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Wednesday, May 14, 2008

Platypus Genome!

posted by on May 14 at 17:51 PM

(Salim Virji)

Who doesn’t love the platypus? This is a creature bizarre enough to make marsupials feel better about themselves. The platypus, lactates (mammal!) and lays eggs (reptile!), grows fur (mammal!) and venom (reptile!).

This might be the single most interesting creature, from an evolutionary point of view, on the planet. About 315 million years ago, Amniotes—a primitive vertebrate with four legs, pretty much resembling a blurry picture of every animal that comes to mind—split into two groups. The Sauropsids eventually became all reptile-like creatures, including Dinosaurs, snakes, lizards and birds. The Synapsids became, well, us and all other mammals. Almost 170 million years ago, the Platypus split off from the rest of the Synapsids and hung out on a little evolutionary twig of its very own.

This is all a bit like reading the Silmarillion or Numbers, so I’m moving on to the big, exciting, point for evolutionary biologists. 170 million years ago, we and the Platypus shared a common ancestor. If you want to reconstruct how we evolutionarily came to have external testicles, nipples, separate opening for pee and poop—all things we have but the Platypus doesn’t—we could compare how a Platypus is put together, its genome, to our own. Our common ancestor probably lacked all these things. Likewise, the Platypus has been busy since departing our common ancestor, figuring out how do things we can’t—like make poison or see the world using only electricity. How’d that happen?

Well, we now have a draft of the Platypus genome. This’ll be fun.

Right off, the male Platypus has five X and five Y chromosomes. Huh? By comparison, every other male mammal has one X and one Y. One of the more pleasant observations is how similar we are to them. Over 80% of the genes in the Platypus strongly resemble those in humans or mice.

The remaining fifth is where all the fun actions occurs! Like what? The genes for chemical receptors, that make the nose work, are totally different. Genes for making eggs? Different from just about anything. The eggs are tiny and the baby Platypus hatches much earlier than is typical in egg-laying creatures. The baby then licks milk off the belly of the mother—remember, no nipples! If you wanted a snapshot of the evolution of mammals, that don’t lay eggs and nurse their young, this is pretty much it.

Ok, enough of my wonderment. Read the paper, if you can! If you can’t, bitch to your representative about the publication of publicly funded research in private, subscription only journals.

RSS icon Comments


Yeah, we have a common ancestor, but we never invite him to family gatherings. It's just ... well ... it would take too long to explain.

Posted by RonK, Seattle | May 14, 2008 6:03 PM

go read some platypus poetry if you haven't already.

Posted by M1 | May 14, 2008 6:12 PM

I love!

Posted by CML | May 14, 2008 6:13 PM

"Parchment-shelled egg-laying monotremes also exhibit a more ancestral glandular mammary patch or areola without a nipple that may still possess roles in egg protection. However, in common with all mammals, the milk of monotremes has evolved beyond primitive egg protection into a true milk that is a rich secretion containing sugars, lipids and milk proteins with nutritional, anti-microbial and bioactive functions."

I need to get me a lady platypus and start licking it's glandular mammary patch, sounds like sweet ambrosia! Anti-microbial, too.

Posted by Gabe | May 14, 2008 6:19 PM

Did you keep saying "the platypus" because you don't know what the plural of platypus is?

fascinating stuff btw.

Posted by sepiolida | May 14, 2008 6:22 PM

Platypuses are fantastic creatures! But very shy; when we went to Australia we went to three platypus exhibits with no luck before getting lucky at the fourth, Healesville Sanctuary: (photo is terrible, I know).

The chromosome thing just freaks me out.

They're not unique, though -- echidnas are monotremes, too, and almost as weird.

Posted by Fnarf | May 14, 2008 6:22 PM

@5: Platypi? Platypussies? In a group, Platypod? I need to look into this. I don't think anyone knows!

Posted by Gabe | May 14, 2008 6:47 PM

Indeed, the echidnas are pretty amazing too. I remember reading their penises have four (five?) heads. Only two are used during sex, the other heads shrivel up only to be used next time, trading off.

Still, the complex electricity-based sensory system in the Platypus wins my heart. That just such a *fish* thing.

And yes, I have no clue as to the proper plural for the Platypus. An hour when I wished Slog was copyedited regularly.

Posted by Jonathan Golob | May 14, 2008 7:00 PM

Jonathan, you are delightful!

Posted by PopTart | May 14, 2008 7:14 PM

@5 and @8:

I love a good unresolvable's neither, it's both.

I'm sticking with Platypi since it's from Latin and that would be correct there. Perhaps Platypae for more than one female Platypus, Platypum for transgendered Platypi.

Posted by Gabe | May 14, 2008 7:15 PM

Hah, I got it!


Score one for the Greeks...and the Geeks who spend time looking this shit up.

Posted by Gabe | May 14, 2008 7:22 PM

Stupid question: how does the platypus lactate if it doesn't have nipples?

Posted by giantladysquirrels | May 14, 2008 9:06 PM


This is too cool! Milk producing glands are just mutated sweat glands!

So, think of it like this:
scales -> skin + hair -> sweat glands to keep the skin cool -> a patch of skin's sweat glands mutate to make something that keeps eggs happy -> this secretion becomes nutritious = milk from a patch of skin! -> the center of this special patch of skin raises up to make it easier to feed from -> The Nipple!

Platypodes are somewhere on the "secretion becomes nutritious" step. I, for one, am pro-nipple. Listing to anyone who has survived giving birth to a human child, I must admit that marsupials have a better strategy for birthing live young.

Posted by Jonathan Golob | May 14, 2008 9:16 PM

@12: This is the first thing I got onto after reading the OP. I thought, "without a nipple, there still must be mammary glands, or proto-mammary glands, does it just secrete through the skin? I'll bet there are sense areas around these "areolas" that respond to a youngun rubbing against it.

Basically, as Johnatan gas stated, this is the evolution of the nipple encapsulated.

Now your Amazons...

Posted by Gabe | May 14, 2008 9:36 PM

@ 14: yeah, has, not gas:)

Posted by Gabe | May 14, 2008 9:41 PM

Just a teensy correction: Not all of the vertebrate land animals are amniotes. The big exception is amphibians. Amniote eggs have extra membranes that protect the eggs and prevent the egg from drying out. Even placental mammals (us) have amniotic membranes -- hence the common gush of fluids when mom-to-be's membranes break. Frogs, newts, etc. don't have those membranes, and hence split away from our lineage way before platypuses, birds and reptiles did. So hey -- how about comparing platypus DNA to frog DNA?

Posted by spudbeach | May 14, 2008 9:45 PM

@16: Correct me if I'm wrong, but aren't the Amphibians the link fron sea to land (other than the Coelacanth)? Wouldn't the Platypoda be more of a throwback or side job than a link?

Posted by Gabe | May 14, 2008 10:03 PM

@13: except for that pouch thing. And the tiny, tiny brains they have (marsupials have the smallest brains for their body weight of anything, I think).

Posted by Fnarf | May 14, 2008 11:25 PM

JG, I understand your frustration with gov't funded studies published in for-profit venues. But I hope you are aware that at least NIH is addressing this problem (and in a way that is remarkably forward-thinking given that we are STILL dealing with the Bush Admin.):

If you didn't know, read the guidance, and check in with your Sponsored Projects office. They are a font of useful information. (Can you tell what I do for a living?)

Posted by Exiled in LA | May 15, 2008 1:47 AM


You're right in that amphibians are the best "link" we have to our marine vertebrate ancestry. Amphibians were the first land vertebrates (tetrapods). They evolved from the same group of fishes (now mostly extinct) that includes the coelecanth. The first reptiles evolved from a long-extinct group of amphibians and (as Dr. Golob stated), birds and mammals evolved from long-extinct groups of reptiles. So, for vertebrates, amphibians were the first to (partially) abandon their sea legs.

And I agree that the platypus is more likely to be an oddity or side job, rather than some throw back to amphibian lifestyle. Most monotremes that have ever existed went extinct long ago. The survivors are few and far between, and may not be the most accurate representations of the typical monotreme lifestyle. What if all placental mammals suddenly went extinct except for humans and the armadillo? The two of us alone would barely be an accurate representation of our group. :-)

So, I'm more intrigued to compare the platypus genome to several different reptiles, since all mammals arose from mammal-like reptiles. Unfortunately, I don't think many (any?) reptile genomes have yet been sequenced. Though I think a lizard and a crocodilian are in the pipeline.

Posted by James | May 15, 2008 6:08 AM

Marsupial evolution is interesting to me because in many ways it mirrors mammalian evolution in the ways it exploits it's enviorments. My own belief is that given the right conditions life will repeat many of the body types and uses to adapt. (i.e. birds vs. bats, tiger vs. marsupial tiger, now extinct, etc.)

Posted by Vince | May 15, 2008 6:46 AM

I love Science 'cause he's so delicious.

Posted by ray | May 15, 2008 8:45 AM

Does anybody remember those informational animal cards that used to be advertised on TV in the 80s? I think you got a box of them or something like that... That is the first time I ever heard of the platypus and I was totally hooked. They are phenomenal.

I saw some at an aquarium a few years ago and they are much smaller than I thought they would be. But, I still couldn't taking my eyes off of them -- so crazy-looking.

Posted by Julie | May 15, 2008 9:10 AM

If you're ever lucky enough to see 'em in the wild (I haven't been, but hope to someday), be careful; they have huge, razor-sharp spurs on their back legs that can cut you up, and poison you. Maybe that's why they aren't invited over to RonK's house anymore.

Posted by Fnarf | May 15, 2008 9:36 AM

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