Opsin coexpression tunes different regions of the retina to spectrally distinct parts of the visual field (#449)
Each part of the visual field is viewed by a distinct region of the retina, and these regions often differ in sensitivity. Variation in spectral sensitivity has been documented within the retinas of a diverse and growing number of vertebrate and invertebrate taxa. In such species, the appearance of objects and signals will depend on the sensitivity of the retinal region that detects them. The retina's spectral sensitivity is determined primarily by visual pigments of the photoreceptors. These visual pigments are comprised of an opsin protein bound to a chromophore, and altering either molecule can modify the pigment's absorbance spectrum. We use dual-labeling fluorescent in situ hybridisation to determine the distribution of opsins within cell types and across the retinas of African cichlids. The cichlids of Africa's Great Lakes are noteworthy because they form new species extremely rapidly, due in part to coevolution of spectral sensitivity and male nuptial colouration. We found that in the cichlid Metriaclima zebra, double cones (pairs of partially fused cone cells) mix different combinations of opsins in retinal regions that view distinct backgrounds. This opsin coexpression increases absorbance of the corresponding backgrounds, and modeling indicates this could facilitate detection of dark objects. Thus, opsin coexpression may be a novel mechanism of spectral tuning that could be useful for detecting prey, predators and mates. However, our calculations show that coexpression of some opsins can hinder colour discrimination, creating a trade-off between visual functions. Additional experiments demonstrate that the location and relative proportion of opsins within the M. zebra retina can be altered by the rearing light environment. Thus, the background spectra at different angles in a cichlid's environment might influence the specific pattern of spectral sensitivity that develops across its retina, biasing signal evolution and contributing to the rapid radiation of this diverse group of fishes.
- Dalton BE, Loew ER, Cronin TW, Carleton KL (2014) Spectral tuning by opsin coexpression in retinal regions that view different parts of the visual field. Proceedings of the Royal Society B: Biological Sciences 281.
- Dalton BE, Lu J, Leips J, Cronin TW, Carleton KL (Under revision) Plasticity of spectral tuning by regional opsin coexpression in the African cichlid Metriaclima zebra. Molecular Ecology.