Study On Opsin Gene Expression And Spectral Modulation Technique In Turbot(Scophthalmus Maximus)

Vision is one of the sensory systems in fish,which is closely related to behaviors such as mating,foraging and avoiding predation.Opsin is a protein which combines with chromophore to form the initial element of vision.Functional diversification of opsin plays a great role in the adaptive evolution of vision.However,related research is rarely reported on how opsin genes adapt to changes in light environment at different stages of life cycle in fish.The turbot(Scophthalmus maximus)is an important aquaculture species with great commercial value.During the development,it experiences metamorphosis and changes from planktonic to benthic life.The shift in living habits is accompanied by a huge change in its habitats,especially the light environment.This research conducted in turbot,we investigated the visual opsin repertoire,analyzed the changes in retina structure and heterochronic opsin expression during the development of turbot.We also measured the opsin differential expression in response to different spectral environments,and compared the spectral preference of juvenile turbot.This study attempted to identify the structural and functional characteristics of the visual organs in the development of turbot,and to explore the adaptability of fish visual characteristics and light environment.This study can provide a theoretical basis for the improvement of light modulation technique in the industrial culture of turbot.The relevant results showed that:1.The ratio of nuclei of the outer nuclear layer(O.N.)to ganglion cells(G.)in the retina of 1-month-old turbot was 2.22.It indicated the low degree of the assembled meshwork and high visual acuity.The numbers of cone cells and G.in 4-and 9-month-old were significantly lower than that of 1-month-old,while the change in the number of O.N.was just the opposite.After metamorphosis,the ratio increased and visual acuity decreased.And the increased number and proportion of rod cells indicated that the photosensitivity of turbot may be enhanced.We speculated that turbot may survive in the dark environment by increasing the amount of rod cells.2.Turbot possess five classes of visual opsin genes: 1)rhodopsin(RH1);2)red-sensitive opsin(LWS);3)ultraviolet-sensitive opsin(SWS1);4)blue-sensitive opsin(SWS2);and 5)green-sensitive opsin(RH2A1,RH2A2,RH2B1,RH2B2 and RH2C).Unlike other single-copy genes,turbot have five RH2 paralogs.Along with Pacific bluefin tuna(Thunnus orientalis),tutbot is one of the species with the most RH2 genes among the sequenced fish to date.3.Heterochronic changes in opsin gene expression were found in different developmental stages.Specifically,during the development of turbot,RH1 expression significantly increased and reached the highest at 9 months of age.In the larval stage,all cone opsin genes were expressed at a low level.LWS was the highest one with a proportional expression level of 47.98%.After metamorphosis,RH2B1 gradually replaced it as the dominant opsin gene,and the highest proportion level of RH2B1 was 63.27% at 9 months of age.The proportional level of LWS slightly increased at18 months of age,but that of RH2B1 showed no significantly different.Turbot may undergo a spectral sensitivity transition from red to green.We speculated that heterochronic changes in opsin gene expression may be a strategy retained in the evolution of turbot to adapt to benthic life.And the increased proportion level of RH1 in 18 months of age may occur in preparation for reproduction in shallow water.4.The treatment of LED lights with different spectral led to changes in turbot opsin gene expression.In all opsin genes,only the expression of RH2B1,SWS1,and SWS2 showed plasticity changes induced by ambient light.The expression levels of individuals under full,green,and blue spectrum light were significantly higher than those under orange and red spectrum light.And the expression levels of individuals under blue and green light were about twice that of the individuals under orange and red light,while individuals under full spectrum light were slightly higher than individuals under orange and red light.The expression levels of the other six opsin genes were not significantly different among the treatment groups.We speculated that the plasticity of opsin expression is a mechanism for turbot to react to rapid changes in light environment.In addition,turbot lacks the plasticity of LWS expression compared to other species such as guppy(Poecilia reticulata)in the juvenile stage.5.The amino acid substitutions of E122 Q and M207 L were found in subtype RH2 C of turbot.They are not only spectral tuning sites,but also closely related to the blue shift of opsin function.The branch-site model tests showed that they were under positive selection during evolution.We speculated that turbot may thus obtain blue-shift green opsin subtype.It may enhance the ability to discriminate blue light,which is beneficial for turbot to adapt to the bluish ocean in the benthic stage.6.The spectral preference of juvenile turbot was investigated.It showed obvious green light preference.The frequency of its appearance in the green light region was48.00%,followed by blue(11.56%)and full spectrum(8.18%).While the frequency of turbot found in red or orange light region was under 1%.The results of this experiment provided evidence for the high sensitivity to green light after metamorphosis.We proposed that the preference for different spectral may be related to the fact that the habitat of its benthic stage in the wild is dominated by short-to-medium wavelength spectrum.