of the host cell. This induces changes in the caterpillar’s immune defenses that allow the developing wasp larvae to colonize and consume it. 

To investigate this relationship further, a team led by Salvador Herrero of the University of Valencia and Jean-Michel Drezen of the University of Tours analyzed DNA from multiple moth and butterfly species using existing databases. The researchers identified bracovirus DNA sequences in the genomes of the monarch (Danaus plexippus), silkworm (Bombyx mori), beet armyworm (Spodoptera exigua), and fall armyworm (Spodoptera frugiperda). 

To confirm these bracovirus insertions, the researchers then ground up frozen larvae specimens and extracted their DNA. They found that bracoviruses incorporate genes from their original wasp host and bring those along. As described in Science, wasp genes riding on a virus have been found within the caterpillar genome. In rare cases where the parasitization didn’t kill the caterpillar – perhaps the wasp laid the eggs in the wrong host – bracovirus genes could become integrated into the genome of the caterpillar's developing egg or sperm, New Scientist explains. And in those cases, they’d be passed down to the next generation of caterpillars. If they’re somehow advantageous, they might be selected for and become a permanent part of the species' genome. 

Furthermore, these integrated genes aren’t just remnants, they’re active. The acquired genes have since become domesticated, and they seem to protect butterflies and moths against other pathogenic viruses.