Symbiotic associations traditionally have been treated as evolutionary curios rather than as a major source of evolutionary innovation. Recent research on a wide variety of organisms is changing this view and is breaking down the barriers between the traditional categories of parasitism, commensalism and mutualism, to produce a more flexible view of multispecific interactions. An especially abundant, but little discussed, mutualism exists between parasitoid wasps in the superfamily Ichneumonoidea and a novel form of DNA viruses known as polydnaviruses. Mutualisms between viruses and eukaryotes are not often reported, although as many as 100 000 species of organisms may exhibit this unusual association. In this review Jim Whitfield considers what is known about the parasitoid-polydnavirus relationship and how (and from what) it might have arisen.
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