Monotremes (egg-laying mammals) are the only extant mammalian outgroup to therians (marsupial and eutherian animals) and provide key insights into mammalian evolution. An international team of scientists has sequenced and analyzed the genomes of the platypus (Ornithorhynchus anatinus) and the short-beaked echidna (Tachyglossus aculeatus), the two extant monotreme lineages, and compared them to those of chickens, humans, rats, Tasmanian devils, and lizards. Their results appear in the journal Nature.
“The complete platypus genome has provided us with the answers to how a few of its bizarre features emerged,” said senior co-author Professor Guojie Zhang, a researcher from BGI-Shenzhen, the University of Copenhagen, the Kunming Institute of Zoology and the Center for Excellence in Animal Evolution and Genetics of the Chinese Academy of Sciences.
“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 eutherian mammals evolved to become animals that give birth to live young instead of egg-laying animals.”
One of the platypus’ most unusual characteristics is that, while it lays eggs, it also has mammary glands used to feed its babies, not through nipples, but by milk — which is sweat from its body.
During our own evolution, we humans lost all three so-called vitellogenin genes, each of which is important for the production of egg yolks. Chickens on the other hand, continue to have all three.
Professor Zhang and colleagues demonstrated that platypuses still carry one of these three vitellogenin genes, despite having lost the other two roughly 130 million years ago.
The platypus continues to lay eggs by virtue of this one remaining gene. This is probably because it is not as dependent on creating yolk proteins as birds and reptiles are, as platypuses produce milk for their young.
In all other mammals, vitellogenin genes have been replaced with casein genes, which are responsible for our ability to produce casein protein, a major component in mammalian milk.
The authors found that the platypus carries casein genes as well, and that the composition of their milk is thereby quite similar to that of cows, humans and other mammals.
“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,” Professor Zhang said.
Another trait that makes the platypus so unique is that, unlike the vast majority of mammals, it is toothless.
Although its nearest ancestors were toothed, the modern platypus is equipped with two horn plates that are used to mash food.
The study revealed that the platypus lost its teeth roughly 120 million years ago, when four of the eight genes responsible for tooth development disappeared.
Yet another platypus oddity investigated by the team was how their sex is determined.
Both humans and every other mammal on Earth have two sex chromosomes that determine sex — the X and Y chromosome system in which XX is female and XY is male.
The monotremes, however, have 10 sex chromosomes, with five Y and five X chromosomes.
“Thanks to the near-complete chromosomal level genomes, we can now suggest that these 10 sex chromosomes in the ancestors of the monotremes were organized in a ring form which was later broken away into many small pieces of X and Y chromosomes,” the researchers said.
“At the same time, the genome mapping reveals that the majority of monotreme sex chromosomes have more in common with chickens than with humans. But what it shows is an evolutionary link between mammals and birds.”
Y. Zhou et al. Platypus and echidna genomes reveal mammalian biology and evolution. Nature, published online January 6, 2021; doi: 10.1038/s41586-020-03039-0
This article is based on text provided by the University of Copenhagen.
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