Viral genes protect butterflies and moths from parasitoids

A virus that infects lepidopteran insects – moths and butterflies – confers protection against the parasitic wasps that seek to lay eggs within the moths’ and butterflies’ still-living bodies by inhibiting the development of parasitic wasp larvae there, according to a new study. The results highlight a previously unknown adaptive weapon in the ever-ongoing evolutionary arms race between parasitoids, viruses and their shared insect hosts that may allow the viruses to outcompete parasitoids within the same host. The findings also reveal a novel gene family in viruses – parasitoid killing factor (pkf) – that encodes proteins toxic to parasitoids, and which, in some cases, are incorporated into the lepidopteran host’s genome. Lepidoptera are routinely targeted by both parasitoid wasps and entomopathogenic viruses that compete for the same host resources, a dynamic that has resulted in an evolutionary chain of adaptation and counteradaptation between pathogen, parasitoid and their shared insect hosts. Since viruses are particularly well-suited to mediate gene transfer between different hosts, the virus-parasitoid-lepidopteran host arms race may reflect selection for horizontal gene transfer between the interacting organisms. Within a subset of insect viruses and several lepidopteran insects they infect, Laila Gasmi and colleagues discovered pkf genes, which, when expressed, inhibit the development of parasitic wasp larvae in virus-infected hosts. According to Gasmi et al., horizontal transmission of pkf genes among viruses and their hosts has provided insects a molecular mechanism of defense against parasites. “Our results suggest that the driving force in the virus-parasitoid-lepidopteran host arms race is derived not only from individual relationships of natural enemies and their hosts but also by gene transfers that provide target-specific weapons to outcompete parasitoid competitors,” write the authors.