The Genomic Landscape of Metallic Colour Variation in Ground Beetles.

The metallic colour variation of beetles is a spectacular feature that has inspired diverse human cultures. However, little is known about the genetic basis of this trait or its ecological importance. In this study, we characterize the geographical distribution, optical mechanism, genetic basis and ecological and evolutionary importance of metallic colour variation in the Nebria ingens complex, an alpine ground beetle in the Sierra Nevada, California. We find that elytral colour varies continuously across two allopatric species (from black N. ingens to green N. riversi), with hybrid populations showing intermediate coloration, and we demonstrate that the metallic colour is generated from multilayer reflectors in the epicuticle of the elytra. By applying association mapping in natural populations (wild-GWAS) using high-density genotype variants, we identify five promising candidate genes covarying with metallic variation, with known roles in cuticle formation and pigmentation pathways. This finding, together with a significant correlation between colour variation and water availability, suggests that metallic variation evolves as a local adaptation to environmental variation in the N. ingens complex.