The Cone Visual Pigments of Two Australian Marsupials, the Honey Possum and the Fat–Tailed Dunnart, With the Potential For Trichromacy

Abstract

Abstract: : Purpose: Microspectrophotometric measurements of photoreceptors in two species of marsupials, the honey possum (Tarsipes rostratus) and fat–tailed dunnart (Sminthopsis crassicaudata), have revealed three cone classes with absorbance peaks at approximately 350, 505 and 557 nm for the possum and 350, 509 and 535 nm for the dunnart. The objective of this study is to identify the visual pigments that underlie these three cone classes. Methods: Total retinal RNA and genomic DNA was used as template for the PCR amplification of the coding and intron/exon regions of opsin genes. PCR products were cloned into an appropriate vector and sequenced using ABI 3730 and 3100 sequencers. Recombinant opsins were generated by cloning of coding sequences into the pMT expression vector, followed by transient transfection of HEK 292T cells and regeneration of the pigment with 11–cis–retinal. Results: The full coding sequences for an SWS1 and an LWS opsin were obtained for both species and the peak absorbance for the dunnart opsins shown by in vitro expression and pigment regeneration to be at 363 nm (UV) and 533 nm respectively. This confirms the identity of the pigments present in the short–wavelength–sensitive and the long–wavelength–sensitive cones as belonging to the SWS1 and LWS opsin classes. The pigment responsible for the middle–wavelength–sensitive cones has proved more elusive. It is not a modified LWS pigment and no evidence for the retention of an RH2 opsin gene could be found. A duplicated rod opsin gene is however present in the dunnart genome. Conclusions: The pigments responsible for the short–wavelength–sensitive and the long–wavelength–sensitive cones in these species are shown as belonging to the SWS1 and LWS classes. In vitro expression of the former pigment confirms a peak absorbance in the UV. The middle–wavelength–sensitive pigment has not been identified but it does not appear to be either an RH2 pigment or encoded by a duplicated copy of the LWS gene. It is possible that the pigment derives from a duplicated copy of the rod opsin gene.

Keywords: photoreceptors • color vision • opsins