Functional analysis of PDV virulence genes

The characterization through genome sequencing of molecules encoded by PDVs gives insights into how they contribute to the parasite's success. The goal of current studies is to define more precisely the role of each gene product in this complex manipulation of host physiology. The fact that reverse genetics is not possible in these hostparasite systems has, however, made functional analysis of PDV genes difficult. Most functional approaches have therefore consisted of a combination of solid expression data (when applicable), in vitro biochemical activity assays, in vitro bio-assays, and finally transient expression assays in cell culture and more rarely in vivo. The use of RNA interference (RNAi) technology is still limited to cell-culture assays (albeit with one exception: Bonvin et al., 2005) but is likely to develop and help us to characterize PDV gene function or function of PDV host targets in vivo in the future.

Another approach for testing the involvement of PDV-encoded factors in parasitism consists of studying their evolution by comparing viral genes in different species of a wasp genus to identify whether selection pressures promoting divergence of the sequences are operating on these genes, thus indicating their involvement in dynamic molecular interactions between hosts and parasites. Using this method we have been able to show that genes belonging to two families present in the CcBV genome (cystatin and PTP) have been under diversifying selection pressures, indicating they most probably play an important role in parasitism success (Serbielle et al., 2008; C. Serbielle et al., unpublished results). Similar results were obtained for cysteine-motif proteins in ichnoviruses (Dupas et al., 2003). Certain PDV gene families encode proteins with characterized domains, which enable prediction of their biochemical activity and possible involvement in host physiology. In the cases of PDV cystatins, cysteine-motif proteins, and PTPs, for example, these groups of proteins are likely to target immune functions and/or development of the lepidopteran host.

We now present an overview of the PDV genes for which the functional evidence of involvement in host immune disruption is the most advanced (see Fig. 9.4. and Table 9.1).

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