BRISCOE, A.D.; University of California, Irvine: Evolution of Color Vision: Butterflies as a Model

The visual pigments (rhodopsins) of invertebrate photoreceptor cells are composed of a light-sensitive retinaldehyde chromophore attached to a membrane-bound opsin protein. Surveys of spectral sensitivity measurements, visual pigment spectra and opsin gene sequences indicate that all butterfly eyes, like those of most other insects, contain UV-, blue- and green-sensitive rhodopsins. Some species also contain a fourth or fifth type, related in amino acid sequence to green-sensitive insect rhodopsins, but shifted in absorbance to provide red-sensitivity. Studies seeking to map the retinal mosaics of butterflies with more than three visual pigments find complex heterogeneity in the distribution of these rhodopsins among the retinular cells of individual ommatidia. Combining the approaches of electron microscopy, epi-microspectrophotometry, PCR cloning and in situ hybridization, we have found that the compound eye of Vanessa cardui has the typical ultrastructural features of the butterfly retina, but contains only the three common insect rhodopsins. Vanessa thus provides an opportunity to assess the retinal mosaic of what is likely to be the more primitive lepidopteran retinal organization. Opsin mRNA expression patterns among the photoreceptor cells of individual ommatidia indicate that there are major differences between the color vision systems of nymphalids and papilionid butterfly families: the nymphalid Vanessa has a simpler, trichromatic, system than do tetrachromatic papilionids that have been studied.