(Image: Dave Watts/Alamy)

It’s part-reptile, part-mammal, part-bird – and totally unique. Two centuries after European scientists deemed a dead specimen so outlandish it had to be a fake, the bizarre genetic secrets of Australia’s platypus have been laid bare.

Platypuses lay eggs and produce venom like some reptiles, but they sport furry coats and feed their young with milk like mammals. The odd creatures are classed as monotremes, with only one close relative – the echidna.

But as primitive mammals that share the same ancestor as humans, a study of the animal’s genome can improve biologists’ understanding of how mammals evolved, while illuminating the platypus’s strange physiology.


Wesley Warren at Washington University in St Louis, Missouri, led the international team that sequenced the platypus genome. As expected, they found an amalgam of some ancestral reptile and some newer mammalian features. But there were also surprises.

Ancient milk

Biologists already knew that the platypus has a weird complement of sex chromosomes, but the team found that the gene sequences responsible for determining sex are more like a bird’s than a mammal’s.

And while the gene that the human sex-determining gene evolved from is present in the platypus genome, it seems to have nothing to do with sex determination. So, that function must have evolved after the platypus split from our common ancestor, about 166 million years ago.

However, by that time, milk production was well-evolved. The platypus has the same repertoire of milk protein genes as a cow or a human. Clearly, milk evolved long before we evolved to give birth to live offspring, says team member Jenny Graves at the Australian National University in Canberra.

The team also investigated the genes for the platypus toxin, which males deliver via a barbed spur on their heel. While the toxin is similar to a snake’s – adapted from natural neurotransmitters and other proteins – it seems to have evolved independently in the two animal groups.

Watery smells

“It looks like the platypus started from the same suite of genes, but did completely different things with them,” says Graves.

In yet another surprise, the team found that the platypus has a bigger repertoire of a particular class of vomeronasal receptors than any other animal.

This finding suggests that their sense of smell is important while they are foraging under water, says Stewart Nicol, a platypus researcher at the University of Tasmania, Hobart. “I don’t think anybody expected that,” he says.

The draft genome sequence should also help to end the platypus’s identity crisis, Graves says.

“We know it is not the head of a duck stitched onto the tail of a beaver any more – but there was still controversy about where it fitted in, and whether perhaps it was a weird marsupial” she says. “Now, with the whole genome, there is no doubt that it split off the branch that led to both placentals and marsupials. At last, we can clear about where it fits.”

Journal reference: Nature (DOI: 10.1038/nature06936)

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