Abstract
Polydnaviruses are obligate symbionts integrated as proviruses in the genome of some ichneumonoid wasps that parasitize lepidopteran larvae. Polydnavirus free viral particles, which are injected into the host at oviposition, express virulence factors that impair immunity and development. To date, most studies have focused on the molecular mechanisms underpinning immunosuppression, whereas how viral genes disrupt the endocrine balance remains largely uninvestigated. Using Drosophila as a model system, the present report analyzes the function of a member of the ankyrin gene family of the bracovirus associated with Toxoneuron nigriceps, a larval parasitoid of the noctuid moth Heliothis virescens. We found that the TnBVank1 expression in the Drosophila prothoracic gland blocks the larval-pupal molt. This phenotype can be rescued by feeding the larvae with 20-hydroxyecdysone. The localization of the TnBVANK1 is restricted to the cytoplasm where it interacts with Hrs and Alix marked endosomes. Collectively, our data demonstrate that the TnBVANK1 protein acts as a virulence factor that causes the disruption of ecdysone biosynthesis and developmental arrest by impairing the vesicular traffic of ecdysteroid precursors in the prothoracic gland steroidogenic cells.
Highlights
Parasitic wasps represent the largest group of parasitoid insects which attack and parasitize a number of insect species, exploiting different developmental stages [1]
We found that four days after egg deposition (AED) the larvae expressing TnBVank1 did not significantly differ from control yw;hairy-Gal4 driver (h-Gal4) (n = 5; t = 0.8557; no significant (NS)) (Figure 1B)
PDVs are among the major host regulation factors used by parasitic wasps to subdue their hosts, which show immunosuppression and a number of developmental and reproductive alterations associated with disruption of their endocrine balance
Summary
Parasitic wasps represent the largest group of parasitoid insects which attack and parasitize a number of insect species, exploiting different developmental stages [1] These parasitic insects have a peculiar injection device, the ovipositor, which is used to deliver the egg along with host regulation factors that primarily disrupt the host immune reaction and endocrine balance to create a suitable environment for the development of their progeny [2,3]. These host regulation factors include viruses of the Polydnaviridae family, obligate symbionts of ichneumonid and braconid wasps attacking larval stages of lepidopteran hosts, and respectively classified in the genera Ichnovirus (IV) and Bracovirus (BV) [4]. Free PDV particles infect the host tissues without undergoing replication, and express virulence factors that alter host physiology in ways essential for offspring survival [5]
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