Parasitic wasps (in the order Hymenoptera) inject their eggs into lepidopteran hosts, where the eggs go through their developmental stages. Along with the eggs, the wasps also deliver viruses carrying genes encoding proteins that inhibit caterpillar immune defenses. Some of these genes are permanently transferred to the lepidopteran host where they have assumed new defensive functions against other viruses.
The viruses that parasitic wasps inject with their eggs, called Bracoviruses, are encoded in the wasp genome. About 100 million years ago a nudivirus genome integrated into the genome of a common wasp ancestor. With time the viral genes became dispersed in the wasp genome. The viruses produced by these wasps today no longer carry capsid coding genes – they are found only in the wasp genome – but only carry genes whose products can modulate lepidopteran defenses. Once in the lepidopteran host, these viruses deliver their genes but no longer form new particles.
An important question is whether wasp Bracoviruses can contribute genes to Lepidoptera – a process called horizontal gene transfer. This possibility would seem remote because the lepidopteran hosts for wasp larvae are dead ends – they die after serving as hosts for wasp development. However, it is possible that some hosts resist killing, or that wasps occasionally inject their eggs and viruses into the wrong host, one that can resist killing.
To answer this question, the genome sequence of Cotesia congregata bracovirus was compared with the genomes of a regular host as well as non-host Lepidoptera. Bracovirus DNA insertions were identified in genomes of the monarch, the silkworm, the beet armyworm and the fall armyworm, but not in the genome of the tobacco hornworm, the usual host of the wasp (C. congregata).
Not only were the Bracovirus sequences found in these varied Lepidoptera, but some appeared to be functional. Two such genes encode a protein that interferes with the replication of baculovirus, a known pathogen of Lepidoptera. This discovery was made in the process of producing the encoded proteins using baculovirus vectors! In other words, viral genes delivered by Hymenopteran wasps were appropriated by the Lepidoptera and used for their defense against a pathogen.
To put it another way, nature has carried out a gain-of-function experiment. Should we impose a moratorium?
The delivery of immunosuppressive viruses by wasps along with their eggs is by all accounts a remarkable story. The appropriation of some of these genes by the wrong hosts should not come as a surprise, yet the finding is nevertheless simply amazing. As long as we keep looking, we will find that the biological world is always full of new revelations.