Molecular Evolution And Functional Study Of Duplicated Ultraviolet Sensitive Opsin Genes In Fishes

As the ancestor of vertebrates,it was the beginning of the division of the great family of vertebrates,mainly divided into two categories: osteichthyes and chondrichthyes.Among them,the class osteichthyes contains the largest number of species,and most modern fish belong to the class osteichthyes.Whole Genome Duplication events occurred two times in the ancestors of vertebrates prior to the differentiation of teleost,and the third time Whole Genome Duplication event occurred in the teleost after the differentiation of teleost.These Whole Genome Duplications provided the basis for the evolution of fish,and left a lot of gene copies in many fish.Over time,some species lost their gene copies under the screening and retention mechanism of the copy gene,and some of the copies which retained lose their original functions.The reasons for the loss of function vary,with some genes being subfunctionalized and others defunctionalized to avoid the accumulation of harmful mutations during evolution.The Short Wavelength Sensitive gene in fish is also affected by duplicated genes,the gene is responsible for UV sensitivity in fish,normally,the opsin encoded by the gene is sensitive to light with a wavelength of about 360 nm and controls ultraviolet vision in vertebrates.Vision is one of the important sensory functions of vertebrates and is of great significance to the activities of vertebrates.Vertebrates have ultraviolet vision in the early stages of origin and differentiation,With the continuous differentiation of species,the ability and habit which is helpful to adaptation have been strengthened and preserved in the course of evolution,while unimportant or harmful functions have been gradually degraded,some animals,such as human,gradually lost ultraviolet vision under the influence of different living environments and living habits.In fish,however,the uv vision is preserved in many shallow-water fish,some of which even have two SWS1 genes that encode uv-sensitive opsin,the exact effects of these genes on the evolution of ultraviolet vision in fish have been poorly reported.In this study,we reconstructed the SWS1 sequence of the ancestor of fish according to the SWS1 sequence of 147 vertebrates and the evolutionary relationship between species,four representative ancestral sequences were selected for gene expression.At the same time,we also selected four modern fish species with replication genes and two more ancient fish species for gene expression of SWS1 gene,and through UV spectrophotometry to verify the function of the visual proteins encoded by these genes.And what we found,experimentally,In one of two copies of the Cyprinodon variegate and Lepisosteus oculatus SWS1 genes,each of them had one copy of the gene that expressed a protein that was not uv-sensitive,the other copy was uv-sensitive(The maximum absorption wavelength of opsin SWS1 in Lepisosteus oculatus is 351 nm while the maximum absorption wavelength of opsin SWS1 in Cyprinodon variegate is 358 nm);both copies of SWS1 opsins of Plecoglossus altivelis and Chanos Chanos are uv sensitive,and their functions have diverged(The maximum absorption wavelength of opsin SWS1-1 in Plecoglossus altivelis was 355 nm while opsin SWS1-2 in Plecoglossus altivelis was 350nm;the maximum absorption wavelength of opsin SWS1-1 in Chanos chanos was 355 nm while opsin SWS1-2 in fish is 366nm).Species with one copy of SWS1 have only ultraviolet vision,while fish with two copies of SWS1 may have a more advanced ultraviolet color vision.The functional evolution of fish SWS1 replicator gene provides new support for pseudogenylation and functional differentiation models.This varying degree of opsin retention may be due to differences in the living environment and habits of fish,however,as far as our study is concerned,there is no single environmental or behavioral factor can determines whether opsin gene function is retained or not.These fish should have developed their current genotypes in their respective environments.Meanwhile,by comparing the opsin function of SWS1 between the ancestors of fish and modern fish species,we found that for most of the time,the SWS1 gene was associated with the evolution of fish.SWS1 gene is highly conserved,which means that ultraviolet vision is an important sensory function for fish,and there is a long way to go.