More platypus

I've been poking about a little bit extra in the platypus genome for a week now in response to the platypus genome release article I wrote about in my previous post. It was a pleasant surprise to notice that so many of the gene families we are interested in in our lab have distinguishing features in the platypus. It has the neuropeptide Y7 receptor, which is absent from other mammals; it has a unique galanin receptor that I've been looking into some more; it has the shortwave-sensitive-2 opsin gene, the visual pigment that detects blue light, which is absent in other mammals (we have modified the ultraviolet-light-detecting opsin to see blue light instead)... it's pretty interesting stuff.

I missed this BBC article last week. It's not spectacular, but it has a nice audio interview with professor Jenny Graves at Australian National University, co-author of the genome release paper. I liked this highlight of what we are able to learn about evolutionary transitions by looking into and comparing genomes:

The platypus is a mammal, it makes milk and it has fur so it is defined as a mammal, but it left the rest of the mammals a long time ago. It diverged 166 million years ago from a common ancestor that probably looked more like a reptile than a mammal. So it's not a reptile, it is a mammal but it's retained a lot of reptilian characteristics like laying eggs for instance... Of course one of the things we wanted to look at was egg-laying and making milk because we want to retrace the steps in how did we get to be mammals? and so first of all we looked at the egg yolk proteins and indeed there is an egg yolk protein there, but there's only one of them whereas birds have three for instance. So it looks as though the platypus is already shifting its allegiance from nurturing their young inside an egg and nurturing their young with milk.

There have also been quite a few blog posts concerning the platypus, which is fun. Pharyngula and Adaptive Complexity both have very comprehensive posts and both also take issue with the purported image of the platypus as a composite creature, "part bird, part reptile, part mammal"; Genomicron and Nimravid focus on the problem with calling the platypus "primitive" or defining its characteristics as "reptilian" or "avian"; Carl Zimmer just wants to know where the platypus' stomach went and The Digital Cuttlefish shares with us a few inspired words.

Swedish blog tags: Vetenskap, Biologi
Technorati tags: Science, Biology, Genomics, Platypus

Ph34r the platypus!

Proof that god has a sense of humor? Or simply an amazing creature whose unique combination of features are the result of an intriguing evolutionary history? You guessed it.

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

Swedish blog tags: Vetenskap, Biologi
Technorati tags: Science, Biology, Genomics, Platypus