The first complete map of a platypus genome has just been released and is as strange as one would expect from a creature with 10 sex chromosomes, a pair of poisonous spurs, a fluorescent skin coat and skin that “His.” milk.
The duck-billed platypus is truly one of the rarest creatures on Earth. Along with the pointed equidna, these two Australian animals belong to a highly specialized group of mammals, known as monotremes, which lay eggs but also breastfeed their young with milk.
The genes of both are relatively primitive and unaltered, revealing a strange mixture of several classes of vertebrate animals, including birds, reptiles, and mammals.
As different as the platypus might seem at first, it is these same differences that reveal our similarities and our shared ancestry with the other vertebrates on Earth.
Scientists think their genome could tell us secrets about our own evolution and how our distant mammal ancestors went from laying eggs to giving birth.
“The entire genome has provided us with the answers to how some of the bizarre features of the platypus arose,” 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 the evolution of other mammals, including humans.”
In previous years, a female platypus had a part of its genome sequenced, but without any Y chromosome sequence, much information was missing.
With a male platypus, researchers have now created a physical map with a very accurate platypus genome.
Today, living mammals are divided into three groups, including monotremes, marsupials and eutherians or “placentas”, humans belong to the latter group.
Together, the latter two form a subclass known as Terian mammals. All Terian mammals give birth to living young, but monotremes are simply too different to be part of this group as well.
It is still unclear when these three different groups began to diverge from each other. Some think that monotremes separated first, and marsupials and eutherians followed suit. Others think the three groups diverged at about the same time.
The platypus genome has helped clarify some of the dates. Data collected from the echidna and platypus lineages suggest that their last common ancestor lived up to 57 million years ago.
Meanwhile, monotremes as a whole appear to have diverged from marsupials and eutherian mammals about 187 million years ago.
Even after all this time, the semi-aquatic platypus has remained extraordinarily unchanged, fitting a niche into the Australian shrub that many marsupials and mammals simply cannot make.
The authors were particularly interested in the animal’s sex chromosomes, which appear to originate independently of other terian mammals, containing a simple XY pair.
The platypus, however, is the only known animal with 10 sex chromosomes (equidnias have nine). The platypus has 5X and 5Y chromosomes arranged in a ring that appears to have broken into pieces throughout the evolution of mammals.
Comparing this chromosomal information with humans, opossums, Tasmanian devils, chickens, 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 terian mammals.
Therefore, it is not too surprising that monotreme genomes contain most of the milk genes that other terian mammals possess.
Casein genes help encode certain proteins in mammalian milk, but monotremes appear to have additional caseins with unknown functions. That said, her milk doesn’t look like it comes from a cow or even a lactating human.
As such, the platypus probably does not depend as much on egg proteins as on other species of birds and reptiles, because it can subsequently feed its young through the lactating glands of the skin.
Its genome supports it. Although birds and reptiles are based on three genes that encode the major egg proteins, the platypus appears to have lost most of these genes about 130 million years ago. Chickens currently have all three egg protein genes, humans do not have them, and the platypus has only one fully functional copy left.
The platypus is a strange medium, and its genome is a kind of bridge to our own evolutionary past.
“It informs us that milk production in all existing mammal species has developed through the same set of genes derived from a common ancestor that lived more than 170 million years ago, alongside the first dinosaurs of the Jurassic period, ”says Zhang.
The entire genome has also revealed the loss of four genes associated with tooth development, which probably disappeared about 120 million years ago. For food, the platypus now uses a pair of horn-like dishes to grind food.
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 the complete genome sequences, appear to have lost this key poison gene.
The authors say their results represent “some of the most fascinating biologies of platypus and echidna” alike.
“The new genomes of both species will provide more insight into Terian innovations and the biology and evolution of these extraordinary mammals,” they conclude.
The study was published in Nature.