Genomic Analyses Reveal Why The Platypus Lays Eggs And Sweats Milk

An egg-laying, semi-aquatic mammal that sweats milk and glows in the dark. Why not? Lukas Vejrik/Shutterstock.com

There are some pretty freaky characters in the animal kingdom but arguably the "Top of the WTFs" has to be the duck-billed platypus. As semi-aquatic mammals who lay eggs, sweat milk and glow in the dark (turns out they're not the only ones), it’s easy to imagine why some early naturalists doubted the authenticity of early specimens which no doubt looked like someone had simply glued a beak to a taxidermy beaver. They are a member of the monotremes where they’re joined by another of Australia’s most unusual critters: the echidna. Now, new research published in the journal Nature has sought to identify the root of these animals’ spectacularly bizarre array of characteristics by mapping their genomes.

Such research is of great academic value as by diving into the genome of what may be the Earth’s strangest mammal we can see what happened evolutionarily speaking to wind up at the bizarre body maps of the platypus (Ornithorhynchus anatinus) and the echidna (Tachyglossus aculeatus). “Together with our echidna sequence, the genomes of the two species allow us to detect the ancestral and lineage-specific genomic changes that shape both monotreme and mammalian evolution,” wrote the researchers in the paper.

Vitellogenin genes are a vital component for animals who produce egg yolks such as chickens, extant animals in whom all three genes have endured. Humans have lost all of these genes which is why embryos aren’t treated to a lovely yolk while during their development (sorry babies). The genome of the platypus reveals they have kept one of these three genes, with the other two falling by the wayside some 130 million years ago. Observational evidence of this surviving gene is seen in their egg-laying. However, unlike reptiles and birds’ eggs, the eggs of the platypus aren’t dependent on yolk proteins because… *checks notes* they SWEAT MILK.

Milk production is a characteristic of mammals and this is explained by our genomes which show the vitellogenin egg-making-trio were replaced with casein genes. These churn out casein proteins which are a major component of mammalian milk and are thought to have first appeared in a common ancestor around 170 million years ago. Platypus also has casein genes which explains why their milk is quite similar to that of cows and humans. Where they differ from us enormously however is in their delivery. Unlike humans, they lack nipples so their young slurp up perspired milk from patches of their mother’s skin.

Perhaps most mind-boggling of all was the discovery that platypus has 10 sex chromosomes: five Y and five X. This is of course worlds apart from every other mammal on Earth whose sex is determined with XX for female and XY for male, say the study authors.

Echidnas are a member of the monotremes alongside the platypus. Jukka Jantunen/Shutterstock.com

The near-complete chromosomal level genomes acquired from the new study means researchers can now theorize that the monotremes’ 10 sex chromosomes were once organized in a ring form which later broke off into many small pieces of X and Y chromosomes. The genome mapping also reveals that most of the monotremes’ sex chromosomes are more similar to those of chickens than humans, demonstrating an evolutionary link between mammals and birds.

"The complete genome has provided us with the answers to how a few of the platypus’ bizarre features emerged,” said Professor Guojie Zhang of the Department of Biology in a release about the research. “At the same time, decoding the genome for platypus is important for improving our understanding of how other mammals evolved - including us humans. It holds the key as to why we and other eutheria mammals evolved to become animals that give birth to live young instead of egg-laying animals.”

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