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Quantifying Abdominal Pigmentation in Drosophila melanogaster
Published on: June 1, 2017
More than a colour change: insect melanism, disease resistance and fecundity
I M Dubovskiy1, M M A Whitten, V Y Kryukov
1Institute of Systematics and Ecology of Animals, Siberian Branch of Russian Academy of Science, Novosibirsk 630091, Russia.
Abstract:
A 'dark morph' melanic strain of the greater wax moth, Galleria mellonella, was studied for its atypical, heightened resistance to infection with the entomopathogenic fungus, Beauveria bassiana. We show that these insects exhibit multiple intraspecific immunity and physiological traits that distinguish them from a non-melanic, fungus-susceptible morph. The melanic and non-melanic morphs were geographical variants that had evolved different, independent defence strategies. Melanic morphs exhibit a thickened cuticle, higher basal expression of immunity- and stress-management-related genes, higher numbers of circulating haemocytes, upregulated cuticle phenoloxidase (PO) activity concomitant with conidial invasion, and an enhanced capacity to encapsulate fungal particles. These insects prioritize specific augmentations to those frontline defences that are most likely to encounter invading pathogens or to sustain damage. Other immune responses that target late-stage infection, such as haemolymph lysozyme and PO activities, do not contribute to fungal tolerance. The net effect is increased larval survival times, retarded cuticular fungal penetration and a lower propensity to develop haemolymph infections when challenged naturally (topically) and by injection. In the absence of fungal infection, however, the heavy defence investments made by melanic insects result in a lower biomass, decreased longevity and lower fecundity in comparison with their non-melanic counterparts. Although melanism is clearly correlated with increased fungal resistance, the costly mechanisms enabling this protective trait constitute more than just a colour change.
Insights
Melanic Galleria mellonella moths exhibit enhanced resistance to Beauveria bassiana fungal infections due to a thickened cuticle and upregulated immune responses. However, these costly defenses reduce their overall biomass, longevity, and fecundity.
Area of Science:
- Insect immunity
- Melanism and disease resistance
- Entomopathogenic fungi
Background:
- The greater wax moth, Galleria mellonella, presents distinct morphs with varying susceptibility to pathogens.
- Melanic strains of G. mellonella display an unusual resistance to the fungus Beauveria bassiana.
Purpose of the Study:
- To investigate the immunological and physiological mechanisms underlying the heightened resistance of melanic G. mellonella to B. bassiana.
- To compare the defense strategies of melanic and non-melanic morphs.
Main Methods:
- Comparative analysis of immune and physiological traits between melanic and non-melanic G. mellonella morphs.
- Gene expression analysis for immunity- and stress-management-related genes.
- Measurement of haemocyte counts, cuticle phenoloxidase (PO) activity, and encapsulation capacity.
- Assessment of fungal penetration, haemolymph infection, larval survival, biomass, longevity, and fecundity.
Main Results:
- Melanic morphs possess a thickened cuticle, higher basal expression of immune genes, increased haemocyte numbers, and upregulated cuticle PO activity.
- Enhanced capacity for fungal particle encapsulation in melanic morphs, prioritizing frontline defenses.
- Melanic G. mellonella showed increased larval survival, retarded fungal penetration, and reduced haemolymph infections.
- Trade-offs observed: melanic morphs had lower biomass, decreased longevity, and reduced fecundity in the absence of infection.
Conclusions:
- Melanism in G. mellonella is strongly correlated with enhanced resistance to B. bassiana, mediated by specific, costly immune and physiological adaptations.
- These adaptations involve a thickened cuticle and augmented frontline defense mechanisms, rather than late-stage infection responses.
- The evolution of defense strategies differs between geographical variants, with melanism representing a significant investment in pathogen resistance at a physiological cost.
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