Opsin proteins are fundamental components of animal vision whose structure largely determines the sensitivity of visual pigments to different wavelengths of light. Surprisingly little is known about opsin evolution in beetles, even though they are the most species rich animal group on Earth and exhibit considerable variation in visual system sensitivities. We reveal the patterns of opsin evolution across 62 beetle species and relatives. Our results show that the major insect opsin class (SW) that typically confers sensitivity to "blue" wavelengths was lost ~300 million years ago, before the origin of modern beetles. We propose that UV and LW opsin gene duplications have restored the potential for trichromacy (three separate channels for colour vision) in beetles up to 12 times and more specifically, duplications within the UV opsin class have likely led to the restoration of "blue" sensitivity up to 10 times. This finding reveals unexpected plasticity within the insect visual system and highlights its remarkable ability to evolve and adapt to the available light and visual cues present in the environment.
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We thank all researchers whose published RNA-seq data were used in this study. We are grateful to Rebecca Plimpton for her assistance with opsin protein modelling. We thank Stephen Richards and the Insect 5000 Genomes Project (Baylor College of Medicine Human Genome Sequencing Center; https://www.hgsc.bcm.edu) for access to data from the unpublished i5k genome for Anoplophora glabripennis. We also thank Dan O’Sullivan for creating the beetle illustrations. Further, we thank the following people for collecting and/or providing specimens: Eric Anton, Rolf Beutel, Dan Young, Margarethe Brummerman, Ralph Peters, Carola Greve, Sven Nekum, Alexander Blanke, Susanne Dobler, Cary Minteer, Dirk Ahrens, Jonas Eberle and Silvia Fabrizi. Finally we thank Alexander Donath, Lars Posiadlowski, Xin Zhou, Karl Kjer, Bernhard Misof, Karen Meusemann, Adam Ślipiński and the 1KITE Coleoptera subproject for allowing access to unpublished transcriptome assemblies, providing sample infomation and assisting with 1KITE coordination. This work was supported by National Science Foundation Division of Environmental Biology grants [#1355169 DDM] and [#1265714 SMB].