The first full map of a platypus genome has just been released, and it’s as strange as you’d expect from a creature with 10 sex chromosomes, a few poisonous spores, a layer of fluorescent fur, and skin that ‘sweats’ 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 and feed their young with milk.
The genes of both are relatively primitive and unaltered, revealing a bizarre mix of different classes of vertebrates, including birds, reptiles and mammals.
However different the platypus may appear at first, it is these very differences that reveal our similarities and our shared ancestry with the other vertebrates on Earth.
Scientists think the genome could tell us secrets about our own evolution and how our distant mammalian ancestors went from laying eggs to giving birth.
“The complete genome has given us the answers to how some of the platypus’ bizarre features emerged,” explains evolution biologist Guojie Zhang from the University of Copenhagen.
“At the same time, deciphering the platypus genome is important to better understand how other mammals evolved – including us humans.”
In previous years, part of the genome of a female platypus was sequenced, but without Y chromosome sequences, much information was missing.
Using a male platypus, researchers have now created a physical map showing a highly accurate platypus genome.
Today, living mammals are split into three groups, including monotremes, marsupials, and eutherians or ‘placentals’ – we humans belong to the latter group.
Together, the latter two form a subclass known as therian mammals. All mammals all give birth to live young, but monotremes are just too different to be lumped together with that group.
It is still unclear when these three different groups first began to diverge from each other. Some think the monotremes split off first, using marsupials and eutherians as an example. Others think that all three groups diverged at about the same time.
The platypus genome has now helped elucidate some of the data. The data gathered from the echidna and platypus lines suggests that their last common ancestor lived up to 57 million years ago.
Meanwhile, monotremes as a whole seem to have diverged from marsupials and eutheric mammals about 187 million years ago.
Even after all this time, the semi-aquatic platypus has remained remarkably unaltered, fitting into a niche in the Australian bush that many marsupials and mammals just can’t.
The authors were particularly interested in the animal’s sex chromosomes, which appear to have evolved independently from other therian mammals, all of which contain a simple XY pair.
However, the platypus is the only known animal with 10 sex chromosomes (echidnas have nine). Platypus have 5X and 5Y chromosomes organized in a ring that appears to have broken into pieces in the course of mammalian evolution.
By comparing this chromosome information with humans, opossums, Tasmanian devils, chickens, and lizard genomes, the authors found that platypus sex chromosomes have more in common with birds such as chickens than with mammals such as humans.
But while platypuses lay eggs like chickens, they feed their young milk like their mammals.
It is therefore not such a great surprise that monotreme genomes contain most of the milk genes that other therical mammals possess.
Casein genes help encode certain proteins in mammalian milk, but monotremes appear to have additional casein with unknown functions. That said, their milk is no different from that of a cow or even a nursing human.
As such, the platypus is probably not as dependent on egg proteins as other bird and reptile species, as it can later feed their young through the lactation glands on its skin.
His genome supports this. While birds and reptiles rely on three genes encoding important egg proteins, the platypus appears to have lost the majority of these genes about 130 million years ago. Chickens today have all three egg protein genes, humans have none, and the platypus has only one fully functional copy left.
The platypus is a strange interspace and its genome is a kind of bridge to our own evolutionary past.
“It tells us that milk production in all existing mammalian species was developed through the same set of genes derived from a common ancestor that lived more than 170 million years ago – in addition to the early Jurassic dinosaurs,” says Zhang.
The entire genome has also revealed the loss of four genes related to tooth development, which likely disappeared about 120 million years ago. To eat, the platypus now uses a pair of horn-like plates to grind its food.
The poisonous spores on its hind legs could possibly be explained by the creature’s defensin genes, which are associated with the immune systems of other mammals and appear to produce unique proteins in their venom. Echidnas, which also had their full genome sequences, appear to have lost this important venom gene.
The authors say their results represent “one of the most fascinating platypus and echidna biologies.”
“The new genomes of both species will allow further insights into their innovations and the biology and evolution of these extraordinary egg-laying mammals,” they conclude.
The study is published in Nature.