Knockout of sws2a and sws2b in Medaka ( Oryzias latipes ) Reveals Their Roles in Regulating Vision-Guided Behavior and Eye Development.

The medaka ( Oryzias latipes ) is an excellent vertebrate model for studying the development of the retina. Its genome database is complete, and the number of opsin genes is relatively small compared to zebrafish. Short wavelength sensitive 2 ( sws2 ), a G-protein-coupled receptor expressed in the retina, has been lost in mammals, but its role in eye development in fish is still poorly understood. In this study, we established a sws2a and sws2b knockout medaka model by CRISPR/Cas9 technology. We discovered that medaka sws2a and sws2b are mainly expressed in the eyes and may be regulated by growth differentiation factor 6a ( gdf6a ). Compared with the WT, sws2a ^-/- and sws2b ^-/- mutant larvae displayed an increase in swimming speed during the changes from light to dark. We also observed that sws2a ^-/- and sws2b ^-/- larvae both swam faster than WT in the first 10 s of the 2 min light period. The enhanced vision-guided behavior in sws2a ^-/- and sws2b ^-/- medaka larvae may be related to the upregulation of phototransduction-related genes. Additionally, we also found that sws2b affects the expression of eye development genes, while sws2a is unaffected. Together, these findings indicate that sws2a and sws2b knockouts increase vision-guided behavior and phototransduction, but on the other hand, sws2b plays an important role in regulating eye development genes. This study provides data for further understanding of the role of sws2a and sws2b in medaka retina development.