Comparative transcriptomic insights into the evolution of vertebrate photoreceptor types.

To explore the molecular similarities and potential evolutionary origins of vertebrate photoreceptor types, we analyzed single-cell and -nucleus transcriptomic atlases from six vertebrate species: zebrafish, chicken, lizard, opossum, ground squirrel, and human. Comparative analyses identified conserved transcriptional signatures for the five ancestral photoreceptor types: red, blue, green, and UV cones, as well as rods. We further identified and validated molecular markers of the principal and accessory members of the tetrapod double cone. Comparative transcriptomics suggests that the principal member originated from ancestral red cones, although the origin of the accessory member is less clear. The gene expression variation among cone types mirrors their spectral order (red → green → blue → UV). We find that rods are highly dissimilar to all cone types, suggesting that rods may have diverged prior to the spectral diversification of cones.