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Who would think that the reclusive platypus would be such a trouble-maker? When the antipodean oddity made its British debut in 1799, it stirred up a ferocious debate that lasted 85 years; one that Darwin himself didn't even see the end of.

At first naturalists thought the furry creature with the webbed feet and the duck's bill was a hoax, the work of a skilful Chinese taxidermist. A specimen in London's National History Museum bears the scars of an attempt to unpick the fine 'stiches'. When none were found, the debate turned to whether this creature was really a mammal at all. Though furry, no nipples or mammary glands were visible.

Even more perplexing, dissection showed a single lower body opening like the cloaca of a bird or reptile. That earned it, and its similarly equipped cousin the echidna, a new classification amongst mammals: the 'monotremes'.

Mammary glands were located in the 1830s. But it took till 1884 for William Caldwell's famous telegram "Monotremes oviparous, ovum meroblastic," to reach the gobsmacked naturalists of the British Association for the Advancement of Science. What the young Scottish embryologist had telegraphed was: the platypus laid eggs and they were yolky. There it was: a furry, suckling, lizard-limbed, egg laying mammal. Who could resist the temptation to see it as a missing link between reptiles and mammals? Darwin, even without the egg-laying information, pronounced: "these anomalous forms may almost be called living fossils."

Now, some 209 years after the platypus first confounded the scientific community, the U.K. journal Nature has published a detailed analysis of the creature's genome; its genetic blueprint revealed in 2.3 billion letters of DNA code.

Revelations in the code

The code certainly offers revelations. "It's a key missing piece from the evolutionary puzzle that allows us to establish what the earliest common ancestors of mammals were like," says Wes Warren, whose lab at Washington University in the U.S. city of St. Louis, Missouri, carried out much of the sequencing.

But it also offers more confounding clues. "It shows the platypus method of sex determination has more in common with birds than other mammals. I'm not sure if I can make sense of this," says Des Cooper, a marsupial geneticist at the University of New South Wales in Sydney, Australia.

Let's start with the revelations that make sense and leave that confounding thing called sex till last.

There are some parts of the genome that show the platypus for what it is: a hybrid of reptilian and mammalian features. For instance the platypus' reptilian roots can be seen in the very dialect of the code. All animal genomes have stutters of repeating DNA. Some are long-lasting stanzas of thousands of letters or more; others are tiny hiccups of a few letters known as microsatellites. It turns out the platypus hiccups bear far more resemblance to those of chickens and reptiles than to those in mammals.

More evidence of the reptilian dialect is seen in a cluster of genes that control the size of the foetus. In mammals other than monotremes, these genes come imprinted with different activities: if they came from a sperm, they are highly active; if they are from an egg they are inactive. But the platypus genes from either egg or sperm have the same activity.

Andrew Pask and Marilyn Renfree at Melbourne University believe the reason the platypus genes don't bear imprints is revealed by their cleaner DNA. While the foetal growth genes of mammals are peppered with stanzas of repetitive DNA, the platypus genes are relatively free of them, as are the corresponding genes of the chicken (overall the platypus genome has about the same fraction of repetitive DNA as other mammals: 50 per cent). "Repetitive [DNA] often causes neighbouring genes to shut down and that may have been useful for establishing imprinting in the other mammals", explains Pask.

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