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article imageGenomic mapping — The platypus is part mammal, bird and reptile

By Karen Graham     Jan 11, 2021 in Science
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 platypus (Ornithorhynchus anatinus), sometimes called the duck-billed platypus is native to Eastern Australia and Tasmania. It is the sole-surviving member of its family (Ornithorhynchidae) and genus (Ornithorhynchus), though a number of related species appear in the fossil record.
Along with four species of echidna, or spiny-anteaters, the platypus is one of the five extant species of monotremes, which existed millions of years prior to the emergence of any modern-day mammals.
Monotremes are a highly-specialized group that both lay eggs but also nurse their young with milk, whereas mammals do not lay eggs. The platypus is unique because it not only hatches its young but feeds them milk that is excreted through its sweat.
Four of the five extant monotreme species: platypus (top-left)  short-beaked echidna (top-right)  we...
Four of the five extant monotreme species: platypus (top-left), short-beaked echidna (top-right), western long-beaked echidna (bottom-left), and replica eastern long-beaked echidna (bottom-right).
Ypna
Researchers have studied in depth the genes of both the platypus and spiny-anteater. The results of their genome mapping revealed the monotremes are a blend of several vertebrate animal classes, including birds, reptiles, and mammals. Their work was published in the journal Nature on January 6, 2021.
A shared ancestry with Earth's other vertebrates
Today, science splits living mammals into three groups; monotremes, marsupials, and eutherians, or "placentals." Humans belong to the eutherian group.
Together, the marsupials and eutherians make up a subclass known as therian mammals. This is because they give birth to live young. Monotremes are just too different to be lumped into this group.
A human (placental) holding two koalas  (marsupial)  representing the two extant infraclasses of the...
A human (placental) holding two koalas, (marsupial), representing the two extant infraclasses of therian mammals.
Bahnfrend
"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."
Scientists still don't know when the three distinct groups began to diverge from one another, however, it is believed that the monotremes split off first. Others think all three groups diverged at roughly the same time, reports Science Alert.
So, this is where the complete genome mapping has helped to clear up the question, sort of. The research data collected from echidna and platypus lineages suggests their last common ancestor lived up to 57 million years ago, while monotremes as a whole appear to have diverged from marsupials and eutherian mammals about 187 million years ago.
The duck-bill platypus lives in deep waterside burrows and is one of only two egg-laying mammals
The duck-bill platypus lives in deep waterside burrows and is one of only two egg-laying mammals
Greg Wood, AFP/File
The platypus has 10 sex chromosomes
The research has also shed light on why it can be difficult to determine the sex of the platypus, according to CTV News Canada, Every mammal on Earth has two sex chromosomes - an X and a Y.
The platypus has 10 sex chromosomes, five Y and five X chromosomes. The echidnas have nine sex chromosomes. The platypus chromosomes may have been arranged in a ring that appears to have broken apart into pieces over the course of mammalian evolution. And interestingly, the researchers found that the broken-up pieces of the monotreme sex chromosomes have more in common with chickens than with humans.
"Decoding the genome for platypus is important for improving our understanding of how other mammals evolved, including us humans,” explained Zhang. "It holds the key as to why we and other eutherian mammals evolved to become animals that give birth to live young instead of egg-laying animals.”​
More about Platypus, genomic mapping, monotremes, 10 sex chromosomes, spiky echidna
 
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