Ectopic Expression of Virus Innexins in Heliothis virescens Disrupts Hemocyte-Mediated Encapsulation and Host Viability

Abstract

1.Polydnaviruses are dsDNA viruses associated with endoparasitoid wasps. Delivery of the virus during parasitization of a caterpillar and subsequent virus gene expression is required for production of an amenable environment for parasitoid offspring development. Consequently, understanding of Polydnavirus gene function provides insight into mechanisms of host susceptibility and parasitoid wasp host range. Polydnavirus genes predominantly are arranged in multimember gene families, one of which is the vinnexins, which are virus homologues of insect gap junction genes, the innexins. Previous studies of Campoletis sonorensis Ichnovirus Vinnexins using various heterologous systems have suggested the four encoded members may provide different functionality in the infected caterpillar host. Here, we expressed two of the members, vnxG and vnxQ2, using recombinant baculoviruses in susceptible host, the caterpillar Heliothis virescens. Following intrahemocoelic injections, we observed >90% of hemocytes (blood cells) were infected, producing recombinant protein. Larvae infected with a vinnexin-recombinant baculovirus exhibited significantly reduced molting rates relative to larvae infected with a control recombinant baculovirus and mock infected larvae. Similarly, larvae infected with vinnexin-recombinant baculoviruses were less likely to molt relative to controls, and showed reduced ability to encapsulate chromatography beads in an immune assay. In most assays, the VnxG protein was associated with more severe pathology than VnxQ2. These results, in light of previous findings, support that Polydnavirus Vinnexin gene family members may provide complementary, rather than redundant, effects. This in turn indicates a need to test gene family member functionality across infected hosts for effects to determine member contribution to host range. 2. ImportancePolydnaviruses are obligate mutualistic associates of highly speciose wasp taxa that parasitize caterpillars. Expression of Polydnavirus-encoded genes in hosts parasitized by wasps is necessary for successful parasitization, and an unusual genome structure including multiple-membered gene families is hypothesized to contribute to host manipulation. We have tested this hypothesis by in vivo expression of two members of a family of Polydnavirus homologues of Innexins, or insect gap junction proteins. Previous findings demonstrated that the two Vinnexins induce different physiological alterations in heterologous systems. Here, in host caterpillars, we observed differential alteration by the two proteins of host immune cell (hemocyte) bioelectrical physiology and the immune response of encapsulation. Not only do our data suggest a linkage between cellular bioelectricity and immunity in insects, but they support that gene family expansion has functional consequences to both Polydnavirus and host wasp success.