Evolutionary ecology of the visual opsin gene sequence and its expression in turbot (Scophthalmus maximus)

Yunong Wang,Xian Li,Xiaona Ma,Lele Wu,Shihong Xu,Jun Li,Tengfei Du,Li Zhou,Changbin Song

doi:10.1186/s12862-021-01837-2

Abstract

BackgroundAs flatfish, turbot undergo metamorphosis as part of their life cycle. In the larval stage, turbot live at the ocean surface, but after metamorphosis they move to deeper water and turn to benthic life. Thus, the light environment differs greatly between life stages. The visual system plays a great role in organic evolution, but reports of the relationship between the visual system and benthic life are rare. In this study, we reported the molecular and evolutionary analysis of opsin genes in turbot, and the heterochronic shifts in opsin expression during development.ResultsOur gene synteny analysis showed that subtype RH2C was not on the same gene cluster as the other four green-sensitive opsin genes (RH2) in turbot. It was translocated to chromosome 8 from chromosome 6. Based on branch-site test and spectral tuning sites analyses, E122Q and M207L substitutions in RH2C, which were found to be under positive selection, are closely related to the blue shift of optimum light sensitivities. And real-time PCR results indicated the dominant opsin gene shifted from red-sensitive (LWS) to RH2B1 during turbot development, which may lead to spectral sensitivity shifts to shorter wavelengths.ConclusionsThis is the first report that RH2C may be an important subtype of green opsin gene that was retained by turbot and possibly other flatfish species during evolution. Moreover, E122Q and M207L substitutions in RH2C may contribute to the survival of turbot in the bluish colored ocean. And heterochronic shifts in opsin expression may be an important strategy for turbot to adapt to benthic life.

Highlights

IntroductionTurbot live at the ocean surface, but after metamorphosis they move to deeper water and turn to benthic life
As flatfish, turbot undergo metamorphosis as part of their life cycle
The tree confirmed the identities of turbot opsin genes: RH1, Short wavelength-sensitive opsin 1 (SWS1), SWS1-like opsin (SWS2), RH2A1, RH2A2, RH2B1, RH2B2, RH2C, and Long wavelength-sensitive opsin (LWS)

Summary

Introduction

Turbot live at the ocean surface, but after metamorphosis they move to deeper water and turn to benthic life. The light environment differs greatly between life stages. Due to water absorption and scattering of light, the photic environment of the aquatic system in which fish live changes rapidly with depth, especially a shift to. Teleost code five classes of visual opsin genes: (1) RH1 (rhodopsin, spectral peak absorbances around 500 nm) for dim light; (2) RH2 (rhodopsin-like opsin, 470–510 nm) for green; (3) SWS1 (short wavelength-sensitive type 1, 360–430 nm) for ultraviolet; (4) SWS2 (SWS1-like opsin, 440–460 nm) for blue; and (5) LWS (long wavelength-sensitive, 510–560 nm) for red. Due to substitution in key sites, the multiple opsin subtypes from duplication generally have different spectral peak absorbances (λmax), which helps enrich the visual system [2, 3, 7, 10]

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