A structural deletion of aspartate 1-decarboxylase underlies a melanic mutation in Tribolium castaneum

Cuticle pigmentation and sclerotization are tightly coupled processes in insects, yet how variation within shared metabolic pathways contributes to phenotypic diversification remains incompletely understood. Here, we characterize a spontaneously arising melanic mutation in the red flour beetle Tribolium castaneum . Genetic analysis combined with bulked segregant mapping localized the mutation to a defined region on chromosome 1, within which transcriptomic profiling identified aspartate 1-decarboxylase ( ADC ) as the gene most strongly affected in mutant individuals. RNA interference (RNAi)-mediated reduction of ADC expression was sufficient to recapitulate the melanic cuticle phenotype. Genomic analysis further revealed a large structural deletion spanning ADC and an adjacent gene, accompanied by a hAT transposable element insertion, coinciding with a marked reduction in ADC expression. Beyond pigmentation, ADC deficiency was associated with altered cuticle ultrastructure, coordinated transcriptomic changes related to cuticle remodeling and molting. Together, these findings link structural genomic variation at a key metabolic node in catecholamine metabolism to pleiotropic effects on pigmentation, cuticle organization, and development, highlighting the integrative nature of cuticle formation pathways in insects. • A melanic mutation in Tribolium castaneum maps to chromosome 1. • A structural deletion disrupts ADC and markedly reduces its expression. • ADC recapitulates melanic pigmentation and cuticle defects. • ADC knockdown causes coordinated transcriptomic changes in cuticle remodeling and molting.