Effects of the polydnavirus of Cotesia congregata on the immune system and development of non-habitual hosts of the parasitoid.

Polydnaviruses (PDV) are obligate mutualistic symbionts found in association with some groups of parasitic Hymenoptera. In these groups, they suppress the immune response of the parasitoid's host and are required for successful parasitoid reproduction. Several PDV effects have been described in different experimental systems, but no clear picture of PDV mode of immunosuppression has emerged. No study to date has directly tested if PDV modes of action are evolutionarily conserved or divergent among parasitoid taxa within the Ichneumonoidea. We hypothesize the divergence in PDV mode of immunosuppression can be detected by identifying points of divergence in the immune response of different host species to PDV from one parasitoid species. This study tests the effects of purified PDV from Cotesia congregata on the immune response of three larval lepidopteran species that naturally are hosts of parasitoid species that differ in taxonomic relatedness to C. congregata. Here we demonstrate that despite associations with distantly related parasitoids (Ichneumonidae and Braconidae), Manduca sexta and Heliothis virescens showed similar patterns of increased glucose dehydrogenase (GLD) activity, suppressed cellular encapsulation in vitro, and increased time to pupation. In contrast, Lymantria dispar showed no response to C. congregata PDV across any of the parameters measured, even though it has an evolutionary association with several parasitoids closely related to C. congregata and within the Microgastrinae. The PDV immunosuppression in H. virescens and M. sexta does not correlate with host molecular phylogeny either. The suborganismal effects shown in M. sexta and H. virescens translated into significantly reduced pupation success in M. sexta only. Results demonstrate that while some PDV modes of immunosuppression in hosts may be divergent, others may be conserved across broad host groups.