Despite their independent origin all PDVs have common structural features, most probably because they have to evolve within the constraints imposed to maintain an effective association with the wasp, leading to successful parasitism. For example, all PDV genomes comprise large gene families: the existence of multiple variants of the same gene in PDVs may provide the means to interact with related signalling pathways in different tissues of the parasitized host. Moreover, the gene content of the particles appears to reflect the physiology of the host-parasitoid interaction rather than the type of virus captured originally. Indeed, no common genes are found within CiBV and other bracovirus genomes (Weber et al, 2007), although they are produced by the same viral machinery and originate from the same ancestral wasp-virus association (Bézier et al., 2008). The particular gene content of CiBV might be explained by the Cheloninae wasp life cycle. C. inanitus females oviposit into the eggs of the lepidopteran host (i.e. they are ovo-larval parasitoids). Although lepidopteran embryos have been shown to respond to parasitism by expressing a number of immune-related proteins (Abdel-Latief and Hilker, 2008), it is possible that Chelonus eggs and embryos are subject to only a limited response by the host cellular immune system. Therefore the genes maintained in CiBV particles are likely to be involved mainly in the control of host development. In contrast, bracoviruses from other wasps that oviposit directly into larval hosts are required to defend themselves against host cellular defences immediately. For this reason they have IkB and cysteine-motif genes in common with ichnovi-ruses (Dupuy et al., 2006; Falabella et al., 2007), as well as PTP genes in common with GfV (Lapointe et al., 2007), suggesting that these factors have been selected by convergent evolution and may thus play a key role in the control of host immunity.
An interesting question concerns the origin of virulence genes and the way in which they have been acquired by PDVs. It was originally proposed that PDV virulence genes might originate from genes encoding venom products involved in parasitism success (Webb and Summers, 1990). Accordingly many potential virulence products are made up of a truncated form of a conserved eukaryotic protein (such as cactus/ IkB proteins) or a single protein domain (such as PTPs). However, surprisingly, they are not particularly close to insect proteins, most of them sharing less than 60% similarity with proteins from insects, birds, or mammals (Bezier et al, 2008). The lack of a clear phylogenetic link between bracovirus and insect proteins may reflect the fact that bracovirus factors are evolving at a very fast rate due to their involvement in host-parasite interactions. In support of this interpretation, bracovirus PTPs and hcB-like proteins are less closely related among themselves than are the corresponding homologous proteins from different insect orders while C. congregata housekeeping genes are closely related to those of Apis mellifera and Nasonia vitripennis, indicating that the high divergence observed for hcB-like and PTPs sequences is not a general trend of the parasitoid wasp genes but a specific feature of bra-covirus genes (Bezier et al, 2008).
It is possible that some bracovirus virulence genes might have been present in the genome of the ancestral virus since viruses are known to pick up cellular genes that are beneficial for their life cycle in infected hosts (Herniou et al., 2003). This might explain the high divergence rate of bra-covirus genes, which, as genes from pathogenic viruses, are likely to have evolved rapidly and over a long period of time. An alternative hypothesis is that bracovirus virulence genes were acquired after the integration of the ancestor virus; originally residing in a non-viral region of the wasp genome, these genes were transferred to the proviral form at different times during the radiation of the microgastroid complex, leading to their incorporation into virus particles. In the case of bracovirus cystatins (Espagne et al., 2005) and ichnovirus vin-nexins (Turnbull and Webb, 2002) PDV genes lack the introns present in the cellular copies, suggesting these genes were acquired via integration of cDNA into the proviruses (Espagne et al, 2005). Human long interspersed element retrotrans-posons have been shown to integrate transcribed DNA sequences in the genome. This results in genes which may fulfil new physiological functions (Esnault et al., 2000). In the case of the aspartyl protease present in the Toxoneuron nigriceps bracovirus
(TnBV) genome, this gene has a clear retroviral origin: it was most probably acquired following the integration of a retroviral element in the bracovirus chromosomal form (Falabella et al., 2003). The fact that this gene is highly expressed in parasitized host haemocytes, fat body, and prothoracic glands suggests it has a physiological function.
Some data suggest the possibility that virulence genes may also have been acquired within lepidop-teran hosts during the development of wasp larvae. Co-infection of hosts by wasps from different species does not result experimentally in detectable genetic exchange between their associated viruses (Stoltz et al., 1986). However, over large periods of time rare events of this type may occur, and additionally genes could also be acquired from other viruses not associated with parasitoids (Drezen et al., 2006; Bigot et al., 2008). In what appears to be an example of the latter type, bracoviruses from the Cotesia genus contain a copy of the baculovirus GP94 gene (Drezen et al., 2006) which is probably functional in CvBV. Surprisingly the bracovirus gene is phylogenetically most closely related to a particular lineage of lepidopteran baculoviruses (Xestia-c nigrum granulovirus) and not to hymen-opteran baculoviruses, suggesting that the gene was acquired from a baculovirus of this lineage before the radiation of the Cotesia genus (Drezen et al., 2006). The bracovirus genomes of Cotesia species also contain a gene conserved in ascoviruses, a group of lepidopteran viruses (Drezen et al,
2006). These potential horizontal gene transfers could be explained by the intimate relationship between the parasitoid and the lepidopteran host, combined with the high concentration of virus particles during pathogenic virus infection. The penetration of virus particles in the wasp tissues might result fortuitously in the integration of a gene at the proviral locus in the wasp germline. The new gene could then be maintained if it provided a selective advantage to the parasite. A hypothetical gene transfer may also explain the intriguing similarities between IKB-like genes from different PDVs. Indeed, bracovirus and ichnovirus IKB-like proteins share a molecular signature, indicating that they have a common history (Falabella et al,
2007). Hyperparasitism involving an ichneumonid and a braconid wasp may have resulted in transfer of IKB genes from one PDV to another, which might explain their common features despite the different viral origins of the PDVs. The alternative hypothesis that they were obtained independently from an endogenous wasp gene having this signature is less likely since the molecular signature of PDV IKB-like proteins is not found in available sequences of cactus/IKBs from Hymenoptera (A. mellifera, N. vitripennis).
Thus PDV virulence factors most probably originated from multiple, different sources: viruses, mobile elements, wasps, and Lepidoptera. To enquire further into how these factors control host immunity it is necessary to describe first what is known of the immune responses of Lepidoptera and the overall effect of PDVs.
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