MBE Advance Access originally published online on May 23, 2006
Molecular Biology and Evolution 2006 23(8):1538-1547; doi:10.1093/molbev/msl014
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Research Article |
Evolution of the Cichlid Visual Palette through Ontogenetic Subfunctionalization of the Opsin Gene Arrays
* Hubbard Center for Genome Studies and Department of Zoology, University of New Hampshire; UCL Institute of Ophthalmology, University College London, London, UK; and Department of Biological Sciences, University of Maryland, Baltimore County
E-mail: spadyty{at}mail.nih.gov.
The evolution of cone opsin genes is characterized by a dynamic process of gene birth and death through gene duplication and loss. However, the forces governing the retention and death of opsin genes are poorly understood. African cichlid fishes have a range of ecologies, differing in habitat and foraging style, which make them ideal for examining the selective forces acting on the opsin gene family. In this work, we present data on the riverine cichlid, Oreochromis niloticus, which is an ancestral outgroup to the cichlid adaptive radiations in the Great African lakes. We identify 7 cone opsin genes with several instances of gene duplication. We also characterize the spectral sensitivities of these genes through reconstitution of visual pigments. Peak absorbances demonstrate that each tilapia cone opsin gene codes for a spectrally distinct visual pigment: SWS1 (360 nm), SWS2b (423 nm), SWS2a (456 nm), Rh2b (472 nm), Rh2a ß (518 nm), Rh2a 
(528 nm), and LWS (561 nm). Furthermore, quantitative reverse transcription polymerase chain reaction at 3 ontogenetic time points demonstrates that although only 4 genes (SWS2a, Rh2a and ß, and LWS) are expressed in adults, mRNAs for the other genes are all expressed during ontogeny. Therefore, subfunctionalization through differential ontogenetic expression may be a key mechanism for preservation of opsin genes. The distinct peak absorbances of these preserved opsin genes provide a palette from which selection creates the diverse visual sensitivities found among the cichlid species of the lacustrine adaptive radiations.
Key Words: cichlid • opsin • visual pigment • gene expression • real-time RT-PCR • subfunctionalization
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