Credit: Wikipedia
Serious smelling power
The platypus may have held on to many ancestral characteristics but its adaptation to its watery world is awe-inspiring. It spends most of its time underwater, blind and deaf, relying on electro sensors on its bill to detect the electric impulses of prey.
A big surprise from reading the genome is that it may also be adept at detecting water-borne odorants. Tsviya Olender and Doron Lancet at the Weizmann Institute in Rehovot, Israel, analysed the platypus code for clues to its system of odour detection.
As Lancet puts it, "animals have two noses". One, which depends on olfactory sensors, can only detect air-borne compounds. The other is the vomeronasal organ (humans probably don't have one). Vomeronasal receptors are like nasal taste buds, able to detect non-volatile compounds.
For instance dogs taste pheromones in urine by touching their tongue to the vomeronasal organ in their upper palate. The platypus turns out to be superbly endowed with vomeronasal receptors. "A typical mammal has a couple of hundred of them; the platypus has 1,000", says Lancet. Since the platypus spends 90 per cent of its time in water, he speculates the platypus uses these receptors for detecting water-soluble odorants.
Which brings us finally to sex and the platypus. Specifically the sex chromosomes of the platypus.
Platypus sex
For most mammals, their chromosomes look pretty similar, all fairly large and about 50 of them. Reptiles and birds are very different though; they tend to have lots of tiny chromosomes that early cytologists referred to as 'chromosome dust'. Shockingly to the first cell biologists who peered down the microscope and saw them, the platypus chromosomes resemble those of birds and lizards. They are a mess of 46 tiny chromosomes and 6 big ones.
But it's not just the outward appearance of the chromosomes that bears a resemblance to birds. Mammals like us (eutherians) or kangaroos (marsupials) all determine sex by having different sets of sex chromosomes. Getting a pair of the two big X's makes a female. An X and a little Y, makes a male. Birds on the other hand have an opposite system. Two copies of a big Z chromosome make you a male, while a Z plus a little w, makes you a female.
But not all animals have different sex chromosomes. In many species of reptile for instance, males and females have the same chromosomes. Their sex is determined by temperature.
If you're a turtle egg, in cool conditions you're a male; if it warms up you're female. So the thinking is that a couple of hundred million years ago, some animals stopped relying on temperature and put their own failsafe sex switch onto one of the chromosomes. At that point the chromosome pair had to stay different, and so they drifted further and further apart, till the point that you have the almost unrecognisable pairs of X and Y or Z and W.
So what do we see in the platypus? For starters, they have ten sex chromosomes! Females have five pairs of X and males have five X and five Y. None of the platypus sex chromosomes are even vaguely related to ours. Searching for the remains of our X chromosome, locates it on a regular chromosome, autosome number six. It's a finding that nicely aligns with the theory that sex chromosomes evolved from ordinary ones.
fascinating
so what happends if there arent a complete transfer of 5 chromosomes?
like 6 x's and 4 y's ?
is the 5 transferred as a package?
is information on the y equivalent chomosomes dominant over each other?
are they duplicates or are they different?
so many more questions
Eliot
platypus sex chromosomes
Hi Eliot,
good questions. You would not get offspring if the platypus had not worked out to sort out the ten sex chromosomes and we showed that it works reliably (Grutzner et al. 2004, Nature). How they do it? The all stick together in a chain of chromosomes X1Y1X2Y2....X5Y5, then all Ys wander off in one direction and all X in the other (in a way it is as a package), how they do it? We don't know,
The chromosomes are not duplicates they all carry different genes.
Cheers, Frank.
Platypus genome
Excellent article, marred only by the common practice of journalists to confuse 'code' with 'information'. With minor and rare exceptions, the genetic code is universal, being the same for cabbages, mushrooms, jellyfish and camels. Just as it was possible to transmit an infinite number of messages using the Morse code, the genetic code makes possible a virtually infinite variety of proteins.
Martin Hanson, retired science teacher