The first complete map of a platypus genome has been released recently, and it is in every way strange as you see a creature with 10 sex chromosomes, a pair of venomous spurs, a coat of fluorescent fur, and skin that ‘sweats’. I would expect milk.
The duck-billed platypus is actually one of the strangest creatures on Earth. Along with the spikey echidna, these two Australian animals belong to a highly specialized group of mammals, known as monotrames, which both lay eggs, but also nurse their young with milk.
The genes of both are relatively primitive and unchanged, revealing a bizarre mixture of classes of many vertebrate animals, including birds, reptiles and mammals.
As the platypus may differ at first, it is these very differences that reveal our similarities and our shared ancestry with other vertebrates of the earth.
Scientists think that its genome can tell us the secret about its evolution and how our distant mammal ancestors went from laying eggs to giving birth.
Evolutionary biologist Guoji Zhang from the University of Copenhagen explains, “The complete genome has given us the answer as to how some bizarre features of the platypus have been revealed.”
“At the same time, decoding the genome for the platypus is important to improve our understanding of how other mammals have evolved – including us humans.”
In previous years, a female platypus made some of its genome sequences, but without the Y chromosome sequences, a lot of information was missing.
Using a male platypus, researchers have now created a physical map with a highly accurate platypus genome.
Today, living mammals are divided into three groups, including monotrem, marsupial and eutherian or ‘placentals’ – we humans belong to that last group.
Together, the latter two form a subclass known as physician mammals. Therian mammals all give birth to live young, but monotrames are too different to simply be lumpe as well as that group.
It is not even clear when these three different groups first started deviating from each other. Some feel that the monotreme diverges first, followed by the suit Marsupial and Eutherian. Others think that all three groups left at about the same time.
The genome of the platypus has now helped to clarify some dates. Data collected from the Echidna and Platypus lineages suggest that their last common ancestor lived until 57 million years ago.
Meanwhile, monotremes as a whole appear to have originated from the marsupials and Eutherian mammals about 187 million years ago.
Even after that time, the semi-aquatic platypus remains remarkably unchanged, fitting a niche in the Australian bush that many marsupials and mammals simply cannot.
The authors were particularly interested in the sex chromosomes of the animal, which seem to have originated independently from other physician mammals, all of which have a simple XY pair.
The platypus, however, is the only known animal with 10 sex chromosomes (echidnas have nine). The platypus consists of 5X and 5Y chromosomes arranged in a ring that breaks apart into pieces during mammalian development.
Comparing this chromosomal information to humans, oposums, Tasmanian devils, chickens, and lizard genomes, the authors found that sex chromosomes of the platypus have more in common with birds such as chickens than birds.
But when platypuses lay eggs like chickens, they feed their young milk like therian mammals.
This is not too surprising, therefore, that the monotreme genome contains most of the milk genes, which other mammals possess.
Casein genes help to encode some proteins in mammalian milk, but additional caseins with unknown functions appear in the monotrame. That said, their milk is not unlike a cow or a lactating human.
As such, the platypus probably does not depend on egg protein like other birds and reptile species, as it can later feed its young through the lactation glands on its skin.
Its genome supports it. While birds and reptiles depend on three genes that encode for key egg proteins, the platypus loses the majority of these genes 130 million years ago. Today all three egg proteins in chickens have protein genes, humans have none and only one fully functional copy remains in the platypus.
The platypus is a strange distinction, and its genome is a bridge to our own evolutionary past.
“This tells us that milk production in all prevalent mammal species has evolved through the same set of genes derived from a common ancestor, dating back more than 170 million years – with dinosaurs dating back to the Jurassic period” Zhang it is said.
The complete genome has also revealed the loss of four genes associated with tooth development, which probably disappeared 120 million years ago. For eating, Platypus now uses a pair of horn-like plates to grind its food.
Poisonous spurs on its hind legs can possibly be explained by the organism’s defensin genes, which are linked to the immune system in other mammals, and give rise to unique proteins in their toxin. Echidnus, which also had their complete genome sequence, has lost this major toxin gene.
The authors state that their results represent “some of the most fascinating biology of platypus and echidna”.
“New genomes of both species will advance innovation and innovation and innovation of Inion egg-taking mammalian organisms.”
The study was published in Nature.