An Opposite Pattern to the Conventional Thermal Hypothesis: Temperature-Dependent Variation in Coloration of Adults of Saccharosydne procerus (Homoptera: Delphacidae)

Current evidence of climate-driven colour change in insects and its impact on sexual signals

The colours of insects function in intraspecific communication such as sexual signalling, interspecific communication such as protection from predators, and in physiological processes, such as thermoregulation. The expression of melanin-based colours is temperature-dependent and thus likely to be impacted by a changing climate. However, it is unclear how climate change drives changes in body and wing colour may impact insect physiology and their interactions with conspecifics (e.g. mates) or heterospecific (e.g. predators or prey). The aim of this review is to synthesise the current knowledge of the consequences of climate-driven colour change on insects. Here, we discuss the environmental factors that affect insect colours, and then we outline the adaptive mechanisms in terms of phenotypic plasticity and microevolutionary response. Throughout we discuss the impact of climate-related colour change on insect physiology, and interactions with con-and-heterospecifics.

Genetic basis of variation in thermal developmental plasticity for Drosophila melanogaster body pigmentation

Seasonal differences in insect pigmentation are attributed to the influence of ambient temperature on pigmentation development. This thermal plasticity is adaptive and heritable, and thereby capable of evolving. However, the specific genes contributing to the variation in plasticity that can drive its evolution remain largely unknown. To address this, we analysed pigmentation and pigmentation plasticity in Drosophila melanogaster. We measured two components of pigmentation in the thorax and abdomen: overall darkness and the proportion of length covered by darker pattern elements (a trident in the thorax and bands in the abdomen) in females from two developmental temperatures (17 or 28°C) and 191 genotypes. Using a GWAS approach to identify the genetic basis of variation in pigmentation and its response to temperature, we identified numerous dispersed QTLs, including some mapping to melanogenesis genes (yellow, ebony, and tan). Remarkably, we observed limited overlap between QTLs for variation within specific temperatures and those influencing thermal plasticity, as well as minimal overlap between plasticity QTLs across pigmentation components and across body parts. For most traits, consistent with selection favouring the retention of plasticity, we found that lower plasticity alleles were often at lower frequencies. The functional analysis of selected candidate QTLs and pigmentation genes largely confirmed their contributions to variation in pigmentation and/or pigmentation plasticity. Overall, our study reveals the existence and underlying basis of extensive and trait-specific genetic variation for pigmentation and pigmentation plasticity, offering a rich reservoir of raw material for natural selection to shape the evolution of these traits independently.

Colour lightness of butterfly assemblages across North America and Europe

Melanin-based dark colouration is beneficial for insects as it increases the absorption of solar energy and protects against pathogens. Thus, it is expected that insect colouration is darker in colder regions and in regions with high humidity, where it is assumed that pathogen pressure is highest. These relationships between colour lightness, insect distribution, and climate between taxa and subtaxa across continents have never been tested and compared. Here we analysed the colour lightness of nearly all butterfly species of North America and Europe using the average colour lightness of species occurring within 50 km × 50 km grid cells across both continents as the dependent variable and average insolation, temperature and humidity within grid cells as explanatory variables. We compared the direction, strength and shape of these relationships between butterfly families and continents. On both continents, butterfly assemblages in colder and more humid regions were generally darker coloured than assemblages in warmer and less humid regions. Although these relationships differed in detail between families, overall trends within families on both continents were similar. Our results add further support for the importance of insect colour lightness as a mechanistic adaptation to climate that influences biogeographical patterns of species distributions.

The Environmental Plasticity of Diverse Body Color Caused by Extremely Long Photoperiods and High Temperature in Saccharosydne procerus (Homoptera: Delphacidae)

Melanization reflects not only body color variation but also environmental plasticity. It is a strategy that helps insects adapt to environmental change. Different color morphs may have distinct life history traits, e.g., development time, growth rate, and body weight. The green slender planthopper Saccharosydne procerus (Matsumura) is the main pest of water bamboo (Zizania latifolia). This insect has two color morphs. The present study explored the influence of photoperiod and its interaction with temperature in nymph stage on adult melanism. Additionally, the longevity, fecundity, mating rate, and hatching rate of S. procerus were examined to determine whether the fitness of the insect was influenced by melanism under different temperature and photoperiod. The results showed that a greater number of melanic morphs occurred if the photoperiod was extremely long. A two-factor ANOVA showed that temperature and photoperiod both have a significant influence on melanism. The percentages of variation explained by these factors were 45.53 and 48.71%, respectively. Moreover, melanic morphs had greater advantages than non-melanic morphs under an environmental regime of high temperatures and a long photoperiod, whereas non-melanic morphs were better adapted to cold temperatures and a short photoperiod. These results cannot be explained by the thermal melanism hypothesis. Thus, it may be unavailable to seek to explain melanism in terms of only one hypothesis.