From rod to cone: functional transformations in the evolution of vertebrate photoreceptors

The vertebrate retina contains two types of photoreceptors: rods and cones, the receptors of nocturnal and diurnal vision, respectively. They have a number of morphological and biochemical differences that determine their functional role. The discovery of intermediate photoreceptor types in a number of vertebrates became the basis for the transmutation theory, which postulates that both rods and cones are capable of changing their functional roles and transforming into their opposite type during adaptation of the visual system to different habitat conditions. Changes in the photoreceptor physiology during functional transition can occur at different levels: morphological (general cell structure), molecular and biochemical (expression of specific protein isoforms of the photosensitive signaling cascade) and electrophysiological (sensitivity and kinetics of the light response). Photoreceptors with a confirmed transitional type are found in basal vertebrates and in groups that have underwent a shift in the habitat conditions towards extremely low or high light levels. In the last two decades, the understanding of the molecular mechanisms leading to the functional transformation from typical rods to cones, and vice versa, has advanced significantly. However, a number of aspects remain poorly understood, primarily because many animals possessing transformed receptors are far from the standard biological model objects. This review discusses the examples of transitional photoreceptors in various taxa, describing the history of their study and current research that sheds light on the molecular features underlying their non-standard physiology.