Evolution of visual pigments in passerine birds: From opsin genes to visual pigment function

Understanding the great diversity of color in the animal world requires understanding how animals perceive color, the environmental context, and the extent to which the color vision and color co-evolve. Here I ask how color vision varies across related bird species with the ultimate goal of evaluating the role of color perception in the evolution of plumage coloration. I focus on the visual pigments (opsins) of New and Old World warblers, two distantly related groups of birds that occupy similar environments but differ dramatically in plumage color. I began by assessing how much warbler opsins vary at the sequence level and found substantial amino acid variation. I expressed these visual pigments in vitro, and found that despite having accumulated amino acid substitutions, warblers visual pigments have very similar spectral sensitivities. I examined the evolutionary dynamics of visual pigments further by integrating these warbler data with that of other passerine birds. For the short-wavelength or "blue" sensitive pigment (SWS2), I found an association of spectral sensitivity and habitat. These differences are in a manner expected from the light composition in these habitats. Moreover, I reconstructed ancestral pigments and thus the history of SWS2 divergence from the Old World warblers to their New World counterparts. I showed SWS2 spectral sensitivity results from shared ancestry, and not convergence.;Divergence in the visual system can occur through changes in opsin gene sequence and visual pigments themselves, or by differences in the regulation of the expression of these genes. I evaluated opsin gene expression divergence in New World warblers and how it relates to plumage sexual dimorphism and ecology. I found differences in the expression of opsins among species and between sexes. More specifically, the degree of sex-bias was different among warbler species: most species have female-biased expression (higher relative expression in females than males), but other species exhibited the opposite pattern or no sex bias, implying sex bias in opsin expression is an evolutionarily labile trait. I evaluated these results using several measures of ecology and sexual dimorphism, and show that females of species with the most sexual dimorphism in plumage also have higher Sws2 expression, a pattern that is not present in males. This suggests a relationship between an aspect of the female visual system and divergence in male plumage, as predicted by classic sexual selection models. I also found that the expression of two additional opsins, Lws and Sws1, varies in birds occupying different habitats and does so in the direction we would expect from the composition and amount of light available in those environments.;Overall, I found visual pigment spectral tuning evolves very slowly while regulation of opsin gene expression allows for fast adaptive evolution of the visual system in response to natural and sexual selection, and, in particular, sex-specific selection pressures.