How a Melanism Mutant Responds to Different Temperatures: A Case Study in Ladybird Beetle Harmonia axyridis...

Many insect species demonstrate thermal-induced phenotypic plasticity in melanin levels as an effective temperature adaptation strategy. Harmonia axyridis (Pallas) (f. succinea 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 (ml) that exhibits elevated melanin levels throughout most developmental stages compared to the wild type (wt). In this study, we examined whether ml displays similar thermal-induced phenotypic plasticity as wt under different temperatures (17.5°C, 25°C, and 32.5°C). Our findings indicated that ml, 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, ml consistently had significantly higher melanin levels than wt across all three temperature conditions. Surprisingly, despite having larger dark spot areas on the elytra, ml had higher carotenoid concentrations than wt at 25°C and 32.5°C, while no significant difference was observed at 17.5°C. These results indicated that the melanism mutant ml of H. axyridis 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.