The monarch butterfly, along with the silkworm and many other types of butterflies, have been found to be genetically modified by parasites, more specifically, genes from parasitic wasps. Researchers discovered that "These genes were acquired through a virus that weaves in and out of DNA...Parasitic insects known as braconid wasps lay their eggs inside the caterpillars of butterflies and moths. The wasp larvae that hatch from the eggs typically kill the host caterpillars" (Live Science). The wasps also inject what is called a bracovirus, which can then incorporate themselves into the genomes of caterpillars. These viruses benefit the parasite, allowing it to live freely inside it's host, the caterpillar, and from there, can then weave themselves into the host's genomes, allowing the virus to inhibit the immune defense of the host, protecting the larvae of the wasp (Live Science).
Butterflies may, sometimes, repel these parasites, by eliminating the eggs, or killing the larvae. They can also repel these attacks if a different species of wasps, that doesn't normally parasitize that species of butterfly, attacks that butterfly. However, this parasite has a mutual relationship with its host. Just as the butterfly, or caterpillar, helps the parasite by giving it a safe place to protect the wasp larvae, the parasite returns the favor by protecting the host of other viruses. The caterpillars and butterflies that survive such attacks, can then go on to produce offspring with the newly modified genes.
I found this to be very interesting because usually we see GMOs (genetically modified organisms) being created within science labs; however, this article showed GMOs be created within nature itself.