Abstract
Catecholamine metabolism plays an important role in the determination of insect body color and cuticle sclerotization. To date, limited research has focused on these processes in silkworm. In the current study, we analyzed the interactions between catecholamines and melanin genes and their effects on the pigmentation patterns and physical properties of sclerotized regions in silkworm, using the melanic mutant melanism (mln) silkworm strain as a model. Injection of β-alanine into mln mutant silkworm induced a change in catecholamine metabolism and turned its body color yellow. Further investigation of the catecholamine content and expression levels of the corresponding melanin genes from different developmental stages of Dazao-mln (mutant) and Dazao (wild-type) silkworm revealed that at the larval and adult stages, the expression patterns of melanin genes precipitated dopamine accumulation corresponding to functional loss of Bm-iAANAT, a repressive effect of excess NBAD on ebony, and upregulation of tan in the Dazao-mln strain. During the early pupal stage, dopamine did not accumulate in Dazao-mln, since upregulation of ebony and black genes led to conversion of high amounts of dopamine into NBAD, resulting in deep yellow cuticles. Scanning electron microscope analysis of a cross-section of adult dorsal plates from both wild-type and mutant silkworm disclosed the formation of different layers in Dazao-mln owing to lack of NADA, compared to even and dense layers in Dazao. Analysis of the mechanical properties of the anterior wings revealed higher storage modulus and lower loss tangent in Dazao-mln, which was closely associated with the altered catecholamine metabolism in the mutant strain. Based on these findings, we conclude that catecholamine metabolism is crucial for the color pattern and physical properties of cuticles in silkworm. Our results should provide a significant contribution to Lepidoptera cuticle tanning research.
Highlights
Diversified body color and sclerotized exoskeleton are essential for the survival and evolution of insects, and catecholamines play an important role in the pigmentation and sclerotization of cuticles [1,2,3,4,5,6]
Investigation of the expression levels of the corresponding melanin genes revealed that after dopamine injection, the Ddc level was significantly lower in the injected group, while the expression levels of ebony and Bm-iAANAT were significantly higher than those in the control group (Figure 1B)
Similar patterns were observed when NBAD and N-acetyl dopamine (NADA) were injected as controls for each other, i.e., the level of ebony was significantly lower in individuals injected with NBAD, while that of Bm-iAANAT was lower in those administered NADA (Figure S6C, D)
Summary
Diversified body color and sclerotized exoskeleton are essential for the survival and evolution of insects, and catecholamines play an important role in the pigmentation and sclerotization of cuticles [1,2,3,4,5,6]. After oxidation by phenoloxidases, such as Laccase, these catecholamines participate in insect cuticle tanning [9,10,11,12]. Varying levels of catecholamines lead to different body colors in insects [13,14,15,16,17]. Expression levels of melanin metabolism genes influence the synthesis of catecholamines that contribute to insect body color [18,19,20]. Different pigment precursors can be produced by diverse genes or vary according to the expression levels of these genes, resulting in distinct pigment patterns in insects [21,22,23,24]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
