Molecular diversity of protostome non-visual opsin arthropsin.

Opsins underlie visual and non-visual photoreceptions in animals. Vertebrate and arthropod visual opsins belong to different opsin groups and convergently show spectral diversity ranging from the UV to the red region for color vision. Recently, uncharacterized opsins called arthropsin have been identified from various protostome genomes. Arthropsin is clustered with arthropod and mollusk visual opsins and vertebrate blue-sensitive non-visual opsin Opn4. Here, we show that arthropsins have unexpected spectral diversity ranging from the UV to the red region. In particular, water flea (Daphnia magna) expresses red-sensitive arthropsins in the optic lobe and brain. Among the non-visual opsins characterized so far, these arthropsins exhibit the most red-shifted spectral sensitivity. Moreover, the molecular mechanism responsible for the red shift of arthropsins is different from that of red-sensitive vertebrate visual opsins. We characterize arthropsin as a different type of non-visual opsin which acquired the spectral diversity independently of vertebrate and arthropod visual opsins.