Possible trajectories of Tribolium retina evolution. Schematic presentation of three mutation scenarios that could have led from an ancestral trichromatic retina to the retinal mosaic of extant Tribolium castaneum. Colors indicate photoreceptor light specificities as in Fig. 3. Numbers above arrows represent number of necessary mutation induced gene regulatory changes underlying opsin expression pattern evolution. L stands for genomic loss of B-opsin. (a) Instantaneous B-opsin loss scenario. B-opsin loss without correlated or preceding expansion of other opsin expression domains is unlikely since it would result in a fraction of light insensitive photoreceptor cells and thereby a substantial deterioration of the resolution of spatial vision (black). Subsequent mutational steps that would lead to the Tribolium castaneum retinal mosaic are therefore just indicated with increased opacity. Genomic B-opsin loss was more likely preceded by either gradual expansion of the LW-opsin expression domain (b) or immediate maximum expansion of LW-opsin expression (c). See text for further details.