Morphogen-driven melanin pathway dynamics regulated by Wnt1 and apontic-like underlie larval spot coloration in...

In insects, conspicuous larval pigmentation patterns serve critical ecological roles such as warning signals and mimicry, yet their underlying genetic regulation remains poorly understood. In this study, I investigated the molecular mechanisms underlying black and yellow pigmentation patterns in three distinct larval spot types of the silkworm Bombyx mori: large, diffuse L-spots of the Multilunar (L) mutant; small, sharply defined +^p-spots of the Normal strain; oval p^M-hybrid spots of an interspecific hybrid with Bombyx mandarina. Each spot type comprises a yellowish center surrounded by a black periphery, forming crescent-shaped pigmentation patterns. Chemical treatments confirmed that both colors are melanin-based. Using quantitative PCR and RNA interference (RNAi), I analyzed six melanin synthesis genes (tyrosinehydroxylase, dopadecarboxylase, laccase2, yellow, tan, and ebony) and discovered that black pigmentation involves both dopa/dopamine- and NBAD-melanin synthesis, whereas yellow pigmentation primarily reflects only the latter. I further examined Wnt1 and apontic-like (apt-like) using qRT-PCR, RNAi, and TALEN-mediated mosaic analysis. Wnt1 expression localized to presumptive spot areas, suggesting a concentration-dependent role in regulating both spot size and pigment composition: higher Wnt1 levels were associated with larger spots with yellow centers, while reduced Wnt1 expression resulted in black pigmentation and smaller spots. Wnt1-activated transcription factor apt-like was required for pigmentation in all spot types without influencing spot size. Taken together, the results of this study reveal a morphogen-driven gene regulatory network in which Wnt1 expression levels and downstream transcriptional cascades orchestrate pigment placement and patterning, offering new insights into the modular genetic control of insect pigmentation.