Light environment drives evolution of color vision genes in butterflies and moths.

Opsins are the primary light-sensing molecules in animals. Opsins have peak sensitivities to specific wavelengths which allows for color discrimination. The opsin protein family has undergone duplications and losses, dynamically expanding and contracting the number of opsins, throughout invertebrate evolution, but it is unclear what drives this diversity. Light availability, however, appears to play a significant role. Dim environments are associated with low opsin diversity in deep-sea fishes and cave-dwelling animals. Correlations between high opsin diversity and bright environments, however, are tenuous. Insects are a good system to test whether opsin expansion is associated with greater light availability because they are enormously diverse and consequently display large variation in diel activity. To test this, we used 200 insect transcriptomes and examined the patterns of opsin diversity associated with diel-niche. We focused on the butterflies and moths (Lepidoptera) because this group has significant variation in diel-niche, substantial opsin recovery (n=100), and particularly well-curated transcriptomes. We identified opsin duplications using ancestral state reconstruction and examined rates of opsin evolution, and compared them across diel-niches. We find Lepidoptera species active in high light environments have independently expanded their opsins at least 10 times. Opsins from diurnal taxa also evolve faster; 13 amino acids were identified across different opsins that were under diversifying selection. Structural models reveal that four of these amino acids overlap with opsin color-tuning regions. By parsing nocturnal and diurnal switches, we show that light environment can influence gene diversity, selection, and protein structure of opsins in Lepidoptera.