Functional interplay of visual, sensitizing and screening pigments in the eyes ofDrosophilaand other red-eyed dipteran flies
Several fly species have distinctly red-coloured eyes, meaning that the screening pigments that provide a restricted angular sensitivity of the photoreceptors may perform poorly in the longer wavelength range. The functional reasons for the red transparency and possible negative visual effects of the spectral properties of the eye-colouring screening pigments are discussed within the context of the photochemistry, arrestin binding and turnover of the visual pigments located in the various photoreceptor types. A phylogenetic survey of the spectral properties of the main photoreceptors of the Diptera indicates that the transition of the brown eye colour of the Nematocera and lower Brachycera to a much redder eye colour of the higher Brachycera occurred around the emergence of the Tabanidae family.
Functional interplay of visual and screening pigments in the eyes of long-horned flies (left; Diptera: Nematocera) and short-horned flies (right; Diptera: Brachycera). The seven circles in the central enlarged facet indicate the rhabdomeres, where the six peripheral rhabdomeres contain green-sensitive rhodopsin (Nematocera) with a blue-shifted metarhodopsin isoform (blue circle around the green dot), or blue-sensitive rhodopsin (Brachycera) with a red-shifted metarhodopsin isoform (red circle around the blue dot). The seventh, central rhabdomere (violet, green and blue spot) contains UV-, blue- and green-sensitive rhodopsins with the corresponding blue- or green-sensitive metarhodopsins. The photoreceptor somas contain yellow screening pigment. The dark brown screening pigment (Nematocera) or red screening pigment (Brachycera) in the pigment cells optically isolate the ommatidia and produce the eye colour. The nematoceran dark brown pigment absorbs all UV and visible wavelengths, while the brachyceran red pigment allows red stray light (red arrow) to reisomerize the metarhodopsin into rhodopsin, thus restoring the peripheral photoreceptors' sensitivity. The red stray light may cause reduction of the angular and contrast sensitivity of the green-sensitive central photoreceptor.