Phenotypic plasticity in insects: the effects of substrate color on the coloration of two ground‐hopper species

Abstract

The question of how phenotypic variation is maintained within populations has long been a central issue in evolutionary biology. Most of these studies focused on the maintenance of genetic variability, but the phenotype of organisms may also be influenced by environmental cues experienced during ontogeny. Color polymorphism has received particular attention in evolutionary studies as it has strong fitness consequences. However, if body coloration is influenced by the environment, any conclusions on evolutionary consequences of fitness trade-offs can be misleading. Here we present data from a laboratory experiment on the influence of substrate color on three aspects of the coloration of two ground-hopper species, Tetrix subulata and Tetrix ceperoi. We reared hatchlings either on dark or on light substrates, using a split-brood design. Although the type of pronotal pattern changed mainly in response to nymphal development, the basic color was strongly influenced by the substrate color. In both species, black and dark olive color morphs were found more frequently on the dark substrate, whereas the gray color morph dominated on the light substrate. These findings have considerable implications for our understanding of color morph evolution as they show that color polymorphism may not only be maintained by natural selection acting on discrete color morphs, but also by phenotypic plasticity, which enables organisms to adjust to the environmental conditions experienced during ontogeny. This facultative morphology is opposing to the prevailing view of color morph adaptation, which assumes a purely genetic determination and co-evolution of discrete color morphs with life history traits.

No evidence for developmental plasticity of color patterns in response to rearing substrate in pygmy grasshoppers

“…Our results and conclusions are incongruent, however, with those of Hochkirch et al (2008). These authors reared T. subulata and Tetrix ceperoi (Bolívar, 1887) on different visual backgrounds using a split-brood design but concluded that whereas the pattern itself was not influenced by plasticity, basic color showed a plastic response to the color of the rearing substrate experienced during growth, with individuals exposed to darker backgrounds developing darker color patterns.…”

“…These authors reared T. subulata and Tetrix ceperoi (Bolívar, 1887) on different visual backgrounds using a split-brood design but concluded that whereas the pattern itself was not influenced by plasticity, basic color showed a plastic response to the color of the rearing substrate experienced during growth, with individuals exposed to darker backgrounds developing darker color patterns. What may account for the discrepancy between our results and those of Hochkirch et al (2008)? First, there may be a biological explanation for the different findings.…”

“…tant degree by intra-observer inconsistencies in morph assignment or by measurement error. That it is more difficult to reliably assign color morph of small nymphs than of adults (see Results) may also contribute to what Hochkirch et al (2008) interpreted as reversible color changes. Finally, that we found no evidence for plasticity of coloration is not a simple consequence of low statistical power due to insufficient sample size, because our study is based on data for a much larger number of families and individuals than the study by Hochkirch et al (2008), which yielded only 14 surviving T. subulata and 6 surviving T. ceperoi at the termination of the experiment.…”

“…The populations included in the study by Hochkirch et al (2008) and in our study do not share the same evolutionary history, and it is therefore possible that they now respond differently to environmental cues. But there are also several important differences in experimental design, methods, and survival of experimental animals between our study and that by Hochkirch et al (2008) that may explain our inconsistent conclusions. In our experiment, survival of grasshoppers was high (~70%) and independent of color morph.…”

“…Although ample evidence suggests a genetic determination of the color polymorphism in tetrigids, it has recently been suggested that developmental plasticity in response to color of the substrate experienced during growth may contribute to the extreme variability in color and pattern seen in pygmy grasshoppers (Hochkirch et al 2008), and it is theoretically possible that correlated plastic responses in different traits, so-called plasticity integration, may contribute to the associations of color and pattern with other traits (Pigliucci 2001). Developmental plasticity has also been invoked as an alternative explanation for fire melanism (i.e., the rapid increase in the relative frequencies of black (melanistic) and dark-colored phenotypes in populations living in recently fire-ravaged areas) in different species of insects (Hocking 1964;Rowell 1971;Majerus 1998), including pygmy grasshoppers (Karlsson et al 2008).…”