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More than a colour change: Insect melanism, disease resistance and fecundity / Tariq Butt

Proceedings of the Royal Society B: Biological Sciences

Swansea University Author: Butt, Tariq

Abstract

This is the first detailed study of the fungal resistance strategies of a dark morph of the wax moth, Galleria mellonella, a lepidopteran pest in beehives. No study has yet properly addressed the question of whether insects can develop full resistance to fungal biological control agents (BCAs). We i...

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Published in: Proceedings of the Royal Society B: Biological Sciences
Published: 2013
URI: https://cronfa.swan.ac.uk/Record/cronfa14620
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Abstract: This is the first detailed study of the fungal resistance strategies of a dark morph of the wax moth, Galleria mellonella, a lepidopteran pest in beehives. No study has yet properly addressed the question of whether insects can develop full resistance to fungal biological control agents (BCAs). We investigated the correlation between melanism and fungal resistance, and by comparing a normal (pale) morph of the insect with the fungus-resistant dark (melanic) morph, we uncovered a complex set of additional traits that underpin this relationship and show that the two morphs have evolved different strategies to deal with fungal pathogens. While the link between insect melanism and enhanced immunity has been reported previously, we believe ours is the first study to uncover its true complexity. Other studies to date have focussed on the darkness of the insect cuticle (degree of melanization), and certain aspects of the insect immune response such as lysozyme activity, phenoloxidase and hemocyte (blood cell) activity, and their trade off with certain obvious life history traits. Furthermore, most studies have utilized an artificial infection route (i.e. injection) to gauge the response to fungi. Our study looks at both topical application and injection in parallel, using the insect-pathogenic fungus Beauveria bassiana, a true pathogen of G. mellonella. We show that the resistant melanic morphs of this insect species adopt a different evolutionary strategy to deal with fungal pathogens; they sacrifice fecundity, size and longevity to prioritize mechanical and immunological defenses on the “front line”, targeting the likely sites of infection or damage. Timing and location is critical, and pale morphs seem to recruit their defenses too late, sealing their fate. We provide preliminary evidence of mutations to an immune-related gene that have probably accrued during a long period of divergent evolution of the melanic morph. This in turn suggests that a low-fecundity strategy works for this insect. The study also goes beyond an assessment of the expression of antimicrobial peptide genes (whose role in this context may be more to do with preventing secondary infections than providing a specific antifungal response), to consider a resistant insect’s need to manage and ameliorate stress (originating from a variety of pathogen or host sources). The novel findings of this study significantly advance our understanding of the field and have obvious implications for the biocontrol strategy. We suggest too that tolerance mechanisms can only be fully investigated by looking at multiple systems in unison, encompassing more than just standard indicators of immunity and color change.
Keywords: Insect, insect-pathogenic fungus, melanism, immunity, resistance, stress, micro-evolution
College: College of Science