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S.S. Deeb, M. Wakefield, L. Marotte, J.A. Graves; The Cone Visual Pigments of an Australian Marsupial, the Tammar Wallaby (Macropus eugenii): Sequence Spectral Tuning and Evolution . Invest. Ophthalmol. Vis. Sci. 2003;44(13):5113.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To determine, for the first time, the sequence and mechanisms of spectral tuning of the cone visual pigments of a marsupial. Methods: Wallaby retinal RNA was used as template in reverse transcription and PCR amplification of sequences encoding the short-wave sensitive (SWS) and the middle wave-sensitive (MWS) cone pigments. The amplification primers were selected to have evolutionarily conserved nucleotide sequences. PCR products were directly sequenced. The rapid amplification of cDNA ends method was used to obtain the complete sequence. Results:The tammar wallaby retina contains only SWS and MWS pigment mRNAs, consistent with dichromatic color vision. The mRNA encodes a SWS1 class pigment of 346 amino acids. Sequence comparisons with eutherian SWS pigments predicts that the wallaby SWS1 pigment absorbs maximally (λmax) at 424 nm. The difference from the mouse UV pigment (λmaxof 359 nm) is largely accounted for by the Phe86Tyr substitution. The wallaby MWS pigment has 363 amino acids. Species comparisons at positions critical to spectral tuning predict aλmaxnear 531 nm, which is lower by 8 nm than that determined by electroretinography (Hemmi, J.M. et. al. Vision Res 40:591, 2000). Conclusions: The tammar wallaby most likely has dichromatic color vision based on SWS and MWS pigments, consistent with published behavioral data (Hemmi, J.M., J. Comp Physiol A 185:501, 1999). As was determined in vitro for the bovine SWS1 pigment (Fasick, J.I.. et. al.,Biochemistry 41:6860, 2002), the Phe86Tyr substitution plays a major role in shifting the spectrum of the wallaby SWS pigment by about 66 nm towards the red, suggesting that this mechanism existed before divergence of the eutherian and metatherian mammals some 130 million years ago. An apparently different mechanism of spectral tuning of the SWS1 pigments, involving 5 amino acids, has evolved in primates (Shi, Y. et al. Proc natl Acad Sci USA 98:11731, 2001). Spectral tuning in the wallaby MWS pigment is based on the same mechanism observed among other mammals.
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