Platypuses do it differently

Of all the mammals, the naturalist George Shaw (no relation) wrote when he first described it in 1799, the platypus ‘seems the most extraordinary in its conformation’, exhibiting ‘the perfect resemblance of the beak of a duck engrafted on the head of a quadruped’. The specimen looked so like a taxidermic prank that ‘I ought perhaps to acknowledge that I almost doubt the testimony of my own eyes.’

Nevertheless, he duly classified it as a mammal despite its beak, ‘which verifies in a most striking manner the observation of Buffon: viz. that whatever was possible for Nature to produce has actually been produced.’ The apparently hybrid nature of the platypus had been recognised long before. An Australian Aboriginal Dreamtime story identifies the platypus as the offspring of a duck and a water rat. Early European colonisers of New South Wales called them ‘duckmoles’.

Last week, the first near-complete platypus genome was published. This is an impressive achievement. Animals and plants have their DNA bundled up in tangles inside chromosomes (fruit flies have eight chromosomes, humans have 46, dogs have 78 and platypuses have 52). To read the DNA sequence – the As, Ts, Cs and Gs – you have to break the chromosomes apart into tiny pieces. You’re then left with the job of computationally placing the pieces back together.

Without a idea of what you’re looking for, this is a challenge. It’s as if somebody had mixed jigsaw pieces from many different puzzles together in a blender, thrown away the boxes with the pictures on the front, and handed you the remnants. To get started on the reassembly, it helps to have a rough idea of what you’re trying to make. A genome from a related species is a good starting point. Unfortunately this isn’t an option for species without close living relatives, such as the platypus.

Biologists have always been fascinated by taxonomically aberrant species, but they don’t necessarily attract general interest. As a masters student, I spent a short time working on Xenoturbella, a genus of small, nondescript marine worms that live in deep-sea mud. For the last few decades, taxonomists have argued over where they should be placed in the tree of life. The scientists who discovered a new species in 2016 called it X. churro, because of its supposed similarity to the Spanish doughnut, but they’re unappetising creatures. Mass appeal has so far eluded them.

The platypus, by contrast, is always a big draw. A few years ago, I went to a talk on platypus genetics. It was well attended. I suspect that, like me, most of the audience were hoping for a light and entertaining discussion with plenty of platypus pictures. As the presenting scientist instead showed slide after slide heavy with confusing data, I sobered up. It turns out that the platypus genome has its own extraordinary conformation, just as bizarre as its outward appearance.

Extant mammals are split into three groups: placentals (like humans, cats and dogs), marsupials (like kangaroos and koalas) and monotremes, an exclusive club of just five species: one platypus and four echidna. (‘Formerly nipples were the mark of a mammal,’ Engels wrote in 1883. ‘But the platypus has none … What a beautiful confirmation of Hegel’s thesis that the inductive conclusion is essentially a problematic one!’) Around 200 million years ago, monotremes diverged from other mammals, but we still have things in common, such as mammary glands. (Engels was still right that platypuses have no nipples: their milk oozes out of their skin like sweat.)

Unlike most mammals, which have one pair of sex chromosomes (X and Y, with males typically XY and females typically XX), the platypus has five. This was discovered in 2004, when researchers showed that monotreme sex chromosomes originated independently from other mammals. Males have five pairs of alternating, different Xs and Ys: X₁Y₁X₂Y₂X₃Y₃X₄Y₄X₅Y₅.

During meiosis, the ten sex chromosomes assemble into a spectacular alternating chain and then separate, creating sperm with either X₁X₂X₃X₄X₅ or Y₁Y₂Y₃Y₄Y₅. This byzantine structure seems to work, but as the researchers admitted at the time, the arrangement of the chromosomes remained ‘deeply puzzling’. How could such a thing have come about? An incomplete genome was published in 2008, but from a female: no Y chromosomes.

This week, we are a little closer to knowing. We now have a genome from a male platypus where 98 per cent of the DNA has been mapped back to chromosomes. As well as the similarity between the X and Y chromosomes, the researchers looked at their physical interactions in platypus liver cells. The patterns suggest a specific order in which non-sex chromosomes were recruited, over evolutionary time, as sex chromosomes.

There is convincing evidence of genetic similarity between the X₁ and Y₅ chromosomes, but no physical interaction. This suggests that the ancestor of platypuses and echidna could have had its sex chromosomes arranged in a ring during meiosis: the alternating chain X₁Y₁X₂Y₂..., but with Y₅ linking back round to X₁. Over time, this last link would have become weaker and eventually decoupled. Circular rings of sex chromosomes have been studied in flowers (evening primroses), but they've never been seen in an animal. In fairness, they haven’t been seen in the platypus either, just inferred in its ancestor, but it seems a plausible hypothesis for the origins of its sex chromosome daisy chain.

With a complete genome, it will be possible to investigate the genetic origins of the platypus’s weirdness in more detail. And more than two centuries after its Linnean classification, its cabinet of curiosities keeps growing: at the end of last year it was reported that platypus fur glows blue-green under UV light.