As vertebrates, the ancestral complement of our cone visual pigments, (cones are receptors in our retina which are responsible for the way that we see colour), are comprised of four classes which range spectrally from ultraviolet to red. Through the course of evolution, eutherian mammals have lost two of these classes, (called the SWS2 and Rh2 classes) and retained the two known as the LWS and the SWS1 classes. What all of this means, is that with the exception of us primates, most eutherians are red-green colour blind. In other words, they are what you would call ‘dichromats’.
What has recently been discovered is that monotremes have managed to retain the SWS2 class – rendering them ‘trichromats’ with colour vision. And why is this such exciting news? This is the first finding of an SWS2 gene in any mammal. This means that the eutherian loss of SWS2 and subsequent reduction to dichromacy must have occurred after monotremes branched off the mammalian lineage.
Through careful analysis of platypus genes, it has also been found that somewhere along the line, they lost the SWS1 class. The fact that both the SWS1 and SWS2 genes are present in eutherians and monotremes respectively, implies that our great, great ancestral grandparents must have had both of these genes. In combination with the LWS class, this would have meant that they were trichromats with full colour vision. Thanks to the platypus, we can now appreciate that the long evolutionary journey towards our ability to see every colour of the rainbow wasn’t quite as black and white as we believed it to be.
Written By Zara Marais
Primary Reference
Davies, W.L., Carvalho, L.S., Cowing, J.A., Beazley, L.D., Hunt, D.M., Arrese, C.A. 2007. Visual pigments of the platypuses: A novel route to mammalian colour vision. March Vol. 17, pp. R161-R163
Secondary References
Explanation of Cone Visual Pigments
Wikipedia Article on Dichromacy
Wikipedia Article on Trichromacy
Recently Released Sequence of the Platypus Genome
http://pre.ensembl.org/Ornithorhynchus_anatinus/index.html
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