The Organization of Genes Encoding Ichnovirus Structural Proteins
Résumé
A large number of genes encoding structural components of polydnaviruses associated with ichneumonid wasps of the subfamily Campopleginae have recently been identified (Volkoff et al., 2010). The genes involved in the production of ichnovirus particle proteins were shown to be localized in specific regions embedded in the genome of the wasp Hyposoter didymator. These specialized regions were named 'IchnoVirus Structural Proteins Encoding Regions' (IVSPERs). Three IVSPERs, representing over 60kb in length, were identified in the H. didymator genome. Although they are not packaged in the virus particles, the IVSPERs can be considered functionally as an integral part of the viral genome and are amplified during virus replication. Their particular genomic organization (high coding sequence density, single-exon predicted genes) suggests that IVSPERs represent fingerprints of an ancestor virus that integrated its own DNA into the genome of an ancestor wasp. The IVSPER genes constitute a set of genes specific to ichnoviruses and conserved among ichnovirus-associated wasps, as shown for the nudivirusrelated genes involved in bracovirus particle production. However, their lack of similarity with genes from known pathogenic viruses suggests that ichnoviruses originated from a virus family that has yet to be described. Altogether these recent findings show that ichnoviruses and bracoviruses derive from independent evolutionary events and represent an example of convergent evolution in the use of virus particles to transfer virulence genes into the host caterpillar.