A wide range of organisms have the ability to see in color, to distinguish between lights on the basis of their wavelength and independently of their intensity. The visual systems that underlie these capacities vary widely in the number of receptors involved, the number of wavelength regions that they can distinguish and the boundaries of the visible spectrum. Through this diversity, it is generally accepted that an organism must satisfy two basic requirements if it is to have color vision: (a) it must have at least two visual receptors tuned to different regions of the spectrum, and (b) its visual system must compare the outputs of these receptors.
I performed behavioral experiments to determine whether the fruitfly, Drosophila melanogaster, has color vision and, if so, what kind of color vision system it has. I focused on Drosophila because the range of resources available for manipulating its nervous system genetically make it potentially a very attractive model system for studying the biological basis of color vision.
At present research into color vision in Drosophila faces a puzzle. On the one hand, phototaxis is the only fly behavior that would appear to satisfy the preconditions for color vision. It is the only behavior that has been shown to implicate more than one receptor type and published behavioral studies suggest that flies may have opponent responses to different wavelengths. There is anatomical evidence for the existence of opponent connections in the underlying neural network that could support comparisons of the sort thought to be necessary for color vision.
At the same time, behavioral experiments over the years have shown that phototaxic behavior depends strongly on the intensity of the stimuli over a wide range of stimulus strengths. Whether flies choose lights of one wavelength over another can depend on the intensities of those lights, which is inconsistent with color vision being at work. Moreover, the network underlying phototaxis does not appear to segregate luminance and chromatic information. Such a convergence is consistent with the observed intensity-dependence of the behavior but is not typical of networks that underlie color vision.
To explore the hypothesis that flies have color vision, I carried out phototaxic experiments with wild type flies using lights of wavelengths across the spectrum over a range of intensities. Tests involved lights on one or both sides of the apparatus. The results indicate that when exposed to light on one-side wild type flies exhibit contrasting behavioral responses to 'UV' (331–355 nm) versus 'blue-green' (442–515 nm) stimuli over a range of higher intensities. Higher intensity UV light enhances phototaxis rates, while higher intensity visible light depresses rates. While this behavior does not satisfy the definition of color vision because the flies are not choosing between two lights, it suggests that flies possess some of the key elements of a color vision system.
In tests with lights on opposite sides of the apparatus, fly preference depends more strongly on intensity than in one-sided tests. In two-sided tests fly responses violate the principle of univariance, which is typical of responses based on color vision, but exhibit opponent behavior at only a limited range of intensities. Thus wild type behavior in two-light tests provides evidence of significant luminance input into elements of a color vision system.
The data on wild types suggest that R1-6, the main photoreceptor class in the fly retina, which respond to low intensity stimuli, may contribute to the intensity-dependence of phototaxis observed in many studies. To explore this hypothesis, I carried out phototaxic tests with Rh1-mutants, which lack functional R1-6 cells. While wild types respond differently to higher intensity UV and blue-green stimuli, the main spectral division in the behavior of Rh1-mutants is between 442 nm ('blue') and 515 nm ('green'). Flies are attracted to higher intensity blue light, while they avoid higher intensity green. Preference for blue over green is consistent over a wide range of intensity settings, though not all. Thus, Rh1 -mutant behavior closely approximates blue-green color vision. These results imply that R1-6 not only contribute to the detection of low intensity light but also have influence over the chromatic discriminations that they flies make.
|School Location:||United States -- New York|
|Source:||DAI-B 72/05, Dissertation Abstracts International|
|Keywords:||Color vision, Photoreceptors|
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