The associations between several thousands of parasitic wasps and viruses (polydnaviruses or PDVs) represent a unique example where a complex virus machinery has been domesticated by one organism (the parasitic wasp) to manipulate the physiology of another (the parasitized lepidopteran larval host). The viral machinery produces particles used to transfer and express virulence genes encoding factors that impair host immunity. PDV particles, injected along with parasite eggs into the host body, manipulate host immune defences and development, thus enabling wasp larvae to survive and develop in a potentially harmful environment. Viral particle production occurs exclusively in specialized cells of the wasp's ovaries and PDVs are vertically transmitted. Two PDV genera have been described, Ichnovirus and Bracovirus, which are associated with thousands of wasp species from the subfamilies Campopleginae and Braconidae respectively.
In the first part of this chapter we will present the characteristics of PDVs and the origin of the genes they encode. We will then describe the general effect of PDVs on the insect immune system, and finally we will review functional analyses of PDV products.
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