Evolutionary Constraint on Visual and Nonvisual Mammalian Opsins

Animals have evolved light-sensitive G protein-coupled receptors, known as opsins, to detect coherent and ambient light for visual and nonvisual functions. These opsins have evolved to satisfy the particular lighting niches of the organisms that express them. While many unique patterns of evolution have been identified in mammals for rod and cone opsins, far less is known about the atypical mammalian opsins. Using genomic data from over 400 mammalian species from 22 orders, unique patterns of evolution for each mammalian opsins were identified, including photoisomerases, RGR-opsin (RGR) and peropsin (RRH), as well as atypical opsins, encephalopsin (OPN3), melanopsin (OPN4), and neuropsin (OPN5). The results demonstrate that OPN5 and rhodopsin show extreme conservation across all mammalian lineages. The cone opsins, SWS1 and LWS, and the nonvisual opsins, OPN3 and RRH, demonstrate a moderate degree of sequence conservation relative to other opsins, with some instances of lineage-specific gene loss. Finally, the photoisomerase, RGR, and the best-studied atypical opsin, OPN4, have high sequence diversity within mammals. These conservation patterns are maintained in human populations. Importantly, all mammalian opsins retain key amino acid residues important for conjugation to retinal-based chromophores, permitting light sensitivity. These patterns of evolution are discussed along with known functions of each atypical opsin, such as in circadian or metabolic physiology, to provide insight into the observed patterns of evolutionary constraint.

Automated summary

Through analyzing genomic data from over 400 mammalian species from 22 orders, this study identified unique patterns of evolution for each mammalian opsins, highlighting the varying degrees of sequence conservation and lineage-specific gene loss. The findings demonstrate extreme conservation for OPN5 and rhodopsin across all mammalian lineages, while showing moderate conservation for cone opsins, nonvisual opsins, and high sequence diversity for photoisomerase and the well-studied atypical opsin, OPN4, within mammals. These conservation patterns are maintained in human populations, indicating the importance of key amino acid residues for retinal-based chromophores in all mammalian opsins. Further discussions on the functions of each atypical opsins provide insight into the observed patterns of evolutionary constraint in circadian or metabolic physiology.