Beyond spectral tuning: A functional hierarchy of cone opsins revealed by analyses of RH2-deficient medaka

Vertebrate color vision is mediated by multiple cone opsins with distinct spectral sensitivities. Although these cone classes differ primarily in wavelength sensitivity, their relative contributions to different visual behaviors remain poorly understood. Here, we generated RH2 (green-sensitive) triple knockout medaka (Oryzias latipes) and compared their phenotypes with those of our previously established SWS1 (violet-sensitive)-, SWS2 (blue-sensitive)-, and LWS (red-sensitive)-deficient lines with respect to viability, optomotor response (OMR), and mate choice. The results demonstrated that different cone types make markedly different contributions to distinct visual behaviors. Namely, loss of RH2 resulted in reduced viability, diminished OMR across short-to-middle wavelengths—including conditions that strongly stimulate short-wavelength cones—and impaired body-color preference. In contrast, loss of other cone types produced more restricted or context-dependent effects. These findings reveal that visual signals originating from RH2 cones play a central role in luminance- and color-guided behaviors, and that cone contributions to visual processing are asymmetrical rather than equivalent. Notably, this functional organization differs from that reported in zebrafish, highlighting lineage-specific diversification of cone function among teleosts. Together, our results indicate that cone-derived signals are weighted unequally in downstream visual processing in medaka, and that this weighting may differ among vertebrate lineages.