Distribution and consequences of heritable viruses in host-parasitoid communities – viromics

Most insects are hosts to bacterial symbiont. These symbiont may deeply affect the phenotype of the insects in various ways. Nowadays, we begin to have a satisfactorily view of their diversity, their phenotypic effects (ranging from parasitism to mutualism), and circulation in insect communities. In addition to bacterial symbiont, insects may harbour heritable viruses. One striking example is the case of polydnaviruses (PDV) found in numerous parasitoid species. In parasitoid species, the adult female is free-living and injects its eggs inside or on the body of other arthropods (usually insect larvae or pupae) where the offspring develops as parasites until emergence. PDVs are also injected into the host during oviposition, and they circumvent the host immune reaction (otherwise the wasp egg would be killed by the host immune system). Thousands of insects are dependant on the virus for successful development.
Viruses may also affect the behaviour of their insect host. For example, we recently discovered an heritable virus called LbFV that forces the female parasitoid (Leptopilina boulardi) to lay their eggs in already parasitised hosts. This behaviour called superparasitism benefits to the virus spread since superparasitism allows the virus to colonize new parasitoid lineages (horizontal transmission). Other examples show that viruses may induce male-killing phenomenon as observed for some bacteria. However, contrary to symbiotic bacteria, we have no clear idea of the diversity, distribution and of the role of heritable viruses in insects, despite their potential influence in shaping the phenotype of their hosts.
One reason explaining this pattern is probably that virus genomes are rapidly evolving precluding the use of “generalist” PCR primers that hybridize with diverse families of viruses, contrary to bacteria (16S screens). However, today, with the development of next-generation-sequencing (NGS) technologies, metagenomics analysis are relatively easy to handle and the question of the functional basis of the interactions, the diversity of interactions and the distribution of viruses may be addressed with limited efforts. Using these technologies we propose to (1) study the functional basis of the behavioural alteration induced by LbFV on the parasitoid and (2) identify all heritable viruses present in a Drosophila-parasitoids community, their respective host ranges and prevalences.
The first aim of this project will consist of obtaining the whole genomic sequence of the manipulative virus LbFV. This will be complemented by the comparison of the transcriptoms of various L. boulardi lineages (infected/uninfected; highly manipulated/slightly manipulated) using NGS technologies. This work should allow us to identify “superparasitism genes” in the parasitoid genome. The evolutionary conservation of these genes will be studied by looking for their possible counterparts in related hymenopteras presenting variable lifecycles (parasites/free-living, ecto/endoparasitoids). In addition to the fundamental interest of this work, this should pave the way for applications in the field of pest management programs based on biological control since superparasitism induces a strong reduction in parasitoid efficiency.
The second aim of the project is to sample all the viruses present in the community of Drosophila and their parasitoids in South-East of France. At this spacial scale, we observe very contrasting environments (variations in temperature, species abundance, relative competition) and evidences of local adaptations. Furthermore, we have evidence that in the only species studied so far (L. boulardi) at least two highly prevalent heritable viruses are segregating in populations (LbFV and an RNA virus). The identification of heritable viruses present in this community, their respective host ranges and prevalences should allow us to identity candidates that could be involved in the adaptation of insects to their local environment.