Cone opsin genes of African cichlid fishes: Tuning spectral sensitivity bydifferential gene expression

Spectral tuning of visual pigments is typically accomplished through change s in opsin amino acid sequence. Within a given opsin class, changes at a fe w key sites control wavelength specificity. To investigate known difference s in the visual pigment spectral sensitivity of the Lake Malawi cichlids, M etriaclima zebra (368, 488, and 533 nm) and Dimidiochromis compressiceps (4 47, 536, and 569 nm), we sequenced cone opsin genes from these species as w ell as Labeotropheus fuelleborni and Oreochromis niloticus. These cichlids have five distinct classes of cone opsin genes, including two unique SWS-2 genes. Comparisons of the inferred amino acid sequences from the five cone opsin genes of M. zebra, D. compressiceps, and L. fuelleborni show the sequ ences to be nearly identical. Therefore, evolution of key opsin sites canno t explain the differences in visual pigment sensitivities. Real-time PCR de monstrates that different cichlid species express different subsets of the available cone opsin genes. Metriaclima zebra and L. fuelleborni express a complement of genes which give them UV-shifted visual pigments, while D. co mpressiceps expresses a different set to produce a red-shifted visual syste m. Thus, variations in cichlid spectral sensitivity have arisen through evo lution of gene regulation, rather than through changes in opsin amino acid sequence.