Opsin proteins covalently bind to small molecular chromophores and each protein-chromophore complex is sensitive to particular wavelengths of light. Multiple opsins with different wavelength absorbance peaks are required for color vision. Comparing opsin responses is challenging at low light levels, explaining why color vision is often lost in nocturnal species. Here, we investigated opsin evolution in 27 phylogenetically diverse insect species including several transitions between photic niches (nocturnal, diurnal, and crepuscular). We find widespread conservation of five distinct opsin genes, more than commonly considered. These comprise one c-opsin plus four r-opsins (long wavelength sensitive or LWS, blue sensitive, ultra violet [UV] sensitive and the often overlooked Rh7 gene). Several recent opsin gene duplications are also detected. The diversity of opsin genes is consistent with color vision in diurnal, crepuscular, and nocturnal insects. Tests for positive selection in relation to photic niche reveal evidence for adaptive evolution in UV-sensitive opsins in day-flying insects in general, and in LWS opsins of day-flying Lepidoptera specifically.
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