Not that we needed another reason to love the platypus - looking something like the cross between a beaver and a duck, being venomous, endemic to Australia and one of the two mammals that lay eggs is awesome enough - but now that we have its genome sequence (the release-paper having been published yesterday in Nature) we have the chance to learn a lot about how modern mammals evolved from more reptile-like beginnings.
The New York Times writes:
If it has a bill and webbed feet like a duck, lays eggs like a bird or a reptile but also produces milk and has a coat of fur like a mammal, what could the genetics of the duck-billed platypus possibly be like? Well, just as peculiar: an amalgam of genes reflecting significant branching and transitions in evolution.
In short, there are elements and patterns in the genome that are very similar to those in birds and reptiles and others that are decidedly mammal.
Several "reptilian" genes involved in vision, circadian rhythm and food intake are present in the platypus but have been lost other mammals, while of course the genes for the milk proteins caseins and their arrangement are as mammal as can be. Some of the genes for the proteins that coat the egg-cell before fertilization are shared with other mammals while others have only previously been found in fish, amphibians and birds - just to give some examples.
But even though its apparent mix of features is reflected in its genome, the platypus is not "part bird, part reptile, part mammal" like some popular science outlets sloppily have written. Science Daily and New Scientist really should know better. The platypus is a representative of a group of mammals that diverged from the rest of the mammal lineage at an important transition from more reptile-like creatures towards modern mammals. By comparing, for instance, our genome with the platypus genome we get an insight into how that transition took place. That's precisely why it's a beautiful addition in our understanding of how evolution has put genomes together. Not because it's a "lizard-bird-mammal" in any sort of way.
There are also some pretty cool unique features to the platypus genome. It has a lot of genes for odorant receptors that cannot be found in other animals - possibly to detect water soluble odorants when it forages underwater, or maybe as part of a highly advanced pheromonal system? I mentioned in the beginning that platypuses were venomous (the males inject the venom through a spur on their hind legs) - it turns out that the proteins that make up the platypus venom are not a "reptilian" character but have evolved independently in reptiles and in the platypus.
I have been using the preliminary platypus genome sequence in my research for a while now, trying to figure out how the genes that make up part of the brain's endocrine systems have evolved. It's been a nice addition to my evolutionary trees and I count on it to continue to be very useful in the future.
>> Edited May 16, 2008.
Warren, W.C. et. al. (2008). Genome analysis of the platypus reveals unique signatures of evolution. Nature, 453(7192), 175-183. DOI: 10.1038/nature06936
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