Abstract
Pea aphids have an obligate nutritional symbiosis with the bacteria Buchnera aphidicola and frequently also harbor one or more facultative symbionts. Aphids are also susceptible to bacterial pathogen infections, and it has been suggested that aphids have a limited immune response towards such pathogen infections compared to other, more well-studied insects. However, aphids do possess at least some of the genes known to be involved in bacterial immune responses in other insects, and immune-competent hemocytes. One possibility is that immune priming with microbial elicitors could stimulate immune protection against subsequent bacterial infections, as has been observed in several other insect systems. To address this hypothesis we challenged aphids with bacterial immune elicitors twenty-four hours prior to live bacterial pathogen infections and then compared their survival rates to aphids that were not pre-exposed to bacterial signals. Using two aphid genotypes, we found no evidence for immune protection conferred by immune priming during infections with either Serratia marcescens or with Escherichia coli. Immune priming was not altered by the presence of facultative, beneficial symbionts in the aphids. In the absence of inducible immune protection, aphids may allocate energy towards other defense traits, including production of offspring with wings that could escape deteriorating conditions. To test this, we monitored the ratio of winged to unwinged offspring produced by adult mothers of a single clone that had been exposed to bacterial immune elicitors, to live E. coli infections or to no challenge. We found no correlation between immune challenge and winged offspring production, suggesting that this mechanism of defense, which functions upon exposure to fungal pathogens, is not central to aphid responses to bacterial infections.
Highlights
Vertebrates have both adaptive and innate immune systems
Immune Priming and Escherichia Coli Infections For Experiment 4 (Figure 2A), the minimal model indicated no significant interactions between cofactors, and, in particular, the interaction between pre-exposure and challenge was not significant (p = 0.27), which would be expected if pre-exposure increased survival upon a second challenge with live bacteria
In Experiment 5 (Figure 2B), using aphid clone 5AR, preexposure had a significant impact on survival after subsequent challenge (p < 0.01), but not in a manner consistent with immune priming based protection; aphids pre-exposed to bacterial signals had lower survival than those not pre-exposed to bacterial signals
Summary
Vertebrates have both adaptive and innate immune systems. The adaptive immune system allows vertebrates to mount highly specialized responses towards microorganisms to which they have been previously exposed. Based on the presence of innate but not adaptive immune systems in invertebrates, it was thought that insects would lack the ability to prime the immune system for later invasion by a pathogen to which they had been previously exposed. Many studies have shown that some insects can have a stronger and more effective immune response after previous pathogen exposure. This phenomenon, known as immune priming, has been demonstrated in response to bacterial, viral and fungal infections [3,4,5,6,7]. The mechanisms underlying insect immune priming remain elusive, though there is evidence for phagocytosis [10,11,12] and known immune pathways [13,14] being involved
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