The first complete map of the platypus genome has just been released and is as strange as you would expect from a creature with 10 sex chromosomes, a pair of venomous spurs, a layer of fluorescent fur and “sweating” skin. milk.
The duck-billed platypus is truly one of the strangest creatures on Earth. Together with the spiny echidna, these two Australian animals belong to a highly specialized group of mammals, known as monotremes, which both lay eggs but suckle their young with milk.
The genes of both are relatively primitive and unchanged, revealing a bizarre mix of several classes of vertebrates, including birds, reptiles and mammals.
As different as the platypus may seem at first, it is precisely those differences that reveal to us the similarities and our ancestors shared with the other vertebrates of the Earth.
Scientists believe that its genome could tell us secrets about our own evolution and how our distant ancestors of mammals went from laying eggs to birth.
“The complete genome gave us the answers to how some of the bizarre features of the platypus came about,” explains evolutionary biologist Guojie Zhang of the University of Copenhagen.
At the same time, decoding the platypus genome is important to improve our understanding of how other mammals have evolved – including us humans.
In previous years, a female platypus had part of its genome sequenced, but without Y chromosome sequences, a lot of information was missing.
Using a male platypus, researchers have now created a physical map with an extremely accurate platypus genome.
Today, living mammals are divided into three groups, including monotremes, marsupials and eutherians or “placentas” – we humans belong to the latter group.
Together, the latter two make up a subclass known as terrestrial mammals. Therian mammals all give birth to live young, but monotremes are simply too different to be crowded with that group.
It is not yet clear when all three of these distinct groups began to deviate from each other. Some believe that monotremes separate first, following the example of marsupials and eutherians. Others believe that all three groups diverged at about the same time.
The platypus genome has now helped clarify some of the data. Data collected from the echidna and platypus lines suggest that their last common ancestor lived up to 57 million years ago.
Meanwhile, monotremes as a whole appear to be divergent from marsupials and eutherian mammals about 187 million years ago.
Even after all this time, the semi-aquatic platypus has remained remarkably unchanged, fitting a niche in the Australian bush that many marsupials and mammals simply cannot.
The authors were particularly interested in the animal’s sex chromosomes, which appear to have originated independently of other terrestrial mammals, all of which contain a simple XY pair.
However, the platypus is the only known animal with 10 sex chromosomes (echidnas have nine). The platypus has 5X and 5Y chromosomes organized in a ring that seems to have broken into pieces during the evolution of mammals.
Comparing this chromosomal information with humans, opossums, Tasmanian devils, chicks, and lizard genomes, the authors found that platypus sex chromosomes have more in common with birds like chickens than mammals like humans.
But while the platypus lays eggs like chickens, they feed their young milk like terrestrial mammals.
Therefore, it is not too surprising that monotreme genomes contain most of the milk genes that other terrestrial mammals possess.
Casein genes help encode certain proteins in mammalian milk, but monotremes appear to have additional caseins with unknown functions. That being said, their milk is no different from what comes from a cow or even a lactating man.
As such, the platypus is probably not as dependent on egg proteins as other species of birds and reptiles, as it can later feed its chicks through the lactation glands on its skin.
Its genome supports this. While birds and reptiles rely on three genes that encode major egg proteins, the platypus appears to have lost most of these genes about 130 million years ago. Chickens today have all three protein genes in their eggs, humans have none, and the platypus has only one fully functional copy.
The platypus is a strange intermediary, and its genome is a kind of bridge to our own evolutionary past.
“It informs us that milk production for all existing mammal species was developed through the same set of genes derived from a common ancestor who lived more than 170 million years ago – along with the first dinosaurs of the Jurassic period.” , says Zhang.
The entire genome revealed the loss of four genes associated with tooth development, which probably disappeared about 120 million years ago. To eat, the platypus now uses a pair of horn-like plates to grind its food.
The poisonous spurs on the hind legs can be explained by the creature’s defensin genes, which are associated with the immune system of other mammals and appear to give rise to unique proteins in their venom. Echidnas, which also had their complete genome sequences, appear to have lost this key venom gene.
The authors say that their results represent “some of the most fascinating biologies of platypus and echidna” alike.
“The new genomes of both species will allow an additional perspective on terrestrial innovations and on the biology and evolution of these extraordinary mammals that lay eggs,” they conclude.
The study was published in The nature.