The Platypus
Dave Smith
Platypus proves even odder than scientists thought
Ian Sample, science correspondent
The Guardian, Thursday May 8 2008 Article history
A DNA study found the creatures to be a mix of mammal, bird and
reptile with, even more strangely, 10 sex chromosomes.
At first dismissed as a prank, and later cited as proof that God has a
sense of humour, the duck-billed platypus has finally given up its
evolutionary secrets.
The creature, considered one of the strangest mammals in the world,
has become the latest to have its genetic code sequenced, revealing it
to be a bizarre mix of mammal, bird and reptile, with very complex
sexuality. While humans have two sex chromosomes, the X and Y, the
platypus has 10, with five of each kind.
An international team of scientists extracted DNA from a female
platypus, named Glennie, reading all 2.2bn pairs of her genetic
"letters". Thought to have begun to diverge from other mammals 170m
years ago, the platypus has been regarded as the nearest thing
biologists have to a missing link between the earliest reptiles and
mammals. It has thick fur and produces milk for its young, yet the
females lay eggs and the males produce venom - the only mammals to do
so.
When first discovered in Australia in 1798, the beaver-tailed animal
caused such bemusement that the zoologist George Shaw declared it
could well be a hoax.
The new study, published in Nature, shows the platypus as both
evolutionary relic and pioneer. Chris Ponting, at the Medical Research
Council's functional genomics unit at Oxford University, said
scientists had had the first chance to see if the platypus's weird
appearance was reflected in its DNA: "Lo and behold, we saw genes like
those in lizards and birds, as well as some like those in other
mammals. It has retained many genes other mammals lost from a time
when all mammals looked much like lizards."
Many of the animal's stranger characteristics are now thought to have
evolved independently. The venom, which is released from the male's
hind leg spurs, is thought to have developed late in the animal's
history. And the remarkable electrosensitive bill, which helps the
platypus hunt underwater while its eyes and ears are covered, appeared
long after the platypus split from its reptilian ancestors.
The fact that the animal has five X and five Y chromosomes is "the
weirdest thing about a very weird animal," said Ewan Birney, a co-
author on the paper, based at the European Bioinformatics Institute,
near Cambridge. "In theory it means there are 25 possible sexes,
though in practice that doesn't happen."
And here are Ogden Nash's thoughts:
The Platypus
I like the duck-billed platypus
Because it is anomalous.
I like the way it raises its family
Partly birdly, partly mammaly.
I like its independent attitude.
Let no one call it a duck-billed platitude.
Peter Brooks
>
>
> The fact that the animal has five X and five Y chromosomes is "the
> weirdest thing about a very weird animal," said Ewan Birney, a co-
> author on the paper, based at the European Bioinformatics Institute,
> near Cambridge. "In theory it means there are 25 possible sexes,
> though in practice that doesn't happen."
>
poor platypus would be like otherwise!
Thank you for this update, I've long been fascinated by the animal. I
was a bit disappointed when I saw my first specimen in a museum, I'd
thought that they were the size of otters, but the one I saw was
smaller than a large rat.
Are there many platypus species, or just the one?
They might be the only mammals to produce venom in a literal sense,
but I know of some that have managed a fairy good metaphorical effort!
Dave Smith
It seems from some quick surfing that the male platypus grows up to
about half a metre long -- females are typically smaller. Wikipedia
states that there is only one species:
"Together with the four species of echidna, it is one of the five
extant species of monotremes, the only mammals that lay eggs instead
of giving birth to live young. It is the sole living representative of
its family (Ornithorhynchidae) and genus (Ornithorhynchus), though a
number of related species have been found in the fossil record."
Dave
Peter Brooks
wrong.
I'm still fascinated by the sexual opportunities. How on earth do they
manage to keep to two sexes with so many possibilities?
Lance
---------------------------
Another article:
NYT
May 13, 2008
Basics
A Gene Map for the Cute Side of the Family
By NATALIE ANGIER
When scientists announced last week that they had deciphered the
complete genetic playbook for the duck-billed platypus, the public
reacted with considerably more enthusiasm than it had accorded similar
bulletins about the sequencing of, say, the mustard plant, the
mosquito or the wild chicken. A “fantastic response,” said Jennifer
Marshall Graves of the Australian National University in Canberra, a
principal author on the report. “More than I expected.”
One reason for the glowing reviews is that people love platypuses the
way they love penguins and panda bears, as adorable, clumsy and
nonthreatening creatures that remind them of kids playing dress-up.
But the platypus trumps its plush-toy costars by adding a kind of Dada
prankishness to the equation, what with its bill that looks like a
Charlie Chaplin shoe, the leathery, thumb-sized eggs it insists on
laying, the Daffy Duck webfeet outfitted with venomous spurs and the
milk that dribbles down its unnippled chest. That the genetic code of
the platypus proved to be as bizarrely pastiched as its anatomy
enhanced the popular appeal of the report, published in the journal
Nature.
Yet for researchers in the burgeoning field of comparative genomics,
the real beauty of having spelled out all 2.2 billion chemical letters
of the platypus’s genetic blueprint lies not with the freak-show charm
of the animal but rather with its sublime ordinariness, positioned as
almost a platonic abstraction of a mammal, yet one with enough
specifically derived features to remind us that it is just an animal
trying to make its way in the world. It is archaic and post-modern,
primitive and refined. By studying the platypus and its close
relatives, scientists hope to better understand the genesis and
evolution of the entire mammalian family tree.
“Modern mammalian diversity is enormous,” said Zhe-Xi Luo, curator of
vertebrate paleontology at the Carnegie Museum of Natural History in
Pittsburgh, “but our view of that diversity has been heavily
influenced by our focus on two of the three groups of mammals, the
placentals and the marsupials.”
That fixation isn’t surprising, he said, given their numbers.
Placental mammals that gestate their young internally, as do we and
most of the pelts we know, account for 95 percent of the world’s 5,600
species of mammals, and marsupials like the kangaroo, koala and our
very own opossum, whose young are born at a grublike stage and do most
of their developing externally, often in a pouch, constitute nearly
all the rest.
The third group of mammals, the monotremes, claims a measly three
distinct animals, and all are indigenous to Australia and New Guinea:
the duckbilled platypus and the long-beaked and short-beaked echidnas,
also known as spiny anteaters. Yet this small club goes very deep, and
with the sequencing of the platypus genome, Dr. Luo said, “the total
scope of mammalian diversity” can now be explored.
The monotremes are considered the most ancient mammalian group, dating
back to the Jurassic period, which began roughly 200 million years
ago. They are not our direct ancestors, but rather split off from the
rodenty line that gave rise to us and marsupials some 180 million
years ago, with marsupials breaking away maybe 40 million years
later.
However ancient the rupture, the last common ancestor between us and
monotremes was clearly mammalian, for we all sport mammalian traits,
some of us more luxuriously than others. The duckbilled platypus, for
example, is jacketed in two layers of fine, dense fur, the better to
keep it warm in its semi-aquatic existence, and “it’s the softest fur
you can imagine,” said a co-author of the platypus paper, Wesley
Warren of Washington University School of Medicine.
Monotremes also have a four-chambered heart, a mammalian jaw hinge and
a set of tiny middle ear bones that, unlike in reptiles, are separated
from the lower jaw and hence lend mammals their highly sensitive
hearing. And though a platypus mother lacks nipples, the fluid that
oozes from two round patches of skin on her belly is the same sort of
rich blend of sugars, proteins, fatty acids, vitamins and
antimicrobial agents lionized by La Leche.
Monotremes also possess features that hark back to a runty mammal’s
lot at play among the giants of Jurassic park. They can dig themselves
into the haven of a hole within moments. They are most active in the
evening and at dawn, in contrast to the diurnal dinosaurs. They have
true or vestigial spurs on their back legs, which the male platypus
still packs with a snakelike venom. While the poisonous spearlets are
now directed largely against rival males, the trait could well have
served among ancestral monotremes as the equivalent of a bee’s sting,
to ward off the Bigfeet that ever threatened to crush them.
At the same time, the monotremes still pay homage to their
premammalian forebears. The group name, monotreme, means “one-holed,”
and so echidnas and platypuses have the equivalent of a bird or
reptile cloaca, a single all-purpose orifice for excretion, sex and
the laying of eggs.
The laying of eggs is, of course, a monotreme’s clearest vinculum to a
reptilian-avian mode of life. After digging herself an underground
nest and padding it with vegetation, a female platypus lays one to
three eggs and incubates them for about 10 days.
On hatching, the babies are barely the size of a gumdrop, and they
will spend the next five months nursing in the burrow, emerging only
when they are close to full-grown, maybe the size of a housecat. It’s
time to start hunting for a living, to dive into a pond in search of
crustaceans, which the platypus does with the help of chemoreceptors
to smell the prey and electroreceptors to detect the minor electrical
field surrounding the prey and mechanoreceptors to track movement of
the prey and a toothless but tough horny bill for seizing and crushing
the prey it has amply sensed.
“They’re voracious, curious creatures, constantly moving and nibbling
on things,” Dr. Warren said.
For its part, the echidna is highly specialized for eating ants and
termites, with a long pointed snout and a wormlike, sticky tongue.
In the specifics of their hunting gear, the platypus and echidna
reveal their highly stylized sides, the kidskin fit between themselves
and their niches, and the ways they have profoundly evolved since
their ancestors and ours started loosening their jawbones and
sweetening the glandular secretions with which they moistened their
eggs. Monotremes are like the fantasy geezer aunt you never knew you
had, breezing in from the other side of the world, with wild tales of
the past and no plans for slowing down.