The first complete map of a platypus genome has just been released and it’s just as strange as you would expect from a creature with ten sex chromosomes, some poisonous spores, a layer of fluorescent fur and a skin that ‘sweats’ ‘. milk.
The mallard duckbill is truly one of the strangest creatures on earth. Along with the prickly echidna, these two Australian animals belong to a very specialized group of mammals, known as monotremes, which lay both eggs but also nurse their young with milk.
The genes of both are relatively primitive and unchanged, revealing a bizarre mix of several vertebrate animal classes, including birds, reptiles, and mammals.
As different as the platypus may initially appear, it is the differences that reveal our similarities and our shared ancestry with the other vertebrates of the earth.
Scientists think that the genome may tell us secrets about our own evolution and how our ancestors of mammals went into the distance from eggs to birth.
“The complete genome gave us the answers to how some of the bizarre features of the platypus emerged,” explains evolutionary biologist Guojie Zhang of the University of Copenhagen.
“At the same time, the decoding of the genome for platypus is important to improve our understanding of the evolution of other mammals – including our humans.”
In previous years, a female platypus had part of its genome, but without any Y chromosome sequences, much information was missing.
Using a male platypus, researchers have now created a physical map with a very accurate platypus genome.
Today, live mammals are divided into three groups, including monotremes, marsupials and euthanasia or ‘placentas’ – our humans belong to the latter group.
Together, the latter two form a subclass known as therian mammals. Therapeutic mammals all give birth to young live, but monotreme is simply too different to be captured in the group as well.
It is still unclear when all three of these different groups first began to differ from each other. Some believe the monotreme first split, with marsupials and euthanasia. Others think all three groups differ at about the same time.
The genome of the platypus has now helped clear up some dates. Data collected from echidna and platypus descendants indicate that their last common ancestor lived until 57 million years ago.
Meanwhile, it appears that monotremes generally deviated from marsupials and eutharic mammals about 187 million years ago.
Even after all this time, the semi-aquatic platypus has remained remarkably unchanged, and it fits a niche in the Australian forest that many marsupials and mammals cannot.
The authors were particularly interested in the sex chromosomes of the animal, which appear to be independently derived from other mammals, all of which contain a simple XY pair.
However, the platypus is the only known animal with ten sex chromosomes (echidnas has nine). Platypus organized 5X and 5Y chromosomes into a ring that appears to have broken into pieces during the evolution of mammals.
Comparing this chromosome information with humans, opossums, Tasmanian devils, chickens, and lizard genomes, the authors found that the platypus’ sex chromosomes had more in common with birds such as chickens than mammals such as humans.
But while platypus eggs lay like chickens, they feed their young milk like therian mammals.
It is therefore not too much of a surprise that monotreme genomes contain most of the milk genes that other mammals possess.
Casein genes help encode certain proteins in mammalian milk, but appear to have monotreme extra caseins with unknown functions. That said, their milk is no different from that of 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, as it can later feed their young through the lactation glands on the skin.
His genome supports this. While birds and reptiles depend on three genes that encode 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 specimen left.
The platypus is a strange in-between, and its genome is a kind of bridge to our own evolutionary past.
“It informs us that milk production in all mammals that currently exist was developed by the same set of genes that came from a common ancestor that lived more than 170 million years ago – along with the early dinosaurs in the Jurassic period, “says Zhang.
The complete genome also revealed the loss of four genes associated with the development of the tooth, which probably disappeared about 120 million years ago. To eat, the platypus now uses some horn-like plates to grind up its food.
The venomous spores on its hind legs could possibly be explained by the creature’s defense genes, which are associated with other mammals’ immune systems, and appear to cause unique proteins in their venom. Echidnas, which also had their complete genome sequence, apparently lost these important venom genes.
According to the authors, their results are ‘some of the most fascinating biology of platypus and echidna’.
“The new genomes of both species will enable further insight into the new innovations and the biology and evolution of these extraordinary egg-laying mammals,” they conclude.
The study was published in Earth.