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During parasitism, virus particles are co-injected with the egg into the lepidopteran caterpillars. The virus is unable to replicate in the parasitized host, but the viral proteins produced throughout the parasitism (which lasts 7-8 days under controlled conditions) neutralize the immune response and control the growth and development of the host, thus providing a favorable environment for the parasitoid to develop.

In particular, the polydnavirus targets the activity of host hemocytes, the circulating immune cells in insects, responsible for encapsulating and neutralizing large foreign bodies. We have shown that infection of caterpillars with polydnavirus from Hyposoter didymator is associated with a significant decrease in the hemocyte population due to the activation of apoptosis. Apoptosis is a process of programmed cell death, which is, in some cases, activated during an attack by a pathogen. The 'suicide' of infected cells thus prevents the pathogen from multiplying and spreading to other cells in the body. In our model, the induction of apoptosis in hemocytes prevents the elimination of the developing parasitoid. We further showed that HdIV has a complex relationship with other pathways involved in insect antiviral defence, also regulating classical antiviral pathways (RNAi, Toll, Imd, and JAK/STAT pathways) depending on the cellular context (Visconti et al, 2019).

We are also interested in the potential involvement of long non-coding RNAs (lncRNAs) in host cells-PDV interactions. lncRNAs have emerged as a new class of functional elements involved in the regulation of many fundamental biological processes, both normal and pathological. They are defined as RNAs larger than 200 nucleotides, lacking coding capacity and characterized by high tissue expression specificity. Our objective is to identify by RNA-seq the lncRNAs of S. frugiperda, whose expression is modified during infection by HdIV. To assess their specificity with respect to the pathogen and the cellular context, we compared their expression profiles in hemocytes and fat bodies in response to the polydnavirus HdIV and the densovirus JcDV infection. We have thus identified a catalogue of lncRNAs regulated by viral infection, and which could therefore play a regulatory role in host-virus interactions (article in preparation).

In order to go further in our study, we are also exploiting single-cell RNA-seq (scRNA-seq) technology to analyze transcriptomic heterogeneity within different populations of lepidopteran cells interacting with the polydnavirus. Our objective is to measure the diversity of molecular mechanisms responsible for the disruption of important physiological functions, such as apoptosis, in the immune cells of the host caterpillar.