Convergent mechanism underlying the acquisition of vertebrate scotopic vision

High sensitivity of scotopic vision (vision in dim light conditions) is achieved by the rods' low background noise, which is attributed to a much lower thermal activation rate (k_th) of rhodopsin compared with cone pigments. Frogs and nocturnal geckos uniquely possess atypical rods containing noncanonical cone pigments that exhibit low k_th, mimicking rhodopsin. Here, we investigated the convergent mechanism underlying the low k_th of rhodopsins and noncanonical cone pigments. Our biochemical analysis revealed that the k_th of canonical cone pigments depends on their absorption maximum (λ_max). However, rhodopsin and noncanonical cone pigments showed a substantially lower k_th than predicted from the λ_max dependency. Given that the λ_max is inversely proportional to the activation energy of the pigments in the Hinshelwood distribution-based model, our findings suggest that rhodopsin and noncanonical cone pigments have convergently acquired low frequency of spontaneous-activation attempts, including thermal fluctuations of the protein moiety, in the molecular evolutionary processes from canonical cone pigments, which contributes to highly sensitive scotopic vision.