These three cone types are a direct result of the expression of three cone opsin genes in adults: SWS2a, RH2a and LWS genes [ 21]. Larval and juvenile tilapia express different subsets of the opsins and have more complex visual pigment complements.
These results suggest that there may be more than two distinct visual pigments in the twin cones, and/or co-expression of opsin genes within the same outer segment, as in other fish[ 19].
If this is true, then opsin expression in individual photoreceptors must change through time resulting in co-expression of more than one visual pigment in a given cell for brief periods.
The slower rate of evolution of the BRh1 opsin is consistent with our observation that this gene encodes the 437 nm visual pigment in Lycaena rubidus, which is a wavelength of peak absorbance that is more typical of the "blue-sensitive" visual pigments in butterflies, than the duplicate BRh2 gene in L. rubidus which encodes the unusually red-shifted 500 nm visual pigment [ 50].
In theory, RPE cells of R91W Nrl −/− mice should receive much more light due to the reduced levels of visual pigments, which are the main light absorbers in the retina.
Given that C. nuchalis and other bowerbird ocular media transmit more UV wavelengths than most other species with VS-type visual pigments, they might represent a transitional link in the evolution from a VS to a UVS visual system [ 39].
Nevertheless, ommatidia in the DRA of honeybees express both UV- and green-sensitive visual pigments [ 25], and the DRA photoreceptors of bumblebees express more than one UV pigment [ 26], potentially broadening spectral sensitivity.
More importantly, the epithelium contains melanin, which absorbs stray light that penetrates the retina without being absorbed by the visual pigments.
Some butterflies have four visual pigments, one of which is maximally sensitive to red wavelengths.
"Clothing is much more visual".
