Now We Know Why Platypus Are So Weird - Their Genes Are Part Bird, Reptile, And Mammal
The first complete map of a platypus genome has just been released, and it's every bit as strange as you'd expect from a creature with 10 sex chromosomes, a pair of venomous spurs, a coat of fluorescent fur, and skin that 'sweats' milk.
The duck-billed platypus is truly one of the oddest creatures on Earth. Along with the spiky echidna, these two Australian animals belong to a highly-specialised group of mammals, known as monotremes, which both lay eggs but also nurse their young with milk.
The genes of both are relatively primitive and unchanged, revealing a bizarre blend of several vertebrate animal classes, including birds, reptiles, and mammals.
As different as the platypus might seem at first, it's those very differences that reveal our similarities and our shared ancestry with Earth's other vertebrates.
Scientists think its 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 provided us with the answers to how a few of the platypus' bizarre features emerged," explains evolutionary biologist Guojie Zhang from the University of Copenhagen.
"At the same time, decoding the genome for platypus is important for improving our understanding of how other mammals evolved - including us humans."
In previous years, a female platypus had some of its genome sequenced, but without any 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 split into three groups, including monotremes, marsupials, and eutherians, or 'placentals'. We humans belong to that last group.
Together, the latter two make up a subclass known as therian mammals. Therian mammals all give birth to live young, but monotremes are simply too different to be lumped in with that group as well.
It's still unclear when all three of these distinct groups first began to diverge from one another. Some think the monotremes split off first, with marsupial and eutherians following suit. Others think all three groups diverged at roughly the same time.
The genome of the platypus has now helped clear up some of the dates. The data collected from echidna and platypus lineages suggests 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 that time, the semi-aquatic platypus has remained remarkably unchanged, fitting a niche in the Australian bush that many marsupials and mammals simply can't.
The authors were particularly interested in the animal's sex chromosomes, which appear to have originated independently from other therian mammals, all of which contain a simple XY pair.
The platypus, however, is the only known animal with 10 sex chromosomes (echidnas have nine). Platypus have 5X and 5Y chromosomes organised in a ring that appears to have broken apart into pieces over the course of mammalian evolution.
Comparing this chromosome information to humans, opossums, Tasmanian devils, chickens, and lizard genomes, the authors found the platypus's sex chromosomes have more in common with birds like chickens than mammals such as humans.
But while platypus lay eggs like chickens, they feed their young milk like therian mammals.
It's not too much of a surprise, therefore, that monotreme genomes contain most of the milk genes that other therian mammals possess.
Casein genes help encode certain proteins in mammalian milk, but monotremes appear to have extra caseins with unknown functions. That said, their milk is not unlike what comes from a cow, or even a lactating human.
As such, the platypus is probably not as dependent on egg proteins as other bird and reptile species because it can later feed their young through the lactation glands on its skin.
Its genome supports this. While birds and reptiles rely on three genes that encode for major egg proteins, the platypus appears to have lost the majority of these genes roughly 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 weird in-between, and its genome is a sort of bridge to our own evolutionary past.
"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," Zhang says.
The full genome has also revealed the loss of four genes associated with tooth development, which probably disappeared roughly 120 million years ago. To eat, the platypus now uses a pair of horn-like plates to grind up its food.
The venomous spurs on its hind legs can possibly be explained by the creature's defensin genes, which are associated with the immune system in other mammals, and appear to give rise to unique proteins in their venom. Echidnas, which also had their full genomes sequenced, appear to have lost this key venom gene.
The authors say their results represent "some of the most fascinating biology of platypus and echidna" alike.
"The new genomes of both species will enable further insights into therian innovations and the biology and evolution of these extraordinary egg-laying mammals," they conclude.