Warning signal plasticity in hibiscus harlequin bugs
Color variation in aposematic (conspicuous and defended) prey should be suppressed by frequency-based selection by predators. However selection of color traits is confounded by the fact that coloration also plays an important role in many biological processes, and warning coloration may be constrained by biotic or abotic factors. Temperature, in particular the importance of thermoregulation, has been suggested as the source of much of the geographical variation in warning coloration we see in natural populations. Differential selection in different thermal environments may lead to developmentally canalized or ‘fixed’ differences between populations. Conversely, inter-population differences may be due to phenotypic plasticity, wherein trait expression is modified by environmental conditions. The hibiscus harlequin bug Tectocoris diophthalmus (Heteroptera: Scutelleridae), is a shieldback bug, with iridescent patches that show size variation between individuals, as well as inter-population variation with geographic patterning. This study aimed to identify environmental factors that drive the expression of this variable trait, using surveys, modeling, and experimental approaches. Surveys were taken at sites throughout Australia in three climate regions (tropical, subtropical, and temperate) at different time periods, and results were modeled with a multilevel ordinal regression. We tested for correlations between colouration and several biotic (density, host plant) and abiotic (temperature, rainfall) factors. We found strong phenotypic plasticity with respect to temperature and rainfall. Higher temperatures and increased rainfall were related to suppressed iridescence. A factorial experiment with tropical and temperate bugs in two climate-typical temperature regimes confirmed phenotypic plasticity in response to temperature, likely due to temperature sensitivity in melanin expression. Tropical and temperate populations showed striking differences between plasticity reaction norms, suggesting local evolution on the shape of phenotypic plasticity. We suggest that studying both biotic and abiotic selection pressures is important for understanding the causes of inter-population variation in aposematic signals.
KeywordsColor variation Phenotypic plasticity Aposematic signals Iridescence Scutelleridae
This project was funded in part by a Joyce Vickery grant from the Linnean Society of New South Wales and an Australia Awards Endeavour Fellowship. We are grateful for the assistance of local museums and curators in helping locate sites for surveying, including Graham Brown at the Museum and Art Gallery of the Northern Territory, Geoff Thomas at the Queensland Museum, Dave Britton at the Australian Museum, and Beth Mantle at the Australian National Insect Collection. We thank the Dhimurru Aboriginal Corporation for granting access to their land, in particular Mandaka Marika and Banula Marika for escort and aid. We thank Colleen and Geoff Keena for their repeated hospitality in visiting their private Hibiscus garden. Andrew Allen provided vital input in building the multilevel ordinal regression model. We thank Ben Parslow for his work as a field assistant, and Laurie-Anne Keller for her work assisting photographic image analysis. Finally, thanks to several anonymous reviewers whose comments greatly improved this manuscript.
Compliance with ethical standards
Conflict of interest
The authors declare no conflict of interest.
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