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Abstract
One of the more unusual groups of insect pathogens consists of members of the family Polydnaviridae, insect DNA viruses that live in mutual symbioses with their associated parasitoid wasp (Hymentoptera) carriers until they are injected into specific lepidopteran hosts. Once inside this secondary host, polydnaviruses cause a wide variety of negative effects that ultimately ensure the survival of the parasitoid larvae. Because of their unusual life strategy and genetic features, it had been difficult to fully characterise polydnaviruses in terms of evolutionary history, replication cycle and functions in the host that might normally be well characterised for more conventional viruses. Recently, our understanding of polydnavirus evolutionary origins, gene content, genome organisation and functions in parasitism has greatly increased. Key findings are summarised in this review with emphasis on evolution of polydnavirus genes and genomes, their functional roles in insect pathology and their potential applications in insect biological control and biotechnology.