PATHOGEN VIRULENCE SHAPES SPECIFIC IMMUNE MEMORY IN INVERTEBRATES

Accumulating evidence reveals that invertebrates can develop immune priming, a form of innate immune memory. Such protection can be pathogen strain- or species-specific and long-lasting. However, immune memory is not consistently observed across studies, suggesting involvement of ecological and pathogen‑related factors. For instance, pathogen virulence may critically influence invertebrate immune memory as it shapes the intensity and duration of the initial immune activation, host survival, and the opportunity for memory‑like mechanisms to develop. To investigate this hypothesis, we used the insect Tenebrio molitor as a model. Larvae were primed with sublethal conidia doses of two strains of the entomopathogenic fungus Metarhizium brunneum that differ in their ability to kill T. monitor larvae (Cherry: low virulence; CAT: high virulence), followed by a lethal homologous or heterologous challenge. We assessed survival, immune responses (including phenoloxidase activity, lytic activity, and hydrogen peroxide production), and the ability of hemolymph to inhibit fungal germination and growth. Our findings support that specific immune memory, demonstrated by enhanced survival and more efficient pathogen clearance, occurs only with the low-virulence strain. Interestingly, the measured immune responses were not directly linked to immune memory but instead correlated with pathogen virulence, showing heightened activity against the more virulent strain.