EVOLUTION, DEVELOPMENT, AND GENETICS OF OPSIN GENE EXPRESSION IN AFRICAN CICHLID FISHES
O'Quin, Kelly E
Carleton, Karen L
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The molecular genetic mechanisms that underlie phenotypic evolution include mutations within protein-coding, cis-regulatory, and trans-regulatory factors. Although many studies have examined how these mutations individually contribute to phenotypic divergence and the formation of new species, none have examined how they may do so collectively. In this study, I examine how these molecular genetic mutations collectively contribute to the evolution of color vision among African cichlid fishes. I show that phenotypic divergence in cichlid color vision is achieved by mutations affecting the coding sequence and expression of seven opsin genes. After contrasting the roles of these two mechanisms, I use bioinformatic-, association-, and experimental genetic analyses to determine what role mutations in cis- and trans-regulatory DNA play in the evolution of cichlid opsin expression. Specifically, I demonstrate that: (1) Protein-coding mutations primarily affect cichlid opsins sensitive to the ends of the visible light spectrum (SWS1 [ultraviolet-sensitive] and LWS [red-sensitive]). (2) Changes in opsin gene expression contribute to large differences in color vision among closely related species. These analyses also reveal that the expression of the SWS1 and SWS2B opsins have diverged among closely related cichlids in association with foraging preferences and ambient light intensity, suggesting that their expression has evolved due to natural selection. Ancestral state reconstructions reveal that changes in opsin expression have evolved repeatedly among cichlids in Lakes Tanganyika and Malawi; further, I find that this repeated evolution has likely been achieved by repeated changes to cichlid development. (3) Bioinformatic analyses suggest that cichlids have diverged in multiple cis-regulatory sequences surrounding the opsin genes, and association mapping identified three putative single nucleotide polymorphisms upstream of the SWS2A (blue), RH2B (blue-green), and LWS (red) opsins that may contribute to cichlid opsin expression differences in cis. (4) Genetic mapping in experimental crosses suggests that divergence in multiple trans-regulatory factors also contribute to the evolution of SWS2B (violet), RH2A (green), and LWS (red) opsin expression. The contribution of these trans-regulatory factors to the evolution of cichlid opsin expression may outweigh those in cis. These results reveal that multiple molecular genetic mechanisms can contribute to phenotypic evolution among closely related species.