Adaptive loss of shortwave sensitive opsins during genomic evolution in cartilaginous fish

Widespread gene loss can occur during evolution. Cartilaginous fish (e.g., sharks, rays, and skates) cannot see blue or violet light, potentially because they lack the shortwave-sensitive cone opsin gene ( sws ). However, the evolutionary mechanisms underlying sws loss remain unclear. Here, we elucidated the whole-genome assembly structures of skate ( Okamejei kenojei ) and blue shark ( Prionace glauca ) and analyzed the distribution characteristics and intragroup differences of opsin-related genes in cartilaginous fish. By using a zebrafish model with sws deletion, we inferred that in the presence of SWS1 and SWS2, blue and violet light can induce cell aging, respectively; this is followed by photoreceptor layer thinning. Thus, sws loss aids in preventing shortwave light damage to the eye. In the retinas of numerous cartilaginous fish, the tapetum lucidum strongly reflects light. Thus, in cartilaginous fish, the existence of tapetum lucidum in the retina and loss of sws may be interdependent; in other words, this adaptive gene loss may increase cartilage fish fitness.