True blue: S-opsin is widely expressed in different animal species.

Summary Colour vision in animals is an interesting, fascinating subject. In this study, we examined a wide variety of species for expression of S-opsin (blue sensitive) and M-/L-opsin (green-red sensitive) in retinal cones using two novel monoclonal antibodies specific for peptides from human opsins. Mouse, rat and hare did not express one of the investigated epitopes, but we could clearly prove existence of cones through peanut agglutinin labelling. Retinas of guinea pig, dog, wolf, marten, cat, roe deer, pig and horse were positive for S-opsin, but not for M-/L-opsin. Nevertheless all these species are clearly at least dichromats, because we could detect further S-opsin negative cones by labelling with cone arrestin specific antibody. In contrast, pheasant and char had M-/L-opsin positive cones, but no S-opsin expressing cones. Sheep, cattle, monkey, men, pigeon, duck and chicken were positive for both opsins. Visual acuity analyzed through density of retinal ganglion cells revealed least visual discrimination by horses and highest resolution in pheasant and pigeon. Most mammals studied are dichromats with visual perception similar to red-green blind people.