based maximum-likelihood analysis indicated that, associated with the two blue spectral shifts, four amino acid sites (Ile 17Met, Ala64Ser, Asn70Ser, and Ser137Ala) have evolved substitutions in parallel and exhibit significant dN/dS >1. Homology modeling of the full-length Limenitis arthemis astyanax L opsin placed all four substitutions within the chromophore-binding pocket. Strikingly, the Ser137Ala substitution is in the same position as a site that in primates is responsible for a 5- to 7-nm blue spectral shift. Our data show that some of the same amino acid sites are under positive selection in the photopigments of both butterflies and primates, spanning an evolutionary distance >500 million years.
Ever since Darwin, there has been intense interest in the evolution of complex traits, with one of the most notable examples being the evolution of the eyes. Only three phyla have evolved image-resolving eyes: arthropods, mollusks, and chordates (Land and Fernald, 1992). Although there are vast morphological and neurobiological differences among the eyes of these lineages, there are also many similarities that may provide new insight into the molecular mechanisms governing the evolution of this complex trait.
Image-resolving eyes are composed of photoreceptor cells, which contain photopigments, and they also have accessory pigment cells, which shield the photopigments from stray light. Photopigments are made up of a light-sensitive chromophore (e.g., 11-cis-retinal) and an opsin protein. Although the isolated chromophore has an absorbance spectrum maximum (λmax value) at 380 nm (Han et al., 1998), the λmax values of photopigments can vary from 360 to 600 nm through the spectral tuning of the chromophore by specific interactions with amino acids in the chromophore-binding pocket of the opsin protein. Because photopigment sensitivities represent clear adaptations to an animal’s light environment (Yokoyama, 1997), the amino acid sites of opsins involved in spectral tuning may be under positive selection, which has been most extensively studied in fish, mammals, and primates (Surridge et al., 2003; Spady et al., 2005; Yokoyama and Takenaka, 2005).
Comparing these photopigment results with those of a more distant lineage such as butterflies could provide a better understanding of the molecular mechanisms of the eye and how these molecules change under selection. The visual systems of butterflies and primates may be under similar selective pressures. Butterflies and primates share a similar light environment and contain species that are nocturnally or diurnally active and species that are fruit-feeders. Like primates that are leaf eaters, butterfly females need to discriminate among foliage types for oviposition, which may have a strong impact on their color vision (see Kelber, 1999,