Cone Opsin Genes of African Cichlid Fishes: Tuning Spectral Sensitivity by Differential Gene Expression

Carelton and Kocher (2001) studied the genetic basis for variation in spectral sensitivity among different species of cichlid fishes. The molecular basis of vision is found in the chromophore, a molecule bound inside a larger opsin protein, that when stimulated by light undergoes a conformation change. When the chromophore changes it induces a larger change in the surrounding opsin protein, which them passes along the signal. Cichlids have 5 classes of retinal opsins: RH2, SWS-1, SWS-2, and MWS/LWS. In this study 2 clades of Cichlids from east African lakes were compared: D compressiceps from the sand clade and M zebra from the rock clade. L fuelleborni, another rock clade cichlid species, and O niloticus, Tilapia. Previous studies have demonstrated that vision plays an important role in the mating behavior of cichlid species.

The experiment consisted of three parts: genomic DNA sequencing, sequence analysis, and real-time RT-PCR. Cone opsin genes were identified and gene-specific primers were developed. RACE was performed in both the 3’ and 5’ directions. The DNA was amplified and assembled. The nucleotide and amino acid sequence for the cichlids was used to create a phylogenetic tree. Also a 3D model of retinal was used to analyze the protein and determine where amino acids differences were most commonly found. The real time RT-PCR was used to study gene expression using a fluorescent tag.

The DNA sequencing showed all five opsin genes: RH2, SWS-1, SWS-2A, SWS-2B, and LWS/MWS. The genes were similar in D compressiceps and M zebra and showed greatest conservation in the area that would make up the chormophore holding pocket of the protein. RT-PCR showed that some genes had greater expression in one species than another. Specifically M zebra and L fuelleborni (both from the rock clade) both expressed RH2 and SWS-2B genes the most, while D compressiceps expressed RH2 and LWS the most. These results suggest that it is gene expression, not variation in chromophore type or genetic sequence that dictates the visual sensitivity of East African cichlids.

This article involves many of the vision related topics we touched on in class. First the article discusses which range in the electromagnetic spectra that cichlids see in. The rock dwelling cichlids see in a shorter wavelength than the sand dwelling cichlids. Another important feature to this study is the absorption of light by water, which changes the available visual range for the cichlids. Radiance is important, because the color of the cichlids could impact visual selection for those colors. The rock dwelling clade which can see in shorter wavelengths, is probably more ultraviolet than the sand dwelling clade. Light filtering is involved again because of the water’s transmitting properties. Light attenuation is also involved with this study, as the amount of light that reaches the area decreases the further one goes down in water. It’s not specifically mentioned, but it seems reasonable to hypothesize that cichlid’s optimal color is designed to stand out from the background hue, that they would have a contrasting pattern, and that they would have a conspicuous shape. I hypothesize this because the article suggests color is used in cichlid mating, rather than avoiding predation or some other selective fore. Opsins and choromophores are discussed, as this article investigates their expression. This study directly investigates mechanisms for tuning photoreceptors for color. Also the rock dwelling cichlids can see in UV, which based on the lecture suggests their ancestral to the sand dwellers who can’t see UV as well.

1.) Carelton, Karen L., Kocher, Thomas D. (2001). Cone Opsin Genes of African Cichlid Fishes: Tuning Spectral Sensitivity by Differential Gene Expression. Molecular Biology & Evolution, 8, 1540-1550