Abstract
BackgroundArthropods have received much attention as a model for studying opsin evolution in invertebrates. Yet, relatively few studies have investigated the diversity of opsin proteins that underlie spectral sensitivity of the visual pigments within the diverse beetles (Insecta: Coleoptera). Previous work has demonstrated that beetles appear to lack the short-wavelength-sensitive (SWS) opsin class that typically confers sensitivity to the “blue” region of the light spectrum. However, this is contrary to established physiological data in a number of Coleoptera. To explore potential adaptations at the molecular level that may compensate for the loss of the SWS opsin, we carried out an exploration of the opsin proteins within a group of beetles (Buprestidae) where short-wave sensitivity has been demonstrated. RNA-seq data were generated to identify opsin proteins from nine taxa comprising six buprestid species (including three male/female pairs) across four subfamilies. Structural analyses of recovered opsins were conducted and compared to opsin sequences in other insects across the main opsin classes—ultraviolet, short-wavelength, and long-wavelength.ResultsAll nine buprestids were found to express two opsin copies in each of the ultraviolet and long-wavelength classes, contrary to the single copies recovered in all other molecular studies of adult beetle opsin expression. No SWS opsin class was recovered. Furthermore, the male Agrilus planipennis (emerald ash borer—EAB) expressed a third LWS opsin at low levels that is presumed to be a larval copy. Subsequent homology and structural analyses identified multiple amino acid substitutions in the UVS and LWS copies that could confer short-wavelength sensitivity.ConclusionsThis work is the first to compare expressed opsin genes against known electrophysiological data that demonstrate multiple peak sensitivities in Coleoptera. We report the first instance of opsin duplication in adult beetles, which occurs in both the UVS and LWS opsin classes. Through structural comparisons of known insect opsins, we suggest that opsin duplication and amino acid variation within the chromophore binding pocket explains sensitivity in the short-wavelength portion of the visible light spectrum in these species. These findings are the first to reveal molecular complexity of the color vision system within beetles.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0674-4) contains supplementary material, which is available to authorized users.
Highlights
Arthropods have received much attention as a model for studying opsin evolution in invertebrates
The first is the chemical environment of the protonated Schiff base, and the second is the shape of the chromophore, which is determined by the structure of the chromophore binding pocket
We detected at least four opsin copies in all buprestids (Figs. 2 and 3a-b)—two ultraviolet sensitive (UVS) and two longwavelength sensitive (LWS) opsins, the most detected in any beetle species
Summary
Arthropods have received much attention as a model for studying opsin evolution in invertebrates. Relatively few studies have investigated the diversity of opsin proteins that underlie spectral sensitivity of the visual pigments within the diverse beetles (Insecta: Coleoptera). Previous work has demonstrated that beetles appear to lack the short-wavelength-sensitive (SWS) opsin class that typically confers sensitivity to the “blue” region of the light spectrum. This is contrary to established physiological data in a number of Coleoptera. Previous studies have suggested substitutions of amino acids in the seven trans-membrane domains closest to the chromophore—the chromophore binding pocket—have the highest potential for altering the peak spectral sensitivity (λmax value) of a visual pigment, known as spectral tuning (see [1–11]). The identification of amino acid variation between putative opsin copies is an important step in predicting spectral tuning of photopigments
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