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Study Finds Parasitoid Wasp, Symbiotic Virus Sequences in Butterfly Genomes

NEW YORK (GenomeWeb) – Parasitic wasps have left lasting marks on some butterfly and wasp genomes, according to a new study in PLOS Genetics, which found braconid wasp sequences and/or related viral genes in several of these species.

Researchers from Spain and France scrutinized genome sequences from organisms in the Lepidoptera order, comparing those from butterflies that act as a host for reproducing braconid wasps and those that don't. Braconid wasps are known for injecting their eggs into the caterpillars of some lepidopterans, they explained, using massive, symbiotic bracoviruses to circumvent caterpillar immunity and control its development while they are colonized by wasp larvae.

The team's results reveal the extent to which such viruses cart wasp DNA into the caterpillar during this parasitic process, producing horizontal gene transfer events that pepper butterfly and moth genomes with bracovirus sequences and even, in some cases, wasp genes.

"Integration of parts of the viral genome into host caterpillar DNA strongly suggests that integration can sporadically occur in the germline, leading to the production of lepidopteran lineages that harbor bracovirus sequences," according to corresponding authors Jean-Michel Drezen, with CNRS, and University of Valencia genetics researcher Salvador Herrero, and colleagues.

"Moreover, some of the transferred bracovirus genes reported here originate from the wasp genome, demonstrating that a gene flux exists between the two insect orders Hymenoptera and Lepidoptera that diverged [around 300 million years ago]," they added.

As braconid wasp eggs are injected into butterfly and moth host caterpillars, they carry with them viral particles put together in the wasp's ovaries, the team explained. But while researchers have known that the viral symbionts play a crucial role in helping the braconid wasp control their caterpillar host, less was known about potential horizontal gene transfer events mediated by the bracoviruses.

Though parasitized caterpillars don't typically metamorphose into butterflies — or even survive the parasitic event — the researchers speculated that there may be instances in which they do successfully complete their life cycles, perhaps carrying viral and wasp DNA along with them.

To explore that possibility, the team started by searching butterfly or wasp genomes for sequences from Cotesia congregata bracovirus (CcBV) — a sequenced virus that's symbiotic with the parasitoid wasp Cotesia congregata.

The researchers did not find CcBV DNA in the genome of the tobacco hornworm (Manduca sexta), a frequent C. congregata host. But they did detect DNA from the bracovirus in the genomes of lepidopterans that aren't used as hosts for the wasp, including the monarch butterfly, silkworm, beet armyworm, and fall armyworm.

In the monarch butterfly genome, for example, the team tracked down three CcBV sequence insertions, including insertions that contained sequences resembling the bracovirus' RnaseT2, Ben9, and Ben4 genes. Targeted sequencing revealed CcBV-like sequences in five more monarch individuals from Canada and Australia.

And the researchers saw further examples of Ben9 and Ben4 bracovirus gene insertions when they tested 88 more representatives from the monarch butterfly and several related species.

Moreover, based on the selection signals associated with these sequences, the group speculated that the genes may be functional and even beneficial in butterfly genomes, perhaps providing some protection against other potential viral invaders.

Likewise, the analysis uncovered the Ben5 gene and other CcBV sequences in the silkworm genome, while the beet armyworm and fall armyworm genomes were home to both bracovirus and wasp sequences.

"Altogether, our results strongly suggest that two acquired genes can confer an advantage against viral infection, although the comprehensive analysis of the molecular function of the identified proteins is awaited," the study's authors noted.