Weak and inconsistent associations between melanic darkness and fitness‐related traits in an insect

Abstract

The idea that the fitness value of body coloration may be affected by biochemically mediated trade‐offs has received much research attention. For example, melanization is believed to interact with other fitness‐related traits via competition for substrates, costs associated with the synthesis of melanin or pleiotropic effects of the involved genes. However, genetic correlations between coloration and fitness‐related traits remain poorly understood. Here, we present a quantitative‐genetic study of a coloration trait correlated to melanin‐based cuticular darkness (‘darkness’, hereafter) in a geometrid moth, Ematurga atomaria. This species has considerable variation in larval appearance. We focus on correlations between larval darkness and fitness‐related growth performance traits. Both a half‐sib analysis and an ‘animal model’ approach revealed moderately high heritabilities of larval darkness and indices of growth performance. Heritability estimates of darkness derived from the animal model were, however, considerably higher than those based on the half‐sib model suggesting that the determination of coloration includes genetic interactions and epigenetic effects. Importantly, on the host plant with the largest sample size, we found no evidence for either genetic or environmental correlations between darkness and growth parameters. On an alternative host plant, there was some indication of positive genetic and negative environmental correlation between these traits. This shows that respective relationships are environment‐specific. Nevertheless, the overall pattern of weak and inconsistent correlations between larval coloration and growth parameters does not support universal trade‐offs between these traits and suggests that physiological costs of producing colour patterns do not necessarily interfere with adaptive evolution of coloration.

Immune function differences between two color morphs of the red palm weevilRhynchophorus ferrugineus(Coleoptera: Curculionidae) at different life stages

“…A study of polymorphic alpine weta Hemideina maori suggested that common immune expression was not heightened with dark coloration. The authors noted that the previous finding of a stronger immune response associated with dark coloration in the high‐density melanic phase of polyphonic insects cannot be extended to insects with other forms of discrete color variation (Sandre et al., 2018). A weak relationship was found between cuticular melanism and the fitness of mountain stone weta Ematurga atomaria , which does not support a universal trade‐off relationship between body coloration and fitness, suggesting that cuticular biological cost does not necessarily interfere with adaptive evolution (Robb et al., 2003).…”

Resource‐based trade‐offs and the adaptive significance of seasonal plasticity in butterfly wing melanism

“…Although tyrosine can be synthesized from phenylalanine, this synthesis itself may not be cost‐free and there is evidence from both mammals and insects that supplementary tyrosine can affect melanization (Evison et al., 2017 ; Morris et al., 2002 ; Watson et al., 2018 ). Indirect evidence for melanin‐synthesis costs in insects also comes from studies that find negative phenotypic (or sometimes genetic) correlations between various melanin‐based traits such as color and immune defense or between melanin‐based traits and other fitness components (Busso et al., 2017 ; Kangassalo et al., 2016 ; Moore et al., 2018 ; Roff & Fairbairn, 2013 ; Sun et al., 2018 , but see also Sandre et al., 2018 ). In the present study, increased dietary tyrosine increased melanization of the spots in cabbage white butterflies therefore adding additional support to the hypothesis of costly melanin‐synthesis.…”

Ontogeny of sexual size dimorphism revisited: Females grow for a longer time and also faster

“…The offspring of the resulting half-sib and full-sib families were divided between two host plants: 12 larvae from each brood were reared on bilberry and three on heather. Using two different host plants was motivated in the context of other uses of the data set [43, 45, 46]. The larvae (N = 1379 reaching pupation) were reared individually in transparent 50 ml plastic vials at 22°C, exposed to a light/dark cycle of 16L:8D, and being provided with food ad libitum .…”