13The Steinernema carpocapsae-Xenorhabdus nematophila association is a nematobacterial complex (NBC) 14 used in biological control of insect crop pests. The ability of this dual pathogen to infest and kill an insect 15 strongly depends on the dialogue between the host's immune system and each partner of the complex. Even 16 though this dialogue has been extensively studied from the two partners' points of view in several insect 17 models, still little is known about the structure and the molecular aspects of the insects' immune response 18 to the dual infection. Here, we used the lepidopteran pest Spodoptera frugiperda as a model to analyze the 19 respective impact of each NBC partner in the spatiotemporal immune responses that are induced after 20 infestation. To this aim, we first analyzed the expression variations of the insect's immune genes in the fat 21 bodies and hemocytes of infested larvae by using previously obtained RNAseq data. We then selected 22 representative immune genes for RT-qPCR investigations of the temporal variations of their expressions 23 after infestation and of their induction levels after independent injections of each partner. We found that 24 the fat body and the hemocytes both produce potent and stable immune responses to the infestation by the 25 NBC, which correspond to combinations of bacterium-and nematode-induced ones. Consistent with the 26 nature of each pathogen, we showed that X. nematophila mainly induces genes classically involved in 27 antibacterial responses, whereas S. carpocapsae is responsible for the induction of lectins and of genes 28 expected to be involved in melanization and cellular encapsulation. In addition, we found that two clusters 29 of unknown genes dramatically induced by the NBC also present partner-specific induction profiles, which 30 paves the way for their functional characterization. Finally, we discuss putative relationships between the 31 variations of the expression of some immune genes and the NBC's immunosuppressive strategies. 32 3 33Author summary 34 Entomopathogenic nematodes (EPNs) are living in the soil and prey upon insect larvae. They enter the 35 insect by the natural orifices, and reach the hemocoel through the intestinal epithelium. There, they release 36 their symbiotic bacteria that will develop within the insect and eventually kill it. Nematodes can then feed 37 and reproduce on the insect cadaver. By using transcriptomic approaches, we previously showed that 38 Lepidoptera larvae (caterpillars of the fall armyworm Spodoptera frugiperda) produce a strong immune 39 response in reaction to infestation by EPNs. However, we do not know if this immune reaction is triggered 40 by the nematode itself -Steinernema carpacapsae -or its symbiotic bacteria -Xenorhabdus nematophila. 41 To answer this question, we present in this work a careful annotation of immunity genes in S. frugiperda 42 and surveyed their activation by quantitative PCR in reaction to an injection of the bacteria alone, the axenic 43 nematode or the ...

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