Abstract
Recent progress in whole genome sequencing has revealed that animals have various kinds of opsin genes for photoreception. Among them, most opsin genes have introns in their coding regions. However, it has been known for a long time that teleost retinas express intron-less rhodopsin genes, which are presumed to have been formed by retroduplication from an ancestral intron-containing rhodopsin gene. In addition, teleosts have an intron-containing rhodopsin gene (exo-rhodopsin) exclusively for pineal photoreception. In this study, to unravel the evolutionary origin of the two teleost rhodopsin genes, we analyzed the rhodopsin genes of non-teleost fishes in the Actinopterygii. The phylogenetic analysis of full-length sequences of bichir, sturgeon and gar rhodopsins revealed that retroduplication of the rhodopsin gene occurred after branching of the bichir lineage. In addition, analysis of the tissue distribution and the molecular properties of bichir, sturgeon and gar rhodopsins showed that the abundant and exclusive expression of intron-containing rhodopsin in the pineal gland and the short lifetime of its meta II intermediate, which leads to optimization for pineal photoreception, were achieved after branching of the gar lineage. Based on these results, we propose a stepwise evolutionary model of teleost intron-containing and intron-less rhodopsin genes.
Highlights
Opsins are photoreceptive molecules that universally underlie the molecular basis of visual and non-visual photoreception in animals[1,2,3]
The conservation of the exon/intron structures of rhodopsin genes isolated from cyclostomes, cartilaginous fishes and tetrapods suggests the possibility that the teleost intron-less rhodopsin genes emerged by retroduplication near the base of the Actinopterygii[11,12]
We propose the evolutionary history of rhodopsin in the Actinopterygii, including when retroduplication of the rhodopsin gene occurred and when different expression patterns of teleost intron-containing and intron-less rhodopsins emerged
Summary
Opsins are photoreceptive molecules that universally underlie the molecular basis of visual and non-visual photoreception in animals[1,2,3]. We characterized the full-length coding sequences of intron-containing and intron-less rhodopsin genes from major lineages of non-teleost fishes in the Actinopterygii and analyzed their phylogenetic relationship. To perform genomic PCR of gray bichir, reedfish and Siberian sturgeon rhodopsin, we designed three pairs of primers based on their mRNA sequences.
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