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It sweats milk and has glowing fur — the genome of the platypus has been sequenced

Science — It has been called bizarre, iconic, unusual, mysterious and strange. Now the whole genome of the platypus has been sequenced. A scientist from the University of Copenhagen led the project.

The platypus has glowing fur, sweats milk, has 10 gender chromosomes, a poisonous claw, a beak to record electrical impulses from its prey in water, webbing for swimming between its toes, and lays eggs even though it is a mammal.

There are not many animals that are as strange as the platypus. And we humans actually share a distant, common ancestor with it some time around 170 million years ago.

In other words, this is a quirky and — evolutionary speaking — durable cousin. Professor Guojie Zhang from the Villum Centre for Biodiversity Genomics at the Department of Biology at the University of Copenhagen believes that we might be able to learn something from it.

Together with researchers from other universities throughout the world, he has mapped out the full genome of the platypus. The project took six months, and the results have been published in the scientific journal Nature.

Platypus has sex chromosomes that are a lot like birds

Their study can give us some answers as to how the weird traits of the platypus arose.

Professor Guojie Zhang is particularly amazed by the 10 sex chromosomes that the platypus turns out to have. We have two of them, but the platypus has 10, divided into five Y and five X chromosomes.

»This is perhaps one of the most important results coming out of our study: that the platypus has 10 sex chromosomes. Our sequencing of them is of a very high quality, and it turns out that the platypus’ sex chromosomes have nothing to do with the human ones, but correlate instead with the sex chromosomes of chickens. Platypuses in other words have bird-like sex chromosomes,« says Guojie Zhang.

The platypus has only one out of three sets of egg genes left

A platypus lays eggs, even though it is a mammal. Why do we humans no longer lay eggs?

»This is a difficult question to answer,« says Guojie Zhang, »but our research has shown that the ancestors of humans and the ancestors of mammals had genes that were both able to form eggs and egg yolk. One of these genes is still active in the platypus, while humans have lost all three of them. Perhaps it is because we, as a species, became so skilled at taking care of our weak, newborn, young that we no longer had to lay eggs,« says Guojie Zhang.

His research has also shown that the platypus carries a so-called vitellogenin gene, which it needs to form eggs and egg yolk, while modern day birds – like chickens – bear all three of the vitellogenin genes.

According to Guojie Zhang, we do not know exactly when the platypus lost the two vitellogenin genes, but this may have happened about 180 million years ago.

The platypus also has genes that allow it to produce milk for its young, after the eggs are hatched – even though the female does not have teets. The young lick the milk from the mother’s fur.

Incredible progress in genome sequencing

The platypus belongs to the group of the earliest mammals called monotremes, and the platypus, and its cousin the ant eater, are a part of the group. The animals use a single duct (the cloaca) for their urinary, defecatory, and reproductive systems.

The sequencing of the platypus genome is 96 per cent complete. It took only six months to complete the platypus project, compared to the thirteen years it took to map out the human genome. The sequencing of the human genome lasted from 1990 to April 2003, and cost USD 2.7 billion.

According to Guojie Zhang, the big difference is not so much that it is easier to map the genome of the platypus than the human.

It is just that the scientific equipment that sequence whole genomes has got so much better since the beginning of the new millennium. A few researchers can now achieve the sequencing of a mammal’s genome in six months and with a relatively modest budget of about EUR 8,000 – even though the final costs, according to Guojie Zhang, can vary greatly due to unforeseen expenses.

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