[Animals] Platypuses reveal secrets about the evolution of mammals

[#Steeven.™]

Genetic analysis of platypuses and echidnas has given scientists some clues about mammalian evolution. These strange creatures are the only mammals known to lay eggs and are now known to have 10 sex chromosomes instead of the two normally found in other mammals, including humans.

A beaver-like, duck-billed species endemic to Western Australia and the island of Tasmania, the platypus exhibits a variety of bizarre characteristics: it lays eggs rather than giving birth to live young, produces milk, and has a few poisonous spurs, plus many more sex chromosomes than other mammals. For something it is considered one of the strangest animals in the world. Now, an international team of researchers led by the University of Copenhagen has carried out an exhaustive analysis of the genome of this strange creature and its first cousins: the echidnas. Their conclusions, recently published in the journal Nature, provide new data on this creature and its evolution.

Ever since Europeans discovered the platypus in Australia in the late 19th century, the peculiar semi-aquatic creature has baffled the scientific community.

It lays eggs, but nurses its young. It's not a bird, but it has a snout in the shape of a duckbill. It lacks teeth and the males have a poisonous spur. Ever since Europeans discovered the platypus in Australia in the late 19th century, the peculiar semi-aquatic creature has baffled the scientific community. Now, according to this new research, it is known that the monotremes, the order to which they belong, diverged from the therians, the subclass of mammals to which we belong, about 187 million years ago, about 21 million years before what was thought, while the two species (platypus and echidna) could have separated from each other about 55 million years ago. The genetic sequencing of both species has allowed researchers to shed new light on the evolution of these strange creatures, and thus of all mammals.

 

The importance of genetics and evolution
To better understand the importance of this study, it is first necessary to go back to the earliest ancestors of mammals: the first synapsids, which diverged from their sister group, the sauropsids, approximately 300 million years ago. These gave rise to today's reptiles, birds and dinosaurs, while synapsids continued to evolve until approximately 200 million years ago, when the first recorded mammals appeared, including australosphenids, the clade to which they belong. the current platypuses and echidnas. Why are they so strange? What genetic characteristics of these creatures survived to this day? Do modern mammals have some genetic trait in their chromosomes that comes from a common ancestor? These are some of the answers that the researchers were looking for.

"The whole genome has given us the answers to how some of the peculiarities that make these creatures such strange creatures arose. At the same time, decoding the platypus genome is important for improving our understanding of how other mammals, including the platypus, evolved. humans. How we evolved to become creatures capable of giving birth to live young,” explains Professor Guojie Zhang from the Department of Biology at the University of Copenhagen.

Platypuses, Zhang says, belong to the class of mammals, although they have retained many of the original characteristics of their ancestors, which probably contributes to their success in adapting to the environment in which they live.

 

Lays eggs but nurses its young
One of their most outstanding peculiarities is that they are capable of laying eggs and nursing their young, two qualities that are apparently incompatible with each other. Although, unlike echidnas, they perform this last function without having mammary glands. That is, the milk they produce 'exudes' directly from the pores of the skin.

The answer to this particularity, as detailed in the study, lies in the expression of the three genes of vitoligenin, a precursor protein in egg formation. All three genes are present, for example, in birds, but not in humans. The study found that platypuses still retained one of those three genes, despite losing the other approximately 130 million years ago, which is why they continue to lay eggs, even though they no longer need that protein for their young to develop. .

In all other mammals, including humans, the genes for vitellogenin have been replaced by those for casein, a protein of high biological value present in milk. In fact, the researchers found that the milk produced by platypuses is quite similar to that of other mammals like dairy cows to humans.

 

A common ancestor would explain milk production
"The results tell us that milk production in all extant mammalian species has evolved via the same set of genes derived from a common ancestor that lived around 180 million years ago, along with the earliest dinosaurs of the Jurassic period." Guojie Zhang says.

Another trait that makes the platypus so unique is that, unlike the vast majority of mammals, it has no teeth. Although the closest ancestors of these monotremes did have teeth, the modern platypus is equipped with a duck-billed snout that is used to grind food. The reason is still not known for sure, but the study reveals that it lost its teeth approximately 120 million years ago, when four of the eight genes responsible for the development of these pieces disappeared.

 

The only animal with 10 sex chromosomes
However, among all the oddities investigated, the most curious dealt with the way in which these creatures determined their sex. Mammals, including humans, have two sex chromosomes: X and Y, the combination of which determines sex. Monotremes, the researchers found, have 10: five Y and five X chromosomes.

Thanks to nearly complete chromosomal-level genomes, the researchers can now suggest that these 10 sex chromosomes in the ancestors of monotremes were arranged in the shape of a ring that later split into many small pieces of X and Y chromosomes. At the same Over time, genome mapping reveals that most monotreme sex chromosomes have more in common with chickens than humans, demonstrating the surprising link between mammals and birds. What new surprises will the study of these creatures bring us? The authors of the study affirm that they plan to carry out a more exhaustive sequencing that will allow us to delve even deeper into the secrets of these creatures that never stop surprising us: the last one to date, published last year, revealed that these creatures are capable of emit bioflurescent light.

 

https://www.nationalgeographic.com.es/naturaleza/ornitorrincos-desvelan-secretos-sobre-evolucion-mamiferos_16297