Rhodopsin (rh1) is the visual pigment expressed in rod photoreceptors of vertebrates that is responsible for initiating the critical first step of dim-light vision. Rhodopsin is usually a single copy gene; however, we previously discovered a novel rhodopsin-like gene expressed in the zebrafish retina, rh1-2, which we identified as a functional photosensitive pigment that binds 11-cis retinal and activates in response to light. Here, we localized expression of rh1-2 in the zebrafish retina to a subset of peripheral photoreceptor cells, which indicates a partially overlapping expression pattern with rh1. We also expressed, purified and characterized Rh1-2, including investigation of the stability of the biologically active intermediate. Using fluorescence spectroscopy, we found the half-life of the rate of retinal release of Rh1-2 following photoactivation to be more similar to that of the visual pigment rhodopsin than to the non-visual pigment exo-rhodopsin (exorh), which releases retinal around 5 times faster. Phylogenetic and molecular evolutionary analyses show that rh1-2 has ancient origins within teleost fishes, is under similar selective pressure to rh1, and likely experienced a burst of positive selection following its duplication and divergence from rh1. These findings indicate that rh1-2 is another functional visual rhodopsin gene, which contradicts the prevailing notion that visual rhodopsin is primarily found as a single copy gene within ray-finned fishes. The reasons for retention of this duplicate gene, as well as possible functional consequences for the visual system, are discussed.
The authors declare no competing or financial interests.
J.M.M. helped design the study, sequenced rhodopsin genes, expressed and characterized rhodopsins, and drafted the manuscript. S.L. helped design the study and performed in situ hybridization. M.D.F. and C.K. performed in situ hybridization. R.K.S. and E.A.G. ran the phylogenetic and selection analyses and provided text for the manuscript. F.S. contributed to the sequencing. V.T. helped design the study and provided guidance and edits for the manuscript. B.S.W.C. led study design, helped draft the manuscript and supervised all aspects of the project.
This work was supported by a Natural Sciences and Engineering Research Council of Canada Discovery Grant (B.S.W.C., V.T.), a Science Without Borders Fellowship (E.A.G) and a Vision Science Research Program Graduate Fellowship (J.M.M.).
In total, 19 new nucleotide sequences were submitted to NCBI GenBank, and are avialable under the following accession numbers: 16 new rh1 sequences (KY026025–KY026040) and three new rh1-2 sequences (KY026041–KY026043).
Supplementary information available online at http://jeb.biologists.org/lookup/doi/10.1242/jeb.145953.supplemental
- Received August 25, 2016.
- Accepted October 25, 2016.
- © 2017. Published by The Company of Biologists Ltd