Characterization of the IκB-like gene family in polydnaviruses associated with wasps belonging to different Braconid subfamilies

Abstract

Polydnaviruses (PDVs) are obligate symbionts of hymenopteran parasitoids of lepidopteran larvae that induce host immunosuppression and physiological redirection. PDVs include bracoviruses (BVs) and ichnoviruses (IVs), which are associated with braconid and ichneumonid wasps, respectively. In this study, the gene family encoding IkB-like proteins in the BVs associated with Cotesia congregata (CcBV) and Toxoneuron nigriceps (TnBV) was analysed. PDV-encoded IkBlike proteins (ANK) are similar to insect and mammalian IkB, an inhibitor of the transcription factor nuclear factor kB (NF-kB), but display shorter ankyrin domains and lack the regulatory domains for signal-mediated degradation and turnover. Phylogenetic analysis of ANK proteins indicates that those of IVs and BVs are closely related, even though these two taxa are believed to lack a common ancestor. Starting from a few hours after parasitization, the transcripts of BV ank genes were detected, at different levels, in several host tissues. The structure of the predicted proteins suggests that they may stably bind NF-kB/Rel transcription factors of the tumour necrosis factor (TNF)/Toll immune pathway. Accordingly, after bacterial challenge of Heliothis virescens host larvae parasitized by T. nigriceps, NF-kB immunoreactive material failed to enter the nucleus of host haemocytes and fat body cells. Moreover, transfection experiments in human HeLa cells demonstrated that a TnBV ank1 gene product reduced the efficiency of the TNF-a-induced expression of a reporter gene under NF-kB transcriptional control. Altogether, these results suggest strongly that TnBV ANK proteins cause retention of NF-kB/Rel factors in the cytoplasm and may thus contribute to suppression of the immune response in parasitized host larvae.

Functional Annotation of Cotesia congregata Bracovirus: Identification of Viral Genes Expressed in Parasitized Host Immune Tissues

“…Expression was also examined for ptpp, detected by 454 sequencing only in hemocytes; vank-4, detected only in fat body; and ptph, for which expression was not detected by 454 sequencing in both tissues. All three genes had previously been reported to be expressed by using a multiplex RT-PCR analysis (33,35). Taken together, these qRT-PCR analyses confirmed the trend observed by transcriptome analysis (Fig.…”

“…There were, however, some exceptions to this trend: two circles (C26 and C23) that are abundantly produced possess PTP and VANK genes that are expressed at low levels by M. sexta fat body and hemocytes. Interestingly, the proteins encoded by these genes lack peptide signals and potentially act intracellularly (35,68). Selection could be acting to maintain high-level production and encapsidation of these circles by the ovaries of female wasps, thereby enabling the widespread intracellular production of CcBV virulence genes expressed at low levels.…”

“…They were then maintained under the same rearing conditions for 24 h, as were five M. sexta larvae from the same cohort, which were used as controls. The time point of 24 h postoviposition was chosen for expression analysis for consistency with previous studies (33)(34)(35) and also because physiological effects on hemocytes are visible at this time point (28). Cat-erpillars were anesthetized for 10 min on ice before dissection.…”

“…This relative CcBV gene expression analysis by 454 sequencing did not allow the detection of expression of certain CcBV genes that had previously been reported to be expressed by using RT-PCR (13 PTP genes and 3 VANK genes) (33,35). The expression levels of these genes were therefore below the limit of detection of the 454 analysis, which had been set to a minimum depth of 5 reads.…”

When parasitic wasps hijacked viruses: genomic and functional evolution of polydnaviruses

“…Among the genes transferred into BVs, two families of virulence genes are particularly remarkable, the ankyrin and the ptp genes [59][60][61][62]. Apart from CiBV (Cheloninae), these genes are found in all BVs studied, including Toxoneuron nigriceps BV (Cardiochilinae) and MdBV, Glyptapanteles BV and Cotesia BV (Microgastrinae).…”