Functional annotation of Cotesia congregata bracovirus: identification of the viral genes expressed in parasitized host immune tissues.
Résumé
Bracoviruses (BV) from Polydnaviridae family are symbiotic viruses used as biological weapons by parasitoid wasps to manipulate lepidopteran host physiology and induce parasitism success. BV particles are produced by wasp ovaries and injected along with the eggs into the caterpillar host body where viral gene expression is necessary for wasp development. Recent sequencing of the proviral genome of Cotesia congregata BV (CcBV) identified 222 predicted virulence genes present on 35 proviral segments integrated in the wasp genome. To date, the expression of only a few selected candidate virulence genes had been studied in the caterpillar host, and we lacked a global vision of viral gene expression. In this study, a large-scale transcriptomic analysis by 454 sequencing of two immune tissues (fat body and hemocytes) of parasitized Manduca sexta caterpillar host allowed detection of expression of 88 CcBV genes expressed 24 h after the onset of parasitism. We could link the expression profile of these genes to several factors, showing that different regulatory mechanisms control viral gene expression in the host. These factors include presence of signal peptides in encoded proteins, diversification of promoter regions, and more surprisingly gene position on the proviral genome. Indeed, most genes for which expression could be detected are localized in particular proviral regions globally producing higher numbers of circles. Moreover this PDV transcriptomic analysis also reveals that a majority of CcBV genes possess at least one intron and an arthropod transcription start site consistent with an insect origin of these virulence genes.