How a Melanism Mutant Responds to Different Temperatures: A Case Study in Ladybird Beetle <scp> <i>Harmonia axyridis</i> </scp> Demonstrating Thermal‐Induced Phenotypic Plasticity

Many insect species demonstrate thermal-induced phenotypic plasticity in melanin levels as an effective temperature adaptation strategy. <i>Harmonia axyridis</i> (Pallas) (f. <i>succinea</i> morph), known for its diverse elytra patterns, generally displays a reduction in the overall melanin level and spot number on both the dorsal cuticle of pupa and elytra as temperature increases. In our laboratory colony, we have discovered a novel melanism mutant (<i>ml</i>) that exhibits elevated melanin levels throughout most developmental stages compared to the wild type (<i>wt</i>). In this study, we examined whether <i>ml</i> displays similar thermal-induced phenotypic plasticity as <i>wt</i> under different temperatures (17.5°C, 25°C, and 32.5°C). Our findings indicated that <i>ml</i>, inherited in a Mendelian autosomal recessive pattern, exhibited much smaller changes in the occurrence frequency of dark spots on dorsal surfaces of both the pupa and the elytra alongside temperature variations. Additionally, <i>ml</i> consistently had significantly higher melanin levels than <i>wt</i> across all three temperature conditions. Surprisingly, despite having larger dark spot areas on the elytra, <i>ml</i> had higher carotenoid concentrations than <i>wt</i> at 25°C and 32.5°C, while no significant difference was observed at 17.5°C. These results indicated that the melanism mutant <i>ml</i> of <i>H. axyridis</i> displays distinct thermal-induced melanism plasticity compared to the wild type, which will enhance our understanding of the thermal responses of insects differing in genetic background.