Global transcriptional profile of Tranosema rostrale ichnovirus genes in infected lepidopteran hosts and wasp ovaries

High temperature induces downregulation of polydnavirus gene transcription in lepidopteran host and enhances accumulation of host immunity gene transcripts

Endoparasitoids face the challenge of overcoming the immune reaction of their hosts, which typically consists of encapsulation and melanisation of parasitoid eggs or larvae. Some endoparasitic wasps such as the solitary Tranosema rostrale (Hymenoptera: Ichneumonidae) that lay their eggs in larvae of the spruce budworm, Choristoneura fumiferana (Lepidoptera: Tortricidae), have evolved a symbiotic relationship with a polydnavirus (PDV), which in turn helps them suppress the host's immune response. We observed an increase in mortality of immature T. rostrale with increasing temperature, and we tested two hypotheses about the mechanisms involved: high temperatures (1) hamper the expression of T. rostrale PDV genes and (2) enhance the expression of spruce budworm immunity-related genes. Dissections of parasitized spruce budworm larvae reared at 30°C revealed that most parasitoid eggs or larvae had died as a result of encapsulation and melanisation by the host. A qPCR analysis of T. rostrale PDV (TrIV) gene expression showed that the transcription of several TrIV genes in host larvae was downregulated at high temperature. On the other hand, encapsulation, but not melanisation, of foreign bodies in spruce budworm larvae was enhanced at high temperatures, as shown by the injection of Sephadex™ beads into larvae. However, at the molecular level, the transcription of genes related to spruce budworm's melanisation process (prophenoloxidase 1 and 2) was upregulated. Our results support the hypothesis that a temperature-dependent increase of encapsulation response is due to the combined effects of reduced expression of TrIV genes and enhanced expression of host immune genes.

Specificities of ichnoviruses associated with campoplegine wasps: genome, genes and role in host-parasitoid interaction

Ichnoviruses (IVs), unique symbiotic viruses carried by ichneumonid campoplegine wasps, derive from integration of a paleo-ichnovirus into an ancestral wasp genome. The modern 'genome' is composed of both regions that are amplified, circularized and encapsidated into viral particles and non-encapsidated viral genomic regions involved in particle morphogenesis. Packaged genomes include multiple circular dsDNAs encoding many genes mostly organized in gene families. Virus particles are assembled in specialized ovarian cells from which they exit into the oviduct lumen; mature virions are injected during oviposition into the insect host. Expression of viral proteins in infected cells correlates with physiological alterations of the host enabling success of parasitism.

Extensive transcription analysis of the Hyposoter didymator Ichnovirus genome in permissive and non-permissive lepidopteran host species

Ichnoviruses are large dsDNA viruses that belong to the Polydnaviridae family. They are specifically associated with endoparasitic wasps of the family Ichneumonidae and essential for host parasitization by these wasps. We sequenced the Hyposoter didymator Ichnovirus (HdIV) encapsidated genome for further analysis of the transcription pattern of the entire set of HdIV genes following the parasitization of four different lepidopteran host species. The HdIV genome was found to consist of at least 50 circular dsDNA molecules, carrying 135 genes, 98 of which formed 18 gene families. The HdIV genome had general features typical of Ichnovirus (IV) genomes and closely resembled that of the IV carried by Hyposoter fugitivus. Subsequent transcriptomic analysis with Illumina technology during the course of Spodoptera frugiperda parasitization led to the identification of a small subset of less than 30 genes with high RPKM values in permissive hosts, consisting with these genes encoding crucial virulence proteins. Comparisons of HdIV expression profiles between host species revealed differences in transcript levels for given HdIV genes between two permissive hosts, S. frugiperda and Pseudoplusia includens. However, we found no evident intrafamily gene-specific transcription pattern consistent with the presence of multigenic families within IV genomes reflecting an ability of the wasps concerned to exploit different host species. Interestingly, in two non-permissive hosts, Mamestra brassiccae and Anticarsia gemmatalis (most of the parasitoid eggs were eliminated by the host cellular immune response), HdIV genes were generally less strongly transcribed than in permissive hosts. This suggests that successful parasitism is dependent on the expression of given HdIV genes exceeding a particular threshold value. These results raise questions about the mecanisms involved in regulating IV gene expression according to the nature of the lepidopteran host species encountered.

Characterization of the polydnaviral ‘T. rostrale virus’ (TrV) gene family: TrV1 expression inhibits in vitro cell proliferation

Tranosema rostrale ichnovirus (TrIV) is a polydnavirus (PDV) transmitted by the endoparasitic wasp T. rostrale to its host Choristoneura fumiferana during oviposition. PDV genes are expressed in infected caterpillars, causing physiological disturbances that promote the survival of the developing endoparasite. The previously sequenced genome of TrIV contains ~86 genes organized in multigene families and distributed on multiple segments of circular dsDNA. Among these, the ‘T. rostrale virus’ (TrV) family comprises seven genes that are absent in other PDV genomes examined to date and whose function(s) remain(s) unknown. Here, we initiated a functional analysis of the TrV family using qPCR, transfection and RNAi approaches. TrV family genes were weakly expressed in wasp ovaries, but some displayed high transcript abundance in parasitized caterpillars. Whilst TrV1 was the most highly transcribed TrV gene in infected caterpillars, transcript levels for TrV5 and TrV6 were nearly undetectable, indicating that they may be pseudogenes. Temporal and tissue-specific patterns of transcript abundance were similar for all expressed TrV family genes, indicative of an apparent lack of difference in function or tissue specificity. Infection of Cf-203 and Sf-21 insect cells with TrIV led to a dose-dependent inhibition of cell proliferation with no sign of apoptosis. Whilst similar inhibition was observed following transfection of cells with a cloned genome segment carrying the TrV1 gene, RNA interference targeting TrV1 largely restored cell growth in TrIV-infected cells, indicating that TrV1 expression was responsible for the observed inhibition. We suggest that TrV genes may contribute to host developmental disruption by interfering with host-cell proliferation during parasitism.