Part mammal, bird and reptile: Genome mapping reveals how the platypus came to be

A new study warns that the platypus population will be halved by 2070 unless action is taken to protect it from the effects of development and climate change. (Tahnael Hawke / University of New South Wales)

EDMONTON — Ever since Europeans discovered the platypus in the late 1700s, the bizarre, duck-billed, semi-aquatic creature has perplexed researchers.

The creature, with its toothless duck-like bill, webbed feet, ankle spikes full of venom, and glowing fur, stunned researchers with its appearance alone, long before it was discovered that it could both lay eggs and produce milk for its young.

Modern day researchers are still trying to understand how the platypus, largely considered to be the world's oddest mammal, came to be – until now.

For the first time, researchers at the University of Copenhagen have mapped a complete platypus genome, providing answers to how some of the platypus' bizarre features emerged.

The platypus belongs to an ancient group of mammals – monotremes – which existed millions of years prior to the emergence of any modern-day mammal.

By comparing the platypus’ genome with that of one of the only other living monotreme species, the short-beaked echidna, researchers can build a better understanding of the genes that led to the creatures’ distinctive traits.

"Indeed, the platypus belongs to the Mammalia class. But genetically, it is a mixture of mammals, birds and reptiles. It has preserved many of its ancestors' original features -- which probably contribute to its success in adapting to the environment they live in," professor Guojie Zhang, University of Copenhagen researcher, said in a press release.

Monotremes are best known for their unique ability to both lay eggs and nurse their young once they’ve hatched.

Researchers note that one of the platypus' most unusual characteristics is its ability to feed its hatched young by excreting milk through its sweat.

According to the research, published this week in the journal Nature, during our own evolution, humans lost their vitellogenin genes, which are important for the production of egg yolks. Chickens, for example, have all three of these genes. Instead, we developed casein genes, which are responsible for our ability to produce casein protein, a major component in mammalian milk.

Platypuses still carry one vitellogenin gene, despite having lost the other two roughly 130 million years ago, which allows them to lay eggs. But they also carry the casein genes, allowing the creatures to develop milk that is very similar to cow’s milk.

“It informs us that milk production in all extant mammal species has been developed through the same set of genes derived from a common ancestor which lived more than 170 million years ago, alongside the early dinosaurs in the Jurassic period,” Zhang said.

ONLY ANIMAL WITH 10 SEX CHROMOSOME

The research has also shed light on one of the most perplexing platypus topics – determining the creatures’ sex.

Every other mammal on earth, including humans, have two sex chromosomes. But the platypus has 10, including five Y and five X chromosomes.

Researchers now believe the chromosomes were organized in a ring form, which was later broken away into many small pieces of X and Y chromosomes in ancient monotreme ancestors.

At the same time, the genome mapping reveals that the monotreme sex chromosomes have more in common with chickens than with humans, suggesting an evolutionary link between mammals and birds.

"Decoding the genome for platypus is important for improving our understanding of how other mammals evolved, including us humans,” explained Zhang of the Department of Biology.

“It holds the key as to why we and other eutherian mammals evolved to become animals that give birth to live young instead of egg-laying animals.”​