Wednesday, April 8, 2009
UV vision
On pages 110-113 Carroll talks about UV vision, especially among birds. Carroll says that “a single change may alter the function of the SWS opsin” (111). In your response explain what SWS opsin is and how it relates to UV vision. Also explain the importance of UV vision. Talk about the serine and cysteine amino acids and what will happen when they are replaced with one another. What other species use UV vision and how does it differ from the UV vision in birds?
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The SWS opsin gene is a short wavelength (detects light <400 nm) opsin that is used to detect the color violet and ultraviolet in many mammals, thus enabling them to have UV vision. UV vision is an important part of the visual systems of many organisms because it plays many roles in mating, finding food, and feeding. Female blue tits (Cyanistes caeruleus) prefer males whose crests are the brightest and reflect the most UV light. Honeybees use their UV vision to find pollen and conversely, plants have pigments that can be detected in UV vision to attract pollinators like bees. Carroll discusses the role of UV vision in the feeding of young nestlings; the chicks have highly reflective mouths in the UV band of light and this draws the attention of their parent when it comes time to feed.
ReplyDeleteCarroll states that “a single change may alter the function of the SWS opsin”; this occurs at position 90 in the bird SWS opsin. Birds with the amino acid serine see in the violet range while birds with the amino acid cysteine have the ability to see in the ultraviolet range. This seemingly trivial difference at position 90 of the bird SWS opsin gene determines whether or not the bird can see UV light. Those with normal, functioning SWS opsin genes (amino acid cysteine at position 90) can see UV light while those with a mutation that causes serine at position 90 can only see in the violet range. This single amino acid difference shifts the absorption maximum by about 35 nm: from around 370 nm to 405 nm.
Some fish, amphibians, reptiles, and mammals have UV vision. Some specific examples are honeybees, butterflies, bats, and blue tits. In general, any species with an SWS opsin tuned to around 360-370 nm is capable of seeing UV light.
SWS opsin or the short wave sensitive opsin is one of the two photoreceptors that each color-vision organism needs. Most mammals have two cone photoreceptors, which are fine-tuned to absorb light from one of the different parts of the visible spectrum (one short and one long). SWS are thought to have been turned to ultraviolet wavelengths in early mammals. However, a change in the SWS opsin has shown to shift the UV sensitive SWS photoreceptors to the violet region of the spectrum. A study done at the University College London studied the molecular changes that occurred in two varieties of tree squirrel opsin, and its shift from UV to violet light. The study showed that the grey squirrel’s (Sciurus carolinensis) SWS opsin had a peak absorption in the violet range of the light spectrum. This was compared to the UV sensitive pigment from a mouse, and a difference in the position of a phenylanine (Phe) was substituted with tyrosine (Tyr). In order to test if this truly changed the vision from UV to violet, the Tyr and Phe were switched back in a grey squirrel. The results showed that the spectrum shifted back to UV. This then proves that a change in the phenylaline, serine or systeine amino acids can cause a dramatic change in the type of vision within an organism.
ReplyDeletehttp://jeb.biologists.org/cgi/reprint/209/11/v.pdf
According to Carroll, “SWS opsin determines whether a particular species [of bird] is sensitive to violet or ultraviolet light” (110). The sensitivity is determined by the position of the amino acid in the SWS opsin. Carroll points out that birds with the amino acid serine in position 90 of the SWS opsin in the violet range, while birds with the cysteine acid in that same position see in the ultraviolet range. To continue, Carroll reveals that if the two amino acids are swapped then, such as serine being swapped with cysteine, a violet pigment transforms to ultra-violet sensitive and vice versa. As Carroll develops his discussion of UV vision in birds, he reveals that ultraviolet vision evolved independently in birds about four different times. Carroll attributes to the repletion of natural selection to sexual selection: “mating preferences in ultraviolet-sensitive species is affected by colors and patterns that are visible only in the ultraviolet range” (111).
ReplyDeleteThis type of sexual selection driven preference for vision has been observed in other species. Recently scientists of the University of Bristol examined the relationship between ultraviolet vision and mate choice in the guppy. They conducted a series of behavioral experiments to investigate the role of UV perception in mate choice in both female and male guppies. In their experiments the visual appearance of potential mates was manipulated using either UV transmitting (UV+) or UV blocking (UV-) filters. Female guppies significantly preferred UV+ males. Male guppies tended to prefer UV- females, but their preferences were marginally nonsignificant. Further experiments investigating the role of luminance, indicate that UV wavelengths are probably being used for color discrimination rather than for detecting differences in brightness. These experiments raise the possibility that UV is used in mate assessment in different ways by male and female guppies. This may reflect the different strategies that the two sexes have in order to maximize reproductive success.
There seems to exists an inherent contradiction between sexual and natural selection when it comes to plumage feathers. While the brightly colored feathers increase the allure of a possible mate, it also seems it would increase its presence to predators. In a recent study conducted by University of Arizona they demonstrated how differences in color vision of passerines, birds, makes them less conspicuous in the eyes of predators. By using a retinal model to compare reflectance from the plumages of Swedish songbirds to the reflectance of their natural backgrounds, they found their color badges to be significantly more conspicuous to other songbirds (which have a UV-tuned visual system) than to raptors and corvids (which have a violet-tuned system) in both coniferous and deciduous forests, consistent with an adaptive private communication system. Their results showed that male songbirds tune their signals to the vision system of the intended, conspecific receiver while reducing color contrast to the background in spectral parts to which their predators are most sensitive. The colors of the forehead and chest are significantly more conspicuous to other songbirds than to avian predators in both the coniferous and deciduous forests. In other words, songbirds use color signals that are more visible to their vision system than to that of their predators, allowing a directed communication channel for displaying male quality.
Bird color vision differs from that of humans in two main ways. As previously explained, birds can see ultraviolet light. It may be that birds are similar to bees in terms of vision. Bees, like humans, have three receptor types, although unlike humans they are sensitive to ultraviolet light, with loss of sensitivity at the red end of the spectrum. This spectral range is achieved by having a cone type that is sensitive to UV wavelengths, and two that are sensitive to "human visible" wavelengths. Because color is the result of differences in output of receptor types, this means that bees do not simply see additional 'UV colors', they will perceive even human-visible spectra in different hues to those which humans experience.
Bees are trichromatic, like humans, so the three dimensions of bee color can be mapped onto the three dimensions of human color. With birds, and indeed many other non-mammalian vertebrates, life is not so simple. As well as seeing very well in the ultraviolet, all bird species that have been studied have at least four types of cone. They have four, not three, dimensional color vision. Recent studies have confirmed tetra-chromacy in some fish and turtles. While UV reception increases the range of wavelengths over which birds can see, increased dimensionality produces a qualitative change in the nature of color perception that probably cannot be translated into human experience. Bird colors are not simply refinements of the hues that humans, or bees, see, these are hues unknown to any trichromat.
The differentiation of UV vision among different organism goes back to Carroll’s discussion of “use it or lose it:” Natural’s selection tendency to purge a genetic code of injurious mutations. According to scientists Si and Yokoyama the presence of UV vision is associated strongly with the availability of UV light in the environment and with the UV-dependent behaviors of the animals. One striking example is the pseudogenization of the SWS1 gene in coelacanth, the so-called ”living fossil” that lives at depths ≈200 m in the ocean, where short-wavelength lights such as UV cannot reach . Presumably for a similar reason, the dolphin also lost the gene. Although UV vision is important to many organisms, UV light may also damage retinal tissues. In humans and many other species, screening pigments in lenses or corneas effectively obviate most UV light so our retinas are protected. It may be because of this and other reasons that humans, cow, chicken, and many terrestrial vertebrates do not possess UV vision.
Another question regarding to evolution of UV vision is whether the common ancestor had UV vision. Scientists Shi and Yokoyama attempted to explore this question by first computationally inferring the protein sequence of the ancestral SWS1 opsin. They then used the recombinant DNA techniques and site-directed mutagenesis to make this ancestral opsin in the laboratory, incubated the opsin with chromophore, and measured the max wavelength of the regenerated visual pigment. The result showed that the visual pigment has a wavelength max of 361 nm, suggesting that the vertebrate ancestor could sense UV. Through more meticulous examinations, they concluded that the ancestral opsin was UV-sensitive. The presence of a UV-sensitive opsin, however, is not equivalent to a color capacity of the organism in that part of the spectrum, because the opsin may detect the light, but its output may be wired into the brain in a manner that would not allow color vision
Sources:
http://beheco.oxfordjournals.org/cgi/content/full/13/1/11
http://www.pnas.org/content/102/18/6391.full
http://www.bio.bris.ac.uk/research/vision/4d.htm
http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=166178