Connection between cuticular hydrocarbons and melanization in Harmonia axyridis revealed by RNAi-mediated silencing of the CYP4G79

Abstract

The insect cuticle is a multifunctional shield providing shape stabilization and threat protection. It has a complex composition of cuticular components. Although cuticular melanization is essential and melanin spots are common for many insects, information about the connection between melanization and other cuticular components is limited. The ladybird Harmonia axyridis adults and pupae exhibit regular and predictable melanin spots on their cuticles. Here, RNAi-mediated silencing of CYP4G79 , which functions as an oxidative decarbonylase in hydrocarbon biosynthesis, decreased the cuticular hydrocarbon contents at different levels of H. axyridis , resulted in enlarging (mild) or breaking (severe) melanin spots on cuticle. In addition, transcriptional analysis showed key genes of cuticular melanization up-regulated under CYP4G79 RNAi; Scanning electron microscope (SEM) and Oil Red O (ORO)-staining results suggested a distribution correlation among Cuticular hydrocarbons (CHCs), lipids and melanin. For severe phenotypes with broken melanin spots of treated beetle’s pupal cuticle, histological examinations showed that abnormal pigmentation in cuticular layers were physiologically caused by cuticular microstructural change. In brief, these two cuticular components of H. axyridis , may function together; and cuticular hydrocarbons are essential to the formation of melanin spots. Furthermore, the orderly formations of cuticular components are important to cuticle construction; this study furthers our understanding of insect cuticle formation and functions.