Cqu

sequence of the opsin protein and its interaction with the chromophore. Modifications in the photoreceptor itself, such as the use of oil droplets by some species, mean that the actual response of the photoreceptor may not correspond to that of the visual pigment alone. Examples are the chicken Rh2 gene that is paired with a green oil droplet and the goldfish Rh2 that is green-shifted because of the modified chromophore it uses (Yokoyama and Yokoyama 2000). Because its different opsins respond differently, an organism can compare the signal strength of adjacent opsins to assess "color" and other attributes of incident light coming from the same part of the environment.

Insects have dual dim-bright and color vision capabilities that vary among the species that have been tested (Pichaud, Briscoe et al. 1999). Flies have independent receptors for the two types of vision, but the ability of other insects to distinguish between the two is less clear because the neural impulses to the brain from the two systems seem to merge. Honey bee and Drosophila eyes express three opsins that absorb maximally in the UV,blue, and green ranges. Some butterflies appear to have

Extracellular

NH2 000'

180, ala/ser, 3-4 nm 285, ala/thr, 14 nm 308, ser/ala, 18 nm

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