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CA Arrese; Trichromacy in a Marsupial, the Honey Possum (Tarsipes rostratus) . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4545.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Previous immunohistochemical investigation of retinal visual pigments in a marsupial, the honey possum (Tarsipes rostratus), revealed the presence of cones sensitive to short (SWS) and medium to long (M/LWS) wavelengths, while microspectrophotometry (MSP) determined the maximum wavelength absorbance of LWS-cones at 557 nm and rods, at 502 nm. In this study, further microspectrophotometric analyses are undertaken to determine the spectral sensitivity of SWS-cones visual pigments, so as to obtain a more complete picture of colour vision in the species. The honey possum, the only terrestrial mammal feeding exclusively on nectar and pollen, has a highly specialised lifestyle suggesting a specific requirement for colour vision. Methods: Three dark-adapted animals were terminally anaesthetized with an overdose of a barbituric anaesthetic (Saffan, 0.1ml/10g body weight, i.p.), decapitated and the eyes removed. Spectral absorbance curves were measured from microspectrophotometric scans of cone photoreceptors (n = 60). All cones contained an oil droplet. Results: In addition to the previously identified LWS-cones, MSP measurements revealed the presence of two distinct cone visual pigments, with mean wavelengths of maximum absorbance (λmax) at 375 nm and 506 nm, representing sensitivity to ultraviolet (UVS), and medium- (MWS) wavelengths, respectively. The visual pigments of MWS-cones showed similar absorbance and photochemical properties to those of rods. Conclusions: The presence of three cone visual pigments in the honey possum not only extends colour vision capabilities to the ultraviolet region of spectrum, but also provides the basis for trichromacy, reported here for the first time in a non-primate mammal. We suggest that the presence of MWS-cones in the honey possum retina is a feature retained from the ancestral reptilian retinal design. Consequently, trichromacy in the honey possum may have a different evolutionary origin from that in primates.
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