Evolution and the origin of vertebrate retinal photoreceptor cells: Insights from tunicates and medaka
The vertebrate retina contains two types of photoreceptor cells, rods and cones, which use distinct types of opsins and phototransduction proteins. Cones can be further divided into four subtypes with differing wavelength sensitivity. The difference in wavelength sensitivity is primarily due to specific opsin subtypes they possess: LWS (red), SWS1 (UV or violet), SWS2 (blue), and RH2 (green). The current diversity of retinal photoreceptor cells seems to have evolved early in vertebrate evolution because the agnathan lamprey has the full repertoire of cone opsin genes. During vertebrate evolution thereafter, particular cone opsin subtypes have been lost in some lineages. For example, a nocturnal ancestor of mammals lost SWS2 and RH2 opsins. Although photoreceptor development has been extensively studied in a variety of vertebrate species, the mechanism by which photoreceptor subtypes are established is still largely unknown. It also remains unclear how vertebrate retinal photoreceptor cells were evolved. In the first half of this seminar, I will talk about development and molecular properties of photoreceptor cells in tunicates, the closest relatives of vertebrates. Tunicates are the only invertebrates that have the vertebrate-type opsins, and therefore studies on tunicate photoreceptor cells are of particular interest in the context of the origin of vertebrate retinal photoreceptors. In the second half of my talk, I will focus on two photoreceptor-specific miRNAs, which are potentially involved in photoreceptor subtype specification in a vertebrate retina. In the medaka Oryzias latipes, the genes encoding miR-726 and miR-729 are located upstream of the red-sensitive opsin gene LWS-A and the UV-sensitive opsin gene SWS1, respectively, and are co-expressed with the respective opsins in specific cone subtypes. The miR-726/LWS pair is conserved between teleosts and tetrapods, and the miR-729/SWS1 pair is conserved among teleosts. Based on transcriptomic analysis of the retina of miRNA knockout fish, I will discuss the possible role of miR-726 and miR-729 in development and evolution of retinal photoreceptor cells.