Interactions between animals and microbes are ubiquitous in nature and strongly impact animal physiology. These interactions are shaped by the host immune system, which responds to infections and contributes to tailor the associations with beneficial microorganisms. In many insects, beneficial symbiotic associations not only include gut commensals, but also intracellular bacteria, or endosymbionts. Endosymbionts are housed within specialised host cells, the bacteriocytes, and are transmitted vertically across host generations. Hostendosymbiont co-evolution shapes the endosymbiont genome and host immune system, which not only fights against microbial intruders, but also ensures the preservation of endosymbionts and the control of their load and location. The cereal weevil Sitophilus spp. is a remarkable model to study the evolutionary adaptation of the immune system to endosymbiosis since its binary association with a unique, relatively recently acquired nutritional endosymbiont, Sodalis pierantonius. This Gram-negative bacterium has not experienced the genome size shrinkage observed in long-term endosymbioses and has retained immunogenicity. We focus here on the 16 antimicrobial peptides (AMPs) identified in the Sitophilus oryzae genome and their expression patterns in different tissues, along host development or upon immune challenges, to address their potential functions in the defensive response and endosymbiosis homeostasis along the insect life cycle. (1,3-7) . These small cationic peptides were shown to target microbial phospholipid membranes, leading to depolarisation, changes in their permeability, and subsequent microbial cell death. Some AMPs can also inhibit viral proliferation by disruption of the viral protein synthesis and the viral gene expression (1,7,8) . Although prokaryotic AMPs, called Bacteriocins, have also been described (2) , we will focus here on eukaryotic AMPs.

Research on insect immunity has been at the forefront of antimicrobial peptide studies since the first description of an animal AMP in the silk moth Hyalophora cecropia (9) . Cecropin was reported as a novel bactericidal agent able to permeabilise the membrane of a limited number of Gram-positive bacteria, without affecting eukaryotic cells (9) . Since the discovery of this first AMP, a large number of AMPs have been identified in diverse insect families as an important part of the immune response (7) . Interestingly, recent availability of functional analyses is currently uncovering the level of specificity and synergy of these immune effectors (5,10,11) .