Abstract

BackgroundOpsins are the only class of proteins used for light perception in image-forming eyes. Gene duplication and subsequent functional divergence of opsins have played an important role in expanding photoreceptive capabilities of organisms by altering what wavelengths of light are absorbed by photoreceptors (spectral tuning). However, new opsin copies may also acquire novel function or subdivide ancestral functions through changes to temporal, spatial or the level of gene expression. Here, we test how opsin gene copies diversify in function and evolutionary fate by characterizing four rhabdomeric (Gq-protein coupled) opsins in the scallop, Argopecten irradians, identified from tissue-specific transcriptomes.ResultsUnder a phylogenetic analysis, we recovered a pattern consistent with two rounds of duplication that generated the genetic diversity of scallop Gq-opsins. We found strong support for differential expression of paralogous Gq-opsins across ocular and extra-ocular photosensitive tissues, suggesting that scallop Gq-opsins are used in different biological contexts due to molecular alternations outside and within the protein-coding regions. Finally, we used available protein models to predict which amino acid residues interact with the light-absorbing chromophore. Variation in these residues suggests that the four Gq-opsin paralogs absorb different wavelengths of light.ConclusionsOur results uncover novel genetic and functional diversity in the light-sensing structures of the scallop, demonstrating the complicated nature of Gq-opsin diversification after gene duplication. Our results highlight a change in the nearly ubiquitous shadow response in molluscs to a narrowed functional specificity for visual processes in the eyed scallop. Our findings provide a starting point to study how gene duplication may coincide with eye evolution, and more specifically, different ways neofunctionalization of Gq-opsins may occur.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0823-9) contains supplementary material, which is available to authorized users.

Highlights

  • Opsins are the only class of proteins used for light perception in image-forming eyes

  • Transcriptomic and phylogenetic analyses reveal four Gq-opsin genes in scallop To determine the number of Gq-opsin genes in scallop, we performed deep transcriptome sequencing of tissuespecific libraries derived from dissected eyes, mantle tissue, and adductor muscle of Argopecten irradians

  • Amino acid percent similarity was more conserved between the aligned Helix 1 (H1) through H7, and ranged from 92.6% (Air-OPNGq2 versus Air-OPNGq3) to 76.9% (Air-OPNGq1 versus Air-OPNGq4) (Table 1)

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Summary

Introduction

Opsins are the only class of proteins used for light perception in image-forming eyes. Gene duplication and subsequent functional divergence of opsins have played an important role in expanding photoreceptive capabilities of organisms by altering what wavelengths of light are absorbed by photoreceptors (spectral tuning). New opsin copies may acquire novel function or subdivide ancestral functions through changes to temporal, spatial or the level of gene expression. If gene duplicates are retained, they can follow one of three evolutionary fates (first outlined by [7]; see expanded models reviewed by [8,9,10]) If both paralogs have the exact same function or suite of functions, the existence of a second copy can increase production levels of encoded protein (“gene conservation” [11]). One copy of the duplicated gene can acquire a novel function while the other copy retains the original, pre-duplication function (“neofunctionalization” [7])

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