Browsing by Author "Hofmann, Christopher M."
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Item Characterization of visual pigments, oil droplets, lens and cornea in the whooping crane Grus americana(The Company of Biologists Ltd, 2014-10-29) Porter, Megan L.; Kingston, Alexandra C. N.; McCready, Robert; Cameron, Evan G.; Hofmann, Christopher M.; Suarez, Lauren; Olsen, Glenn H.; Cronin, Thomas W.; Robinson, Phyllis R.Vision has been investigated in many species of birds, but few studies have considered the visual systems of large birds and the particular implications of large eyes and long-life spans on visual system capabilities. To address these issues we investigated the visual system of the whooping crane Grus americana (Gruiformes, Gruidae), which is one of only two North American crane species. It is a large, long-lived bird in which UV sensitivity might be reduced by chromatic aberration and entrance of UV radiation into the eye could be detrimental to retinal tissues. To investigate the whooping crane visual system we used microspectrophotometry to determine the absorbance spectra of retinal oil droplets and to investigate whether the ocular media (i.e. the lens and cornea) absorb UV radiation. In vitro expression and reconstitution was used to determine the absorbance spectra of rod and cone visual pigments. The rod visual pigments had wavelengths of peak absorbance (λmax) at 500 nm, whereas the cone visual pigment λmax values were determined to be 404 nm (SWS1), 450 nm (SWS2), 499 nm (RH2) and 561 nm (LWS), similar to other characterized bird visual pigment absorbance values. The oil droplet cut-off wavelength (λcut) values similarly fell within ranges recorded in other avian species: 576 nm (R-type), 522 nm (Ytype), 506 nm (P-type) and 448 nm (C-type). We confirm that G. americana has a violet-sensitive visual system; however, as a consequence of the λmax of the SWS1 visual pigment (404 nm), it might also have some UV sensitivity.Item Evolution of Sexual Dichromatism. 1. Convergent Losses of Elaborate Female Coloration in New World Orioles (Icterus spp.)(American Ornithological Society, 2008-02-05) Hofmann, Christopher M.; Cronin, Thomas W.; Omland, Kevin E.Studies of sexual dimorphism often focus on the evolution of elaborate male traits, whereas the evolution of elaborate females has been largely ignored. Yet a phylogenetic perspective suggests that changes in either male or female traits may lead to the evolution of sexual dimorphism. Furthermore, changes in the degree of sexual dichromatism can be caused by gains or losses of elaboration. One common form of elaboration found throughout the animal kingdom is the use of highly saturated and contrasting colors. To investigate further the evolution of female elaboration and sexual dichromatism, we took quantitative measurements of color from New World orioles (Icterus spp.) and then used ancestral-state reconstruction to infer evolutionary changes in male and female elaboration. Our findings suggest that male elaboration is ancestral and strongly conserved but that female elaboration has changed repeatedly, especially through the loss of saturation and contrast. Thus, changes in female—rather than male—color appear to lead to the evolution of sexual dichromatism in orioles. These repeated gains of strong sexual dichromatism through the loss of female elaboration were supported using multiple methods of character coding and reconstruction. Our phylogenetic results suggest that studies of sexual dichromatism cannot assume that color dimorphism arises through increased male elaboration. These findings have important implications for future studies investigating the ultimate causes of sexual dichromatism.Item Evolution of Sexual Dichromatism. 2. Carotenoids and Melanins Contribute to Sexual Dichromatism in New World Orioles (Icterus spp.)(American Ornithological Society, 2008-02-05) Hofmann, Christopher M.; Cronin, Thomas W.; Omland, Kevin E.Several recent studies have investigated how different proximate mechanisms of color production contribute to sexual dichromatism. These studies suggest that carotenoid pigments—which are frequently subject to sexual selection—are more strongly associated with sexual dichromatism than melanins. This reasoning implicitly assumes that increased male elaboration leads to sexual dichromatism. However, sexual dichromatism can be generated through multiple evolutionary pathways, including decreases in female elaboration. We examined whether evolutionary changes in carotenoid- and melanin-based plumage were correlated within New World orioles (Icterus spp.), a genus in which male elaboration is ancestral and only female elaboration varies. We found a significant correlation between evolutionary changes in the degree of carotenoid and eumelanin sexual dichromatism. These findings differ from those of previous comparative studies and suggest the possibility of interesting differences when different evolutionary pathways—such as changes in male versus female coloration—lead to sexual dichromatism.Item The Eyes Have It: Regulatory and Structural Changes Both Underlie Cichlid Visual Pigment Diversity(PLOS, 2009-12-22) Hofmann, Christopher M.; O’Quin, Kelly E.; Marshall, N. Justin; Cronin, Thomas W.; Seehausen, Ole; Carleton, Karen L.A major goal of evolutionary biology is to unravel the molecular genetic mechanisms that underlie functional diversification and adaptation. We investigated how changes in gene regulation and coding sequence contribute to sensory diversification in two replicate radiations of cichlid fishes. In the clear waters of Lake Malawi, differential opsin expression generates diverse visual systems, with sensitivities extending from the ultraviolet to the red regions of the spectrum. These sensitivities fall into three distinct clusters and are correlated with foraging habits. In the turbid waters of Lake Victoria, visual sensitivity is constrained to longer wavelengths, and opsin expression is correlated with ambient light. In addition to regulatory changes, we found that the opsins coding for the shortest- and longest-wavelength visual pigments have elevated numbers of potentially functional substitutions. Thus, we present a model of sensory evolution in which both molecular genetic mechanisms work in concert. Changes in gene expression generate large shifts in visual pigment sensitivity across the collective opsin spectral range, but changes in coding sequence appear to fine-tune visual pigment sensitivity at the shortand long-wavelength ends of this range, where differential opsin expression can no longer extend visual pigment sensitivity.Item Using Color to Define Species Boundaries: Quantitative Analysis in The Orchard Oriole Complex Supports the Recognition of Two Species(American Ornithological Society, 2007-03-15) Kiere, Lynna M.; Hofmann, Christopher M.; Tracy, Ian E.; Cronin, Thomas W.; Leips, Jeff; Omland, Kevin E.The recent divergence of Orchard (Icterus spurius spurius) and Fuertes’s Orioles (I. s. fuertesi) makes them an ideal system for investigating species boundaries. Orchard and Fuertes’s Orioles differ in several respects. They have distinct breeding ranges—Fuertes’s Orioles breed in eastern coastal Mexico, whereas Orchard Orioles breed throughout eastern and central North America—and differ in plumage coloration, with adult male Orchard Orioles appearing ‘‘chestnut’’ and Fuertes’s Orioles ‘‘ochre.’’ However, no detailed quantitative color analyses have been conducted. To characterize these differences we quantitatively measured plumage color using reflectance spectrometry. The colored plumages of adult male Orchard and Fuertes’s Orioles have unique spectral characteristics, with no color overlap between them. Combined with life history differences and previous molecular studies, these findings support the classification of Orchard and Fuertes’s Orioles as separate species. Additionally, this study demonstrates the utility of quantitative color measurement as a tool for examining species boundaries.Item Using Spectral Data To Reconstruct Evolutionary Changes In Coloration: Carotenoid Color Evolution In New World Orioles(John Wiley & Sons, 2007-05-08) Hofmann, Christopher M.; Cronin, Thomas W.; Omland, Kevin E.Carotenoid‐based colors are thought to play an important signaling role in many animal taxa. However, little is known about evolutionary changes in carotenoid coloration, especially among closely related species. We used a phylogenetic perspective to examine carotenoid color changes within New World orioles (genus Icterus). Oriole color was quantitatively measured using reflectance spectrometry. We found continuous variation from short‐ to long‐wavelength carotenoid colors in extant orioles–perceived by humans as ranging from yellow to scarlet–suggesting that these carotenoid‐based colors have evolved as a continuous character. Ancestral state reconstruction suggests that short‐ and long‐wavelength carotenoid colors have evolved independently multiple times, likely from a middle‐wavelength ancestor. Although color showed considerable lability, we found a significant amount of phylogenetic signal across the entire genus. This implies that while labile, the colors of closely related taxa tended to resemble each other more than would be expected due to chance. To our knowledge, this is the first study to use quantitative character states derived from reflectance spectra in ancestral state reconstruction. Reflectance spectra provide an unbiased quantitative description of color that allowed us to detect subtle changes among closely related taxa. Using these quantitative methods to score and reconstruct color changes among closely related taxa provides a better understanding of how elaborate animal colors evolve.