Evolution of the activation steps in the vertebrate phototransduction cascade analysed using agnathan eye transcriptomes

Vertebrate phototransduction represents the best understood example of response activation in a G protein cascade. Not only have the protein components been characterised comprehensively, but in addition the molecular mechanisms that mediate high amplification and rapid response kinetics are understood in sufficient detail to predict the onset phase of the response to light absorption. Although the phylogeny of the phototransduction proteins has been studied extensively in jawed vertebrates (gnathostomes), there is a paucity of information about the corresponding proteins in the jawless branch of vertebrates (agnathans), from which gnathostomes diverged around 500 million years ago. The only surviving jawless vertebrates are lampreys (around 40 species) and hagfish (around 80 species). We have applied high-throughput sequencing to eye tissue from one species of hagfish, two species of lamprey and five species of gnathostome fish, to obtain mRNA sequences for the components of the phototransduction cascade