Parasitic wasps, which lay their eggs inside living caterpillars and other insect larvae, carry viruses that help protect their eggs from the immune defenses of the host. According to a new Science Advances study, these wasps have captured viruses in their DNA repeatedly during their evolution, and the process was likely essential for their survival. The findings suggest that virus domestication – when organisms harbor viruses that benefit them – may be a widespread evolutionary mechanism.

Wasps carry viruses called polydnaviruses to help mediate the transfer of virulence genes. Integrating into the DNA of the host cell causes changes in the caterpillar’s immune defenses, allowing the parasite larva to colonize and consume it. There are some wasp species that use virus-like particles (or VLPs) to protect their eggs against encapsulation – an immune response that involves engulfing foreign bodies in a sheath of immune cells. But unlike polydnaviruses, VLPs don’t contain DNA. Why some wasps do this is still unclear, and whether or not VLPs are even of viral origin has been hotly debated since the 1970s.

To investigate, a team led by Jean-Michel Drezen from CNRS-Université François Rabelais de Tours and Anne-Nathalie Volkoff of Université de Montpellier studied Venturia canescens parasitoid wasps that come from populations collected in the Valence area of France. This species uses VLPs to protect its eggs from the immune responses of the host, or in this case, the larvae of the Mediterranean flour moth (Ephestia kuehniella). Using a comprehensive, deep sequencing analysis of RNA synthesized in the ovarian tissues that produce VLPs, the team found that these do, in fact, originate from viruses. 

The particles are produced from the domestication of a virus from the nudivirus group, and when they sequenced the whole genome of V. canescens, the team found that all of the nudiviral genes were grouped into clusters flanked by regions containing wasp genes – indicating how they’re an integral part of the wasp’s genetic material. 

The VLPs introduce immunosuppressive proteins into the immune cells of the host caterpillar. Specifically, they operate like natural viral “fat bodies” (or liposomes) that wrap virulence proteins in a lipid bilayer and transport them into the target cells of hosts. That means virus integration occurred repeatedly during the evolution of parasitic wasps – conferring different functions such as gene transfer and protein delivery.

^{Venturia canescens female with Ephestia kuehniella host. Thomas Steinemann/IRBI-CNRS}

^{Image in text: Colorized transmission electron microscopy image of Venturia canescens VLPs budding from the wasp oviduct cells. Marc Ravallec/DGIMI-INRA}