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. Author manuscript; available in PMC: 2018 Dec 1.
Published in final edited form as: Curr Opin Insect Sci. 2017 Sep 18;24:75–83. doi: 10.1016/j.cois.2017.09.007

Figure 2.

Figure 2

(A) Subfunctionalization of an Opsin due to specific amino acid substitution: (Ai) Two views (180° rotation) on a 3D model of the Coleopteran UVS2 (Acmaeodera diffusa) with relevant amino acid residues marked in green and the retinal chromophore in orange (taken from [9*], https://creativecommons.org/licenses/by/4.0/). (Aii) The dramatic blue shift in the Opsin sensitivity (shown here for Drosophila Rh3) is caused by a single amino acid mutation (taken from [89]). Spectral sensitivity curves are shown as solid lines and the absorption spectra of the unaltered or predicted visual pigment as dotted lines, curves are normalized to have a peak value of 1. (B) Co-expression of a red and a green Opsin yields a photoreceptor with broadband sensitivity (taken from [36]). (Ci) The sensitizing pigment, 3-hydroxyretinol attached to the outside of the Opsin, absorbs UV light and transfers its excitation energy to the main chromophore, adding a sensitivity peak in the UV (shown in Cii for Drosophila Rh1; modified after [37]). (D) Elements of spectral-tuning circuits in a model insect ommatidium. Schematic of a longitudinal view of the ommatidium and micrographs of relevant structures: (i) transmission electron micrograph of a longitudinal section of the facet lens, (ii) and (iii’) fluorescence micrographs, and (iii) light micrograph of transverse sections at different positions along the longitudinal axis of the ommatidia. Fluorescing (here UV and yellow), perirhabdomal, and filter pigments alter the light spectrum that reaches the Opsins. The three circles in (ii-iii’) indicate three different ommatidial types that exhibit unique combinations for spectral tuning. (Figure adapted from [74**] (https://creativecommons.org/licenses/by/4.0/legalcode); micrograph (i) taken from [41], and (ii-iii’) from [74**]). (E) Examples of how spectral-tuning affects the sensitivity of photoreceptors. Spectral sensitivity curves in solid lines and dotted lines as described above for (Ai, B, and Ci). (i) Colored lenses reduce transmittance of a small range of wavelengths by selective reflection (gray dashed line; figure modified from [41]). (ii) Fluorescent pigments and (iii) rhabdomeric pigments function as cut-off filters (grey dashed lines) isolating the Opsin from a broad part of the spectrum, thus causing a shift in spectral sensitivity (figures modified from [65]).