Adaptive melanism and immunity to fungal infection in the migratory grasshopper

Background/Question/Methods

Many ectotherms respond to cooler temperatures by increasing their ability to absorb sunlight via adaptive melanization of exposed surfaces.  Conversely they reflect more sunlight in warmer habitats. In insects, phenoloxidase is a key enzyme for both cuticular melanization and the generalized immune response of insects to invasion. Hence, increases in temperatures might impact susceptibility to disease making populations more vulnerable to climate change.  We asked: is decreased melanism in response to higher temperatures associated with less enzymatic immunity and greater vulnerability to fungal attack?  Migratory grasshoppers Melanoplus sanguinipes were reared from the third instar in either a hot (39°C) or cool (28°C) environment.  To control for physiological age, grasshoppers were moved to a common 33°C on the day they molted to adults. We assayed phenoloxidase (PO) and prophenoloxidase (proPO) activity of the blood and measured body mass and cuticular darkness.  In a second group of adults, we assayed survival to attack by the entomopathogenic fungus Beauveria bassiana strain GHA.  We topically applied a dose of fungal spores that typically causes high mortality within 14 days (1*10⁵ conidia / 1 µl sunflower oil per insect) and followed survivorship for 14 days.  Application of sunflower oil alone served as a control group.

Results/Conclusions

Grasshoppers reared in the hot environment were larger and lighter in color than those reared at the cool temperature.  Blood PO and proPO titers of those reared in the hot environment were significantly less than those reared in the cool one. Although temperature did not have a significant effect on survivorship of the controls, it did affect survivorship of those grasshoppers treated with B. bassiana.  Consistent with differences in PO and proPO activities, those reared in the hot environment had greater mortality from fungal infection and shorter median survival time relative to those reared at the lower temperature.  We have shown experimentally that melanism in response to ambient temperature is also associated with changes immunity and the ability to defend against fungal attack.  Although the change in coloration is generally considered adaptive for thermoregulation, its association with enzymatic immunity makes it detrimental from the standpoint of combating invasion.  More importantly, adaptive melanism may be detrimental to a population’s resilience to climate change.