Abstract
Microevolutionary adaptations and mechanisms of fungal pathogen resistance were explored in a melanic population of the Greater wax moth, Galleria mellonella. Under constant selective pressure from the insect pathogenic fungus Beauveria bassiana, 25th generation larvae exhibited significantly enhanced resistance, which was specific to this pathogen and not to another insect pathogenic fungus, Metarhizium anisopliae. Defense and stress management strategies of selected (resistant) and non-selected (susceptible) insect lines were compared to uncover mechanisms underpinning resistance, and the possible cost of those survival strategies. We hypothesize that the insects developed a transgenerationally primed resistance to the fungus B. bassiana, a costly trait that was achieved not by compromising life-history traits but rather by prioritizing and re-allocating pathogen-species-specific augmentations to integumental front-line defenses that are most likely to be encountered by invading fungi. Specifically during B. bassiana infection, systemic immune defenses are suppressed in favour of a more limited but targeted repertoire of enhanced responses in the cuticle and epidermis of the integument (e.g. expression of the fungal enzyme inhibitor IMPI, and cuticular phenoloxidase activity). A range of putative stress-management factors (e.g. antioxidants) is also activated during the specific response of selected insects to B. bassiana but not M. anisopliae. This too occurs primarily in the integument, and probably contributes to antifungal defense and/or helps ameliorate the damage inflicted by the fungus or the host’s own immune responses.
Highlights
Insects are predominantly dependent upon cuticular, humoral and cellular defenses to resist fungal pathogens
Survival assays conducted on cohorts of the selected (S) and non-selected (NS) lines revealed significantly increased resistance of the 25th generation of the selected for increased resistance to the pathogen (S line) to B. bassiana compared with the NS line (Figure 1A); (P = 0.01)
During the early stages of topically-applied fungal infection, the cuticular PO activity in infected S, but not infected NS larvae, became elevated above uninfected larval levels at 24 h post inoculation for both B. bassiana and M. anisopliae infections (P,0.05, P,0.001 Figure 2A), with this being more pronounced in M. anisopliae treated insects
Summary
Insects are predominantly dependent upon cuticular, humoral and cellular defenses to resist fungal pathogens. If the pathogen is able to breach the cuticle, it has to contend with the host’s humoral and cellular defenses [2]. The latter consist of hemocytes, which will participate in wound healing and encapsulate fungal elements too large to be phagocytosed. A wide range of antimicrobial peptides (AMPs) have been reported in insects with most of them showing antibacterial activity while relatively few have been identified with antifungal activity [6,7,8,9]. Some AMPs appear to be peculiar to specific insect species while others have been reported in several insect species suggesting that some share a common ancestry while others have evolved independently [10]
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