Evolution of the vertebrate phototransduction cascade activation steps

• Published in: Developmental biology 2017-11, Vol.431 (1), p.77-92
• Main Authors: Lamb, Trevor D., Hunt, David M.
• Format: Article
• Language: English
• Subjects: Animals; Cyclic Nucleotide Phosphodiesterases, Type 6 - genetics; Cyclic Nucleotide Phosphodiesterases, Type 6 - physiology; Cyclic Nucleotide-Gated Cation Channels - genetics; Cyclic Nucleotide-Gated Cation Channels - physiology; Cyclic nucleotide-gated channel; Evolution; Evolution, Molecular; Gene Duplication; Humans; Models, Genetic; Opsin; Opsins - genetics; Opsins - physiology; Phosphodiesterase; Photoreceptor Cells, Vertebrate - physiology; Phototransduction; Phylogeny; Transducin; Transducin - genetics; Transducin - physiology; Vertebrates - genetics; Vertebrates - growth & development; Vertebrates - physiology; Vision, Ocular - genetics; Vision, Ocular - physiology
• ISSN: 0012-1606; 1095-564X
• DOI: 10.1016/j.ydbio.2017.03.018

We examine the molecular phylogeny of the proteins underlying the activation steps of vertebrate phototransduction, for both agnathan and jawed vertebrate taxa. We expand the number of taxa analysed and we update the alignment and tree building methodology from a previous analysis. For each of the four primary components (the G-protein transducin alpha subunit, GαT, the cyclic GMP phosphodiesterase, PDE6, and the alpha and beta subunits of the cGMP-gated ion channel, CNGC), the phylogenies appear consistent with expansion from an ancestral proto-vertebrate cascade during two rounds of whole-genome duplication followed by divergence of the agnathan and jawed vertebrate lineages. In each case, we consider possible scenarios for the underlying gene duplications and losses, and we apply relevant constraints to the tree construction. From tests of the topology of the resulting trees, we obtain a scenario for the expansion of each component during 2R that accurately fits the observations. Similar analysis of the visual opsins indicates that the only expansion to have occurred during 2R was the formation of Rh1 and Rh2. Finally, we propose a hypothetical scenario for the conversion of an ancestral chordate cascade into the proto-vertebrate phototransduction cascade, prior to whole-genome duplication. Together, our models provide a plausible account for the origin and expansion of the vertebrate phototransduction cascade. •We examine the phylogeny of phototransduction genes for agnathan and jawed vertebrates.•The results are consistent with gene expansion during 2R, followed by speciation.•We present a specific scenario that accounts for the observed phylogenies.•We also propose a scenario for the origin of the proto-vertebrate cascade prior to 2R.•Together, these models can account for the origin of vertebrate phototransduction.