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The wasp, Hyposoter didymator, lays its eggs and develops in the body of caterpillars (OR, larvae) of the Noctuidae family, a family of moths which includes numerous species of economic interest. The strain of the wasp studied comes from the south of Spain, where it is one of the natural enemies of the cotton tree worm, Spodoptera littoralis, a polyphagic pest from Egypt which is present throughout the Mediterranean basin. In the laboratory, H. didymator is grown on the species, Spodoptera frugiperda, (fall armyworm is its common name), a pest which is present on the American continent.
Like other parasitic wasps (see Cotesia congregata),H. didymator requires a symbiotic virus of the polydnavirus family for its development in the caterpillar. This virus is produced in the ovaries of the female wasp (Figure 1), where it is stored. Therefore, when the female wasp lays an egg in the caterpillar, the viral particules are injected simultaneously (Figure 2). In the caterpillar, the viral particles infect numerous tissues and the viral genes are expressed. The viral proteins thus produced act at different physiological levels in the caterpillar, modifying its physiology in an advantageous way for the parasite. In fact, the virus is indispensable for the development of the wasp in the caterpillar’s body. The major effects on the parasitized caterpillar provoked by the presence of the polydnavirus are the inhibition of the defensive immune response (which is normally triggered following detection of a foreign body such as the parasite’s egg) and the perturbation of larval development.
The polydnaviruses are classified in two families, Bracovirus and Ichnovirus, which are associated with wasps from the Braconide and Ichneumonides families respectively. The polydnavirus associated with H. didymator, HdIV, belongs to the Ichnovirus family, whereas the one associated with the wasp, C. congregata, CcBV, belongs to the Bracovirus family. Thanks to phylogenetic studies, scientists are in agreement that these two polydnavirus families are the result of independent association phenomena between an ancestral virus and and an ancestral wasp (Figure 3). In addition to the evidence from phylogeny, several other types of evidence contribute to this conclusion: (i) the morphology of the virus particles and different morphogenesis in the two families and (ii) the genomes of the ichnoviruses have practically no similarity to those of the brachiviruses (not detected with BLASTN or BLASTX). These divergences, which are associated with a convergence of effects on the physiology of the parasitized caterpillar, raises the important question of whether these two polydnavirus families have distinct evolutionary origins and perhaps different modes of production.
In contrast to the situation with brachovirus, the genome of ichnovirus is still poorly known because this polydnavirus family has been studied by only a few laboratories in the world. Only the virus associated with Campoletis sonorensis, CsIV, has been sequenced: it consists of 22 segments of 6 to 20 kbp (total size of 223 kbp). Furthermore, the genome of CsIV contains some molecules which are closely related (“nesting”, in which some segments are generated by recombination from a larger segment), a phenomenon which has not been widely documented in Bracovirus. As for HdIV, the number of segments which constitute its genome is not known, but is around 20, with sizes which vary from 2 kb to more than 10 kb (Figure 4).
In order to understand the role of these viruses in the parasitic success of the wasp it is important to identify the proteins expressed in the lepidopteran host. We have identified 28 genes to date, which have been classified in 12 families. Of these genes, 70% have no homology with genes which have already been described and 40% are genes which have not been described in other ichnoviruses. At the moment, the organization of the genome in the viral particles seems to follow the same criteria as for the bracovirus CcBV: genes organized in families, genes for virulence factors, characteristics of the eukaryotic genome, absence of genes which are similar to classical viral genes. However, only the complete sequence of this genome will make it possible to make solid comparisons between ichnovirus and bracovirus.
This data will also make it possible to address the question of the evolutionary origin of the polydnaviruses, in a comparison with the bracovirus associated with C. congregata. They will be complementary, and necessary for the analysis of the genes and mechanisms implicated in the production of viral particles in the parasitoid wasp which the Montpellier group is also studying.