Visual pigments and colour vision in marsupials and monotremes

Abstract

The sequences and spectral characteristics of the cone visual pigments of several Australian and a few South American marsupials have been determined. Based on genomic and mRNA sequence analyses, it was found that all of these species have only two cone-specific photopigments: one belonging to the short-wave-sensitive 1 (SWS1 ultraviolet or violet), and the second belonging to either the middle-wave-sensitive (MWS) or long-wave-sensitive (LWS) visual pigments. This predicted that colour vision in these species is dichromatic. However, behavioural studies have indicated that at least some Australian marsupials have trichromatic colour vision. This is consistent with the observation by microspectrophotometry that three classes of cone photoreceptors exist in the retinae of some of the Australian marsupials. Retinal mRNA and genomic DNA sequence analysis approaches did not find a third cone photopigment. These approaches used PCR primers that are specific to cone photopigments. Based on spectral sensitivity, it was suggested that a rod photopigment could be expressed in some cones to establish trichromacy. Sequence analysis of the photopigment genes and corresponding mRNAs in the platypus and echidna indicated the presence of only two opsins: SWS2, typically found in reptiles, birds and fish, instead of the SWS1 pigment typically found in most mammals, and a LWS, suggesting that monotremes also have dichromatic colour vision. Unlike in other mammals, the monotreme SWS2 and LWS genes were tandemly arranged and separated by a locus control region, indicating retention of ancestral reptilian opsin genes. The gene structure and expression of monotreme cone opsin genes bridge the phylogenetic gap between reptiles and other mammals. A proposed evolutionary pathway of the opsin genes is described.