Venomics of the ectoparasitoid wasp Bracon nigricans.

BACKGROUND: Venom is one of the most important sources of regulation factors used by parasitic Hymenoptera to redirect host physiology in favour of the developing offspring. This has stimulated a number of studies, both at functional and "omics" level, which, however, are still quite limited for ectophagous parasitoids that permanently paralyze and suppress their victims (i.e., idiobiont parasitoids). RESULTS: Here we present a combined transcriptomic and proteomic study of the venom of the generalist idiobiont wasp Bracon nigricans, an ectophagous larval parasitoid of different lepidopteran species, for which we recently described the host regulation strategy and the functional role of the venom in the induction of physiological changes in parasitized hosts. The experimental approach used led to the identification of the main components of B. nigricans venom involved in host regulation. Enzymes degrading lipids, proteins and carbohydrates are likely involved in the mobilization of storage nutrients from the fat body and may concurrently be responsible for the release of neurotoxic fatty acids inducing paralysis, and for the modulation of host immune responses. CONCLUSION: The present work contributes to fill the gap of knowledge on venom composition in ectoparasitoid wasps, and, along with our previous physiological study on this species, provides the foundation on which to develop a functional model of host regulation, based both on physiological and molecular data. This paves the way towards a better understanding of parasitism evolution in the basal lineages of Hymenoptera and to the possible exploitation of venom as source of bioinsecticidal molecules.

Host regulation by the ectophagous parasitoid wasp Bracon nigricans.

The host regulation process has been widely investigated in endophagous parasitoid wasps, which in most cases finely interact with living hosts (i.e. koinobiont parasitoids). In contrast, only very limited information is available for ectophagous parasitoids that permanently paralyze and rapidly suppress their victims (i.e. idiobiont parasitoids). Here we try to fill this research gap by investigating the host regulation by Bracon nigricans, an ectophagous idiobiont wasp species. Parasitism, mainly by venom action, is able to redirect host metabolism in order to enhance its nutritional suitability for the developing parasitoid larvae and to provide the required metabolic support to host tissues. The observed alterations of the host titers of haemolymph proteins, carbohydrates and acylglycerols are associated with a parasitoid-induced mobilization of nutrients stored in the fat body. This tissue undergoes a controlled degradation mediated by a close surface interaction with haemocytes, where a cathepsin L activity is localized, as demonstrated by immunolocalization, biochemical and transcriptional data. B. nigricans parasitism does not markedly influence the survival of haemocytes, even though a persistent suppression of the immune competence is observed in parasitized hosts, which show a reduced capacity to encapsulate and melanize non-self objects. These immune alterations likely allow a more efficient food uptake and use by the ectophagous larvae. The obtained results indicate that the host regulation process in basal lineages of parasitic Hymenoptera is more complex than expected and shares functional similarities with adaptive strategies occurring in derived koinobiont species.